Bending tight radius in top purfling

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Craig Bumgarner
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Location: Drayden, Maryland

Bending tight radius in top purfling

Post by Craig Bumgarner »

I just can't seem to get this. I sorta fudged it in past guitars but need to get it RIGHT! I'm trying install BWBW purfling (.020" each, .080" total) on the top inside a .080" black fiber binding. The guitars are Selmacs and the cutaway involves two tight radius bends, one roughly 3/4"-7/8" radius, the other a little larger. It keeps breaking at these radii, sometimes not completely, but any grain failure leads to a bump in the fairness of the line. Here's what I have tried.......
  • Read the relevent posts in the old MIMF Archive :D
  • Tried a single wood BWBW purfling ready made. (I think this is all maple, the black is dyed)
  • Tried two pieces of BW purfling (likewise all maple)
  • Tried installing straight pieces without prebending. Actually, this works about the best. It still breaks, but the breaks don't fail completely and with clamps and cawls, I can sorta crush it into place. Still noticeable though.
  • Tried pre-bending w/ hot pipe and hand bending.
  • Tried pre-bending a male wooden form with channels to support binding and heat gun to supply heat
  • Tried pieces wet, damp, light spritz and bone dry
  • Tried using a support strap of spring steel
  • Tried all 8 fingers to support purfling during bending
No luck at all. Any thoughts? I see other guitars with wood purfling and this kind of radius in the cutaway, so I know it can be done. No doubt my technique is at fault, but so far, can't figure where.

One thing I have not tried is single strips of .020" purfling. Another thought is to try .020" fiber sheet instead of wood for the black at least.

I'm pretty committed to wood purflings, but plastic and fiber sheet are looking better all the time.

CB
Steve Senseney
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Re: Bending tight radius in top purfling

Post by Steve Senseney »

I have a cartridge heater with a 1/2" diameter which I use for my tightest curves.

This is usually on a "f" hole binding.

Use the outside form, along with the small diameter heater is the only idea I have.

When I was binding the "f" hole, I had a corian form shaped to the "f" hole pattern, and pushed the binding into the form while the heater was pushing against the binding.
Craig Bumgarner
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Location: Drayden, Maryland

Re: Bending tight radius in top purfling

Post by Craig Bumgarner »

Thanks for the reply Steve. Never heard of cartridge heaters, didn't know there were such things. Just googled them, very interesting.

You are using this to bend purfling to the f hole, which is a much tighter bend than I'm trying to get, but what I have experienced before trying to bend into a concave shape is the purfling breaks on the side closest to the mold for lack of support. The break usually happens well before I get close to the mold. What kind of purfling are you using. I have had better luck bending over a convex shape, but not sufficiently better, either way is a bust.

I've been think this morning maybe the mold should be a curved slot in wood, the width and depth of the purfling, similar to the shape on the top, but with a ledge wall on both sides. Like a narrow rosette channel. Then heat and bend progressively into the channel. The only difference between this and doing it cold on the guitar with straight pieces is heat could be applied during the bending.

C
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Mark Swanson
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Re: Bending tight radius in top purfling

Post by Mark Swanson »

Are you gluing the thinner strips together to get your .080 purfling? And then bending? If you glue them yourself or buy them, that's pretty thick to bend in that small of a radius all together. I think you should bend them and install them one at a time. Tape them in place and then when you get them all bent glue them in place and tape up real well. By the way, the Stew-Mac brown tape is the best tape!
  • Mark Swanson, guitarist, MIMForum Staff
Craig Bumgarner
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Re: Bending tight radius in top purfling

Post by Craig Bumgarner »

Mark Swanson wrote:Are you gluing the thinner strips together to get your .080 purfling? And then bending? If you glue them yourself or buy them, that's pretty thick to bend in that small of a radius all together. I think you should bend them and install them one at a time. Tape them in place and then when you get them all bent glue them in place and tape up real well. By the way, the Stew-Mac brown tape is the best tape!
Yes, I've been starting with straight glued up strips. I'm glad to hear you consider it a challenge too 8-)

Okay, so bend individual .020" pieces and glue them. You say "tape them in place" then "glue them in place"..... Stupid questions: 1) do you mean "in place" on the guitar? 2) How do you glue them in place if they are taped in place? 3) Would you glue the binding and the side purfling in at the same time as you glue up the top purflings? I usually do, but with this many pieces going at a time, sounds pretty sporting.

