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Making outside mold...
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=50375		 Page 1 of 1
Author:  bcombs510 [ Fri Mar 02, 2018 1:26 pm ]
Post subject:  Making outside mold...
Hello,

The last time I made a mold myself I used the acrylic template to draw half the body shape onto a board, cut it close with a bandsaw, then sanded to the line with an oscillating sander. It worked out OK. I then used that as a template to make 3 more layer using a pattern bit.

I'm curious though, is there a way to incorporate the router into the equation to get a more precise shape on the first board? I can use a pattern bit to make the cauls for the inside of the mold easy enough, by using the acrylic template as the pattern, but then they don't mate super cleanly with the outside mold portion. Maybe I need to get better at controlling the sanding portion of the process. ;)

I hope that makes sense. I'm trying to have a nice fit the full side of the body, like the LMI molds work, but without access to a CNC. :)

Brad
Author:  Colin North [ Fri Mar 02, 2018 2:08 pm ]
Post subject:  Re: Making outside mold...
LMI molds have the inside spreader/caul, so I assume(?) you are looking to mate this to the outside mold?
I've just made a new mold for my own shape, and a wanted a similar inside spreader/caul to fit.
Getting a perfect fit is problematic without CNC of course
Part of the problem is also the thickness of the sides, which have to fit between inside and outside mold/cauls, and may sometimes vary between woods, but mine average about 2mm.
So what I've done is to run a line (5mm washer with a pencil) round the inside parts (with outside mold assembled, both side of inside caul wedged in centrally)) and then sanded them to that line.
I'm going to fit 3mm cork strips 2 cm wide vertically, spaced round the inside spreader. thus allowing 2mm for the thickness of the sides, and any small variatins should be taken up by the spring in the cork.
Weather's held the cork delivery up, so waiting to see how it goes.
Author:  dofthesea [ Fri Mar 02, 2018 2:34 pm ]
Post subject:  Re: Making outside mold...
you could use a biting bit in a router. Make the first cut with a pattern bit, then make the second cut with a rabbiting but. the go back to your pattern bit and slowly take it down. Be sure and take out small amounts each to insure accuracy. Most people have problems with patterns and templates because they try to take out too much material with one pass. FYI MLCS has the best prices on router bits.
Author:  stumblin [ Fri Mar 02, 2018 4:42 pm ]
Post subject:  Re: Making outside mold...
As Colin said, the thickness of the sides needs to be taken into account. I`ve found this also helps when making the bending form. I made my last mold, bending form and spreaders at the same time. I first cut the boards roughly to shape on the bandsaw and made the mold in the usual way. Then with the original template as the pattern I used an old binding rebate cutter to cut the first board for the form 2mm undersize. This gave me a rebate almost the full thickness of the board and I used a pattern/flush bit riding in the rebate to remove the remainder.
This served as the pattern for both the bending form and the spreaders.
Author:  Bryan Bear [ Fri Mar 02, 2018 5:33 pm ]
Post subject:  Re: Making outside mold...
If you want your cauls and/or bending form to be exactly x amount smaller than the template to allow for sides make a second template by routing with a smaller bearing like you would for cutting the binding channel. Then you pattern rout your parts from this new template.

I do things a bit differently than probably most and this might not pleas everyone. When I make a new shape I make an outside template instead of a template of the body shape. I use this template to make my mold. I then make my bending form (if I'm bothering to make one) with the offcuts and am not super careful about getting them perfect. They need to be about side, blanket and slats thickness smaller than the mold but in reality, minor imperfections go away when you put them in the mold and put the spreaders in. Full sized length spreaders are probably less forgiving of minor variations. I use a waist caul and one in the widest part of each bout and I only have one set for all shapes. The waist caul insures the waist is located well and the bout spreaders push on the bouts allowing the curves to fair themselves in.
Author:  wbergman [ Fri Mar 02, 2018 6:56 pm ]
Post subject:  Re: Making outside mold...
My two cents is that the caul will never be exact. So, I try to allow room for a layer of compressible material, such as cork or rubber--doesn't really matter what. When I build up layers for a mold, I use a router cut off bit (is that the correct term?). My hand is not so steady to avoid tipping the router, so I built a little router/shaper table using a laminate trimmer, just because that is what I have. The router is stationary and I slide the mold around on the table.
Author:  Alex Kleon [ Fri Mar 02, 2018 7:08 pm ]
Post subject:  Re: Making outside mold...
I made this for 2, 3, and 5mm offsets. Works good for side thickness allowance when making an outside mold.Image

Alex


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Author:  bcombs510 [ Fri Mar 02, 2018 8:40 pm ]
Post subject:  Re: Making outside mold...
Thanks folks, I’m going to try to tackle it this weekend. I’ll post up some pics of the results.


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Author:  Woodie G [ Sun Mar 04, 2018 7:45 am ]
Post subject:  Re: Making outside mold...
Most hardware stores and big box stores will have nylon spacers - we've found that a variety of wall thicknesses are available, and use the 0.100" to transfer the shape of the mold's master ply to the form stock. With 0.085" side thickness and 0.020" slat/foil/paper added, the offset due to the spacer wall thickness and 0.5mm pencil lead is very close to the 0.105 target offset. We have 12" disk and a large oscillating spindle sander in the shop to shape the master plies of the outside mold and bending form, but working these in 1/2" MDF with a drum sander in the drill press is how one of the former students handles the job without a stationary tool-rich shop...after the masters are done, the 3/4" center plies are cut with the flush trimming bit and the mold and forms are built up (two outer rough plies are glued and nailed to the center ply and trimmed in place on the router table.
Author:  gregorio [ Sun Mar 04, 2018 8:58 am ]
Post subject:  Re: Making outside mold...
On this topic, as someone who potentially has access to a nice big cnc machine, are there drawings (cad) to be had or purchased out there for moulds?
Author:  rlrhett [ Sun Mar 04, 2018 8:22 pm ]
Post subject:  Re: Making outside mold...
gregorio wrote:
On this topic, as someone who potentially has access to a nice big cnc machine, are there drawings (cad) to be had or purchased out there for moulds?


Georgia Luthier Supply sells their guitar plans as DFX files, I believe. It would be a trivial matter to take a DFX outline and create a mold from it.
Author:  Colin North [ Tue Mar 06, 2018 9:42 am ]
Post subject:  Re: Making outside mold...
Sorted.
Used an orphan side re-bent to my shape, 2mm thick, cut into 2 x 50mm wide pieces.
Tested gaps by loose fitting the cork pads first, and had to use 3mm and 4mm sections of cork as required, then rasped/sanded thicknesses to touch "where it fits".
Author:  Colin North [ Mon Mar 12, 2018 2:15 pm ]
Post subject:  Re: Making outside mold...
Actually, I prefer rubber. Cork was awkward to size and not really compressible enough.
Used some old foam play-matting, thicknessed to just under 6mm.
Author:  SteveT [ Mon Mar 12, 2018 2:57 pm ]
Post subject:  Re: Making outside mold...
How do you get that caul through the soundhole? idunno
Author:  Colin North [ Mon Mar 12, 2018 3:00 pm ]
Post subject:  Re: Making outside mold...
Pull really hard.
:shock:
Ok, it's fitted to hold the sides in place for fitting neck block, tail block and linings, then for sanding the rims to radius.
I use a different "system" to secure the sides while fitting top and back, which is removeable though the soundhole.
Author:  Colin North [ Mon Mar 12, 2018 4:52 pm ]
Post subject:  Re: Making outside mold...
Dang, should have said it's inflatable...
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