MDF benchtop & holdfasts

[Ben Beckham]

My benchtop is 6 layers of 3/4" MDF (3 sheets halved and glued) for a total of 4.5" of thickness. Heavy as crap and ugly as sin, but extremely functional. A great bench on which to build a better bench later. Dog holes are 3/4" round holes.

I just got a pair of custom made traditional holdfasts meant to work with 3/4" holes and they aren't working great yet. I think my problem is one of two things. Either the top is too thick for holes of that size to work, or the holdfasts just won't grip the MDF well enough, or maybe even some combination of the two. If the MDF is the problem, I can use the <cough>circular saw and router</cough> to remove a strip on top and bottom and inlay pieces of beech to grip right. If the top is too thick, I think maybe I can just make the holes bigger on the underside, so the grip length goes down.

Thoughts? Anyone ever have similar problems?

[Jim Matthews]

I wonder if the process burnished the hole and rendered it slick.

If the holdfast doesn't grip the side, the flex in the arm will pull it free.
Can you rough up the column of the holdfast?

What if you spray on some 3M 77 (http://solutions.3m.com/wps/portal/3M/en_US/3M-Super-77/Super77/) to test the idea?
It should be easy to remove, if it doesn't help.

[Harvey Pascoe]

I use one sheet of MDF on my bench as a sacrificial surface. When it gets all crapped up I just flip it over. I use a variety of different type of stops and all have similar problems. The MDF is not strong enough and the holes distort and the stops break out. To fix this, I cut a mortise in small blocks of maple, then glued them into an enlarged hole in the MDF. No more stop break outs.

[Simon Frez-Albrecht]

I agree the issue is probably the softness of the MDF. I use my Gramercy holdfasts in a 5 1/4" thick benchtop no problem.

[Chris Griggs]

Yep the MDF bruises/distorts/crushes - had the same problem with my old bench which was made of 2 layers of MDF. Traditional holdfasts aren't a great option for an MDF top. You may want to look into some of Lee Valleys modern holdfast/holddown clamps.

[glenn bradley]

+1 on the smack-em-in holdfasts not being the best idea in MDF. I use these.

[Ben Beckham]

I agree the issue is probably the softness of the MDF. I use my Gramercy holdfasts in a 5 1/4" thick benchtop no problem.

Great to know the thickness of the top is not the problem. Looks like I can saw and rout out and replace a strip of the MDF on top and bottom with some beech, maple, or SYP and they ought to work fine.

Wife wanted to go to a haunted house tonight. I know this is a hand-tool forum, but I can't help but chuckle thinking about what she'll say when she gets home from the office this evening and finds me destroying the shop (aka the florida room off the kitchen in the house) with power saws and routers....

[Mario Soldevilla]

my bench top is a single layer of 1 1/2" mdf sandwiched between 1" thick baltic birch ply. my holdfasts work great in this set up, perhaps you could inlay some plywood discs around you dog holes top and bottom to provide grip.