Tap and Clearance Drill Sizes-Clarification Needed

Hi gang,
I am acquiring quality drill bits for general use but mostly to fabricate and repair low carbon steel, cast iron and aluminum.
I have a drawer full of bits but, I recently purchased a good set of fractional Rushmore drill bits (short-the name eludes me). I also buy quality tap and threading dies.
My next purchase is to fill in the gaps for lettered and numbered bits.
Example, to drill a 1/4" hole, there are 6 possible drill bits required;
For 75% Thread in Aluminum, Brass and Plastics>>
1/4 x 20 .1887 =#7
1/4 x 28 .2062 =#3
1/4 x 32 .2117 = 7/32

For 50% Thread in Steel, Stainless and Iron>>
1/4 x 20 =7/32 (.2188)
1/4 x 28 .2280 =#1
1/4 x 32 .2280 =#1

For Close Fit,
Drill size .2570=F

For Free Fit,
Drill Size .2660 =H

It makes no sense to buy cheap because the drilled hole will not be round, it's anyone's guess what size you end up with.

This leads me to my confusion.
Do I really need to follow the printed guides? Is there a rule of thumb from you experienced machinists to follow?
For example, when drilling a hole to be tapped in (steel/iron) either 1/4 x 20 or 1/4 x 28, how important is the .009" difference in hole size?
When using a hand tap and watching that thing twist, the .009" seems quite significant! (don't break!)

I look forward to your advice, opinions.
Thanks,
Jeff

 

Machine screw size, or stub size. There is latitude in what tap drill you use. Smaller drills will make for more complete threads, which are stronger, but less so than you might think, and will put more strain on the tap. Larger drills leave a somewhat weaker result, though again, less than you might think, and much less likely to break a tap. Use the chart size for normal tapping in common metals like mild steel and 6061 aluminum for ordinary work, but don't be afraid to change sizes, up or down, for gummy metals or harder and tougher metals or for ultimate strength. The standard charts usually give 75% threads, though they do vary from size to size and from chart to chart. You can use your own percentage number as you wish, the math is easy.

 

They make those charts for a reason . Different class threads have different minor and major diameters as well as pitch diameters . Why not just go and purchase a good set of cobalt split point drills as a number , letter and fractional set ? Save yourself some headaches ! But to answer your question , if you're not doing DOD work and it won't be scrutinized by an inspection department , cheat a little bit on the minor diameter if necessary . If someone's life or health isn't on the line , who cares .

 

When you bust your tap in an undersized hole you'll figure it out.

 

I generally use the "rule of thumb" that the tap drill size should be the nominal screw size minus the thread pitch distance. Metric threads specify pitch. Inch threads specify threads per inch, so the pitch distance is the reciprocal of TPI.

Example: for ¼-20, 1"/20 = .050", so the tap drill should be .250-.050 = .200". The nearest drill size is #7, which is .201". Coincidentally ( ), this is the one you'll find on the printed charts.

By the way ... nominal diameters of numbered screws follow the formula of 0.060" + 0.013" X screw size number. Examples: #0 screw has a nominal diameter of 0.060", #10 a nominal diameter of 0.060 + 10 X 0.013 = 0.190"

 

I do things the wrong way most of the time. In steel, I go one size drill larger than the 75% chart calls for unless it is a critical application, in which case I use the 75% strength recommendation. In harder stuff like stainless or medium/high carbon steels I go one size larger and have gone two sizes up for small holes. For aluminum and brass, I use roll taps only and those require their own drill sizes; I follow their recommendations.

When I say one drill size, I mean the next one up; it might be a fractional, letter or wire size.

I haven't broken a tap in over 25 years because of this, nor have I had a thread pull out or strip. I've done 00-90 threads to 3/4-16 threads this way and have had zero issues so this works for me.

 

Just be aware it isn t the minor diameter your concerned with although it does impact it but it is the pitch diameter. Fine threads don t have the cutting depth like course. Unless a class of thread is needed I just use the major diameter subtract the pitch and like mikey go up in drill size in the harder materials. Doesn t affect the thread as you may think.

 

Hi gang,
I am acquiring quality drill bits for general use but mostly to fabricate and repair low carbon steel, cast iron and aluminum.
I have a drawer full of bits but, I recently purchased a good set of fractional Rushmore drill bits (short-the name eludes me). I also buy quality tap and threading dies.
My next purchase is to fill in the gaps for lettered and numbered bits.
Example, to drill a 1/4" hole, there are 6 possible drill bits required;
For 75% Thread in Aluminum, Brass and Plastics>>
1/4 x 20 .1887 =#7
1/4 x 28 .2062 =#3
1/4 x 32 .2117 = 7/32

Jeff

You should note that the number (in red above) you have quoted for the 75% thread are the minor thread diameter, not the drill diameter. Minor diameters of threads are of little use in deciding tap drill size. Major diameters and pitch are the best determinant.

I would just buy a decent quality 115 pc. set and be done with it. As to sizing the hole, the rule of thumb major (nominal) diameter minus the thread pitch is an easy to remember formula that you always have with you. For metric threads, it is fairly simple as pitch is in mm. For Imperial, you have to convert tpi to inches of pitch so a calculator may be needed depending on your ability to juggle numbers in your head.

Long ago, I wrote the thread size along side the recommended tap drill in my index so the information is always at hand. Like others, I will go up a few thousandths on drill size for difficult to tap materials.

As to clearance fit, my tendency is to drill close to the major diameter or 1/64" larger. If alignment is an issue, I will go larger as the situation dictates.

If I were to add any drills, it would be to add a set of metric drills since metric is becoming more common place. If you anticipate needing larger holes, a Silver and Deming set by 1/16", 9/16" to 1" should fill out your needs.

 

Here s a simple way I look at threads. Pitch Diameter determines class of threads. Pitch Diameter is an imaginary tube running the full length of the thread. Say the thread is a half inch and the pitch diameter is say .460 (randoms numbers just for theoretical purpose) this is where the length of the sides are the same as the distance across the thread. Then the tolerance goes off of this until it s junk. Internal is more rigid with drilling and tapping operations most often and equal cutting depth but single point one must be aware how this changes with cutting depth and major diameter on a lathe . Drills are ways more forgiving in this regard.

 

I just use the tap'n'drill chart from Littlemachineshop. It has drill sizes for 75% (or 70%) - alu, brass, plastic - and 60% - steel, cast iron and other hard materials, plus tight and loose clearance holes. I scribbled roll form tap drill sizes on to that. Works fine by me.