Need Lathe Pipe Thread Cutting Ideas

[Linstrum]

Lee sizing dies use a 1/4" or 3/8" National Tapered Pipe Thread for the collet that holds the primer punching mandrel and I need to make a new collet for my .30-06 sizing die. Pipe threads have a 3/4" per foot included-angle taper and I'm trying to figure out a way to cut that tapered thread on my lathe without having to offset the tailstock and machine it between centers. Mounting something that short between centers is a bit difficult to begin with plus I am very reluctant to disturb my tailstock because I just managed to get it adjusted so my lathe can make a 40" cut without any taper in that length, which took about an hour plus required making trial cuts on a 40" length of 1.5" steel bar stock to get it adjusted, and a 40" piece of 1.5" steel bar stock isn't exactly cheap.

Besides machining between centers, a taper bar is the other obvious way, but unfortunately my lathe isn't built with taper bar mounts on the bed or cross feed. It appears that I'm up a creek without a paddle unless there is some obscure way of cutting tapers on short lengths of bar stock on a lathe like mine, although in the past I have faked it a few times by hand-cranking the crossfeed to approximate a taper that was not too critical.

All suggestions welcome, I'll try anything once! Well, almost anything.


rl597

[richbug]

As agrivating as that would be to do, I would just cut straight threads a bit shallow on the lathe, and chase it with a die.

[Red River Rick]

FWIW, the thread is 1/8" NPT, not 1/4" or 3/8". Just cut the straight threads, the internal thread is tapered so it'll still tighten up. Otherwise, get yourself a 1/8" NPT threading die and
use it to cut the threads.

RRR

[S.R.Custom]

...Just cut the straight threads, the internal thread is tapered so it'll still tighten up.

What he said. And make sure your cuts in the collet are generous enough that they don't close too early when you thread the collet in.

[Calamity Jake]

Wouldn't it be a lot easer to get one from LEE????

JM 2¢

[richbug]

Wouldn't it be a lot easer to get one from LEE????

JM 2¢

unless he wants a non-standard size

[Red River Rick]

Check here, if you don't already know, for the threading die: http://www.kbctools.com/usa/main.cfm

"1/8" NPT, Adjustable Split Die, HSS.................$10.95"

If time, is of value, this would be the cheapest and easiet way to go.

RRR

[S.R.Custom]

This is pretty common stuff for your local hardware store, too.

[Pavogrande]

How about using a 1/8" pipe plug as raw stock?

[machinisttx]

Set the compound to the proper angle and cut the taper, then use the appropriate thread die to cut the threads.

Without a taper attachment you're out of luck.

[Linstrum]

Thanks guys! Never any problem with getting plenty of good ideas at this site.

I want to try some ideas to improve the Lee collet, which is why I want to make my own instead of getting a replacement, although since Lee does have a limited lifetime warranty, I can get a new one for half price, but I might have to pay $8 for shipping.

Hardware store pipe plugs come in three “flavors” - cast iron, brass, and stainless steel. Of those three only the stainless steel type would work well to make a new collet from since type 304 stainless used for plumbing fittings has all the right characteristics to work very well. Unfortunately pipe plugs are way too short to make a new collet from, so that is out for a starting point. Good idea, though, you know, K.I.S.S. Principle - keep it simple, shooter!

This morning I machined a collet with straight threads. With the compression relief cuts made into the collet it bends in on itself and creates its own taper. Later on tonight when I have time to resize a few .30-06 cartridges and it doesn't work out I'll take the suggestion of chasing the threads with my 1/8” pipe die on the next one I make.

It would still be nice to be able to cut my own taper threads, though, since I do have another use for doing that. One of the hydraulic control valves on my tractor skip loader uses British taper pipe threads in combination with National Fine bolt thread JIC/ANC male hydraulic 37° flare fittings machined on the other ends of the nipples for the hydraulic hose fittings to screw onto. British pipe threads employ an oddball thread pitch of 19 turns per inch for 1/2” pipe, along with the 55° Whitworth thread profile. It isn't any trouble to regrind my threading tool bit to 55° - I just use a regular parting tool bit ground to a point instead of a square end for thread cutting anyway – and my lathe can do 19 tpi no problem. I just need to be able to do the tapered thread cutting part. Maybe I will figure out how to put a taper bar on it since I keep needing one.


rl598

[lathesmith]

Lin, if your lathe has an MT3 tailstock taper, why not just mount a boring head with a 1/2" straight shank dead center in the tailstock? You can easily dial in the exact offset you want with one of these. It's not like you're going to be turning at high speed, and you can keep it oiled, besides. I've done this a few times on my 13x, it works fine. I found a MT3 taper shank with 1 1/2" threads to replace my R-8 taper shank on my boring head, it was about $18 if I remember right. Beats the heck out of messing with the tailstock. Like you, I hate doing that!
lathesmith

[Morrison Machine Shop]

Check out this link, might be what you need

http://www.green-trust.org/junkyardp...etCenterSM.pdf

[DLCTEX]

Shipping from Lee a month ago was $4.

[Cap'n Morgan]

It is indeed possible to cut a tapered thread on a manual lathe - I did it many moons ago, but it is quite a bit of work.

Basically, what you do is disengage the cross slide spindle nut from the crossway, so the cross slide can move freely. Then you mount a guide plate on the back of the carriage. The guide should be mounted in the same angle as the taper on part you want to cut. You'll also need a ball bearing or similar mounted on the cross slide to follow the guide bar while cutting.

