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Blackfoot Accuracy?

Blackfoot Accuracy?

Postby thercplanebuilder » Wed Apr 22, 2015 5:17 am

I am wondering if anyone out there that has a black foot machine can let me know what tolerance they get from there machine?
I calibrate my x axis(length of table) at 75 inches and everything is good but when I move the x axis 1" it measures .985 at 2" 1.989", 3" 2.992, 4" 3.992, 5" 4.989, 8" 7.982 . The machine, at small movements, seems to me off .01- .02. I have tried it at 1/16 micro step and 1/8 micro step. I tried the 1/8 thinking I was loosing to much torque at 1/16 and all numbers came out the same. Is this as good as it gets or do I have a problem?
I have been thinking of upgrading my drivers to Gecko but do not want to spend the money on this machine if I can not get better accuracy.
thercplanebuilder
 
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CNC Machine: BlackFoot v4 4x8

Re: Blackfoot Accuracy?

Postby Awesomeness » Wed Apr 22, 2015 7:38 pm

Something is wrong, and it 99% surely not in your drive electronics (or at least not that replacing them with a Gecko would make a bit of difference).

Some things to check:
  • Something mechanical is loose. This is especially suspect since you get different results each time. It is also the solution to 90%+ of peoples problems, usually after they SWEAR a dozen times they've checked that everything was tight.
  • Interference on your cables. Try shielded cables?
  • Cutting forces are deflecting the z-axis. Are you measuring a cut part? If so, try making some very light cuts, to see if less deflecting gives better results.
  • Part moving, or expanding/contracting with heat.
  • "Small" 1/4" drive axle shafts are twisting. Some people have replaced them with 3/8"-1/2" shafts.
  • Chain sag, looseness, or vibration.
  • Inaccuracies in the chain links' length. Could be caused by wear, or even just the fact that chain is not really sold as a precision drive component, it just happens to do the job "ok".
  • Calibration is off. If you're calibrating at 75" then measuring at 4", you're obviously not using the same tool to do both. While calibrating at 75" is a good idea, try doing it differently. For example, use a different tape measure, calibrate in the other direction, or calibrate to the far side of the 75" mark/line. You could even go as far as calibrating to 75 1/8", and telling the computer it's going to 75" (or similarly subtracting from the measurement, so that when you measure with your precision calipers at 4" you get the answer you want).

Ultimately, I think you may be being overly worried, too. Being off by 0.018" on an 8" cut, on a soft part (e.g. wood, foam, etc.) is hardly a big deal. Even professional CNC routers only claim tolerances in the 0.002-0.015" range usually. Notice ShopBot's tricky wording... "Positional accuracy of +/- .002”.", which means the position will only get within 0.002", and the bit may deflect further away from that! Many of these materials (e.g. wood) shrink/swell more than this.
If you don’t have time to do it right, when will you have time to do it over?
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CNC Machine: BlackFoot v4 4x8


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