Chasing Backlash

Well, I was a hardware engineer (EE, not mechanical) so hardware guys thinking software guys "just don't get it" is about as big a cliche as you can imagine.

It seems like it ought to be easy. Computer (do your best impression of Disney's Goofy, the idiot dog): "Oh, I got to change direction, time to crank out those special backlash steps". If it needs to crank out something like 12-1/8 steps, that's different. That's a screw that's too steep, and a system design problem.

I could go on, but I don't want to bore everyone.
 
silly question since we are allready wandering off topic:

Where did you get your import ballscrews? I've been poking around at them for my bridgeport and prices are elusive while import screws seem to come with lots of horror stories.
 
Metal said:
silly question since we are allready wandering off topic:

Where did you get your import ballscrews? I've been poking around at them for my bridgeport and prices are elusive while import screws seem to come with lots of horror stories.
Click to expand...

I got mine from a place called Linear Motion Bearings. They are Chinese imports, so if that disqualifies them you should know. The main way to buy from them appears to be through eBay,
http://stores.ebay.com/linearmotionbearings

In my case, I chased around for them for a day or so, finally getting an email link here, in the "Grizzly G0704 CNC Conversion Thread", which I eventually seemed to have stolen from the thread starter as I documented mine.

I emailed the contact given on that page and when I told him I wanted to do a G0704 conversion, he knew exactly what to do.


Hope that helps,
Bob
 
MontanaAardvark said:
I agree that rapids are pretty meaningless because you can never cut at those speeds. They really only matter if you have to do something with a lot of movements above the work and your time is money. I did an array of holes on the front and read panels of my motor controller box. Drill, retract bit, move to next hole, repeat, a large number of holes, followed by a "carriage return, line feed" to the next row of holes. Rapids sped that job up nicely. (Rapids on my Sherline were 15 IPM, a far cry from these speeds).

Pardon if this is like a rant, but I'm an engineer and I always see things this way. I just don't see why cutting a circular cutout should be something backlash compensation can't fix. In fact, it should be a really easy case. Backlash only shows up when direction is changed, and in a circle, each axis changes direction twice at the most. The CNC controller should know everything it needs to know. It doesn't show up when the axis stops and resumes the same motion, or slows down and resumes. Torque in a stepper is maximum as lowest speed so if the motor just slows down or stops without reversing direction, backlash doesn't apply. It's not going to slip. If it does, that's a different problem.

I guess I just don't get it.
Click to expand...

I cut a 2-1/2" through pocket in 3/8" thick aluminum at .100" DOC, .220"stepover, 8000 rpm,
MontanaAardvark said:
I got mine from a place called Linear Motion Bearings. They are Chinese imports, so if that disqualifies them you should know. The main way to buy from them appears to be through eBay,
http://stores.ebay.com/linearmotionbearings

In my case, I chased around for them for a day or so, finally getting an email link here, in the "Grizzly G0704 CNC Conversion Thread", which I eventually seemed to have stolen from the thread starter as I documented mine.

I emailed the contact given on that page and when I told him I wanted to do a G0704 conversion, he knew exactly what to do.


Hope that helps,
Bob
Click to expand...
MontanaAardvark said:
I got mine from a place called Linear Motion Bearings. They are Chinese imports, so if that disqualifies them you should know. The main way to buy from them appears to be through eBay,
http://stores.ebay.com/linearmotionbearings

In my case, I chased around for them for a day or so, finally getting an email link here, in the "Grizzly G0704 CNC Conversion Thread", which I eventually seemed to have stolen from the thread starter as I documented mine.

I emailed the contact given on that page and when I told him I wanted to do a G0704 conversion, he knew exactly what to do.


Hope that helps,
Bob
Click to expand...

That's the same place I got mine. Price and service was excellent as was delivery. Keep in mind these are not top of the line screws and nuts. If you want to machine within a few tenths you should look elsewhere.

Tom S.
 
