Drills for tapping threads in wood

[Doug Shepard]

I need to put a few threaded non-through holes in wood for machine screws. Haven't totally determined what size so I ordered bottoming taps for #6, #8, and #10 machine screws. Now I'm trying to figure out how to drill for them. I'd like to use a brad point but cant find anything all that close to the wire gauge drill sizes. The #8 & #10 closest smaller brad point are 2 wire sizes smaller which I think maybe is OK for threading wood? But the closest smaller brad point for the #6 is 6 wire sizes smaller which I'm sure is way too tight. Do they make brad points in wire gauge sizes??

6-32 #36 drill
8-32 #29 drill
10-32 #21 drill

Here's a compararison chart link
http://www.mcmaster.com/param/html/DrillSizeComparisonTable39921/

[Bill Houghton]

Regular drill bits with the points for metal will work fine on wood, although it's true that brad point drills will wander less when they start. I've got just a couple of brad point drill bits, and can't count the holes I've drilled with the regular jobber's drill bits.

If you want a brad point bit, and you're agile at the bench grinder, you can re-point a conventional jobber's drill bit to be a brad point drill bit by grinding on the corner of the wheel. Not easy for small drill bits, though.

[Bruce Page]

Doug, first off ditch the 6-32 if you can. The 6-32 has the worst root pitch diameter to body diameter ratio and is very easy to break – I’ve broke a ton of them.
A regular twist drill will work fine in wood just use a center drill or punch mark to keep the drill from wandering.
The drill sizes you have listed are for 75% thread, in metal. They should work fine but if you have a drill set available you might try experimenting with the next size smaller to give you a little more thread strength.
In any case, the threads won’t be as strong as if in metal so don’t over tighten them.

[Randall Scott Smith]

Have you checked out W.L. Fuller & Co?

http://www.wlfuller.com/html/brad_point_drills.html

I'm thinking that a 7/64 bit would work for the #6 (0.1094 hole dia for tapping), 1/8 bit for the #8 (0.1250 is a little undersized - should be around 0.1360, but wood should tap a bit easier than plastic), and 9/64 for the #10 (0.1406 - again a little undersized, should be 0.1490). A 5/32 might work okay for the #10 as well.

Here's a handy conversion table for tapping machine screws. I would assume wood taps similar to plastic.

http://www.marfas.com/machinescrewtapping.shtml

Machine screws have a notoriously poor holding power in wood due to their fine threads. That said, have you considered inlaying a nut in the wood as an alternative? Just cut a shallow hexagonal hole, drill an oversized clearance hole in the center of the hexagonal inlay, and epoxy in the nut. That should hold as good or even better than tapping the wood itself. Alternatively, if you're using this for hanging hardware and you don't want it to show, but you can't access the back to use a nut from behind, you might want to consider using a hanger screw.

http://www.whitechapel-ltd.com/tech/fasten_hardware.shtml

Regards,
Randy

[Doug Shepard]

Thanks guys
I've already got the 5/32" (for the 10-32) and 1/8" (for the 8-32) bradpoints which was why I was asking about using 2 sizes down. Actually I guess it's more like 1-1/2 sizes down. I figured those 2 would probably be OK. Since I'm going to heed Mr. Pages advice (No not Jimmy - the other Page:D)


Doug, first off ditch the 6-32 if you can. The 6-32 has the worst root pitch diameter to body diameter ratio and is very easy to break – I’ve broke a ton of them.
...


the other one is now a moot point. I was mainly thinking brad point so I could get a flat bottomed hole. I've only got about 3/8" thick hardwood and was going to use short brass machine screws and washers to attach a leather piece to the wood. I'm doing a couple of scabbard type cases for straight edges. The cap end will be held onto the main piece with small diam rare earth mags but I just wanted a leather strap to keep the cap attached to the body.