Mitre or Chamfer?

[kirknelson]

I am about to begin the construction of cabinets and am wanting to cut the joints at 45 degrees. I want to do this to provide a stronger joint but mainly to hide the edge of the MDF, which we all know acts as a sponge for any type of moisture. I do intend to apply a veneer so perhaps it isn't worth the effort, but I am wondering what the best way to accomplish this is.

Should I:
a) Set the table saw at 45 degrees and cut the panels (how do I measure this sut acurately?)

b) Cut the panels to size then use a chamfer bit to provide an acurate 45 degree angle.

c) Something else I haven't thought of.

Thanks for your input. :T

[---k---]

I wouldn't bother with 45 degree joints, but if I did I would use the table saw - 1/2 the number of steps.

[wkhanna]

Kiss

My personal experience is that MDF is rather soft and will chip easily. A feathered or sharp, thin edge will not last long, and will probably get damaged before, or while you’re gluing up. Even if using biscuit joints as a means for strengthening the joint, a simple butt joint would be easier and just as effective.

Using a simple butt joint (no biscuits) will provide all the strength you need, so long as you use a high-quality glue such as Tite-bond, etc.

You’re creating needless work and making it harder than it need be. I subscribe to the KISS (Keep It Simple, Stupid) principal. Things will go smoother and easier for you, and as long as you glue and clamp properly, your boxes will never fall apart.

BTW, Simply putting a few coats of sealer on the MDF will provide the barrier your looking for.

As always, JMHO, YMMV

[tf1216]

Recently a builder showed me a neat trick to seal up the edges of MDF.

His trick is using a sponge dipped in Tite-bond (I) cement to cover the wanted areas with glue. He kept a tin container of glue for this job and when he needed it he would mix it with water to obtain the milky consistency he wanted. Simple and effective.

[BigguyZ]

If you're veneering, the exposed edge of the MDF won't matter. If you are that worried about it, you can always use a sanding sealer to soak up into the "end grain" and solidify it. But it's not necessary at all.

[stangbat]

I agree that mitering MDF is not worth the trouble if you are going to veneer. I sealed the end of my MDF with PVA glue (Titebond). If you are veneering using the PVA glue-iron on method, the ends will get sealed with the glue anyway when you roll it on. If you are using contact cement, then you can seal the ends as tf1216 suggested. For baffles that are painted, I smear on the glue with my finger, let it dry, then sand it.

[collo]

Butt joints are so much easier to do than mitred.

To add strength, use edge bracing which doubles the area for the glue to act.

[kirknelson]

Thanks for the comments all, sounds like I'm making the project more difficult than it need be.

I agree that mitering MDF is not worth the trouble if you are going to veneer. I sealed the end of my MDF with PVA glue (Titebond). If you are veneering using the PVA glue-iron on method, the ends will get sealed with the glue anyway when you roll it on. If you are using contact cement, then you can seal the ends as tf1216 suggested. For baffles that are painted, I smear on the glue with my finger, let it dry, then sand it.

As this will be my first attempt at veneering I plan on using the PVA method since it seems more forgiving of mistakes.

I do plan to paint the baffles a high gloss black instead of veneering them. If I understand your comment correctly you just smear glue on the edge and not the face of the baffle? Do you worry at all about the edges around the drivers?

I'm probably being overly worried but I built an equipment rack out of MDF awhile back and painted it without sealing the edges. Within a month several of the shelves had grown moisture "bubbles" at their edges. I don't want to repeat that mistake.

[stangbat]

Thanks for the comments all, sounds like I'm making the project more difficult than it need be.



As this will be my first attempt at veneering I plan on using the PVA method since it seems more forgiving of mistakes.

I do plan to paint the baffles a high gloss black instead of veneering them. If I understand your comment correctly you just smear glue on the edge and not the face of the baffle? Do you worry at all about the edges around the drivers?

I'm probably being overly worried but I built an equipment rack out of MDF awhile back and painted it without sealing the edges. Within a month several of the shelves had grown moisture "bubbles" at their edges. I don't want to repeat that mistake.

Yes, I rounded over the baffle edge and then smeared glue on the edge. It soaks in and sands very smooth with a 320 grit paper. You may need to do a coat, sand lightly, then another coat and sand. I sealed the flat portion of the baffle with a couple of coats of sanding sealer. I did not worry about the portion around the drivers, but it wouldn't hurt to seal them with glue to prevent any problems. To date, after a year of service, my baffles still look as they did when I finished them.

