TAGB 140-D3 is a cold saw machine with high performance utilized in plants for production of welded tubes. The machine can be supplied in different versions according to the customer requests but, independently from differences that characterize one version compared to another one, the main feature which makes TAGB 140-D3 a technologically advanced machine is the blade shifting motion by brushless motor.Limited size and weight, their high dynamics – therefore their capacity to react in very short time to required.
Speed variations of saw cutting machine- make this type of motors particularly suitable for this kind of application.
The possibility to change in a few milliseconds rotation speed during cutting process allows, together with a specific control software, to remarkably reduce cutting time and, in general, to optimize blade work according to the profile to be cut.
General Description
In this paragraph a synthetic explanation of machine is made just with the aim of defining the main units which compose the machine itself.
The TAGB 140-D3 is a flying cutting machine with cold saw, appropriately designed to be inserted in a production line of tubular tubes where cut high quality and production high speed are required.
The machine basically consists of a base (ref. 3) on which a carriage (ref. 6) slides, carrying the cutting head (ref. 5), the vices unit (ref. 4), a tube bearing and guide system (ref. 1) located just before the vices and an exit support (ref. 2), located just after clamp outlet and leading the profile to the run-out table for the ejection. At the initial part of the cutting bed is placed an entry tube guide (ref. 8), which functions to guide the tube coming out of the mill and a safety sensor (ref. 12), which stops the machine in case the tube strikes on the vices or finds an obstacle during its shifting towards the ejection zone.
Appropriate screw anchors warrant the fastening of the support to the ground. On the sides of the base appropriate coupled decelerators (ref. 10) are properly placed in order to stop the carriage in case the motor drive can’t control it.
In the rear part of the base, the carriage motor drive, consisting of an electric motor (ref. 9), is housed and protected from the working area of the cutting head.
A cable conveyor chain (ref. 7) allows the electric, pneumatic and hydraulic supply to the users on carriage. Appropriate anchor bolts (ref. 11) warrant the fastening on the ground of the base (ref. 3).
Technical Specifications Of Cutting Machine
Raw Material Cold Saw
| Materials | Hot and cold rolled steel/ galvanized steel |
| Tube thickness (minimum – maximum) | 1.0 ÷ 6.0 mm |
| Tube length (minimum – maximum) | 3000 ÷ 12000 mm |
| Tube diameter (minimum – maximum) | 19 ÷ 114.3 mm |
| Profiles | Minimum square: 15×15 mm Maximum square: 90×90 mm Minimum rectangle: 20×10 mm Maximum rectangle: 120×60 mm |
Technical Data Cold Saw
| Maximum production speed | 140 m/min |
| Cutting length tolerance | ±1,5 mm on bars from 6 m |
| Break-down style | fixed tube centre/ constant bottom line |
| Tube passage height (bottom line) | 920 mm |
| Type of cut | electric cold saw |
| Type of blade | TCT = tungsten carbide HSS = high speed steel (super-rapid steel) |
| Blade maximum diameter | 520 mm |
| Blade maximum peripheral speed (D520 mm) | 550 m/min |
| Maximum blade thickness | 2,5 mm (Ø 450) 3,0 mm (Ø 520) |
| Blade rotation motor | Brushless 10,7 kW |
| Blade translation motor | Brushless 3,0 kW |
| Carriage motor | 60 kW |
| Carriage stroke | 3500 mm |
| Carriage transmission | Special inextensible belt |
Software specifications of cold saw
- Continuous cycle
- Start-stop cycle
- Rounding
- Cusp
- Downgrading
- Single measure immediate cut-off
- Clamps extension
Optional devices of cold saw
- Lubrication: oil-spray blade lubrication
- Blade cooling with emulsifying water
- Tube mobile supports
- Liftable supports on run-out table
- Predisposition for hot saw
- Interchangeable hot saw cut-head
- Remote adjustment for the vices pressure
- Sound-proof cabin
- Rack carriage launch
- “Diagnostics” package
- “Quality” package
- “Advanced optimization” package
- Tools Cutoff manager (management of cut parameters)
- Tools Blade Manager
Cutting Machine Noise Level Of Cold Saw
The cold saw machine noise level without the material being processed is brought about only by the moving mechanical organs, to which it’s necessary to add the peak noise produced by cutting operation, resulting prevailing and beyond Leq 87÷95 dB (A), with measurements executed at distance of 1m from the machine, with or without protections.
