Author Topic: #29 or #28 drill bit (Read 9067 times)

jcovais · « **on:** March 14, 2015, 05:54:01 PM »

As I mentioned in another post I'm getting ready to start my first build, a Chambers kit. I have viewed both videos. in Ehlert's video he says use a #28 drill bit because in his experience it works better in tapping 8 -32. in Turpin's video and in the Chambers instructions it recommends a #29 drill bit ....any thoughts?
Jim

Nate McKenzie · « **Reply #1 on:** March 14, 2015, 06:01:13 PM »

I have only used #29 but why not try them both on a piece of scrap metal and see which one works best for you. Let us know.

David Rase · « **Reply #2 on:** March 14, 2015, 06:01:56 PM »

All my drill charts say to use a #29.
David

PPatch · « **Reply #3 on:** March 14, 2015, 06:09:07 PM »

I've never used a #28 but the 29 works just fine.

dp

Ky-Flinter · « **Reply #4 on:** March 14, 2015, 06:36:34 PM »

The diameter of a #29 drill is .136"
The diameter of a #28 drill is .1405"

The #28/.1405" hole will be a little easier to tap, but will result in less thread engagement.

-Ron

jerrywh · « **Reply #5 on:** March 14, 2015, 06:38:50 PM »

Never assume someone is correct just because they wrote a book or are well known. I have found many many mistakes in books and heard a lot of famous gun makers say things that are incorrect.
Writing a book makes you an author not a authority. Same goes for Videos.

P.W.Berkuta · « **Reply #6 on:** March 14, 2015, 06:39:02 PM »

When drilling with a 118 deg twist drill the hole will end up slightly larger than the drill bit diameter because the drill point wobbles ever so slightly even with a good drill press. The way to correct this and get a hole very close to actual drill diameter is to first drill a pilot hole 0.010 (or so) under the finished size hole you want. Example: say you want a 0.136" hole (#29 drill) you first drill a smaller hole say 0.125" then use you #29 drill to open it up to final size. The drill point "wobble" is in the 0.125" hole and when you drill with the #29 drill the drill point "wobble" is not a factor. If you want a "close to exact" size then you will ream the hole (with lube) by first using the 0.125" drill bit.

jerrywh · « **Reply #7 on:** March 14, 2015, 10:19:56 PM »

Also if you sharpen the drill off center it will drill a bigger hole.

Mark Elliott · « **Reply #8 on:** March 14, 2015, 10:20:39 PM »

The tap drill for 8-32 is a #29. I always use the tap drill specified by the charts that represent standards that have been in place since well before WWII. How loose your screw is in the the threads depends on the tolerance tap you use and how the screw was cut. Taps are rated by tolerance as to how tight the threads will be. Most of the the ones sold are in the middle and should work just fine. Maybe Ron had a set of 8-32 taps with a tighter tolerance and didn't realize it.

Jim Kibler · « **Reply #9 on:** March 14, 2015, 10:26:27 PM »

I assume you are tapping your lock plate or trigger plate. Either will work just fine.

jcovais · « **Reply #10 on:** March 14, 2015, 10:44:29 PM »

Thank you all for all the good info!!

davec2 · « **Reply #11 on:** March 14, 2015, 11:05:27 PM »

A # 29 will give you a 75% thread (i.e. 75% of the full thread form). A #28 will give you about 60% and a #27 about 50%. Also, there are charts that are more complete in that they recommend a different "percent of thread" for different materials. For aluminum, brass, and plastics (for an 8-32 tap), the #29 is recommended to give a 75% thread. For steel, stainless, and iron, some charts recommend a #26 or #27 drill for a 50% thread.

But, as Jim Kibler says, either will work just fine for what we are doing here. In my aerospace world, the amount of thread engagement is a little more persnickety.

Bob Roller · « **Reply #12 on:** March 15, 2015, 02:25:01 AM »

Quote from: David Rase on March 14, 2015, 06:01:56 PM

All my drill charts say to use a #29.
David

On the rare occasions I have to use an 8x32 thread,I use a #28 drill and countersink the hole the depth of one thread on both sides.Tap matic fluid is also used.

Bob Roller

jerrywh · « **Reply #13 on:** March 15, 2015, 08:24:56 PM »

There are some metals that will not thread very easily like some stainless or heat treated steels. In trying to get a 75% thread you take too much risk of breaking the tap. In that case it pays to use a slightly larger drill.
Remember this, The treads in the hole need not be any stronger than the threads on the screw.
I always try for a 75% thread on screws. On breech plugs more.

