WM18 CNC Mill Conversion

Hi

Anyone with experience of converting a WM18 mill to CNC know if its needs ballscrews? I have got a quote for a conversion kit but it seems they are suggesting I just use the existing lead screws. Does it not require ballscrews due to the backlash?

Cheers

I don't think you'll find many people advocating use of the std leadscrews. You simply can't make them accurately enough to give acceptable backlash whilst still being able to assemble the nut to the leadscrew. Even if they seem OK when brand new, they will soon bed in and you will have measurable backlash. The friction is also pretty high relative to ballscrews.

Ballscrews are readily available from AliExpress and ebay for starters. It's one of the first things people usually buy when embarking on a conversion.

Makes you wonder what else they have cut corners on? Check out the hundreds / thousands of conversions done on similar machines. I assume the generic design used by Warco is available under any number of other brand names.

Murray

Or read the 2 part article in ME on converting a WM18 to CNC complete with ball screws that has recently been published

Edited By JasonB on 29/06/2018 13:49:13

As well as cutting backlash ball screws reduce the torque required from the motors which gives the choices of smaller motors or higher traverse speed. Also kits mean someone else is making a profit at your expense, it may be worth costing up doing everything yourself. It's not too hard and there are plenty with experience on this forum. The CNC cookbook website also has a lot of useful information for someone starting out in CNC.

Martin C

16mm dia x 5mm pitch x 700mm long ballscrew cost me £35 from China. Thread efficiency 85% compared to 35%, equivalent to more than doubling the motor torque while also almost eliminating backlash. Makes sense to do it.

It also makes sense to do it yourself - then you know the machine inside-out, which will help if there are problems later.

Regards

Richard.

I do have those issues, which is the reason I am asking. It seemed odd to me that the company would not recommend ballscrews, and the replies are confirming their usefulness. If they can be bought cheaply online then thats not a massive issue.

Thanks Martin I will have a look at that site

Thanks Martin I will have a look at that site

Thats quite reasonable. Did you look into 4th axis chucks (not sure if right term) by any chance? £200 seems to be about the going price but I am not sure if that should include a motor or not.

Hi Murray!

They do sell ballscrews, which again made me wonder. Maybe they were going for a minimal / low cost build, I don't know. This is the full listing:

Stepper Motor 8.7Nm Nema34
Stepper Motor 4Nm Nema23
Digital DSP Stepper Driver 5.6A, 50V CWD556
Digital DSP Stepper Driver 7.2A, 80VDC or 60VAC CWD872
Breakout board KK01 CNC Stepper Motor 5 Axis with Relay
600W PSU 48V 12A CNC Power Supply Stepper Motor Milling

£580 inc vat and postage. Good deal or no?

Cheers

Edited By Ross Lloyd 1 on 29/06/2018 22:59:31

Only 2 motors and 2 drivers for a 3 axis machine? Looks a poor deal to me.

IMO overpriced and likely more power than required for the machine.(8.7Nm)

Emgee

I missed off the quantities of each. Per line above: 1, 3, 2, 1, 1, 2

8.7Nm is for a potential 4th axis

Ross

Any problems with back-driving of the ballscrews on these conversions?

No!

MYCNCUK have a few threads on converting this type of mill, which may be of assistance..

John

I built the electronics for my Novamill, BoB + PSU + 4xDM542 drivers (but I've only ever used 3, haven't used A axis yet) for less than £200 buying individual units from eBay. This didn't include the steppers (already fitted to the machine) but Zapp Automation have NEMA23 steppers at up to ~£30 and NEMA34 at ~£60. You may not need a 34 for the Z drive. Lower prices than these available on the web.

I assume the 4th axis would use a rotary table or similar to achieve a reduction ratio, in which case 8.7Nm must be wonderfully OTT. Calculate what torque would be available at the work and ask what you could possibly need that for. You could destroy significant parts of the machine with that. Even 4Nm for the main axes must have a pretty large safety factor.

One thing to understand about steppers is that their torque falls off rapidly with speed. When I plotted a variety of stepper motor characteristics (torque vs speed), I noted that they display a characteristic that is very close to constant power. What that means in practice is that you might as well drive the ballscrews directly rather than gear the motor down through a reduction stage. That's the most common arrangement for steppers but you do see some with a reduction stage. With the latter you simply reduce the all-important(?) "rapids" whilst doing nothing to increase the torque / force available at a given speed of movement. For a willy waver, the stall torque at the ballscrew might be legendary but if you ever tried to develop it, something would break....

If you spend more money and go for a proper servo motor rather than a stepper, you will find that the stall torque is less than he headline figure for a similar size stepper but generally that torque will be achievable over most of the speed range. So you would probably need a reduction (typically 2 - 2.5 is achievable) but it will achieve much better performance over the whole speed range.

Murray

I did one of these (but labelled "King Canada" using a ball-screw kit from a supplier south of the border here in Canada. Thoughts:

- the underside of the table needed grinding to give more room for the X axis nut;

- the dowel pin holes for the X axis end plates did not match my mill;

- the y axis kit assumed that the threaded holes in the castings were not off-kilter (sigh);

- the gearbox was noisy, so I obtained a belt-drive replacement from benchtop precision in the USA, HUGE difference;

- added ball oilers to all axes;

- I replaced the speed control with direct drive from my CNC bits, and used the tachometer bits to feed back into my CNC computer, to set and hold speed, independent of drive belt setting;

- I added a wired XHC HB-04 pendant - fantastic bit of kit.

Overall results?

The best part was (and is) the LinuxCNC computer code. All I use is a trackball and the pendant, no keyboard in site.

The worst part was the mill - I obviously got one put together from a kit of parts, the table was not flat, but humped down the X axis (could have been because of my slight relief grinding for ball nut) but there are hard spots on the table, and, I think the ways, so the accuracy at some points is debatable.

I did the same on my (BusyBee) version based on your recommendation and I agree. The difference is astounding. I wouldn't mind kitting it out with the Benchtop-Precision 3-ph motor adapter + VFD if I could find a suitable motor locally whose price didn't assume gold-plated windings.

I hope King are better to deal with than BusyBee. When I was looking at fitting ball-screws, I tried to order a spare table and cross-slide from BB (to make it easy and allow me to use the same mill to do the mods) - which they said they were OK with but would have to order from the Far East. They (supposedly) went ahead but in two years I could never pin them down and then they went away completely.