Threading a long rod and keeping it straight.

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Tim Wescott

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I'm trying to make needle valves per the drawing below, and I keep screwing up the spraybar.

Basically, I get a nice machined straight brass rod, but as soon as I thread it I end up with a nice, threaded, gracefully bent, brass rod.

How do people do this? I'm currently using a die in a holder, held in my hand -- if I made up a die holder to be held in a chuck in my tailstock would it work? I've considered trying to single-point machine it, my lathe isn't well set up to make such a cut in such confined quarters (but I'll figure it out, if it's the only way to make it work).

(Edit: I'm trying to tap it at 5-44 -- is that just too coarse a thread?)

Thanks.

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dnalot

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Most threading dies are for chasing threads. It would be best to single point machine about 80 percent deep and then chase to the final size using a die holder in the tail-stock.

Mark T
 

fabricator

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dnalot is right, generally split round dies are for cutting threads. Hexagonal dies are for chasing threads. But if you have a tailstock die holder you would stand a much better chance of good results.
 

Tim Wescott

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It's a round die, bought as a thread cutting tool.

Mark, I hear what you're saying about single-point threading and then chasing -- but I think I'm going to make me a tailstock die holder anyway and try it, because getting my lathe to cut threads over such a short span is going to be murder.

Then if that doesn't work I'll go back and figure out how to single-point turn threads on something while supporting it with a center.
 

fabricator

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I've had good luck using a handheld die holder if I put the piece to be threaded in the chuck or a collet then keep the tailstock up against the die to keep it perpendicular.
 

Tomcat

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I'm trying to make needle valves per the drawing below, and I keep screwing up the spraybar.

Basically, I get a nice machined straight brass rod, but as soon as I thread it I end up with a nice, threaded, gracefully bent, brass rod.

How do people do this? I'm currently using a die in a holder, held in my hand -- if I made up a die holder to be held in a chuck in my tailstock would it work? I've considered trying to single-point machine it, my lathe isn't well set up to make such a cut in such confined quarters (but I'll figure it out, if it's the only way to make it work).

(Edit: I'm trying to tap it at 5-44 -- is that just too coarse a thread?)

Thanks.

View attachment 116075
You can also machine a short section and thread it, then reduce the next section and thread it, etc. Also watch the diameter of the rod - a very small reduction on the diameter makes a huge difference in the threading forces required.
 

clockworkcheval

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In my experience some finer and longer die-cut threads wil not follow a nice straight path, even if cut with a die held in a tailstockholder. Too many elements can be slighty amiss adding up to scrap. So I fully agree with singlepoint cutting, if possible at all supported with a running support on the saddle.
 

terryd

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Hi,
I made myself a taistock die holder and have made a straight 5/32" threaded rod 11.5 inches long. However in the meantime you can make a thread in the lathe with a die and standard hand held holder very accurately. How? With the work centered in the chuck and the end chamfered push the die against the work and lock the tailstock. Turn the chuck with left hand and use right hand to feed the tailstock just enough to keep light pressure on the die. The handle of the die holder should be prevented from turning by resting on the saddle or cross slide of the lathe, I've just been making a series of threads of varying size from M2.5 to M8 (5/16) metric fine using this method as all of my tailstock die holders were full of other dies and I couldn't be bothered to keep changing dies for a few one fs. I've added a picture to show the set up, this is not a working set up just a demo - N.B. very important - the lathe must NOT be powered, isolate it. Note the handle restrained by the cross slide, with a larger die holder it could rest on the bed.

I've also added a picture of my tailstock die holder. It is very simple and slides on a shaft which has a morse taper for the tailstock. I usually use it hand held without the screw in handle and under power and when I release the holder it simply spins with (slowly) rotating work piece.

TerryD

2020-05-08 11.55.19.jpgDSCN2500.JPGLathe Threading tool complete.JPG
 

tornitore45

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A die holder that can slide and stay on axis is mandatory.
If the die starts crooked the result is a "drunken" thread, being the part hollow one side is thinner then the other and the part bends.
Visualize an angle between the part axis ant the die axis.
The thread will eventually get deeper and deeper on one side and shallower and shallower on the opposite side as the thread progresses. This is an exaggerated visualization but it does not take much for the die to attempt to right itself because there is more resistance on the deeper side, in doing so it bends the part.

The smaller the diameter the harder is to start square and smaller error are enough to give trouble.
Tho start a die well aligned I usually turn down 4 or 5 thread pitches to the minor diameter and then cut that stub off. The die and the part have little choice to move as the thread starts and place an offset force until the first thread is formed.
 

Mainer

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You ARE turning the rod down to 0.125" before you thread it 5-44. aren't you? I'm a bit confused by the drawing, as it looks as though the thread is spec'd at 6-32. Am I missing something?
 

terryd

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I just came across this picture of a long thin thread - 5/32 diameter - which I cut under power using my tailstock die holder - it was dead straight. This is obviousy brass but I've done the same job in mild steel.

TerryD

Small thread cut with tool.JPG
 

tornitore45

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The fact that the part is hollow and the wall is fairly thin aggravates the situation substantially.
 

terryd

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Die holder hand held for small thread (5/32" BSW) And just to clarify, the Morse taper sliding shaft for die holder to slide on which I didn't show earlier

TerryD

DSCN2511.JPGDSCN2498.JPG
 

terryd

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The die holder sliding on a shaft is an excellent way to go, I'm going to have to brazenly copy that.
That's fine, I copied the basic design but made up all the dimensions to suit. If you note there is a hole down the centre of the sliding Morse taper bar which accomodates longer parts. I actually made another bar, turning an integrated Morse taper which I drilled all the way through so that even longer threads could be produces. The longest I've made is 11 3/4"
 

terryd

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The fact that the part is hollow and the wall is fairly thin aggravates the situation substantially.
Of couse another solution is to turn a bar- even hardwood would suffice, close fitting in the hole and Loctite (I use cheap 'superglue' - cyanoacrylate) it in when cutting the thread. Heat up gently to break down the Loctite, pull out the support bar and clean up. Smiles all round :)

TerryD
 

teeleevs

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You can also machine a short section and thread it, then reduce the next section and thread it, etc. Also watch the diameter of the rod - a very small reduction on the diameter makes a huge difference in the threading forces required.
Tim, Tomcat is right, if the rod is slightly under size it will cut easier it will also expand out to the right size, having said that brass is difficult as it will cut tight and the rod will slightly twist which also contributes to bending, a good new die is better than an old one. I know none of this helps you, just keep trying.
 

awake

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Then if that doesn't work I'll go back and figure out how to single-point turn threads on something while supporting it with a center.
Tim, I pretty much always single-point threads - in part because the only dies I have are only good for chasing threads, not for cutting them from scratch. But I want to call attention to the last thing above - if you go this route, with a long, thin part, you really need the end to be supported by a center; otherwise the threading force will bend the rod, and/or give you lobed threads as the part flexes. DAMHIKT!
 

JohnBDownunder

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All the above reminded me of this item for turning slender rod and perhaps if you had a few to do it might be possible to adapt the idea to support the threaded diameter behind the die???
John B
 
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