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manofgresley
19-05-2013, 04:41 PM
Hi all.

As anybody a strong opinion on whether or not to change the standard drive shafts on my cnc converted milling machine to Ballsrews or leave the originals in.

Can anyone recommend which Stepper Motor Drive couplings are best for CNC application.

Regards

Ray

birchy
19-05-2013, 06:41 PM
I'm dithering about with a Warco WM16 mill which I plan to convert to CNC but was planning on leaving the standard lead screws in and seeing how it performs. If it turns out to be crap, then I'll look at anti-backlash nuts, followed by ball screws. Ball screws cost about 5x the standard acme lead screws, but are they *really* 5x better?

C_Bubba
19-05-2013, 08:11 PM
All I can say is when I converted my RF-31 back about 2000, I left the original lead screws in and within about 2 months (even with a one shot oiler), I had worn the bronze nut out enough to "walk" it down the screw with no rotational movement of either the screw or nut.
Worth 5x, Yep!

My thoughts anyway.

i2i
19-05-2013, 08:23 PM
ballscrews all the way...worth 20 times the cost of the leadscrews.

Jonathan
19-05-2013, 08:35 PM
Leadscrews are OK to get started, as it's generally very easy to just add motors and use the original screws. You can probably then change to ballscrews without wasting much if any money on parts. However, you'll soon find that backlash, low efficiency and wear (to name but a few) make the overall machine much less useful than it could be.

Ross77
19-05-2013, 10:21 PM
ballscrews all the way...worth 20 times the cost of the leadscrews.

What he said..

You can easily get rid of backlash in lead screws by tensioning (or compressing) two nuts together but the resultant friction is often more of a problem than the backlash. My little mill has has this type of anti-backlash and I can get pretty much zero backlash but the friction is so high it is difficult to turn the hand wheel and impossible to get smooth motion. as already pointed out this will also result in the screw wearing out quickly.

The halfway house is to use brass nuts and a hydraulic tensioning system to self adjust for the wear and then replace the nuts as when they totally wear out. but by the time you set that up it is probably easier and cheaper to get a ball screw.

Don't be tempted to use delrin, been there done that and it has no place in milling machine. I've learnt from my mistakes and I'm getting ball screws for the next one.

John S
19-05-2013, 11:24 PM
Done a few WM16's and one thing to bear in mind is that some convert easily and some don't.
The stumbling block is the bed in the middle, the casting when being cast, sags and make the gap tighter in the middle.
No problem with the top, that's machined so no sag. From the ones I have done it's 50/50 if a ballnut clears.

So worst scenerio is to get someone with a big mill to take a lick out or angle grind a clearance grrove.

WM14 are usually OK, I haven't come across a bad one but WM 18's 'always' need machining

manofgresley
20-05-2013, 07:00 PM
Many Thanks, Any thoughts on the couplings?

Ray

John S
20-05-2013, 08:28 PM
Oldhams, they are the only non backlash coupling that works CORRECTLY in two planes.
All the others are fück ups.

russell
21-05-2013, 10:49 AM
As anybody a strong opinion on whether or not to change the standard drive shafts on my cnc converted milling machine to Ballsrews or leave the originals in.
Clearly, yes some people have strong opinions.

Personally I think it depends on what you intend to use the machins for. I am using Delrin anti backlash nuts on my converted Chinese mill but then it is only a toy for learning and occasional use in model engineering. I have 1 thou backlash on both X and Y axes and it gives very little friction at that setting.

If you are going to use the mill every day for production purposes there is no question that ballscrews are necessary.

... just my 2 centimes worth.

Russell.

Ross77
21-05-2013, 11:41 PM
Clearly, yes some people have strong opinions.

:whistle::whistle:




I am using Delrin anti backlash nuts on my converted Chinese mill but then it is only a toy for learning and occasional use in model engineering. I have 1 thou backlash on both X and Y axes and it gives very little friction at that setting.

Delrin will have reduced friction But I found that it just acted as a damper and masked the backlash. measuring low backlash only under a no-load condition. Once the load is applied it compresses until the the other edge of the screw is in contact( ie play in the screw/nut). The amount of compression is dependant on the load so you end up with varying amounts of error on the work piece.

russell
22-05-2013, 11:26 AM
Delrin will have reduced friction But I found that it just acted as a damper and masked the backlash. measuring low backlash only under a no-load condition. Once the load is applied it compresses until the the other edge of the screw is in contact( ie play in the screw/nut). The amount of compression is dependant on the load so you end up with varying amounts of error on the work piece.
Agreed, this can be a problem but like everything it depends on the design. Some use springs to compress the split nut and of course these flex under load. I am just using screws to adjust it with no flexibility. My first attempt did have a flexibility problem which I traced to the flange on the nut flexing. It was cured by backing it with a substantial steel washer.

Again, horses for courses.

Russell.

i2i
22-05-2013, 11:57 AM
at the end of the day, you're going to cnc a manual mill for what reason. You want an accurate computer controlled milling machine that will cut metals from ally to probably steel.

The first need is accuracy, which means ballscrews.

You don't need to go any further.

manofgresley
24-05-2013, 10:33 AM
Hi John

I agree, i have used Oldham couplings for many years, i just thought things may have moved on a little, they seem so expensive these days.

Regards
Ray

John S
24-05-2013, 12:54 PM
Unless I'm out of touch with other couplings, not that I'd use them, I thought they were about the same price.
I get mine from ARC, used to buy off RS but they have cut their range right down.

JoeHarris
24-05-2013, 02:10 PM
Not sure if you have looked at buying your ballscrews from China? There is a massive cost difference between China and the UK. Check out Linearmotionbearings2008 on ebay.

manofgresley
25-05-2013, 10:46 AM
Many thanks i2i.

Yes commonsense must prevail, i am converting to CNC for greater control and accuracy, so i will go down the ball screw route, if i can find screws and nuts that will fit under my slides. My Lead screws are 14 mm diameter, what would be the smallest diameter ball screw i could use i wonder?

Regards

Ray

ptjw7uk
25-05-2013, 11:02 AM
You could go down to 12mm dia ball screw.
The biggest problem will be finding enough room for the ball nut!
On my Sieg X1 conversion I used RSW ball nuts which have the smallest dia, when I did mine I thought you could not get them with wipers fitted but you can.
RSW Ballnut (http://www.worldofcnc.com/ballscrews/marchant-dice/rsw-ballnut-p-1511.html)

Just an idea to think about
Peter

Jonathan
25-05-2013, 12:11 PM
You can get 12mm (RM1204) ballscrews from linearmotionbearings2008 on eBay. The steel the ballnuts is made from is just about soft enough to mill, so if you need to make them a bit smaller that's a cheap option. It's probably easier to grind it and glean the nut thoroughly afterwards. Clearly the stiffness and critical speed of the screw are both reduced if it's smaller, however in this instance neither is significant since it's a small machine.

Ross77
25-05-2013, 09:48 PM
Have you got any space between the base and end plate on the bottom axis (Y- I think on a mill)? I'm just looking to put ballscrews on my mill and want the biggest diameter possible as it is only supported on one end. The channel between the ways is too small for 20mm dia. ball nut but I have space to move it forward and mount the nut flange on the face of the base.

Any way just a thought.