Desperate newbee

[dogfuel]

On larger peices, my shopbot works fine, but I can't cut 'good' small curves. This happens in a variety of material, with a variety of bits over a range of speeds.

Am I missing something?

Thanks in advance for any suggestions - attached is a pic of a 6"x3" ellipse that demonstrates the issue.


Joe

[dogfuel]

Here's the pic.

Thanks
3871

[cnc_works]

Well, I'm far from an expert here, but it looks to me as if your problem occurs during a direction change on an axis. I would suspect too much play maybe in your rack & pinion. Is your backlash spring properly adjusted? You might even be able to grab your z axis and stress it in one direction or another and feel the play.

[srwtlc]

Check your set screws on the pinions to be sure that they are tight and holding on the flats of the motor shaft. They could be slipping at each change of direction.

[gerald_d]

A supplement (http://www.talkshopbot.com/forum/cgi-bin/discus/show.cgi?tpc=312&post=6946#POST6946) to Scott's post.

[Brady Watson]

Joe,
The elipse itself, what software did you use to create it?

What are your ramp speeds set at?

What cutting speeds have you tried? Be specific XY/Z.

1st thing I would do is make sure that my control box is getting a timing value of at least 45 when starting up the control software. If you are using the DOS version, it will be right there when firing up. If you are getting low numbers, exit the software, turn off the control box and try it again. If the machine doesn't sound quite right, chances are you have a low number...and this affects stepper performance.

The next thing I would check is if the elipse is nice and clean in software. You have to select the object in PW and zoom ALL the way in. Make sure everything is symetrical. For the heck of it, create a new elipse in PW and transform to the size you need. Try cutting that.

The next thing I would check is your ramping speeds. If they are set too high it is possible that it can't accurately hold tolerance if you are moving too quickly. Recommended/factory ramping values are .85/.45 XY/Z respectively.

Try cutting that same curve at 1,.25 inches per sec if your elipse is CLEAN and SYMETRICAL.

If you are ONLY getting this sort of thing on curves and not straight cuts, I doubt it is a mechanical issue.

Hope that helps,
-Brady

[dogfuel]

Thanks for all the direction.

It appears the problem was a loose pinion set screw on the Y axis compounded by a too high XY ramp speed (the ramp speed that seems to minimize problem is in the .35 - .4 range).

Thanks for everyones input ... tomorrow I'll try Gerald D's method for tightening the screws (I found the issue before reading his posts) - hopefully this will keep it from happening again as soon.

Thanks again

[gerald_d]

Joe, we tightened ours carefully over 3 years ago, without Loctite, and have never had reason to check them since. It is critically important that the screw sits perpendicular to the flat spot on the shaft.


3872

How tight is tight? Well, a properly-fitting new allen key will not strip out the hole and you can tighten till your hand hurts. That is about as scientific as I can go. (I think that the allen key will get damaged before the little screw does.)

[scott_smith]

I like that, "tighten till your hand hurts".