trouble with smooth bottom surface

[jsmithconstruct]

I am trying to recreate a bronze plaque cut from mdf. The background is supposed to be flat but I am not getting the desired affect. I am using a bottom surfacing bit that is supposed to leave a flat surface. Has anyone run into this problem before. I have attached a photo of the problem.

[billp]

Jonathan,
With some materials as SOON as you break through their surface, they will start to absorb moisture from the atmosphere ( as well as some more air ). That would mean that each of your passes could actually have a different "Z" surface starting level ! This happens in MDF, styrofoam,etc. It can then be a little tricky to find a suitable place to re-zero your bit for successive passes.
This is also another reason why MDF can be problematic when building a table for a 'Bot. It can go from a flat piece of sheet goods to a corn flake after just a few minutes of exposure to moisture.

[jsmithconstruct]

This was created with one toolpath so I only zeroed it once. Is the information you provided still true with this condition?

[paul_z]

Jonathan,

I agree with Bill but there are a couple more things to consider.

MDF canl take a "set" (curve) if stored on it's edge. Depending how you mount it, the center of the piece may not be touching the spoilboard. If so, the mdf will deflect up/down depending on the bit. An upspiral bit will cause upward deflection while a down spiral or mortising bit will press down hard on the MDF, forcing the MDF to deflect down until is touches the spoilboard. This bowing can also happen if you screw the MDF overly tight to the spoil board. Bowing can also occur as you cut the MDF. I don't know if it is humidity or just strain relief.

Yet one more possible cause is the top side of the material heating up from a down spiral or mortising bit. If the top becomes warmer than the bottom, the material will try to bow upward. I see this a lot in acrylic. It is a "sly" problem because as soon as the material cools back down, the bow disappears leaving no clue as to what went wrong.

If any of these are the cause of your inaccurate Z cutting, I'd expect it to be worse at the center of the work than near the screw down points.

Another possible cause is how and where you ZZeroed the bit. If you ZZero off the top of the material and the material has distorted from a previous cut, you're in trouble. If the previous cut left "fuzzies" on the edge of the cuts, lightly sand these down before ZZeroing from the top. My preference for this type of work would be to ZZero off the top of the spoilboard and ZZero in the same place each time. Even then, you might want to run each cut twice to try to accomodate any distortion during the cutting process.

Paul Z

[Brady Watson]

I'll add a few more possibilities:

1) Material moved or cupped (as others pointed out) because your hold down strategy failed.

2) You have excessive backlash in your Z axis, causing it to fluctuate a little between teeth.

3) You are running too fast a plunge speed and you lost a few steps.

4) Your lower V-roller bearings are not tight (if using a PRS)

-B

[jsmithconstruct]

Thanks so much for all the helpful responses.