Plywood carcase & shelf questions

[Dukester]

Looking at building a hutch for a friend that wishes it to be around 6' high by 5' long & probably 2' deep. She wants to paint the piece, so question 1) is cabinet grade plywood what I want to use regardless of finish?

The shelves on the top of the hutch are to span the entire 5' width without breaks. They will be approximately 5' x 20". Assuming I use a lipped edge of solid wood to cover the edge of 3/4" ply for shelves in the front, (question 2) will the shelves need reinforcement at that length & width? They will probably be holding china, bowls, dishes, etc. and lots of them. I've never used plywood for shelves in that length and am concerned they will sag. What are my options?

Cheers! Duke

[David]

[George]

Answer to number one is MDO if painting.

Answer to 2 is to have a couple inches of wood on all edges, and at least one, though I'd go two, up the middle underneath.

[Woodchuck34]

Duke,

Fine Woodworking Magzine recently published an article about painted cabinets. The main point of the article was that if you are painting the finished product, there is no need to use anything but MDF. It won't warp and is much cheaper then cabinet quality plywood. Not sure why you would use MDO, unless you were building an outdoor cabinet. Here in Hershey, PA, MDO goes for $75/sheet, compared to $20 for MDF. Also, if you paint it according to another article FWW did, you can acheive excellent results.

I have made floor to ceiling cabinets that are 4' wide on which I store books and have not had a problem with sagging. Some of those are 3-4 years old. When I made those cabinets, I routed dados in both sides, as well as reinforcing the front with a Face Frame through the midpoint of the shelf. On the back I used a backer board and some L brackets at the midpoints, to provide additional support, but also to secure the cabinet to the walls. MDF will sag over a 5' length if not supported properly, but most hardwoods also will, just not to the same extent that MDF will.

You could always make the carcase out of MDF and choose another material for the shelf. The end result is using MDF will save you $$$$$.

Good Luck,

Chuck

[Dukester]

I'll check this out; do you remember the issue number by chance?

Not having worked with MDF, what is the appropriate joinery method for constructing a carcase made from one? Are you saying to use a standard shelf edge of solid wood on the MDF edges?

Cheers! Duke

[George]

Well, he's building for someone, which means they're paying the bills.

Second, check the sagulator referenced above.

Third, good plywood joints like combination joints are _much_ stronger than the same in MDO.

Lot of benefit for small cost?

[SonomaProducts.com]

Current issue

[Woodchuck34]

Duke,

It was only 3 or 4 issues ago. The cabinets I made are completely manufactured with MDF. It sounded like you wanted to use a hardwood shelf. I suggested you could make the carcase out of MDF and just construct the actual shelf piece out of hardwood.

On any edges of MDF that have been cut or routed, I apply a thin coat of spackle, lightly sand, and use an oil base primer before applying the final coat. Once again, this is outlined in the FWW article.

The only joinery method I use is dados with regular woodworkers glue. Sometime I uses "a couple of brads", but I just started trying dowels based on another article I read. Prevents any splitting.

Chuck

[Woodchuck34]

Duke,

I just checked FWW's index and can't find the issue. When I get home tonight, I'll pull it out and post the #.

Chuck

[Woodchuck34]

Yeah, but he's building for a friend vs. a "For Profit". Maybe they'd like to save some money.

Chuck

[PDQ]

If it is to be painted MDF is the way to go. A waste of money buying ply unless you are really after strength. I doubt you will be able to do the 5' without some kind of centre support. You might want to make the centre support a "selling" point.

A few facts on sag:

1. Typical shelves vary from 8" to 12" deep

2a. 3/4" Solid Wood has a maximum span of 36" before it starts to sag 2b. 1 1/16" solid wood has a maximum span of 48"

1. Veneer Core ply is the same span as solid wood cited in (2)
2. MDF of the same thicknesses is 32" and 42" span respectively.

The following chart illustrates the weight required to make a 12"-wide shelf bend 1/4" across a 36" 3/4" thick span and a 48" 1 1/16" thick span.

Shelf Material

[Dukester]

Thanks for this info and the rest of the posts on the sagulator. Another (!) question on MDF fastening: attaching solid wood face frames - do you do this the same way? Is glue & brads the best way on this?

