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Drill size for 5/16 lag screw

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PostPosted: Mon Jul 04, 2005 3:43 am Post subject: Drill size for 5/16 lag screw Reply with quote

For attaching a newel post for the first time with 5/16" lag screws into Douglas fir, the screw is going into the doug fir carriage lengthwise. Usually you can use a drill about equal to the body size of the screw. But since you have only one chance to get this right, and since you need very good holding strength, you need to know what to do. You know you also have to get a relatively expensive extra long bit so you can use the newel (with holes already drilled) as a pilot hole.

Also, when you removed the old stairs, you noticed that the lag screw threads have squared edges, not a knife edge like the replacements. Is this significant?
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PostPosted: Mon Jul 04, 2005 6:52 am Post subject: Reply with quote

First let's define: a lag screw is a heavy wood screw having a square bolt head:
Screws are usually produced by one of three methods: machined, rolled or stamped. At times two screws with the exact same TPI and the exact same pitch -- and by the same manufacturer -- will appear to have different edges: one sharp and one squared. This isn't an optical illusion; it is caused from the wear on the processing machinery and/or coatings.

Use a pilot drill a bit smaller than the screw body. In the case of edges, the sharper edge offers greater holding power. If you carefully compare the old screws with the "squared edges" to the new ones with the "knife edge" you will very probably see that they have different TPI (Threads Per Inch), as well as different thread pitch. Basically, the more TPI of a screw, the "sharper" the thread edge will be. As far as thread pitch (or angle) goes, a thread pitch of 55 degrees, for example, will produce a sharp-edged thread, but 60 degrees -- a simple 5 degree difference -- will produce squared-edged threads. As far as "holding power" goes, there are -- technically speaking -- four (4) main "strengths" to a threaded fastener: yield, tensile, shear, and pullout. There are several factors that are very carefully considered when establishing these ratings, and thread pitch -- which in part affects the thread's embedment into the material (full or partial) -- is a key to factoring pullout strength. Thread embedment is also affected by the diameter of your pilot hole. Too large a hole will offer only partial thread embedment and possibly poor performance. Too small a hole will very possibly lead to unwanted fractures in the material, and a too small a pilot hole will also create dynamic friction when the screw is driven, and that heat will cause very minor but very relevant expansion in the screw resulting in poor thread embedment when it cools. Basically, then, there is only slight performance differences between the sharp and squared edged threads; the key is your pilot hole and the surrounding material it leaves for full thread embedment.
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