RE: z-axis depth problems

[skfisher]

I have a BlueChick CNC machine from Buildyoucnc.com. 


Here is the situation I find myself in frequently. I am carving a sign first with an 1/8" endmill for the rough pocketing and then I switch to a 1/16" endmill for the final profiling of the letters. I have a basic auto-zero plate to zero the z axis to the work surface between tool changes. For the 1/8" endmill if my final depth of cut is set to -.125" when I am done I measure anywhere from -.125" to -.130" which for me is ok. Then I switch tools to the 1/16" endmill which is programmed to cut to the same final depth of -.125". By the time the 1/16" endmill MOP is finished the overall depth can be anywhere from -.16" to -.18".

I have tried the following:
1. Cleaned the lead screw and reapplied a dry lubricant.
2. Slowed the z axis velocity down to 20ipm, accel to 2, step pulse to 3, and dir pulse to 5.
3. Tightened the collet to ensure the end mill is not slipping.

So far I have been unable to erradicate this problem. My x and y axis (belt drive, bluechick) are fine.

I am at a loss. Any help would be appreciated.

I have the 270oz in motors and should have the 425ozin motors in the next week. I had some concern that the torque on the 270ozin motors was causing some lost steps. I am also puzzled by why the problem seems more pronounce with the small endmil.

Sincerely,

Stephen

[Tweakie.CNC]

Stephen,

Just a shot in the dark but if it was lost steps (caused by whatever) then the finishing pass, with the small endmill, may have more small Z axis movements and thus show more error.

If you are satisfied that the problem is not mechanical then you should perhaps start looking for electrical noise and thoroughly check all electrics etc. (I know this is easier said than done   ) A search for electrical noise should bring up some suggestions.

Tweakie.

btw. Welcome to the forum.

[skfisher]

Tweakie

Thanks for your reply.  Since my original post I have done some test adjusting the velocity and accel of my z-axis motor.  I also switched the x-axis and z-axis drivers.  All to no avail.  Same problems.  I am now back to thinking this is a mechanical issue.  I am going to dissassembly my z-axis lead screw, couplers, motor and tighten everything back down.  I am loosing between 1/16" and 1/8" on my z-axis over the course of running a program.  I am thinking now that the couplers connecting my z-axis motor to my leadscrew might be slipping a little bit and perhaps my motor is a little underpowered and having a little trouble overcoming the initial static friction when it starts to move.

Ah well.  That's why we chose to play with these machines because it's fun/frustrating trying to get them to work just right.

Stephen

[Tweakie.CNC]

Hi Stephen,

I Hope you find the problem easily.

Please report back when you have found it as we all like to know the solutions / causes to issues like this.

Tweakie.