My MDF Questions: Q4: Is there any reason I should not use MDF for M&T joints?

[Tom Overthere]

Q4: Is there any reason I should not use MDF for mortise-and-tenon joinery?

It occurs to me that 1/4" mortise walls with a 1/4" tenon thickness might not be strong enough in 3/4" MDF.

These are the likely scenarios:

a) MDF tenon into an MDF mortise

b) MDF tenon into a hardwood mortise

c) Hardwood tenon into an MDF mortise


I bought a Delta mortiser and tenoning jig some time back. Both are still in boxes and I'd like to put them to work.

Thanks

[Peter Quinn]

Any reason you should not use MDF for mortise and tennon joinery, or vice versa, ie not use M&T for MDF? Yea, because its cheap junk, barely able to support its own weight over any span. It is one step up from cardboard. You will need a carbide solution to make any great number of mortises in it, standard mortising chisels may not go far. Think router or slot mortiser. And it will be dusty as all get up. Why bother to use M&T joinery for such a low grade product?


I'm not a 'No MDF' guy, it has its place, in my quick and dirty paint grade kitchen doors as panels for instance. Best way to join it in 3/4" thickness? butt joints, glue and screws. If you are going to take the time to bend it, form it, and make mortise and tennons in it, you should make "IT" something a bit better than MDF.

[Frank Drew]

Tom,

Expanding a bit on Peter's answer, MDF has no grain strength since it's just wood particles and dust held together with glue, and particularly once you cut through MDF's hard face (as in making a tenon), you'd end up with an extremely fragile item prone to snapping off at the first stress. IMO mortising wouldn't be much better since MDF wants to split in its thickness anyway.

I think with MDF biscuit joinery might work as well as anything.

[Paul Ryan]

I am not the great craftsman like many here are, but why would you want to spend your time making mortises and tendons with MDF? I have never tried it I have never had the reason too. MDF is cheap junk it has its place but I cant think why I would need to do a M&T with it. If you tried I think it would make a mess and neither piece would be very strong. It would probably weaking the material and joint. It is not sold wood you are working with it is saw dust glued together.

[Myk Rian]

I made an MDF box using locking rabbet joints once. Just ONCE. It's still together, but only because it doesn't get much use. MDF is pressed cardboard, or close to it.

Anyway, back to your Q. Some have sealed it with watered down glue. My router table is sealed with furniture paste wax.

[Chris Padilla]

Biscuits work very well for attaching hardwood to MDF. When I edgeband MDF with hardwood, this is what I use.

I think, as others have said, that MDF simply isn't strong enough to use it in a place where one might think a M&T joint is needed but it really depends on how you are building the project. I can think of instances where a hardwood tenon might be fine into a mortise in MDF but I have a hard time thinking that an MDF tenon would be useful in any situation. There is just no strength there, I think.

[glenn bradley]

OK Tom, your other questions were plausible but this one has to be just for fun, right. :D:D:D Your just joking around to see what we'll say :confused: :rolleyes:

Just in case . . . . no on the M&T with MDF from me :).

Now, all that being said . .. I used MDF to frame a 1/4" plywood panel for a shop cabinet door. I used bridal jointes and they are still going strong. It was just for practice but it refuses to die.

[Tom Overthere]

Yikes. Looks like I touched the "MDF Nerve" with this question...

I'm just asking MDF questions because I'm not set up yet and haven't yet dulled all my own blades :D experimenting with MDF. Also, I am willing to adjust my designs to accommodate the least expensive viable material available to me. If MDF will do what I need, I'll re-design with MDF in mind.

I don't know about everyone else, but I seriously doubt I'll ever be commerically successful making and selling limited-production specialty pieces built from actual hardwoods. I recently studied hardwood prices at a local supplier...and had to pick myself up off the floor :eek:.

I'm hoping to receive good advice from those of you who may have already experimented with MDF, successfully or not.


E X A M P L E

Imagine a simple end table consisting of a top, four square legs and a four-inch-tall apron below the top. If I were using hardwood, I'd mortise two adjacent sides of each leg. And I'd tenon both ends of each apron segment.

