1/4" MDF

[nat_wheatley]

Bit for cutting this? I tried a 2 flute straight bit, which didn't work very well. Next attempt will be with a 1/4" downspiral, but I wanted to check here first. Thanks.

[coach]

Nat, I cut 3/4" MDF yesterday with my 3/8 bit I use for plywood.
It actually gave me the best results I have had in MDF.
I am going to the shop tomorrow and will get the number off the bit.

[Ryan Patterson]

I would us an up spiral if hold down was not a problem. The diameter would be the 3/8" if the detail would allow. Not sure of your shapes but would cut at 8"/sec at 12000 two flutes. What issue did you have with the straight bit?

[coach]

Nat, I just found the invoice with the bit I used.
It is Onsrud 60-123c it's 3/8" with chip breaker
expensive but I have cut a ton of plywood, some with HPL on it.

[pfulghum]

1/4" downshear is the only bit I use for this.
MDF held with vacuum table.

-- pat

[Gary Campbell]

Nat...
Try one of your mortise compressions for a test cut
Gary

[knight_toolworks]

a downcut works great. though you may have to cut a bit into the table to get a clean cut on the down facing side. if you are doing small parts I would use a 1/8" or what is becoming my standard a 5/32" downcut from centurion. it's beefy enough for faster cutting but small enough for the shavings to stay in the cut to help hold the part in place.

[eaglesplsh]

Gary is right...

You'll need a compression bit to keep the top and bottom surfaces from becoming a total mess.

Look for a "mortise" compression bit - the transition from up shear to down shear on these bits is closer to the tip compared to standard compression bits. They work great when you need to cut a shallow dado/rabbit/mortise. They should work equally well for your thin stock.

We use 1/4" and 3/8" mortise compression bits from Centurion Tools. The transition on both diameter tools is below 0.250 They also have the best prices on compression bits that I've found.

[knight_toolworks]

I cut a lot of mdf and I never use a compression bit. but to get a clean bottom face you need to cut a bit into the spoilboard if you use a downcut bit. maybe it is cut speed but I seldom cut about 4ips and that may make a difference. I even get clean both sides on melemime using a downcut. unless spoilboard is chewed up that is. but my method would not give as clean a bottom face if your spoilboard is chewed up. but spoilboard is cheaper then bits.

[pfulghum]

I agree. Downcut and cut into spoilboard 1/64 or so. Much cheaper than a compression bit.

[nat_wheatley]

I'll try the mortise compression, I do most other cuts with these. I did try one, but am only now realizing why it didn't work. I had cut the parts in 2 passes, leaving a 1mm skin on the parts for the first pass. That 1mm skin would have been enough to land the upspiral part of the bit against the top surface of the material, which explains the poor cut quality. I'll try again, leaving no skin (not needed as the parts are generally fairly large) and would guess that this would do it.

The straight bit seemed to 'shred' the material somewhat. Out of curiousity, I may still try the downshear. Where my spoilboard is generally fairly chewed up, with the downshear I think I may run into issues with cut quality on the bottom face of the parts.

Thanks very much for the posts.