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Just about to jig up making a load of moulds, bending forms etc as I know have access to so CNC equipment and want to know what materials people prefer I was thinking 3 or 4 layers of 18mm MDF or Plywood any preference? My early moulds were made of anything I could find chipboard mainly, but want these to last.

Thanks

John

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Formerly JJH

I learn more from my mistakes than my successes

My experience is that MDF doesn't work so use plywood. Baltic birch is my choice while MDF machines nicely it tends to flex.

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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

Baltic birch works very nicely. It's a pretty high quality plywood. Don't know about plastics, and aluminum might be pretty heavy.

I've used epoxy sealed MDF for twenty years without an issue. ALL molds will flex, including "baltic birch" which I put in quotes because what folks are selling as baltic birch ain't your mother's baltic birch. I've gotten it from suppliers large and small and most often its no more stable than the birch faced plywood at the home centers. If you over tighten your spreaders all molds (other than metal) will flex.

The OLF sponsor LMII uses "light" density fiberboard for their molds. So I would assume that there is no doubt that "Medium" density fiberboard (MDF) will work just fine, perhaps an overkill?

While there is no doubt that even the the Russian or Chinese sourced "baltic birch" is stronger than mdf, I would question my approach and/or the precision of my side bending if I need the strength. Exerting a lot of force with my spreaders to keep my rims where I want them probably means I need to tweak my sides for a better fit. That said in our furniture/cabinet shop we use Jaycore and Kaycore exclusively and I have enough drop for more molds than I'll ever need. MDF makes great utility work table surfaces and thats about the only thing I use it for. Chipboard is worthless IMO.

Any economical plywood would be my choice.

Tim

I just wouldn't worry. Use whatever sheet material you can get easily and cheaply.

I've used the lot over the years: ply, MDF, chipboard, and OSB. They all dull your cutters, but they are all strong enough and stable enough. No issues with any of them.

That said, two years ago I had all my main model moulds remade by CNC in 50mm deep MDF (yes, you can buy 50mm MDF) glad to say I didn't have to handle the boards, but they are great. Best moulds I've ever had.

Mainly because I didn't have the hassle of making them!

Nigel

http://www.theluthierblog.com/articles/
http://www.nkforsterguitars.com/luthier-book/

I prefer to make my molds and bending forms out of hardwood plywood. It seems to have the best combination of weight, strength and toughness properties. If the CNC router uses a small diameter cutter (1/4 inch) you could use the off fall from the mold to make the bending form for the same model. That would save a little time and material.

You guys are the experts here. I've only had one experience but I'll chime in anyway. I made a drum sander using 3/4" plywood from Home Depot also 2x4's. After finishing it And were I to build it again. I would use the red oak and MDF. The plywood isn't square won't lay flat(bowed up etc). I made a mold from MDF am perfectly satisfied. If you could get a better grade of plywood that would be my choice though.


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Inexpensive birch ply for me. MDF will swell a bit with time and and it's pretty messy to work with.

I use MDF too...

Baltic birch is a lot more stable and will flex a lot less than MDF. However MDF and Ply sandwich will also work. If you use spreaders plywood would be better

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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

laminations of baltic or pine for me -depends upon the balance on my credit card, and what's on the shelf at lowe's.

i hate mdf

If you have a CNC that can cut all of the laminated mold in one operation, that is good. However, if you plan to cut the layers individually and then glue them up, that is not good. It is not that easy to get them to register. If you cannot cut it already glued up, then you might be better off using a shaper table to cut each successive layer after it is glued. Some people use a hand held router, but I am not steady enough to do that.

If using MDF (plywood, too, I guess), after it is all done, the surface must be sealed. One post above uses epoxy. I am too allergic to epoxy to use it. I use multiple layers of "solvent" based polyurethane Do not use water based sealer, because it will swell the grain. It seems minor, but the surface will not be smooth and therefore more difficult to clean.

i've many molds this way. the way to do it is to use pins. each layer gets two holes drilled in it, then the layers are sandwiched together using the pins to locate each lamination. pound in some dowels/round stock into each hole completely through the stack and you're done.

at your cnc machine, clamp up to do the hole operation, then use the holes to clamp up for the contouring operation. remove outer clamps and cut away.

Construction ply, 18mm here. I have used MDF, but hate the dust worse than ply.
The ply seems better for the flush router cutters I've been using too.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

But don't rely only on the pins! They'll make things a million times easier but still check that it's all square before clamping. I made this mistake only last week! Wish I had a bandsaw!

So far I've use baltic birch ply, and I cut it with my scroll saw (16 inch throat). It's a little slow, but really not too bad. With the scroll saw blade you can get a very thin kerf line so that the cut offs of the mould can be used for the bender form.

quite. perpendicularity is key.

Yes, a bandsaw sure is pretty handy.
Even a smallish one as long as it can be set up properly.
I don't resaw, so mine is only Max Depth Of Cut 120mm, Max Width Of Cut 245mm, but I use it for almost everything I do.
I had to fit new tyres recently and was almost completely stuck without it for 3 weeks until they came back into stock - just as well I had a guitar to finish, or I would have been hell to live with.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I think the one you have is £250 at Axminster now. I'd be tempted to save another coupleahundred just so I could resaw backs and tops occasionally. Still, I'd love to have one of any size! So much extra work using a coping saw!

And it really can be a hassle! Now I've made one (well, two including the one I messed up), and I know I can do it, I think I'll be farming it out in the future!

I love this debate! Over the past 12 years I have built a couple of hundred radius dishes out of laminated MDF. They are in place all over the world, including in my shop. They don't swell and move anymore than any other wood. I mean really? How many are making instruments in environments with huge humidity swings? At the end of the day, use the material you most comfortable or confident with. I have been woodworking professionally for 35 years and have not issues with any of these sheet materials when used within the scope of their design. I personally don't like plywood because of the huge variation in density and integrity between the layers, but that is just my bias for certain aspects of mould making. I do use baltic and standard birch for cabinet making though.

Shane

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Canada

Someone made a mold for me using CNC for individual layers and then pinning them together. They did not register. If you were to clamp against the mold, you would not have good results. I wanted to use them in a vacuum bag to laminate sides and linnings, which just would not have worked.

For me, the dust issue with MDF is greater than the flex issue. You can sandwich it with ply to stiffen it. Or....just use ply. Anything will flex--some materials more than others. Seems to me that MDF is a bit harder to seal, too. I've read that glue-sizing it will seal it against "striking in" of other finishing materials, but have not tried this. Finally, it seems to me that MDF is heavier than most of the plies I've used. I've used a lot of baltic birch (the older stuff) for other projects, but I've never made a mold from it, so I can't comment on that.