Materials for jig making.

[Robert Smallwood]

Hi. are ply & MDF freely interchangeable or are there constraints for their use in making jigs?

Thanks,
Rob.

[Warren May]

It may depend to some extent on the quality of the plywood. I often use hard 1/4" masonite for templates that aren't going to be used with a router. MDF is just okay for router templates and it is easy to cut/sand but it seems more susceptible to damage when bumped. Cheaper than plywood so it's alright for one-off work, in my opinion. Finishing MDF with polyurethane seems to give me peace of mind but not sure if it really strengthens it. Harder to write on after finishing, though, so mark it up before applying poly. All plywood isn't created equal. A good quality 9-ply cabinet grade birch works well for me . You can fill the occasional void with plastic wood before you sand to the final shape. Plywood seems to take rough handling a little better than MDF.

[Barry Daniels]

MDF is very stable and will remain flat while plywood can warp. The face of MDF is hard and wears well but the edges are soft. If you need the edge to be hard apply several coats of CA glue. MDF also carves easily in case you need to make an arched form. The disadvantages of MDF are that it is heavy and the saw dust or sanding dust is a nuisance.

[Bob Francis]

MDF is very stable and will remain flat while plywood can warp. The face of MDF is hard and wears well but the edges are soft. If you need the edge to be hard apply several coats of CA glue. MDF also carves easily in case you need to make an arched form. The disadvantages of MDF are that it is heavy and the saw dust or sanding dust is a nuisance.

Soes it swell when CA is applied? Specifically thinking about a carved top mold. Would varnish be a better sealant?
Sorry to hijack.

[Bob Gramann]

I have had MDF warp when I cut out one face (like making a dish) and leave the other intact. I had to laminate it to another piece to keep it flat and even then, it pulled in a little. Cutting an outline around it is no problem. I used to use it to make my outside molds. But carving curves of an archtop top across it may give some stability issues. You may not end up with the shape you carved. I don't use MDF for anything but a workbench surface anymore. I just couldn't handle the dust when I cut it. I use many-ply plywood for my molds now. Three layers of 3/4" plywood stacked and glued stays pretty stable.

[Bob Matthews]

The face of MDF is hard and wears well but the edges are soft. If you need the edge to be hard apply several coats of CA glue.

Be careful when applying CA to MDF edges, it soaks up a vast amount very quickly and then proceeds to smoke as the CA cures. DAMHIKT
Once applied tho, it does produce a very hard edge.

[Mario Proulx]

For my outside form these, days, I use 1/2" spruce plywood(AKA spruce sheathing). It is lightweight, inexpensive and works. I use 2 or 3 layers, separated by spacers to build them up to the height I want/need, but with solid spacers at the waist to support the spreaders. Keeping my forms and jigs light in weight became a real goal when I blew my back 9 years ago....

My favorite template material is the stuff cheap flooring is made of. I want to call it laminate flooring, but it's not a laminate. But y'all know what I'm talking about. The stuff is HARD, stable, and of course, inexpensive. Free, most of the time, if you ask someone who's installing some to save you some off-cuts and scraps. Or just buy a box of whatever's on sale and you're good for a long while... It's hard on band saw blades, so use an older one.

[Clay Schaeffer]

Masonite (high density fiberboard) is just MDF on steroids , but I think it is a better choice for templates. For jigs I prefer multi-ply or baltic birch plywood. It is stronger and lighter than MDF. For "one off " stuff I use whatever scrap is at hand.

[David King]

Call me a snob but I can't stand MDF for anything. I'd much rather buy the occasional sheet of Euro Birch 11ply and make a template that won't dent or crumble when it hits the floor. If I were doing production then I'd swing for 6061-T6 sheet stock to add a few decades to the life. I actually saw up and reuse old templates all the time for new, smaller ones. I wouldn't be doing that with MDF...

[Gordon Bellerose]

I have taken a liking to plexiglass. It' s easy to cut and sand to shape, and is also light.
The other advantage is you can see through it.

[Dennis Duross]

I would agree that plexiglas OUGHT to be a good template material, but I have enough difficulty finding things in my shop. The last thing I need is templates made from invisible materials.

[Chris Paulick]

polycarbonate over plexi , machines and taps much better, just need to fine a plastics supplier for cost reasons. MDF just too heavy and the dust sucks when cutting, I pretty much just use it for radius dishes and malamine for some cabinates but preferr laminate plywood for that now. I've used some stuff called sand ply from Home Depot that was 5 ply and fairly cheap around $35 a sheet back when. I also like jig plate Alu. for jigs with no curved cuts. I've used masonite for stuff too. Sort of depends on what I'm making, what I have around and how much cash I have to spare. Scraps of maple, ash and walnut are nice for stuff too.

