Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: yurtman on May 30, 2013, 01:38:55 PM
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I've just been making my first cuts on my recently finished CNC router. I cut a small table top earlier today, and had 3 small bumps in a profile cut (g-code attached). The timber is maple, I did 15 x 1.5mm passes and each pass had the bump in the same place. I'm confident there are no lost steps as the rest of the geometry was smooth. There is no uniformity to where these bumps are in relation to the rest of the geomtery, and I would expect if there was mechanical culprit for these bumps to be elsewhere in the toolpath. I've taken a close look at the g-code toolpath and this seems to be fine. I don't think there is any backlash, and the bump positions don't seem to be on curves. Any insight would be much appreciated!
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g-code attached
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Looks like it may be flex or slackness in your machine that is causing that.
Hood
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Looks like every instance is near what looks like "Tabs" along combined X,Y moves.
Out of curiosity, I'd try setting all of the tabs along single x and y axis moves to eliminate X,Y,Z combined moves.
What did you generate the code with ? (just curious) These tabs have basically 0 length .... just ramping up, then dwn.
Russ
:)
and ... Welcome to the group ! :)
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Also, even the upper passes that do not change the Z for the tabs, still break the otherwise "straight" move into segments un-necessarily.
This could be the area of your problem.
Possibly ::)
dang ... I was in machine coords, disregard the dro's :D
Russ
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Thanks for the suggestion overloaded, it could explain 2 of the three bumps. I will run the same code tomorrow without tabs and see if this makes a difference. FYI the tabs are 3D tabs in VCarve Pro.
Hood, I have run other geometry without these bumps , I would expect machine flex to be more consistent. Even with this tool path, there's a similar feature that has been cut blemish free.
Thank you both for your input.
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If its tabs is it raising in those positions? Have you trammed your spindle by chance?
Brett
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Are these bumps present before the last few passes? If they are then dont think its tabs.
Hood
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....... each pass had the bump in the same place.
If each pass followed the same path, then there would be the same jog (bump) on the opposite side of the cut.
At least ONE path is different. Maybe a finish pass ? ? ?
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Cv settings?
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Problem found: the pulley wheel on my X axis had play in it due to a loose grub screw. This was causing a very small delay in the X axis responding to moves, I guess a bit like inconsistent backlash. Thanks for all your help on this, I really appreciate your input.
On a hardware note, I have nema 23 steppers with 3/8 d shaped shafts, and the 22 tooth pulley wheels with 3/8 round bore with a grub screw to keep in place. I have read on Joes CNC forum that some people have glued in the grub screws to stop them loosening, which has worked but I feel isn't an elegant or long term solution. Is anyone else having a problem like this? Are pulley wheels available with a d shaped 3/8 bore?
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Good job !
Glad you found it.
Russ
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Use thread lock, you get it in different strengths, right up to permalock which you really dont want if you ever plan to remove the setscrews in the future.
Common brand of such products is loctite.
Hood
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You can also use Sleeve Retainer (by Loctite also).
Clean the shaft and bore with carb cleaner or alchahol first.
A little dab l' do ya.
Pretty permanent though .... will be a bi*ch to get off if you ever need to, but it won't slip.
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If your threads are deep enough in the pulley you can also use 2 set screws, one on top of the other. Put first in tight as the devil plus 2 rounds and the next the same. This acts like a jam nut, only its more like a jam bolt. ;D Another thing that will help is the type set screws you use. Some have knurls on the point that act like a lock washer and will run in cut the will have to cut their way out (just slightly). I would use those against the shaft and flat nose for the back up that would be bottoming out on the first set screw in the thread. I used this method along with medium or some call it screw grade Loctite. If you do use any form of liquid lock though be sure everything is good and clean like RC said.
Brett
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Thanks for the suggestions. The grub screws are bad, they have soft heads that just allow the Allen key to mash up. I'll be getting new grub screws for sure, and may try Chaoticone's idea for jamming two screw in. It may be a bit tight, I think there's about 4-5mm of thread. As far as loctite compound goes, is 638 OK?
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I tend to use 270 if I want it to be fairly permanent, especially as I use a lot of A4 stainless setscrews.
Hood