Cutting an Arc?

[Jim Egan]

I have to cut an arc in a piece of 3/4" hard maple that is about 70 inches long. The arc itself isn't that severe. From the start to the apex (35 inches) the rise is only about 8 inches. I'm sure I haven't described it very well. You'll have to trust me that the arc is pretty gentle.

I have a template made out of 3/4" plywood to use so I know exactly where the arc has to be. Oh, and I need to do this for six pieces.

What is the best way to do this? Should I use a router? I have a nice Freud triple flute trim router bit available. But I'm concerned that there would be a lot of tear out on half of the arc. The arc is for the most part parallel to the grain. Well, at least the apex of the arc is. It's not like most of the edge area is perpendicular to the grain. Would it work OK to use the router on half the arc where tear out is unlikely and then flip it over and do the other half in the same manner?

Or should I cut close to the template with a jig saw and then sand the rest of the way? That doesn't sound like a lot of fun.

Thanks, Jim

[Brett A. Thomas]

Jim,

Whatevever you do, don't just use a router. I did that on a headboard once and got a big ol' split where the grain changed direction. If you're going use a router and template, rough it out with a jigsaw, first, so the router has much less material to remove. If I had it to do over, that's probably what I'd do.

At the time, I glued up a new headboard, cut to the line as best I could with a jigsaw, then used a spindle-sander attachment chucked in my drill press to sand to the line. I was dreading doing it that way, but it was actually pretty easy and came out very nicely (if I do say so, myself).

If you don't have a drill press (or don't want to use it as a sander), I understand some people here have liked the Ryobi oscillating spindle sander that Home Depot has for not much money right now. I've never used one myself, but I'm sure a Google groups search for "Ryobi OSS" will turn up a lot of reviews.

Good luck...

-BAT

[Morris Dovey]

Jim...

I don't normally work with maple; but I think I'd go with the router and template strategy. I'd go in the "climb cut" direction for smoothest edge; but I'd prefer a template plus bushing arrangement that let me remove about 1/4" of maple on each of three passes. I might even be tempted to do all that with a piece of tape on the edge of the template, then make a final "cleanup" pass with the tape removed.

[Charles Spitzer]

jigsaw. beltsander.

[Drew Eckhardt]

Rough cut with a jig saw, finish with a router and flush-trim bit.

[George]

Jeez! Doesn't anybody watch "The Router Workshop?"

Half template, flip the piece so you're always cutting downhill. Makes things symmetrical, too.

[TWS]

I'd go along with this but only use the top bearing for half of the arc. Put a matching bearing on the bottom of the trim bit and flip the template/board over to do the other half of the arc so you are always cutting across the grain. No tear out this way.

I assume you are only making an outside curve so sanding is ok to finish. If you are doing an inside curve use a scraper to finish.

TWS

[Bob]

I don't understand all the chagrin and horror stories mentioned by others in using a router to cut an arc. You can do superb work with a

1/2" straight bit, a plunge router and a circle cutting jig. The key to the whole thing is making light, multiple passes (same principal as cutting mortises with a router). Someone in our area did a big doorway arch using this principal. It had a 30 foot radius and he did it in the parking lot!

Bob

[Brett A. Thomas]

Well, my chagrin and horror story is that it was about the first time I'd ever used a router, and I didn't take "light, multiple passes." Don't do it and *not* take "light, multiple passes" because you'll split it like I did.

-BAT

[Ba r r y]

Cut close to the template with a jig saw, and then ROUT to the finish line using a router and pattern bit.

Mind the grain direction with the router to avoid tearout.

Barry

[Andy Dingley]

Why ask us? We haven't handled the timber, we don't know what tools you've got. There can be a fair bit of variation in how maple behaves for tearout.

So work it out for yourself. You've got plenty of scrap, carve a chunk out roughly, then saw a smaller arc out of that, trim it by some favoured method, and see what the results are like.

If it were mine, I'd saw them within 1/16" on the bandsaw, then probably trim to exact size on the router with a bearing trim bit. In general, this will give a perfectly good finish on most bits of maple, so long as you're only taking a whisker off.

