Cutting quality issues...

[harryball]

I'm running a new PRTAlpha96. I tuned the Z axis today and got it to .005" square to the table top. I ran these two plaques.

On the left I used a Bosch 1/4" down spiral bit, the edges of the letters and plaque are crisp, but I get tooling marks in the area clear. These are not ridges like a Z out of square... it's odd I can't figure out what's happening. I even put a shop vac on the table to pull the board down and 4 screws on each corner. I can't understand how the bit could plunge up and down... it'd make you suspect something was wrong with the bot...

Now look a the right, the area clear was done with a 1/4" 2 flute straight bit, a cheap-o at that... the area clear is clean but the letters and edges are not clean.

Here's what I know...
My down spiral keeps the edges clean but compacts chips on a profile cut rather than cleaning them out and leaves tooling marks.

My Freud up spiral clears chips well, leaves the edges in tatters.

My el-cheap-o 1/4" straight bit leaves an almost perfect area clear with not so good edges.

What am I battling here? Is this just a factor of the bits? How is that one down spiral bit responsible for such tooling marks?


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[beacon14]

Part of the problem with the steps may have something to do with internal stresses in the material that are relieved as you machine away part of the workpiece - in other words the workpiece is relaxing as you cut more and more of it away, and moving into or away from the bit. Can you give us more info on your hold-down? If you had to use 4 screws that leads me to believe that your vacuum table is not strong enough to hold the part really flat during machining. Screws have a tendency to add stress to a soft material like MDF, and can make the center (unscrewed area) of the part raise up slightly.

You say the piece on the right has a perfect background but there appears to be a step in the photo along the left border, and a dwell mark next to the detail at the bottom - I'm not sure the bit is as much of a factor as you think.

[pfulghum]

I would tend to aggree with David when cutting MDF like you are. I almost always will cut in two passes.

One rough (leaving about 1/32" in x,y, and z)
Then a final cut.
No need to change cutters, just let the part move.

-- pat

[gerald_d]

For that type of job you can use a cutter with slightly radiused tips. We sometimes touch the cutter to a grinder before using them.

[Brady Watson]

Personally I would use the upcut. The little hairs that stick up are quickly and efficiently knocked off with an orbital sander and a 220 grit pad.

As others point out, do 2 passes. I would do one with .02 allowance and then the same TP with 0 allowance to cleanup with little load on the cutter.

-B

[thecustomsignshop]

I had a similar issue.

I found using a compression bit works the best, thus addressing the hair issue and forcing the material to cut clean in the cleared out areas.

My step over on a .25 inch bit is .125.

I also setup the tools paths with an allowance of .02 then a cleanup pass of 0 as Brady described.

I found that holding the material to the table using vacum reduces internal stresses that may cause the materal to arch or move. Vacum keeps the material flat to the table.

The only issue I still have is with the smoothness of curves, but we are working on that one.

[harryball]

Sounds like good information here... I'm going to try changing the cutting direction to climb for the up spiral and a few other suggestions from here and let you know how it goes.

Robert