Dog/Holdfast hole size tolerance

[Ryan McKenna]

Hi All,
I am working on a workbench with a 2x4 top. The top will end up about 3" thick. I am at stage to start drilling my dog holes and need your guys' advice on drill bit size tolerances. My plan had been to use my brace and #12 auger bit for the holes. I have bought a pair of Gramercy hold fast and drilled a hole in the bench top to use one when working on the apron dovetails. Granted I was only using one holdface but I did not find it held that great. i roughed up the shaft a but using sandpaper like i have seen recommended before but it did not help much. I noticed there was a bit of space between the holdfast and the hole when inserted so I measured the auger bit and it is 0.755. So this week I stopped at Lee Valley and picked up a 3/4 auger bit but when I went to use it tonight realized the drill I had that I thought could use 1/2" shanks was actually only 1/4".

So before I exchange this bit for something else I thought I would check with you guys to ask if my original bit too big to use with the holdfasts? Should I try roughing up the holdfast shaft more? Or should I get a different bit from Lee valley?

Thanks for your advice.
Ryan

[ken hatch]

Three inch thick slab should be no problem.....that said, holdfasts are a little black magic and some times one will give problems for no apparent reason. The #12 should work fine, what wood is the slab?

ken

[Jim Koepke]

What is the measure on the holdfast shafts?

My recollection is mine are ~5/8" and work fine in a 3/4" hole.

When abrading the hold fasts, go around the shaft, not the length.

jtk

[Matt ONeill]

Ryan,

I've got a number of the gramercy holdfasts, and sandpaper never worked for me.. I tried counterboring my holes (4" thick bench), using sandpaper on the holdfasts, nothing would work. Someone somewhere suggested using a center punch on the shaft instead of sandpaper, and boy did that do the trick. I just used one of those cheapo spring loaded center punches, and went up and down the shafts real quick. Now they grip like the dickens. It made such a difference, I think Joel should change his instructions on the TFWW website.

- Matt

Hi All,
I am working on a workbench with a 2x4 top. The top will end up about 3" thick. I am at stage to start drilling my dog holes and need your guys' advice on drill bit size tolerances. My plan had been to use my brace and #12 auger bit for the holes. I have bought a pair of Gramercy hold fast and drilled a hole in the bench top to use one when working on the apron dovetails. Granted I was only using one holdface but I did not find it held that great. i roughed up the shaft a but using sandpaper like i have seen recommended before but it did not help much. I noticed there was a bit of space between the holdfast and the hole when inserted so I measured the auger bit and it is 0.755. So this week I stopped at Lee Valley and picked up a 3/4 auger bit but when I went to use it tonight realized the drill I had that I thought could use 1/2" shanks was actually only 1/4".

So before I exchange this bit for something else I thought I would check with you guys to ask if my original bit too big to use with the holdfasts? Should I try roughing up the holdfast shaft more? Or should I get a different bit from Lee valley?

Thanks for your advice.
Ryan

[Richard Hutchings]

I have the the exact bench top as you. 3" thick made from 2X4s, and gramercy holdfasts. I used a 3/4" auger bit and I never even cleaned or rough sanded the holdfasts as many suggest. I wanted to see how the work right out of the box. Perfect everytime.

[Bob Snyder - Austin]

I have the the exact bench top as you. 3" thick made from 2X4s, and gramercy holdfasts. I used a 3/4" auger bit and I never even cleaned or rough sanded the holdfasts as many suggest. I wanted to see how the work right out of the box. Perfect everytime.

I have this experience also. However my Veritas dogs needed a slightly larger hole.

[Ryan McKenna]

I'm not sure what the wood 2x4 are up here. I believe it is either spruce or a type of pine.

[Ryan McKenna]

Thanks for the replies everyone. Sounds like my original auger bit should work. I will try roughing up the holdfasts a bit more. I think last time I sanded in a spiral down the shaft, this time I will go around it more.

Thanks for the tip Matt, if sanding doesn't work will try the centre punch on one of them. I haven't seen that tip before.

[Marc Seguin]

I'm in the process of making a bench right now as well. I used one of my flea market #12 auger bits without checking it first and found that it measured 0.770" after I had most of the holes drilled. The grammercy holdfasts I have come in at 0.717". At first I was pretty worried about the sloppy fit, but after trying them out there hasn't been a problem. My bench top is only 2" thick Douglas Fir though.