But what if I employ veneered+edgebanded MDF for the top and apron segments, and use hardwood for the legs?

What are my options for joining the MDF apron to the hardwood leg?

Biscuit joinery, as mentioned by Chris?

Pocket Hole screw joinery?

Thanks

[Russ Boyd]

I use MDF a lot and contrary to the "hater of mdf" it's a VERY stable material. I don't think I would use it in the m&t joint however. I have used it successfully in rail and stile applications, which isn't too far off. I think an application where there wasn't a huge stress on the joint would be fine. Experiment.....it's not expensive stuff. This is my OPINION formed by EXPERIENCE with mdf. Like I said, try it and stress test it.

[glenn bradley]

Yikes. Looks like I touched the "MDF Nerve" with this question...

Tom, I hope my response wasn't taken badly. I was trying to be amusing in my own twisted way. I think the resounding chorus has made the M&T position known.

I'll vote as NOT being an MDF hater. It certainly cannot be worked like some woods but, many materials require a different apporach. I don't use it for furniture but many folks that work with veneer do use it as a substrate. It is quite stable, is usually reliably flat. I have several shop cabinets made out of it but you aren't likely to find it in my living room. I primarily turn to rabbets and dado's with or without screws for shop cabinets.

MDF does fail to support its own weight over a span but this just requires proper planning. My router table top is MDF as are many other folk's. Pat Warner could hardly be called uninformed and his router table top is MDF.

My top did sag before I learned that an inch and a half of MDF will sag if unsupported over about a 15" span with a 15 pound router hanging from it :D. Fortunately, it is flexible and after adding proper support and a bit of shimming, it is fine.

[Doug Hobkirk]

I believe (but am too lazy to confirm) that FWW mag found bridle and half-lap joints were stronger than M&T joints. Half-lap joints would be easy on MDF projects and offer a huge glue surface. I think M&T joints usually require finesse and tweaking with hand tools which is not something I want to do with MDF for aesthetic reasons - I like the tactile feel of wood.

PS - I think MDF is excellent for many, many applications.

[Russ Boyd]

Very good point on the tweaking of m&t joints. I wouldn't want to do that either with mdf.

[Jeff Bratt]

I use MDF where its good qualities make sense - it's flat, stable, and especially smooth. However, as a structural member in a piece of furniture, it is highly deficient to solid wood (or plywood). Even with the full thickness of a sheet of MDF, its mechanical strength is significantly less that hardwoods (3-5 times weaker) or softwoods (2-4 times weaker). Once you cut through the harder outer surface of MDF, this only gets worse.

In my opinion, mortise and tenon joints using MDF are a complete waste of time and effort. In a table, the leg-to-apron joints are the most critical for the strength of the piece. Using MDF for either part of this application is not a very good idea. Also, glue soaks into the interior of MDF faster than you can squeeze it out of the bottle. I seriously doubt you could get a reasonable glue line inside a M&T joint (or many other joints) when using MDF.

If you want to build something using MDF, consider things with a cube-like design, where the stresses are spread out over larger areas, not concentrated like at a classic table leg joint.

[Tony Bilello]

Since MDF does not have any grain and normally it glues up well, I would think that a M&T joint may work as well as any other kind of joint with MDF since all surfaces would glue up the same, unlike hardwood with end grain.
If I was in my shop now, I would try it for the heck of it.
I say "Do it" as a test and let us know what happens.
Apparently no one here yet has done it and we are all guessing. Make that test piece, bust it up tomorrow and then report back to us.
Then, tomorrow you will be the expert.

I have made lap joints with MDF and they have worked real well.
MDF glues up very easily.

Hope to hear from you tomorrow

[Tom Overthere]

Thanks all, for the follow up information.

Looks like MDF is maybe NOT what I'm looking for afterall (in some cases). I do appreciate you guys guiding me on this.


So now I'm falling back to maybe using plywood instead of MDF.