I recently built a sub and decided to paint it rather than veneer. I sealed the MDF (inside and out) by rolling on thinned Titebond glue as recommended by lunchmoney in his ZRT thread. I did this to prevent the MDF from absorbing moisture and causing the joints to show up after a few years. It seems to seal the MDF well. It can leave a bumpy surface depending on how you apply it, meaning some sanding is necessary before finishing. The glue melts from the heat if you sand too quickly so this means sanding had to be done by hand with a block.

The PVA iron on method for applying veneer is almost foolproof. Just make sure you use paper backed veneer. And use some extra care on the edges and corners to get the veneer stuck tight. I would heat the edges/corners and apply pressure with the iron, then remove the iron and quickly press down HARD with a block since I couldn't put the same force on the iron without the risk of breaking it. I also found that regular Titebond seemed to work better than Titebond II. I was amazed how good the results were. I even veneered a friend's sub after doing my towers.

[DancesWithBeers]

Although biscuits might not be necessary for strength, biscuits, dowels, and dadoes are very useful when trying to keep things aligned and square during glue-up.

[gimpy]

One of the hardest parts about beveling 45* is alligning and gluing. Make sure you have a plan/jig in mind or it will be very difficult (speaking from experience/rather inexperience).

[vinceb]

butt joints, esp if you're veneering. Overlap them a bit (or use biscuits for alignment) and use a router to flush trim them for a near-perfect fit. I used biscuits for the initial side/top joints then did the fronts and backs oversize and flush trim. Bondo, sand, veneer. the 45s will kill you trying to line it up, listen to the guys here, they will steer you right.

[wkhanna]

......the 45s will kill you trying to line it up....

Bingo!

If you are not using hardwood and have dedicated jigs......step away from the miters.

As always, JMHO, YMMV

[JoshK]

Yeah, from (in)experience, I can say that 45º joints are a bitch! All boards have to be perfectly square on all four sides and all four parts have to be perfectly sized and then aligning them perfectly is no easy feat either.

Follow up question, while we are on the topic. I am painting my front baffle. It will have a large roundover. What sealer do you suggest is best to seal the MDF before painting? Some said sanding sealer, others glue.

[brianpowers27]

I am painting my front baffle. It will have a large roundover. What sealer do you suggest is best to seal the MDF before painting? Some said sanding sealer, others glue.

I smear the entire surface with one coat of undilluted wood glue. I then do a second coat for the raw edges. (Buy the gallon. :T )

[Brian Walter]

As JoshK said, using miters successfully requires that you cut all sides perfectly square, most people are not set up properly to do that. A highly accurate crosscut sled or sliding table is almost a must unless you are making fairly short cuts, then an accurate miter guage should suffice.

I used a router once and made a rabbit/miter joint using a combination of a rabbit bit and a 45 degree miter bit. This was very involved and took a lot of trial and error to set up properly. I did this for all 6 sides, so there there was no visible end grain any were. After doing that I bought a locking miter bit in hopes that it would simplify things. That was about 10 years ago and I've never used the bit so I can't say how well it works.

I can attest that the diluted wood glue works fairly well at sealing the end grain of MDF, much better in my mind than Minwax Wood Hardener. Zinsser Universal Sealer (dewaxed shellac) also works fairly well, but again, I like the diluted wood glue better.

Brian

[bluewizard]

If your goal is to minimize edge grain and increase the glue surface area, there are alternatives to both miter and butt joint.

The attached diagram indicates just a few.

Steve/bluewizard

[oldloder]

How much do you folks dilute the glue?

[mikela]

There was a thread at diyaudio on sealing MDF. One person had pretty good success using thinned Varathane as a sealer.

[Brian Walter]

How much do you folks dilute the glue?

When I did it I only diluted it about 10% - 15%, just enough to thin it out so that it went on like thick water.

Brian

[oldloder]

^^^ Thanks Brian. :T

[cyberspyder]

As JoshK said, using miters successfully requires that you cut all sides perfectly square, most people are not set up properly to do that. A highly accurate crosscut sled or sliding table is almost a must unless you are making fairly short cuts, then an accurate miter guage should suffice.

I used a router once and made a rabbit/miter joint using a combination of a rabbit bit and a 45 degree miter bit. This was very involved and took a lot of trial and error to set up properly. I did this for all 6 sides, so there there was no visible end grain any were. After doing that I bought a locking miter bit in hopes that it would simplify things. That was about 10 years ago and I've never used the bit so I can't say how well it works.

I can attest that the diluted wood glue works fairly well at sealing the end grain of MDF, much better in my mind than Minwax Wood Hardener. Zinsser Universal Sealer (dewaxed shellac) also works fairly well, but again, I like the diluted wood glue better.

Brian

I don't think he meant angling the pieces end-wise, but rather edge.