Installing twin screw vise...oversized holes?

[Andrew Levine]

I am planning to install my twin screw vise on my 36" wide bench as a huge end vise. I don't have a skirt on my bench (it's a huge douglas fir slab), so I will install one that will act as the rear jaw. The vise comes with 3/8" bolts and round barrel nuts to secure it to the bench (see picture below from the directions).

This skirt will be installed with the grain opposite that of the benchtop - but the directions have me drilling 13/32" holes. The bolts are 3/8" (or 12/32"). This only gives me 1/32" of expansion room for the benchtop. Should I oversize these holes? There is a washer so they should be held in securely even with a bigger hole (right?). How much should I oversize them?

I'm surprised the directions missed this...maybe they assume you are installing it as a face vise, but the pictures in the manual show it as an end vise.

Thanks.


vise.jpg

[Lee Schierer]

You can check the amount of expansion contraction at this site: Wood Movement Estimator Be sure to pick the correct type of Douglas Fir.

As far as the instructions go, they are generally correct, but the manufacturer does not know that you are mounting the vise cross grain. Once you calculate the seasonal wood movement for your slab you should be able to determine how much side play you need to add in the holes. Personally, I would slot the holes in the vise back board instead of trying to elongate the holes in the slab.

[Dave Richards]

The manufacturer also has no way to know how much expansion and contraction your bench top will experience so they leave it up to you to make the determination of how much of an allowance you need to make.

I agree with Lee. I would slot the holes in the vise backboard and let the bolts move with the top. Actually, you can probably just slot one of them and leave the other round.

Typically the end pieces on a bench top are fitted with a tenon or tongue on the bench top and mortises on the end piece. In this case, since you're retrofitting, it might be easier to reverse the joint. I would be inclined to do something like this.



If you want to glue it, only do so at the front end for a few inches. Slot the bolt hole toward the rear.

[Andrew Levine]

Oh I was definitely planning on slotting the apron, not the bench, that makes sense. I wasnt planning on gluing it.

Is the tenon necessary? I suppose it will give more gripping power than the bolts alone, but seems tricky to do. I guess I could use a rabbeting bit and a router equipped with either a straightedge or fence?

[Dave Richards]

As I said, it is typical to put a tenon on the end of the bench to fit into a mortise in the end cap. I think at this point it might be easier to cut a dado into the bench. I probably doesn't make any difference which way you go but I would do something. You could also use a spline if you put dados in both parts. The key thing is to keep the end cap from moving up or down