Power feed - 'which motor shall I buy'?

Here's the background. Having bought a biggish mill (Amadeal VM32L) I find I really would like a power feed. I knew that, of course before buying it, but manufacturers power feeds are not cheap. In addition I would like to consider CNCing this machine at some point and thought a stepper motor would be a good approach.

Also as a software chap, I quite fancied the coding challenge - I've already built a CNC like system for my micromill.

Finally, I've been overspending my engineering budget recently so cost is a factor to avoid a swift blow from the good lady.

I'm struggling with the maths. I found various places where I could calculate the power of the motor I needed, but came away more confused than not.

The table is around 840x210 and I calculated the mass at around 60kg with a 20kg vice on it making 80kG.

The most useful site I found was this which had a calculator on it (a spreadsheet).

This reckoned I needed a 12Nm motor. Which is big and expensive and needs big power supply, drive and so on.

I'd been hoping that the PSUs and drives I already had (15A 24V PSU and 3.5A 6600) would do the trick.

I then started looking at DC motors. In particular cordless drill motors. I have one that I took apart eariler ( a drill), though the gearing is missing some cogs.

I found that I could make the table whizz along good and proper by attaching my 18V cordless drill to to the leadscrew. So enough power and controllers seem cheapish.

However, I was still confused about how much power I actually needed, so I build myself a poor man's dynamometer (if that's the right word.

This consists of a pulley on the X-Axis leadscrew, a bit of string and a luggage weigher.

the pulley radius is about 20mm.

I found I could move the table at milling speed with about 5kG force.

I then took a 50mm facemill and cut a 1mm deep channel in some mild steel. the first time round I'd set the cutting speed too low (450rpm) and it needed 7/8 kg Force. The second time at 800 rpm it only took 5/6kg again.

I think that's 50 Newtons at 2cm which is 1 N.m well shy of what the estimates worked out. Clearly a fast traverse would require more (I shall experiment).

There seems to be a wide range of views of what sort and power of motor is needed for this and I'm struggling to make sense of it.

What I would like is to use a 3./ 3.5 A stepper such as this claiming 3Nm, but I fear that it will struggle. Can you confirm my fears.

On the DC motor side, can I control the speed well enough? I would think so though I'm still left with power requirements. The motor from my (dismantled) drill claims to take 380A at stall speed according to a spec sheet I found, so power becomes a challenge. Specically it looks like I need 30 or 40 A at least at 18V.

I did think of running it off the original battery but with the charger on all the time. Thus the battery acting more like a capacitor.

I'm sorry for all this dribble, but any clarity you can provide would be most welcome!

Iain

You can trade speed for torque if you don't need high speed feeds. For hobby use where time is not money a table travelling at high speed is probably not necessary. I would suggest trying the 3Nm stepper and if it is slightly underpowered you can use the toothed timing belts and pulleys to reduce the speed and increase torque. Another option is to consider hybrid stepper servos where the stepper has an encoder on it and the matching driver reads the encoder. They increase the power as required to achieve demanded position (within limits) and are a good compromise between cheap stepper motors and traditional dc servo motors.

Martin C

I recently posted https://www.model-engineer.co.uk/forums/postings.asp?th=148282&p=14 a couple of pics of mine. Really it needs a higher torque motor but not that much.

I used a Mondeo wiper motor & it takes so little current that the speed controller on the 20a dc unit does not control properly. I takes on average less than 8A @12v DC. I have driven 63mm 6 insert face mills with it & no issues. See my other postings of facemilling hard steel.

Steve.

My KX3 uses a 3Nm stepper on X & Y which may give you a starting point, as your mill is a bit larger then maybe a 4Nm would be better, that is what mine has on Z.

A lot will also depend on how you tend to use the machine if keeping to smaller cutters that can be run at a faster speed then you will not need so much feed power but one that will run faster. On the other hand if you want to use bigger tooling, deep cuts and slog your way through at lower revs then that is going to need more power from teh stepper.

Thanks, All.

