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jdunmyer
10-27-2007, 08:01 PM
A quicky search didn't turn it up, so here goes:

I want to make a 5/16-36 tap. Presumably, the OD needs to be somewhat larger than the nominal size. How much larger?

My effort involved turning a piece of drill rod to more or less nominal size, cutting threads, then using a 1/8" mill to cut 2 grooves ("flutes") down the length, centered on the diameter. No attempt was made to harden it, as it is only to be used for threading brass, and only a couple or 3 times at that.

Although it made nice-looking threads in the brass, it wouldn't begin to screw onto the male thread.

What did I do wrong?

TIA:

jimmstruk
10-27-2007, 08:47 PM
If I were making a tap I would measure the existing thread (male) and add clearance to my tap. A store bought tap has an H number written on it. Example= H3 would be 3 half thousants over size. Also machinerys handbook has lots of information on making taps. Good luck. JIM

oldtiffie
10-27-2007, 11:35 PM
A quicky search didn't turn it up, so here goes:

I want to make a 5/16-36 tap. Presumably, the OD needs to be somewhat larger than the nominal size. How much larger?

My effort involved turning a piece of drill rod to more or less nominal size, cutting threads, then using a 1/8" mill to cut 2 grooves ("flutes") down the length, centered on the diameter. No attempt was made to harden it, as it is only to be used for threading brass, and only a couple or 3 times at that.

Although it made nice-looking threads in the brass, it wouldn't begin to screw onto the male thread.

What did I do wrong?

TIA:

Hi jdunmyer.

How did you check the "male" thread for size?

It would not be the first time that a thread has been "off-size" for what-ever reason.

Perhaps "enlarge" the tap by putting 1 or several layers of "Tissue" or similar paper over one "land/side" of your "tap" and try it again. The will force the side opposite the paper "packing" to try to cut "larger". Also "roll-your-own" cigarette paper works quite well sometimes.

If the "brass" was not "hard" enough it may well be a "bronze" and if so increasing the "hook angle" (aka "top rake") on the tap might help. I suspect that the tap is "rubbing" a bit instead of "cutting".

Using a decent cutting fluid (even saliva!!) will assist here.

It is also possible that the "teeth" on the "tap" are "too narrow" in which case the "tap" thread may need to be re-cut.

I hope this helps.

FWIW, Marv Klotz has an excellent utility for use of/with "wires" to measure screw threads at:
http://www.myvirtualnetwork.com/mklotz/fckeditor/UserFiles/File/3wire.zip
http://www.myvirtualnetwork.com/mklotz/

But - a caution: the 60 deg included angle of the tool and thread are assumed to be both correct (angle) and symmetrical.

I hope this helps.

Bguns
10-28-2007, 04:53 AM
I like to make tools as needed, but for small special taps, I get em from Victor
http://www.victornet.com/cgi-bin/victor/subdepartments.html?did=100&id=gqNGHjIF
No affiliation Blah Blah, just a gunsmith that has to deal with odd screws all the time...

Their part number: TAST-5/16-36 $8.50

oldtiffie
10-28-2007, 05:30 AM
I like to make tools as needed, but for small special taps, I get em from Victor
http://www.victornet.com/cgi-bin/victor/subdepartments.html?did=100&id=gqNGHjIF
No affiliation Blah Blah, just a gunsmith that has to deal with odd screws all the time...

Their part number: TAST-5/16-36 $8.50

Thanks BGuns.

That sure is some catalogue.

I've never seen one so comprehensive.

Those die-heads are really "it" for production threading but you'd sure need a machining centre or a good turret/capstan lathe or tail-stock - or a very good mounting system in a tool-post!!

jdunmyer
10-28-2007, 07:56 PM
I've ordered the 5/16-36 tap from Enco and should have it this week. Although I knew it was available, I wanted it yesterday, so thought I'd give it a try.

