Cutting an Arc?

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

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.
<g>

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

Jim

"Morris Dovey" <[email protected]> wrote in message
news:[email protected]...
> Jim Egan wrote:
> > 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 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.
>
> 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.

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

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

"Lew Hodgett" <[email protected]> wrote in message
news:e%[email protected]...
>
> 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.

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.

On 9 Dec 2004 11:37:12 -0800, "Jim Egan" <[email protected]>
wrote:


>Or should I cut close to the template with a jig saw and then sand the
>rest of the way?

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

In article <[email protected]>,
Jim Egan <[email protected]> wrote:
>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?

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

--
<a href="http://www.poohsticks.org/drew/">Home Page</a>
9/11 was a premptive attack

Bob wrote:
> 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).

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

On 9 Dec 2004 11:37:12 -0800, "Jim Egan" <[email protected]>
wrote:

>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?

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" <[email protected]> wrote in message
news:[email protected]...
>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

jigsaw. beltsander.

"Jim Egan" writes:
> I have to admit that I have had this project ready for cutting the arc
> for quite some time.
<snip>

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

Jim Egan wrote:
> 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 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.

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.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/solar.html

I used a router in standard direction for half of the wood with grain
going "downhill" and climb cut the other half without moving the
template.

On 9 Dec 2004 22:13:23 -0800, "Jim Egan" <[email protected]>
wrote:

>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.

Jim Egan wrote:
> 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.

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

On 9 Dec 2004 22:13:23 -0800, "Jim Egan" <[email protected]>
wrote:
<snip>
> 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)
>
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.

>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.
>
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

Jim Egan Wrote:
> 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 i
> 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
> 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 th
> 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

I once had to cut a similar piece for an oak mantle I was building -
laid out the arc with a flexible batten, then cut it with a jig sa
(and a new blade). Then, cleaned it up using an orbital sander unti
it looked right.

I'm getting ready to build another similar mantle (this time for my ow
home) and will probably use the same technique. If this is somethin
that you will be reproducing in the future, you may want to conside
making a template from 1/4 inch hardboard. Just make sure that yo
pre-cut the arc to within 1/8" to 1/16" of the line with your jig saw
then finish it off with a pattern cutting bit (bearing on top) in you
router.

Regardless of how you finish your cut, make sure that you are using
good blade in your jigsaw

--
makesawdust

On 9 Dec 2004 11:37:12 -0800, "Jim Egan" <[email protected]>
wrote:

>I have to cut an arc in a piece of 3/4" hard maple that is about 70
>inches long.

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.
--
Smert' spamionam

On Thu, 9 Dec 2004 13:45:31 -0700, [email protected] (Drew
Eckhardt) wrote:

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

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