Assuming the same simple end-table-with-apron example:
If I were to use a 3/4" plywood for the apron and table top, do you suggest biscuits to join apron to legs (and to the underside of plywood table top)?

Or (dare I ask), would anybody here consider making a tenon on an apron segment made of a 5-ply plywood?

And, what configuration of plywood do you guys buy for furniture applications? Assuming I edge band and veneer some visible surfaces, is there a defacto/customary minimum grade for "quality furnishings". I used to buy lumber core years ago in another part of the country, but have no idea what that costs now, or if it's even available to me. Is 5-ply the standard, or will the next higher ply count do (7-ply? 9-ply? 11-ply?)

Ah, if only lumber were cheap and plentiful...sniff...

P.S. Glenn, your account of MDFsagging over a 15" span is pretty discouraging. And Tony (your post appeared while I was typing mine), my garage is FROZEN so you won't be hearing from me tomorrow. I appreciate the good advice but the way things are looking, I'm hoping for a thaw in about three weeks (or maybe June...brrrrrrrrrr!)

[Mats Bengtsson]

I believe (but am too lazy to confirm) that FWW mag found bridle and half-lap joints were stronger than M&T joints. Half-lap joints would be easy on MDF projects and offer a huge glue surface. I think M&T joints usually require finesse and tweaking with hand tools which is not something I want to do with MDF for aesthetic reasons - I like the tactile feel of wood.

PS - I think MDF is excellent for many, many applications.

If a half lap joint is strong on MDF, then it should follow that a long tennon joint with a long mortise is strong on MDF, since there is strength in either the long rail part of the half lap, or the glue part.

In MDF, I favor doing long rail joints myself, easy to do with a router, which works well in MDF. But it has to be a straight tennon, angled cutouts will not hold.

Loose tennons (Domino) will work well in MDF as well (relying on the glue strength of MDF, and leaving both strong outer faces intact.

Regarding strength and sag, the sagulator is a good tool for those calculations.

--- Mats ---

[Jeff Bratt]

My vote would be for making a tenon on the end of the ply, and a mortise in the solid wood legs. Biscuits into the edge of ply will tend to split it if the joint is stressed. I would also consider making the tenon thicker than you might otherwise, since around half of its thickness will be wood layers with the grain going the wrong direction. Since a table leg is usually thicker than the apron, this should not be a problem. If you use 5-ply sheets, I'd even consider a full thickness tenon, since only the middle and outer layers would have the grain going the favorable direction. With more plies, this would be less of a problem. Ply for the top will also tend to sag less and weigh a lot less. Just make sure you get flat plywood, you can't plane ply aprons to take a bow out of them...

[Todd Hoppe]

Tom,

Good luck with your projects. I think your ideas are quite good about using the most inexpensive available materials. I actually like MDF and use it frequently.

I'd keep shopping too, though. You may be surprised at what you find is actually the least expensive (or close to it). For example, a sheet of cabinet grade plywood can run $60-75 easy. If you build your project from a low cost wood, say Ash, poplar, hickory, etc. you may find that your total cost is very close.

I personally use a mix of materials whereever appropriate. For example, I built this coffee table:

http://www.woodworking.com/dcforum/User_files/4637194512043f45.jpg

...and I used MDF cores for the aprons, with a kerfed solid maple skin. It is incredibly strong. I could have used maple ply, but the quantity needed for the whole project was so small that it really didn't make much difference.

http://www.woodworking.com/dcforum/User_files/463719621ef85b6a.jpg

The weakest part of the project is, by far, the way the aprons are attached to the leg (mdf to solid with pocket screws). I just didn't feel I had the skill set to do an MT joint. If a joint fails, I'll rebuild it with a different core, and MT.

Another example, this is the price list for my favorite yard.

http://www.theworkbench.com/pdf/current.pdf

Ash, Poplar, Pecan, Hickory, and wormy soft maple are all quite inexpensive (depending on quantity). All are more fun to work with than dusty MDF. If you compare these to cabinet grade ply, some can even be cheaper on a square foot basis.

