Over the past many years I have cut many threads with good results. More recently I have come across mention of polishing threads for scientific instruments such as telescopes where precision and smooth operation are important. I have never seen a thread polishing demonstration in the many different shop classes, machine shop jobs or in a friend's home shop.
With a future threading task soon to find its way into my shop where a polished thread would make for smoother operation in the long term, I was wondering what some of the members of this board might have to offer on the seldom seen or done operation of thread polishing. Any wisdom in this regard will surely help my very limited knowledge of this task.
How does one polish a thread?
Re: How does one polish a thread?
Sir
Many ways to skin this cat, from the sublime to the ridiculous.
Is the thread to be used with a nut, or does it function as a worm?
There is the question of fit/tolerance, and whether you are attempting to correct or minimize lead error.
Material to be used is another question.
Many ways to skin this cat, from the sublime to the ridiculous.
Is the thread to be used with a nut, or does it function as a worm?
There is the question of fit/tolerance, and whether you are attempting to correct or minimize lead error.
Material to be used is another question.
Re: How does one polish a thread?
One thing might be to use a split nut to polish a threaded rod. It would be a lapping operation. You would use a nut softer than the threaded rod so that the compound embeds in the nut and not the rod.
Steve
Steve
Re: How does one polish a thread?
The thread in question is a .775 outside diameter Acme thread of 8 threads per inch. An odd ball size from Baldwin Locomotive works for an injector steam supply valve off of the turret casting. The current thread is worn almost paper thin and it is amazing that it did not fail in service before the rebuild was started several years ago.
Many of the crew members are ham fisted with the valves. Having a smooth operating thread might insure that they have a good feel for when the valve is fully open or fully closed and not torque it beyond all reason which seems to be standard practice. Some items never wear out, just get abused and busted finding their way into the shop long before normal wear would have.
The internal thread is a bronze alloy casting and the new stem thread piece will be aluminum bronze.
Many of the crew members are ham fisted with the valves. Having a smooth operating thread might insure that they have a good feel for when the valve is fully open or fully closed and not torque it beyond all reason which seems to be standard practice. Some items never wear out, just get abused and busted finding their way into the shop long before normal wear would have.
The internal thread is a bronze alloy casting and the new stem thread piece will be aluminum bronze.
Re: How does one polish a thread?
My opinion?
A waste of time. This would be very much akin to putting lipstick on a pig, as there is nothing to be gained for the intended purpose.
A very decent finish can be produced by simply selecting a material that machines well. That would naturally exclude the use of mild steel. Beyond that, understanding how to take fine finish cuts with proper lubrication will go a long ways towards improving surface finish. With a sharp tool (I prefer HSS), you can take half thou passes until you've achieved the desired pitch diameter, along with a very nice finish. That's assuming you select a good to machine material.
If you are willing to go beyond steel, you might consider using pre-heat treated 416 stainless. It has excellent corrosion resistance and is readily machined. Best of all, 416 is the nicest of all the stainless alloys to machine, so surface finish will be outstanding assuming you use a sharp tool.
If you intend to use steel, you might consider using Stressproof. It, too, machines very nicely and will yield excellent finishes, especially with HSS.
Harold
Edit: If you have an interest in learning to lap threads, you might consider reading the book published for the Moore Jig Borer Company. They used to hand lap lead screws achieving a level of precision that boggles the mind (millionths). Sorry, I do not know the proper title of the book.
I just read the previous post, which details the use of aluminum bronze for the screw. Finish most likely won't be an issue, again, assuming you use sharp tools. Aluminum bronze doesn't tear.
A waste of time. This would be very much akin to putting lipstick on a pig, as there is nothing to be gained for the intended purpose.
A very decent finish can be produced by simply selecting a material that machines well. That would naturally exclude the use of mild steel. Beyond that, understanding how to take fine finish cuts with proper lubrication will go a long ways towards improving surface finish. With a sharp tool (I prefer HSS), you can take half thou passes until you've achieved the desired pitch diameter, along with a very nice finish. That's assuming you select a good to machine material.
If you are willing to go beyond steel, you might consider using pre-heat treated 416 stainless. It has excellent corrosion resistance and is readily machined. Best of all, 416 is the nicest of all the stainless alloys to machine, so surface finish will be outstanding assuming you use a sharp tool.
If you intend to use steel, you might consider using Stressproof. It, too, machines very nicely and will yield excellent finishes, especially with HSS.
Harold
Edit: If you have an interest in learning to lap threads, you might consider reading the book published for the Moore Jig Borer Company. They used to hand lap lead screws achieving a level of precision that boggles the mind (millionths). Sorry, I do not know the proper title of the book.
I just read the previous post, which details the use of aluminum bronze for the screw. Finish most likely won't be an issue, again, assuming you use sharp tools. Aluminum bronze doesn't tear.
Wise people talk because they have something to say. Fools talk because they have to say something.
Re: How does one polish a thread?
Ditto what Harold said, you don't need to go beyond good thread cutting practice for your application and the materials used.
Re: How does one polish a thread?
Foundations of Mechanical Accuracy by Wayne Moore, the son of one of the founding brothers. Hard to find and expensive when you can. You might be able to get one on interlibrary loan. Very good read.Harold_V wrote: Edit: If you have an interest in learning to lap threads, you might consider reading the book published for the Moore Jig Borer Company. They used to hand lap lead screws achieving a level of precision that boggles the mind (millionths). Sorry, I do not know the proper title of the book.
Steve
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LIALLEGHENY
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- Location: Bohemia, NY
Re: How does one polish a thread?
The book is readily available through Amazon or other online bookstores, starting at around $225.00 or you can buy directly from mooretool.com for $150.00
Nyle
Nyle
Re: How does one polish a thread?
A comment (based on experience) in regards to good thread finish and difficult to machine materials.
I spent a huge amount of time chasing (single pointing) threads when I was in training. No die heads were used at any time.
Amongst the myriad of items that got threaded were some timing shafts for the launch set for the missile. The shafts were threaded 5/8"-16, and were made of manganese bronze---a very difficult to machine material, although it yielded nice finishes. I do not recall the precise length of the generated thread, but it was in the vicinity of 4". Tool life suffered, as it is extremely tough material, much tougher than aluminum bronze.
That said, with a sharp tool, one could take fine plunge cuts for finish passes, the results of which would yield a V shaped chip, very thin, as depth of cut was often just a few tenths.
When a good finish and proper form is a requirement, plunge cutting the final few passes tends to improve surface finish, as the trailing edge of the thread is machined, along with the leading edge. Chip flow can be an issue, so only light passes (no greater than .002" on the diameter) are recommended.
Harold
I spent a huge amount of time chasing (single pointing) threads when I was in training. No die heads were used at any time.
Amongst the myriad of items that got threaded were some timing shafts for the launch set for the missile. The shafts were threaded 5/8"-16, and were made of manganese bronze---a very difficult to machine material, although it yielded nice finishes. I do not recall the precise length of the generated thread, but it was in the vicinity of 4". Tool life suffered, as it is extremely tough material, much tougher than aluminum bronze.
That said, with a sharp tool, one could take fine plunge cuts for finish passes, the results of which would yield a V shaped chip, very thin, as depth of cut was often just a few tenths.
When a good finish and proper form is a requirement, plunge cutting the final few passes tends to improve surface finish, as the trailing edge of the thread is machined, along with the leading edge. Chip flow can be an issue, so only light passes (no greater than .002" on the diameter) are recommended.
Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
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Magicniner
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Re: How does one polish a thread?
I always said that you could, but that when done it would just be a shiny one!
Re: How does one polish a thread?
Similar to off shore tools.