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Depth issuse(s) with pocket operations

Greetings all, first time poster here! Also very new to CNC machining so bare with me. :grin:
(Now I know new members can only add one photo to posts haha)

I’ve been making some test cuts which particular emphasis on pocket operations and have been having a couple of issues.

1. The bit seems to completely miss some spots during a pocket operation.

Target Depth: 1/4in Feed: 30in/min Plunge: 30in/min Depth Per Pass: 0.125in RPM: 14,000 Bit: 1/4in compression bit

I can obviously sand those spots down but was curious what could be causing it? I’m using a 1/4in compression bit which, from what I understand, is not ideal but I’m not sure how that would account for this issue. The Z-axis appears to be operating as expected in any other situation so I don’t believe that to be the problem.

  1. Aside from the bit not hitting some spots (as you can also see at the bottom of the following image), it appears as if the bottom of the “L” was either cut off or, more likely, chipped off somehow.

Image # 2

Target Depth: 1/4in Feed: 30in/min Plunge: 30in/min Depth Per Pass: 0.125in RPM: 13,000 Bit: 1/4in compression bit

Here’s an easel sketch of what I hope to create for my friends newborn.

Image # 3

Any ideas on what would cause of either of these and/or advice for solutions? Could it be a case of wrong type and/or size bit for the job? Does my machine need to be re-calibrated? Error in the Z-axis? Workshop on an ancient haunted burial ground? I wish I knew.

Specs for my setup: Maslow default frame / Ridgid R22002 / Ring system / Z-axis w/ mod posted by @geeklimit / Bricks replaced w/ 5lb weights / Firmware 1.27 on a Windows 10 machine / G-code created in Easel.

Thanks for reading and I look forward to hearing from some of you smart folks out there in Maslow Land. :slight_smile:

A bit of background on compression bits. They are a combination of an upcut bit (on the first part, often somewhere between 1/4 (or less) and 3/8 of the shaft) and then it transitions to a downcut profile. So the upcut portion pulls the material up and it meets the sawdust from the downcut portion pushing material down. The goal is to avoid tear out on the bottom of a through cut (thus the upcut portion) and tear out on the top surfce (thus the downcut portion). [EDIT: I forgot to mention that this packs the kerf area with sawdust so tightly you sometimes have to use a screwdriver to loosen it before it releases when you pick up a piece. I forgot to put tabs on a project this week on a table CNC and I was fortunate in that the pieces being cut were fully help in place by the packed sawdust. Not recommended but gives you an idea of how tightly it can pack (that was 1" baltic birch).]

I see you’ve got quite a bit of fur around your edges. Ideally that should be clean with a sharp bit, especially a compression bit, but only if you are plunging the compression bit in past the upcut portion, otherwise you’re only cutting with the upcut and you’re going to tear out along the top and get fuzzy edges.

My experience with compression bits is on a 4x8 table machine (and limited at that) but often times (on very rigid machines with steel frames and rigid grantrys) they are cutting 50%, 75%, 95% or even 100% through in a single pass. With a very rigid machine and a powerful spindle this is possible. It’s a bit hard to fathom, but a compression bit on the right hardware is capable of this, and delivering a clean cut in a single pass (!).

If you want to see it in action, check out this video from YouTube posted by Onsrud, who make CNC machines and tooling. They are doing 1000 inches per minute (ipm) with a compression bit in a single pass on 1/2" plywood: https://www.youtube.com/watch?v=ymwBdZsDP2Q

I think for most jobs on the Maslow a downcut bit (so you have a clean top surface) is likely going to be the way to go. None of this caused the missed areas in the pocket, but I’ll defer to an expert on what might be happening there.

-Jeff

1 Like

Many bits are offsize, possible your bit is a tiny bit smaller than the assumed size. What did you use for a step over (amount the bit cuts over the previous cut) for your pocketing? What does it look like in a gcode viewer?

Make sure you have a bungie over your router as well.