A question for you gunsmiths and others who make screws on the lathe.

What sort of fixture do you use so you can remove the screw from the lathe, check it and return it to the lathe and continue cutting the thread?

Typically you would make a nut. Drill and tap a small piece of steel and use that to check the threads as you really do not want to remove it from the chuck or collet.

But if you are machining threads and you must remove the part to check the threads here is how I do it.

All you have to do is set up some sort of stop in the chuck or collet. I prefer a collet with a collet stop. Then you will need to index the screw some how so as not to loose the position on the diameter. I.e. where it is located within the 360 degree rotation of the spindle.

This can be as simple as a pencil mark on the screw head or something on the collet stop to engage the screw head. Keep in mind you can still foul things up by indexing 180 degrees out on a straight screw head or 60 degrees out on a hex head you still need to mark it so you know where to index it.

The finer the threads the more accurate the location of indexing needs to be.

I've had to machine threaded inserts for an injection mold this way because the engineer could not give me accurate enough dimensions to machine the inserts too.
I made a set of soft jaws with a step to butt the part up against. then scribed a line on the diameter marked with dykem. Worked great and I was able to insert and remove the part as many times as it took to get the finish dimension correct (about four times).

But in reality you would want to use a nut or a thread gage to check the threads.

Never use a jig. Just chuck up your part and catch your lead.
Make sure your threading tool wont hit your threads, come in on the thread dial and then stop the lathe when you get close to the threads.
Move your cross feed in and your compound over to line up your lead and then set your cross feed dial to zero.
Make sure you come out of the threads and take the half nuts out before you start the lathe back up.
Never a day goes by that someone doesn't walk in here with buggered up threads.

Turning box tool and a geometric die head.

quote:

Originally posted by brad may:
Never use a jig. Just chuck up your part and catch your lead.
Make sure your threading tool wont hit your threads, come in on the thread dial and then stop the lathe when you get close to the threads.
Move your cross feed in and your compound over to line up your lead and then set your cross feed dial to zero.
Make sure you come out of the threads and take the half nuts out before you start the lathe back up.
Never a day goes by that someone doesn't walk in here with buggered up threads.

This is exactly how Mike Jines taught me to do it.

quote:

Originally posted by brad may:
Never use a jig. Just chuck up your part and catch your lead.

I've heard this called "picking up the thread" where I've worked. Same idea. Also, you can get pitch micrometers that let you measure the thread without removing the part.

quote:

Originally posted by jpl:

quote:

Originally posted by brad may:
Never use a jig. Just chuck up your part and catch your lead.

I've heard this called "picking up the thread" where I've worked. Same idea. Also, you can get pitch micrometers that let you measure the thread without removing the part.

If...you have the old screw to measure.

If the screw head is on the chuck side you can test fit, just as you would a receiver to a barrel. Otherwise the most accurate method is to measure over thread wires and compare to the screw you are replicating or to the thread standard. I think the tables are listed in Machinery's Handbook. Using wires the work never has to be removed from the lathe. http://www.kanabco.com/vms/eng...ng_mhandbook_07.html

Now if for some reason you have to remove from the lathe to check fit, I think an easy way to accomplish that and be able to restart machining without picking up the thread would be to do it between centers with a dog driver. If the dog is not removed from the work, it will index back correctly when put back in the machine and you can continue cutting the thread without having to pick it up.

quote:

Originally posted by Jim Kobe:

quote:

Originally posted by jpl:

quote:

Originally posted by brad may:
Never use a jig. Just chuck up your part and catch your lead.

I've heard this called "picking up the thread" where I've worked. Same idea. Also, you can get pitch micrometers that let you measure the thread without removing the part.

If...you have the old screw to measure.

Or if you have a copy of the machinery's handbook Long list of pitch diameters in there.

And on odd pitches or special pitches a little math and any pitch diameter can be found just use the tolerances of a thread that is similar.



Pitch diameter calculation

D= Major diameter

p= Pitch

Pd= Pitch Diameter class 2 A/B threads.

Pd= D-(.64952/p)

Or .4662" = .4987 - .032476

For a 1/2" 20 tpi bolt

Works on any 60 degree thread meteric or standard just convert all dimensions to inches and threads per inch.

Thanks for all interesting ways you guys do it. I’ve struggled trying to get the work back into the lathe correctly after trying it to see how it fit.
A while back I watched my friend John Wills make a screw and remove it and replace it in the lathe several times. Each time it only took seconds to be back cutting the thread.
John designed and made these little collars for ¼”, 5/16” & 3/8” screw stock. I’ve made one for ¼” and 5/16”. When you loosen the chuck and remove the work just replace it between the same lathe jaws each time and rotate the little dog against jaw No. 1 (on my lathe). The length is set by the back of the collar and rotation is set by the dog. Simple, quick and accurate.
John also cuts a screw slot at the back end of the stock for installing and removing the screw, also a good time to mark for the new screw slot.

quote:

Originally posted by gzig5:
Now if for some reason you have to remove from the lathe to check fit, I think an easy way to accomplish that and be able to restart machining without picking up the thread would be to do it between centers with a dog driver. If the dog is not removed from the work, it will index back correctly when put back in the machine and you can continue cutting the thread without having to pick it up.

What size "gun screw" are we talking about that you could run between centers and drive with a lathe dog?

