A Few Quick Threading Questions...

[formula156]

I've only ever threaded with taps and dies, but I'm ready to start on my lathe (MicroMark 7x14 mini lathe) and I've got a few questions I haven't found answers to yet.

1) I've heard that the cutting tool should have a flat tip 1/8 the size of the thread pitch. Does anyone do this, and is it necessary?

2) Where could I find some charts that show all the info I need for the thread (pitch diameter, major and minor diameters for OD and ID threads), basically everything I need. The thread I need to cut is 1/2-28, which isn't a standard UNC or UNF thread, so I'm having trouble finding the information I need.

Thanks for the help guys!

 

[dan s]

1) I've heard that the cutting tool should have a flat tip 1/8 the size of the thread pitch. Does anyone do this, and is it necessary?

I personally don't, because IMO It's to difficult a task to perform on a manual grinder accurately.

2) Where could I find some charts that show all the info I need for the thread (pitch diameter, major and minor diameters for OD and ID threads), basically everything I need. The thread I need to cut is 1/2-28, which isn't a standard UNC or UNF thread, so I'm having trouble finding the information I need.

A copy of the Machinery's Handbook will have most normal & fine threads. However what your after is an ultra fine thread, and can be found here:
http://www.efunda.com/DesignStandards/screws/unified.cfm?start=64&finish=147

 

[DickDastardly40]

When I cut a thread (internal or external) I grind a V tool with a sharp point. I cut the thread to depth, then open out the flat on the bottom using side cut removing from the left flank of the tool (the tool is at full thread depth for each pass).

This is easier if you use the direct approach method with the compound parallel to the lathe bed but also do-able using the compound at 29 1/2 deg using jiggery-pokery with the cross slide to move the tool sideways.

I always make the external thread first, then fit the internal thread to it.

Hope the above is clear.

Al

 

[formula156]

I'll run this by you guys, and if you could correct me (or hopefully agree) I'd appreciate it.

For a 1/2-28 Class 2 fit, I could do a .499 Exterior thread major with a .456 minor. For the interior thread, I could do a .500 major and .461 minor. This is with everything rounded to the nearest thou. The would mean the majors are only .001 apart and the minors are .005 apart. Should the major and minor have the same difference between them?

Should I perhaps cut the external thread as I specified, start the internal with the major I specified, and just keep advancing the internal thread till I get the fit I am happy with?

Dick,

I understand how you would flat the bottom. Do you flatten in 1/8 of the pitch? I assume you thread and flatten the enternal first. Then, you thread the internal, test the fit, and then flatten the minor of the internal thread?

Thank guys!

Oh, when grinding a threading tool (external for example), you grind the side to a 60° point, and put a little rake on the point. There is no grinding on the top, correct?

Oh again! When advancing the compound at an angle, I can do the trigonometry to figure how far to advance it, but is there a simple formula or a chart with this info.

Do you guys do a couple final passes advancing the cross slide to clean up the threads?

 

[DickDastardly40]

When opening out the thread, I remove material until the flat on the crest of the bolt (external) is 1/8 of the pitch gauged by comparison to the correct feeler guage.

The flat at the root of the thread on the bolt should be (perfect world) 1/4 pitch. It is very difficult to measure internally which is why I make the internal to fit the one I can see rather than vice versa.

If you need to make threads to be as backlash free as possible, the clearances you have stated will work but may be very tight in getting to rotate. The slightest burr may cause it to stick. Also which materials you are cutting bears on the finish which can make them tight.

If you need interchangeability between several parts a little extra clearance may help as there will always be some minor differences which may add to cause sticking.

Grinding: top rake depends a lot on the material, brass doesn't need any. You will need side clearance to take into account the pitch of the thread on the left side (the bottom of the thread you have just cut angles to the right and on larger threads can impinge on the tool).

I generally don't do trig to work out when at full depth, if possible I leave a narrow diameter strip at the start of the thread which is at the minor (external) or major (internal) size. When the tool just marks this I know I am on size. If need be this can be machined off afterwards or sometimes just leave it on if it can't be seen or doesn't affect utility

Again Hope this helps

 

[gbritnell]

Here's a page out of my Craftsman-Atlas lathe book. I think it has the type of information you want.
gbritnell

 

[steamer]

There is some good downloadable information and some free video's on threading on the lathe here.

http://www.homemodelenginemachinist.com/index.php?topic=4025.0

Hope that helps.

Dave

 

[formula156]

Here's a page out of my Craftsman-Atlas lathe book. I think it has the type of information you want.
gbritnell

I actually have an online version of this chart. Is a regular 60° tool a national form tool or a V tool though?

 

[DickDastardly40]

60 Degrees included angle is the standard form for National , Unified, and metric in both coarse and fine pitches

55 Degrees included angle is the standard form for Whitworth (BSW and BSF), BSP (Pipe), ME and BSTP (Taper Pipe).

47.5 degrees included angle is the form for BA

All the above are V forms.

Non-V forms include Acme (sometimes 14.5 degrees chamfer), square threads and buttress threads

 

[formula156]

60 Degrees included angle is the standard form for National , Unified, and metric in both coarse and fine pitches

55 Degrees included angle is the standard form for Whitworth (BSW and BSF), BSP (Pipe), ME and BSTP (Taper Pipe).

47.5 degrees included angle is the form for BA

All the above are V forms.

