Making a prototype and ordered a wacky European thread tap. What SAE pitch is close?

[Clausing4900]

So I ordered a 3/4-40 tap thinking it was 40 TPI. Turns out it's a G 34. I have already tapped the hole and now realize it's some obscure pitch. I can measure tpi because the tap is a spiral tap and hard to get a read on.

Can someone tell me what pitch would match this best? I need to cut some threads in a matching cap and can only cut standard threads with my clausing.

I can bore out the threads I already cut and find a less obscure tap if I need to. The part is not critical tolerance wise.

Thanks guys.

 

[john.k]

I dont see what you are getting at.........however to convert any TPI to metric lead,simply divide 25.4 by the TPI.........so for 40tpi,the metric lead is 0.635 mm.....for 34tpi?,its 0.747mm,or 0.75 near enough.Now its very likely your Clausing has a .75mm lead in the gearbox.........the old rule in machining is that if a thread is too tight ,get a bigger wrench.

 

[Nmbmxer]

G 3/4? That’s a common European hydraulic port thread. It’s 3/4-14 BSPP.

 

[crazygoat]

I'm pretty a 3/4-40 tap is 40 TPI. Why do think otherwise?

 

[Hazzert]

The pitch isn’t the only issue, if the tap really is a G(BSPP) the thread angle is 55* not 60.

 

[Peter.]

If you need to measure the thread pitch tap a hole in some scrap then cut the piece in half through the hole.

 

[Clausing4900]

Thanks. As far as measuring the thread. Tapping some scrap and bi-secting was going to be my next step. I think my solution is going to be to bore the thread out and shrink-fit a threaded sleeve in the bore. The thread I tapped is not very even either. The thread is offset, It is a cylindrical component with the threaded hole off the axis of the OD. I had to use a center in the quill and tap it with a wrench. I couldn't hold the tap in a collet as the shank is larger than .750". Due to this, it walked off axis and cut heavier on the low side of the bore.

Does anyone have any tolerance numbers for a shrink fit here. The OD of the sleeve will be 1.250" what should the bore that receives it be dimension wise. I was going to cool the part with dry ice and heat the cylinder in an oven.

What temp can I bring up the 4140 cylinder up to without harming the tolerances after cooling? Will this heat cause difficulty in machining afterwards?

Thanks again for the assistance.

 

[Bill Simmons]

A simple way to check the pitch of that tap would be to just take some 1/4 inch plate (make it aluminum for ease of machining) and drill and tap. Then turn the tap 10 turns and measure the travel of the tap. A little arithmetic will provide the pitch.

Steel expands about 7 parts per million per degree F. Press fits are usually about .001 inch of interference per inch of diameter. 200 degrees would be required as an absoulte minimum but I would go to about 400 degrees to give yourself some working time. While the dry ice would help it can (will) cause condensation to form on the part. This may reduce the amount of time you have to position the part as the thermal conductivity between the two parts will be increased.

 

[MattiJ]

Something stinks here. G3/4 would be parallel thread and you should be able to tell 14 tpi from 40 tpi even by staring at it with your butthole.

 

[LKeithR]

...G3/4 would be parallel thread and you should be able to tell 14 tpi from 40 tpi even by staring at it with your butthole..

You have a way with words. But I agree. As it turns out I just bought a G 3/4 tap earlier this week for a project we're
starting to do. If you can't tell the difference between 14 tpi and 40 tpi by eyeballing it there's something wrong. I don't
have anything around the shop with a 40 tpi thread but when you compare the 14 tpi to a 1/2 -20 tap you can see the
difference and a 40 tpi would half the pitch...

 

[Hazzert]

I don't have anything around the shop with a 40 tpi thread...

Bet you own a micrometer.

 

[john.k]

Two very common small screws are 40tpi.......1/8x40 Whit,and #5x40 screw....both commonly used in older appliances .

 

[Hazzert]

Two very common small screws are 40tpi.......1/8x40 Whit,and #5x40 screw....both commonly used in older appliances .

I don’t think old appliances and common screw size go together nowadays.

 

[Aejgx6]

Be it 40tpi is handy for shop made fixturing stuff.

 

[Comatose]

Whitworth and #5? What's wrong with 4-40? That's probably the most common thread in my shop.

 

[LKeithR]

Bet you own a micrometer.

OK smartass, you got me--never thought of that...

 

[fciron]

Thread gauge?

Stand the tap on end next to a rule and rotate it so you can count all the threads in half an inch, or even a quarter?

Put it it your part with an indicator on top and measure the travel of one complete turn? Or just hold a ruler next to it in count the number of turns to back it out one quarter inch?

Look up the spec in the machinery's handbook?