Drill size for 6-40 screw

[pushboater]

I'm a little confused in that I just downloaded a tap and clearance drill chart and it's showing two drill sizes for a 6-40 tap. The first, a #33 drill bit, is what I have always used for this size screw. However, on this chart it states that the #33 drill is for a 75% thread for Aluminum, Brass, or Plastic. They say to use a #31 drill bit for a 50% Thread for Steel, Stainless & Iron.  Maybe this is why I've broke so many 6-40 taps in the past. Any thoughts before I switch to a #31 bit? Any comments or thoughts would be appreciated.

Capt. David

[James Rogers]

Most all normal charts I am familiar with use a 75% thread as standard. The Starrett card I keep in my pocket calls for a no. 33 (.1130). I think I recently learned anything over say 60% is basically overkill as far as strengthening so 50% of thread engagement is I think sufficient if tapping more than one and a half times the diameter..... IIRC.
Take this with a grain of salt until someone with more knowledge replies  

[Smoketown]

Of course one could also use a #32  drill bit.   

The quality of the tap has a tremendous bearing on how many taps one breaks.


Cheers,
Smoketown

[pushboater]

I thought about splitting the difference and using a #32. I might try that and see how it works. Is there a specific brand of tap that you've found that seems to be of superior quality than any other Smoketown?

David

[Bob Roller]

I don't think I have ever seen a carbon steel tap in 6x40 and I use it
for tumbler threads for the hammer screw.The #33 is the right size and
I always countersink the hole to be threaded one thread deep. It makes
the tap easy to start and I also use threading fluid

Bob Roller

[dmb25]

My instructors at the machien shop say if your worried about breaking taps and when working with tool steels especially with small taps in tool steel to use only a 50% engagement and if you at tensile test of different threads it is still more then Strong enough. 

[JLBSparks]

I usually drill .004"-.010" larger than Starrett card recommendations depending on thread  size and material used. I don't power tap anything smaller than 5/16"-8mm.

   -Joe

[Smoketown]

I typically buy my taps from Brownells.

They usually have most taps available in carbon steel.

http://www.brownells.com/gunsmith-tools-supplies/general-gunsmith-tools/taps-dies/taps/wire-gauge-carbon-taps-prod6876.aspx

Cheers,
Smoketown

[Jesse168]

There is NOT a perfect drill size for any tap.  The material you plan to drill most of the time determines the hole size for any tap.  For example tapping for a 1/2" X 13 thread the standard drill is 27/64" except if the material you are tapping is stainless steel then if your tap isn't extremely sharp and should be a 3 flute you use a 7/16" drill.  The reason for making the hole larger is to allow the chips to flow from the flutes.  If the chips don't flow from the flutes of a tap they build up and your tap will break.  Also you need to use very good tapping fluid designed for the type of material you are tapping.  I like to use TAP HEAVY this a true tapping fluid that reminds you of honey it's that thick.  On most small taps they only come in 2 flutes because there isn't enough diameter to make more flutes.
Don't be afraid to back the tap up and restart tapping several times.  When you are tapping you are expecting one piece of steel to move metal from another piece of steel, brass, aluminum, etc.  and some will cut easy while others will be gummy or hard when you tap.  One of the best fluids to use on aluminum when you tap it is kerosene.
Find a machinist or go to your local library and find a MACHINIST HAND BOOK.
The older books explain things a lot better than the newer ones do.

Just a bit of advice from a machinist with 46 years  experience before I retired back in January.  I still have my own Southbend Lathe and a Bridgeport Milling machine at home.

[pushboater]

Thanks to everyone who replied. Lots of good info.  Smoketown, I ordered some of those 3 flute taps from Brownells last night. They sounded like some high quality taps so I thought I'd give them a try. Thanks again everyone.