# Pilot hole sizing for wood screws



## jharris (Jan 8, 2011)

Hello all, hope everyone's fat and happy!

I'm putting together the tenoning sled for a router table featured on page 44, volume 30, #180 of Woodsmith magazine. The plans call for a 3/16 x 1 1/4" screw to attach the fence clamp to the fence. 

I have a chart from online extras re: pilot holes but it doesn't refererence screw diameter in fractions. It refers to screw diameter by "screw gauge". 

How do I convert the screw gauge shown in the chart to fractions?

Since I'll be using this tenoning sled until I can afford to purchase top quality dado blades for the TS and will be changing out the sacrificial fence frequently I want to get the pilot hole just right so it doesn't get worn out and loose.

Help please.

Jeff


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## cabinetman (Jul 5, 2007)

Try this chart.












 







.


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## BWSmith (Aug 24, 2010)

I have these little Starrett fractional/decimal drill charts stuck to several pcs of equip here...........BS,DP,ect.

Sure wish I had a few of those charts like C-man posted.3"x5",white background with black lettering,printed on heavy cardstock.BW


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## jschaben (Apr 1, 2010)

Quick way to convert decimal to fractional is to multiply the decimal number by an arbitrary denominator an use the nearest whole number. ie 0.167 * 64 = 10.688 or 11/64.:smile:


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## BassBlaster (Nov 21, 2010)

jschaben said:


> Quick way to convert decimal to fractional is to multiply the decimal number by an arbitrary denominator an use the nearest whole number. ie 0.167 * 64 = 10.688 or 11/64.:smile:


 Say what!?!:huh:


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## jharris (Jan 8, 2011)

jschaben said:


> Quick way to convert decimal to fractional is to multiply the decimal number by an arbitrary denominator an use the nearest whole number. ie 0.167 * 64 = 10.688 or 11/64.:smile:


This wouldn't be a decimal to fraction conversion. The pilot hole reference chart that I have lists the screw size as a "gauge" value. For example an 8 gauge screw requires an ......... pilot hole.

What the heck is an 8 gauge screw? 

Jeff


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## Jammer (Jul 15, 2009)

BassBlaster said:


> Say what!?!:huh:


I'll try--

Given a decimal number less than one, and wishing to convert it to a fraction, multiply it by the fraction you wish to end up with.

That is, if you want sixteenths, multiply it by sixteen, if you want eighths, multiply it by eight, or, as in the example above, by 64 if want sixtyfourths.

You will end up with some number, which is the numerator (the top number) for the fraction you are seeking.


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## jharris (Jan 8, 2011)

cabinetman said:


> Try this chart.
> 
> 
> 
> .


Got it Cabman. Thank you very much. You da man.

Jeff


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## Jammer (Jul 15, 2009)

jharris said:


> This wouldn't be a decimal to fraction conversion. The pilot hole reference chart that I have lists the screw size as a "gauge" value. For example an 8 gauge screw requires an ......... pilot hole.
> 
> What the heck is an 8 gauge screw?
> 
> Jeff


Gauges are "in between" fractions, and are standard sizes.

They run both smaller and larger than fractional sizes, as well as in between fractions. They include numbered sizes as well as sizes denoted by letters.

I use a chart.

Once you pick them up, you'll wonder how you ever lived without them.


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## jschaben (Apr 1, 2010)

What I actually do is grab the drill index and start plugging the screw in the holes until I find the closest one to the screw "root" and use that bit:thumbsup:


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## jharris (Jan 8, 2011)

cabinetman said:


> Try this chart.
> 
> 
> 
> ...


Just finished up. Used Cabmans chart. Had to go to Lowe's for a 9/64 bit. I drilled my pilot holes using this bit but the hole was to tight for the screw. I attribute this to the material. 
The fence on this jig is maple. Had I used a softer wood the 9/64 pilot hole would have worked fine.

All I did was re-drill the pilot hole using another bit a fraction larger and it worked nicely.

I saved the chart Cabman suggested as a PDF and will always start with it, consider the hardness of the workpiece and go from there.

Thanks everyone!

Jeff


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## jharris (Jan 8, 2011)

jschaben said:


> What I actually do is grab the drill index and start plugging the screw in the holes until I find the closest one to the screw "root" and use that bit:thumbsup:


Hell, I can't do that. Way too simple and easy. Great idea! Doh! Smile

Jeff


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## rrich (Jun 24, 2009)

For a pilot hole, hold the screw up to a light background. Hold increasing smaller drill bits up behind the screw. When the body of the screw just barely hides the drill bit, you have found the pilot hole drill bit.

For a body (clearance) hole. Use the same technique as above except use the larger of the screw shank or outside diameter of the threads.

BTW - Slightly countersink the pilot hole. Slightly countersink the exit side of the clearance hole. This will prevent bridging.


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## Jammer (Jul 15, 2009)

The body, not the threads.

I do it the other way around, and hold the bit over the screw, and pick the one that leaves the threads showing, but not the body.

You say tomato, I say tomato.

Lately, I've needed a magnifying lamp. But that's another story, and the *real* answer to that problem is to hire an apprentice and teach him to do it.

After you get a few holes of experience, you'll be able to tell by the feel when you drive the screw if you have the right bit.


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## rrich (Jun 24, 2009)

Jammer....

Words /words

I got the "body" word from drill and tap guides for metal working.

Not tomato, CABBAGE!


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## jharris (Jan 8, 2011)

Finished the tennoning sled today. Drilled the pilot holes according to the chart but I had to re-drill a fuzz larger to get the screws in.

I haven't tried it out yet as I have to make a stop for my router fence so I don't cut past my sacrificial fence and into the sled fence.

I'll post photo's as soon as my mobile app stops crashing on attachment.

Jeff


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## jharris (Jan 8, 2011)

Here ya go. I added the tapered handle.


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## woodnthings (Jan 24, 2009)

*Nice looking setup*

Just a few questions:
Does the particle board "sacrificial fence just slide in from the end ? Or do you have to take off the aluminum angle to replace it?
I see no miter slot track/guide bar on the bottom, and that's good, other wise you couldn't adjust the jig closer to the router table fence. :thumbsup:
Finally, what are those gizmos in the grey plastic container? :blink: bill


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## jharris (Jan 8, 2011)

The sacrificial fence slides in from the sides or bottom after slightly loosening the clamps screws.

The holes in the aluminum angle are over-sized so that you can pull the angle into the rabbet in the top of the sacrificial fence and tighten the screws. 

I found that as I tighten the screws the clockwise rotation of the screws actually helps to draw the clamp snug to the sacrificial fence.

The gadgets in the grey box are Kreg set-up bars. They're one of the best purchases I've made. 

I use them primarily on the router table for vertical horizontal cut depth but they have proved very useful for hand held routing as well. 

Highly recommend!!!!


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## Pirate (Jul 23, 2009)

I just use a dial caliper to measure the root, and shank diameter.
Faster than using a chart, even if I knew the screw size.


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