Could use some help with rough cut settings

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 have a short video to demonstrate the jitter. 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 the software to produce a toolpath that will rough cut without stressing the machine? I appreciate any suggestions or help.

rough_cut_jitters.mp4 (6.1 MB)

That bit looks really long. The stock 24v 300w spindle has can have alot of flex in the shaft especially with long bits. When you start cutting with that much stick out the bit will start vibrating, it’s crazy you can actually see it in your video!

When I had my stock spindle I really only could use the 1/8" bit with about .875" stick out. With that I was going about 60ipm at 0.03" DOC and it gave me pretty good results (in MDF).

Yeah, the bit is pretty long, but it’s going down an inch and a half into the wood, so I need a longer bit. I could try finding one that’s just a little shorter, but I do need a full 1.5" cutting depth.

Ya, that’s going to be tough with the stock spindle. I’d try a shorter bit just to see if that is actually the problem. If the stick out ends up being the issue, you could use a shorter bit and go down as far as you can (ie 3/4") and then use a bandsaw/jigsaw to cut the part free roughly and then use a flush trim bit on a router to clean up the remaining. That process might not work depending on how intricate your design is.

You could also consider cutting your project out of two 3/4" piece and laminating them together.

I agree with @Rusty, that bit and the spindle aren’t good together. You may be able to minimize some of the flex and such by decreasing the depth of cut and the speed, but ultimately if you want to go down 1.5" you’ll want to very seriously consider getting a router to replace the spindle.

The length of stick-out is one of the biggest problems. If the silver part of the bit is solid shank, you may be able to push it another 1/4" in, leaving just enough sticking out to make the cut. You don’t want to push it in far enough that the collet is on the flutes though, since that’ll damage the collet.

It looks like that’s a 1/4" shank, you’ll definitely want the largest shank you can get for deep cuts.

I can’t wait to see the finished product, wooden landscapes are always interesting to see!

The bit is 1/4 x 3", I found (and ordered) a 2.5" tall bit which might be able to help with the wiggle. Since most of my issue is when the bit hits the left or bottom areas after a deep cut, I was thinking maybe sloping the outer edge down might help when it hits the sides.

That bevel is a bit wide, but you get the idea. Think that might help alleviate the wiggle issue?

Have you considered doing it as a two-pass? We do this in laser cutting any time the material to be removed is too deep or heats up too fast.

A video I was watching about planers yesterday recommended cutting relatively shallow passes, as too deep would cause the cutter to slow down unacceptably while cutting, and put a lot more stress on the machine.

So, to follow up, I’ve made some progress and discovered a new issue (which looks like it’s software related). Thanks for everyone’s advice.

  • Turns out I had a screw loose (again, story of my life). The spindle mount was slightly loose against the screw rod.
  • Got a shorter 1/4" bit, just long enough to reach the bottom. The shaking and shimmying reduced substantially; the only time it made noise was when it drilled down at the edges, so I can adjust the plunge rate.
  • I added a beveled outside, which seemed to help a LOT in reducing jitter when the bit reached the edges.
  • After witnessing the latest run, I don’t know that doing two runs would make a difference. But I can try later.
  • Discovered a new cause for much of the vibration problem: the toolpath was not honoring the .05" pass depth sometimes, and would cut down as much as an inch in one pass, which caused a godawful noise and vibration. I’m going to post on Vectric’s forums and ask the devs if this is intended behavior, because it seems weird.

At the end of the run, a few screws were loosened, but not to the extent they were before. So if I can get the toolpath issue resolved, I’m pretty sure I’m FINALLY ready to do the work I bought the machine for.

Here’s a video of the toolpath issue. The path properly goes along the right edge of the mountain, doing .05" passes, but when it goes back left over the top of the mountain, instead of going back and forth at .05", it plunges all the way down. You can see just how much stress that creates on the machine. Hopefully the Vectric team will be able to suggest a way to fix that.

20151114_194425_1.mp4 (2.3 MB)

Two steps forward, one step back.

My guess is you need to set up a roughing pass and then a detail/finishing pass. The roughing pass will take into account the settings for the bit…a 0.05 depth of cut (or whatever you have selected).

A detail pass/finishing pass cuts the whole thing in one pass. So run the roughing pass, get the bulk of the material out then run the detail pass to clean it up and get it looking right.

As for the loose screws, get some loctite. Use the stuff that’s repositionable/removable, since you don’t want to break parts or have to heat them up to make tiny adjustments.

Good suggestion on the Loctite, I think I’ll try to find some.

Here’s a graphic of the latest issue I described:

  1. Cross-section of my project
  2. Green lines are the toolpath, which correctly routes out the right side of the design.
  3. Problem area: The red line shows how the toolpath dives deep on the left side, routing out the entire area in red in one pass. This is the problem.
  4. Here’s the expected toolpath, but it’s not happening. I’ve asked the forums over on Vectric, but no response yet.