Thread - Pilot Drill Charts

[Jacques Reed]

Originally Posted by Lionelgee

Hello All,

Just wondering if there is some form of "Quick Reference Guide" or chart available; online or elsewhere, which identifies what size thread or drill should be used when working with different material. For example, when drilling a pilot hole for a 6 G screw into wood what size should the pilot drill be?

Once upon a time this information used to be printed on the screw's packaging. This information is getting rarer to find each time I go into a hardware store.

Also, is there some form of table that shows if a piece of steel rod of a certain dimension needs to be threaded - what size thread is the most suitable for the application?

Similarly, a chart that shows what size tap to use to cut a thread into a specific drill diameter?

Any assistance would be greatly appreciated.

Kind regards
Lionel

Hi Lionel,

With reference to wood screws in softwoods, it is not that critical to just eyeball a drill size the same or slightly larger than the minor diameter of the screw (diameter at the bottom of the threads). With hardwoods, use a larger drill with a diameter up to half the depth of the threads. I have broken off woodscrews in hardwood by using too small a pilot hole. With a set of fractional and metric drills you will always find a good size. If you find the screw very tight going in just back it out and run the next larger size drill down the pilot hole.

The diameter of holes for tapping and diameter of rods for threading are always determined for the specific tap or die and not the other way round. There is some tolerance with threading a rod slightly oversize, but little or no tolerance for a hole undersize for tapping. A broken tap will likely ensue.

Studying a tapping and drilling chart will show you what is possible.

Hope this is of some help.

Cheers,