Martin, I do want high speeds - for return and moving from one end of the table to the other for jobs needing different clamps. I can be quite patient when need be, but I don't relish winding a wheel 200 times just to move the damned table!

Thanks Steve. I think my drill motor idea may be overkill torque-wise, though it does have some interesting challenges for reducing the speed and may give me a chance to use my gear cutters.

Thanks, Jason. One of the complications appears to be that it's not just the torque, but how it behaves electrically. The sources mentioned consider inductance and voltage - the higher the induction, the higher the voltage to run at a given (higher) speed. And, as above, I'm keen to get a fast return.

As far as power is concerned, my preference is for carbide tools on the mill, though they are expensive when you break them. I've not really tried to see how much grunt I can get out of the mill, but I should like to be able to run hard if I need to. Though, again for occasionally deep cuts, I can run by hand if I need to.

I suspect my sweet spot is in the 5 - 8 Nm range for a stepper. Perhaps I should just save my pennies

Iain

I'm with Steve but I prefer the Vauxhall Astra wiper motor to the Mondeo cost £15, 12V 10A power source £10, speed controller, fwd/rev switch and box £10.......... all told £35 and of course you have to make the drive unit

Have one on my SPG miller and one on my Super 7.

Colin

iain, a typical Align power feed won't go over 400mm/ min I regularly cut at that rate on the KX3 or higher and can do rapids of 5-6 times that if I'm brave enough so don't rule out a stepper if you want a fast return. The stepper will also give plenty of grunt at 1mm/min which a DC motor even geared is unlikely to do, certainly the DC Gear Head motor on the X3 lacks low end grunt but will still move the table fast on full speed.

I use 2 of these stepper motors which came from Arc euro they are
rated at 6.5 Nm .This Warco mill table size is 30 by 8 inches (760 x 205 mm)
so a similar size to yours ,there is a slight reduction in the output of these
motors as the machine uses a unipolar system but it still moves the table as
fast as the Align power drive .
Here is a copy of the specification sheet as a guide.
John

Hi Ian, Details of mine are in my albums. I used a car wiper motor that I got from a scrapyard for free.

Well done Iain for measuring the force needed to move the table! The problem with your first calculation is the mass of the table isn't the main factor. It's the force needed to overcome sliding friction and that of the leadscrew mechanism. The sliding friction of a milling machine table is oily low, and although the lead-screw and nut are relatively inefficient, the screw is a favourable torque converter. No way could I move a 20 ton concrete block, but I have pulled a 20 ton canal barge - the barge being on a slippery surface matters much more than the weight.

Type of motor used depends on how much control is wanted. A windscreen wiper motor and DC speed control will wind the table to and fro as a convenience, but there's no accuracy in it. Like power steering in a car, it's down to the driver to get it right. A requirement for accurate stop-start positioning and speed is more easily met with a stepper motor (or hybrid or servo) and computer control.

Having measured the minimum force needed to move the table, it's necessary to allow for cutting force as well. It's also worth repeating the measurement with the table heavily loaded, say with a vice, rotary table and the biggest object you're likely to cut. As you mention, my experiments also found cutting doesn't need lots of extra oomph on the table, presumably because the mill-motor does most of the cutting work.

You're in the right ball-park, but I suggest doubling the maximum load measurement to size the motor. Triple if nervous! Shouldn't need a massive motor but make sure the power-supply has plenty of grunt.

Dave

Edited By SillyOldDuffer on 20/05/2020 10:56:23

Well done Iain for measuring the force needed to move the table!

[quote]A windscreen wiper motor and DC speed control will wind the table to and fro as a convenience, but there's no accuracy in it.[/quote]

I disagree here. I use a window screen motor and a variable voltage power supply. and can control the cutting speed from stop to as fast as the motor will run for fast traverse. I do that at 30 volts. The average voltage when fly cutting with a 2.5 inch diameter cut is about 6 volts and less than 2 amps current draw. That gives me a repetable 50 or 60 mm a minute. Though I do admit that I adjust the voltage and therefore the speed to suit the work being machined.

