Going to drill/tap for a young Mfg side charging handle, 1/x28 threads.  Just need to know the final drill size to use.  I could finish the hole w a chuck reamer I guess.   Thx

It's stamped on the tap.  IIRC it is a No.7.

It depends on desired thread engagement.
And that depends on material hardness.

Drill and Tap chart

1/4 28  no. 3   (0.213) in Aluminum, brass, plastics, (75% engagement)

no. 1 in Steel (0.228) (50% engagement)

(1/4 20 is no. 7 in Al)

Rule of thumb- Major D minus 1/pitch.

1/4-20=

1/4" - (1/20) = .200 so you use a #7 drill.

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1/4-28. Good example though, we'll let him do the math.

from brownells:
http://www.brownells.com/userdocs/skus/l_395142801_2.jpg


her eis the description:
395-142-801WB
Taper Tap, 1/4-28, 3, 17/64
Mfr Part: 1323


whast does that say?  

sorry, I need to order the tap and the drill bit together.

ETA: I did NOT order that tap.  I ended up getting a cobalt steel tap from McMaster which worked easy.

#3 is what it says, which is correct.

That would be a "wire gauge" size right?

would this be a good bit or should I get a carbide bit or soething.  This carrier is hardened:


http://www.brownells.com/gunsmith-tools-supplies/power-tools-accessories/drill-bits/wire-gauge-drills-short-length-prod26269.aspx

Yes.

and what tap would one use to put extra fine threads in an existing .125" hole?  

I had hoped to use 6-48 set screws to plug somehitng...

That would work.

That tap won't like hardened steel.

About the 6-48 holes, here is an answered question on brownells for this taper tap:
395-648-001WB
Taper Tap, 6-48, 31, 25**
Mfr Part: 1021




So, what I do not understand is what "clearance drill size" and "tap drill size" means?  I wonder if I might need to drill these little holes slightly so that the tap will work.  

I really am appreciating all you help sir, and the others too.

Clearance drill is if you want a hole for the screw to go through without threading - like a washer, or if you're clamping through one part.  The clearance drill gives proper clearance so the threads don't bind on the hole.

The tap drill size is the hole you want to drill & then tap for threading.

Clearance is oversizing the hole in the part that you are bolting through. Like when you are screwing your grip on. It just has a thru hole, it's not tapped. So that hole is at the clearance size. It allows for any mismatch in manufacturing, between locations, bolt dimensions, etc.

In your example with the .125 hole and threading it to 6-48, the tap drill size is .120 so your hole is already bigger than you would drill, thus no need to drill.

A threaded dimension is important at the pitch diameter which is not crests of the thread. Thus the .005 that's missing on your .125 hole isn't a big deal, and is really only .0025 per side anyways.

The tap drill is just the drill size that correlates to being the correct one for a tapped hole. It usually allows for approximately 75% engagement. According to people that are much smarter than I am, that is plenty for most applications and a good rule of thumb.

thank you so much guys.  Obviously, I am not a GS or machinist and you have helped me alot.

I'm no machinist, I just fool with things a bit.  I think if you're planning on trying to tap a hardened steel bolt with an Irwin tap you're going to have problems - including a broken tap.

I've never tried to tap hardened steel, but from what I've read you may need carbide taps - and they are very brittle and very easy to break.  I've also read the better taps, like what you get from Brownells will do it even though they are high speed steel not carbide.  YMMV

I would not try to tap a hardened steel bolt as a first try at tapping something though.  I'd suggest getting get a few cheap taps like the irwin and some mild steel plate and practice.

No carbide tap needed.  Just a good drill and tap, both are unlikely to be found at Brownells.

Try www.maritool.com

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I will practice.  I'm planning on doing it by hand rather htan the press.  I have never before in my life tapped a hole, I am curious how it will go, I guess it has to suck itself in and you have to take care not to rotate it w/o it advancing or it will eat up the threads it just cut.

