Machining Curved Base and Crown for Applying Over Bullnose Cornerbead

Shaper operation tips for making a challenging custom moulding. December 2, 2006

I am shaping parts for a three piece base moulding application that allows curving around each 1.5" radius jumbo bullnose cornerbead. I've devised a sound sliding fixture for the shaper, but can't seem to avoid burning. With such a small radius, I can't physically feed fast enough on the shaper to avoid burning, and nothing seems to help!

A) After processing, I cut the inside radius of 1.5" on the W&H with a convex knife set.
B) Next I cut a 3" radius rough profile with a quarter-round knife set (W&H again).
C) Bevel on T.S. completes the blank, which is held vertically in the sled, started against a starting pin and shaped on the weighted sled with bearing offset accounted for.
D) Part is cutoff on the table saw with a miter sled.
E) Part either burns horribly, or burns beyond very horribly.

On the upside, the parts aren't blown out (or blown up!) and are of perfect dimension/fit. I am using corrugated back knives of about 5" swing and a 4" rub collar. I have tried 3600, 5100 and 8000 rpms. When pushing as fast as I possibly can, the burning is minimized, but still too much to sand out. The species is cherry, and I'm afraid to attempt climb cutting freehand with this application. What is the next step?

Forum Responses
(Architectural Woodworking Forum)
From contributor B:
The reduced cutterhead rpm speed, as you have tried, is the right direction to be going. Also, try a larger bearing for a first pass that leaves 1/32" to be removed with the final size bearing on a second pass. This might help.

From contributor T:
How many knives are you running in the corrugated knife head? If you are running four knives, you may want to try only running two. Other than that, contributor B's last idea may help significantly by reducing how much material is removed at a time.

From contributor H:
If these parts are on a radius, a turner can make them on a lathe. Not only do they not burn wood, but they can sand it when they're done. I have had curved moldings up to 12" radius made. It's quicker than cutting the wood out and shaping it. The turner wants the wood rough cut, and mounted so that the ends meet to make turning a circle smooth.

From the original questioner:
Thank you for the helpful advice! As of 8:33pm MST, my first order of parts are complete. Shaper speed was backed down to 3600rpms and produced a "chomping." Some practice and an increase back to 5500rpms along with an indexed sub template that allows infinite multiple passes (1-3 for hogging, and pass four got climb-cut), and I made perfect cuts about 1 out of 7 times with no blowout, no burning and dead on consistency. The parts look like something bought from Enkeboll!

Note... I made my templates, which comprise the base of the sled, out of Lexan. The rest of the sled is made of Corian, hickory and fiberglass. It has a "hopper" which is filled with lead shot for stability and reduced chatter.

I consider using a lathe (or a competent wood turner and his/her lathe) frequently, but lack all resources to carry this out. I'd love to try it sometime, and would further really love the time for the learning curve.

Accurately cutting machined wood parts from a turned product perplexes me. I'm aware you only get 3 90 degree parts from a turning due to kerf. Accurately cutting those is the challenge. Could anyone elaborate?

As I am challenged with 3" inside radius, 7 1/8th" 52/38 crown outside corners which will come up in about two weeks. I'm planning to rough on the table saw and hand scrape, but would be likely to try turning if I thought I had a snowball's chance at yielding precise results.

From contributor B:
There are two ways I know of to do your 3" radius inside crowns:
1. 3d software on a CNC, which is how we do them when absolutely necessary.
2. Turned on a lathe, which I outsource to an area turner and is my preferred method for these.