Flush trim bits

[Tom Gattiker]

What is the best flush trim bit to use to trim a circular table (1" hard maple) using a pattern? In particular:

--what daimeter bit (is bigger better for this type of application)?

--Bearing on top or bottom?

Thanks

[Mike Vermeil]

As long as you cut off as much waste as possible prior to routing, you'll be OK with a 1/4" bit as long as you take it slow (that's 1/4" shank & cutter).

If you go with a 1/2" shank and cutter, the chances of vibration occuring will be reduced - but still remove as much waste as possible prior to routing. As you cut across the grain at the strange angles encountered cutting circles, odd vibrations can be a problem.

[Tom Gattiker]

...than 1/2 inch make sense?

[Dave Arbuckle]

Larger diameter is better. Reason is that the angle the cutter meets the stock is more acute with a larger cutter, for any given amount of removal.

Bearing near the router is better. Reason is that with the bearing away from the router, you have a lever that can help lift the router and spoil the cut. With the bearing up against the bottom of the router, no similar force.

Dave

[Tom Gattiker]

That's the way I had it figured, (but I have been known to figure wrong.)

[Mike Vermeil]

As you start getting bigger than 1/2", you get into speed problems - meaning you've got to slow the router down. I don't have a variable speed router, so I try to stay away from large bits as much as possible.

Now having said that, I've got a big ol' rabet/pattern bit form Jesada that I use when necessary. It's about 1 1/8" in diameter, and 2" inches high. It's a brute, but it cuts very smooth when used at the proper speed. I found that it vibrates less than the 2" long, half inch diameter bit I have when cutting stock thicker than an inch or so. I was cutting some redwood just under 2" thick, and the smaller bit just didn't cut as well. I think it's got something to do with the cutter's angle of attack in relation to the wood being cut. The cutters on the large bit come in at a much lower angle than those on the smaller bit. It may also have something to do with the effective length of time the cutting edge stays in contact with the wood, but that's getting a bit over my head.

[Mike Vermeil]

Dave - excellent point on the location of the bearing. All my flush trim bits have the bearing at the end of the bit, and lifting up the router and "leaning" the bit into the workpiece is a definite problem, errr, or so I've heard.

I never really considered how having the bearing at the other end would stop that, but there's no way you could lift up the edge of the router closest to you and lean the bit into the workpiece. Of course you could lift up the back edge and do the same, but accidentally lifting the front edge is much more common as you scoot the router along.

[Lars Thomas]

Tom, when you say 'circular' do you mean the table is a circle? or does that mean round-ish? If the table is a circle, then it might be easier to make a trammel base for your router (so you can use the router like a big compass). Then the pattern (nor the bearing) is an issue. If the table is round-ish, yea, then the pattern is your best bet.

Dave, good point about the lever on the bottom bearing bit. Never thought of that. Thanks. Lars

[Steve Jenkins]

I'm with Lars. flip the top upside down and drill a small hole in what will be the center. Mount your router to a piece of 1/4" ply to use as a trammel and just travel around the top. Mark it out first with a pencil and cut near the line with a jig saw or on the band saw. From the center of the end grain area to the long grain try to cut a little closer to the line. This is where you are most liable to run into problems when routing.Even with trimming first I often set the trammel about 1/32-1/16 big and make the initial cut in stages routing a little deeper each pass. After getting all the way through I will then reset the trammel and make a final pass full depth. Steve

[Tom Gattiker]

It is Really 2 semi circles and 2 rectangular leaves. I am using Kim Carleton Graves book as a guide. He says rough cut with a band or jig saw. Then use a quarter circle pattern to flushtrim. I am thinking the trammel is unworkable since he recommennds joining the 2 semi-circular halves with the table's locking mechanism (& pins) before finishing off the circle with the router. Presumably, the locking mechanism would be in the way.

Would like to hear what others say on this. Or maybe kim will weigh in if he's out there.