Case construction with MDF

[Anna]

I'm building a mobile base for a benchtop jointer. I was originally going to follow the plans in ShopNotes 48, but I've since changed my mind. Unfortunately, the change came after I bought the MDF.

What I would like to do now is the one in ShopNotes 54, which uses plywood. I have used plywood in my two other mobile bases so far, and I'm getting pretty familiar with it.

The question is, can I just substitute MDF for the plywood and follow the plans accordingly? I know MDF is supposed to be strong, etc, but how is it for case construction? Do I have to put in cleats to reinforce the joints? Can I use pocket hole joinery on MDF? Are biscuits preferable? How do I attach hardwood edging to MDF? With plywood, I've tried the rabbet-and-groove joinery, but I'm not sure if that will work with MDF.

Thanks for any help.

Anna

P.S. Am attaching a picture of the cart I'm now planning to build.

[DaveW]

I haven't used MDF for case construction, but I think you'll be ok with most of the construction- the thing I'd be concerned with would be screwing the piano hinge on the case...

But someone with more MDF experience should chime in soon enough

[RayintheUK]

I use MDF almost exclusively for cabinet and carcass construction. I prefer it over plywood - no splinters in MDF!

I don't get to see Shopnotes over here, but from a quick look at the picture I can see no problems whatsoever in using MDF for the construction, although I'd want it to be 3/4" rather than 1/2" I wouldn't use pocket screw technology for anything structural in MDF - there are better ways.

I don't know what their plans suggest for the back of the cart. As this is going to be potentially the most stressed component, it needs to have good construction integrity. What I'd do is to inset the back into dados all round, using rebates (rabbets) where the sides meet the top and base. This will have the additional benefit of preventing racking - an excellent tradeoff for the 1.25" of depth it will cost. You will not need any cleats to re-inforce this method.

I'd hang the doors off sprung clip-on European hinges (my favorites!) - that way, you'll have no worries about the screws of the piano hinge into the MDF edge.

Hardwood edging can be attached any way you like - I use biscuits quite a lot, but glue alone will do the job as long as you make sure to coat the raw edge well, otherwise there is a possibility the joint will be starved. Brush or roll the glue on the MDF edge first, then coat the hardwood, then return to the MDF and see if it needs a "top up" before joining.

If you're in any doubt - or have additional questions - drop me a PM. If the above is not clear, I could knock up a quick model in Google Sketchup.

Ray.

[Anna]

Thanks, Ray! Good information to know. I'm using 3/4" MDF. I was originally going to just use rabbets to attach the sides and back to the top and bottom, but I'll dado the sides, bottom and top to accommodate the back instead. I will use rabbets to join the top and bottom to the sides.

How deep should the dadoes/rabbets be? For 3/4" plywood, I've always used 1/4" deep dadoes/rabbets. Is it the same with MDF, or should it be closer to 1/2"? And do I have to reinforce the joints with anything besides glue? I wasn't sure I understood that part. I've heard of confirmat screws being mentioned in this forum, but are they necessary? Is glue enough to join two MDF pieces?

And what I'm getting from your post is that the hardwood edging is really more aesthetic than structural, right? Can I put a hardwood edge on the tops of the sides, for example, then rabbet the HW edge to accommodate the cabinet top? I'm a little worried about dinging the edges of the MDF when I'm finally using it.

I was originally thinking of putting hardwood edging on the doors to attach the piano hinges to, but I'll use the European hinges instead.

One other thing: the slide-out drawers are quarter-inch hardboard base that rides in dadoes on the sides. Is that okay with MDF? I've used the same concept with plywood, and that works fine. I'm a little worried about the edges of the MDF dadoes deteriorating over time. Will an oil finish fix that?

I do feel much better about using MDF now, though. I'll start my new mobile base today, and I will come across more questions, so if you don't mind, you will probably get a few PMs from me.

Thanks again!

[pber2025]

I just finished building a mobile base for my BT3100 with two drawers. I built it using 3/4" MDF assembled using glue and biscuits. Looks very solid so far...

One thing to keep in mind, MDF makes a lot of fine dust. Make sure you use all the protection needed.

[Anna]

Thanks, pber. Did you just butt joint the MDF and used biscuits to hold them? I used biscuits when I built my bench, and they're fine, but I since discovered pocket screws and would rather use them if I can. Ray (in UK) mentioned it's not necessary, though.

