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General Mach Discussion / Calibration Issues - odd results

« on: June 13, 2011, 09:19:57 PM »

I was trying to fine tune the calibration on our machine today.

I thought I had it nearly right-on for the X and y axis, but have some confusing results.

I am using two techniques for the calibration - as I don't have a set of gauge blocks I am "roughing in" with big moves and measuring with a ruler (1/20th" scale - 50 mil/division) and then finally "fine tuning" with short moves (<= 1") using a dial indicator and magnetic base.

The problem I have is that the measurements between the two techniques do not agree.  I don't think it is a machine problem - I think it is my measurement technique.

I can get the calibration right on the money either way - I can set the counts per inch using the dial indicator for a 1" move +/- 1 mil - and this is for both directions - I will measure 1" and then do the opposite move and return exactly to 0 - +/- 1 mil for the opposite direction move.  I have the same results with both G0 and G1 moves.

If I go with the numbers I get from the 1" move with the dial indicator - I will have, as a example for an 8" move on the X-axis, a bit more than a 1/8" error.  Yet I can return to zero position where I started and I've got a zero on the dial - and a 1" move will still yield +/- 1 mil - either direction.

Here the travel is too far - and the cal value for the counts per inch is ~8100.  I am using velocity between 10 and 20, based on how the stepper motors respond.

So - using the "ruler technique" - with a "calibrated eyeball" I will perform the cal again - and come up with something like 7975 counts/inch.  Now the 8" move is nearly perfect (with my eyeball) - and also incremental 250 mil, 500 mil, and 1000 mil move are right on the money!

I certainly understand that my eyeball is not as accurate a a dial indicator - yet am perplexed by my results - as certainly I can discern two divisions of the ruler (100 mil) with my eye.

As I have the same results in both directions on both X and Y axis - I don't think this is a mechanical thing like backlash - I think I have a flaw in my measurement technique.

Tomorrow I will first get the machine cleaned and lubricated - as I have found that the proper lubrication makes a big difference.  I have been just spraying WD-40 on the crosslide and bed rails for the short term, but the manual recommends 10 weight machine oil.

I will get this machine as clean as a rifle in boot camp, oil it well according to the manual, and try again.

Is white lithium grease OK for the drive screws?  This adheres better and longer in my experience with engines.

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General Mach Discussion / Is there a thread dedicated to n00bs here?

« on: June 12, 2011, 12:46:51 PM »

I mean like a thread for somebody like me that is not a machinist!

I design electronics for a living - but am currently tasked with getting a combo mill/lathe (Shoptask) machine up and running to run prototype quantities of mechanical parts for our products (LED lighting).

I need to get this done by next week - so no pressure

What I need to do here is use this machine as a sort of automated drill press to drill holes in a sheet metal "pan" that is ~18" square.  These holes will be for mounting 4X LED PCB to the pan - the finished product being a 24" square light fixture.

I would imagine that my required accuracy would be on the order of +/- 10 mil across a dimension of ~18" in the Y axis and ~8" in the X axis.  I am basing this requirement on the size of the fastener (screw) and the size of the hole in the PCB.  Due to the 10" limitation of movement about the X-axis - the pan will need to be rotated 180 degree during the process.

This machine has three stepper motors to drive three axis - X,Y, and Z.  Currently I just need to use for drilling sheet metal which I would imagine would be a maximum of 100 mil thickness (probably less) and the material would be steel.

I have managed to perform a cursory calibration of the machine as far as movement goes with the Mach3 software.  I do not have a set of gauge blocks so first "roughed in" the calibration using a steel ruler for 4" moves - and then fine tuned it using a dial indicator for 1" moves.  I currently have the Y-axis +/- 1 mil, X-axis +/- 3 mil (this can be improved), and Z-axis +/- 6 mil (the Z-axis on this machine has a problem).

Is this calibration technique adequate for my needs?

Will the "pan" need to be pre-punched to guide the drill?

Should I use normal drill bits or pilot drill bits?

Is there some "convention" as to what is "zero" for each axis?  I currently have dead center of the X and Y axis as zero - and Z-axis zero is fully retracted - i.e. Z only moves in a negative direction.  Is this "normal"?

What about feed rate?  I know from using a manual drill press that this matters - but I am not sure I understand the concept using Mach3.  Is this simply a function of the acceleration profile setup during the motor tuning calibration - or are there other ways to control this?