Stophel · « **Reply #14 on:** March 15, 2015, 08:46:13 PM »

I would not even dream of using a 28 bit for an 8-32 screw hole. Loose, wobbly screws do not make me happy. So, 29 it is. I wish I could get them even tighter! But the taps tend to break... I'm very particular about such things.

flehto · « **Reply #15 on:** March 16, 2015, 07:22:37 AM »

I always use a #29 drill for an #8-32 tap . When I was a toolmaker, if a #10-24 thread was spec'd, a #10-32 was used instead. A #10-24 tap because of the deep threads was the most "breakable" tap. .....Fred

Keb · « **Reply #16 on:** March 16, 2015, 02:29:38 PM »

Quote from: Ky-Flinter on March 14, 2015, 06:36:34 PM

The diameter of a #29 drill is .136"
The diameter of a #28 drill is .1405"
The #28/.1405" hole will be a little easier to tap, but will result in less thread engagement.
-Ron

That's a difference of .0045" on the diameter. That's .0022" per side of the hole's diameter. A human hair is roughly .003" thick. Just think about how much difference using a #28 or #29 will make. We're just holding a lock plate or a trigger plate on with #8 screws. When we get to the breech plug using a greater percentage of threads is important but on a lock plate, not so much, IMHO.

moleeyes36 · « **Reply #17 on:** March 16, 2015, 05:35:12 PM »

Here is a chart I found on line some time ago that provides a lot of information on drill and tap sizes. Whether the information is helpful or confusing, you can decide for yourselves.

http://www.shender4.com/thread_chart.htm

Mole Eyes

Scota4570 · « **Reply #18 on:** March 16, 2015, 06:22:39 PM »

I normally measure the nose of the tap. Whatever that is to get a full thread is the bit I select. If I have any trouble with the tap not starting right I use my drill press or milling machine to guide the tap straight. Put a pointy thing int he chuck. The back of the tap or tap handle will have a place to put that point. That will keep it straight while you start the thread. The absolute best way is to drill the hole in the drill or mill then tap without moving the part in the drill press or mill. when working by hand, and you want the best shot at an easy full depth thread you could cut a partial thread with an 8-32 then chase out full depth with a 10-32.

al56 · « **Reply #19 on:** March 16, 2015, 06:25:59 PM »

The formula that I teach students is as follows:
decimal size - (1/pitch) = tap drill size
.164 - (1/32) = .13275 the closes size to that would be either .128 or .136. I would use a .136 or # 29 drill. Mostly we just use a tap chart and the size on those is dependent on the % of threads listed at the top.
Al

B Shipman · « **Reply #20 on:** March 18, 2015, 06:57:57 AM »

What works is what matters. You try taping a #29 through a bolster and you have a JOB sometimes resulting in a broken tap. A slight countersink and a #28. As a matter of fact, when the rifle is all done, and you have any resistance when placing the side lock screws, which you will, I run the tap thrpugh a final time.

What do you think is going to happen. The screws don't secure the lock securely. Not once in what? 37 years. THey should work easily without jamming. #28 Your rifle should not be a pain in the ass with buggered screws for the sake of a preconceived notion of precision.

Mark Elliott · « **Reply #21 on:** March 18, 2015, 07:34:51 AM »

I have never had a problem drilling and tapping the holes for the lock nails(screws) using the normal tap drill and tap. Of course, I don't normally use an 8-32 screw in the bolster. I use a 10-32 in the bolster, and an 8-32 in the front of the lock, through the web between the barrel and the ramrod hole.

I mark the hole locations with the lock in the gun; drill the holes with the lock out of the gun using a drill press and then tap the holes through the stock with the lock in the rifle. I may have on occasion, countersunk the holes one thread on the inside of the lock to facilitate installing the lock nail. However, I don't do it as a normal practice.

I can't speak to what problems others may have. The only problem I ever have installing lock nails is with the barrel being a little out of position such that the rear lock nail/screw doesn't want to go through the recoil lug. Usually, a rap on the muzzle with an assembly hammer takes care of that problem.

Now, if I didn't carefully "feel" the taps through the holes a thread at a time, with tap free, I would break them. It is just a difference between it taking a minute or two to thread a hole as opposed to less than a minute if I just tried to ram it through. I think I have only ever broken one tap threading a hole in the lock plate for a lock nail/screw. Normally, I break taps in a blind hole.

Of course, as Bill said, you do what works. Problem solving is part of making anything. So, if you find you need to go up one wire drill size, by all means do that. There is no orthodoxy about these things.

B Shipman · « **Reply #22 on:** March 19, 2015, 05:59:41 AM »

Good answer Mark.

Mark Elliott · « **Reply #23 on:** March 19, 2015, 06:08:48 AM »

I am glad you liked it, Bill.

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Author Topic: #29 or #28 drill bit (Read 9067 times)

jcovais

jcovais