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View Full Version : Need advice on making NPT threads on the o.d. of a turned part in the lathe...



Brian Gale
10-28-2003, 02:45 PM
Howdy-

Need some thoughts/leads/advice etc. on threading NPT on the o.d. of a part on the lathe. I'll typically have a threaded area butted up against a shoulder when I'm making a fitting/etc. and need some advice on how to create quick and easy pipe threads on the o.d. of my turned part...1/4-18 NPT is pretty typical. Right now I'm turning the part and then clamping it in a vice and trying to use a die to form the threads...this is a waste of time and difficult to get straight. I'm pretty ignorant on this and want to do it with minimal setup, etc. The parts are small so taper turning doesn't seem realistic. Any readily available trick adapters etc. for a die that make this easy...

Can you help?

Thanks-
BG

Evan
10-28-2003, 02:53 PM
I have an adapter for my SB9 tailstock that is a MT2 taper with a circular plate of brass about 3" in diameter and 1/4" thick on the end. It is useful for holding work straight when presenting it to a bit in the spindle. Nice for drilling large holes in the end of long pieces at low rpm. It also works well for aligning a die in a die wrench to the work in the chuck.

Edit:

BTW, I don't power thread like that but place a block on the ways and let the die handle rest against it. Use the tailstock crank to apply pressure and rotate the chuck by hand.

[This message has been edited by Evan (edited 10-28-2003).]

JCHannum
10-28-2003, 03:24 PM
They make die holders that are basically a shell that holds the die with a shank that can be gripped in the tailstock chuck to guide the die and keep it on center.
1/4" pipe thread is pretty hefty to do in this manner however, as it will probably slip in the lathe chuck.
Taper threading is not that difficult. For pipe thread, the cutting tool is set up the same as for machine threads, at 90* to lathe centerline. The work can be set at an angle with tailstock setover method, or a taper attachment used to move cross slide.

Dr. Rob
10-28-2003, 03:47 PM
...and FWIW, I once found reason to check the accuracy of commercially available (=plumbing store) NPT's and fittings.

There was none. The darn things were hopelessly askew. Completely useless by any machinists measure.

(point is, if your attempts are a little crooked, maybe they're still good enough)?

SGW
10-28-2003, 04:54 PM
Setting up for taper pipe threads:

JCHannum, I think the cutting tool is set up at 90 degrees to the axis of the PIPE, not at 90 degrees to the taper? I'll have to check Machinery's Handbook....

Forrest Addy
10-28-2003, 05:12 PM
Lemme guess, no taper attachment?

JCHannum
10-28-2003, 05:45 PM
The threading tool is set up as if cutting machine threads. 90* to axis of lathe. Compound is set at 29*, 29-1/2* or 30* depending on your persuasion. Taper is 2*-22', or 3/4" to the foot.

wierdscience
10-28-2003, 08:25 PM
If you want to do it quickly but still accurately you have two choices,#1 a geometric style die head and #2 a Ridgid quick opening die head like you see on a threader.

Both can be had on ebay,I would go for the geometric head as they are cheaper on ebay than the Rigid head and more versatile.

I got a 9/16""D" geometric head and two sets of dies for $45.00,and the dies can be had new from KBC for something like $120.00 a set.

curley
10-28-2003, 08:41 PM
Brian, thread the part 18 TPI as normal, except stop short of full depth, then use the die to finish the thread. I would use the same setup in the lathe and the tailstock to support the die to start.
Works pretty good on straight threads too.
Regards, Curley

C. Tate
10-28-2003, 10:01 PM
Curley's way is best IMO without cost of die head as mentioned. The die head will work very well and is fast but tooling can be expensive and taper threads require a tremendous amount of horse power as compared to single point many times part will slip in three jaw chuck or simply stall light machines. I like to rough it with single point and finish with some type of die. Remember that any type of die will love lots and lots of oil the thick, dark and stinky is best.

Jim Hubbell
10-28-2003, 10:09 PM
Set up the lead screw for the TPI of pipe thread you wish to cut. Set compound etc. as you normally do. For a cutting tool use a single die from a power threading machine. Set it 90deg. to lathe axis. The correct taper will be built in. Go slow towards the end as there is a lot of torque involved.

Brian Gale
10-29-2003, 01:05 PM
Thanks all...I don't have a taper attachment and haven't been able to find the tailstock device that will hold a die to help get it started. Anyone have pictures of the taper attachment or have a lead on where to buy the die holder?

Many thanks for all of your responses!
-BG

Evan
10-29-2003, 03:28 PM
Brian,

Buy a Morse Taper blank arbor and make your own tailstock attachment. The blank end of the arbor is unhardened so you can drill and tap it, then attach a disk to it with a countersunk socket head screw. Cost about $30 or less for a MT2 blank end arbor.

Hellbender
10-29-2003, 03:50 PM
Anyway you can turn things around...ie... put your threading die in your 3-jaw and secure your part in a tailstock or carraige mounted fixture or chuck or....??

All you need to do is get the die started good and straight and then you can finish it by hand.

------------------
NRA Lifetime Member

curley
10-30-2003, 10:53 AM
Brian,I have a fixture I made to hold a die (nothing but a piece of pipe, plate welded on ends, one end center drilled, the other counter bored to take die. Turn with pipe wrench), but I have used the tailstock chuck with the jaws opened back into the body, and just push the die with the flat, front surface of the chuck, using a regular die stock. If you single point most of the thread, you will lessen the load on the die, get a straight start, and won't need the tailstock for more than a couple or three turns.
Regards, Curley

Gator
10-30-2003, 01:41 PM
I do it the way Jim Hubbell said and if the torque is heavy I set the leadscrew to the pitch being cut and make multiple passes as if cutting a machine thread.

Larry

Brian Gale
10-30-2003, 03:59 PM
Thanks guys...makes sense...