Non-V forms include Acme (sometimes 14.5 degrees chamfer), square threads and buttress threads

Hmm...that chart lists different dimension for a form tool and a v tool, but they are the same? Which dimensions should I use?

 

[Maryak]

1) I've heard that the cutting tool should have a flat tip 1/8 the size of the thread pitch. Does anyone do this, and is it necessary?

IMHO grind the tool to the correct included angle, with no top rake. Stone the point to remove the sharp tip. Start cutting.

2) Where could I find some charts that show all the info I need for the thread (pitch diameter, major and minor diameters for OD and ID threads), basically everything I need. The thread I need to cut is 1/2-28, which isn't a standard UNC or UNF thread, so I'm having trouble finding the information I need.

A 28 tpi thread will have the same depth of thread regardless of the diameter on which it is being cut and in this case, external thread = 0.022" deep and internal thread = 0.019 deep.

Hope this helps ??? ???

Best Regards
Bob

 

[Tin Falcon]

Here is a link I put up a while back to a couple of military training manuals this should cover a lot of the basics.
http://www.homemodelenginemachinist.com/index.php?topic=3216.0
Tin

 

[georgeseal]

national form tool has a blunt point to corespond with the bottom of the thread
V form has a sharp point there for you have to go in further than with a National form tool

As Maryak said TPI is what is important Diameter dose not matter

Have been making some 1" 20 TPI plugs

 

[formula156]

A 28 tpi thread will have the same depth of thread regardless of the diameter on which it is being cut and in this case, external thread = 0.022" deep and internal thread = 0.019 deep.

Hope this helps ??? ???
Best Regards
Bob

Where did you get those figures from? The charts I have seen contradict that. And, what about the major diameters? If I buy something externally threaded 1/2-28 is it going to have a .500" major with .022" deep threads?

I am making a flash suppressor for a rifle, and I am planning to thread the barrel and the part, but I want the part to fit on anything that already has 1/2-28 threads.

Thanks!

EDIT: Nevermind! I wasn't thinking correctly. Given a major and minor, I need to divide by two to find the cutting depth. I've been looking at this too hard I think, and I confused myself.

But...one question still remains...if the thread I need is 1/2-28, will the external thread have a major diameter of .500? Damn, I guess I am still slightly confused. I guess what I am asking is, how do I figure my outer diameters for the internal and external thread. I understand the rest from there.

 

[Maryak]

If I buy something externally threaded 1/2-28 is it going to have a .500" major with .022" deep threads?

Yes

But...one question still remains...if the thread I need is 1/2-28, will the external thread have a major diameter of .500? I guess what I am asking is, how do I figure my outer diameters for the internal and external thread.

Yes and the internal thread will have a bore equal to 0.500" - 0.038" which is 0.462"

Is the thread you are after 1/2" UNEF?

Hope this helps ???

Best Regards
Bob

 

[formula156]

Yes

Yes and the internal thread will have a bore equal to 0.500" - 0.038" which is 0.462"

Is the thread you are after 1/2" UNEF?

Hope this helps ???

Best Regards
Bob

I think 1/2-28 is considered a Unified Ultra Fine Thread.

From what you said, that would make the external and internal majors the same at .500, which would be an extremely close fit. But the external minor would be .456 and internal minor .462. Sorry to sound redundant but that doesn't seem right, unless cutting the external thread brings the external major down a few thousands (if the left and right sides of the points of the thread overlap.

Again, sorry to keep asking the same question.

Thanks!

 

[DickDastardly40]

If I have your question correctly about the overlap, the bolt (external) will have a minor diameter of 0.456 and the nut (internal) 0.462. Therefore the bolt is smaller and you have 0.006 clearance.

 

[formula156]

If I have your question correctly about the overlap, the bolt (external) will have a minor diameter of 0.456 and the nut (internal) 0.462. Therefore the bolt is smaller and you have 0.006 clearance.

Right...but what about the major diameters of both the internal and external. If they are both .500, there would be no clearance. That's the part I am unsure about. Unless cutting to the correct minor depth brings the major depth down a little from where it started.

 

[bmuss51]

with my experience in thread cutting over 40 yrs now. all your charts and table will only get you close to a class 2 fit for a one of thread unless you plan to do multi items of the same cut. so what i have done in the past is use your thing you want to fit as a gage for it's mate. and cut the mate till you get a fit that your happy with. and keep in mind the the starting end will be smaller and may take several cuts to get the thds straight. and for the final fit a little lapping might be required. and when that part is done, you will be able to do the other part with the same sequence.
just remember that the machine your doing this on is not a high precise machine and i doubt if you will be able to hold your cuts to the tenths. so alittle lapping compound will get you to the final end point.
don't make this harder on youself. just cut the thds with the compound at 29.5 degrees and your tool at a included angle of 60 degrees. and the correct thds per inch . and then get your major dia to size if it's external and cut till you get a fit you want or very close and then lap to a final fit

 

[kvom]

I am making a flash suppressor for a rifle, and I am planning to thread the barrel and the part

The above post gives excellent advice IMO. In your position I would make the suppressor. Then trial thread a piece of rod the same diameter as the barrel to make sure you are comfortable with the fit and the process before cutting the threads on the barrel.