Moreover, during production, sound emissions are determined by the noise generated by the tubes being processed and by noise sources immediately upstream and downstream cold saw machine. Sound levels therefore depend on a number of factors: operating conditions, layout of the factory where the machine is employed and product loading/unloading machines upstream and downstream of the unit. When measuring sound pressure levels, use instruments conforming to I.E.C. 651 and I.E.C. 804 Class 1 standards, and take measurements from the points shown in Fig. IV-4 at the prescribed height from ground level (1,60 m) and distance (1 m) from the machine base:
Cutting Machine Cycle Of Cold Saw
This chapter describes the cold saw cutting machine functioning cycle.
This description is general, which means that all cutting machines produced by FIVES OTO S.p.A. in the last years follow this logic of functioning independently from constructive choices that differentiate the various models and from tube running direction.
That’s the reason why in all figures in this chapter a symbolic representation of the cut-off machine will be utilized.
In drawing V-1 a cut-off machine for tube line with right-left sense of direction is shown.
It shows the base (ref. 1), on which the carriage (ref. 5) slides and on which the cutting head (ref. 6) with the relative blade (ref. 7) and the vices unit (ref. 8) are assembled.
The two segments in ref. 2 and ref. 9 represent the limits of carriage normal functioning. It is a particular element of each cut-off machine and it represents a distance which the PLC utilizes to make the calculations of cycle. If the carriage exceeds these limits, the machine will generate an emergency signal and the production will be stopped.
The reference in ref. 9 (that is always on the side where the tube arrives) is also called “home position”: it represents the zero of carriage positioning measurement system. The reference in ref. 2 is also called “overtravel”.
The two indicators in ref. 3 and ref. 4 are utilized to find out the two fundamental positions of cutting head: cold saw cutting start point and motion reversal position, that is the point in which, after having cut the tube (ref. 13) the blade comes back to cutting start point (the one shown in figure).
These indicators don’t exist physically on the machine (blade positions are calculated by PLC according to data received by encoder) but they are represented with black triangles for an easier explanation. Ref. 14 finds the tube head coming from the tube mill with constant speed.
In ref. 10 the sensor (wheel connected to encoder) which detects the profile speed (production speed) is shown. Once the cut is carried out , the tube is supported and then ejected by a discharge system (belt conveyor or run out table) of which only the first two rolls are represented (ref. 15).
The two arrows (ref. 11 and ref. 12) are a symbolic representation of production speed (ref. 11) and carriagespeed (ref.
The arrow 11 will never change, which means that production speed will be always considered constant forcut cycle explanation, while arrow 12 will change as in length as in direction.
In the case shown in figure, tube and carriage are running at the same speed.
Cutting Time Of Cold Saw
It’s fundamental to explain before describing carriage cycle what cutting time means.
Differently from what logically could be thought cutting time is not the time spent by blade to cross (and then cut) the tube.
Cutting time means the time elapsed from the moment in which the blade leaves cutting start point to descend to tube direction to the moment when, after having reversed its motion, it recrosses that point again.
In drawing V-2 blade sliding cycle is shown
- Fig. A: cutting start point, it shows the situation before controlling the cut.
- Fig. B: cut control has been given (cutting time computation starts), the vices are closed, the blade has approached the tube and the proper phase of cutting is starting.
- Fig. C: tube cut is over, but the blade is still in descent phase to motion reverse point
- Fig. D: blade descent run is over, the vices open and the blade reverses its motion to rise with maximum speed.
- Fig. E: the blade reaches the start point in full return speed. In this moment cutting time computation is over: deceleration phase is starting.
- Fig. F: the return run is over and the blade reverses again its motion to reach cycle start point.
Carriage Cycle Of Cold Saw
The cold saw machine has the function of cutting, at prearranged lengths, the tube which is continuously produced by tube line and moreover it has to guarantee that the cut bar is correctly ejected from the cutting zone. Let’s assume to be in front of the machine that is regularly producing. Let’s start analysing the cycle in the moment when the carriage reverses its motion to synchronize itself with tube speed.
It’s shown the carriage reversal at a level C0 (C = carriage, 0 = observation start moment).
The profile head is at a level T0 from blade. The carriage is reversing its sense of direction, therefore it has no speed while the tube runs always with production speed.
The blade is in its position of cut start and vices are open.
