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machine not accurate
« on: November 23, 2011, 04:16:39 PM »
hi there i have a strange problem:

i did the "set steps per unit" process in the settings page of mach3 on x and y axes of my router. i did let the machine travel 50mm and measured it - it tells me 50.01 on the x and 50.02 on the y axis which is fine for me. problem is that when i draw a 40x40mm rectangle to be cut out in cut2D, export it as mach3 gcode and let it machine this the cut out part measures 39.56mm (x) and 39.73mm (y). i did try this with different speeds but the cut out piece is never near 40x40mm.

strange thing also is that when i change the steps per unit in the motor settings and then do the "steps per unit" process in the settings page of mach3 it still travels the right measurement. shouldnt it travel differently if i change the steps per unit in the motor settings?

of course i checked if my cutter is exactly 6mm (which is the cutter i have chosen in cut2d).

hmm, this is strange. it would be great if someone could send me some tips or ideas on what this f.... problem could be.

have a nice day out there!

Offline Hood

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Re: machine not accurate
« Reply #1 on: November 23, 2011, 06:32:47 PM »
Did you remember to save the axis settings when you changed them before you went on to change the next axis?
Re: machine not accurate
« Reply #2 on: November 23, 2011, 07:20:52 PM »
also, the verified distance could be correct, but the presence of backlash could make the actual part come out wrong ... possibly smaller as you are seeing.

Offline ger21

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Re: machine not accurate
« Reply #3 on: November 23, 2011, 08:02:59 PM »
What kind of machine is this? tool deflection or machine flex can also cause this. Even cut direction can effect size if the machine is flexing. Try cutting in the opposite direction and see if the size changes.

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Re: machine not accurate
« Reply #4 on: November 24, 2011, 07:01:07 PM »
i built the machine myself. the whole construction is quite solid (sorry for my bad english by the way:). i would never expect my machine to be as precise as for example +/- 0.01mm but i expect it to be in the range of +/-0.1mm. currently the precision is about +/- 0.5mm.

just to get it right: from your kind answers it seems more likely that the problem is the machine? for me this also would be logically the solution at the moment. what is strange to me is that even at very low feedrates it doesent get better and even at high feedrates it does not get worse.

so it seems that i will have some fun thinking, refining and maybe redesigning parts of my machine.

if anyone still has some ideas which do not regard to the mechanics but to common software or electronic problems please let me know. i think it would be helpful if i post a summary about the machine including the electronics and mechanics next time.

anyway, have a nice day and thank you so far.

ps.: yes, i did save the axis settings.

Offline Tweakie.CNC

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Re: machine not accurate
« Reply #5 on: November 25, 2011, 05:22:44 AM »

It's still on the mechanicals, sorry, but do you have thrust bearings between your motors and leadscrews ??
Often, the motor bearings are internally spring loaded (spring washers) and will give quite a bit of backlash if subject to axial loading.

Just a thought.

Re: machine not accurate
« Reply #6 on: November 25, 2011, 08:04:14 AM »
Depending on your design, there can be multiple areas where backlash can creep in.
Pulleys, gears, belts, chains, rack and pinion and ... the leadscrew (or ballscrew) and nut.
Plus the thrust bearing that Tweakie mentioned.
Here is a good way to check it out.
I think you should.
Re: machine not accurate
« Reply #7 on: November 25, 2011, 08:06:23 AM »
...... also, shaft to coupling connections as well as the couplings themselves.
Re: machine not accurate
« Reply #8 on: November 25, 2011, 08:43:10 AM »
thank you all for your nice replies. my question is answered already. i just was wondering if i did something wrong in mach3 or if it could be something with my crappy ebay controller beside of some of my "build my own cnc router design flaws". i am now pretty sure that the problem comes from the machine. this is what i wanted to know. my machine at the moment is work in progress and i know the things which have to be changed. thank you very much so far!

next thing i will do is to change the spindles, couplings,... for the y-axis. at the moment i do drive the y-axis with two spindles and two motors. running the two motors in sync is something that causes troubles all the time so i will change this to work with one motor and two spindles with a belt drive. i also will use professional couplings then. i will see how this works out and if it turns out good, i will then replace the x-axis and last but not least the z-axis.

at the moment i am happy with the machine because it works like it can and i am looking forward to make it better.

have a nice day, all you nice people out there.