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 Post subject: Mold design
PostPosted: Tue May 17, 2016 10:53 pm 
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Koa
Koa

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I've been using the old carriage-bolt-and-nut system for holding my molds together for many years. Flipping through the LMI website recently, I noticed their new design, which basically has interlocking sides coupled with a piece of PVC. Very clever. Makes me feel stupid for not thinking of it myself!

So I'm building some for my latest builds. If anyone has used or made something like this: Is it prudent to thin the middle layer of the "sandwiched" side by .005" or so for the saking of getting it into the other side easily? or is it sufficient to soften the edges in that area and add a bit of paste wax?


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 Post subject: Re: Mold design
PostPosted: Wed May 18, 2016 8:49 am 
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Brazilian Rosewood
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I see pluses and minuses to both types of molds. The LMI mold looks pretty slick and lightweight (I'm not sure I'm sold on the stability of LDF).
For someone who always builds to the set dimension they would work fine.
I sometimes vary the width of the guitar bodies. The non interlocking mold halves allow me to put shims between the halves to change the width. Using tapered shims allows me to change the proportions of upper/ lower bouts. My molds are simpler and a bit more clumsy to use I'm sure.



These users thanked the author Clay S. for the post: Ken Jones (Fri Jun 03, 2016 2:26 am)
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 Post subject: Re: Mold design
PostPosted: Wed May 18, 2016 9:43 am 
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Brazilian Rosewood
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Like Clay, I have used shims between the mold halves to change the body shape on my mando/uke mold and it is a great solution. I recently made a new guitar mold that is interlocking and adjustable. I designed the lower bout curve so that it could fold into itself when the shape is hinged at the heel. This gives me 3 sizes (O, OO and OOO) from one mold. Since the pivot is at the heel, all three shapes have the same curves but different proportions (the lower bout shrinks but the upper bout and waist close up less). This was by design, the interlocking mold could easily be made to adjust at both ends equally.

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mold.jpg


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 Post subject: Re: Mold design
PostPosted: Wed May 18, 2016 9:48 am 
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Koa
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With all due respect to LMII -- I see the fact that these molds do not have defined halves -- AKA, an actual distinct center line as a bit of a design flaw. Plus the very same thing (hinged opening) can be accomplished and constructed easier using a regular large/wide pinned door hinge at both ends. My $.02

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 Post subject: Re: Mold design
PostPosted: Wed May 18, 2016 10:29 am 
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Koa
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I'm not sure that a pinned, hinged joint that lacks adjustability is a positive, or that lacks the easy centerline alignment that Mr. Cierpilowski mentioned. When I fit sides in the mold, having a fine adjustment and defined centerline to get the right fit in the waist and trim for a tight fit elsewhere is helpful.

From what I have seen here with 7-8 guitars moving through assembly at any given time (3-4 plus student guitars in molds), being able to clamp the mold into a repair vise for access to all sides for linings, side tapes, and cleanup, the ability to clamp tail block and neck block to the mold without interference from a toggle clamp, and the ability to place assembled rim in mold into radius dishes to profile the sides are all very useful capabilities. In addition, the ability to hang the mold with rim installed keeps things off the shop floor and out of harm's way. Being able to directly clamp the waist into the mold with little or no flex is another desired quality. Finally, any spreader system seems to me to be less useful if it cannot be left in the guitar during the process of the box being closed and removed through the soundhole.

The LMI mold and some of their other fixtures were discussed here a few weeks back, and the concerns were with weight versus stiffness of low density MDF, the lack of an adjustable closure, the size and lack of flexibility of the spreader system, lack of reference centerline, and adjustment for wear in the pinned joints.

It looks to me like, if that pinned joint is desirable, it does not have to quite so intentionally cute - Mr. Bear's approach is an interesting one where a builder might like to adjust body width on some body shapes. I would miss the easy centerline and alignment of the dod-eared molds we use, but for something like a Taylor dreadnought shape with flat areas at neck and tail, it would allow some flexibility.