We can observe that the carriage starts following the tube not necessarily from “home position” (that is C0= 0): the distance C0 is computed by microprocessor which controls the cycle and depends on cutting time and on the length of the bar to be cut besides other variables which are not specific parameters of the cut-off machine (carriage maximum speed, maximum acceleration, carriage available stroke) and therefore they don’t change according to the changing of the tube to be cut.
On the basis of these data check system computes the space required to the carriage to reach tube speed, to synchronize itself for a time equal to at least cutting time, then to decelerate, reverse its sense of direction and come back to start point to repeat the cycle from the beginning without stops.
When the time we call “1” is elapsed, the carriage will be in acceleration phase. Its speed is still lower than production one.
The profile head has moved further away from the blade because its speed is still higher then carriage one.
In figure the initial references (C0 and T0) are always kept, for an immediate comparison with the situation from which we have started.
Once the time we call “2” is elapsed, the carriage reaches the synchronization with the profile.
The cut control is given, the vices close and the blade starts its cycle.
From this moment starts the computation of cutting time, at the end of which the carriage will be able to start its deceleration phase.
After a time we call “3”, the blade has finished its descent stroke, the profile has been cut with at a T3 = T2 length and it’s in most part on the run-out table already.
The vices can open now or they can stay closed according to the option chosen on control console.
Cutting time is not yet over, therefore the carriage and the tube have always the same speed.
Carriage and tube have still the same speed but now the carriage can start the deceleration phase, which will lead it to motion reversal in correspondence of overtravel.
The cut profile leans with its most part on the rolls of ejection zone and it’s still supported by cut-off machine.
After a time we call “6”, the carriage is at the overtravel, that is the reversal point.
In normal functioning conditions the reversal point, which precedes carriage return phase, always coincides with the carriage forward limit position, over which an emergency sensor signals a functioning anomaly and stops the cycle (and the production).
The reason of this functioning is just to reduce at the minimum extent the overhang of the tube, which is coming out from the vices and must be located on the first roll of the ejection zone and, at the same time, as previously described, to guarantee the best possible support to the cut profile.
After a time we call “7” the carriage has reached its return speed, which not necessarily coincides with possible maximum speed. After a time we call “8” the carriage has returned in its initial condition.
The one previously described is a schematic and very easy representation of the various phases of cut-off machine working cycle.
Without explaining in detail the laws about carriage motion, it’s enough to know that by defining cutting time and tube length, the system microprocessor, that controls cut-off machine motion, calculates the maximum production speed and the stroke utilized by carriage.
For short cutting times there will be high production speeds and carriage strokes lower than the available maximum one (these are the conditions shown in the previous figures where the carriage doesn’t exploit all its stroke C6 but the difference C6-C0).
In these cases production maximum speed is limited not by available stroke, but by carriage maximum speed, that is a project value.
For long cutting time there will be lower production speeds and carriage strokes always equal to the value C6, that means that the cut-off machine exploits all available stroke.
In these cases production maximum speed is not limited by carriage maximum speed but by available stroke that is a project value.
Just to make some examples, the following graphs represents the three operative modes of the cut-off machine, the project data of which are:
- carriage maximum speed: 230 m/min
- carriage maximum stroke: 3400 mm
These data refer to a bar length of 6 meters:
- Cutting time: 1.4 s
- Production maximum possible speed: 130 m/min
- Return speed: 230 m/min (machine limit)
- Utilized stroke (not shown by graph): 3360 mm
The stroke utilized by the carriage is approaching the limit one: it’s enough that cutting time increases a bit to utilize all the stroke and in that case the limit will be not the maximum speed, but the stroke itself.
- Cutting time: 1.7 s
- Production maximum possible speed: 110 m/min
- Return speed: 180 m/min
- Utilized stroke (not shown by graph): 3400 mm/min (machine limit)
What is now limiting the performance is not carriage maximum speed but its stroke. Over this cutting time, the carriage will always utilize all its stroke.
Cold Saw Cut
INTRODUCTION
In this chapter the general aspects of cold saw cut and the main parameters connected to it are reported. Two main categories of blades utilized in steel tubular profiles production sector are explained: high-speed steel blades called HSS and tungsten carbide tips blade.
In the end guide lines for a logic choice of blades and cut parameters according to production requirements will be introduced.
The informations reported have general characters and they are not exhaustive;
they have the sole purpose of informing the reader about different problems connected to the cold saw cut and therefore of guiding him to find out a parameter range- to be used to cut a determined tube- which can be considered a starting point for following optimized choices.