For dust protection, I use surgical masks. It's worked pretty well so far. The first time I cut MDF, I had no idea about the problem with dust and didn't wear any protection at all. I had a runny nose for a couple of days after that. Now I know better.

Maybe you can post a picture of your mobile base?

Thanks again,
Anna

[RayintheUK]

How deep should the dadoes/rabbets be? For 3/4" plywood, I've always used 1/4" deep dadoes/rabbets. Is it the same with MDF, or should it be closer to 1/2"? And do I have to reinforce the joints with anything besides glue? I wasn't sure I understood that part. I've heard of confirmat screws being mentioned in this forum, but are they necessary? Is glue enough to join two MDF pieces?

I'd use a 3/8" dado throughout - same for the rabbets. No other reinforcement necessary.

And what I'm getting from your post is that the hardwood edging is really more aesthetic than structural, right? Can I put a hardwood edge on the tops of the sides, for example, then rabbet the HW edge to accommodate the cabinet top? I'm a little worried about dinging the edges of the MDF when I'm finally using it.

Yes, you could do that. Seal it with Tung Oil or similar when the glue has cured.

I was originally thinking of putting hardwood edging on the doors to attach the piano hinges to, but I'll use the European hinges instead.

That would work, but the Euro hinges make for easier assembly and the doors come right off in an instant for spring cleaning, etc.

One other thing: the slide-out drawers are quarter-inch hardboard base that rides in dadoes on the sides. Is that okay with MDF? I've used the same concept with plywood, and that works fine. I'm a little worried about the edges of the MDF dadoes deteriorating over time. Will an oil finish fix that?

I wouldn't do that. Even with an oiled finish they'd tend to skew and jam. I'd use 1/2" bottoms and ride them on metal runners - either kitchen drawer types, or even full extensions. Workshop cabinets tend to get well loaded and I think the drawers as you described them might be a bit flimsy.

I do feel much better about using MDF now, though. I'll start my new mobile base today, and I will come across more questions, so if you don't mind, you will probably get a few PMs from me.

Absolutely no problem - just remember to allow for the UK time difference, please.

Ray.

[cabinetman]

Anna

Ray has a lot of good info. Just some more input for you to ponder. Use coarse thread screws in MDF. When tightening down, just bump them tight instead of just spinning them in. Once you strip a hole, it's done. You'll have to add a tooth pick or matchstick to reinstate the hole. On dadoe or rabbet depth, I don't usually go deeper than 1/4". All you've got to create for the joint is a pocket providing for a glue surface. The balance of the remainder of material after a dadoe or rabbet gets a little flexy much thinner than 1/2". I'm not saying you can't machine deeper than 1/4", but this is just my practice. Sometimes I get a little clumsy in my old age and in rough handling or bumping have broken off pieces that were machined too deep. The same principle applies to dadoe (grooving) a center divider for shelves on both sides. I would machine an 1/8" dadoe (groove) on both sides, leaving 1/2" in between (for 3/4" material). Once glued and clamped you have a good joint. No need for pocket screws. And to repeat what Ray said, pocket screws in MDF aren't that structural.



"I'M NEVER WRONG - BUT I'M NOT ALWAYS RIGHT"

[Popeye]

Just to add alittle more.... screws don't do much for you in the end with mdf but..... in the absence of enough clamps or long enough clamps they are a good substitute till the glue dries. Especially for shop cabinets or in places they won't be seen. A note with screws.... predrill on mdf and use either coarse thread screws or the conformat screws you mentioned. Pat

[Anna]

Thanks, Ray, Cabinetman and Popeye. I had been thinking about the middle divider and how deep the dadoes/grooves will be. I even thought I should use TWO MDF sheets instead of just one, to make sure it's not too weak because of grooves on both sides. I've since decided to use hardwood strips as shelf/drawer runners instead. I'll cut shallow grooves on the MDF (probably 1/8") for the runners which will be attached with glue and screws. I know the screws are probably not necessary in this case either, but I do like the security.

The one part I'm currently stuck with is whether to dedicate one side of this jointer cabinet for dust collection. I saw a plan where they cut a hole through the top and put a garbage can in the cabinet to catch the chips. They built a hood out of hardboard to direct the chips into the hole. I can do that, or build a drawer that will fit that space to collect the chips, or just attach a shopvac to the jointer directly.

But otherwise, I think I now have a pretty good idea of how to build this with MDF. Thanks to everyone for the great advice.

Anna