[Tom Overthere]

Loose tennons (Domino) will work well in MDF as well (relying on the glue strength of MDF, and leaving both strong outer faces intact....Regarding strength and sag, the sagulator is a good tool for those calculations.

--- Mats ---Thanks Mats ==

I will indeed try a couple of MDF mortise+tenon experiments, in line with your comments.

When you say "Domino" do you mean Festool specialty tool - looks sort of like a biscuit joiner? For future reference, any tools that are "black with lime green accents" are well outside my budget. :D

What is a SAGULATOR? The older I get, the less I want to get near anything like that...:eek:

[Tom Overthere]

...I would also consider making the tenon thicker than you might otherwise, since around half of its thickness will be wood layers with the grain going the wrong direction...If you use 5-ply sheets, I'd even consider a full thickness tenon, since only the middle and outer layers would have the grain going the favorable direction. With more plies, this would be less of a problem...Excellent point. I didn't even consider how the alternating grain directions will affect tenon strength.

I'll endeavor to make tenons in plywood (or MDF) as thick as possible, which should not be a problem as most mortises WILL be cut into heavier solid hardwood.

[Mike Henderson]

Since MDF has no grain structure, butt joints are as good as anything. You won't add any strength doing a M&T from MDF.

Any butt joint with MDF is like gluing two pieces of lumber side by side.

Mike

[Tom Overthere]

...You may be surprised at what you find is actually the least expensive (or close to it)....If you build your project from a low cost wood, say Ash, poplar, pecan hickory, wormy soft maple, are all quite inexpensive (depending on quantity). All are more fun to work with than dusty MDF. If you compare these to cabinet grade ply, some can even be cheaper on a square foot basis.

I personally use a mix of materials whereever appropriate. For example, I built this coffee table...and I used MDF cores for the aprons, with a kerfed solid maple skin. It is incredibly strong.


Another example, this is the price list for my favorite yard.
http://www.theworkbench.com/pdf/current.pdf Thanks a lot, Todd. I appreciate the .PDF price sheet, and I really like you coffee table. Thanks for the pics.

Q1. Is that an opaque glass top set in a rabbet, or is it something else? 'Looks great!

Q2. The core of your apron appears to be 3/4" MDF. How'd you bend it? And considering that it's a CIRCLE you didn't have any room springback or other "X factors"... So the question is really, "How'd you bend it so accurately?"

Q3. Judging by the accurate spacing of those kerfs, can I assume you bought that maple banding ready-made? If so, do you recall who makes/sells it?

[Tom Overthere]

Since MDF has no grain structure, butt joints are as good as anything. You won't add any strength doing a M&T from MDF. Any butt joint with MDF is like gluing two pieces of lumber side by side.Not sure what kind of butt joint applies, Mike. I think maybe some kind of lap or half-lap is called for, but a mortise+tenon would be easier than trying to improvise and hide a lap or half-lap joint.

Specifically, I'm thinking of an MDF (or plywood) apron segment's tenon mated to a mortise cut in a hardwood table leg having a square cross section (as viewed from above).

[Mats Bengtsson]

Thanks Mats ==

I will indeed try a couple of MDF mortise+tenon experiments, in line with your comments.

When you say "Domino" do you mean Festool specialty tool - looks sort of like a biscuit joiner? For future reference, any tools that are "black with lime green accents" are well outside my budget. :D

What is a SAGULATOR? The older I get, the less I want to get near anything like that...:eek:

Yes, sorry referring to something out of budget, Domino is a "black and lime green" thingy.

Sagulator is a web page where you easily can calculate shelf sag depending on shelf size and weight and material, thus comparing material and thickness influence on a thought application. http://www.woodbin.com/calcs/sagulator.htm

--- Mats ---

[Tom Overthere]

Thanks, Mats ==

That Sagulator (heH heh) will come in really handy. I appreciate the link.

[Todd Hoppe]

Thanks a lot, Todd. I appreciate the .PDF price sheet, and I really like you coffee table. Thanks for the pics.

Q1. Is that an opaque glass top set in a rabbet, or is it something else? 'Looks great!