General Aspects Of Cold Saw Cut
There are many different technologies to cut steel tubes.
The most utilized ones, in increasing order of cut quality, are:
- hot saw cut, also called friction saw cut because the cut is obtained by metal fusion;
- cut-off;
- cold saw cut.
Each of these cut technologies has its well defined place in the production field of steel tubular tubes, but in the last years market request is surely orienting on product quality, and therefore the cold saw cut-off machine, considering plant simplifying it concerns and the continuous progress in the development of ever more performing blades, seems to be the winning technology.
where it’s possible to see the blade with three grasping teeth (teeth in position 1, 2 and 3) is shown. Tooth in position 4 has already came out of the tube, bringing the chip removed during the cut.
In the drawing the chip is represented a single piece and with a size different than the true one, in order to emphasize that the tooth goes along with cutting, the quantity of removed metal, conveyed in the space which separates it from the tooth that precedes it, is more and more increasing.
For example, when the tooth passes from position 3 the length of the removed (and therefore conveyed) chip is approximately equal to value L13.
Between cutting start point and cutting end point the tooth has therefore removed a chip with a length approximately equal to the arc Ac.
Fortunately, things are actually different, which means that tooth profile is made in a way that the chip has the tendency of disrupting than bending or remaining in one piece.
This is not the best place for delving into tooth profile analysis, let’s just say that the two main parameters are cut angle (α) and rake angle (γ), which sensitively change according to material to be cut.
Once the material to be cut is established (remember that for ferrous materials utilized in most cases in tube mills, the value of cut angle α is about 18° and rake angle γ is about 8°-12°) the three determinant factors which influence cut result, both in therms of quality and in therms of blade duration, are:
- blade tooth pitch (p);
- cutting speed (Vt);
- tooth load (az).
The pitch p (fig. A) is the distance between two teeth and it’s connected to blade teeth number z by the relation:
| z = (Blade diameter x 3.14) / p (A) |
Cutting speed Vt coincides with blade peripheral speed and it’s measured in meters/minute (m/min). It’s obviously linked to the number of blade revolutions per minute n by the relation:
| n = Vt / (Blade diameter x 3.14) (B) |
Tooth load az is emphasized in fig. A and represents the feed made by the blade in arrow Va direction (progress or feeding speed) in the time spent by a tooth to take the place of the previous one. It’s indicated in millimetres per tooth (mm/tooth)
In other words, it represents the thickness of the “thin slice” removed by each tooth.
Tooth load value together with cutting speed value determine blade feeding speed Va explained in millimetres per minute (mm/min) according to the relation:
| Va = (az x z x Vt) / (3.14 x Blade diameter) (C) |
By a rapid observation of the relation (C) we can find out that, once the blade is chosen, therefore once diameter and teeth number z are defined, there are only two ways to increase progress speed:
- increasing of tooth load;
- increasing cutting speed.
It’s required to remember that the choice of pitch, cutting speed and feed can’t be increased unconditionally because they depend on:
- tube type and size;
- material features.
For example, Cold saw a recommended pitch for a 40×40 square cut with thickness 3 will be surely lower than the one for a round cut with a 38 diameter, with thickness 3 which is the mother tube of the one 40×40. To cut a profile made of a material with a yield stress near to the ultimate strength, a certain pitch will be utilized; to cut the same profile made of the same material and with the same ultimate strength but with a very lower yield stress, the choice will involve a bigger pitch.
Types Of Blades Utilized In Cold Saw Cut
There are two different types of blades utilized on flying cold saw cut-off machine:
- high-speed steel blades (HSS – High Speed Steel)
- blades with added inserts (normally called TCT – Tungsten Carbide Tips – because the inserts are normallyrealized with tungsten carbide)
Fig. A: HSS coated with chip-breaker teeth.
Fig. B: HSS coated with BW teeth.
Fig. C: TCT Kinkelder.
Fig. D: TCT Stark.
HSS blades are made of high-speed steel and they are treated with special lining with the aim of improving their surface hardness and lowering their coefficient of friction.
The friction between teeth side and the tube generates heat which is transmitted to blade body increasing its temperature.
HSS blades give no problem until about 500 °C, while higher temperatures can cause transformations in steel intrinsic structure. That’s why it’s very important the choice of cutting speed and of cooling type utilized according to geometry and material of the tube to be cut.
