Lag screws or bolts for vise in mdf bench

[Ken Cohen]

Hi --

I'm building a slightly revisied Woodsmith workbench (plan from several years ago) with an ash base and edge banding and triple layer 3/4" MDF top with an added 1/4" sacrificial hardboard layer.

Plan calls for lag screws through the MDF (including a fourth mounting layer) to support a 10" vise.

Since I've added the hardboard layer (screwed to the MDF since my hardboard in not flat), it dawned on me that I could bolt the vise to the MDF base with 1/2" carriage bolts recessed in the top of the MDF + washers to spread the point load. Hardboard would cover the bolt heads and remain available for any needed adjustment.

More than a few posts question the long term holding power of lag screws in MDF and recommend carriage bolts. However, I'm fine with lag screws if they work over time.

Question: Am I better off sticking with lag screws or switching to lag bolts?

Thanks for the help.

Ken

[Gary Breckenridge]

Lags have great holding power; I like them going into the wood at right angles. Mdf is a different animal and I would go with bolts and lock washers. I also countersink for the nut and washer with a spade bit large enough for a socket to fit in.;)

[Larry Edgerton]

Carriage bolts. Drill the holes and treat with epoxy to harden the area around the bolts. Just paint some on and it will soak in instantly, keep painting until it quits soaking in, then stick in your carriage bolts. This will make the area around your bolt holes basically a big piece of plastic and your vise will not work lose because of the MDF giving way. I use this method with all of the MDF jigs I make and have had zero failures since I adopted this method I picked up from the West System newsletter.

[Myk Rian]

Carriage bolts or big, thick washers.

[Jamie Buxton]

If you're going to use through-bolts, don't use carriage bolts. Use bolts with real heads that you can put a wrench on. Carriage bolts have the nasty habit of spinning in the hole when you torque them down. Yes, you can go through the exercise of infusing the wood with epoxy, but why bother?

[Ken Cohen]

Thanks to all for the responses.

Clear winner: Hex head bolts with washer countersunk into MDF top (covered by hardboard).

Lingering question: Coating/filling the hole + countersink area with epoxy -- necessary, helpful or just belt and suspenders? (irrespective of the answer, a very helpful suggestion.)

Nit: Do I need to worry about the bottom nut loosening? If so, lock washer or other idea

Thanks again for all the great advice.

Ken

[Bill Huber]

I have a bench some what the same, I used Flathead bolts on my vise. I just used a normal countsink on the top, put the bolts all the way though the top, hardboard and all. This way I don't have to take the hardboard off to remove the vise if needed.

I got the bolts at Ace Hardware, they are 5/16 and that is enough to hold a vise with no problem.

[Jeff Dege]

Lingering question: Coating/filling the hole + countersink area with epoxy -- necessary, helpful or just belt and suspenders? (irrespective of the answer, a very helpful suggestion.)
I'd think that would be dependent upon how much work it is to get to the heads of the bolts, should they come loose.

I mounted a couple of vises on a bench, a month or so ago, using bolts. I countersunk them on both ends - at the top for the bolt heads and washers, and at the bottom, for a set of nuts and washers. Then mounted the vise on the end of the bolts and added another set of nuts and washers.

I figured that unscrewing the layer over the top of the bolt heads would be sufficient work that I wouldn't want to do it very often, so I used loktite on the intermediate nuts, and filled the bolt-head countersink with glue.

If your hardboard top layer comes off easily, you may not need to bother with all of this. My top layer, though, was 1-1/2" of Ikea oak countertop, and unscrewing it from the MDF is a bit of work.

[Lee Schierer]

I would use blots with hex heads and use T-nuts http://www.woodpeck.com/media/main_tnut.jpgon the underside of the mounting surface. This arrangement will hold your vice while putting the MDF in compression with no danger of screws pulling out. T-nuts come in a variety of thread sizes so yu can use any size bolt you want.

[Ken Cohen]

Thanks again for the helpful responses.

For my circumstances, hex bolt with T-nut seems best/easiest solution. Kind of a dumb question, but just want to confirm set-up:

Hex bolts and washers countersunk on MDF top of bench top (carry load) -- no need for epoxy

T Nuts countersunk into bottom of MDF to hold bolt in compression. Bolt cranked down tight.

T Nut surface slightly below MDF surface so that bottom of vise can rest on MDF.

Vise attached to remainder of bolt as follows : T-Nut, vise, regular washer, nut. (no lock washers or lock nuts).

Thanks for the quick once over -- would hate to get it wrong.

Ken

[Lee Schierer]

I sent you a PM , but then found this link that may help everyone. T-nut installation (http://www.threeballclimbing.com/install4prongtnuts.htm) You don't need the glue.

[Garth Keel]

Very helpful to know how to install t-Nuts.

[Bill Huber]

Thanks again for the helpful responses.

