Threading Silver Steel rod

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picclock

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I am having trouble threading silver steel on my lathe. The problem seems to be that the point of the threading tool (HSS) seems to break off. I have never managed to cut a 'good' thread in silver steel, the finish is always rubbish despite trying different lubricants. The cut surface is rough almost as though the metal has been torn away. The swarf appears quite normal. My current procedure is to rough out the thread on the lathe and then finish with a die, which produces acceptable finish results. I think my technique is OK as it works fine on other steels, so its clearly something to do with the hardness of the material. I have tried different cut speeds and depths, the last tip break being with a 7 thou cut at low speed.

Any advice most welcome

Many Thanks

picclock
 

Ken I

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Picclock,
I've had similar problems - the tip of the tool goes when you disengage at the end of the cut - try screwcutting into an undercut - only disengaging in the undercut.

I presume you are using the half angle method and not "plunge" cutting. Plunging on SS produces bad tearing.

Regards,
Ken
 

tel

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My experience is that silver steel is best finished with a die. No trouble to get a good finish on plain turning, but it eludes me with threading.
 

arnoldb

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Hi Picclock

When threading silver steel, I just make sure the cutting tip on the tool is sharply honed on a fine oilstone. If you use the tool straight from the grinder, the edge is too rough.
I use slow speed, lots of good tapping fluid as cutting fluid/lubrication, and small infeed increments; The deeper the groove gets the less infeed for each pass. My first pass is never more than 5 thou infeed, and that gets reduced to 2 thou at a time. Every third or fourth pass I make a spring pass without any additional infeed. My top slide can't swivel around enough to use the angle offset method, so I just plunge straight in.
This photo shows an M14x0.75 thread just finished in 16mm silver steel as described above - nice and clean:


Regards, Arnold
 

tel

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That is a nice finish Arnold - verging on excellent! ;)
 

John Rudd

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I made a new cross slide lead screw from silver steel but used an M8 left hand die to cut the thread.........lots of cutting compound...
 

picclock

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@ Arnold

That's a very good finish - One I would like to emulate.

I always have a 30 degree lead in chamfer. Undercut the end section of the thread prior to starting. Use the angled compound slide method. Finish the tool with an india stone. Radius the tool tip (the last one was M6 so I tried for a 6 thou radius). And the tool tip broke just over halfway along the thread with 7 thou advance of the compound on about the 5th cut. Breaks always seem to occur when the thread depth is half or more.

The main difference I can see between your process Arnold, and mine is that you take more lighter cuts. I thought 7 thou was a fairly light cut, but I will try to reduce it to see if it improves things.

I think I may make/buy/try a swing up tool holder as this might speed things when thread cutting with such small increments.

Glad to hear I'm not alone with this issue.

Many Thanks

picclock
 

pete

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Silver steel, Or to use the North American term of Drill Rod for the ones wondering exactly what it is, It does have more than a few types avalible. Air, water, And oil hardening to mention the types. And then there's different alloys depending on who makes it. My very limited experience shows the sharper the tool then the slightly better the finish. No I'm not trying to state the complete obvious. I mean sharp as in far sharper than you would normally think of as a sharp threading tool. Honed as fine as your capable of does seem to work slightly better. Arnoldb's picture is some of the best work I've seen so far for single point threading silver steel. I've yet to read about anyone who doesn't have a finish problem with lathe threading it though.

Maybe? it's a material that would finish well at far higher speeds than most without CNC or a Hardinge can or are capable of? There's got to be a few members here with high end commercial experience who have single point threaded it at those higher speeds with carbide. Going by some of my commercial hardened tooling. Someone knows far more than I do about how to do it right.

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In my experience, and I do thread silver steel occasionally, silver steel is not meant to be threaded.

It is a fairly easily worked tool steel for the manufacture of tooling, not shafts etc. If you hardened it after screw cutting, the threads would just break off, as they would be too hard to withstand tightening pressure.

People use silver steel for other things because it is fairly accurately ground, unfortunately, another property of raw silver steel is that it will rust very freely (not too bad after hardening and tempering), so no use in steam or even air driven (water down the airlines) engines or exposed shafts. You would be much better off going for ground stainless instead, which is what I tend to do.

I have forgotten the number of steam engines sent to me for refurbishment that have been fitted with silver steel shafts and piston rods that have completely ruined an engine internally because of rust formed over the winter non use period. The engines are usually locked up solid with heavy rust scale.


John
 

mu38&Bg#

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picclock said:
The main difference I can see between your process Arnold, and mine is that you take more lighter cuts. I thought 7 thou was a fairly light cut, but I will try to reduce it to see if it improves things.

.007" is quite heavy if you've made 4 passes at .007 already. Sandvik recommends 5 passes for 1mm pitch for their inserts. .006, .006, .006, .005, .003" radial infeed for their inserts. I also thread with reducing passes using carbide inserts, I don't do anything in drill rod though.

I think lighter feeds will solve your troubles. How far is the material hanging out past the chuck?
 

picclock

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@ dieselpilot
Last one was 15mm long thread with 5mm unthreaded to the chuck so 20mm total. I dont think its an overhang issue and any longer than that I would support with a centre anyway. Although it seems counter intuitive, I might grind up a tool for straight entry threading, not using the compound. It certainly seemed to work for ArnoldB, and its a fairly easy thing to try. Although with the extra tool loading one might expect the tip to break more easily, however with only 2 thou of infeed the load on everything is much reduced. If I have good success I will post pictures. May have to get a swing up toolholder first (tool envy rises here ;D). Wondered about using indexable thread insert tool, but never worked out how you would get the correct root radius.

