Cylinder sizing?

[Jay Pique]

I need some large quarter-round molding. The finished size must have have a "radius" of 3" - that is, the distance from the point to the cut edge must be 3". I thought of turning a 6 1/8" cylinder, and losing an eight to the kerf on the TS. Then I remembered it'd be quartered, so I'd lose another eight. Then I realized that as I take the kerfs I'm actually shortening the legs by somewhat more than an eight due to the curvature of the piece. I haven't done a google search and I haven't made any effort whatsoever to do the math. Anyone up to the task? tia JP

******************************************************* I'll be on the rowing machine if you need me.

[boorite]

A cylinder of 6 1/8" *will* give you sections with radius 3" when quartered, assuming a 1/8" kerf.

Molding? That's a log!

[marks542004]

draw a cross section on a piece of paper to see your errorw.

61/8" diameter , one cut at 90degrees to diameter leaves you a length of 3" for a 1/8"kerf after the first cut, you have two half round pieces, 3"high and 6 1/8" across make a second cut and you have two quarter round pieces 3" radius. more or less.

I believe but have not drawn it , that the curve will not be a 3" radius , but will be the remainder of the 6 1/8" curve. And not a proper 90 degree circle segment.

Probably close enough for a moulding though.

[Jay Pique]

But not quite 6 1/8" across, because I've lost the 1/8" section from the middle.

snip

It's to sit vertically below another, smaller quarter-round piece. The whole assembly is for a corner detail on some wall panels.

I know I can sneak up on it, and I could rip in on the band saw for less loss, but I'm sort of keen on knowing the math behind it. Time to start digging. JP

[boorite]

There's hardly any math involved. Quarter a 6-1/8" cylinder with a 1/8" kerf, and you come out with 4 quarter-rounds, which, when you gang them together, make a 6" cylinder.

You *do* come out with a proper 3" radius and a 90-degree arc.

Every point on that arc is 3" from the center of the new circle or the vertex of that 90-degree angle. So we say you have a 3" radius there.

Nothing to see here.

[Doug Miller]

No, you don't. It's pretty close, but not exactly. The radius of curvature is

3-1/16", not 3", and the arc does not meet the flat quite at 90 degrees.

Not correct. But close enough it doesn't matter.

[Andy]

Not exactly. Yes, the cylinder will be 6" in diameter when measured across the cut lines, but it won't be a perfect cylinder. The radius on the outside curves will still be half of 6 1/8" - JP is right. Take it a little further - say you start with the 6 1/8" cylinder, and keep cutting 1/8" sections off until the straight parts of your "quarter round" are only 1" long. Then you put the 4 quarters together, and you do NOT have a round 2" diameter circle - you have pieces with a 1" flat face, but the radius of the outside curve doesn't change. So, Jay, the question is whether it's more important that you have a radius of exactly 3", or flat sides of exactly 3". If you start with any diameter cylinder, and remove any material at all for a kerf, you can not end up with 4 pieces with 3" faces AND a 3" outside radius. Unless you're up for a lot of sanding... I hope I explained that well enough

- draw it out, and keep taking away kerfs until it makes sense. If you're looking for opinions, I'd say that nobody could tell the difference between a 3" radius curve and a 3 1/16" radius curve, so I'd say the 6 1/8" cylinder would probably work. It would be even closer to use a bandsaw for the thin kerf and reduce the cylinder diameter accordingly. Good luck and let us know what works, Andy

[Lew Hodgett]

RE: Subject

Ever consider 1/4 circle from plywood.

3" radius is NBD.

If interested, check out Anderson International here in SoCal.

Lew

[CW]

A 6 1/8 cylinder has a radius of 3 1/16. Quarter it and each piece still has a radius of 3 1/16. You could use a 1/2" blade to cut it. The radius still won't change.

[B A R R Y]

I'd do it with a shaper.

[Jesse R Strawbridge]

You could do it as a glue up, using paper as a release between the quarters. Turn to 6" and separate along the paper.

