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Milling Machine Low Speed Torque

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Peter Gain24/07/2009 14:53:23
103 forum posts
I have a Warco WM16 mill/drill. When used at slow speed for fly cutting, even in "backgear" there is very little torque. Either the machine stalls or the speed has to be increased, not desirable when flycutting. I prefer fly cutting as I obtain a good finish & the tooling is easily sharpened in the shop.
Is the lack of torque due to having electronic speed control? Would a mill having belt drive provide better low speed torque? Are there any retro fit electronics that would effect an improvement?
All suggestions will be welcome.
Peter Gain.
 
Jim Nolan24/07/2009 18:08:21
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77 forum posts
My understanding is that with some electronic speed controllers they do not have much torque at the low end. How you overcome it I don’t know, perhaps a Google search would help?
 
Jim
Peter Tucker24/07/2009 23:13:52
183 forum posts
I have a small lath-mill combination mathine with wich I fly cut a 4.5" dia. at 390 rpm. The cutter tool is a 0.5"dia rod with a TC saw blade tip brased to it.  Very light cuts need to be taken (0.25mm max).
 
Peter
Steve Sparrow26/07/2009 10:38:13
8 forum posts
Hi Peter,
 
Yes, an inverter (Electronic Speed Control) will give a lot less torque at low speeds simply because of the way it works.
 
One way to get round the problem (if you have mechanical means of changing spindle speeds) it to arrange things so that the motor is rotating as fast as possible (up to 50Hz)whist still allowing you to still have a low spindle speed. This can often be achived by moving belts or gears and is akin to having your car's gearbox in first gear - Lots of torque and engine rev.s but not much speed at the roadwheels.
 
Inverters work by varying the frequency (Hz) that run the motor and your aim is to get as close as you can to getting the required spindle speed when the inverter is running the motor at 50Hz. This is where your motor will produce its maximum torque.
 
If you don't have the means to change spindle speeds in the usual ways (belts, pulleys of gears) then a change of motor and spindle pulleys may be the only alternative. You might even be lucky enough to be able to simply swap them to the opposing poistion and use the same drive belt....
 
Regards
 
Steve
Peter Gain31/07/2009 09:59:02
103 forum posts
Hi Steve,
Thanks for your comments. My machine does not have any belts & I cannot change the gearing. The only practical speed change method is by electronic control. Do you have any data to enable me to upgrade the existing controller?
Regards,
Peter.
Donald Lunn31/07/2009 22:14:58
9 forum posts
Hi Peter
 
When you say that you are using electronic control do you mean that you are using an inverter,or just a speed control.I use an inverter on my Mill/drill and lathe but i don't lose hardly any torque at low speed on mine,simply because i paid extra and purchased the the one that keeps torque through the whole range.Thats why i have never quite understood, when i read that people who have inverters lose the torque at low speeds,and i have read this on a number of occasions.Please feel free to contact me on Message member if you wish.
 
Regards
     Don
Peter Gain01/08/2009 11:24:43
103 forum posts
Hi Don.
Thanks for your reply. The speed controller on my mill/drill is just an electronic type, not an inverter. The supply is 240vac 50hz. As I understand it, the circuit rectifies to DC then when the speed control is turned down the current is chopped into a series of ever shorter pulses until they are so short that the motor stops. Maybe an over simplification but that's the basic principle. (I believe that a similar principle is employed by model railway enthusiasts to provide a reasonably smooth operation at low speeds). The machine works well with end mills, solid or indexable which I run at revs as per Tubal Cain's workshop book, ie several hundred rpm. When the speed is set to (say) 50/60 rpm for fly cutting m/s with an HSS tool anything greater than a 10 thou cut causes the machine to stall as soon as the cutter hits the work piece. I have timed the revs & the digital counter is accurate. It may be the case that I am expecting too much from a Far Eastern product.
Regards,
Peter.
Richmond01/08/2009 11:41:00
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73 forum posts
632 photos
64 articles
Hi Peter,
 
One thing you dont mention is the diameter of the swing of the flycutter .....
 
