Taking the CNC Plunge

[TomS]

I tried a multitude of Mach3 CV settings such as CV Distance Tolerance, bumped Look Ahead to 200 lines, made sure G100 Adaptive Nurbs CV was turned off, Shuttle Acceleration, changed speeds, feeds, cutter size, number of flutes, DOC, WOC, feed direction and spindle drive belt. No change in surface finish. Even went back and checked gib adjustment. No improvement in surface finish. I'm at a loss as to what's causing this.

 

[TomS]

With nothing to lose I played with my microstepping configuration to see if it had any effect on surface finish. First test was a change from 1000 steps to 400 steps. No change in surface finish. I then went the other way and tried 5000 steps. Again, no change. Next test I turned off microstepping. That was a mistake! When I started Mach I couldn't get the X and Y axis to move. The X and Y DRO's had an error message that would not go away. The message was "-1.#IND". Shut down Mach and configured my drivers for 1000 microsteps (my original setting). Started Mach and the error message was still there. After trying for 30 minutes to clear the error message I gave up and uninstalled Mach. I keep a backup copy so it was a simple matter to reload and get my smoothstepper communicating. So not a very productive day.

At this point I'm out of configuration options so I'm back to looking at mechanical contributors. I did find that when I rotate the spindle by hand the clearance between the spindle shaft splines and the spindle drive hub splines gets loose and then gets snug. Hmmm, looks like I have some runout but I'm not convinced it's the source of my problem, yet. All this being said I disassembled the head and measured spindle shaft runout at the drive end. I got .0025" TIR. Checked the drive hub splines and got .0023" TIR. I'm still not convinced this is the cause of my poor surface finish but at this point I'm looking at everything.

I gave up for the day because it's 100+ now and working it's way to 112 deg F. I'll reassemble it with the high spots opposite each other and see what happens. If that doesn't change anything I'll try something else.

This is so frustrating!

 

[jbolt]

At this point I would suggest posting in the CNC forum for more exposure. You might consider posting over on cnczone as well.

Sent from my SM-G955U using Tapatalk

 

[TomS]

At this point I would suggest posting in the CNC forum for more exposure. You might consider posting over on cnczone as well.

Sent from my SM-G955U using Tapatalk

Good idea. I've got a couple more things to try out first. One is the runout issue I discovered yesterday. The other is a post on the Mach Support Forum that discusses having G90 and G91.1 on the same line or separate lines. The Mach forum says they should be on separate lines. My CAM program (CamBam) puts them on the same line. I've got a writeup by RGSparber that addresses this issue. Just have to read through it again to absorb and decipher all the computer geek language.

 

[Uguessedit]

Apologies to TomS. We are kind of hijacking this thread, but I think this a very pertinent question.

The most common and least cost conversion uses stepper motors that are either direct or timing belt coupled to the lead screws. With the stepper motor powered down, it is possible to overpower the inherent mechanical (magnetic) resistance of the stepper motor manually. It takes a fair amount of force to do so and the stepper motors cog due to their design. Assuming that you are using a standard 200 step/rev motor and have a 5 pitch lead screw that is direct coupled, then the motor will cog at 0.001 inch increments. Irritating at best on the X and Y axis. On the Z axis, without decoupling the quill from the lead screw, it is impossible to use the quill manually. Without providing a very convenient way of decoupling the stepper motors from the lead screws, manual operation is not very practical. If I couldn't switch over in under a minute, I would find it very irritating.

Another and more costly option is to use DC servo motors. These have almost no mechanical resistance when powered down. The Z axis lead screw still must be decoupled from the quill to manually operate the quill. The down side of this is the cost and the more complex control system.

I know this is an old post but the 1600oz hybrid servos he mentioned do not cog when powered down and will rotate freely. I have the same ones on my machines and went with a through shaft and installed hand cranks for the occasional manual use. The hybrids are excellent in that they can be locked when or fully released. Both benefits and if you try cranking it when it’s locked you will find you can’t. Just wanted to mention and clarify for those reading who are unaware.

 

[7milesup]

Hi Ugessedit:
Could you provide a link to the hybrid servos that you use, or would recommend. I am seriously considering making my PM833T a CNC machine and am just starting to gather information on how to do this.

THank you.