Problems with Bending Slats Cupping & CA Side Bending

What're You Doing?
Post Reply
PSmill
Posts: 111
Joined: Sun Sep 09, 2018 12:23 pm
Location: British Columbia, Canada

Problems with Bending Slats Cupping & CA Side Bending

Post by PSmill »

Hi, Just about to make my first attempt at bending a cut-away side. I thought I would be smarter this time and ask questions before I try, rather than after :) I have have some problems with the bending slats cupping (see photo) while working the upper bout bend. Once it cups it becomes ridid and I have to back-off and try again, sometimes loosening the waist ram helps. Putting hand pressure downwards at the end of sandwich helps, as once the proper bend has begun it won't cup. I am wondering about good solutions for this.....I've considered adding a 3rd form in the middle, or placing a shim on top of the aluminium cross rods to add support in the middle of the slat. Thoughts?

While doing a dry run of this bend, I see while pushing the cut-away ram in the sides start to push against the hard 90 degree corner at the end of the form (see photo). I'm considered filing this into a rounded prfofile to avoid the sharp pressure point. Or maybe I have the ram at the wrong angle? Also trying using the spare shoe to lift up on the end of the sides and then using some bungees to hold that in place (see photo).

As always, any advice is invaluable, thanks! P
You do not have the required permissions to view the files attached to this post.
carld05
Posts: 170
Joined: Mon Mar 19, 2012 4:19 pm
Location: Forest Ranch, CA

Re: Problems with Bending Slats Cupping & CA Side Bending

Post by carld05 »

On the molds I make, the cross supports are flush with the outside edges of the mold. I've been using closet pole cutoffs and sand flat any high ones to be flush with the edges. Haven't had any issues with sides cupping. Haven't tried to do any cut-aways yet though.
Stray Feathers
Posts: 677
Joined: Sun Sep 08, 2013 11:39 pm
Location: Ladysmith, BC

Re: Problems with Bending Slats Cupping & CA Side Bending

Post by Stray Feathers »

I only recently made a side bender so am new at this. Like carld05 my moulds have wooden crossbars flush with the sides. I have wondered why many use metal bars, whether it had something to do with heat inside the earlier Fox-style benders with high-wattage light bulbs inside possibly affecting glue in the forms? I had some issues with cupping on a set of walnut guitar sides (which sanded out fairly well) and some cherry ukulele sides (which were too deep to sand out). So I built up the ukulele form to be almost solid all the way across. I still had a little cupping on the only set I did after that, but an improvement. Will keep an eye on it on future projects. I am tempted to rebuild my only present guitar form, and future ones, to be solid. There is a thread describing the ukulele modification I made to the Fox bender (below). Originally the uke form had two sides and a centre piece; imagine it now with four more layers of plywood sandwiched in the form to make it almost solid (don't have a current photo and it is buried . . . .)
http://www.kitguitarsforum.com/board/vi ... f=4&t=9186
tippie53
Posts: 7011
Joined: Fri Mar 03, 2006 7:09 pm
Location: Hegins, Pa
Contact:

Re: Problems with Bending Slats Cupping & CA Side Bending

Post by tippie53 »

the best thing to do is to take a 2 by 4 cut it to support that slat as you bend the cupping is going tp flatten out once you glue that end on the block.
you will be find. Let a little more slat over hang .
Also clamp that end flat overnight it works back pretty well.
feel free to call me I am always able to help
jh
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com
Diane Kauffmds
Posts: 3246
Joined: Sun Mar 09, 2014 8:13 pm

Re: Problems with Bending Slats Cupping & CA Side Bending

Post by Diane Kauffmds »

How are you thinning your sides? Do you use a thickness sander, or are you thinning by hand, sanding and/or planing? I had trouble with sides cupping when I thinned them by hand. Since getting a thickness sander, I no longer have the problem.
Diane Kauffmann
Country Roads Guitars
countryroadsguitars@gmail.com
PSmill
Posts: 111
Joined: Sun Sep 09, 2018 12:23 pm
Location: British Columbia, Canada

Re: Problems with Bending Slats Cupping & CA Side Bending

Post by PSmill »

Thanks everyone. Diane the problem was with the metal slats not the sides. I came up with this idea yesterday and managed to successfully bend the cutaway side last night. I simple clamp made of 2 pieces of scrap 1x3 with bolts and wing nuts, I slid this over the bending sandwich, positioned it near the cutaway ram and tightened it so it was tight enough to keep the slats from cupping, but loose enough I could slide it down as the ram came down. Seemed to work :) My side came out one piece although the cutaway bend is not quite as deep as it should be, about 3/8 shy, not sure why as the ram seemed to have it flush to the form. I'll take it! Tnx.
You do not have the required permissions to view the files attached to this post.
Stray Feathers
Posts: 677
Joined: Sun Sep 08, 2013 11:39 pm
Location: Ladysmith, BC

Re: Problems with Bending Slats Cupping & CA Side Bending

Post by Stray Feathers »

Diane Kauffmds wrote:How are you thinning your sides? Do you use a thickness sander, or are you thinning by hand, sanding and/or planing? I had trouble with sides cupping when I thinned them by hand. Since getting a thickness sander, I no longer have the problem.
Diane - I have only used a thickness sander and have had varying results. I started ( more than a little blindly) with a bending iron, had difficulties getting things bent, but had no cupping. When I invested in a heating blanket and slats, I found the bending much better, but I started to see some cupping. I think the wood also may play a role, but I have done too few instruments to be able to come up with any solid clues yet. Does anyone have any thoughts on cupping being caused more or less by edge grain versus face grain woods? Species? Walnut and cherry (sometimes) have cupped on me, rosewood, no cupping. Water versus Windex versus dry? Open versus solid forms? Temperature or time in the bender? (I have followed John Hall's guidance pretty carefully). Bruce W.

Edit: Here is a cherry ukulele side, thickness sanded, bent with blanket and slats on a more open form with a bit of Windex. If I sanded this out there would not be much side left in places. I ordered new cherry side sets and bent one set, same method except using a more solid version of the form, with better results but still a little cupping.
You do not have the required permissions to view the files attached to this post.
phavriluk
Posts: 554
Joined: Mon Jul 09, 2012 9:49 pm

Re: Problems with Bending Slats Cupping & CA Side Bending

Post by phavriluk »

I'm all ears on this one. My first two sets of sides bent with the Fox bender cupped a lot.
peter havriluk
Diane Kauffmds
Posts: 3246
Joined: Sun Mar 09, 2014 8:13 pm

Re: Problems with Bending Slats Cupping & CA Side Bending

Post by Diane Kauffmds »

Other than uneven thinning, I've found wood will cup if it's too wet when it's bent. When i first started bending, I made the wood entirely too wet and it cupped every darn time.

To add insult to injury, my wood wasn't thinned evenly too.

I've had straight and funky grain cup on me. The majority of my guitars
are built from figured and/or woods with crazy grain. It might be a contributing factor, I don't know. But, I'm positive that uneven sanding and/or wet wood, caused my cupping problems. After correcting these two things the cupping went away.

As far as wood species, I've not found a correlation between cupping and specific wood species.
Diane Kauffmann
Country Roads Guitars
countryroadsguitars@gmail.com
Post Reply