Even though my holdfasts worked well from the get go, I did do the sandpaper thing to them. It didn't seem to make a massive difference. What DID make a world of difference was epoxying some leather onto the pads. They grip much more tenaciously with the leather face, and it's friendlier to the work pieces.

I'd try the leather upgrade with your existing brace holes before trying another bit.

[lowell holmes]

Auger bits are 1/64" oversize according to the booklet that came with my Irwin bits. I would suggest you drill the holes with a forstner bit.
Check the inserts here in this link.

http://www.leevalley.com/US/wood/pag...,41637&p=72602

[Jim Koepke]

Auger bits are 1/64" oversize according to the booklet that came with my Irwin bits.

There are different types of auger bits. The common types for the construction trades are oversize. Dowelling bits will be very close to exact size.

Here is a post of mine where eight 1/2" auger bits are compared:

http://www.sawmillcreek.org/showthre...t-About-Augers

jtk

[Stephen Clement]

I had great success with my gramercy holdfasts in my 3" bench (Douglas fir) using a #12 auger and sandpaper on the shafts. I have found they hold better if you put your thumb on the pad while you strike the crook. This keeps the holdfast from bouncing instead of setting.

[ian maybury]

Even our esteemed mag writers treat the topic of holdfasts gripping as though it's a 'mystery'. It isn't...

There are two primary variables involved - the clearance/diameter difference between the shank of the holdfast and the dog hole in the bench, and the thickness of the bench top. (= effective depth/length of the dog hole). These combine to determine the angle the holdfast can lean out of vertical by when forced to tilt by the action of the clamping force under the pad that bears on the work. This angle in turn determines how effectively the end of the shank protruding out of the bottom of the dog hole can hook under the lower surface of the bench top to resist its lifting out.

If the angle is too little, then the clamping force (having caused the shank to deflect a hair) will result in its sliding upwards in the hole and letting go. Roughening the shank will delay this effect/permit a grip at a somewhat shallower tilt angle, and any trace of oil or wax on the shank will likewise reduce the grip - but these are secondary factors. The real fix is to ensure enough tilt angle.

While mild roughing up is a good idea, heavy roughening of the holdfast shank is probably not - in that it's likely to lead to heavy wear of the dog hole. It's in effect barking up the wrong tree as this is not the primary means by which a holdfast grips.

The mechanism is described in alternative language in post no. 8 here: http://www.sawmillcreek.org/showthre...light=holdfast Follow the link from it to post no. 20 in a previous thread for a more engineering based explanation and drawing.

The Grammercy hold fasts pop up ( accidental pun!) in this sort of discussion more often than others because they are made from bright steel bar - the normal tolerance of which is just a hair under nominal. Which means that they end up being a pretty close fit in a hole drilled by anything that produces a close to nominal size hole - so the tilt angle ends up too small in the thicker bench tops most are going for now. Leading to poor grip. The problem isn't the hold fast, but the hole diameter - they fairly clearly work very well in holes drilled with a brad point bit in tops up to a little over 2in.

It's counter intuitive (how can a holdfast grip better in a sloppier hole?), but as Richard/Jim/Marc/Bob the answer for thicker tops is almost certainly to use a bit or drill type that delivers a larger diameter hole/greater clearance/more tilt angle. Try some test holes first though just in case.

Thicker bench tops reduce the tilt angle for a given hole diameter, so thicker tops need larger diameter dog holes/more clearance over the shank of the holdfast - or alternatively counter bore the dog hole from underneath to reduce the effective thickness the holdfast sees to one that works. This also makes available more clamping height/capacity - and as above don't expect a holdfast to grip properly unless the end of the shank protrudes from the lower end of the dog hole...

ian

[lowell holmes]

There are different types of auger bits. The common types for the construction trades are oversize. Dowelling bits will be very close to exact size.

Here is a post of mine where eight 1/2" auger bits are compared.

jtk

Where is the post? It didn't make the trip.

I don't doubt at all that there are different diameters in auger bits. However, I wouldn't know where to find auger dowelling bits. I've never seen any bit manufacturing company other than Irwin post such information.

[Brian Holcombe]

My holdfasts sort of made their own clearance.