For the cut of ferrous materials the most utilized teeth are the ones with chip-disrupting device and the oneBW type, with teeth alternatively relieved.
These blades can be sharpened again for many times if during their using they are not seriously damaged (complete braking of one or more teeth) and performances after a careful sharpening are not far from the ones of a new blade. The life of a HSS blade, or better, of a sharpening, is measured in m2 of removed chip, but it’s not possible to quantify it for sure because it depends on:
- type of cut-off machine where it is assembled;
- type of cut-off machine where it is assembled;
- type of treatment in blade surface;
- quality of blade sharpening;
- material and size of the profile to be cut;
- utilized cut parameters;
- utilized lubricant type.
Blades with added inserts are appropriately created for cutting solid materials but technological progress and requests of some enlarging market sectors have pushed their use also in tubular profiles sector. They consist of a less hard steel body but tougher compared to the one of HSS blades and in teeth side, where the first contact between blade and tube occurs, a seat is machined where inserts generally made of tungsten carbide are welded.
These inserts have the feature of being extremely hard (but relatively fragile) and resisting to high temperatures, so they are suitable to work at very high cut speed.
Blades with added inserts are created to satisfy the requirement to cut in short time materials with high carbon percentage (> 0.2%), top ultimate strength at 600-800 N/mm2 and big thickness, but they require a very accurate check of cut parameters, very rigid transmissions without clearances to reduce at minimum every vibration which could damage insert fragility.
These blades generally can’t be sharpened again, or it will be possible to carry out one or two sharpening if the inserts are not damaged or splintered during cuts and if their utilization is not very heavy. The average life of a blade with added inserts is surely lower, at the same conditions, compared to the one of a HHS blade because it works in harder conditions and it’s more sensitive to vibrations always present during cut.
Basic Guidelines For Blade & Cut petameter Cold saw
The essential aspects of the two principal blade types utilized on cut-off machine in a tube mill were previously pointed out.
It’s important to underline again that HSS blades have a lower cost than TCT blade one and above all they can be sharpened again.
There is no defined limit which separates the utilizing of HSS blades from the utilizing of TCT blades; surely there are conditions of use in which a type of blade is more recommended than the other one.
The choice of blade type depends on the following factors:
- dynamic and controls of cut speed and feed of cut-off machine;
- chemical and mechanic specifications of profile materials;
- tube size;
- specific requirements linked to tube production;
- production speed.
The type of cut-off machine can still impose the kind of blade to be used which means that if the maximum permissible cutting speed is not high (for example lower than 250-300 m/min), the feed is not completely controllable and motion transmission system is not enough rigid, the use of TCT blades will not surely be recommendable.
In cut-off machines which run with a control system at closed loop, equipped with motors at high dynamics and rotation speed (for example brushless motors) and with transmissions enough rigid, the use of TCT blade guarantees performances not possible using HSS blades.
In this paragraph we refer only to this last case because cut-off machines of TAGB series are equipped with a system for blade motor drive and control, purposely devised to optimize both HSS and TCT blades employment.
About tube material we are considering only the case of carbon steels.
The chemical composition of metal, in particular the percentage of carbon, sensitively influences material mechanic features and therefore the most suitable type of blade to be utilized.
The type of profile (square, rectangle or round) and its sizes are factors that intervene above all in tooth pitch choice more than in the choice of blade type.
Thickness, instead, can be a discriminant element for blade choice.
The requirements linked to production must be also considered, which means that there can be cases when the tube must be dry-cut (maybe it has just been painted in line) or the blade can’t be cooled with water jet at high pressure in order not to remove the protective oil which has been previously applied on the tube, and so on.
An element surely of extreme importance is production speed.
It’s required to value if the primary aim is to obtain production high speeds to the detriment of an elevate cuttingcost or optimizing production speed with the scope of maximizing blade efficiency (lower cutting cost).
In drawing VI-2 each column represents a different combination of factors which ends with a final number indicating the choice of recommended blade.
1: we are surely in the field of HSS blade;
2: in most cases HSS blade is utilized but also TCT blade can be an alternative to be considered case by case;
3: it’s the intermediate condition when there are the necessary prerequisites to utilize both HSS blade and TCT blade;
4: in most cases TCT blade is utilized but also HSS blade can be an alternative to be considered case by case;
5: we are surely in the field of TCT blade.
In most cases the materials utilized for the production of tubular profiles (from hot or cool rolled strips) are steels with carbon low percentage (< 0.2%) and ultimate strength lower than about 500 N/mm2 . In these conditions generally HSS blades are used.
