Getting a smooth pocket in HDU

[michaeld13]

I've tried a lot of different bits and settings but cannot get a smooth look when I pocket for some reason. Does anyone have any suggestions on how to avoid this? You can see the image of what I usually end up with... Thank you!!

[Adrian]

Is that done with one bit or two?

[michaeld13]

settings

It was done with one bit - .125" (1/8")....

settings2

[pixl8ed]

Could the HDU be flexing as it's cut?
What kind of bit -- upcut, downcut, ...?
How thick is the material?

[SteveNelson46]

Try changing the stepover to 10%. 50% is pretty high.

[michaeld13]

I've tried upcut and downcut- the bit I'm using is upcut. Going to try again with a smaller stepover. 10% seems like it would take forever- but I will give it a go. Sorry for the late reply. I do not get notifications to email when my thread is replied to- not sure why that is not a default setting. But thank you gentleman for taking the time to reply. I'll give it a go!

[adze_cnc]

The triangular shape at the nexus of the middle arm and the vertical of the E will be the last thing to be cut. That it is left proud implies that either a) the bit is not cutting as deep at that point or b) that the material is moving farther away from the bit at that point.

Some questions to answer:

1. How thick is the HDU?
2. How is the material being held against the spoilboard (vacuum, clamps, free-floating)?
3. Is there a specific reason for using a 1/8" bit only? Could you use a 1/8" bit to minimize the inside corner radius but use a larger area clearance tool such as 1/4" or 3/8?

Is 50% stepover bad? Perhaps not. The rest of the cut looks quite acceptable. Perhaps 40% or 33%?

[michaeld13]

Thank you ADZE. The material I'm cutting now is 2" - it's held with screws into spoil board- no vacuum method. I'm trying at 25% stepover right now- if that works then yes- strategy to use a bigger bit for larger clearance would be the way to go!

[michaeld13]

I think it's the hold down. I can see the board flexing upwards from the dust vacuum when the head moves up and down- I'm sure that has something to do with it... its better with a lower step over but the pattern s still there and not a smooth pocket. I've been using 2-sided tape for hold downs when I need that- but I don;t think it would stick to HDU very well... maybe it will not sure- I do know it's going to be a pain in the butt to tape HDU down

[Rcnewcomb]

If you are going to cut a lot of it then a vacuum fixture might be worth the time and effort.

In the past I've even used something as basic as a hole in the table with a shopvac attached and some gasket material on the spoilboard.

[GEdward]

I'm not convinced that the problem lies entirely with the hold down method. Two inch thick HDU is not exactly flimsy and the flaws are remarkably consistent. If flexing is the culprit, then the closer to the hold down screws the cutter gets, less flex should be exhibited.
I am kind of wondering how it is that the cut depth of project being .140 and the pass depth of the cutter being .125, that the pockets are cut in one pass.
If improving the hold down technique solves the problem then very well. I am just saying that I would not be surprised if it doesn't. I am leaning toward machine flex being the problem.

Ed

[Leo]

Yeah. I get that also.

Fortunately HDU is REALLY easy to sand. I often times use a chisel as a scraper.

Someday, I'll figure something out, but for now I just deal with it. If you find a solution I would LOVE to know about it.