Need Help to Improve Surface Finish

[TomS]

At the suggestion of another forum member I'm starting this thread here for more exposure. A few week ago I opened up this discussion on my PM-932 build there here - http://www.hobby-machinist.com/threads/taking-the-cnc-plunge.24858/. It starts on page 12, post #344.

Now to the issue at hand. A few months back I installed a 3ph motor and VFD to my CNC converted PM-932. This also included upgrading the stock tapered roller spindle bearings to angular contact bearings (ABEC 5 for the top bearing and ABEC 7 for the lower bearing). Then I followed up with new electronics: a PMDX-126 breakout board, a PMDX-107 spindle control board, and a ethernet smoothstepper. All these "improvements" are working well except that since then the surface finish on angular and circular cuts is terrible.

I've 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. Also made some changes to my CAM program but still no change in surface finish. Even went back and checked gib adjustment, twice. No improvement in surface finish. I'm at a loss as to what's causing this.

Any help is appreciated.

Here are a couple of pictures for reference.

X and Y straight cuts give me a very smooth surface finish.


This is the surface finish I get with angular and circular cuts.

 

[countryguy]

There are a few ideas in this post:
https://www.machsupport.com/forum/index.php?topic=19101.0
.If you are cutting the circle with GCode created from a CAD program then there is a good chance that the circumference consists of many straight lines and depending on the resolution it could be quite rough and lead to the problem you describe.
Try cutting a circle using the Mach wizard which uses circular interpolation, which is as near to a curve as you are going to get, and see if that improves the situation

Some detail in this post maybe could help?
http://www.cnczone.com/forums/mach-mill/213832-cnc-software-posts.html

I was wondering what design CAD tool and which CAM is being used?

 

[gradient]

Why not mill the hole almost to size and finish up with a boring tool to get to final size? The boring tool will leave a very smooth and uniform finish since it is not interpolating circular motion.

 

[TomS]

There are a few ideas in this post:
https://www.machsupport.com/forum/index.php?topic=19101.0
.If you are cutting the circle with GCode created from a CAD program then there is a good chance that the circumference consists of many straight lines and depending on the resolution it could be quite rough and lead to the problem you describe.
Try cutting a circle using the Mach wizard which uses circular interpolation, which is as near to a curve as you are going to get, and see if that improves the situation

Some detail in this post maybe could help?
http://www.cnczone.com/forums/mach-mill/213832-cnc-software-posts.html

I was wondering what design CAD tool and which CAM is being used?

I'm using CamBam for CAD and CAM but have used Fusion 360 for both as well with the same results. I'll try the Mach wizard and see what happens.

That CNCZone thread was quite interesting. I need to study it further to find out how it compares to my gcode.

Thanks

Edit: I did zoom in on the arcs and did not see any straight line segments. CamBam has an "arc fit" setting. I set this to .002".

 

[TomS]

Why not mill the hole almost to size and finish up with a boring tool to get to final size? The boring tool will leave a very smooth and uniform finish since it is not interpolating circular motion.

The rough/faceted surface is on all internal and external angle and arc cuts. There is an underlying problem that I'm trying to nail down so I can correct it. I didn't have this problem before the VFD/electronics upgrade.

 

[spumco]

Ditto above from countryguy.

A very quick way to see if it's hardware or software is to look at the code section with the hole that has unacceptable surface finish. Is the code posted as arc moves or is it G1 X/Y moves? If it's arc moves and it looks like that, you'll be chasing backlash, vibration, chattering, or some other mechanical gremlin. Something to consider is your steps per rev or steps per inch - a coarser setting (no micro steps) will not be able to move in a curve as smoothly as if you're at 5x or 10x microsteps. Linear moves along the x or y will be fine unless you're at extremely low feed rates where the stepper poles are cogging along: thump thump thump.

If you still suspect hardware, either use the Mach wizard or your CAM program to generate a very simple boring operation.

If the moves are posted as G1 linear moves, the the threads above should be helpful. If you don't get it right away, do a little math and see how long each linear segment is. Like this:

001 G1 Fxx X1.002345 Y1.00566 (or whatever)
002 X1.003456 Y1.00666 (again, whatever the values are)

In this case, you can estimate the length of each segment between each code line, given the angle of movement. If the segments are more than .002 (ish) long they'll show up visually in a fresh cut.

With a small diameter cutter longer segment lengths will show up as faceting. [I know, it shouldn't matter about the cutter, but cutter deflection can make it look worse).

So... what CAM are you using? I'm using Fusion 360, and while it's great so far there are a few settings that can output some funky code.

 

[spumco]

You beat me to it on the CAM.

 

[spumco]

So, assuming you're not getting a series of coarse resolution linear moves from your CAM, and Mach is displaying the code as arc segments... the next check is the hardware I mentioned above. How many steps per inch is your machine set to?

 

[TomS]

Ditto above from countryguy.

A very quick way to see if it's hardware or software is to look at the code section with the hole that has unacceptable surface finish. Is the code posted as arc moves or is it G1 X/Y moves? If it's arc moves and it looks like that, you'll be chasing backlash, vibration, chattering, or some other mechanical gremlin. Something to consider is your steps per rev or steps per inch - a coarser setting (no micro steps) will not be able to move in a curve as smoothly as if you're at 5x or 10x microsteps. Linear moves along the x or y will be fine unless you're at extremely low feed rates where the stepper poles are cogging along: thump thump thump.

If you still suspect hardware, either use the Mach wizard or your CAM program to generate a very simple boring operation.

If the moves are posted as G1 linear moves, the the threads above should be helpful. If you don't get it right away, do a little math and see how long each linear segment is. Like this:

001 G1 Fxx X1.002345 Y1.00566 (or whatever)
002 X1.003456 Y1.00666 (again, whatever the values are)

In this case, you can estimate the length of each segment between each code line, given the angle of movement. If the segments are more than .002 (ish) long they'll show up visually in a fresh cut.

With a small diameter cutter longer segment lengths will show up as faceting. [I know, it shouldn't matter about the cutter, but cutter deflection can make it look worse).

So... what CAM are you using? I'm using Fusion 360, and while it's great so far there are a few settings that can output some funky code.

Heading to the shop this morning to test out the wizards.

I have microstepping set at 1000. My machine has been cutting good surfaces for the past couple of years at this setting. I did try 2000 and 5000 but no change in the finish quality.

CamBam is generating arc moves as you can see in the attached gcode file. Here's a picture of the finish. It's rougher than it looks. The picture I posted above is more representative of the finish.

 

[TomS]

As countryguy suggested I read through the cnczone thread and changed my configuration as follows. Turned off CV Distance and Feedrate on the Settings screen. Went into General Config and made sure all of the CV selections were not checked. I also upped my acceleration from 15 to 25 on the X and Y axis. I then ran the Mach3 circular pocketing wizard. The finish was only marginally better. Tried it again in absolute mode and again with Exact Stop enabled. No noticeable difference in the surface finish. What is interesting is I was expecting a herky jerky motion using Exact Stop but it was very smooth.

If it was mechanical I would expect to see surface roughness on straight line cuts also. But I'm not. Even if I had excessive backlash I would think that it would be taken up on 90 deg. arc cuts. Arc cuts are very rough. Still at a loss.