If thickness is not particularly high or if there are no specific production requirements which prevent the use of emulsifying water for blade cooling or the use of nebulized oil for spray lubrication, the choice of HSS blade is the best solution, both economically and for cut quality.
Obviously, when tube size and thickness increase, the TCT blade can be utilized only if it’s required to keep extremely high production speeds or if dry cut is required.
Example:
A 60×4 tube has a cut time of about 3.3 seconds using HSS blade and a line speed of about 60 m/min, thesame tube has a cutting time of about 1.3 seconds with TCT blade and a line speed of about 135 m/min.
carbon high percentage (> 0.2%) and ultimate strength higher than 500 N/mm. The utilizing of TCT blade always guarantees cut short times because its cutting speed can remain very high also with hard materials.
Tooth Pitch Cold Saw
Once the type of blade to be utilized and its diameter, which depends on cut-off machine size, are chosen, it’s required to define tooth pitch and therefore teeth number.
The choice of the pitch depends on two factors:
- contact arc maximum length;
- chip deforming
Contact arc Ac is the space covered by a single tooth while it’s removing the chip, between the moment it comes into contact with the profile and the moment in which it comes out of the profile.
In drawing two kind of tubes are shown, a round one and one of its derived rectangles and the positions of the blade in 4 different points.
In position 1, for the round tube, blade teeth start working in point 1.1 and finish in point 1.2: the contact arc is therefore the whole length from point 1.1 to point 1.2.
In position 2, blade teeth start working in point 2.1, finish in point 2.2, restart working in point 2.3 to finish then in point 2.4: contact arc is therefore the length from point 2.1 to point 2.2 added to the length from point 2.3 to point 2.4.
Position 3 is analogue to position 2 and position 4 is analogue to 1, with the only difference that the length of contact arc is a bit higher in point 4 than in point 1.
In general for round tube the contact arc changes very much during the cut, there is a peak in the initial phase and another peak in the final one, while in the central part the arc keeps always very low.
Things are different for rectangular tube (conceptually similar to the square one) derived from the round tube previously described.
In figure we can find out that contact arc doesn’t change a lot during cut and, above all, its length never reaches the values pointed out for the round tube.
The two graphs show very clearly what is described in the previous page.
They represent the variation of contact arc according to the variation of blade position, that in this case has a diameter of 350 mm.
The first graph refers to a tube D.60 thickness 4, while the second one refers to a rectangle derived from it, size 60×30 thickness 4.
To find out the maximum length of contact arc that from now on we will define as Ac.max some simplified formulas will be used.
The simplification means considering blade diameter infinitely big compared to the sizes of tube to be cut. It’s obvious that the higher the blade diameter/tube diameter ratio is, the better is the approximation.
In case of round tube the following formula is applied:
In figure VI-7 the several graphs show as contact arc length changes since the blade starts to cut the rectangle. It’s therefore impossible to find out an easy but approximate enough formula to allow to consider all these factors calculating Ac.max in rectangle case.
As described below, Ac.max value sensibly influences the choice of blade pitch.
It’s also true that it’s impossible to have a number of blades each equipped with a pitch appropriately chosen for a cut of a determined rectangle or square: normally the utilized pitches are three or four for all the production in a tube mill (just to give an example, a production range could be from 20×1 round to 76×4 round and relative derived tubes).
For these reasons the exact calculation of Ac.max in case of rectangles, that would be very complicated, has no practical interest. The following simple procedure allows to find out a value approximate enough for case requirement:
- calculate Ac. max of mother tube from which the rectangular (or square) profile is derived using the simplified formula (D)
- multiply the obtained value by the corrective factor K which depends on the ratio Mother Tube Diameter
Thickness and it’s included between two limit values:
K = 0.4 for values of the ratio Mother Tube Diameter / Thickness > 20
K = 0.7 for values of the ratio Mother Tube Diameter / Thickness < 10
For values included between 10 and 20, K values included between 0.7 and 0.4 must be chosen.
Remember that these multiplicative values derive from approximations considered in a range of tubes with maximum diameter of 89 mm and maximum thickness of 6 mm and with blade inclination of 30° compared to horizontal plane.
Once known the maximum length of contact arc the choice of pitch p is immediately calculated prearranging the maximum number of teeth in progress Zp.
| p= Ac.max /Zp (E) |
- type of cut-off machine, in particular if it can manage different feed speeds;
- tube sizes and material;
- type of blade (HSS or TCT).
