Diy cnc machine is having trouble with speed and accuracy.

Hello. I have been running my diy cnc wood router for about a year now making mostly hobby stuff for myself. I an stsrting to get a lot of requests from people asking me to make items for them, and am thinking about getting more serioys about my machine.

Since day one I have had trouble with the speed my machine would run at. At around 500 mm per mimute, the motors sound terrible, abd the accuracy which has also always been bad goes to hell.

Currently I run at 300mm per minute and the accuracy is still off by at least a mm at 100mm.

What suggestions do you guys have to improve these issues? What additional info do you need? I will get it asap.

Thanks,

Jason

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Say a bit more about your machine - construction, linear rails, screws, stepper size and so on.
Here is a list of typical "accuracy" errors:

1. after cutting a rectangle machine misses starting point - probably losing steps - steppers or drivers to weak, linear rails not parallel causing excessive drag
2. rectangle looks fine but side lengths are off, error proportional to rectangle size - scaling off - wrong configuration, really "cheap" screws
3. lenghts are ok but sides not perpendicular - "twisted" machine
4. terrible sounds and random errors at specific speeds - faster and slower somewhat works - resonance problems (google "midband resonance"), you need better driver board or damper disks
5. sides longer/shorter than should be - difference constant regardles of rectangle size - backlasch or programming error (cuttercomp)

That's about all i encoutered on my machine :)

That sounds really slow for a wood router. Are you getting a lot of scorching and dwell marks? Wood likes to be cut a lot faster than that; at least 48 inches (1200mm) per minute. Lots of people run them at 500 ipm or faster. If you're really scaling up your operation to do work for other people, you might look into a better motion control system that will give you higher feedrates - a new power supply could help, if your drives can handle it, and if they can't, think about replacing them with something that can.

What about your accuracy (or lack thereof) is giving you a problem? Are your cuts randomly too short or too long, are they always too short or too long, are rectangles out of square, or is there any other pattern you can see in the results you've been getting?

Andrew Werby
www.computersculpture.com

Some pics of your machine may help. Machine construction, motor specs, drives, controller... Material you're cutting, spindle and spindle speed, bits you're using...

Thanks for the responses guys. I will get some pics of the machine and specs on the motion control system up shortly.

I will say it was mentioned that cheap screws and resonance could be part of my problems, and I would agree. I will look into that.

Also you guys also keep asking about the accuracy. I really notice it when I try to cut a circle. They are always oblong. My x and z axis are both driven with acme screws, and my y axis is driven with a rack and pinion system. The ratio of steps to mm on these systems is substantially different, but each axis is callibrated independantly.

I am seeing a large overhaul in my future. I am okay with that I just want to make sure I make the correct changes.

Thanks again

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Alright, here are the pictures of my machine. Please be kind....remember it was built as a hobby machine from spare parts. Taking these pictures made me realize I have a long way to go here.http://img.tapatalk.com/d/13/10/03/y9ybatat.jpghttp://img.tapatalk.com/d/13/10/03/usuza9a8.jpghttp://img.tapatalk.com/d/13/10/03/se2e5ysy.jpghttp://img.tapatalk.com/d/13/10/03/2u3ybeda.jpghttp://img.tapatalk.com/d/13/10/03/ju3adyqy.jpghttp://img.tapatalk.com/d/13/10/03/a5abubas.jpg

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They are pretty large motors....65vdc each. I'm running a 150watt power supply for all three. They are running on pac sci 5310 drivers.

I noticed the acme threads that are driving the x and z are 6 rotations per inch. Motors max at 1350rpm so max speed of travel would be about 5700mm per minute.

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I think that looks pretty good! Envious of yoour steppers, those are really good ones!

Looking at the mechanicals however, I see that the leadscrews are supported on one end only by the spider coupling. An upgrade would be to "fix" the screw near the coupling by usng even radial ball bearing (two angular contact would be better but not necessary) in a bearing block attached to the frame, and secured by shaft couplings from movement. There is quite a bit of flex in the spider couplings, depending on the hardness of the spider, as well as some axial play on teh stepper shaft, which might cause some slight positioning errors. Same thing with the R&P drive; I suspect some slop or play there under load. Theoretically you would want to isolate the stepper motor shaft from axial loads or twisting, which is what the bearing blocks will help accomplish.

There could be computer/software issues as well. Some parallel ports on newer PCs may not put out 5V necessary for some drives, though if you have a breakout board that should solve that. Could be pulse stream from slower PC, maybe too small microsteps? If pulse stream is good I would check LookAhead setting in Config if it's set too low; I had that issue with USB controller and cutting curves at high speed.

Also in ref. to the spiders, they do make them of different hardnesses, so you can try swapping them out, or making shims out of thin brass stock at the hobby store to tighten them up. But it's probably not a good idea to fully support your jack shaft and leadscrew with just the coupling alone.

Never thought I'd see steppers bigger than a PC 690 :eek: Those are 3130oz-in motors!!!
What's the voltage and current rating of the power supply? What current is the drive set too?
The 65V and 1350rpm are maximums for those motors. They are rated at 3.2V and 7.9 amps when wired bipolar series, and 1.6V and 15.8a when wired bipolar parallel.
I'd guess that with a 150w power supply that you'd be lucky to see 300rpm. Those motors are really waaaay too big for what your using them for, and they are probably drastically hurting performance. They should be run at 8 amps and 115V, which is 900watts each. If your powers supply is really 150 watts, than it's only about 5% of what it should be.

Ger,

I kinda thought that might be the case. They were freebies along with the drivers.

What size motors and drivers would you suggest. I wouldn't be opposed to selling these to purchase new.



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