Q2. The core of your apron appears to be 3/4" MDF. How'd you bend it? And considering that it's a CIRCLE you didn't have any room springback or other "X factors"... So the question is really, "How'd you bend it so accurately?"

Q3. Judging by the accurate spacing of those kerfs, can I assume you bought that maple banding ready-made? If so, do you recall who makes/sells it?

Tom,

Q1 - Its 3/8" tempered glass. its frosted on the back side. I picked it up at a local glass supplier.

Q2 - I didn't bend the mdf. I made a trammel arm for the router and cut out 1/4 circle arcs. I laminated 3 together to make a core. The outer radius and inner radius were set to be 1" apart. I just used the same pivot point for each arc. Because I cut 1/4 circle arcs, I could get all from about 1/2 sheet of MDF.

I made a glueing jig that matched the inner radius, and clamped all pieces to it. The maple was so strong that there was almost no spring back.

Q3 - I made the banding from 1/4" maple using a sled with a spacer pin located about 3/32 from the blade. I would cut one notch, move the notch onto the pin, and repeast. If I remember right, there are about 120 cuts per strip. Each took about 10 minutes to kerf once I was all set up.

[Curt Harms]

I haven't used MDF much at all-its reputation for dustiness and my basement shop may not be particularly compatible. Having said that--biscuit joinery was originally intended for sheet goods, not solids. It seems like MDF would be an optimum use for biscuits. The other thought is that because MDF has no grain there are no cross-grain glueup issues so wide half lap joints should pose no risk, unlike half lap joints in solid wood. Just some musings on a cold morning.

Curt

[Alex Shanku]

Just off the top of my head, I am having a hard time seeing how you will save any time/money trying to build a small occasional table like you are envisioning.

A few ideas to toss around.

Is buying veneer (or jointing/planing/resawing/sanding), substrate, glue and the other materials cost effective when a few board feet of real lumber would be all you need?

What about all the edgebanding you will have to do?

Damage and repairs down the road? Harder to fix, imo.

I think using biscuits and glue is a bad idea when attempting to join your aprons to your legs. A normal M&T joint would work here, even if you do use plywood for your aprons.

Also, if you do use plywood for your top/aprons I would still not just glue/screw/biscuit the top down. Movement may be virtually nonexistent, but Its not that hard to use figure 8's or clips or buttons; why take the chance.

My 2 cents, anyway.

Good luck on your projects!!

[David Keller NC]

"I don't know about everyone else, but I seriously doubt I'll ever be commerically successful making and selling limited-production specialty pieces built from actual hardwoods. I recently studied hardwood prices at a local supplier...and had to pick myself up off the floor :eek:."

Just a comment here, Tom, that's related but not exactly on-topic to your question. It's extremely doubtful that you'll be able to make any money as a one-man or small shop turning out low-priced items out of MDF. Remember that you will then be competing with large factories that buy their materials in bulk and have a squad of people on-staff whose only job is thinking up ways to lean out the production process. As hard as it is to sell higher-end furniture that's hand-crafted, it's the only way to make any money - with the exception of kitchen cabinets, and even there you need to position yourself as turning out a higher-quality product than is available on the cheap from Home Depot.

In regards to M&T joints in MDF or plywood - remember that a M&T joint was developed to accomodate solid wood's movement with changes in humidity, and to provide a geometric way to overcome the shortcomings of animal hide glue (witness the "pinned" or "drawbored" mortise and tenon joint).

In the case of MDF, there is no linear grain structure, and there's therefore no reason to make a M&T joint. Given a properly prepared surface and a modern glue, a butt joint will be every bit as strong as a M&T, Biscuit, Dowell, or other construct, because the glue's stronger than the material.

In the case of plywood, there's also no reason to make a M&T. Because plywood's built of cross-grain veneers (assuming, of course, that we're discussing veneer-core plywood and not MDF core), movement with humidity changes is not much of an issue, and once again, because a modern glue is stronger than the material if applied and cured properly, you don't need and there's no advantage to a mechanical all-wood joint. That said, a lot of plywood constructions use biscuits or pocket hole screws as an alignment aid - it makes assembly much faster and reduces weird clamping geometries.

