Help with Rough Cut settings in PVC

[Binkmeister]

I'm using PhotoVcarve to create a landscape based on a grayscale heightmap, so the depth of wood I need to remove in some areas for a rough cut is 1"+. I've been having problems with the settings for the rough cut, and could use a little expert advice; so far I've broken one bit and nearly shaken my machine loose. It probably goes without saying that I'm new to CNC routing.

The 1/8" bit broke; I thought I was going too fast, so I slowed the IPM and plunge depth settings, to about 20 inches per minute. The spindle speed was 12,000 RPM, the depth of cut was set to 0.07", and finally it used a 50% stepover. Still had a tough time biting into the wood without bending the bit, so I tried a 1/4" bit and the machine nearly shook itself apart before I aborted the run. I've tried both flat bottom bits as well as ball nose. The wood is medium hardness.

With those settings, most of the cut seems to work without undue stress, but when the bit reaches the end of the bottom edge or right edge of the row it's cutting (at 45 degrees), it seems to bounce and chatter against the uncut material on the bottom and right sides, vibrating the entire machine and creating a lot of stress. I can upload a short video if that would help demonstrate. FWIW, I was able to do a final pass with 1/16" ball nose on a partially-roughed out piece, and it came out awesome, so it's just the rough cut that I need help with.

Are there any settings that I can/should alter in PVC to produce a toolpath that will rough cut without stressing the machine? I appreciate any suggestions or help.

[dhellew2]

For each pass the depth of the cut should not exceed the diameter of the bit [as a starting guide]. Once you are more familiar with your machine and material you can push the limits beyond the bit diameter guide. Raster cuts with the grain are much easier to cut than cross grain. I recently made several cut .85" deep, raster cut, in red maple, with a 1/8" bit, 16,000 rpm, 55 ipm.
Dale

[Binkmeister]

Do you think maybe the spindle speed is too low at 12000 RPM? One other comment I've gotten is the bit is long, since it has to cut 1.5" deep. Any methods for decreasing the wobble? Here's a video of the problem in action:

https://youtu.be/XQoj8b4Mnio

[IslaWW]

It appears that much of the deflection may be in the machine itself. Not that an extra long bit in an extension doesn't contribute, but when the end of cut is hit you can see that the collet nut jumps also. Look to see if there is a way that you can tighten up the mechanics of the Z axis (at least)

You may simply be putting more leverage against the Z than it can take. Have you tried a shorter bit without the extension cutting the same file?