[Steve Graves]

Masonite and HDF are two different materials. All materials have their outstanding qualities and limitations. If a jig is worth making it is worth making out of the best material you have available for that purpose. I tend to like using baltic birch plywood and Nylon sheet goods. Wear and strength are my measures. Why try to make a jig that allows you to save time and perform repetitive tasks without using materials that will last long enough to pay for themselves?

[Louie Atienza]

I've been using primarily paper-based phenolic for my jigs, as it is rock hard, a bit lighter than aluminum but just as stiff, and can be cut reasonalby well with woodworking tools (though I usually use my CNC now for these tasks.) I also use quite a bit of mic-6 cast aluminum plate, which is ground flat and machines nicely. I work at a bowling alley, and every year the head panels on some lanes need to be changed; they're only 5/8" thick, but these panels are incredibly dense and I use it for a planer platen and other jigs around the shop. HDPE makes nice jigs where you need ease of sliding (like against a fence or bearing) and you can buy it easily at Sam's Club or other big box stores as chopping board. An 18 x 24 board is like $10.

For other jigs I use Baltic birch or other high-ply-count plywood (like ApplePly.) Imported Home Depot plywood has so many voids it's ridiculous. But if you go to their cut plywood section, 2 x 4 pieces, they're surprising Baltic birch-like, not like their full sheets.

Bob G., MDF warps when face-cut since its basically 'case-hardened,' the core is a lot softer than the outer surface. and will take on moisture at a different rate.

[Derek Dang]

I've been using primarily paper-based phenolic for my jigs, as it is rock hard, a bit lighter than aluminum but just as stiff, and can be cut reasonalby well with woodworking tools (though I usually use my CNC now for these tasks.) I also use quite a bit of mic-6 cast aluminum plate, which is ground flat and machines nicely. I work at a bowling alley, and every year the head panels on some lanes need to be changed; they're only 5/8" thick, but these panels are incredibly dense and I use it for a planer platen and other jigs around the shop. HDPE makes nice jigs where you need ease of sliding (like against a fence or bearing) and you can buy it easily at Sam's Club or other big box stores as chopping board. An 18 x 24 board is like $10.

For other jigs I use Baltic birch or other high-ply-count plywood (which I bought here https://sheetmaterialswholesale.co.uk/) Imported Home Depot plywood has so many voids it's ridiculous. But if you go to their cut plywood section, 2 x 4 pieces, they're surprising Baltic birch-like, not like their full sheets.

Bob G., MDF warps when face-cut since its basically 'case-hardened,' the core is a lot softer than the outer surface. and will take on moisture at a different rate.

Yes I like Baltic birch in use too!

[Alan Carruth]

One of the guitar tool suppliers uses transparent colored plexi for templates. The color and the 'glow' of the edges really helps locate them. He also color codes them: all the templates for one size are the same color, and others are different. The first time I saw that was a 'duh' moment.

Plexi and the ilk work reasonably well for some types of cutting guides. I like to use 1/4" stock for router templates, for example. You can also use plexi for drill guides for a limited number of uses. One nice thing about it is that you can simply lay the plexi stock over a drawing and scratch the line into the plastic. It's easy to see, and you can cut and sand right to the line.

Another good use for it is gluing guides, if you want to be sure you're getting the bracing down in the same place every time. Make slots in the plexi that are wider than the braces, with a few 'fingers' sticking in for locators. The plexi can be located on the center line of the top, say, and you just have to butter up the braces and drop them into the appropriate slots. WE used this when I worked in a shop that had a vacuum clamp setup, but it would also work well with a go-bar deck. You need the make the slots wide enough to clean up the glue, of course. The plexi should not stick to the usual wood glues, and you can always wax it.

Masonite router guides work OK, but you have to harden the edges.

+1 on Bob's shout out on the way the contour of an MDF or Masonite part will change when you mill off one face. I suspect that the rolling process introduces stress in the surface. I've had good luck with laminating a piece of 1/4" Masonite to the under surface of a dish form before routing it to shape; it pretty much keeps the edges from curling up, and seems to be more stable with humidity changes. I just used Titebond, and allowed it to dry for 24 hours before taking it out of my go-bar deck.