Now all you have to do is keeping the cross slide pressed against the guide plate while cutting the thread. Use the compound for feed in. The system can also be used for simple contour copying - like a barrel profile.

[3006guns]

Just a helpful hint, nothing to do with threading........

The simplest way to verify or reset your tailstock is to obtain a piece of heavy tubing or pipe of an appropriate length, say 24".

Machine two steel, brass or aluminum plugs that will press fit in the tubing ends. These plugs should be of a larger diameter than the tubing and have center holes.

Put this assembly between centers (you need to make provision for a drive dog on the headstock plug) and take a trial cut on each plug, then measure the diameters with a micrometer. Obviously if the tailstock plug is smaller, it needs to be moved away from you. If larger, move the tailstock toward you.

Once you have both plug diameters the same, your centers are in line. Remove the assembly and store it away. Next time you need to reset your tailstock, just put it back between centers and use a dial indicator on your carriage and keep checking each end as you adjust the tailstock. Once the indicator shows the same measurement, you're done. Easy, and a lot quicker/cheaper than turning down a length of solid stock.

[Linstrum]

Hi, Dale, $4 isn't too bad, thanks.

The photo is the mandrel collet I just made.

I punched out some dead primers using my slightly improved version of the mandrel collet and it works like it should. The original collet wouldn't keep the mandrel from slipping when punching out military primers and after tightening it up a few times the collet threads began to strip. Fortunately the die is hardened and only the collet threads gave out. The improvement was to make the threaded part of the collet a bit longer to give it a longer grip engagement on the mandrel. I do need to keep in mind that the mandrel does need to allow slippage if I get a Berdan-primed case mixed in, the punch pin could break or bend over if the mandrel can't slide. Been there, done that – too many times!

If anyone would like the dimensions I used in order to make a slightly improved mandrel collet for Lee dies, I used a piece of 3/4” round AISI 1045 medium carbon steel alloy (I'm sure that plain old everyday hot rolled AISI 1018 mild carbon steel will work just fine, or even a 5/8” square railroad spike), the collet's over-all length is 1” with 0.590” of it turned down to a diameter of 0.390” to make the threaded shank. The 0.390” diameter part is threaded with regular 27 tpi 60° straight threads to a length of around 0.555”, which is 15 threads. The threads are cut to a depth of 0.0303”, which is 0.035” on the compound feed set at 60° (0.035” x sine 60° = 0.03031”). After cutting the threads, I used a #7 drill (0.201”) to drill a hole all the way through its axis for the mandrel to fit in. The mandrel I have measures 0.202” in diameter, but since drills usually cut slightly oversize holes in steel a drill that is 0.201” in diameter worked in my case but YMMV and a #6 might be necessary to chase out the hole. After drilling the through-hole for the mandrel I cut a slot with a hacksaw the whole length of the threaded part of the shank. Clean up the burrs and that's it, unless you want a pair of wrench flats on the 3/4” diameter part, I milled flats on mine for a 5/8” wrench so the collet wouldn't get boogered up from using vise-grips on it.

Cap'n Morgan's suggestion for cutting tapers and profiles, and 3006gun's method of using a length of pipe with center plugs in the ends to indicate in the tailstock are duly noted, next time I need to do those operations they'll sure make the job quicker and easier. Also, lathesmith and Morrison Machine Shop's ideas for making adjustable offset tailstock centers are other excellent ideas. I have pondered doing that for years and now I know how to do it quick and easy with one of my Criterion boring heads for the Bridgeport mill. The boring head I have in mind has a screw-in shank for the mill quill that I can replace with an MT3 shank, and its boring tool bit holder has three 1/2” holes I can machine and harden a 60° x 1/2” center to go into.

Man, every one of those ideas and suggestions are pretty slick, thanks, guys!


rl599

[Morrison Machine Shop]

Looks good, as I always say "Ought to work, if it don't, it shoulda."

[JIMinPHX]

I'm not going to say that pipe threads can't be single pointed on a manual lathe, but I've never had good luck with it myself. I've single pointed a few passes straight thread, to keep things straight, then gone to a taper die. I've done taper threads on a CNC with good results. I've had my best efficiency cutting taper threads on a manual lathe with a GEO head, but they ain't cheap.

Several good ideas have been posted here by several people but I think that in this situation, the price of a die is less than you would spend on any single pointing taper setup.

If you have other uses for the taper gizmo, or if you just want one to play with & learn from, then it is worth going that route. Otherwise, I would just turn a push plate with a 1 inch face & a 3/8 shank that you can chuck up in the tail stock to push the die onto the work in the main chuck straight & turn the die by hand.

...just my 2 cents

[grumpy one]

It won't solve this problem but here is a semi-weird kind of live center that I use for a whole lot of strange purposes, including holding things that can't have a center-hole drilled in them, or things I want to offset for one reason or another. The right-hand end is MT3 to fit my tailstock, and left-hand end is a great big recess with the work-piece held in it by the four radial screws. In between are some fairly stout ball-races (note the grease-point on the outside). I've been using it as necessary for years. I substitute different screws somtimes, so they won't stick out further than necessary. If I used it more I'd have more screw-holes for the radial screws, in patterns to suit various irregular shapes of workpiece.