...

so here is a 1/4-28 tap at mcmaster made for hardened steel:

http://www.mcmaster.com/#2626a28/=11n77lf




ETA: this is probably better, a hand tap:
http://www.mcmaster.com/#standard-taps/=11n7ehl



Dang, this DIY project is getting expensive

ETA: I got the second tap in this post, it didn't have any problems tapping in the hardened carrier.  It is cobalt steel.

nvmd

At some unstated thread engagement that depends on teh hardness of the material.


Dia   Dim           thread%
#3      0.2130     79.8%
7/32   0.2187     67.5%

If you want a different thread engagement depth break out the reamers.

The harder the material the fciner the threads can be cut.

Well, ill scribe a pair of straight lines, drill a hole undersized where it will end, Angle grinder w cut off wheels and finish w files I guess.  I have no mill.  I am doing it ejection port side so I won't have to measure/cut blind.

Wrong forum.  AeroE

Lol... there's a joke that runs the circles in machine shops all over the country that pretty much goes, "Machinists don't use taps that list the drill size".

There is actually a lot of truth to it for several reasons.
#1 drill size isn't a fixed thing, as mentioned above, different drill sizes can be used for different engagement amounts.
#2 the quality of taps marketed to amatuers and hobbyists is not even close to the same level of those used by professionals. Most hobby/amateur level taps lists the drill size because the user doesn't know the difference and sometimes needs to be coached through the process...

No offense intended to anybody, just that the facts are facts...

So here is how I did it.  


That is the hold it Jr clamp from van de lay industries.  I put a couple pieces of that little aluminum angle bar under the top of the upper there to make the angle of the slot match the angle of the charging handle, which is basically perpendicular to the line between the bottom and top carrier rails.  After I got the bottom edge of the ejection slot opended a RCH, I tried just milling the slot wholly by sliding the upper across the burr but it was a bit much even w/ the dremmel on max speed, so I scribed a line, made a few punch marks and drilled some holes in a line.  I then milled it out w/ a double cut burr and finished w/ a single cut burr, gradually increasing the height of the dremmel or the piece by adding index cards under one or the other to gradually finish the top or bottom edge.  

The whole process of belt griding down first half, then all the shell deflector, and milling slot took several hours as I was figuring it out as I went.

Here is how it is so far.  Still have a lot of work to do blending in where were the shell deflector and port door hinges etc.  


Next time, I will make the knob in the center line of the ejection port at the same angle as the ejection so that the CH will be better positioned to deflect brass as leaving part of the deflector just interferes w/ the CH too much and the extending the CH out farther would make it too long.  Also, it wold allow me to use a nord lock washer rather than thread compound.  I'll have to make sure the gas system is just right for good ejection.

if I didn't say it earlier, that is a young mfg handle,  like a shoulder bolt but, 1/4-28 threads, diameter was about .300 or something between the threaded tip and the knurled handle, all one piece.   The thread engagement seems a bit sloppy, I dont' know if I tapped it too much, wishing that I had not run the taper tap all the way through, still plenty of engagement I guess.  I got a "medium" strength but hight temp loctite so it will stay put till I need to take it down.   I got a class three SHCS from the local machineist supply place and it fit just thee same as the young bit so like I said, I don't know if the tap or muhc technique needs improvement to get tighter engagement.  

Anyway, had a lot of fun doiing it.

I guess I changed my mind.  I am lefty.   for a while I was quetioning the modern notion of having the CH for use by the off hand, but I experimented a bit and became convinced.

i know regular tiles are not truly flat, but I just mostly needed something smooth for sliding as the only thing that mattered there, was having a conisistend distance between the surface and the burr.

I want a mill terribly, when I get an "extra" grand laying around I'll get one of those minimills from little machine ship probably, but that would be a while.

yea, this.

 

i tried with the regular tools i have in my garage, decided to send my bcg off to a guy who does it for a living...

Who and how much?

And another grand for the tooling.