Its not just the last thread, its most every thread I make on silver steel. In fact If I cut a whole thread without the tool breaking its unusual, and the finish is always bad. I don't feel so concerned about it now I know that its a problem other people have had, but I will try the other method to see if it gives any improvement.

Best Regards

picclock
 

Ned Ludd

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Hi Picclock,
Just a thought but what are you making your threading tool from?
I am assuming you are using HSS, if so are you using one with too much Cobalt?
I seem to remember reading somewhere, possibly Gadgetbuilder's site, that the more Cobalt you have the more brittle the steel becomes. We all assume that the more expensive ie Cobalt content, the better an HSS is but if the extra hot hardness is not needed, as in slow speed threading, a lower Cobalt content might be a better bet.
Ned
 

picclock

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@ Neil
I'm just using ordinary HSS, some from Arc, some from RDG and some from shows and even some imported. I've probably used 2 or 3 types. Definately found that for turning the ones marked SOBA seem better - Chronos I think.

Need to cut another one today so I thought I'd make a straight tool as per ArnoldB and give it a go.

Note to self : must make swing up toolholder ;D

Best Regards

picclock
 

picclock

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Hopes dashed ???

Well I've just tried threading using the arnoldb method, first cut 5 thou no problem, second cut 3 thou and the tip breaks. About 4 threads in, tool height spot on, everything sorted. Tool from hss supplied by ctc. freshly ground, use jig for 10 degrees side clearance, 60 degree tip angle 5 degree top relief.

It may be the type of silver steel is not the same as yours arnoldb. However I think I will go back to the 60 degree method as it feels like there is less load on the tool using this method.

I think I will just have to live with it - I have been doing so for a while but I just hoped that a better solution would be available.

P1000642.jpg


Best regards

picclock
 

Ken I

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Aaaaarrrrggghhhh - horrible !

If you are using the half angle method - you are only cutting on the leading edge of the tool - try using a bit of side rake - ie the RH side of the top face is lower than the cutting edge - might help.

2c

Ken
 

arnoldb

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:( - and What Ken said.

Normally you should not have top rake on threading tools - as technically it will change the thread angle a bit. For home/hobby threading that shouldn't matter though as the angle is negligible.

From your photo, it looks like your threading tool is not honed sharp enough - there's too much of a burr getting raised around the threading grooves indicating shearing rather than cutting. You really need to hone the sides up very sharp for silver steel. Your silver steel might be of a slightly different composition as well; I'm not sure how the silver steel I get here compares to that in the UK.

What were you using as cutting fluid ? - dry-threading silver steel will not work.

As Ken said - try the angle offset method; it places less of a strain on the toolbit; the only reason I'm plunging straight in is because my lathe's topslide can't swivel around far enough to do it.

If you want, I'll take a close-up photo of my threading bit tonight to show you what the cutting edges should look like.

Another thought: when grinding your threading tool - let it cool down often and do not allow it to heat up at the tip to the point where it changes colour. That can cause the tip to become brittle or develop very fine cracks.

Kind regards, Arnold
 

picclock

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Hi

The tool was very sharp and honed, what you are seeing as raggedness is mainly the remains of the tissue I used to remove the lubricant. I was using some random motor oil which seems to work best when turning Silver steel. The cut edge is rough, in that the tissue catches on it, but nowhere near as it appears in the picture. I've also used lard which is OK, but works very well on mild steel. You can see that the tool got about 4 threads in before breaking. I will regrind to get rid of top rake although I doubt that's the problem. The straight in method (Arnoldb) obviously works for him so there must be some other differences.

As I said before I will go back to the angled compound feed method and live with it for now.

Best Regards

picclock
 

gbritnell

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Hi Picclock,
In your last posting you mentioned using 'random motor oil'. Any type of lubricating oil is positively a no-no for threading. I use any of the modern cutting fluids and still have some of the old dark sulphur based thread cutting fluids that my plumber friend gave to me. I thread silver steel, 4140 steel and others with no problems. As has been mentioned throughout this thread the tool needs to be very sharp at the edge and the final couple of passes just need to be basically spring cuts to clean up what has been cut.
gbritnell
 

steamer

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Another part of the equation is the rigidity of the set up....do you have a tailstock center for that?

If the part climbs up over the tool, it will break the tool for sure......I would second Arnolds/others thoughts on the angled compound approach, but back that part up with a center if at all possible. I know it seems like belt and suspenders, but its been my experience that the little shafts squirm away from the tool during a cut, and don't cut to full depth as a result. The next pass ends up at way deeper depth of cut, and then the part climbs up over instead of pushing away....and snap!

Sometimes you need to do a few "dead passes" between putting the cut on to take the spring out of the part

Motor oil is OK.....but there are proper cutting oils out there too
Dave
 

picclock

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Hi steamer

Yes the end was held in a dead centre, just not shown in the picture. Will have another go now as I said before with angled compound. Re lubricants, the only reason for using this oil is that it seems to work well when turning so I figured threading should be the same.

Thanks for the comments.

Best Regards

picclock
 

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