Query on bending the back plate after tapering sides

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Quinny
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Query on bending the back plate after tapering sides

Post by Quinny » Wed Jan 07, 2015 12:01 pm

Hi,
I brought tenor uke plans from Georgia Luthier Supply and their construction manual - I've only previously made two non-arched soprano ukes. The plans use arched back braces to form the arch of the back plate. They also have a template to use so that the sides taper from the heel block to the neck block. I was surprised that the back plate would bend from 75mm at the heel to 47mm at the neck - as per the plans. I emailed Georgia Luthier Supply and this was their response:

"Begin by first creating your back plate with arched braces glued to the back. This will create the arch across the bouts. Use the side template to trace the side contour on the outside of each side after they are bent (note this is the most important step to get a good arch from tail to head block). Hold the sides securely in a form with spreader jacks and carefully plane down the sides to within about 1mm of the traced lines. Install the kerfing from the back. Finish sanding the sides and now the kerfing down to meet the traced side contour lines. Now you can normally glue the back to the sides and the arch from the tail to head blocks will be natural and graceful, as well as the arch across the bouts in the opposite direction, without any special tools."

I can understand the wood bending side-to-side across the bouts, but am surprised to think it would bend heel-to-neck - from 75 to 47mm - without heating.
- Can anyone comment as to whether this works fine to bend the back plate heel-to-neck with tapered sides?
- Do ukes normally have a long arch heel-to neck in addition to across the bouts?
- is there another way to achieve the heel-to-neck arch?

Any comments welcome

Thanks

Andrew

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Re: Query on bending the back plate after tapering sides

Post by johnparchem » Wed Jan 07, 2015 12:21 pm

The arch is not that big, probably 15 foot radius in their plan. Most of the difference you see top to bottom is a taper. No issue at all bending the wood to a 15 foot or five meter radius. Their instructions sounds OK for getting the radius without a radius dish.

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Re: Query on bending the back plate after tapering sides

Post by colburge » Wed Jan 07, 2015 12:24 pm

Wont be a problem Quinny, that is not a tight radius.

Col

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Re: Query on bending the back plate after tapering sides

Post by Nick » Wed Jan 07, 2015 12:58 pm

Hi Andrew, your back isn't curving from 75mm down to 47mm, it's your sides that are tapering down so in effect the back's plane isn't parallel with the soundboard's to start with, it sits on an angled plane and then just curves at each end to meet the head and tail blocks. The 75 down to 47 sounds a lot but that's a vertical height difference between the respective ends, if you imagine an angled plane that sits at the highest point of the side (usually at the waist) and is parallel with a similar plane that touches both blocks, there isn't much of a bow at all (that last paragraph goes to prove that a picture really is worth a thousand words! :lol: ).
As others have pointed out probably a 12-15' radius which, over it's length, a back can achieve easily no heat required. Make the tapered sides up as described and sit your back blank on, it doesn't need to have any braces on it at this stage if you want & press down at the tail block and flex the other end down to meet the head block, you'll wonder what all the worry was about :wink: .
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Re: Query on bending the back plate after tapering sides

Post by Quinny » Wed Jan 07, 2015 3:19 pm

Thanks guys. :cl The consensus is clear that the heel-to-neck arch won't be an issue for the back plate to conform to.
Thanks Nick for the detailed reply - it was very reassuring that there won't be a problem.

A follow-up query:
I understand the side-to-side back arch helps the wood adjust to humidity/temperature as well as better reflecting the sound.
- Is the idea of the side taper for better sound reflection as well?
- Is it standard for ukes to have the heel-to-neck taper?
- Basically, is a side taper worth the bother?

Ta

Andrew

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Re: Query on bending the back plate after tapering sides

Post by lauburu » Thu Jan 08, 2015 6:00 am

Hi Andrew
Don't overthink your first instrument. That will do your head in.
You have invested in plans and a manual which is sensible. Follow the plans carefully and try to avoid cockups. Most tenor ukes seem to have a variation in dimensions between head and tail which I have always believed to be due to a variety of factors including the ones you mention. Once you've made your first tenor (of which, I'm sure, you will be immensely proud) start experimenting and change a few variables to see what works best for you. And be sure to show us some pictures of the finished result.
All the very best on your project.
Miguel

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Re: Query on bending the back plate after tapering sides

Post by johnparchem » Thu Jan 08, 2015 8:06 am

Quinny wrote:
- Is it standard for ukes to have the heel-to-neck taper?
- Basically, is a side taper worth the bother?

Ta

Andrew
If you make a Martin style tenor a taper is standard if you make a hana lima tenor the taper is not standard ... Both had a radius around 15'.

I recently finished 2 one to a Martin style plan and one to the hana lima plans. I find the tapered uke a little more comfortable to hold. I also like the look.
If your plans have the side profile it will be easier to follow the plans which will give you both the radius and the taper.

