drills for tapping Q

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I've been leafing thru Machinery's Handbook and saw the chart for drills needed for tapping different holes. I was surprised by the number of letter and number drills that are used for these purposes. So I went to Lowe's and Harbor Freight and Home Depot and a lot of other places and they don't seem to exist.

COuld regular fractional bits be used? The handbook lists tolerances allowable for tapping based on the tap size and # of threads (it gives a minimum and maximum allowable hole).

I guess I could try my hand also at making a boring bar from my 3/8" HSS blanks and custom my own hole too. I have a South Bend 9A lathe, btw.

Oh, and I plan on tapping a variety of steels for some projects.

As you can tell I'm a beginner so your suggestions appreciated. I don't have much on my credit card so please don't suggest anything that's too expensive. LOL

Dave V
 
Harbor Freight sells sets with number drills. HSS, TiN coated and Cobalt. (edit) They've got a 60 piece set on sale now for $20.

What I've always done is look up the decimal inch or metric size of the recommended drill and scrounge through what drills I've got to find the closest size using my caliper. This gives also gives you some choices on how tight you want the hole to be.

I've got a number set now but I still scrounge through my box of mixed bits when tapping because I have some I like.
 
Depends on how much strength you require from the internal threads. Can't remember where I've seen the info, but you do not give up a lot of strength as the tap drill size increases. I went for a few years without number and alpha drills with little to no problem for holes 1/4" and up. That was for coarse threads. It is much better to make the hole oversized than undersized. Undersized holes lead to hard tapping and broken taps.

Sometimes we get our underware all knotted up over stuff that is really unimportant. Make chips and see what you can live with.

Bill
 
thanks, all for the tips! So some of you get by without number/letter bits I take it.

Alan, you mention scrounging. I used to do that as well, but one day I got tired of that and hauled out the table saw and made little box holders for drill bits.

The frustrating part was some bits have numbers stamped on the shank in ink and they tend to become unreadable after some time. I actually had to use a micrometer to determine which bit was which. Now they're in little compartments and as long as I return them to compartment where they belong, I should be fine.

I also did that with taps and dies in a separate holder.
 
Bill, very useful! I see they've got other charts as well. This tap chart is better than what's in 19th Ed of Machinery's Handbook. Thanks.

Dave
 
Those ubiquitous drill/tap charts are kind of "middle of the road", usually for 75% thread, as mentioned. That has to do with the amount of surface contact on the thread flanks between the screw and the threaded hole, for instance. The greater the contact area between the two, all other things being equal, the stronger the connection will be. But the smaller the hole, the harder it will be to tap, as you are cutting "deeper" threads to achieve greater than the standard 75%. It becomes a more important if you are pushing the limits on the strength of the material, or perhaps having to compromise on the ideal diameter-to-depth ratio. Like in thinner material, you should probably try to use a smaller drill to gain a little more flank engagement and hence strength from the thread.

Machinery Handbook does give more data regarding the upper and lower limits for the hole size, and major diameter size in the case of OD threads, but the tightness or looseness of the thread ft is more controlled by the pitch diameter, also in MH. Generally referred to as class (i.e. 2A, 3A or 2B, 3B, etc.) of fit. That has little to do with the drill, unless you actually drill larger than the specified PD. Hole size limits do change a little with class. You are free to choose any method or size to make the hole, as long as it stays within the limits given in MH. Choosing the tap, with it's own sizing system will control the thread fit with the mating part.
 
Yes you can get by with just fractional bits. Just select the nearest size. I usally go the next size larger. It dosent make the thread loser, thats controlled by pitch diameter, which is controlled by the tap itself. You give up strength due to less engagement but not usally enough to even worry about. I have several sets of numbered and letter drills, i just rarely use them unless im trying to hold a specific hole dia. Tapping dosent have to be that close in the majority of applications.
 
The one thing I would add is to always go a little bigger if picking a decimal equivalent (rather than smaller). It's very easy to break a tap if you go too small.
 
the chart pdf worked like a charm! I just tapped 12 holes with a 3/8-16 tap and a 5/16 drill.

DaveV
 
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