Drills

[extropic]

I failed to make myself clear it appears, sorry about that. I drill holes for tapping at or near the ANSI Unified Screw Thread internal minor diameter standards for the thread class required.

Using a hardware store tap hole chart does not provide this information and gives but one choice. 1/4-20 for instance, the drill size will be #7 (.201), the ANSI 2B and 3B standard, Minimum Minor Dia. is .196 the Maximum Minor Diameter is .207, I would therefor choose a #6 drill (.204) which is well within this range. the added .003 can make a big difference when machine tapping difficult materials.
Here are the ANSI 2B-3B standards. They may also be found in Machinery's Handbook.
https://www.engineersedge.com/thread_strength/internal_screw_threads_chart.htm

Thank you for the elucidation and the reference to the applicable standards. That's the kind of information that armatures need.

 

[extropic]

Let's not have any ridicule here.

The rag trick is one of many old timers' ways to get things done a certain way. Just because you haven't heard of it, don't knock it.

Same with the oversize drill. There is a tolerance for the minor diameter of a threaded hole, and despite all the charts, the hole size can vary and still be within tolerance. Obviously, going too large weakens the thread, but to each his own. And it's not illegal. Unethical maybe, but not illegal. And a good way to lose a customer.

Anyway.....ToS do not allow personal attacks or mockery. Consider this a warning.

If a customer drawing requires a specific class of thread and the manufacturer knowingly violates the tolerance, that's fraud. If a Certificate of Conformance is issued, the fraud is compounded. Those practices are definitely illegal.
Wreck™Wreck has clarified that he selects tap drills within the specification tolerance, not in violation of it. That's knowledge that armatures can build on.

 

[dlane]

Thanks for replying, I don't need to buy things I'll never use, I'm thinking I'll look at the tap drill charts
Order screw tap drills for common thread sizes and one size smaller , and the actual final hole size and one size larger, Unfortunately a lot of drill sizes you have to buy in lots ,can't just buy one or two from places I've ordered things from
so the drills could be # ,letter ,fraction for the common tap sizes .
It would be nice to have a full set of all denominations but there kinda pricey.
I have several tap drill charts I'll compare them for size differences
Just want something easier to get in mill chucks ,I'm not making spacecraft here (yet)
Guess I'll Craig's list most of the long drills , they are all US made and sharp

 

[Rob]

Guess I'll Craig's list most of the long drills , they are all US made and sharp

You might list them in the classified area of the forum first.

 

[Dan_S]

For a basic tap drill size reference you can use this. If memory serves this chart lists the drills for 75% thread engagement.
http://www.brokenbolt.com/images/starrett-inch-metric-tap-drill.pdf

You can also get this as a wall poster for free from Starrett, if you go here and create an account. http://starrett.qconnect.com/.

With regards to buying drills I recommend the following approach. Buy decent sets of import drills (A-Z, 0-60, 1/16" - 1/2"), and then replace the ones you use most frequently with high quality ones. I did this 10 years ago when I got into machining, as it was recommended to me by several people including my father who was a toolmaker early on in his career. After 10 years I still have a lot of bits that have never been used.

 

[John Hasler]

This covers both 50% and 75% engagement: http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

 

[Bill Gruby]

I hadn't read this whole thread till now. When I got to the rag trick I smiled. That IS the way it was done and it STILL works, the rag type is what your shop rags are. We used prewashed cotton. I was taught that as an apprentice. As far as don't buy sets, it's a personal thing. My sets are all Cobalt. I won't buy imports, again a personal thing. Never had a problem with variation.

"Billy G"

 

[Tony Wells]

You can also wrap twine around a tap to make it cut oversize.

 

[Wreck™Wreck]

I typically use # 50, 40, 30, 20, 21, 11 and 10's the most. Generally I buy packages of 10 each in jobber, 6" and 12" lengths, as well as A,B and C length threaded. I rarely buy anything but Clevelands, as I can get them locally and priced very close to what mail order runs. I keep mostly HSS on hand but on occasion have need for cobalt bits too. Cobalt, I buy as needed. Careful use and I can make the bits last a long time.

Please explain what this means. I am curious, take pictures as I want to see what a 12" long # 50 drill looks like.

 

[Bill Gruby]

It looks like this. They are not uncommon. They come smaller in diameter also.

http://www.globalindustrial.com/p/m...mpaignId=T9F&gclid=CNHDmOLI5ccCFQgRHwod310IkQ

"Billy G"