Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: dphens on August 25, 2010, 08:24:23 PM

Title: Backlash or something else??
Post by: dphens on August 25, 2010, 08:24:23 PM
Hi,

I have a new CNC machine and need some assistance. I am fairly new to the CNC world - been dabbling in it for about a year now and just upgraded to a 24x36 machine. I've been running some tests, making basic geometric shapes and verifying the size. Everything had been ok for the most part except circles. Larger circles appear fine, but smallers circles have flat spots on the left and right of the circle (along the X axis). I assumed I had some backlash, but when I use a dial indicator to try an measure it, I am getting results of .003 or less on the X axis. Is that enough to throw off a small circle (.5" or so)?

Are there any other issues that may also cause this?

If it matters, I am using Vcarve Pro to crate the gcode along with Mach3.

Any help is appreciated!

Dave
Title: Re: Backlash or something else??
Post by: RICH on August 25, 2010, 08:41:05 PM
If you have backlash you will "egg" instead of a true cirlcle. I use backlash comp on the mill and it works well. Now i would suggest that you experiment some to find just what your machne will do. This way you can cut under or over and use finish cuts to get the od or  id very close to dimension.

Did you really mean " .5" off...........? 

RICH
Title: Re: Backlash or something else??
Post by: Hood on August 25, 2010, 08:50:26 PM
I would think you would probably  notice it more in smaller circles as you change direction in a much shorter distance.
How much of a flat is there? Also worth checking for flex as their will likely be more pressure in small circles as opposed to large.
Hood
Title: Re: Backlash or something else??
Post by: ger21 on August 25, 2010, 08:58:28 PM
It could be flex in the machine. It won't show up when measuring backlash, but can act like backlash when cutting.
Title: Re: Backlash or something else??
Post by: dphens on August 25, 2010, 09:01:26 PM
No, I was indicating that circles .5" or so in size is where I was seeing the problem.

I'll have to measure the flat spot. How can spot or measure flex?

I'll play around with the backlash comp too and run a few tests.

Thanks for the replies!

Dave
Title: Re: Backlash or something else??
Post by: RICH on August 25, 2010, 09:08:26 PM
Sorry about that......I can't read but certainly can type ???

RICH
Title: Re: Backlash or something else??
Post by: dphens on August 25, 2010, 09:10:14 PM
No problem Rich. I think that sentence was a little misleading.  :)
Title: Re: Backlash or something else??
Post by: dphens on August 26, 2010, 08:30:22 AM
OK, I added .003 to the X axis for backlash comp and it's definitely helped. Still not perfectly round, but I can seem to find a number the eliminates it all. I've toyed with the idea of a DRO for the X axis so that I could see how far the gantry moves. Do you think this would help me determine more accurately how much backlash I have?

I was/am hoping to use the machine for small engravings, with letters less that .25" high. The backlash my make this unfeasible though.
Title: Re: Backlash or something else??
Post by: Hood on August 26, 2010, 10:39:01 AM
Not really sure what you mean by a DRO for the X axis?
Hood
Title: Re: Backlash or something else??
Post by: dphens on August 26, 2010, 12:32:26 PM
Sorry. I've seen digital readouts that you can add to tooling equipment. An external device that measures actual axis movement. Seems to be something more geared to retrofitting a manual mill, but thought it might be a neat project to try (especially if it gave me more accurate distance measurements).

Like these: http://www.hobbymetrix.com/scales/digitalscalemain.htm (http://www.hobbymetrix.com/scales/digitalscalemain.htm)
Title: Re: Backlash or something else??
Post by: Hood on August 26, 2010, 01:18:35 PM
Ah ok I understand now.
Really you shouldnt have to use something like that but if it can be used on some other project at a later stage it may be worthwhile.
Have you tried testing for flex in the machine? For example you could put a dial indicator on the axis and put the plunger against something solid, then push a bit on the axis and see what kind of reading you get. Also worth testing for backlash like you have done but at the same time pushing a bit against the motion as that may show the flex up.

Hood
Title: Re: Backlash or something else??
Post by: dphens on August 26, 2010, 01:41:47 PM
I am sure I could always use it somewhere else! :D

I have not tested flex as I was not sure how to do that. I'll try it the way you've described. I do need to get a more accurate and reliable indicator though. My current one is cheap and not very good. I'll report back once I've tested this.

Thanks for the help!
Title: Re: Backlash or something else??
Post by: Hood on August 26, 2010, 01:48:23 PM
Not sure what kind of machine you have but also test for quill/Z axis play by putting the dial on it and gently pushing either way, shouldnt need too much force to see movement if there is any.
Hood
Title: Re: Backlash or something else??
Post by: dphens on August 26, 2010, 02:37:08 PM
OK, thanks! I'll try that too.
Title: Re: Backlash or something else??
Post by: Coops on March 24, 2011, 03:16:07 AM
Hi Dave
We have a CNC addition to a profile cutter and have noticed the same thing start to happen.  It was OK for quite a while (more than a year) and has just started to do this.
Ona 300mm diameter circle we end up with two flat spots on opposite sides of the circle - each flat spot is around 50 - 60mm long.
I also not that circles are no longer "perfect" and don't seem to match up as well as they used to.
May be the same problem as yours?
I'll try and check backlash.
Kind regards
John
Title: Re: Backlash or something else??
Post by: Hood on March 24, 2011, 06:41:37 AM
John sounds like backlash and quite a lot at that. Check the bearings, mounting brackets and couplings/pulleys on the motors, if its top/bottom then its the Y axis, left/right then X axis.
Hood
Title: Re: Backlash or something else??
Post by: Coops on March 24, 2011, 07:34:32 AM
Hi Hood
I'll try and spend some time tomorrow and check for backlash - hopefully something has just come a little loose!
Thanks for the tip.
Kind regards
John