Updated Tap Drill Sizes

I was going through my copy of machine shop essentials by Marlow and ran across a comment where it was stated that the tables listed in machinery's handbook were from the 50's and 60's, where most twist drills drilled oversize. These same figures used today are producing thread engagements of over 70%, leading to increased tap breakage. Is there a quick guide listed online that provides updated tap drill sizes?

-John

jlakes85 said:

I was going through my copy of machine shop essentials by Marlow and ran across a comment where it was stated that the tables listed in machinery's handbook were from the 50's and 60's, where most twist drills drilled oversize. These same figures used today are producing thread engagements of over 70%, leading to increased tap breakage. Is there a quick guide listed online that provides updated tap drill sizes?

-John

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1/4-20 thread you take the 20 and get 1/20" or .050" so .250-.050=.200" tap drill for about 75% thread. (metric 6x1mm thread is 5mm tap drill size)
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hand sharpened drills tend to drill more over size. and can drill from 0 to 4% or more oversize
2% of .200 is .004" or .204
4% of .200 is .008" or .208
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there are many charts on tap drill size in machinerys handbook. there is a chart that recommends deeper holes use a bigger tap drill size and shallow depth holes use a smaller size tap drill. this not compensating tap drill size for depth of hole often will break taps. that and trying to use a hand tap rather than a spiral tipped or spiral fluted tap for chip control
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i have seen badly sharpened drills bits in 1.500 size easily drill oversize more than .030", it really matters how good the sharpening of drill bit is

It has always bothered me with 3/8 x 16 suggested tap drill being 5/16.Busted several taps over the years when tapping materials like 304 SS. Now I use letter O drills instead. And a P drill if deeper than .5"

Some drill and tap tables include a value for Probable Hole Size that corresponds to the percent full thread for the recommended drill.

Those tables give useful insight into optional selections. It's always a good idea to check the hole size after drilling the first hole with any drill bit. Tools materials and machines can give differing results under differing conditions.

Blindly following tables is ....well..... blind.

Toms Wheels said:

It has always bothered me with 3/8 x 16 suggested tap drill being 5/16.Busted several taps over the years when tapping materials like 304 SS. Now I use letter O drills instead. And a P drill if deeper than .5"

Click to expand...

SS you go to 50% thread. Not 75%.

jlakes85 said:

I was going through my copy of machine shop essentials by Marlow and ran across a comment where it was stated that the tables listed in machinery's handbook were from the 50's and 60's, where most twist drills drilled oversize. These same figures used today are producing thread engagements of over 70%, leading to increased tap breakage. Is there a quick guide listed online that provides updated tap drill sizes?

-John

Click to expand...

John,go to "Little Machine Shop" website. You can download two charts,metric and imperial.Screw sizes from 0 to 1" in standard sizes,every thing you need to know.
There are formulas that I have forgotten but you need too many to remember anyhow.The chart gives you drill sizes for 75 or 50%.
There are 3 thread sizes listed for each screw size,it gives you the correct drill size for each thread.Sometimes I might want 65%, I pick a drill between the 50% and 75% on the chart.

mike

Sounds good. I've been referencing an old MSC shof chart that only had 75% and I think that gave me tunnel vision..

Most of the tap drill charts that I have seen mention THEORETICAL TAP DRILL SIZES. That means that if the hole is the size it is supposed to be you get a thread engagement the same as the chart shows.

I can't imagine that in the 50's and 60's the technology wasn't available to sharpen drill bits to cut to size.

Rule of thumb:
75% engagement for free cutting materials
60% engagement for hard materials.

If it's hard to tap:
Try a new tap
Drill hole bigger (sometimes, there isn't a size you need.)
Tapping fluid does make a difference.
JR

With the right type of tap I never need to go below 65% thread in any material in my home shop. For a lot of threaded holes I do at work, same story because customers specify an ANSI standard thread and they have a minimum I.D. for threads. See Machinery's Handbook. ISO/metric threads do too. I had parts returned once for having too big a minor diameter on a roll form tapped thread. Fixing it tool going only 0.0004" smaller on the drill, thread minor diameter came out 0.002" smaller. I now check pitch with go/no-go gauges and minor diameter with gauge pins.

Few customers actually check it. This one did and I was told to make sure we don't ever get returns for that again. It's harder to achieve with form taps but controlling the drilled hole size is everything.