so... some questions about cncest 6040 and running slight angles...

so, I bought my cnc knowing I'd end up here at the school of hard learning... fully admit it was a terrible idea to be a newbie and get this machine.

at this point... I've done 6 or 7 little projects to test my machine... all went fine... and then started on a big project - a guitar.
things were going just fine... setup mounting holes, planed the top and back, cut the body outline, cut my control holes... then it came to cutting a neck pocket. it is at this point that I hit a wall.

over the last few days this is what I've learned... my machine cannot accurately cut things with a gradual taper.
Cut the body shape fine with no dreaded "steps" so it seems like the hardware is functioning fine and lots of big curves there. I would think I would have seen issues emerge over the run but I didn't.

For the neck pocket - I started with a typical spiral toolpath... and this ended up 1/2" off after two passes.
then, I ran my pickup cavity (just a box) and it is perfectly centered and lovely. so... what's the difference? well, I guess it's the taper on the neck pocket. seems like the gradual shift in x over the y length... it actually loses where x is and if I go back to my 'x' marked on the stock... we're off. when I cut anything square - we go back to x and we're fine. this makes me think I must be losing steps.

I've slowed my feed from 100 to 85 to 70 to 40 and doesn't seem to improve. again, it is ONLY the neck pocket. anything else I do ends up centered and no steps.



the machine specifics - well I don't know much but I do know it's all ball screws, and it's the red usb board. 4 axis 1.5kw spindle with extended z access. I have it running on a win 7 32bit laptop with nothing else and no inet connection. Full mach3 license. I've set the machine up with the config file that I got from the vendor for mach3.



so... is this the 'missing steps' I hear so much about? Is this more likely an issue with the coupling between the motors and the ball screws? any info appreciated... threads you can point me to, advice on next steps... shoulder to cry on... whatever you have.

After checking for any mechanical issue which it sounds like you don't have because the other cuts were fine. Although something could have come loose just before the neck pocket. Try reducing your acceleration rather than the cutting speed. Cutting wood too slowly will burn bits. Too high of an acceleration rate will cause lost steps.

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Originally Posted by rcheli

After checking for any mechanical issue which it sounds like you don't have because the other cuts were fine. Although something could have come loose just before the neck pocket. Try reducing your acceleration rather than the cutting speed. Cutting wood too slowly will burn bits. Too high of an acceleration rate will cause lost steps.

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thank you very much for the response. so... acceleration is in mach3 settings correct? I will check that next. I cut my pickup cavity (which is a box) after my initial botched attempt at the neck pocket, and several attemps at running different toolpaths for the sm cut of the neck pocket in air. it was perfect after the neck pocket. I had stopped the neck pocket midstream on the initial attemp... so after cutting the successful pickup I then took another shot at the neck pocket and similar issues. the last toolpath was much improved fromt he first in that it was only 'off' by a few 1/32" steps. progressively worse tho.

again, very much appreciate the response. I will go look up acceleration (is that the one that is supposed to be '320'?) and see if I can make headway there.

There should be a setting in Mach3. I use LinuxCNC so I am not directly familiar with Mach3 but all control software should have a setting for acceleration. Just like letting the clutch out too fast on a car with a small engine. Too quick and you will stall it. That is what happens here. Another possibility could be taking too heavy of a cut. You would have to work on your tool path to fix that. My brother is a Luthier and that is the only reason I an familiar with the guitar parts you are talking about. I did once mill a wider slot for a ball bearing nut mod for him. Good luck.

Hi MikeV - How are you generating your tool paths? If you are generating the square pocket one way and the taper another it could be in the programming. With a fitted pocket you would normally do a roughing cut(s) then a finishing cut(s). Peter

thank you again. I'm sure there is a setting, just new to it so will have to consult the manual.
afa heavy cut... I changed my cut depth from .125 to .08 and still the sm issue but further... even when cutting in air - it was going off.
afa luthier... am I a luthier yet? I dunno! I've built 7 from scratch and oodles of partscasters. have been a woodworker all my life... and did solid surface fab while in college. Still finding there is so much to learn!!
in case you want to have a look at what I am into... https://www.projectguitar.com/guitar...ber-2020-r115/
ball bearing nut... right on. nothing I did in my cab/fab days was nearly as demanding as what I've learned building guitars. is really a passion for me. cnc was supposed to be a way to add some rock solid precision to my builds (so much for that for now!!).

thank you for your response sir.

generating tool paths in aspire using the mach3 output driver. Initially since this cavity has a 1 deg incline as well, I was using a rough and finish path output (I thought those were only used for 3d stuff). that one... is the one that drifted 1/2". so I figured I'd eliminate the possibility by angling my work piece and going with a simple pocket.
I've generated it about 5 different ways as a pocket (and as mentioned above with several dif feed and depth settings. I've tried doing it conventional, rastered at 0deg, raster at 90deg, raster with and without a final profile pass and tested it 'in air' quite a few times. what I've found is that the profile pass is what will causes the machine to go off the most.

with the pickup pocket (the one that works flawless) I just used mostly default settings on a pocket setup. that's the conventional "spiral out from center" cut... that sm cut on this shape does not work well and similarly kept drifting right with each revolution... but the left side stayed at ctr? I even went and deleted all reference to the original, other than the vectors, an re-created it... sm problem. then created a whole new program, copied in the vectors... sm issue.

I know I probably need to learn to read g-code... and I have a pretty good idea of how it goes... but so laborious just looking at "go to this x and this y". Hard time seeing anything wrong there.

one more tidbit... the way I'm finding zero on my work piece is to follow a long centerline jogging the machine from one end to the other on the y axis to ensure the workpiece is centered. does not exibit any issue and follows the line well. if I find my zero, then move x axis several times to either extreme then back to zero - no issue... at least not one that is detectable by eye. iow when I advance to the ctr point... I get within a few thousands of 0... and I figure my eye is probably not accurate to more than 4/1000 if that.