Opposite sides machining offset

[Scott Schroer]

Hi all, I've been using my pocket NC and really enjoying it, but have found I have a few offset issues. If I machine a part from opposite sides, there seems to be about a 0.05" offset issue. For example, I cut a hole through a test block, halfway through from opposite sides, and the holes are about 0.1" off from each other on the X axis. I've been adjusting for this when I'm cutting parts by post processing every tool path individually and moving the model in Fusion 0.05" one way or the other depending on which side I'm machining from. It's a fix, but its kind of a pain, and if I start machining on angles on the B axis that aren't 0 deg or 180 deg, I'm not sure how to account for the offset. Is this a machine calibration issue? the only thing I can think of is that the B-axis center of rotation is physically 0.05" off from where the machine thinks it is, although I'm sure you guys check for that. Do I need to adjust the center of rotation by 0.05" in Fusion? (I don't think that would make a difference...). Has anyone else encountered this? Thanks!

[mic...@cyto365.com]

Hi Scott,

I had exactly this problem and I have found a way to calibrate it quite easily (after much try and error).

Drill a hole completely through a part with an end mill having same endmill size as its shaft. I used 6mm endmill with 6mm shaft. Once I have drilled the hole through, I programmed to drill from opposite side. But I stopped the PNC when the PNC is moving in z direction towards the stock. Then I manually jogged the PNC back in the Z direction and removed the endmill and turned it around so the shaft was facing forward and tightened it just slightly so the endmill can move if making contact with the offset hole. Then I jogged towards the part in z direction just before making contact. I noted the current Xvalue from the DRO. I then manually jogged in x direction until I was aligned with the hole. Then I noted the new x value. The new x value was calculated like this:
Xold - (-Xcorrectionx0.5) = Xnew
Then I open up the Linux CNC setup and updated the HOME_OFFSET of AXIS0 which is X axis.
Then save and restart LinuxCNC.

This is only required to be done once. It should work now but please let me know if it doesn’t because I haven’t had time to check the effect if milling from different angles and planes. But at least the opposite hole is now aligned.

Best regards
Micael

[ke...@pocketnc.com]

Hi Scott,

Micael has the correct instructions and just to add a bit more detail, to get into the CNC interface go into the Conf. tab of the software UI > Server > Linux CNC Server Web Configuration. To change the X offset go into axis 0. If you're off by 0.5mm or 0.02 inches, then change the offset by 0.01 (half the total offset), save it. Go into 'system' tab and restart the CNC and Rockhopper, refresh the software and then try. If it's off more, change to 0.01 in the other direction and see if that helps.

When working through this with Micael, we took a piece of 1 inch delrin stock and drilled 1/2" into the stock, rotated 180 degrees and drilled 1/2" in the other side. It was with a 1/8" tool, and so when we put in a 1/8" dowel pin it did hit in the middle, but I could push it all the way through with just a bit of force and couldn't feel a shelf or ledge with a tool. Ultimately, it seemed like a slight tolerance stack up, but it wasn't such a large shelf that we couldn't get the 1/8" dowel in to the other side. 

We'll look into why this issue is happening a bit more, since we're not sure why after the machine is calibrated it is off by so much in these cases. We check the homing repeatability in all axes 10 times to ensure that both the home offset is set correctly and within a certain tolerance. We also cut a square on a 1" piece of delrin and measure it in X and Y and make sure that the block measures within 0.001 inches although this doesn't address the rotation of the B axis and then repeatability of the X and Y.

Please let us know what you find.

Thanks,

Kerry

[Scott Schroer]

Great. Exactly the fix I was looking for. I must be looking in the wrong place for the Config tab to input the new offset. Where do I go from the AXIS command window? I don't see a UI tab... Thanks for the quick response!

[ke...@pocketnc.com]

Hi Scott,

If you take a look at the instruction PDF on this thread of the tutorial it guides you through the process with a bit more specifics for accessing the Linux CNC, you can ignore the actual hardware checks, and go to Axis 0, which is for the X. 

Thanks,

Kerry

[Scott Schroer]

I dont know where the user interface is even to start. Is that a V2 thing? I have the V1. The only interface I've been using is the AXIS software.

[ke...@pocketnc.com]

I replied to Scott via email - these instructions are for the V2 user interface, so it will be different for our V1 users.

[Ben Arcand]

Hi Kerry,

I'm running through the B-axis compensation tutorial, but I'm blocked from getting on the LinuxCNC server.  What's the login info for the LinuxCNC server?  I've tried the "pocketnc" login that gets you in through SSH in a terminal.

Also, I'm not able to update to v2.0.1-beta-9.  When I "Check For Updates", the list remains the same.  I'm on beta-8 currently.  I've checked the ethernet connection and verified that it works

[ke...@pocketnc.com]

Hi Ben,

It's default, default to get into the Linux CNC. Sometimes it takes a good 5-10 minutes to populate the updates for the software, so you may want to give it a few minutes. Let me know if it still doesn't come up after plugging it in and waiting a bit.

Thanks,

Kerry

[Danny Blanchard]

Kerry,

Could I get the same instructions you referenced to Scott and his V1?

On Monday, July 23, 2018 at 2:37:15 PM UTC-7, ke...@pocketnc.com wrote:

I replied to Scott via email - these instructions are for the V2 user interface, so it will be different for our V1 users.

I recently acquired a V1 and the x-axis is off by about .031" in the same fashion that Scott described when the part is rotated.  I've done a bit of Googling about how to find the Linux CNC settings, but to no avail.  If you could pass along those instructions I'd greatly appreciate it (or maybe just post them here?).

Cheers,

Danny