Any suggestions on how to get reduce .100" x .020" strips down to .070 x .020?

Roger on the brown tape. I'm using LMII brown tape. Any different from the StewMac tape? LMII occasionally breaks over the binding edge when pulled tight, but otherwise okay.
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Mark Swanson
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Re: Bending tight radius in top purfling

Post by Mark Swanson »

Ok, let me offer some thoughts...
Tape them in place on the guitar. Don't worry about making them fit perfectly at this point, and tape with just a few pieces of tape. You just want them in position while they dry from being bent, and bend them damp of course. When you have all the strips bent, then undo some of the tape while you glue a few inches at a time, the tape over the un-glued part will hold it all in place while you get a start. Glue the binding as well, all at once. You can use a brush to get the glue in where you want it. Also, sand or scrape the edge of the binding just a little bit to make it less sharp and it won't cut through the tape as easily.
  • Mark Swanson, guitarist, MIMForum Staff
Dave Stewart
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Re: Bending tight radius in top purfling

Post by Dave Stewart »

Couple of things. For a "cartridge heater", I use a 35w pencil soldering iron, held in a vise, with a 7/8"OD x 4" brass "nipple" wedged in place overtop. Works great for tight bends. (I find, using an aluminum flashing "backer" - without which things WILL crack - over a wet rag placed over the nipple generates enough steam to really make things pliable, although it takes practice.) I think you'll definitely have more luck with individual strips rather than pre-glued. I use a simple "slitter" to reduce binding & purfling width. Did a tutorial a while back over on OLF.
http://www.luthiersforum.com/forum/view ... it=slitter
Attachments
slitter5.jpg
Dave
Milton, ON
Craig Bumgarner
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Re: Bending tight radius in top purfling

Post by Craig Bumgarner »

Mark: Thanks! I did a test just now using a piece of scrap cut to the top profile and routed in a suitable ledge. Laid in four pieces of .020" x .100" dry, cold, all together, went right in no problem at all. I taped it in all around and will just let it sit for a day to let it take a set. Pretty sure this is going to work fine as long as I can handle gluing all the pieces (morning job, no coffee). Roger on the gluing, roger on the sharp edge. Thanks again.

Dave:
Thanks for the details on the mini bending iron. As above, I'm going to try to use individual pieces which seem to want to take the bend okay. Thanks for the idea on the slitter, I'll give that a try. I think on this particular top which is 4mm, I think I can route a .070" deep ledge which will accept the .100" deep purfling okay. Just a bit more trouble knocking it down afterwards. "Purlell" first, build more tools second :lol:
Craig Bumgarner
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Location: Drayden, Maryland

Re: Bending tight radius in top purfling

Post by Craig Bumgarner »

Dave: Read your post on OLF in detail, great idea, easy to set up. I've already bent the purfling for this one, but will try your setup at the next opportunity.
Bill Raymond
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Re: Bending tight radius in top purfling

Post by Bill Raymond »

Craig, I have been laminating the strips in my mold for laminating the sides. I laminate wide strips of veneer, then slice off several pieces of purfling from one glued up strip. I use a slotting saw mounted in the drill press (carefully--you could get cut up with this setup).
Craig Bumgarner
Posts: 377
Joined: Mon Jan 09, 2012 3:03 pm
Location: Drayden, Maryland

Re: Bending tight radius in top purfling

Post by Craig Bumgarner »

Bill: Another great idea! I had thought about laminating to the curve, but the cutting process had me stumped. Your slotting saw drill press concepts sounds like it would work well. Would also work to slice off laminated linings. I've been using a bandsaw for this, but it can be hard to support the laminate because of the geometries. The slot saw swinging in a drill press should solve that. Thanks!
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