Question on threading from neophyte lathe owner

D

Daver

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I am the relatively new mini lathe owner. I am attempting to make a part that a friend of mine drew up. Not too complicated. all turning, no drilling, and no milling (which is good, cause I don't have one!). I don't have a machinist handbook yet (its next purchase) or I would try to look it up and figure this out myself.

Anyway, my question is this: I have a piece of 1.2" stock and I want to thread one end of it to 5/16-18. What do I turn that end down to before I try to thread it using a die? or before I try to cut threads with the lathe itself? would it be the same initial major diameter? Is there a preference to one method over another? How would a production shop do it? (I cant see a production shop not using a Die unless they have a CNC to cut the threads...) But I'm not production, I'm only making one of these (unless I screw it up) and I am learning... so I am thinking I will cut them with the lathe, and the 'chase' them with the die to see how close I was. (?)

ok, enough of my chatter... let the wisdom flow!
(thanks for all your help!)
 
If you're doing it with a die, cut it 5/16 or a couple thou less. Depends if you want a tight thread or not. Don't go over 5/16. Using dies, you'll suffer the problem of not getting the threads straight. Lathe is a better way to go unless your die handle is outfitted with a good rig to keep the die flat and perpendicular to the stock. Also, if you're working against a shoulder the die can't thread right up to the shoulder because the starter threads are tapered. You could flip the die around but you'll risk busting the cutting teeth unless you're working with very soft material like aluminum.

If doing it on the lathe, cut the piece about 5 thou under 5/16. The threads usually raise a little. Most factory bolts are a good bit under nominal size. A typical 1/2-13 usually measures .486 in dia. A 1/4 inch bolt usually measures about .245 or so.

Sure, you can chase it with a die after you lathe cut it...

Ray


Daver said:
I am the relatively new mini lathe owner. I am attempting to make a part that a friend of mine drew up. Not too complicated. all turning, no drilling, and no milling (which is good, cause I don't have one!). I don't have a machinist handbook yet (its next purchase) or I would try to look it up and figure this out myself.

Anyway, my question is this: I have a piece of 1.2" stock and I want to thread one end of it to 5/16-18. What do I turn that end down to before I try to thread it using a die? or before I try to cut threads with the lathe itself? would it be the same initial major diameter? Is there a preference to one method over another? How would a production shop do it? (I cant see a production shop not using a Die unless they have a CNC to cut the threads...) But I'm not production, I'm only making one of these (unless I screw it up) and I am learning... so I am thinking I will cut them with the lathe, and the 'chase' them with the die to see how close I was. (?)

ok, enough of my chatter... let the wisdom flow!
(thanks for all your help!)
Click to expand...
 
Turn the end down to 5/16" OD, then cut a thread relief groove at the base so your threading tool or die has someplace to run off. Not doing so will likely leave a burr at the end of the thread. Cut a 45 degree chamfer at the leading end to provide a lead in for the die or cutter. If you use a die it is best to use a tailstock die holder so it goes on straight. Lacking that, you can screwcut it with a lathe tool. The benefit will be that you can tailor the fit with the mating part, if you have it.
 
If it doesn't have to be a supper tight fit thread, just get a 5/16-18 nut from the hardware store and use it to test fit. Start taking light cuts with the threading tool. Try start testing the nut as the "peaks" of the threads start looking like points. It can be helpful to mark the surface of the part with Dykem or Sharpie so that you can tell where you have cut.

And yes, turn the part down to the major diam, or a few thousands less. Run the lathe at the lowest speed possible. If you are threading to a shoulder... I would recommend practicing on scrap before you work on the real thing, things go real fast when threading, and if your hands aren't programmed to hit the half nut at the right time, it can get "interesting".

I would also recommend using a cutting fluid. If it is steel, ordinary motor oil, mineral oil, and even peanut oil work fine, though vegetable oil will get sticky if you leave it on the lathe, so clean up after. If you are cutting aluminum, WD-40 makes a great cutting fluid, just a light spritz every so often. Unfortunately it will likely remove your layout marks.

There are lots of good videos on youtube about single point threading. Check out mrpete22's channel, he has some great videos on this subject.

One last thing, I usually hit the surface of my threads with a couple strokes of a mill file, and then go over them with a wire brush. Helps remove chips, and take of any burrs. Single point cut threads can be _very_ sharp if you don't smooth them out.
 
If using a die, try this. Turn to 5/16" (or a little less), remove the center from the tailstock and use the face of the ram to hold the die (or diestock) square. Lock the lathe spindle and turn the die by hand, keeping light pressure on the die, using the tailstock handwheel.

I have found this proceedure to work very well. Gives a stright thread eveytime.


Bill
 
I suggest you make yourself a die holder. Use a pice of stock at least .38 larger than th die you use . make it several inches long, drill and ream a .38 hole throgh the center, co'bore to dia. & depth of your die, drill & tap 2 holes from the outside to suit the split and drill point hole in your your die for set screws Hold a length of .375 drill rod in your chuckand slide the die holder onto it. If you need leverage you can drill a couple of cross hole and use a pice of round stock as a handle.
Start by forcing the set screw into the split in the die to expand it as far as it will go. Then take another pass and close the die if needed until you get the deisred fit on a nut.
Hope this helps
Ron
 
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