OT: Drill and Tap question

[Larry Heflin]

Hey all,
I'm probably a little OT here, but I'm pretty sure someone here can help. I'm going to do a little drilling and tapping in cast iron. If I want to tap for a 1/4 - 20 bolt hole do I use a 1/4 bit? Is there a rule or guideline to follow for proper sizing? Thanks for any advice.

Larry

[Alan Turner]

For a 1/4 x 20, use a No. 7 bit, which is .2010", or just under a 13/64" bit. Tap and Die charts are everywhere, and oft included with a tap and die set, and are oft printed in industrial supply catalogs.

[Bobby Nicks]

Here's a link to a tap & die chart. http://www.procunier.com/tap_eng_chart.html

There are several different charts out there on the net.

Bobby

[Alan Turner]

Bobby,

Very interesting. Your chart says to use a #6 for 1/4-20, but mine said a #7. So I googled for a minute and the ones I found also said a #7. I did not realize this was a debatable point.

I am not a metal guy and only cut threads when need be. I probalby should not have posted, instead leaving this question to the real experts.

I guess the bottom line is that there is very little that is not debatable.

[Steve Clardy]

#6 and #7 are only a few thousands difference. Either will work, though I use a #7

[JayStPeter]

I just drilled and tapped some 1/4"-20 threads into my bandsaw table. I didn't have the recommended #7, so I used a 7/32". Worked just fine. Cast iron is soft enough that you can get away with it being a little snug. Had I been doing steel, I'd have probably bought the correct bit.

Jay

[Larry Heflin]

Thanks for the replies.

Larry

[Bruce Page]

For a 1/4-20 thread, a #7 (.2010) tap drill will give you approximately 75% thread pitch diameter - the industry standard. A #6 (.2040) yeilds 70% and a #4 (.2090) will give you approx 60%.
Normally, you will back down from the 75% standard only if you’re trying to drill & tap harder materials like tungsten or hardened steels like 17-4ph or 15-5 etc.

Jay, using a 7/32 (.2187) tap drill is only going to give you about 40% thread pitch diameter which makes for a pretty weak thread. Be careful not to over tighten.

[Greg Mann]

There is actually another variable that comes into play (just to confuse things a little more). If you have access to a Machinery Handbook you will find charts that specify larger hole sizes based on the length of thread, the concept being that longer thread engagement would allow larger holes, commonly referred to as minor diameter. Jay's larger hole, at .2187, is a little more acceptable if he is using a lot of thread engagement, i.e., a longer bolt.

Curiously, drill sizes specified are somewhat behind the times. Today's drill manufacturing techniques usually result in holes closer to actual drill size because drill points are more geometrically accurate than they used to be. Today's new #7 will generally drill a smaller hole than a new #7 from 10 years ago. Conversely, when a drill is resharpened with poor technique a larger hole is the inevitable result. It used to be common for a drill size to be specified right on a part print along with the tap size for a given threaded hole. Now, a thread size and class of fit is the most common approach. This will dictate a minor diameter range and it is up to the supplier making the part to determine what drill size will accomplish the desired result.

There is an significant difference in torque required to tap a hole depending on the minor diameter. Small increases in hole size will improve the odds of not breaking a tap. Best rule of thumb for the occasional user? Give yourself a little beak when you can and drill the hole toward the high side IF you can use a longer bolt with 2 1/2 or 3 times diameter thread engagement.

Greg

[JayStPeter]

Jay, using a 7/32 (.2187) tap drill is only going to give you about 40% thread pitch diameter which makes for a pretty weak thread. Be careful not to over tighten.

Uhhh, I guess I was off by one bit size. It was probably a 13/64. I made a number of test holes in a spare piece with different sized bits and was a little sketchy on which I actually picked. I thought it was 2 bits down from 1/4, but must've been 3. I felt like I was going to snap the cheap tap I have with the next smaller size.

Jay

[Larry Heflin]

All done! Drilled, tapped, and working well. I looked over a couple of the charts as referenced above. I'm almost certain that I am over complicating things a bit (no pun), because I'm still a little confused on some of the numbers.

There are several types of numbers involved that didn't seem to match up with my taps in a way that makes complete sense to me. A 13/64 drill bit I understand; 1/4 x 20 I understand; the decimals on the charts I think I understand but have no bits that are marked in such a way and there seems to be some rounding up or down involved; and I have no numbers on taps that are numbered as e.g. #6, #7, etc. Then there is National Course, National Fine, and National Pipe Thread.

In the end I used the 13/64 drill bit as suggested and a tap that, conveniently was marked 1/4 x 20. I'll do a little more research to try and make more sense out of the numbers before I go metric on my next holes for 9mm bolts.
The bottom line, it's done and you all helped. Thank you.

Larry

[Carl Eyman]

Larry:

Numbered drill bits range in size from #1 to #80 (to my knowledge). They give the user a wide choice of sizes. Numbered machine screw sizes are also quite common and also give the user a wide variety of choices. Common numbers are #10-24 which is the equivalent of 3/16" NC , for instance. Commonly used sizes range from #10 down to #4, but they go below that. Numbered tap drills are virtually a necessity when using numbered machine screws. Most hardware stores carry them. Hope I haven't added to the confusion.

[Bruce Page]

Larry, for the 9mm coarse thread (9 X 1.25) you will want to use a 7.8mm drill (.307 inch). Not everyone has a metric drill set in their tool box so I would use a 5/16” (.3125 in) instead. The 9mm fine thread (9 X 1) calls for an 8mm drill (.315 in) so I would use either the 5/16” (.3125 in) drill or a letter “O” (.316 in).

Now I'm confused!! :eek: :rolleyes: :eek:

[DEAN ELZEN]

:confused: I GUESS MY QUESTION IS, NOT ALL DRILL BITS HAVE THE ABC'S ON THEM. WHERE DO I GET THAT CHART FROM. DEAN

[Dave Richards]

Dean, not all drill bits have the letter sizes on them because not all drill bits are letter size bits. You can buy a set of drill bits that include letter and number sizes in addition to fractional sizes.

As was posted, here's a chart: http://www.procunier.com/tap_eng_chart.html

Welcome to the Creek. It's a great place.

By the way, you don't have to yell. All caps in a post is considered yelling.

[Jim Dunn]

One other thing to consider, IMHO, is weather or not you are hand drilling or using a drill press. I've found that hand drilling a hole you have a real good chance of making it bigger than the drill size. Therefore I opt for a slightly smaller hole when I am hand drilling. I can always open it up slightly if need be. Further more, hand sharpening drills is an art! The last thing you want is a handsharpened 1/4" drill that drills more like .265. My 02

Jim

[Barry O'Mahony]

The borgs typically just stock the fractional bit sizes. For smaller holes, they tend to be problematic, as the size increases by a fixed 1/64", which is a huge precentage increase for smaller diameter holes.

I have a couple of Ace Hardware's near me that are "real hardware stores". They have numbered drill bits, letter-size, metric, left-hand, etc. You can try one of those rather than a local Home Depot.

It sound like there are a few machinists here. Any pointers on tapping 304 stainless steel? I need to put a couple of holes in a piece for a project I'm doing.

[DEAN ELZEN]

:) I'm sorry, I'm not a great typer. Its just easier to leave the cap locks on way or the other. I will try to do better next time. Thanks for the reply though. This is a wonderful site!
Dean

[Frank Chaffee]

Welcome to the 'Creek Dean.
As I'm sure you've noticed, if you ask a question here someone will help you out.
Come often.
Frank