Use single point cutters at low speed IF you have to, BUT, multi point facing cutters are probably more suitable with the type of mill and electronic speed control you have.
 
However, in my experience, and I use inverters a lot in my workshop ( 7 at last count ), torque compensation is NOT good at low speed, but is manageable on an AC motor with belts / gearbox etc.
 
From memory the speed control in your mill are of the DC type and hence are more prone to stalling at low speed, not becasue of the supplier, but more because of the constraints of the design i.e. using DC chopping.
 
So, to recap my advice would be to buy a multipoint facing cutter, and increase the speed accordingly.
 
Rgds
Richmond
 
 
HasBean01/08/2009 14:29:18
141 forum posts
32 photos
I once accidently ran a fly cutter at a far greater than recommended speed only to find that the finish was excellent and that the cutter hadn't suffered and was as sharp as before.
Consequently I run them now at two or three times the accepted rate, as long as the toolsteel is Cobalt  HSS and the cuts are light I don't have any problems (apart from the chips flying off all over the place!)
Paul
Donald Lunn01/08/2009 21:24:15
9 forum posts
Hi peter
 
Yes i see the problem with flycutting and use of a speed controller.I agree with richmond on the multipoint facing cutters,looks like that is the best bet.But i like using a flycutter at times,although it's not the fastest cutter to use.But i do find it is the cheapest.So it must be a pain for you if you like to use it.
However,i must agree to disagree with richmond about the torque on the inverter at low speed,at least as far as my inverter is concerned.If you ever think about changing to inverter use i can certainly point you in the direction of the right man to ask for advice.(not me i might add).
 
Don
Richmond01/08/2009 22:14:58
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73 forum posts
632 photos
64 articles
Hi Don,
 
When I say inverters are not good at torque compensation at very low speed I mean that whilst most will offer 150%,and I was basing this on a 1:1 gear ratio ......not via belts and pulleys ......machine spindles can be easily stopped.
 
I'm not talking cheap and cheerful inverters here I am talking Fujitsu, Siemens, Danfoss...... and ALL have the same issues at low speed with a 1:1 motor spindle ratio.
 
So I stick by my comment that a very low speed 50 RPM inverters are problematic without belts / gearboxes to correct.
 
And no, I don't want to get into a "war" of words ....but my knowledge is based on practical expereince in my own workshop
 
Happy swarf making
 
Richmond
John Stevenson01/08/2009 23:43:00
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Moderator
5068 forum posts
3 photos
After fitting well over 300 plus inverters over the years and working closely with IMO, Telemechanique and Yaskawa technical let me explain a few truths about how these work and how the ratings are arrived at.
 
First how they work, they take AC in, turn it to DC and chop it into pieces, organise it then change it back to AC but 3 phase. Its how the chopped up bits are organised that's the magic bit .
 
Now the BS part. the all singing all dancing feature is called Vector drive, where this differs from the old type of variable frequency drive is that it monitors the load and when it drops it bungs a bit more down the line to play catch up then drops back as it does.
 
The next bit they all push is Constant Torque and we need to explain a bit more about these chopped up bits. Technically at 50 Hz [ in the UK ] 240 volt single phase in with spit 240 v out 3 phase but as you drop frequency it drops voltage so at something like 15 Hz it probably putting out 60  to 70 volts AC three phase.
 
Now they say it will produce constant torque at all speeds, what they NEVER tell you is that torque is based on output voltage so at 50 Hz you get full torque of whatever the motor is rated at.
At 15 Hz you get full torque based on 60 to 70 volts, so a 3.3 actual reduction but in theory Constant Torque .
 
Most machine tools get torque by gearing, a 2:1 reduction doubling the torque but this cannot happen electronically, only mechanically .
 
 
What happens in practice and I can prove this as I have a CNC  X3 here that has a two speed direct belt drive and inverter, no names but well know torque vector all singing all dancing. At low speed 20 Hz in the high range which equates to about 1,000 rpm when the cutter enters the work it slows as the Vector drive part kicks in and speeds the spindle up to compensate for load.
 
