Radius Limitations when Machining Curved Mouldings

Very tight curves introduce shape distortions in moulding profiles. March 4, 2006

Question
We need to make some S-curved moldings with a 2-1/2" wide profile about 1" deep into 1-1/2" thick stock. My question concerns a Williams and Hussey molder. What is the minimum radius this machine and attendant jigs can machine? We'll have some radii down around 6" from the inside of the profile. Does anyone have any alternate thoughts on how to do this? We like the US Concepts and other types of overhead shapers, but will need substantially more than the 8 pieces we currently require to justify the bigger machinery. I know about outsourcing and/or CNC. The problem with CNC is the tooling and programming costs. I find it impossible to get real quotes from anyone on this type of work. The other problem with outsourcing is that I have little success in finding suppliers that care about the quality, timing, etc. as much as we do.

Forum Responses
(Architectural Woodworking Forum)
From contributor A:
I just did a search on the Bonyman Jig that is used on a W&H to run ellipses and S curves and I was unable to find documentation of the smallest radius. I did dredge up an old WOODWEB discussion where I determined that the smallest radius I could run on my RBI molder was about 7". My RBI has a 4 point bed elevation mechanism so I would assume with the W&H having an open side it can run a much smaller radius. Hopefully that answers the question on how small a radius the jigs for ovals and S curves can run.


From contributor B:
I think a limiting factor you also have to consider is the geometry of the cutter. You can only go so small without changing the detail because you are cutting from the top. Of course this all depends on your detail but if you are cutting 1" deep you should consider this.

From the original questioner:
To contributor B: You are correct to mention the profile being recut in tight radii. This is something I have not been able to predict - it's the old "try it and see" for us. We have this profile designed to minimize the recut, but it becomes more of a factor as the cutting head diameter increases. Hence, a CNC router with a small diameter tool, with the cutting axis parallel to the molding would be ideal. We often do tight radii on the lathe - up to 8" radius, but now that we need an S with tight radii - 8" to 20", I'd rather not join three pieces to make the curve, but rather do it in one fast go.

Do the W&H and others have a controllable feed rate? Should one do multiple passes to get a deep cut? Do the jigs keep the molding in alignment? I have never been near one of these machines, much less operated one, hence my ignorance.



From contributor A:
I don’t know about the W&H, but the RBI that I use has two speeds alternated by pulley choice. My experience after using both a Foley Belsaw molder and the RBI is that cutting at full depth in one pass is best because there is more wood backing up the cut to prevent tearout. Although it is more of a pain, depending on how you create your curved molding blanks, feeding your parts with the grain is a big plus. I run molding 1" deep and deeper in one pass using rock maple and others and get about 99% of perfection. Sometimes I will run a final pass at, say, 1/64" deep to improve millmarks, with a cut so light that the shavings sort of float in the air.

The last trick up my sleeve is that I flatten the backs of my knives through 600 grit carborundum and hit the bevels with Arkansas files. I know that all sounds arcane, but I rarely get any tearout even when I am forced to run the blank contrary to the grain.


From contributor C:
As stated above the real limiting factor for the radius is the profile you are cutting. We typically consider 7" to be the minimum radius we can successfully profile on the W/H. Anything tighter than that and you'll get severe profile distortion on deeper cut areas of the profile. Some grinding modifications can be made to the knives to help with this but then they won't be able to be used on straight work.

Another issue with doing serpentine curves on the W/H is keeping them properly positioned under the knives at the transition point of the curve. Over the years we've developed a backup pressure fixture to help keep the molding in position under the head. We also have a custom 3-point spring loaded system similar to the Bonyman, W/H and other jigs. I designed it to eliminate the need for backup templates to lift the profile off the table. As such, we can put a backup template on serpentine pieces to help the backup fixture in holding the piece properly under the cutterhead.


From contributor D:
I have a W&H machine and 7" is the tightest radius they recommend. We've tried tighter and they do distort. Also, I haven't discovered a trick to get deeper profiles than 7/8" and that is very tight to the housing. I do have an adjustable feed rate attachment on the W&H that allows us to run from 0-16ft/min. It was an add-on option worth every dime.
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