Which router bit to use?

[Todd Franks]

I am in the process of building a custom transition and cleanout for my cyclone project and I need to make some larges holes and rings (6-14') in some baltic birch and MDF. I am planning on using a Jasper jig. However, I'm a little confused as to which router bit I should be using.

Obviously, I need plunge through capability, and a solid carbide spiral cut bit would do the job, but I've read they tend to be brittle and susceptible to breakage. I also understand that a straight cutting carbide tipped bit won't work because it cannot plunge all the way through. I see in the Whiteside catalog that they have single and double fluted solid carbide straight cutting bits. Are these bits capable of plunge cuts similar to a spiral bit? Are they less prone to breakage compared to a spiral bit? Would the straight or spiral cut bit be more versatile for other uses?

Thanks,
Todd

[Art Mann]

I think a larger solid carbide spiral upcut bit is what you want to use. I think you are overly worried about bit breakage. If you use a 1/4" bit, it is subject to breakage no matter what the material. Use a larger diameter bit. Solid carbide spiral upcuts are all I use for 3/8" and 1/2" mortices. Don't try to plow through 3/4" material all at once.

[glenn bradley]

I have the best luck with a 1/2" shank. 3/8" diameter solid carbide spiral bit. I take 1/8" of depth per pass. Many folks go 1/4" of depth per pass. I'm not in enough of a hurry to break a $40 bit ;-) Rule of thumb I was taught was 'remove no more than 1/4" by 1/4" worth of material per pass'. This has served me well, I have (so far) never broken a bit, not even the little guys.

[Peter Quinn]

I make curved templates in 3/4" mdf and plywood pretty regular using a 1/4" solid carbide spiral upcut and a bosch colt on a home made tramel arm, never had a problem. Keep the depth of cut to just under a 1/4" per pass and use a reasonable feed rate. Don't go too deep/too fast and you'll have no problems.

Typically for inside circles I use two bits and two tools, a spiral carbide up cut, a 1" high top bearing flush trim bit, and a jig saw. Using your jig make two complete passes with the 1/4" spiral (or whatever plunge bit you choose or have) dropping 1/4" depth per pass for a total depth of 1/2". This establishes your circle. Punch a pilot hole in one spot for the jig saw blade. Use the jig saw to cut out the waste in the middle following close to the circular rabit you have created. Finally take one pass with the flush trim bit using your initial rabit as your bearing reference.

Much safer than making the whole cut with the router/spiral as you don't have to worry about that center waste bouncing around the router bit when it is cut free.

[Jim O'Dell]

snip
Much safer than making the whole cut with the router/spiral as you don't have to worry about that center waste bouncing around the router bit when it is cut free.

Yeah, the first time I cut a hole in melamine/particle board for a blast gate, I thought I was going to die. Or at least soil my pants. :rolleyes:
What I ended up doing is marking the aproximate hole that was to be cut out, drilled 3 holes and countersunk some srews into my sacrificial table (waste ply or osb). I also marked the 4 holes at the corners, and put screws there also. That way when I went all the way through with my cut, nothing moved. I felt much safer, and didn't have trips to the house interrupt my shop time. :D If the ring you're cutting out isn't supposed to have any holes in it, you can always build stops around the edge of the piece that is equal or lower than the height of the piece being cut to hold it in place while you cut the circle.
If you are trying to cut a circle out and save the inside, use a bandsaw. :) Jim.

[Todd Franks]

Thanks for the replies. I'll just go with a bigger spiral bit. By the way Peter I like your 2 step method you describe. I was going use lots of double stick tape to prevent the scenario Jim describes, but it still was making me uneasy. I think I'll try your approach.

Thanks again.
-Todd

[Eddie Darby]

This was posted here about having to make big circles for a cyclone.

http://www.cgallery.com/jpthien/cj.htm

The lid:

http://www.cgallery.com/jpthien/cy.htm