Routing a Large Radius with a Low-Power Router

Advice on how to sneak up on a radius corner rout using a great big bit in a smaller router. Tips on speed control, custom router plates, jigs, and roughing out the piece by hand. February 18, 2007

I have a PC 690 router and need to do a 1-1/4" radius on mitered birch fascia panels of a desk I'm building. I'm guessing it would be nearly insane (if not incredibly dangerous, both to myself and the piece) to try to attack the raw corner directly with this bit. Should I start with a smaller radius and build up? Or should I try to cut off the corner first by some means? Doe anyone have any recommendations?

Forum Responses
(Architectural Woodworking Forum)
From contributor E:
That is an enormous bit to put in a 690. It won't pass through the stock base, so you'll have to make one. It’s also going to have a lot of torque (I assume you're free handing this).

First off, use a bigger router. Second, if at all possible, do it on a router table. If you must freehand it, take small passes and work your way up to the full cut. Start shallow, make a pass, lower the bit some, make another pass, and etc. Also, be aware that the ends of your piece are going to want to blow out as you exit the cut. Be safe, and if you can, find a different way to do it.

From contributor B:
That large router bit should have come with a maximum rpm specification. Do not exceed that rating. When we run large bits like that we use a speed control (typically around $25) unit. It plugs into the wall and the router plugs into it. They are available from most woodworking mail order houses. You can also replace the bearing guide (if applicable) with a larger one for a first pass, and then go back to the original bearing for a final pass.

From the original questioner:
Actually - it does appear that there's no way this thing's going to even clear the router base - whose opening diameter is approximately 1.25". So - failing some other creative router solution, I'm going to have to do something else. I thought maybe building up to increasing radii might be good.

So, short of purchasing a new (and expensive) router which I won't need for anything else - what are my other options? Should I just try to glue a machined rounded piece in the corner instead? (I think it would be ugly). It's only two 5" long corner joints I need to cut down (crosscut).

From contributor B:
We make custom acrylic bases for the router all the time. Just take your existing base off and trace it on a piece of plastic, even plexi-glass will work. Drill a larger center hole and you are all set. But, again, don't exceed the recommended rpm's of the bit.

From the original questioner:
Yes - of course - though I'll opt to use a light dimmer I think. Same thing as far as the router's concerned.

From contributor L:
Unless the pieces are curved, you could use a block plane and touch it up with sandpaper.

From contributor M:
Your 690 isn't going to do this. Unless yours is variable speed the bit will spin at 20K RPM - much, much too fast for a bit the size you're intending to chuck into the router.

I would suggest that you go low-tech. Trace the intended arc onto a small piece of cardboard and cut that out. Then use it as an indexing jig. Trace the arc onto both ends of the desk top and use a block plane to nibble down near the line. Fair the curve with a patternmaker's rasp, Shinto rasp, or whatever, then sandpaper. Use that cardboard curve as an indexing check jig and slide it down your curve to check for high and low spots.

From the original questioner:
I'm thinking I'm going to amalgamate all the advice into a solution and act on that. Matt, I'll use your advice to get it most of the way to where I want to (I'll try the block plane, though - experience has taught me, despite any natural skills I might have, to rely on readymade solutions whenever possible - to lessen the chance of muffing it up). I'll try to get things to the point where the router bit doesn't need to take off more than a millimeter of material. My only big fear is having the bit catch on something - and calamity ensuing.

I purchased a Lutron cord dimmer that I'll use as a variable speed device. After looking to the router base, I'm thinking I'll just remove the base plate entirely and tape something like Teflon onto it so that the frame slides nicely across the wood surface. Maybe I can fabricate some sort of guide across the base so that lateral play, and hence eating into the piece won't be a possibility.

From contributor L:
If you have a problem with your block plane, it might be because of the wear bevel on the back of the blade - the blade can be sharp but not cut.

From contributor J:
When running large bits free hand I typically climb cut part of the waste away. Go backwards against the bit rotation taking off a small portion of the waste without letting the bearing get close to the work piece edge. Then go forward wasting a little more. Repeat this process until there is about 1/16" of waste left and then make final pass. It is basically like using a router in a table and moving the fence progressively back to increase cut without all the time spent on setups. I do this frequently with thumbnail bits, anything over a 1/4" round-over, rabbiting bits, large ogees, etc. It also helps with woods that have tricky grain prone to splintering like oak. The 690 is a work horse of a router. Don't be afraid to push it a little.