The two graphs represented in Fig. VI-8 (valid for HSS blades) show as Zp indicatively changes according to Ac. max in case of rounds and rectangles (remember that the square is a particular case of rectangle). The upper straight line (red) points out the suggested maximum value while the lower straight line (blue) points out the minimum value.
Examples:
Blade diameter: 350 mm
Tube diameter: 32 mm
Tube thickness: 2 mm
Important:
The approximations introduced in calculations to have easy and quick formulas to be managed, generate not correct results in the lowest extremes of the considered productive range.
Just to give an example, for a square 15×15 thickness 0,7 a very little and absolutely insignificant pitch would be pointed out.
That’s why it’s required to remember the functional and constructive limits of blades.
In the particular case, let’s consider that:
- HSS blades type have a minimum pitch of about 3.5 mm.
- TCT blades type have a minimum pitch of about 8 mm.
For the other examples only the results of calculations previously described are reported
- Blade diameter: 350 mm
- Tube diameter: 60 mm
- Tube thickness: 4 mm
- Ac. max = 29,93 mm
- Zp min = 3.2 ⇒ p max = 9.35 ⇒ z min = 117
- Zp max = 4 ⇒ p min = 7.48 ⇒ z max = 146
- Blade diameter: 450 mm
- Tube diameter: 76 mm
- Tube thickness: 2 mm
- Ac. max = 24,33 mm
- Zp min = 3.1 ⇒ p max = 7,84 ⇒ z min = 180
- Zp max = 3,9 ⇒ p min = 6.23 ⇒ z max = 226
- Blade diameter: 450 mm
- Rectangular tube: 80×40 mm
- Tube thickness: 2 mm
- Ac.max = 24,2 mm
- Ac.max for rectangle = 24,2 * 0,4 = 9,7 mm
- Zp min = 1.8 ⇒ p max = 5,4 ⇒ z min = 260
- Zp max = 2,7 ⇒ p max = 3,6 ⇒ z max = 390
- Blade diameter: 450 mm
- Rectangular tube: 60×30 mm
- Tube thickness: 4 mm
- Ac. max = 28,5 mm
- Ac.max for rectangle = 28,5 * 0,6 = 17,1 mm
- Zp min = 2.2 ⇒ p max = 7,8 ⇒ z min = 180
- Zp max = 3,2 ⇒ p max = 5,3 ⇒ z max = 260
In the choice of pitch and therefore of the blade number of teeth, besides considering tube sizes, as we have made up to now, it would also be required to consider the material and in particular way its deforming. Moreover the possibility to change blade feed speed during cut is very important. The space which separates two consecutive teeth has the function of collecting the chip that is detached from tooth.
If the material is very deformable the chip will kink and therefore the space occupied inside the room will be bigger than the space required by a chip made of fragile material, which, on the contrary, would shatter because it wouldn’t be able to stand the strains imposed by the tooth.
Tooth maximum and minimum calculated values can be considered using this point of view:
for the same size of the tube to be cut, logically, a bigger pitch should be kept for very deformable materials and a lower pitch for more fragile materials.
Choices oriented to a high number of teeth will be recommendable only if mechanics and control system of the cut-off machine guarantee repetitiveness, accuracy and possibility to change feed speed according to blade position during cut.
A high number of teeth in progress gives more stability to the blade and a better superficial finishing, reducing vibrations and extending the life of the blade itself.
But at the same time a high number of teeth means a relative small pitch that limits the possibility of collecting the chip, therefore it imposes blade low feed speeds and consequently too long cutting times. As shown in the previous pages the major changes of contact arc occur in the cut of round tubes rather than in square and rectangular tubes.
In these conditions, the possibility of advancing only in tube entry and exit zones (maximum contact arcs) and increasing the speed in the central part reduces the whole cutting time and at the same time dispenses more uniformly the load on blade (see the following paragraphs).
Cut speed Vt cold saw
It depends mainly on:
- Type of blade;
- Material to be cut;
- Maximum length of contact arc.
In the sector of welded steel tubes production, the currently achievable cut maximum speeds using HSS blades by a quality coating is about 250 m/min.
This speed is commonly utilized in the cut of tubes with small sizes in material St37or St43.
For the same material, cutting speed is reduced as the maximum length of contact arc is increased.