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Re: Query on bending the back plate after tapering sides

Post by johnparchem » Thu Jan 08, 2015 8:36 am

As a follow on here is a post that shows a tapered uke and a non tapered uke both with a 15' back radius. viewtopic.php?f=25&t=6547

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Re: Query on bending the back plate after tapering sides

Post by Quinny » Thu Jan 08, 2015 12:19 pm

Thanks Miguel for your explanation and encouragement - it helps.
Thanks John for the photos. I agree the side taper looks more elegant.

I can see from the replies that a side taper is a choice rather than a standard, whereas a back arch is probably standard.

Thanks guys

Andrew

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Re: Query on bending the back plate after tapering sides

Post by mooshalah » Fri Jan 16, 2015 2:42 am

Hi Andrew.

I've just read your post.

I have just finished two identical (5-stringed) ukuleles based on the plans I purchased from Georgia Luthier Supply. I didn't buy the construction manual because I'm familiar with guitar and ukulele making techniques. Rather, I purchased them for the dimensions, and basically, I didn't want to reinvent the tenor ukulele, so to speak.

Like you, I got to wondering about the curvatures.

Essentially, one is asked to create a cylindrical back by first creating a side-to-side curvature when one glues the back braces to the back, and then a top-to-bottom curve by gluing the back to a curve that extends from the heel-block to the end-block. over the sides.

It all looked pretty severe to me, but I did it anyway, although I was a bit concerned that I might somehow create a bad "kink" in the back.

As John, Col and Nick have replied, and I can attest, this really isn't a big problem. Wood is pretty flexible and compliant within certain limits. Essentially, this is what we do whenever we force a flat piece of wood like a back or soundboard into a spherical (rather than cylindrical) mould; we force the wood to take up a modest curve in three dimensions. While this obviously builds in a modicum of stresses (or are they strains?), the wood can handle it.

In fact, the biggest issue I had was with the sharpness of the "crown" of the curve, at around the level of the waist, where the top-to-bottom curve peaks as it rises from the neck side of the body and returns down toward the end-block (bottom) of the instrument.

In the drawings ("uke14tenorplan_5.pdf") there is shown a side template with both a thick and a dashed line, and one is instructed to cut the sides of the instrument to the profile of the thick line. I did this but found the "peak" too severe, and I rounded and flattened it off quite a bit more, before I was satisfied that the cylindrically curved back would indeed shape to, and adhere securely to the sides and their attendant linings.

All in all, it wasn't a problem, but I can well understand why this caught your attention.

Here are a few snaps I've just taken of the curves involved.

Kind Regards,

Frank.
Attachments
Top to bottom curvature.jpg
Side to side curvature.jpg
Back and side.jpg

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Re: Query on bending the back plate after tapering sides

Post by Quinny » Tue Jan 20, 2015 4:35 pm

Thanks for the detailed reply and photos Frank. It is reassuring to hear what other people have done and faced the same problems - and overcome them!
I am presently ready to trim down the tapered sides and glue on the plates, which left me with two queries:

1) Kerfing: due to the side-to-side arched back, does the kerfing need to be sanded to an angle to accommodate the arch, or is the curve over the 5mm of the kerfing too small to worry about?

2) Cutting the rebate channel for binding: As I don't have a more expensive router/binding setup to adapt my rebate to the curve of the channel, how do I rebate the back plate for binding? - I assume this difficulty is caused by the side-to-side arch, not the heel-to-neck curve - or both?

Thanks

Andrew

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Re: Query on bending the back plate after tapering sides

Post by 56nortondomy » Tue Jan 20, 2015 7:56 pm

Hi Andrew, it dosen't take much to make a binding jig, i've made mine from some plywood offcuts and some channel from Bunnings 2 sizes that fit into each other nicely to act as runners and a spring on the back to help take the weight of the router. I reckon it only cost about $10 to make and i've used it on about 12 guitars and it works great.
Wayne
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Re: Query on bending the back plate after tapering sides

Post by Quinny » Wed Jan 21, 2015 11:38 am

Thanks Wayne. A picture is worth a thousand words. I get the idea now that it is the cutter/router that needs to be held vertical. I should be able to sort something out.
Thanks
Andrew

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Re: Query on bending the back plate after tapering sides

Post by kiwigeo » Wed Jan 21, 2015 2:44 pm

The other option is to use attach a home made jig to a laminate trimmer that registers off the sides of the instrument. There are drawings for same in back of Jim Williams' book.

Ive got a bunch of jigs for cutting binding channels but I often find myself using the Luthier Tools version of the above jig with my Porter cable laminate trimmer.
Martin

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Re: Query on bending the back plate after tapering sides

Post by Quinny » Thu Jan 22, 2015 9:13 am

Thanks Martin. I checked out the Jim Williams binding jig and can make something like that for my trimmer. I like one idea I found of fixing the trimmer upside down in the vice which allows your focus to be on keeping the side firmly against the jig.

Thanks
Andrew

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Re: Query on bending the back plate after tapering sides

Post by mooshalah » Sun Jan 25, 2015 2:12 am

Hi again Andrew.