The slow down switch is excellent advice as is the larger base. Custom bases are easy to make. You can also find stock bases with large diameter holes readily available at any wood working supplier and most non big box tool suppliers for the PC 690.

If you feel more comfortable cutting out the waste with hand tools then by all means do so. I would be lost without my hand planes. I keep my block plane in the leather hammer loop on my nail bags and reach for it constantly. For what you are doing I think the router would produce more consistent results in a lot less time.

From the original questioner:
Contributor J - that was my reasoning - consistency of cut and less time wasted. But alas - in cutting x-grain with a block plane, I was actually very impressed with how clean I was able to get things (apart from starting to get chipping at the end of travel it should have occurred to me to sandwich the work with sacrificial pieces).

Anyway, it seemed to me that any slight inconsistencies would be hidden by the sanding passes. The only real mishap was a miter-fastening brad that I shot through at an unfortunate angle - it went a bit too deep and got revealed by my planning - efforts to punch it back with a brad punch just gave me a nice fat hole! Should I pack it with sawdust and glue?

From contributor R:
I had to do one of these a couple of years ago. Both long grain on the corner, and cross-grain on the trim. While I have a PC690, and variable speed DeWalt 618, I went with my old Makita 3612 since it is bigger and heavier. I used an aftermarket external speed control.

I was only using a 1" radius roundover, but I'll tell you, when I fired that baby up, it was like standing in front of a DC3! It was very scary business. I used light passes by dropping the bit and a very steady hand and foot placement. It worked out nicely, but my nerves were vibrating for some time after.

The bit you are talking about is 50% larger. That would be even more intimidating. All due respect to Justin, but I would never try that by climb-cutting with the bit at full depth. If there was any grab, you could be in a very bad place in a very short time. Keeping the bearing in contact and dropping the bit a little at a time gives you a much more stable and safe operation in my opinion.

Click here for full size image

From the original questioner:
Wow - that's nice! How did you get the miter line not to show? Or is that a different kind of joint?

From contributor R:
Thanks. The trim is a miter, the corner is, well, hard to describe. The picture will show you. As for the tight miter, just accurately cut, biscuit, and a bunch of clamps. I also took some pains to ensure the miter cut(s) took out as little of the wood length as possible. Because of the grain of this piece, it looks like it wraps right around the corner. That helps a lot in making it look seamless.

Click here for full size image

From contributor R:
I forgot to mention - where the diagram shows corner molding and corner cleat, I ended up making it one piece - just two big rabbets.

From contributor J:
To the original questioner: Contributor R is correct. Climb cutting increases the chance of kick back. My earlier response was incomplete as it failed to note the potential risks. If it makes you feel at all uncomfortable don't do it. The same holds true for any operation with a power tool. If any part of a cut ever spooks you kill the power and bail on it. Always treat any power tool with respect they don't know you are there.

Progressively lowering a router into a cut does allow you to control the router with the bearing always in contact with the material and lends itself well to the use of a plunge router like the 3612. I often use this method as well but tailored my response to your current tooling and my most frequent practices.

If you know the risks and where problems can occur you are better able to anticipate them and account for them with your technique. The climb cut only wastes away a small amount of material. Most of it is removed with forward passes.

I have done this with radius bits up to 1" and feel comfortable with the process. I find it faster and easier to just set the router depth once and be done with it and the risks manageable with technique, planning, and experience.

If the climb cut makes you nervous then just waste the material away with slow shallow cuts following the feed direction of the bit. When cutting in either direction a clear view of the bit is a must with this technique. Also clamp the material securely to a bench or horse. Try a cut like this first with smaller diameter bit. If it works for you add it to you bag of tricks.

From the original questioner:
I don't know why I didn't think of doing my corner like that. I considered it when I thought of bailing on the router option - though cutting it away (the miter) in place would be a bit of a fiasco. I should have asked for advice first!

Anyway - I actually have a plunge base I could have used for the 690. But it would be only slightly faster than incrementally setting the depth on the normal base, I suppose. I agree with the safety comments. Time and again - it's been clear to me that being freaked out by an operation has, most of the time, resulted in bad work and/or danger.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor S:
I regularly round over with a 7/8" and I always climb-cut. Always cut with the router coming towards you - it's much easier to control. I'm not sure how easy it might be on the bigger bit but it's worth a try. I always leave my bit at full depth and take light passes until I reach the finish cut (2-3 passes on the 7/8" bit). Unlike on a router table where the fixed router can grab and throw the piece when climb-cutting with such a big cutter, a hand held router usually climbs its way out of the cut. It's very easy to control.