Dirks 4x8 CNC Router Build

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Re: Dirks 4x8 CNC Router Build

Postby orcinus » Mon Feb 04, 2013 12:40 am

Just out of curiosity... The raw data looks like a pretty regular sine pattern for both planes.
Are my eyes deceiving me (seeing patterns where there are none) or is it really so? And if yes, where does it come from?

When i see a sine, the first thought that usually occurs to me is "eccentric rotating element".
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Re: Dirks 4x8 CNC Router Build

Postby dirktheeng » Mon Feb 04, 2013 1:49 am

I saw the same thing. I think there is a bearing that may be out of spec or something. I'll have to check to see if the period corresponds to the circumfrence of the bearings. It is also very possible that the steel has some periodic character too as it is cold rolled... if one of the rollers was out when it was made that could do it too. The deviation is so small though that it may be within the tolerance of the manufacture.
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Re: Dirks 4x8 CNC Router Build

Postby orcinus » Mon Feb 04, 2013 4:32 am

Yeah, it doesn't look too significant.
I just found it interesting.
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Re: Dirks 4x8 CNC Router Build

Postby macona » Mon Feb 04, 2013 10:00 am

Everything looks fine. The sine-wave line error is probably from the gear mesh. The gear might not be perfectly round or one of the timing pulleys might be. But it is nothing to worry about. It's easy to get worked up about these tiny measurements when it wont matter in the end product. Remember that the machine will change dimensions more than these measurements just by changes in the shop temperature. Plus the materials that you cut with a router, wood and plastic, change dimension as well with humidity (wood) and temperature.

Thats why when people give me drawing that say .001 tolerance I have to ask at what temperature.

Dont worry about it. Just use the machine.
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Re: Dirks 4x8 CNC Router Build

Postby dirktheeng » Mon Feb 04, 2013 3:42 pm

The z axis is driven by lead screw so it is likely not gear mesh... it could be related to the nonlinearity of the lead screw though.

I fitted a sine wave to the data and found that it has a period of about 0.622". This isn't long enough for it to be related to the bearings. Given that it appears in both the XZ and yZ planes, I am inclined to think that it is related to the lead screw somehow. They should be 90 degrees out of phase though. Maybe it is my reference square?

Also, the machine will change by about 0.0012" over 96 inches for every degree F shift in temp. Moral of the story: keep your work close to where you home if you want really high acruacy. Making the table out of MDF doesn't really help anything though... needs to be sealed well.

Maybe I should invest in a dehumidifier and attempt to keep the shop at about 70F year round.
Last edited by dirktheeng on Mon Feb 04, 2013 4:21 pm, edited 1 time in total.
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Re: Dirks 4x8 CNC Router Build

Postby cvoinescu » Mon Feb 04, 2013 4:20 pm

It could well be half the bearing circumference, too.
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Re: Dirks 4x8 CNC Router Build

Postby BenJackson » Mon Feb 04, 2013 7:15 pm

dirktheeng wrote:Also, the machine will change by about 0.0012" over 96 inches for every degree F shift in temp.

Easy to compensate in software. :)
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Re: Dirks 4x8 CNC Router Build

Postby dirktheeng » Mon Feb 04, 2013 8:14 pm

BenJackson wrote:
dirktheeng wrote:Also, the machine will change by about 0.0012" over 96 inches for every degree F shift in temp.

Easy to compensate in software. :)


This got me thinking about corrections in software that could be made.

What if we could set up a very simple measurement of the linear accuracy of the mill that is very cheap and easy... say imaging a line laser beam down the length of an axis. It should be very easy to apply a transform in the machine interpretor (mach 3, emc, etc) to compensate for that error. This, of cource, would only work for regular errors... random error cannot be compensated. Then the expensive rails would not be necessary. you could use a rail that had a 1 inch bow in it and it wouldn't make much difference. You could also theoretically compensate for temperature and machine expansion and all that.
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Re: Dirks 4x8 CNC Router Build

Postby macona » Tue Feb 05, 2013 1:29 am

Could be the screw was not turned down concentric to the screw pitch and you have some wobble. What kind of lead screw did you use?

Seriously, dont worry about it. See how far the machine is off when you actually start cutting thing. Machine flex under load can be more than these tiny errors.
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Re: Dirks 4x8 CNC Router Build

Postby dirktheeng » Thu Feb 07, 2013 2:02 am

All,

I got the router shimmed and trammed now. I adjusted the proximity sensors so that I get a very square gantry. Over 6 inches, I am within about 0.001". It seems repeatable. I will be replacing them when I get some holders made. I will be using pogo pins put into bolts and that will provide a direct contact way to close a switch which will not depend on temperature to any great extent. The best thing is that the entire switch system will only cost about $2 and give accuracy in sub 0.001"

Anyhow, I now have the table completely surfaced and I drilled holes for reference pins so I could align and place work on my table in known locations. I am using carpet tape for now, but will eventually switch to vacuum when I get a chance and a little more money.

On average, the CNC is much quicker at getting projects done than multiple hand operations. I have started building a project with a freind and the cnc is awesome!
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