Carriage bolts in MDF

[Jeff Dege]

What's the best way of keeping carriage bolts from spinning, when installed through MDF? This is in an installation with 5/16" bolts through two layers of 3/4" MDF where the heads are countersunk and covered, and hence will not be accessible. I'm concerned that the square part of the shank will eventually strip out the MDF.

Would it be entirely off-the-wall to put some epoxy in the countersink hole, as I set the bolt?

Or is there some other technique that would work better?

Or am I worrying about nothing?

[Jamie Buxton]

Carriage bolts spin in real wood. In MDF, it is almost hopeless.

You have lots of thickness to play with. So lose the carriage bolts. Use real hex-head bolts -- ones that you can really get a wrench on, and tighten up. Counterbore the back side, and put a washer and a nut there. Tighten the bolt against the nut. They should not come undone. Then cover the bolt head with the plug.

[Mark Smith, too]

Have you seen the torque washers at McFeely's? Might help with the spinning problem.

http://www.mcfeelys.com/carriage-bolts

[Jim Kountz]

Try some t-nuts if the application allows.

[glenn bradley]

I've used the McFeely's solution in soft fir with success.

[Danny Burns]

Try some t-nuts if the application allows.
Then use some epoxy on the "T" nuts and the bolt.

File/rough-up the bolts head and "T" nut's surface, for a good grip with the epoxy.

"T" nuts at Lee Valley. It would seem that the more prongs the better for soft materials.
1/4-20
http://www.leevalley.com/hardware/page.aspx?c=2&p=40361&cat=3,43715,43727

5/16-18
http://www.leevalley.com/wood/page.aspx?c=2&p=61645&cat=1,43455,61995

3/8-16
http://www.leevalley.com/wood/page.aspx?c=2&p=53225&cat=1,43455

If you go the "T" nut route, you can use regular bolts, and just glue the bolt into the "T" nut.

[Frank Drew]

Carriage bolts spin in real wood.

That's always been my experience; I hate the damn things. What McFeely's seem to have done is in effect redesign the carriage bolt, making it a two-part affair with a washer that actually makes the bolt stay put. Good idea.

[Jeff Dege]

You have lots of thickness to play with. So lose the carriage bolts. Use real hex-head bolts -- ones that you can really get a wrench on, and tighten up.
I do have room to counterbore both sides, and to use real bolts. The torque washers look like a great idea, but bolts, nuts and washers I can pick up at the hardware store on the way home from work.

[Tom Veatch]

...Would it be entirely off-the-wall to put some epoxy in the countersink hole, as I set the bolt?...

That's what I'd do if I didn't have access to the bolt head when it came time to torque the nut.

[Cliff Rohrabacher]

MDF has a rather hard skin and gets softer and less dense toward the middle.

That of course means that once you are past the skin there's not a lot of integrity to the stuff.

You can use a high penetrating epoxy like West with the 209 hardener and an overnight soak to get the counter bored hole to absorb the epoxy and toughen up.

But other than that you are pretty much playing hit or miss with MDF once beneath the skin.

[Jeff Dege]

MDF has a rather hard skin and gets softer and less dense toward the middle.

That of course means that once you are past the skin there's not a lot of integrity to the stuff.
We're talking about two layers of 3/4" MDF glued together, with 1/4"-3/8" counterbores on each side. That leaves a minimum of 3/4" in the middle, including two intact skins. The forces in play would be the squeeze the head and the put put on the washers in each counterbore, which would be pure compression, and I'd think that MDF would hold up to that, and pull on the bolt as a whole, which I'd expect to result in tension on the skins, and I'd not expect adding epoxy would help with that very much.


You can use a high penetrating epoxy like West with the 209 hardener and an overnight soak to get the counter bored hole to absorb the epoxy and toughen up.

But other than that you are pretty much playing hit or miss with MDF once beneath the skin.
Now you've got me wishing I'd layered some glass cloth between the two sheets of MDF before I glued them together. Still, epoxy is cheap enough.

[Chris Tsutsui]

Just yesterday I was chiseling MDF making square cutouts for some hanging brackets on my router table. It chisels ok as long as you're careful not to knock a huge chunk off because MDF tends to act like laminated cardboard. I don't see why you couldn't drill a hole, then use a small chisel to make a square cutout to accept the carriage bolt to reinforce whatever method you choose.

[Jeff Dege]

I don't see why you couldn't drill a hole, then use a small chisel to make a square cutout to accept the carriage bolt to reinforce whatever method you choose.
My concern was that MDF wouldn't be strong enough - that the corners of the square section of the carriage bolt would crush the sides of the square cutout, resulting in a round hole that the bolt could spin in freely.

[Larry Edgerton]

Epoxy. Mdf is surprisingly tough when treated with West System. I make outdoor signs with it that have been in Northern Michigan weather for 20 years, treated with West. It soaks right in as fast as you brush it on.

Run the bolt in, then back it out and treat the impression with West System until it quits soaking in. Let cure. Put the bolt back in so that it is not cutting new edges, and you should be fine.

[Joe Jensen]

I recently saw some special washers for carriage bolts. It's a washer with teeth like a T-nut, and a square hole in the middle instead of a round one. The carriage bolt engages the square hole, and the teeth sink into the wood and old. They were at McFeeleys.com I think they are perfect for your application...joe