Fine-tuning rim bending

[GaryDavis]

Greetings from Montana. I am building my first mandolin - an F5. I'm not a complete novice but it feels that way. I built a baritone ukulele last year that turned out pretty good according to those that play it. I don't play any instrument.

I have been bending rim pieces over the last couple of weeks. Its highly-figured cherry, and it has been breaking and scorching pretty easily. I've had to sand it down to 0.050-.052, and it still breaks BUT I finally got the three pieces bent and in the form. My problem is that there are some minor flat spots in/near the tight bends at the scroll and on the s-piece that I'd like to smooth out, but I'm reluctant to keep messing with them. I only have one relatively long piece left that I can use but its not long enough to replace all three pieces. Should I be concerned about these small flat spots? Will they be obvious once the top and back are on? How would one attempt to smooth out these areas if they might be obvious? Or, given my bending experiences to date, should I just leave them be?

Thanks in advance for any advice.

Gary Davis

[Charles E.]

Photos would help if you can. Are you using a stainless steel backing strap while bending?

[GaryDavis]

i'll take some images tomorrow. i did use a steel backing strip.

thanks

[Tom Haywood]

There is no better way to get answers to these questions than to get more practice bending.

I bend sides by hand over a hot pipe - no back strap. I generally get to the final shape with maple in two or three rounds, with usually a days rest between each round to dry and cool into shape. Flat spots tend to occur while drying in the form if not pressed to the edges well. The real difficulty I've seen is that trying to make that tiny bend to round out a small flat spot can result in sort of a "crimped" appearance.

My only build with cherry, so far, was an Irish bouzouki a few years ago. The sides are much wider than a mandolin - approximately 3/4 the width of a dread naught guitar. I remember the cherry (straight grain) being very hard and it had to be bent very slowly with a lot of rolling action across the pipe to get a smooth curve. Instead of cooling in the form, I tried the technique of laying out the bend with a line and nails on my table. It worked well for large bends.

Highly figured wood is more difficult to bend without cracking, because the "resistance" of the wood grain is not very even. I think going thinner than .07 or .06 makes it more likely you will crack it. Slowing down, creating more steam, and keeping the piece moving is usually the answer.

My main personal gigging mandolin has a slight flat spot that is visible when the light hits it just right. It doesn't matter since I don't intend to sell the mandolin, but I think buyers now expect to see factory perfect work from small shop builders.

[GaryDavis]

This is the material with which I'm working. Not started on the soundboard yet.

[IMG]Untitled by Gary Davis, on Flickr[/IMG]

This is the "top" s-piece of the rim. There appears to be only one really noticeable kink and that seems to be where the curve changes from one direction to the other. The scroll area seems smooth enough for a "personal" instrument. My son will be here in a few days, and he'll let me know what he'll accept.

[IMG]Untitled by Gary Davis, on Flickr[/IMG]

Thanks for the suggestions.

Gary

[Tom Haywood]

The grain in those rib pieces looks just like what I had. Is the crack taking place at the end of the piece? If so and it will be glued to a block, you can put some CA glue in the crack, finish the bend, glue up the rib, and likely never have any trouble with it.

[GaryDavis]

Thanks Tom. I managed to get the three pieces bent without cracking. The piece above has been trimmed, and its clamped in the form with the neck block. Not a perfect fit, but pretty close.

My concern is the degree of "smoothness" I've achieved with these tight bends. I think the figured areas bend a little quicker than the non-figured areas and so aren't as smooth as I would like. If there's not a relatively easy solution to smooth the curves better than they are, I'm willing to live with it. I sanded the rim pieces to 220 before bending, but I've got some scorthing to touch up and hopefully that might smooth things up a bit. But I don't have much thickness left to sand.

How about some time clamped in the form in the oven at low temperature? Might that soften the cherry enough to have it better conform to the smoothness of the form? I really appreciate your time and advice.

Gary

[Tom Haywood]

I haven't had much success with general softening of a piece of wood prior to bending, such as with boiling or steaming the entire board. Focused heat with spot steaming works best for me. It seems like slow heating in the oven might make the piece more brittle and subject to cracking, but I haven't tried it so it is just a guess. What are you bending the tight curves over?

[GaryDavis]

Hey Tom. I don't remember whether my pipe is 1" or 1 1/4". How about using a heat gun on the rim while in the form?

You've been very helpful, and I'll refrain from bothering you further.

Many thanks and stay safe - Gary