Lock-Miter and Miter-Fold Joinery with MDF

Detailed discussion of creating good corner joints when building up a column out of flat MDF. June 28, 2007

I need to set up for a production run of MDF mitred columns, and am thinking of using an Amana indexable cutter. Is there a better way (CNC)?

Forum Responses
(Architectural Woodworking Forum)
From contributor I:
Looks to me like you would have to run one of the mating surfaces vertical with that one. I like the freeborn stack that you run everything horizontal and cut the mortise afterwards on the table saw.

From contributor T:
Careful set-up and tall fences with feather boards for holding it tight. It may not be the easiest, but it works.

From contributor P:
Miter fold. Lock miters and MDF are difficult.

From contributor J:
If you have a lot to run, consider diamond tooling. I have had bad experiences with carbide and MDF. The stuff is brutal, and a dull cutter turns an already delicate setup into a nightmare. Changing tooling, or stopping in the middle of a run to send out and resharpen, can cost a lot more than just investing in the right stuff in the first place.

From contributor L:
I'm with contributor P - miter fold. The part that must run vertical on the shaper setup for lock miter has a very sharp corner that damages very easily!

From contributor J:
What is a miter fold? I've seen it in handcrafted boxes, and I think Betterly has a setup for p-lam, but how does one miter fold a production run? It's always good to learn something new and better, so can one of you explain some more about this joint? Do you mean 45 degree miter cut corners somehow joined together?

I agree that the vertical element in the double tongue and groove lock miter is a challenge, but so are many of the millwork operations that we get paid for while others give up because, yes, it's tricky.

I really enjoy the assembly of the DT&G lock miter as it is so easy and foolproof, with a corner so clean and tight, that the effort to get there is well worth it, especially for production runs. The two halves seem to just jump together, and the finished product is extremely strong.

From contributor L:
From WOODWEB 's Knowledge Base: Miterfolds for clean corner joints

We miter for solid surface on our CNC router. Auto-V-groove and Star make machines just for the purpose. A lot of speaker boxes are made that way. BHK makes drawers that way. Lots of laminate countertops have their drop edge formed that way. See them run at the next major show - bet they will be in LV.

From contributor C:
Has anyone run a lock-mitre using their moulder? I bid a job once that didn't come through, assuming I would grab a pair of lock mitre heads and run it. Really doesn't seem overly complicated in that scenario, but perhaps I'm missing something?

From contributor L:
We made a set of knives for our Weinig molder so we could run quartered white oak legs displaying the qtr all around. Works well! Easy to change sizes of leg.

From contributor A:
I agree with the part about diamond tooling, but carbide is plenty fine for even a long run of MDF. Lock miters aren't that difficult to set up once you have done it a few times. I run lots of lock miters with MDF and the results are fantastic. If you do decide to miter fold, you may want to consider getting a 91 degree cutter, especially if doing four or more sided boxes. Gives a little more fudge factor including taking into account the glue thickness.

From Gary Katz, forum technical advisor:
Ditto contributor L's advice. I've run a lot of lock miters on poplar and pine and solid wood, but not MDF. I don't think you're going to really use MDF anyway, though, unless these are inside? If you're making exterior columns, use Extira or something that's meant for exterior use.

But anyway, the sharp little miter edge resting on the table during the vertical cut can be damaged easily and if the workpiece descends even a little bit, the joint will be mismatched.

From contributor O:

Not a guru, but I tried to miterfold some MDF and the skin cracked with a jagged edge when I went to fold it. Tried with various thick skins. Tell us more for doing this on MDF.

From contributor L:
You don't actually "fold" MDF. The cut is clear through and just touches the tape. Or you can lay the fully separate parts on top of the tape after machining. You can fold aluminum that is glued to MDF but the MDF is cut clear through. For a while we were making 8-sided columns with aluminum and P Bd. The Bd was glued to the aluminum, then the whole mess rolled up and held with band clamps. Looked good. The joint was kept to the back.

From contributor J:
Running both the vertical and horizontal pass to create this joint, the infeed always goes fine, as does most of the pass, because there is a square edge supporting the whole thing on the infeed side. The trouble starts as the part feeds through and this support edge gets smaller. A steady hand and eye can do wonders in supporting and guiding the outfeed end, but that last bit of the pass without support will snipe and chomp. I learned to leave extra sacrificial material for this, and not to cut to final length until after the parts are run. This is not practical with a tapered column, so I sometimes use offset support strips on the outfeed side made to just the right thickness to catch the places that lost material. It's a pain to set up, so I only use it if I have to.

From contributor M:
Contributor J raises a good point regarding the last part of the board sniping. Especially when you consider that point wants to go under the outbound fence if there's even a small gap there. I get around this by adding a spacer to the outbound fence (elevated off the table a bit) so the tongue of the piece has something to make contact with.

The 3 piece cutter by Freeborn is really the way to go. Easier to set up, and no issues with loose joints when the bit/cutter gets sharpened.