The data summarized in the table below point out the recommended cutting speeds according to the maximum length of contact arc.
| Tube diameter | Thickness (mm) | Ac. Max (mm) | Sp (m/min) |
| 20 30 38 50 60 76 89 | 1 2 3 3.5 4 4 4 | 8.7 15 20.5 25.5 30 34 37 | 230 200 180 160 140 120 110 |
They become absolutely unacceptable when it’s required to cut high strength materials, with ultimate tensile strength for example of 800-1000 N/mm2.
Moreover, remember that also not particularly hard materials can become critical during tube processing phase because the welding zone subjects a proper hardening, therefore if carbon percentage is high enough, important changes of superficial hardness (and then strength) might occur on the finished product. In these cases and generally when material specifications involve an unacceptable wear of HSS blades, it’s required to use TCT blades.
For this type of blade, cutting speeds up to 800 -1000 m/min. can be reached.
These values can be used for the cut of materials with ultimate tensile strength up to about 600 – 800 N/mm2 while for higher loads, for example 1200 N/mm2, it’s required to sensibly reduce the speed to guarantee an acceptable blade life.
Passing then from carbon steels to stainless steels the speeds decrease up to about 200 – 300 m/min.
Tooth load az of Cold saw
Once the pitch and the cutting speed are chosen it’s required to calculate tooth load (az) which mainly depends on:
- Type of control the cut-off machine is equipped with;
- Tooth pitch;
- Length of contact arc.
In figure three particularly significant points through which the blade passes during the cut are represented:
- point B corresponds to minimum contact arc in tube central part;
- point C corresponds to absolute maximum contact arc – exit from tube phase.
- point A corresponds to maximum contact arc during the entry phase into the tube;
The same points are emphasized in the graph that reports in abscissa the distance of blade from the point of first contact with tube and in ordinate the length of contact arc: in the particular case tube diameter is 50 and thickness is 3.
The two remaining graphs show blade load changes – curve 1 – in case of cut at constant feed speed – curve 2 (graph on the left side) and in case of cut at variable feed speed – curves 3 and 4 (graph on the right side).
In both cases the same blade (Diameter 350 and tooth number Z = 170) and the same cutting speed are used, but the tooth load changes: while it remains constant in the first case, in the second case it changes to reduce the peaks in point A and C (lower feed speed) and to raise the load in central area B (higher feed speed) where contact arcs are very small. The result is a more uniform blade load but, above all, an actual sensibly lower cutting time (a bit more than 1,5 seconds against a bit more than 2 seconds).
To calculate tooth load start from blade pitch p.
Through the graph shown in figure value A (it indicatively represents the space of chip that can be accumulated between each tooth) can be pointed out in correspondence of a certain blade pitch p. Tooth load is calculated using the following formula:
| az = A / Ac.max (F) |
Let’s assume as an example a tube 50×3 in St43, which also the cases reported in figure VI-11 refer to. Consider blade diameter of 350 mm (0.35 m).
Zp min = 3 ⇒ p max = 7,9 ⇒ z min = 140
Zp max = 3.8 ⇒ p min = 6.2 ⇒ z max = 175
Choose a blade with z = 170 ⇒ p = (350 * 3,14) / 170 = 6,47
For p = 6.47 the graph emphasizes a value A = 1.6 from which using the formula (F) the following tooth load is obtained:
az = 1,6 / 23,7 = 0,0
As cutting speed it’s possible to choose Vt = 160 m/min
Using the formula (C) blade feed speed is obtained:
Va = (0.07 * 170 * 160) / (3.14 * 0.35) = 1732 mm/min
This speed, constant along the whole cutting time, is represented by curve (2) of figure VI-11 and generatesthe blade load shown by the curve (1).
Decreasing tooth load in zones in inlet (A) from 0.07 to 0.045 and in outlet (C), from 0.07 to 0.04 and increasing it in the central zone from 0.07 to 0.14 it’s emphasized that the curve (3) of blade load is more uniform and the actual cutting time is reduced.
In the easiest case, that is a cut at constant feed speed, the actual cutting time (time spent by blade to cross the tube) expressed in seconds, it’s calculated using the approximate formula (because it doesn’t consider accelerations):
where L is the length covered by blade from the moment of contact with tube (cutting start point) to the moment in outlet (cutting end point).In the specific case it coincides with tube diameter, therefore:
t = 50 * 60 / 1732 = 1.73 seconds.
