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Hello all, I've been lurking for a while gathering information (and courage) to start my frist build. I am trying an OM style guitar losely following Jonathan Kinkead's book. I built my mold today out of 3/4" mdf. I made an acrylic template by gluing a copy of the plan to the acrylic, cut it out on the bandsaw and sanded to the line on the spindle sander. Then this was outlined on to the mdf and cut out, sanded, and this piece was used with a pattern bit in my router to make 5 more just like it. Pretty common way of doing it from what I've read. After I got it glued up I took some measurements and I'm off a bit. The length is suppsed to be 19 3/8" and I'm off 1/8" at 19 1/2". My question is: is their an easy fix to make up the 1/8"? Should I start over with better measurements or suck it up and purchase one? I've since read about the problems with "tracing" from the plans with variations and all.
Probably the first of many questions im sure so any help would be appreciated!

Jeff

If your length is off 1/8" that's 1/16th at each end. If the mold came out well and looks good I wouldn't sweat it. I reference the important stuff like soundhole center, bridge plate placement, and X brace center from the north end of the guitar so it's all relative. I assume your acrylic template and bending form are the same size as your mold. There'll be a few more cc's of air inside the guitar. That could be a good thing!

Terry

_________________
It's not what you don't know that hurts you, it's what you do know that's wrong.

What Terry said.

As long as both sides of the mold are the same (or close) and the curves are smooth, I wouldn't sweat small errors.
As Terry mentioned, just measure things from the neck/body join line to make sure you don't get the soundhole and fingerboard end misaligned.


Cheers
John

Thanks a lot guys. I haven't made my bending form yet (thats next) but the template and the mold are the same size. Other than the length difference, it came out pretty good. I think what I was afraid of was positioning on down the road with what you mentioned about the sound hole and bridge placement etc. but it didn't click that they would all be measured from the same point up north anyways. Whew! I can get some sleep now....

Jeff

Jeff-
Recently I've started bending sides the way it's shown in this video:
http://www.youtube.com/watch?v=gDQs4rnLbZU
(I don't have the cigarette hanging from my lip, but it works anyway...)
I don't have a 'temperature control with thermocouple, just a 1000W dimmer control.
If you have a heating blanket already, it can save the trouble of making a separate bending form. I just (temporarily) screw the two halves of the mold together to give a nice wide form to bend the sides into.
(I bandsaw my molds out of a solid glue-up, so I already have the 'male' part of the bending form roughed out.)

If you haven't already gotten the rest of your Fox-style bender together, it might be worth considering.
(My 'bending machine' is collecting dust under the bench.)
Cheers
John

Thanks again everyone for all the help. This seems to be a great community of luthiers and a wealth of information.
John, thank you for posting the video explaining the alternate bending technique. I don't have anything for bending yet but my plan was to put together a fox or doolin style bending machine. I think that I will give this method a shot. Do you wrap your sides in the foil and place the blanket on top as the author does or do you use any other barriers? Either way it seems like a great way to do it.

Jeff-
I wrap in foil as in the video- I was doing this before, with the bending machine as well. I also wet the sides well- this is for woods like Indian rosewood which are easy to bend and I find bend much better if well wetted. (Very curly woods can tend to 'fall apart' with water...)

I tried just laying the heating blanket on the wood and that works OK. I didn't use the strips of foil to tie together the sandwich and blanket, which I should have done (I was going from memory of the video). The last couple of sets I've bent (IRW and YellowCedar, in the last week) I've laid a piece of wood on top of the heating blanket (to prevent sagging and separation) and clamped the whole sandwich together with a few spring clamps. Then when the underside of the wood/foil feels hot, I remove the wood/clamps and gently press the sides/foil into the form with heavy gloves as in the video. I usually have a small block of wood handy to use in pressing if things get hot through the gloves.

As far as other barriers/additions to the sandwich- I had one set of IRW sides with a knot (these were low-grade sides) and I was concerned there might be cupping at the knot (there was, in the 1st side from the set- corrected on the iron later by hand). So I added an aluminum flashing 'slat' secured by bending around the ends of the side before it went into the foil. It helped to keep things flat.

I use the blanket at 100% till things are bent and the male 'keeper' is clamped in place, then turn down to about 70% of the 'slider distance' on my 1000W light dimmer for another 5-7 min. Then out of the form, foil off, and back in for about 10 min of heating- a minute at 100% to get things heated up and then the rest at about 50% on the slider. If I'm in a hurry, the side comes out and goes into a simple 'holder' (3 pc 2x2 end-screwed to a scrap of plywood) to keep the shape. Otherwise, I just leave the side in the mold for a while to cool.

The thing I like about this (in the video) method is that it keeps a lot of the 'feel' of hand bending (you are not just cranking blindly with a press screw or clamp) with the benefits of the mold.

BTW- the method Mario Proulx shows with his 'simplified bending machine' also keeps the 'hand feel' to the process as the waist caul is pressed onto place by hand after hand-bending the sides/slat around the form.

When you make the 'male' part of the form, remember to leave allowance for the thickness of the blanket and sides - somewhere in the 3/16-1/4" range.

Cheers
John

You should also check out the videos Todd Stock has on YouTube and John Hall has on his Blues Creek Guitar website. A conventional bender with foil covered blue tempered shim stock for slats and wet brown wrapping paper on either side of the wood is an extremely reliable method. I use two blankets, one on top and the other underneath the sandwich but that's probably overkill. Having the patience to leave the side in the bender overnight is a good thing. there's always something else to do while you are waiting.

_________________
It's not what you don't know that hurts you, it's what you do know that's wrong.

Mario Proulx's simplified bending machine is shown in the video at
http://www.youtube.com/watch?v=sUJbB0pa7Zs

and discussed at
http://www.mimf.com/cgi-bin/WebX?128@1.EMXPaG03Fvm.20@.2cb6cf19

Thanks, Mario!

Bending wood against the heat blanket will raise the grain in the wood in a pattern of the heating elements. Also too much water can cause warping if the wood isn't well quartered. There are many techniques out there for bending wood , find the one you like. I can tell you that too much water is often worse than too little.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Thanks for all the info John.

There are (at least) two different styles of heating blankets being sold- I have one of each. One blanket (from LMII??) is smooth and the other has raised heating element wires 'above' the blanket surface.
Though I personally haven't found any grain-raising problems with either type of blanket, I much prefer the smooth blanket in practice. My guess is that the 'rough' blanket style would be more prone to problems with uneven heating.

To my mind, 'the jury is still out' on the wetting wood before bending issue. Most of the advice you will read from 30 yrs ago advocates getting the wood quite wet before bending, and many folks still do that. However, nowadays people are using all sorts of woods (and quality of wood) that builders would not have considered in the past, so probably new bending techniques are required.

Cheers
John