Excuse my ignorance...

I got a tap and die set and was wondering if there is a general rule of thumb as to what size bit to use when drilling a hole to tap it. I've got a decent set of drill bits in 1/16 inch increments, but couldn't find anywhere in my tap and die set as to what size drill bit to use for certain taps. Any help would be appreciated.

TIA

Rookie

I was going to recommend that you pick up a "Machinery's Handbook," but I didn't realize that the current edition is going for $85. It's overkill for what you need to know right now, but it will answer any question that you may have in regards to machining/fabrication ... in addition to what size hole to drill for tapping UNC, UNF, Metric, BSP, ACME, etc. It's a good reference to keep in the garage, especially for those of us with poor memories. :smokin:

In case you do have $85 burning a hole in your pocket, here's the amazon.com link for it:

http://www.amazon.com/exec/obidos/ASIN/0831126256/qid=1015779273/sr=2-1/ref=sr_2_1/103-5518892-3016669

cm "got beadlocks?" k

The rule I always lived by is when tapping a hole drill a hole 1/8" smaller than the tap size, that's what I was taught from working in construction for many years & it's always worked fine. If you have a 1/4" bolt or tap then drill a 1/8" hole.

If you go by that rule you stand the chances of ruining your taps. (Please note that it is the rule I go by anyway since I don't know the proper mathematical formula. :) ) But there is a precise mathematical formula that my machine shop guy knows by heart to figure out what size to drill. I am tapping 1-18 threads right now and, after going through that formula, my friend told me that I needed to drill a hole that was EXACTLY 15/16".

J. J.

I have a craftsman set of tap and dies, the drill bit that is required should be written or stamped on the tap. All the ones i have ever seen are this way. Yours are not marked at all? The smaller ones are generally listed in a 2 digit number instead of a fraction or decimal size.

The best thing to do is use a Tap Drill chart. 1/16" under is what a lot of people use (to give you a 75% thread). Here is a link to a basic UNC/UNF/UNS chart...

http://www.microprecision.com/chart_tap.html

Here's another link to tap/drill info. It contains the same info that Firetoad's link did, but adds quite a bit more to it as well.

Tap-Drill info (http://www.offroaders.com/info/tech-corner/taps-drills.htm)

There is no accurate "rule of thumb". As the others stated you either waste your time calculating it for 75 % thread or you look it up in a standard chart. Half the tap drill sizes you need are not in a standard set. For instance, 3/8-16 (that's course thread) uses a 5/16 bit, but 5/16 course uses a F bit. 1/4 course uses 13/64 (not commonly in your ordinary sets) and 7/16 course uses a 3/8, while 1/2 -12 (course thread) uses a 27/64. Once again, some you will have in your set, others not. You can always drill oversize, but that will weaken the treads. If you drill undersize, you will likely break your tap. Use a lube appropriate to the metal you are taping (for mild steel use thread cutting oil or ordinary mineral based oil), turn and backout to keep the threads and flutes clear as you tap. Consider using a drill press turning the chuck by hand to start the tap straight, if you can. It's easy to get off to a bad start.

If you don't have a drill press, or can get the drill press under your vehicle, you can make sure you start the tap straight by taking a scrap block of material and drill a hole in it that is just big enough for the tap to fit into and set this block over the hole that you need to tap, insert tap through hole and start turning. Once you get it started good you can back out the tap, remove the block and continue tapping.

Tom

Starret has a handy little red and white card that has all of the "Tap-Drill" sizes on it from #2-3/4-11 or something like that. You want to counter sink the hole before you tap. And take $5 invest in a cheap tap block. Some common tap drill sizes off the top of my head are: 1/4-20 #7 drill(.210"), 5/16-18 F drill(.245"), 3/8-16 5/16 drill, 1/2-13 27/64 Drill. Those will give you about 75% thread strength. One other hint, if you are tapping Stainless, go a little bigger(.010) as SS is gummy and likes to break taps. If you need to ream, just go 1/64th under your ream size, e.g. 1/4-1/64=15/64ths drill size. I was taught .002-.005, but find a drill that will drill that close!

EASY MATH PROBLEM
TAKE 1 DIVIDED BY THE NUMBER OF THREADS
AND SUBTRACT THAT FROM THE THREAD SIZE = TAP DRILL SIZE.

1/4-20
.250-20
1 DIVIDED BY 20 = .05
.250 - .05 = .200
.200 DRILL
ACTUAL DRILL FOR A 1/4-20 IS .207:beer:

Originally posted by DAVID
EASY MATH PROBLEM
TAKE 1 DIVIDED BY THE NUMBER OF THREADS
AND SUBTRACT THAT FROM THE THREAD SIZE = TAP DRILL SIZE.

1/4-20
.250-20
1 DIVIDED BY 20 = .05
.250 - .05 = .200
.200 DRILL
ACTUAL DRILL FOR A 1/4-20 IS .207:beer: \


Yeah that gets you close. Closer than you think. Tap drill is 13/64 for 1/4-20 ( I have been using it for a week now in the shop) 13/64 = .203125 (off the top of my head :D)

Back to the orginal Q though. Most Tap and Die sets you buy (even el cheapo model) comes with a chart.

I got a nice big one at work. Its fun to tell people to "go get me an F drill" they just kinda look at you like WTF :D ;)

Off the top of my head means exactly that, I don't have a chart, hand book, or anything else! .207-.210 :rolleyes: Are we that picky when we are welding on perches!? BTW 13/64 works, but you'll break more taps, and in a machine shop that is as bad as a :rainbow: newbie spouting off on a post, oh...........

Originally posted by Magoo
Off the top of my head means exactly that, I don't have a chart, hand book, or anything else! .207-.210 :rolleyes: Are we that picky when we are welding on perches!? BTW 13/64 works, but you'll break more taps, and in a machine shop that is as bad as a :rainbow: newbie spouting off on a post, oh...........

Ummm was there a point to your post. It generally helps if you form your words into sentances. Makes it easier to comprehend....:flipoff2: :flipoff2: :flipoff2: :D

No, no rule of thumb...why don't you just go ahead and drill a 9/16" hole for a 1/4" tap? If the rest of the world followed this logic we'd still be riding horses.

Why do so many people continue to kill ants with a sledghammer around here? You do not need a rule of thumb if all the info is available and free. No offence meant to you that like to do the math.
Any Snap-On, MAC or Matco truck will be happy to give you a little laminated card with all of the info on drills and taps you will need. Most shops that sell cutting eqipment will have them for less than a buck. No more guess work or broken equipment.