I gotta tell ya, I'm looking at my smallest "miniature" lathe "dog", the smallest one that will reach the chuck, and I think you could lose complete access to the entire screw just clamping to that.

Personally I think brad may's method is the quickest and most effective. Doesn't take any time to reacquire the thread.

I never seem to have enough hands and fingers to use the three wire method to measure OD threads. Thread pitch mike's would be nice if you could ever find the one you need, and they only make a few limited sizes.
Neither one would help you if you were cutting the box (box is oilfield trash speak for ID threads, pin is OD threads).
Most everyone now uses Gagemaker out of Houston for their thread gages. Their tools flat out work.
Although I own and operate CNC equipment, it seems more and more to stay alive these days I'll have my blacksmith hat on.
Starrett makes what they call their thread spring calipers (Nos. 179 and 184). There just their cheap flat calipers, except the OD ones are sharpened to a knife edge to measure the root pitch diameter and the ID ones are bent and sharpened to a pencil point to measure the diameter on the box. These will get you out of trouble more than anything you have in your tool box.
Back when I started, I threaded many a pin where the shaft was 10 feet plus and 3inch or bigger OD. Hydraulic cylinder rods. My lathe had a 1 1/2" hole and only 5 foot bed. Once it was finished there was no way to put it back in the lathe. My thread calipers never let me down.

Not exactly correct.
Starrett makes just about every pitch and range thread mic known covering UNF, UNC, & UNS

The problem is to get a set to cover a decent range you'd have to mortgage your house.

They ain't cheep to say the least

<Edit>: It has been brought to my attention that some may be mistaken about the difference between a thread Mic and a thread caliper.

What I'm referring to is an actual Micrometer with a "V" grove ground on the anvil and a point ground on the spindle. They cover a narrow range of pitches and you need three or four Mics in each size to cover the entire range of thread pitches from say 60 TPI to 8 TPI Guessing here as I have not used them since trade school. Same with thread wires too Haven't used them since trade school.

There is a some what handy little device that work well but is a little cumbersome to use.
Thread triangles. They come with a little chart and fit on nearly any mic made. I think $30 or $40 from MSC or Enco

While we are on the subject what steel do you use for gun screws?

I've about run out of screw stock and had it for so long not sure what it was.

I've ordered some 1144 and waiting on it before I make a set of screws for a Ballard.

quote:

Originally posted by brad may:
Never use a jig. Just chuck up your part and catch your lead.
Make sure your threading tool wont hit your threads, come in on the thread dial and then stop the lathe when you get close to the threads.
Move your cross feed in and your compound over to line up your lead and then set your cross feed dial to zero.
Make sure you come out of the threads and take the half nuts out before you start the lathe back up.
Never a day goes by that someone doesn't walk in here with buggered up threads.

When I pick up a thread like this, I always make the first "cut" with the crossfeed back say .100 or something (with a DRO any distance is easy, but on dials .100 out is easier just to eb sure). Then I engage the half nuts like I am going to cut the thread, shutting the machine down in the middle of the "cut." After the spinning stops, i run the cross feed in. if everything is right you bottom out in the thread when the DRO or dial hits zero.

It is a lot more convenient to make screws turning them complete using a 3 jaw or a 5C collet. I much prefer the collet for small stuff.

As far as the thread depth goes you can eventually just eyeball the thread and tell when it is getting sharp enough. It just takes a practiced eye. Or you can use the half depth or double depth for reference.
After several thousand single point threads you will be able to do it without much thought.

quote:

Originally posted by Westpac:

quote:

Originally posted by gzig5:
Now if for some reason you have to remove from the lathe to check fit, I think an easy way to accomplish that and be able to restart machining without picking up the thread would be to do it between centers with a dog driver. If the dog is not removed from the work, it will index back correctly when put back in the machine and you can continue cutting the thread without having to pick it up.

What size "gun screw" are we talking about that you could run between centers and drive with a lathe dog?

I gotta tell ya, I'm looking at my smallest "miniature" lathe "dog", the smallest one that will reach the chuck, and I think you could lose complete access to the entire screw just clamping to that.

Personally I think brad may's method is the quickest and most effective. Doesn't take any time to reacquire the thread.

My first lathe came with a lousy three jaw that was completely clapped out and bell mouthed and until I saved enough to by a good chuck I did a lot between centers. Making #6 or #8 screws from 1/4" stock with a homemade dog was pretty straight forward but I'm glad I don't have to do it that way any more.

Personally I struggle to pick up small threads and having a dog or something like Michael's collar to index off of, simplifies things.

For material I usually grab 0-1 or W-1 because I have a lot in the small sizes, it cuts nicely, and is strong.

If any of you feel unqualified or it's simply not worth your time to make screws . This fellow might be your ticket .

I had some strange metric obsolete thread, from an RSA manufactured weapon that I needed longer screws for , He did a Superb job

and was MORE than reasonable !. http://www.antrinonline.com/profile.htm

Many moons ago I work at a Screw shop nearly next door to those guys.

quote:

Originally posted by gzig5:
My first lathe came with a lousy three jaw that was completely clapped out and bell mouthed and until I saved enough to by a good chuck I did a lot between centers. .

For a good tutorial on resurfacing/truing chuck backing plates and jaws, please see some of Guy Lautard's writings in his Machinists' Bedside Reader series, available from Brownell's. His explanations and procedures are simple and easily achievable by almost anyone.
Regards, Joe