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Author Topic: Lathe turnings  (Read 13833 times)

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Offline Steve Scott

  • AFTE Member
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  • Posts: 643
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Lathe turnings
« on: July 18, 2016, 12:59:33 PM »
Has anybody read or heard of a study on how quickly a lathe tool/cutter changes sufficiently enough that the signature of the turning changes?  Restated, how long (in length) can a turning be ID'd to itself?
Steve Scott

Offline Humpy

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Re: Lathe turnings
« Reply #1 on: July 25, 2017, 01:28:28 PM »
This could be very difficult to determine and a few variables that could change the surface being turned come to mind.

1. some lathes are variable speed, some fixed speed. All have different feed rates and wear on the gears could likely cause variations on the surface finish.
2. tooling can range from high speed steel to carbide inserts.  For instance I have used carbide inserts for weeks and all of a sudden will go through two in a day just by changing stock.
3. if the stock being turned has been previously heat treated or stress relieved can present a different surface.
4. some stock has hard spots  and soft spots.
5. if it has been rusted can present problems.
6. if the lathe has lots of wear it is likely to not turn round stock evenly.  Some lathes in order to get them to turn true the operator has to rest hand/arm on the different places on the lathe.  Operators that work with one lathe every day and can meet tolerances have found the (sweet spot) it likes.  I have heard of guys that would spike a lathe (for lack of a better term) and remove shims installed here and there while they are on vacation so no one else can be put on their machine.
7. some lathes run coolant (either spray or solid stream) which has an effect on the metal being turned, the life of the tool etc.
8.  The tail stock being out of alignment could present a problem. 
9. A shop floor being subjected to heat/cold may change the bed.
10. if the head is a 3 jaw or a 4 jaw can cause variation in repeatability.
11. The cutting fluid applied can have a big difference on cutting.  For instance I will run used Mobil 1 synthetic chambering rifles and for the last .010" a different cutting fluid is used to get a smoother surface.

Hope this helps.

And there is probably 50 other things that can alter the repeatability of how a lathe turns.

If you can find a guy that runs CNC I am pretty sure they will tell you that inserts are changed at least daily and maybe during a shift.

When I was touring Smith & Wesson I noted a guy was turning yokes on a CNC and gaging about every third one and writing down the measurement changes and he was having to tweak the machine about every three to five yokes.  Smith & Wesson revolvers are basically on the soft side.
« Last Edit: July 25, 2017, 03:21:05 PM by Humpy »

Offline Steve Scott

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Re: Lathe turnings
« Reply #2 on: July 25, 2017, 02:54:15 PM »
Thanks for the info!
Steve Scott

Offline Mark Bennett

  • AFTE Member
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  • Posts: 133
Re: Lathe turnings
« Reply #3 on: August 08, 2017, 06:03:42 PM »
Evan Thompson handed out lathe turning samples at the 2006 AFTE. I don't know if he published anything.
Mark Bennett
Criminalist III, Firearms Unit,
Oakland Police Department,
Criminalistics Division,
455 7th Street, Room 608,
Oakland, CA,


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