Screws and pilot holes--I'm confused

[chuckt]

I don't understand the Fuller tables on pilot holes for screws. Lets take a number 10 screw for example. It has a 3/16 shank diameter and a 1/8 root diameter. The Fuller chart recommends a 13/64 taper drill to do the pilot hole. Okay, 13/64 equals 3.25/16. That's bigger than the shank size so there's nothing for the threads to bite into at the top of the hole?! Or does the bit taper so quickly that the thread soon finds wood to bite into. But even this would leave a minute gap along the shank of the screw? Wouldn't I want no bigger than 3/16?


I guess drilling depth is very important. If one goes too deep, the screw won't have any wood to grab.

[chuckt]

And maybe the Fuller bits aren't the best way to do a pilot hole for refastening?

[Peerie Maa]

I use an adaptor that takes the correct twist bit for the core of the threads, then follows with the correct size hole for the shank, and fits into a counter bore with a depth collar.
Like these http://www.screwfix.com/p/erbauer-countersink-set-4-pieces/33260

[Gerarddm]

Yes, you need to set the stop collar to be no longer than the depth of your screw penetration, and perhaps even slightly less.

I've never had problems following the Fuller sizing guidance.

[Bob Smalser]

Stop using the chart and drill some test pilots in scrap instead.

A combination of soft screws and hard wood requires some fine adjustments no chart will provide you. You should even slice a few pilot holes in half on the bandsaw to see exactly what you are doing by comparing the hole with the screw. Then after some test drivings so you're confident you won't either twist a screw off or have one pull out when driving screws on a production basis, organize your pilot drills, screws and bungs for the task to minimize the chances of a mistake.

http://pic20.picturetrail.com/VOL12/1104763/7534648/369032714.jpg

And obviously, the reason collars are provided is to prevent drilling too deep.

Further, insure you dribble some red lead or copper bottom paint in the pilot holes to minimize the chances of rot and bed your screws with goo to minimize water ingress. Here after a dozen years in the water, you can easily tell which galvanized screws I bedded properly and which ones I didn't:

http://pic20.picturetrail.com/VOL12/1104763/9745605/397619693.jpg

[chuckt]

Thanks guys. Bob--I think I will try a little experimentation as you suggest. Gettign ti right I think is fairly important.

[chuckt]

Bob--I saw in an old thread you recommended driving the screw in and the backing out to align the slots? I don't follow how this could be done while keeping the right torque.

[Bob Smalser]

I saw in an old thread you recommended driving the screw in and the backing out to align the slots? I don't follow how this could be done while keeping the right torque.

That's only for fancy work with light-duty hardware. Bung your refastening job without trying to time the screws.

[chuckt]

Okay--last question. I'm tightening by feel. The best I can explain it is that I crank the screw in until I think that any more turning will cause the bit to walk out of the slot. So its pretty snug.

The ones I am taking out come out pretty easily. They are snug but not stuck at all. I'm not really having much problem with the bit walking out of any slots.

[Bob Smalser]

Okay--last question. I'm tightening by feel. The best I can explain it is that I crank the screw in until I think that any more turning will cause the bit to walk out of the slot. So its pretty snug.



Sounds OK. Run 'em in close with a power driver and finish with a brace and screwdriver bit. I like a small, 6" brace for most screws. Power driving them in all the way isn't the best practice, as they are often overdriven.

[Mrleft8]

I sometimes use soap (like Ivory, or Dial...) as a lubricant on screws in very hard woods, like Maple, Live Oak, and the such.... I drive them in until they seat properly, then I back them out a turn, and re-seat them again holding back just a hair on the force. I don't know if this helps in any way, shape, or form, but it's what my father used to do, and so I, like most good monkeys, do the same... ;)

[Tom Lathrop]

I don't understand the Fuller tables on pilot holes for screws. Lets take a number 10 screw for example. It has a 3/16 shank diameter and a 1/8 root diameter. The Fuller chart recommends a 13/64 taper drill to do the pilot hole. Okay, 13/64 equals 3.25/16. That's bigger than the shank size so there's nothing for the threads to bite into at the top of the hole?! Or does the bit taper so quickly that the thread soon finds wood to bite into. But even this would leave a minute gap along the shank of the screw? Wouldn't I want no bigger than 3/16?


I guess drilling depth is very important. If one goes too deep, the screw won't have any wood to grab.

When I read the Fuller pilot hole table for 3/16 #10 screws I get 9/64 for hardwod and 1/8 for softwood. These seem reasonable to me.

[chuckt]

Your looking at the size for a straight bit Tom which I did not even notice. I was looking at the tapered bit size which I have found out is the diameter at the widest part of the root. It tapers down so should be okay. As Bob suggested, I will do some practice pilot holes to make sure it works.