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 Post subject: Re: Mold design
PostPosted: Wed May 18, 2016 10:58 am 
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Brazilian Rosewood
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I'm not understanding the lack of center line issue with the hinged mold. Likely, this means there is some mold related technique I am missing out on so I would like to hear more and refine my practice. . . It looks to me like the LMI mold has a center line marked on the mold. As does mine (3 center lines in fact depending on the size it is locked into. When using my mold with shims between the halves, the center line is either where the two halves meet or marked on the shim itself. Is there some reason a marked line is inadequate. You all have me second guessing myself as if I am missing something that would make my life easier (though building a rim in a mold is pretty easy as it is).

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 Post subject: Re: Mold design
PostPosted: Wed May 18, 2016 1:28 pm 
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Koa
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Thanks for the insights.

* The molds do have a clear centerline. You may have to blow up the LMI picture to see it. It bisects the hole in which the pin sits.

* I don't know whether LDF is a negative, but I'm making mine out of MDF, so that's not an issue for me.

* I can understand why some might prefer adjustable width. Or adjustable heighth, depth, or any other dimension. But I'm fine with a fixed size mold. Having a CNC, it's not much work to make another one stretching out the same CAD design if I want a wider or narrower body.

Which brings me back to my original question about thinning the sandwiched piece. I went ahead and didn't thin it, and it seems to work just fine. By the time I add some paste wax, it will be pretty slick.


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 Post subject: Re: Mold design
PostPosted: Wed May 18, 2016 1:32 pm 
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Brazilian Rosewood
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Oops, I meant to comment on the thinning of the tabs. I didn't think mine even though they are made from fairly rough plywood. I had no trouble as you have already experienced.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.


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 Post subject: Re: Mold design
PostPosted: Thu May 19, 2016 8:11 am 
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Koa
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Just an additional note on low density and medium density fiberboard for molds...we changed our mold rim width from 2" to 2.5" and added a fillet through the waist area to 3" width on our newest molds. As we adjust body shapes or need new molds, we are replacing the older molds with these stiffer fixtures. We're also replacing the few sande plywood molds with birch, both for greater bending resistance and better durability.

From the updated appendix on mold-building in my building notes, this is an engineer's take on materials choices for molds:

Quote:
I recommend against using medium or light density fiberboard for molds, primarily due to the increased weight of these materials for the same stiffness. While this additional weight can help with profiling the rim, it really is a pain the rest of the time. While at first glance, it might seem that LDF at 38 lbs/ft^3 is less dense than birch plywood at 44 lbs/ft^3, and MDF is only a little denser than plywood at 48 lbs/ft^3, neither material is particularly stiff, so you need more of it to make a mold that is as stiff as one made with plywood.

MDF is about half as stiff as hardwood plywood - modulus of elasticity in the relevant direction is about 580,000 psi versus birch plywood's 1,100,000 - 1,300,000 psi, and LDF is about 1/3 to 1/4 as stiff as hardwood plywood in bending, so the rim width of molds needs to be increased to maintain the same level of distortion under load. For our nominal 2.5" rim width mold design in birch plywood, an increase in rim width to 3.25" is needed in MDF and 3.75" to 4" in LDF to maintain identical stiffness. In both cases, mold weight roughly doubles when using either MDF or LDF, so a dreadnought mold that weighs about 7 to 8 lbs with hardware should be expected to come in at 14 to 16 lbs in either the LDF or MDF versions.

The other reason why I avoid MDF where possible is that it is a dusty, abrasive material that is just not much fun to work with. Even with a good dust collection system, the fine dust from MDF gets into everything and leaves a slick, hard to remove coating on floors, tools, etc. If you do use MDF or LDF for tooling, fixtures, and dishes, try to do the work outside in a breeze...you may still end up covered in fine dust, but at least the shop will be spared.