I find that it is almost impossible to stop the wiper motor without shearing the internal gear off the shaft and even then the motor doesn't stop, just labours a little.

Thanks, all.

JOhn - ARC seem to have quit selling steppers, but I reckon that set you back a few quid. Closed loop steppers cost more, but apparently have a broader powerband. I've come across this one which is probably somewhat overkill but seems 'cheap' for 12NM with drive at £124. I will attempt to broaden my research.

The car motor guys. Thanks for this. I'm particularly pleased to get examples of motors as it was looking a bit pot luck. I don't know what I'm going to do just yet, but certainly that is going to be a cheaper option. And doesn't requirement to hook my mill up to a 20,000 rpm drill motor...

Oh and I did test under load. At a slow spindle speed with a 50mm facemill cutting 1mm, it needed about 7.5 kg. ram up the speed to 800rpm and it took little over a traverse.

Iain

Ah, I'm not being clear again. Yes, the windscreen motor can be speed controlled and very useful it must be too.

What I meant was that type of motor and hence table cannot be stopped dead at a particular point. In the example below a speed controlled windscreen motor could cut the through top slot easily, but it would be hard to stop it at an exact point as needed to cut the blind-ended slot.

A stepper motor can do both traverse cuts and pin-point stops. And if X and Y are both driven by steppers the table could steer the cutter at right angles or in curves.

The advantage of steppers is they open the door to a basic form of Numerical Control.

Dave

Edited By SillyOldDuffer on 20/05/2020 18:05:59

Hi Dave, Guys,

Yes I agree with you, the motor does not stop dead like a stepper would ! I freely admit that the situation you describe of coming to a blind end had never occured to me. I'll have to check just how fast the traverse stops when it hits a limit switch.

A far as car window screen wiper motors go, they are amazingly powerful things. Back in the early days I tested one using a pair of mole grips to try and stall it. I needed two hands and struggled to restrain it. The motor just laboured and carried on turning. It stripped the centre out of the worm wheel.

As it happened I got several motors and found that some are left handed and some are right handed. All the ones that I got were dual speed bar one.

If you have have a look at my album you will see the tumbler reverse mechanism copied from the Myford.

Edited By John Baron on 20/05/2020 20:09:10

OK. I've had enough of dithering. I'm going to treat this as an experiment and be prepared to throw it away.

I've already got a 24v 15A power supply and some 3.5/4A drives, so what I'm going to do is to buy this which gives me a £27.00 introduction to the art and see it it works. God knows why it's sold from Austria, but that's the modern world.

If it's not powerful enough I'll change the gearing.

I can get more speed out of it (I gather) by increasing the voltage which is a relatively cheap thing to do.

If I want to go full CNC later then this is an investment I can comfortable discard.

Gentlemen of the wiper motor variety. I'm afraid that my digital background has betrayed me. I want something I can control and £27 quid seems a reasonable investment.

I will report back once the motor has arrived and I've assembled the bits and build the controller. Hmm. Don't start any short novels!

As always, thanks for all the helpful input.

Iain

Whatever you make, be sure it costs less than the purpose made power feeds which go for about £ 130.

Stefan Gotteswinter had a series of videos using a wiper motor for the X drive.

Myfordboy made videos with a stepper for the X drive. Both are on YouTube

Well I don't think you'll have any lack of power with that motor! My feed uses a titchy Minibea size 23 motor, years old, and the torque is under 1 Nm, with a 3:1 reduction to the screw. It does stall at one end of the table travel where the friction increases markedly (Myford engineering for you) but I'm never likely to be milling much there. I am building a better drive system using newer drive ICs (TI DRV8825s) driven by an Arduino running grbl - overkill but it's free so why not? As an experiment I milled a dodgy mild steel bar end with a blunt cutter taking a 0.5mm cut with no lube, no sign of any lack of power.

Just a quick vaguely related question - I'm planning to connect the motor and lead screw with a timing belt. There are a vast range of options.

Is 10mm width sufficiently strong? Is 5mm pitch too big - should I try for 2mm?

Iain