However the feed isn't linked to the vector drive and that slowing down doesn't take into account the feed has remained constant with the result it's chipped, rubbed the cutting edge and we are now playing with half a deck.
 
 
It can be over come but it's acase of working on available power and gear ratio's, I have even got the technical side agreeing with my findings.
 
Whilst a inverter drive is nice and to be honest I wouldn't be without them it's not the answer to a straight swap on something like the small Chinese millers with DC drives which have inherently more torque at low revs anyway .
 
 The above is a very advanced technically description of how these things work, for a more laid back description you need a Heavy Goods license.
 
John S.
 
Peter Gain02/08/2009 11:28:55
103 forum posts
Hi All,
Thanks for your comments.
Re-fly cutter dia. For FCMS I have been using an HSS cutter with a 3" dia & 30 thou cut @ 50rpm. I culled these recommended figures from "Vertical Milling In The Home Worshop", Arnold Throp, Workshop Practice Series No. 2.
At 30 thou cut the mill will usually stall as soon as the cutter hits the workpiece. With a 10 thou cut the machine can cope but of course the job becomes prolonged.
Having read the forum comments, the penny has dropped. Throp was writing over 30 years ago & the machines to which he refers were good quality British products that employed belts & pulleys.
I will try Has Bean's tip & run the mill at a higher speed. The attraction of fly cutting is that I can easily grind & sharpen HSS using the simple rest described by Harold Hall. For some unknown reason I do not seem to be able to get on with indexable tips. I never seem to obtain as good a finish as with HSS. (I find this with lathe tools as well).
Regards,
Peter.
 
Ian S C02/08/2009 12:05:41
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7468 forum posts
230 photos
Hi Peter,with your indexable tips try 3-400rpm,not too deep cut and feed reasonably fast.How when the motor is running so slow is it kept cool,the fan inside isn't going fast enough. IAN S.C.
Peter Tucker03/08/2009 07:01:05
183 forum posts
Hi Peter,  a HSS cutter on FCMS shoud beable to cut at 100 feet per minute, wich for a 3"dia is about 135 rpm. this may help however if you can get brazed TC you could cut at as much as10 times this speed.  A brazed TC cutter can be sharpened just like a HSS one, however a green grinder weel is helpfull.
Regards,
Peter.
Ian S C05/08/2009 13:37:35
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7468 forum posts
230 photos
When I bought my mill(Rexon 400)I asked the sales man about using the three cutter 50mm face mill,and his reply was,OH about 900 rpm,and crank like h**l,I usually run a bit slower than that,and I have a home made power feed,and thats quite sedate even at its top speed.The first cutters I had,had chip breakers,but I was told much later that I should use plain tips to get a better cut.The tips I use are plain with no hole-held in with a bridge piece,when they are too warn for the mill I sharpen them on the green grit wheel,polish the edge with a diamond lap and use the on the lathe in a tool holder made from a bit of car back axle,sometimes broken bits get brazed on boring bars or other tools.
Ian S C05/08/2009 13:38:11
avatar
7468 forum posts
230 photos
When I bought my mill(Rexon 400)I asked the sales man about using the three cutter 50mm face mill,and his reply was,OH about 900 rpm,and crank like h**l,I usually run a bit slower than that,and I have a home made power feed,and thats quite sedate even at its top speed.The first cutters I had,had chip breakers,but I was told much later that I should use plain tips to get a better cut.The tips I use are plain with no hole-held in with a bridge piece,when they are too warn for the mill I sharpen them on the green grit wheel,polish the edge with a diamond lap and use the on the lathe in a tool holder made from a bit of car back axle,sometimes broken bits get brazed on boring bars or other tools.IAN S C
John Haine16/10/2009 20:27:34
4428 forum posts
264 photos
Peter,
 
Going back to your original question about improved drive electronics, you could look at "Pentapower" units from KB Electronics (http://www.kbelectronics.com/).  I got one of these with a DC quarter-horse motor from Model Motors Direct (not used yet but may be fitted to my CNC mill conversion).  I gather this has compensation for motor speed reduction with load so could solve your problem.  The one I have is the KBWM240.
 
Cheers, John.
 
 

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