This is for making the dummy cartridge for a Stoney Point cartridge overall lenth measuring gadget. I have one dummy cartridge that I sorta used as a guage. It would thread on a turn or more, but not all the way. So, I proceeded to cut a couple of grooves with a 1/8" mill and tried it. The threads in my new cartrige case looked quite good to my untrained eyes, but it wouldn't even start onto the S.P. guage assembly, and I didn't want to force it because the thing is made from aluminum. I had miked my "tap" before removing it from the lathe, and it appeared to be close enough. Besides, with my sample case not fitting, I figured it HAD to be slightly oversize yet, so would work.

I used my carbide threading tool because I couldn't find my HSS job that would have probably worked better. Although, the thread on my tap did look pretty decent. Will have to try again, just for the heckuvit, to see if I can make it work.

Thanks, fellas!!

J Tiers
10-28-2007, 08:09 PM
Drill rod is funny.

The only way to be sure you are "on" size is to use thread wires, thread mic, etc. Wires are best.

Outside mic will read over the fins and other garp thrown up by the bit when threading. That makes you THINK you have it on-size but you don't "know" for sure. Even the thread mic may be hung up on that stuff

If you KNOW the thread mics right with the wires, you can clean off the crap and know it will work. You can also adjust to a different fit class or "H" number easily.

I have made some taps, including a 1/4-36 a while back, for a carburetor thread (didn't want to wait). Worked out fine.

I recommend hardening, even for a short use tool, because that way the edges are sharp and it will cut. Plus, you still have it afterwards, and why not have it sharp and usable after that work?

If it rubs, it may not make the threads on-size. It's easy to do with W-1 or O-1 steel. Heat, dunk in appropriate stuff, draw back to a straw color, you are there. Shank is probably best down to a blue or a very dark straw, it wants to be tough and not shatter.

JeffKranz
10-29-2007, 04:47 PM
Jim,

I have never found that a carbide thread tool will work on very small work. I always grind up a high speed tool bit for really small threads. Carbide seems to want to see fast RPM and usually doesn't have the right angle to remove the chips on manual machines.

Jeff

andy_b
10-29-2007, 10:34 PM
I've ordered the 5/16-36 tap from Enco and should have it this week. Although I knew it was available, I wanted it yesterday, so thought I'd give it a try.

This is for making the dummy cartridge for a Stoney Point cartridge overall lenth measuring gadget. I have one dummy cartridge that I sorta used as a guage. It would thread on a turn or more, but not all the way. So, I proceeded to cut a couple of grooves with a 1/8" mill and tried it. The threads in my new cartrige case looked quite good to my untrained eyes, but it wouldn't even start onto the S.P. guage assembly, and I didn't want to force it because the thing is made from aluminum.

you wouldn't believe the number of guys who try to find out what that thread is for the Stoney Point case gages to make their own. :)
i think i'll have to be ordering up one of those taps myself.

andy b.

Scatterplot
10-29-2007, 11:46 PM
Part of your problem could be some rubbing. I've never cut a tap myself but I would think the profile you're looking for might be something like this:

http://img.photobucket.com/albums/v240/bertmcmahan/threads.jpg

where the top one is the one you cut as I think you said. You could try the bottom one (although admittedly yours would give a sharper point, just depends on the material). The red is a path to grind, only the front faces are doing the cutting and the back is just riding there, you might be getting some extra friction that's causing some wobble in there.

Granted, you might have to do multiple flutes for more stability without the extra cutting edges to guide it in there, which seems a little further than the scope of your project lol (not your abilities, but you did say you wanted quick!)

Just my 2 cents :)

Joel
10-30-2007, 01:26 AM
Good stuff:

http://bbs.homeshopmachinist.net/showthread.php?t=22687

jdunmyer
10-30-2007, 09:43 PM
Andy,
I had measured the S.P. thread and came up with 5/16-36, then did a search that linked to a discussion on a forum someplace. Messages there said "5/16-36 and here's the link to Enco's part number".

Scatterplot,
Someone sent me an email suggesting your method/ideas.

Frankly, I didn't expect it to work, or at least not well. I did expect to learn something, and that was certainly accomplished. That means that the effort wasn't entirely wasted. :-)

Thanks again!!