I've spent a bit more time on mine and reduced the Y axis backlash to .001. I made a tool to hold the AC bearings in place and under compression while I tightened the jam nuts on the ballscrew. Just a little piece of 1/8" thick aluminum scrap I had. It looks like the motor mounts, but that's just by coincidence. I cut some long edges off it and was left with the more or less triangle shape.
Tool.JPG

I tried to do the same thing on the X-axis but had a fit problem. Somewhere along the way, I tapped the X-axis end cap for 10-32 screws instead of 8-32, so these 8-32x 3/8 screws didn't work. I'll get some 3/8" 10-32 screws and try again.
 
MontanaAardvark said:
I've spent a bit more time on mine and reduced the Y axis backlash to .001. I made a tool to hold the AC bearings in place and under compression while I tightened the jam nuts on the ballscrew. Just a little piece of 1/8" thick aluminum scrap I had. It looks like the motor mounts, but that's just by coincidence. I cut some long edges off it and was left with the more or less triangle shape.
View attachment 226288

I tried to do the same thing on the X-axis but had a fit problem. Somewhere along the way, I tapped the X-axis end cap for 10-32 screws instead of 8-32, so these 8-32x 3/8 screws didn't work. I'll get some 3/8" 10-32 screws and try again.
Click to expand...

I understand what you did but not sure why you did it. Your bearing housing has a cap, right? Does the cap not compress the AC bearings in the housing?

Tom S.
 
TomS said:
I understand what you did but not sure why you did it. Your bearing housing has a cap, right? Does the cap not compress the AC bearings in the housing?

Tom S.
Click to expand...

The cap is part of the motor mount, which sticks out far enough so that when it's in place, I can't get a handwheel onto the ballscrew shaft.
 
TomS said:
I cut a 2-1/2" through pocket in 3/8" thick aluminum at .100" DOC, .220"stepover, 8000 rpm,
That's the same place I got mine. Price and service was excellent as was delivery. Keep in mind these are not top of the line screws and nuts. If you want to machine within a few tenths you should look elsewhere.
Tom S.
Click to expand...

Absolutely not, I'm trying to resolve on the BP:
1: my screws are worn out in the middle so if I adjust the nearly .1 backlash out I lose like half my table
2. constant adjustment due to 1: means chasing a moving target of backlash comp depending on where I'm cutting which is a huge pain in the butt.

If I could get accuracy down to a few thou reliably all across the table I would be very happy.

What size screws did you get (or the code if you are feeling nice)? from what i can tell 25mm is about as big as he gets which (to me) looks like a far cry from the size of these leadscrews
 
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I'm running 2005 (20mm) ball screws on my PM-932 conversion on all 3 axis. The lead screws were 25mm. The size of the ball nut restricts what will fit on the X & Y axis on my machine. I would email Chai and see if he sells larger ball screws if that is what you have. He does not list all of what he sells on his ebay page. cgxfred@aliyun.com

I recently ran across this company that also does ground ball screws. I have not ordered from them but have been thinking about getting a quote. https://www.aliexpress.com/store/437307

A lot of people use the Nook ball screws in lieu of the imports. I have not priced them to see what the cost is vs import but it might be worth looking into if you are concerned about the imports.
 
Metal said:
Absolutely not, I'm trying to resolve on the BP:
1: my screws are worn out in the middle so if I adjust the nearly .1 backlash out I lose like half my table
2. constant adjustment due to 1: means chasing a moving target of backlash comp depending on where I'm cutting which is a huge pain in the butt.

If I could get accuracy down to a few thou reliably all across the table I would be very happy.

What size screws did you get (or the code if you are feeling nice)? from what i can tell 25mm is about as big as he gets which (to me) looks like a far cry from the size of these leadscrews
Click to expand...

I'm using 1605 ball screws on the X and Y and a 2005 on the Z. I had Chai machine the ends for my particular application so they would not directly transfer over.

Tom S.
 
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