Up or down cut spiral bits?

[tas99]

I’m just getting my new PRTalpha 48 up and running and I have a question on spiral bits; up or down cut? Which is the best to use for plywood or melamine cutting. I’m not using a vacuum system and the work pieces will be held in place with screws and/or clamps.

[richards]

I use downcut spirals for both plywood and melamine, but I don't cut all the way through the material. If you are going to cut all the way through the material, you will get better results using a compression cutter (which has an upcut section at the bottom of the cutter and a downcut sectin at the top of the cutter - which, in effect, pulls the chips towards the center of the cut).

Using an upcut spiral with melamine, in particular, is messy. You'll probably have chipout all along the cut.

[harryball]

I use downcuts mostly, I do cut all the way through zero'd on the machine bed when possible. I've had good results.

Robert

[Brady Watson]

Instead of spending the money on a compression spiral (they get REALLY pricey), you can also run a downcut spiral 1/8 to 1/4" deep and then switch to an upcut to do the rest. The downside is there is a toolchange, but you won't mind the extra time if quality of cut is your primary concern.

-B

[harryball]

The pitfall on that one is bit size differences and deflection changes. You can end up with a part that has a ridge on the edge to sand off. If you try running a small offset then bringing in the upcut, the same chipping issue can occur. This may or may not be an issue depending on what you are cutting.

Robert

[Brady Watson]

Mic your bits...it's a good idea no matter what you use as tools. I've never experienced a ridge from doing this, but I don't use cheap tooling either...I used to, but found that 9/10 of the time, they were under or over sized.

-B

[harryball]

I admit, Brady, on my first and worst case the offending bit was a Freud. Not the best of bits to use I've discovered. I don't use Freud anymore but I've experienced the ridge on another project using Onsrud bits.

It was a very small ridge, but on a smooth surface with grain it stood out. I didn't investigate the bits mainly because I expected some kind of ridge or texture change and was prepared to finish the edge with the spindle sander. I don't know if bit size or deflection differences were the culpret.

In the last case, if it had been on plywood for cabinets or formica, I'd have ignored it.

Robert

[Brady Watson]

The only ones that I have found that 100% of the time are dead on size are end mills (same grind as upcut spirals, but LOTS cheaper!)

You are right...even the more pricey bits that you would expect to be dead on, aren't always that way. The cheap bits (like pricecutter.com) are still good for some things...but when you need ultra accuracy, you gotta mic the bit. I learned that the hard way early on!

-B

[bill.young]

I guess this is slightly offtopic but I generally cut the way that Mike does..cut most of the way through and then flip over to trim the rest...and just stumbled on some piloted trimmer bits that are available in upspiral, downspiral, and compression. The upspiral and downspiral are available in 1/4" diameter which is hard to find in a bearing-piloted bit.

No personal experience with the company but thought I would pass it on.

http://www.bladesllc.com/solidsurfacebits1.html#SE2954

Bill

[brian_h]

I've had a small CNC for the last 8 yrs. I typically cut at a depth of .85 into solid Ash. My programs are written to either reach that depth in three passes, or do it in one pass at a very slow speed (1/4" spiral upcut). I almost never use the roughing program because you can see the 'water marks' of each roughing pass. (I know I could write a clean-up pass, but that's not the issue for cutting melamine.) I don't know if the 'water line' would happen with melamine using two different bits, but I expect it would. I would probably do some testing before settling on that method.