Erratic Cutting

[Mikebpeters]

31398I am fairly new to the shopbot (buddy 24x32) world and am working on a fairly complex project. I’m making a military crest that is that is the full size of my table. I cut the edges and pockets and rough cut it and all went well. Today I started the finish cut with a 1/8 inch ball nose and it started well. I watched it for a long time and then puttered at other chores. When I next checked on it I discovered it had gone erratic with the depth varying far too much too sand out. What could possibly have happened and what can I do to make sure this doesn’t happen again?!?!

I’m using a raster pattern if that matters

[Brady Watson]

What speeds and RPM on the 3d finishing cut?

-B

[Mikebpeters]

Rpm is whatever the porter cable router runs at. Feed speeds are 150 x and y and 50 z

[bleeth]

As all that material was cut from the face of your project the board likely changed shape from the change in the internal tension of the wood and bowed up in the middle (even if it looks like the edges are still down). Projects of this type are very difficult due to this. For a table project like this when you are done with your rough cut, turn the piece over and do a shallow back-cut area clear under the cutting area. Wait a while before you start your final pass. I would bet if you flipped it over now and laid a straight edge on it you will find the back is not flat any more.

[Mikebpeters]

Thank you for this insight. However, I'm not sure how it could have impacted the erratic nature of the cutting? It was running fine and went erratic in a single pass - a matter of a second or so. could the material have shifted so much that it each pass could vary that much?

[Mikebpeters]

I was thinking about this more. As I was explaining what happened to some coworkers and showing them this picture it became clear that my problems started when the file got to the point where there was considerable 'Z' motion. Could my speeds being to high have caused this?

[srwtlc]

You could have several things going on here. You could have a loose pinion on the Z axis, you could have something on the underside of the table going on that is causing the table to jump up/down, like some debris on a roller (if you have that model variation) or some out of adjustment power stick bearings. You have a lot of abrupt vertical (Z axis) movements and this can cause loss of steps if the Z axis feed rate is too high and also it the ramp settings are too aggressive. Is the tool having to raise all the way up to the surface at the end of each stroke? You could be topping out on the Z axis just a little and losing step there as well. Proper finish pass boundary settings can eliminate that. Adding a draft (10º-12º) to the model can alleviate the steep edges of components, making the steep edges be slight ramps up/down.

Not that this will cause the exaggerated Z variation, but your finish pass stepover % is too high, try 8% -10% instead. 8% will take longer, but the finish will be much smoother.

[srwtlc]

50ipm on the Z shouldn't be too much, but try to match the XY and Z feedrates a bit closer so that the XY doesn't have to slow down so much to wait for the Z. Refer to this doc for some insight on ramping, its effects on each axis, and suggested settings to try. With the proper ramp settings, you should be able to do 2,2 on a Standard. If not, more is wrong with tool adjustment.

http://www.shopbotblog.com/2008/03/a-ramping-the-vr-command-and-how-to-tune-your-tool-for-maximum-performance/

[guitarwes]

I've always heard to match the XY and Z feeds on 3D toolpaths. That may be your problem along with the wood moving a bit after hogging out all that material.

[Brady Watson]

Have a look at this article: VR Settings (http://www.shopbotblog.com/2008/03/a-ramping-the-vr-command-and-how-to-tune-your-tool-for-maximum-performance/)

Specifically the part about 3D speeds:

"I want to take a moment to talk about move speeds [MS] and 3D cutting. Getting your move speed set correctly in your part file is just as important as getting your [VR] dialed in. If you set a MS, 6,3 in your file that cuts a 6 inch wide relief, cutting will not be smooth or as expected for two reasons. One, the XY speed is too fast for the size of the ‘parking lot’ and two, the XY speed is 2X the speed of the Z axis, forcing the XY to slow down while the Z axis tries to keep up. Depending on the type of relief you are cutting and the amount of detail, here are a few speed combinations I have found to work, without symptoms of the tool slowing down to wait for the Z axis: 2,1 – 2,2 – 3,2 – 3,3 – 5,3. Unless you are cutting very large or long 3D parts, you probably won’t get over about 5 IPS. 3D cutting is very different from 2D cutting since the tool has to fight gravity and push and pull a gyroscope (your router) into your work at a high rate of speed. If you make sensible decisions about your move speed and adjust [VR] by observing the movement, sound and smoothness of your tool, you can transform your CNC to match the type of work that you do."

-B

[Mikebpeters]

Thanks all. This has been very educational for me