For my circumstances, hex bolt with T-nut seems best/easiest solution. Kind of a dumb question, but just want to confirm set-up:

Hex bolts and washers countersunk on MDF top of bench top (carry load) -- no need for epoxy

T Nuts countersunk into bottom of MDF to hold bolt in compression. Bolt cranked down tight.

T Nut surface slightly below MDF surface so that bottom of vise can rest on MDF.

Vise attached to remainder of bolt as follows : T-Nut, vise, regular washer, nut. (no lock washers or lock nuts).

Thanks for the quick once over -- would hate to get it wrong.

Ken

I am not sure I follow everything but for what you are doing a 5/16 T nut, stud, flat washer, lock washer and a nut would hold just about any vise you could put on the bench. The vise I am sure has 4 mounting holes so you have 4 5/16 bolts holding it on. Just put a little red locktite on the stud when you put it in and it will hold forever.


121694

[glenn bradley]

I followed the same general plans and balked at the lag screws in MDF. I was going to do as Bill H shows as that is how my rolling worktable vise is held and it has been 5 years or so with no problems. Some wise person here pointed out that if the lag screws didn't hold, I could always drill out for bolts later. The lags have been rock solid for two years of almost daily use. Coulda fooled me ;-)

[Ken Cohen]

How many times can you say thank-you to everyone who responded?

Lee and Bill propose similar, logical approaches. (As you can tell from my earlier post, I got confused between top, underneath, etc.). Only difference is stud/nut vs. hex bolt. Either approach works for me. If anyone has strong opinions, please let me know.

Much appreciated.

Ken

PS: FWIW, my new Jorgenson vise has two 5/16" mounting holes which I assume will be more than adequate.

[Greg Hines, MD]

I used lags for my bench, but the top is 4 layers of plywood, not MDF. I would think that either would probably work, but the bolts would seem more secure to me.

Doc

[Bill Huber]

How many times can you say thank-you to everyone who responded?

Lee and Bill propose similar, logical approaches. (As you can tell from my earlier post, I got confused between top, underneath, etc.). Only difference is stud/nut vs. hex bolt. Either approach works for me. If anyone has strong opinions, please let me know.

Much appreciated.

Ken

PS: FWIW, my new Jorgenson vise has two 5/16" mounting holes which I assume will be more than adequate.

If you do it the way I show then you want to use a stud, just get some all thread and cut it to your needs. If you use a bolt you will have to get one that is exactly the right length or it will push up though the hardboad.

By using all thread you can screw it in until it hits the hardboard and then put the washers and nut on and it makes no difference how long it is.

[Ken Cohen]

I used Bill's suggestions to mount the vise today. Attached are a couple of pix. Nothing pretty, but easy and effective.

Note that I ended up using T-Nuts from Rockler that screwed to the MDF substrate rather than embedding with chisel points. No particular reason -- it's what my Rockler had in stock.

Thanks again to all for the help.

Ken

PS. New post to follow re: after mounting, vise face now toed out in wrong direction.

http://lh3.ggpht.com/_zGAEh1KA3WM/SkbPl8pVPjI/AAAAAAAAAwU/4rerOxxUyrg/s512/vise.jpghttp://lh5.ggpht.com/_zGAEh1KA3WM/SkbPlx9HSiI/AAAAAAAAAwY/Xj9i9iPi_LY/s512/t%20nut.jpg

[Art Bianconi]

As a general rule I try to avoid using MDF for anything except disposable fixtures. The exception to this rule are devices that benefit from the high density of the material which lowers their natural resonance over pine. Thing like Cojones drums and base speaker enclosures are not likely to see high stress but the tone they develop with MDF is hard to match. And, after my bad experiences with wood screws, when I do use MDF, it's a glue only application.

I am suspicious of the dimensional stability of MDF so when it came time to design a table top for my router bench, I went with two pieces of birch veneered 3/4" plywood, glued together and then covered the top with high gloss Formica. (The bench does double duty as a vacuum bagging bench for smaller parts).

A similar method was used in building a purposefully over-designed mirror polishing bench (I needed the weight for stability). There I installed "T" fasteners on the top facing down before applying the black laminate and used 3/8 x 24 threaded bolts from underneath to hold it.

Another bench top used MDF and over time I found the bolts coming loose, not because the nuts had loosened (I use Loc Tite on all machine threaded fasteners) but because of changes in the geometry of the MDF. Maybe it was a moisture issue. Maybe the bolts stretched under the tension (unlikely). Not sure but in any event, I make sure to spray a sealer on all exposed wood and MDF surfaces to maintain moisture content and help insure dimensional stability. All these devices need a flat and true surface that will stay flat and free from warpage. I can't count on that with MDF to the degree I can with good plywood.

Depending on the application, large fender washers or a 3" square piece of 3/4" oak with a thru hole insures that the compressive loads are spread over a large area.

Art

PS: Disregrad the featherboard.