An adjustable compass plane would work too, but maple can be a bit of a pain for tearout sometimes, especially for a varying angle..

If I didn't have a bandsaw, I'd buy a _good_ jigsaw (Bosch barrel-body, about £120). They do make a difference for reduced vibration and thus better cut accuracy and quality.

[Guess who]

If using a rounter I'd suggest (i) cut close [really close] to the line with a jigsaw first. (ii) if feasible, clamp a piece of thin ply next to the one you are cutting, and mark out [same method as for the original] as a guide for the router. The ply, being thin, can be cut directly with the jigsaw and light sanding. Then take your time for a nice smooth finishing cut.

[Jim Egan]

I have to admit that I have had this project ready for cutting the arc for quite some time. After the amount of time put in to constructing these six items (they are shelves) to this point I have been extremely reluctant to start cutting on them because I just knew I was going to screw them up. It was only as I was writing up my question that it dawned on me that I should be able to flip the board over and route the second half of the arc. However, I hadn't given any thought to which end of the router bit the bearing was on. (I do have a bearing on this bit)

I'm not sure that I agree that two bits are necessary for this operation. The bit has the bearing on the end so for the first pass I'll have the template on the bottom. When I finish cutting across the grain (Into the grain? I'm not sure what the correct terminology is here.) on the first half of the arc I'll flip the board end for end (widthwise) and finish the second half and will still be cutting across the grain. Won't I? So two bits won't be necessary. Right? Or am I missing something.

Thanks to others for their replies... Charles: A belt sander I have. A light touch I don't. Brett: A drill press I have also. But at 70 inches of edge to sand multiplied 6 times I think I would be there a long time. Bob: I did use a circle cutting jig to construct my template. But at about 7 feet in length it is a bit unwieldy and took up a buttload of space in my garage. I didn't feel real comfortable with the amount of flex I had over 7 feet of jig. I wasn't sure that I was going to get a clean, consistent cut every time. With my template I think there is less room for error. Time will tell. Morris: I like the tape method you suggested. Great idea! I don't have a template bushing for my router (Bosch) but Christmas is coming up and SWMBO needs specific catalog numbers when I submit my wish list.

Thanks guys, you have given me the confidence that I can complete this project without generating expensive firewood.

Jim

[Lew Hodgett]

I'm getting into this late so may not have a full understanding of the problem.

Laying out large arcs in boat building is quite common, often as much as 16 feet long.

The usual approach would be to layout the complete arc on 1/4" hardboard, then trim proud with a saber saw and then finish sand to the line of the arc which may require hand work to finish smooth.

You now have a full template you can clamp to the finish piece and finish using either a pattern bit (top bearing) or a flush trim bit (bottom bearing) to finish the total piece in one opoeration.

Attempting to do 1/2 the piece then flip the pattern over to do the other half will come home to bite you in the rear end IMHO.

I wouldn't do it.

Every piece will be just a little bit different.

Just enough different to look like ugly on an ape.

HTH

Lew

[George]

You mind telling us how you manage to change the outcome without changing the input?

Those who rout regularly would like to know what is about to bite us.

[nospambob]

[FEngelman]

sounds like a good excuse to buy that compass plane I've always wanted/needed....

[Andy Dingley]

Compass planes tend to have quite large mouths - they can be a problem to use on maple.

[George]

Veritas spokeshave works a treat.

[TWS]

Sorry, I wasn't clear in my response. You can get a bearing set to mount bearings on the shank of your flush cutting router bit (assuming the diameter is at least 1/2 inch diameter). So the single bit would have a bearing on the top and the bottom of the bit.

Attach the template to the workpiece using double sticky tape. Route half the arc with the board positioned so the template is on the opposite side of the board and using the top bearing. Flip the board over (don't move the template) and route the other half of the arc with the bearing on the shank side of the bit. As other point out, make sure you don't move the template between the two cuts, this will give you the best match to your pattern.

If I was doing just one of these I would probably close cut with a bandsaw (jig saw if you don't have a bandsaw) and sand the edge to finish. Given that you are making six identical pieces I think the router is the best choice - the bearing set is a good investment for this type of application.

TWS