Cheers! Duke

[Woodchuck34]

Duke,

You can use normal methods. I know people use biscuits. I just glue mine on and admit that I use "a couple of brads". When working with MDF, you just have to be careful about the placement of the fastners. Too close to the end of a board might split the MDF. Keep brads 2 - 3" from the ends of any boards. Biscuit don't really cause this problem since you are essentially routing out the exact size of the biscuit.

I also actually have heard of people the make MDF cabinets with construction adhesive, but I haven't tried that myself.

Chuck

[PDQ]

6' high=20 | by 5' long & probably 2' deep. She wants to paint the piece, so = question 1)=20 | is cabinet grade plywood what I want to use regardless of finish? |=20 | The shelves on the top of the hutch are to span the entire 5' width = without=20 | breaks. They will be approximately 5' x 20". Assuming I use a lipped = edge=20 | of solid wood to cover the edge of 3/4" ply for shelves in the front,=20 | (question 2) will the shelves need reinforcement at that length & = width?=20 | They will probably be holding china, bowls, dishes, etc. and lots of = them.=20 | I've never used plywood for shelves in that length and am concerned = they=20 | will sag. What are my options? |=20 | Cheers! | Duke=20 |=20 |=20

I forgot myself earlier and sent this in HTML, so, here it is again. As = long as you put a dropped edge on your shelves, MDF seems the material = of choice for your hutch. Given the 5' span envisioned, it seems a = mid-span support is the best bet as this approach will afford the most = capacity. You might have to use this idea as a selling point. It can = be quite an accent to the shelf. Be sure to include a "plate groove" on = the shelf in case your friend wants to stand some of the plates up.=20

Extracted by "Popular Woodworking" from the "Architectural Woodwork = Institute" for their "Winter of 2000" edition.

In general here are the guidelines for how long a shelf can be before it = starts to sag:

Material ------------- Maximum Span 3/4" Thick ------ Maximum Span 1 =

1/6" Thick Solid Wood -------- 36" ------------------------------------- 48"=20 Veneer Core Ply - 36" ------------------------------------- 48" MDF ----------------- 32" ------------------------------------- 42"

The following illustrates how much weight it takes to make a 12" wide = shelf bend 1/4" across a span.

3/4" Thick Material ------------- 36" Span ------ 48" Span Yellow Poplar -------------------- 284 lbs. -------- 117 lbs. Hard Maple & Red Oak ------ 313 -------------- 232 Birch ------------------------------- 348 -------------- 146 MDF ------------------------------- 87 ---------------- 38 Birch Veneer-Core Ply ------- 129 -------------- 54 Birch Veneer MDF ------------ 109 -------------- 46 MDF .05" thick Laminate ---- 205 -------------- 87=20 MDF 1/8" wood edge --------- 79 --------------- 33 MDF 3/4" wood edge --------- 90 --------------- 38 MDF 3/4"x1 1/2" wood=20 edge, dropped ----------------- 241 -------------- 107

Hope all this bumph helps.

--=20 PDQ

[Prometheus]

If for some reason you're using screws (and it's certainly possible, it's MDF, not expensive cherry ply) it's not a bad idea to drill out a hole a little way from the edge you're screwing into and plug it with a hardwood dowel, and then drill some pilot holes for the screws from the edge into the dowel. That way your screws have something tangible to bite into (MDF will just strip out). Used that method for lag screws going into 2x4 end grain on my router table, and it worked great.

[Andy McArdle]

MDF? Nasty stuff; I work with it all too often, particularly for shelving, etc. in cabinetry with a nice timber faceplate. Occasionally I've made entire cabinets solely from 18mm for cheap housing projects. I wouldn't use it on anything I plan to keep for myself...

There're dedicated screws for use with MDF which use a coarser thread than the norm, this helps improve the grip and prevent tearout. I recommend these, although I haven't tried the dowelling method. IMHO biscuits aren't suitable for edge joiunting as MDF has little lateral strength and tends to split with very little help. Not good for kiddies toy-boxes, for example.

Two tips: ANY time screws are used with MDF a pilot hole should be used, especially for edge-screwing. and: Always use a dust mask! It really is 'orrible, 'orrible stuff...

- Andy