One final comment - the Fine WoodWorking test of joints had a major scientific flaw that greatly reduces its applicability. In almost all of the tests, the wood (cherry, in this case) failed, not the glue or the joint per se. The correct conclusion is that the FWW test investigated the strength of cherry in different joint geometries - not the strength of various joints.

What this means is that the particular joint geometries and measurements that FWW tested would be useful to someone working solely in cherry and with roughly the same parts sizes and layouts. Despite their insistence to the contrary, the results cannot be applied to general conclusions about woodwork joints in solid wood of another species, nor can it be applied to manufactured sheet goods.

Moreover, the FWW test (and they acknowledged this in the article) made no attempt to evaluate how strong the various tested joints would be under stress of wood expansion/contraction, which is, of course, critical in furniture construction.

The FWW test was a very clever idea for a magazine article, it was just very poorly designed and executed, and the results were radically mis-interpreted. I design and evaluate engineering and scientific tests as an everyday part of my career, but this skill is a specialized one, and unfortunately FWW did not get very good advice when setting this test up.

[Chris Padilla]

Take a look at the Tansu (http://www.sawmillcreek.org/showthread.php?t=92396) I'm building, Tom. MDF core, walnut veneer prepared by moi. Walnut edgebanding biscuited on...panels Domino'ed on (sorry, a black/lime green accented tool).

Personally, I feel learning to veneer may be a valuable way to afford the higher-end woods moving forward on the planet. They are likely to only get pricier and pricier.

[Josiah Bartlett]

If you really want strength (well, as much as you can get anyway) of a M&T joint in either plywood or MDF, I would use a loose tenon made of hardwood. A plywood tenon should hold up just fine for a table apron, though, especially if you use a corner block to reinforce the legs.

[Chris Friesen]

If you're in it to make money, the only way you're going to be able to compete as a small shop is on design/quality, not price. You just don't have the economies of scale of the larger operations.

The amount of time invested in a M&T joint makes no sense when using MDF, and it will be a weak joint when completed. Given the lack of grain direction, when fastening MDF to itself, plywood, or long grain of solid wood you may as well use just a butt joint with glue. Biscuits/dominos/dowels may help with alignment but will add little strength. (They may help when the glue fails though.) Only if fastening to solid wood end grain would they buy you any strength.

Similarly, there's no point in using M&T joints with plywood since it means you're thinning out your workpiece. You'd probably get better results with a glued butt joint, maybe with dominoes/dowels/biscuits. As others have noted, cabinet-grade plywood can be more expensive than cheaper solid woods and it will still be weaker.

[Chris Tsutsui]

If I'm working with factory laminated MDF such as this maple laminated MDF, and I want strong AIR tight boxes.

I would use a lock miter, clamps, and titebond.


http://img91.imageshack.us/img91/8336/cutmaplepanels.jpg

I doubt that a M&T MDF joint would be any stronger at all than say a miter glue joint.

I mean.. the GLUED portion is going to be stronger than the MDF itself...

[Paul Johnstone]

I don't know about everyone else, but I seriously doubt I'll ever be commerically successful making and selling limited-production specialty pieces built from actual hardwoods. I recently studied hardwood prices at a local supplier...and had to pick myself up off the floor :eek:.



No offense, but if you are doing this for a business, I don't see how you'll be able to compete with factories if you are building stuff out of MDF. I am just a hobbyist, not an expert. I've seen MDF bookcases, etc at the stores for less $$ than I could buy the materials.

I think if you are making specialty items, you'd want them made out of higher quality material. Look into Red Oak, it's relatively inexpensive. I can get sheets of A-1 grade Red oak plywood for between $50-60 sheet. I can get Red oak in FAS for about 1.80 a board foot if I buy in quanity. Sometimes I can get it for less.

Yes, if you are looking at cherry and walnut, materials are going to be considerably more expensive. Probably best not to learn on those materials. Make your mistakes on something less expensive :)