OR

Fasten 4 pieces together with screws at each end, outside the area to be turned. Trim the unturned ends off. (Note: I have never tried this but it should work.)

Jess.S

[joeljcarver]

"radius" of 3" - ...

Make a split turning --- Glue up 4 pieces of 3"x3" stock (or slightly over sized). Glue paper between the joints. This will act as a separator. Turn a 6" cylinder and split the four quadrants along the paper seams with a tap of a chisel. It's done frequently.

[boorite]

Yeah, I wasn't thinking straight. Of course the *arc* still has a

3-1/16" radius, i.e., bisect the 90-degree angle, and the distance to the point on the arc is 3-1/16". But the distance from either leg of that right angle is only 3". I swear I got honors in plane geometry, but that was a long time ago. I was thinking more in terms of getting the piece he wanted.

So the answer is, there's really no way to section a cylinder and get a quarter-round of radius X that's exactly X on each side. You'd have to live with an arc whose radius is X + .5*kerf, or start with an X by X section and cut radius X onto it.

[Jay Pique]

"radius" of 3" - ...

Now THAT's what I'm talking about. Thanks to you two for bringing it to my attention. I'm not a turner, but I have heard of it. I'll definitely relay the message to our lathe guy. I'm a little surprised he hasn't thought of it. He does turnings for us from home. Is there another use for this trick that I would have used myself, but not involving a lathe? For some reason I vaguely remember knocking apart some pieces that had been glued up in this manner.

JP

[Darrell Feltmate]

[Jay Pique]

a "radius" of 3" - ...

Just called him. He said "oh I know that" and seemed to think it wouldn't work. The typical way we make large turning blanks is to glue four pieces of thick stock into long square tubing using 45d miters. So it's hollow in the middle. I don't know, he was pretty dismissive. In any event, if I want to hop on the lathe at work (after I dig it out from under two tons of crap), is this the process by which I should do it.....

I need (4) three foot lengths of quarter round with a 3" radius. So I take (4) 3 foot lengths of 3.125"x3.125" stock and glue them together with a piece of grocery bag in between. Then I screw on a faceplate, being sure that it's centered on the intersection of all four pieces. Then I mount it on the lathe, and align the interesection at the other end with the tailstock center. Then I turn it round to 6". Once done, I simply take it off the lathe and split it apart - I'm assuming with a chisel pounded into the glue joints?

Sounds very nifty. Have I missed anything? JP

[Doug Miller]

In article , "Jay Pique" wrote: [...]

I think the *first* step in your process should be to crosspost this thread to rec.crafts.woodturning . You're likely to get more ideas from the folks over there, and probably better ones than you're going to read in my post...

OK so far, AFAIK...

.. but I don't think I'd do that...

.. owing largely to the difficulty of being sure that it's centered. If it's not centered *exactly*, you're not going to have quarter-round when you separate the pieces.

My first thought is to grip it in a four-jaw self-centering chuck. If you don't have one that opens wide enough, use stock an inch or so longer than needed, and mill a stub tenon on one end, small enough to grip in whatever chuck you do have.

Or you could use a steb center in the headstock. (Yes, that's spelled correctly.) Probably work better than a spur center.

Sounds ok to me... but I've only *read* about doing this. Never actually did it. Ask over at r.c.w .

[Darrell Feltmate]

Jay Not a face plate, use centers, but otherwise, okay. Tell your lathe guy there is no reason for it "not" to work unless he cannot get the centers in the right place. The intersection is hard to miss though. This is one of those cases where "we have always done it that way" because it works :-)

have have a "radius" of 3" - ...

[Jay Pique]

Should I be concerned about the points on the centers acting as a wedge to split the blank prematurely? Stebcenters, as someone said, would probably prevent this. Not sure we have them though.

JP

[Doug Miller]

Nope -- not if you drill a little recess right at the center for the point to slip into. It's *much* more important to avoid lining up the spurs on a spur center with the joints in your glue-up. :-)

Cup centers would work too.