Troubleshooting Tool Marks in MDF

      A CNC process leaves circular tool marks in an MDF substrate that telegraph through the pressed finish lamination. Unfortunately, this may just be in the nature of the materials. August 29, 2005

Question
Does anyone have an answer for removing small tooling lines? I am dishing MDF -.0625, 300ipm@15k with a 2.5" diameter fly cutter, and am getting small circular score lines. We vacuum press and it telegraphs through very badly. Does anyone have any suggestions?

Forum Responses
(CNC Forum)
From contributor H:
This could be a number of things. I will list several possibilities with the most probable first:
1) Head out of alignment
2) Unbalanced tool / tool holder
3) Bad spindle bearings
4) Vacuum not holding firmly
5) Excessive backlash in X /Y ball screw or rack.



From contributor A:
I have seen the same problem in our membrane pressing operation. The problem is due to the nature of MDF. Even if everything on your machine is perfect, you will still likely get lines that, though barely visible, telegraph badly through your surface material. The only solution I have found (as well as other manufacturers I have talked to) is to put an insert in the panel (rout the panel deeper by 1/8" and cut an insert panel of 1/8" MDF to glue in before final pressing).

Sanding works okay, but depending on the emboss, stipple, or ticking on your vinyl you may not be able to hid an orange peel finish. This too is a result of MDF's construction. MDF is started with a very thick layer of wood fibers that sort of resemble cotton candy. These fibers are all pressed at one time together, and the surfaces are finished with some sort of sealing process. If you look closely at a cross section of the MDF you will see that as you get towards the center of the panel (thickness) the board becomes less dense. This lack of density leads toward the orange peel look, and the fiber raised lines.



From the original questioner:
It seems like a lot more variables to work out than I had anticipated. Hopefully I can rule out some of them. The CNC is only a couple of month old, but we know how that can go. The tooling marks seem to be consistent with every part, so I have played with the programming, with some luck. I have also been told that different inserts could help, but Andrew really hit a sore spot. This MDF warps really badly when you remove the surface skin, sometimes so bad that it will curl up off the table bed.


From contributor A:
To curb the warping problem we store all parts flat before pressing, and we press within hours of machining.


From contributor G:
It sounds like you have only outside inserts to do this job - depending on how many teeth you have in the tool. We manufacture a 2 3/4 (70mm) we developed for another firm, which has a 2+2 tooth design. You have two inserts on the bottom of the tool and two inserts with an up shear on the outside of the tool. This allows for faster feed speeds and a perfect cut on the bottom and no lines.


From contributor A:
To contributor G: I tried that tool with no luck. Having no lines is virtually impossible. I probably tried ten different types of cutters to no avail. We sanded for a while, but then switched to inserts.


From the original questioner:
To contributor A: I have tried a few different cutters also, but yet to find one that will be smooth enough not to show after pressing. Even after sanding you have to inspect them with some sort of high intensity lighting. Have you tried the pre-stressed MDF? For now I will give your panel insert a try - it should work but it’s still not the ideal solution. Contributor G - the tool is a standard 2.5" fly with two outside inserts, with the corners slightly chamfered. Who has the tool, and who can guarantee it? That would be great.


From contributor R:
To the original questioner: As Contributor A has stated machining this material and pressing it is very difficult at best. It sounds like they have it worked out to perfection. Although it sounds like Contributor G can guarantee the tool to work, I would be hard pressed to guarantee our tool to work in this situation. We offer a surface cutter with 110 mm diameter with an up-shear on three wings using radius inserts. The up-shear will cause the fibers in the MDF to lift slightly which will then show up after pressing by the orange peel look. It doesn't take much of an imperfection to show up after pressing.


From contributor A:
We do use a pre-stressed MDF. We use GP superior MDF that Flakeboard presses with melamine on one side. There is a built in warp to the board that helps to compensate for warpage off the heated press.



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