There are advantages to MDF and LDF - it is more dimensionally stable than plywood and costs about 1/3 to 1/2 of what big box store hardwood plywood runs. While the stability aspect sounds attractive, we're talking a couple thousandths of an inch less movement with RH changes through the thickness of the sheet stock, so not really applicable to our application. And given that a single sheet of 4' x 8' x 3/4" material will build two molds and a bending form with a little prior planning, I'll spend the $15 per guitar shape to save the wear and tear on my body that comes with heavier shop fixtures. Your decision making process may be different, but you now have the information to make an informed choice on fixture materials.

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Watch what people are cynical about, and one can often discover what they lack.
– General George S. Patton Jr.



These users thanked the author Woodie G for the post: Pmaj7 (Thu May 19, 2016 10:02 am)
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 Post subject: Re: Mold design
PostPosted: Fri Jun 03, 2016 2:05 am 
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Koa
Koa

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The MDF seems to be working well. I'm only making a few guitars, so I'm not too worried about durability. But they seem plenty strong enough.

I am also extremely happy with the LMI-inspired design. It's working very well.

The CNC has, once again, made it very straightforward to experiment with a new design. These guitars will be a lot of fun.

Thanks to everyone for their insights.


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 Post subject: Re: Mold design
PostPosted: Sat Jun 18, 2016 7:31 pm 
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Cocobolo
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[quote="Bryan Bear"]Like Clay, I have used shims between the mold halves to change the body shape on my mando/uke mold and it is a great solution. I recently made a new guitar mold that is interlocking and adjustable. I designed the lower bout curve so that it could fold into itself when the shape is hinged at the heel. This gives me 3 sizes (O, OO and OOO) from one mold. Since the pivot is at the heel, all three shapes have the same curves but different proportions (the lower bout shrinks but the upper bout and waist close up less). This was by design, the interlocking mold could easily be made to adjust at both ends equally.
Attachment:
bryan's mold.jpg


Bryan
I am planning a new mold at the moment and, forgive me, I intend to totally steal this idea. Any tips for the construction of this? I imagine that you glued up 6 layers of ply sheeting? I can see two additional holes at the tail end so I assume you insert the pin to anchor it at one of three different levels of overlap, for 3 different lower bout sizes. Like you say, a similar expand-ability could be built in at the neck end. It is a very neat idea! I think I will have a removable cutaway insert as well. Is there anything you would do differently from your first model if you were doing it again?
Thanks
Mark


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These users thanked the author Mark Mc for the post: Bri (Sat Jun 18, 2016 8:54 pm)
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 Post subject: Re: Mold design
PostPosted: Sat Jun 18, 2016 8:49 pm 
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Brazilian Rosewood
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Steal away, that's why we post stuff. Making it pivot at the heel end takes a lot of planning. I had to work out a shape that would allow the tail curve to fold into itself nicely. Also the heel block end gets a little pinched in the smaller sizes so you have to be willing to work with that limitation. Both of those problems go away if you are just adding equal amounts of width to heel and tail. Yes 6 plys of 11/32" plywood. Remember that they need to be much longer than just half the outline. You need 12 of them and have to work out the right amount to cut each of them short (alternating which end you cut short). Remember ro make alignment holes in the sides so they all glue up flush. . .

You will also need to measure for all the center lines which can get confusing when they start overlapping. I wish I would have planned my cutaway inserts before I made it. I would have left a gap in a couple layers for the inserts to slide in and lock with dowels. You will, of course need a different cutaway insert for each size because the fretboard edge will be in different places for different widths.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.


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 Post subject: Re: Mold design
PostPosted: Sun Jun 19, 2016 12:42 am 
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Cocobolo
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First name: Mark
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Thanks Bryan. All of that makes sense. Good point about needing 3 different cutaway inserts. I hadn't thought it through that far yet, but it is obvious now that you mention it. I will see if I can come up with a nifty solution for that design issue.
Best wishes
M



These users thanked the author Mark Mc for the post: Bryan Bear (Sun Jun 19, 2016 8:02 pm)
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