Sorry for my late reply to your questions about 1). Kerfing (more correctly, kerfed linings - sorry to be a pedant!), and 2). cutting the channel for the bindings.

Others have already answered with regard to cutting the channels, so I'll not say more, other than that even for a smaller instrument like a ukulele, if it has a curved back (or soundboard), getting an accurate cut - that is, a square, exact, uniform distance inwards, and down the side - is always a challenge, and I've long accepted that the only comfortable and predictable way to do this is with some kind of setup that will hold a router (whether a "laminate" router or even a Dremel) securely so that it will move up and down in the vertical plane, as one goes around the profile of the instrument.

If the curve (and it can be an issue either way; side-to-side or head-to-tail) is very gentle, then one can sort-of get away with a simple, hand-held router and a way of ensuring a constant thickness and depth of cut. The more pronounced the curvature, the more the problems compound.

Like others, I've tried my share of hand-holding a Dremel or another form of small router, but it never quite works - and occasionally spectacularly doesn't! I guess that if one intends only to ever make one instrument, it's possible to do this while hand-holding a Dremel attached to one or other small guide such as available from StewMac or other supplier, but I've not found this secure or very satisfactory, and it can sometimes be nerve-wracking.

With regard to subtly sanding the linings at an angle to allow (or improve) the mating of gluing surfaces, I've tried a number of ways - and always because I'm a little too scared not to, rather than because I know for certain that doing it will make a strong joint, while not doing it, will not.

Sometimes I tell myself that it's all likely to be unnecessary, because after all, I'm going to cut a channel for bindings and purflings, thereby removing quite a bit of this back / lining joint, and then stuff this full of - well - bindings and purflings and glue, and squeeze all this tightly together. On other occasions, I fret that if I don't get this just right, a guitar that I've made is going to spontaneously disassemble itself in 30 years time! Certainly, I've made an instrument where I didn't attempt to angle the linings, and this fact was noticeable when one looked at the smoothness of the curvature of the back once the instrument was polished; but this was a cosmetic-, and not necessarily a structural issue. But it didn't look right.

So I'm not one to be able to give a definitive, or even sensible answer.

Ways for accomplishing this sanding at a correct angle include:

1). Sticking sandpaper into a radiused dish (the same dish as used to impart the curves while preparing or gluing in the braces) , and sanding all the linings, sides and neck- and tail-blocks;
2). Sticking sandpaper to a single bar of wood into which a curve equivalent to the "doming" of the back has been cut, and running this the bar from the neck-block to tail-block, sanding everything in its path, to the correct angle(s). This is my preferred method; you can get away with using a minimal amount of sandpaper;
3). Guessing it, or not worrying about it;
4). Using some other form of device or system - and in this regard I mention the "Ukulele Crown Sander.pdf" available from the same website whence you bought the plans (and it took me a while to figure out how to use this device, once I'd made it!) This is supposed to assist with the neck- to tail curvature, not the side-to-side curve, or angle, but I guess one could modify the device shown in the drawing, to do both!

Regards,

Frank.

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Re: Query on bending the back plate after tapering sides

Post by Quinny » Wed Jan 28, 2015 9:28 am

Hey Frank, your posts are so well thought out and clear, thanks for your thoughts, it all makes good sense. The photos show my version of a Jim Williams jig.

I made a board for the trimmer to sit in to clamp to the vice. I then used the edge attachment of the trimmer as a back plate and screwed the jig to that. Under my jig I screwed in two halves from a thick bracket I had in the shed as I found I needed a larger 'tray' to stabilise the base, while I focused on keeping the sides vertical while rebating. As I had a bearing on my rebating bit, I just needed to mount one above it and could adjust its position by sliding the edge attachment in and out. It worked well in the end.

Thanks everyone for your advice in the posts. It is extremely helpful to a newbie!
Attachments
binding jig 2.png
binding jig 1.png

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Re: Query on bending the back plate after tapering sides

Post by mooshalah » Wed Jan 28, 2015 4:10 pm

Hi yet again, Andrew.

I must say that I don't quite get your jig, but it's great that it worked for you. Of course, using an appropriately sized bearing on your rebating bit is an extremely important and valuable technique (that I completely forgot to mention when I last posted a reply!), although I didn't use one for a long time after I started to use my jig. I now find that using both a jig that lets the router rise and fall vertically, and a bearing on the router, while a belt-and-braces approach, makes the process into a very predictable "ho-hum; no problems here" affair.

Frank.

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Re: Query on bending the back plate after tapering sides

Post by kiwigeo » Wed Jan 28, 2015 4:27 pm

Nice simple solution there Andrew, just watch for any play and slopiness on settings on that Ozito router. I bought one of those a few years ago at Bunnings and the thing ended up in the trash can after 5 minutes use...the depth stop kept slipping and I ended up routing a rosette channel right through a spruce top.
Martin

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