3/8" Versus 1/2" Compression Bits

      The smaller-diameter bits reduce waste, but may deflect in thick material. September 30, 2010

I've been using a 1/2" compression bit for outline cutting on mostly 3/4" pine core 2-sided melamine. Recently I ran a 3/8" compression bit and liked the results. What are the advantages and disadvantages of using a 3/8" compression bit versus a 1/2" compression bit?

Forum Responses
(CNC Forum)
From contributor H:
The disadvantage I've encountered when cutting size-critical slots is that the 3/8" bit will deflect when it enters and exits the board, forming a small nib there. A rule of thumb I found is that the diameter of the tool should not be less than half of the depth of cut. A 3/8" tool is on the borderline for a 3/4" cut. If you're producing acceptable cuts with 3/8" tools, then there's no disadvantage that I know of.

From Brian Personett, forum technical advisor:
I started out using 1/2", but switched to 3/8" for cutting 3/4 and even 1" material years ago and have never looked back.

From contributor G:
Less dust with the 3/8.

From contributor M:

25% less dust.

25% less waste... The difference in normally nested sheets of casework on 4 by 8 melamine can amount to more than one full sheet of material per lift, on average. That is a real yield, and it is not in the dust collector.

25% less expensive tool. See Vortex.com for a typical example.


I have found that 1/2 inch tools last a little longer, but not 25% longer.

My experience is that 1/2 inch tools cut a little nicer as they begin to get dull in VC plywood, perhaps due to rake angles, perhaps for other reasons. The difference is not huge here either.

Yes, there is less deflection, but it is very important to distinguish bit deflection from machine deflection. Often these two effects get confused. You may find that machine deflection occurs in both cases, negating much of a difference. It is an unusual machine that has no deflection at all.

Larger and heavier ones usually have less machine deflection; small, light machines have more.

Contributor H's method is best. Try the 3/8 and see if it is a problem, then try the 1/2. If the problem goes away, it is bit; if not, it is machine.

You can usually run 1/2 inch tools a little faster due to a larger gullet that takes a larger chip. Part of this difference, though, is negated by cutting a larger kerf and thus having more chip to remove. My experience is that 1/2 inch bits can run faster by a few percent, all other factors being equal, though a strict interpretation of chip loads often says 1/2 inch should run slower.

For all of these reasons, I normally run 3/8 for both compression and mortise compression. I do use a 1/2 inch corrugated hogging bit for rough cutting MDF and plywood.

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