Loading ...
Sorry, an error occurred while loading the content.

Re: [multimachine] Re: Romig style lathes

Expand Messages
  • David G. LeVine
    ... Chris, I think you missed something critical: Absolute accuracy does not matter very often. Let s assume a lathe with 0.0002 of wear. Can you determine
    Message 1 of 9 , Aug 31, 2013
    • 0 Attachment
      On 08/31/2013 02:40 PM, Chris Tofu wrote:
      > I don't understand at all. The most wear is .001" and not on the back way, and things still turn out barrel shaped? That degree of error is virtually meaningless. That can't be your only problem if your works bulges in the middle.

      Chris,

      I think you missed something critical: Absolute accuracy does not
      matter very often.

      Let's assume a lathe with 0.0002" of wear. Can you determine how bad
      the slop in a bore 0.005" larger will be? This is not uncommon in a
      greased bore where heating may cause contact if there is not enough
      clearance.

      On the other hand, a machinist whose lathe turns to +/- 0.002" who
      finishes the work with files and abrasives to 0.0002" will not see any
      issues related to LATHE accuracy.

      Sometimes technique compensated for poor tools.

      Dave 8{)

      --

      "A word to the wise ain't necessary - it's the stupid ones that need the
      advice."

      Bill Cosby
    • Dennis Shelgren
      I should clarify, the entire top of the bed is worn nearest the headstock. The front and rear edges of the bed are still o.k. In a way the front to back
      Message 2 of 9 , Aug 31, 2013
      • 0 Attachment
        I should clarify, the entire top of the bed is worn nearest the headstock. The front and rear edges of the bed are still o.k. In a way the front to back accuracy is o.k. but the toolpost effectively "sinks" as it hits the worn area. The tool, therefore, is cutting lower on the work piece and also farther away. It isn't something I measured other than to lay a test piece on a straight edge and and I can see light on both sides of a short piece. It is so slight that I still used the lathe until the headstock itself became unaligned.
        If I wanted to use this machine for high accuracy work I would just buy a replacement bed.
      • Chris Tofu
        Right but if you were turning very short brass say near the h/s, what would you say the repeatable tolerance is?
        Message 3 of 9 , Aug 31, 2013
        • 0 Attachment
          Right but if you were turning very short brass say near the h/s, what would you say the repeatable tolerance is?

          ------------------------------
          On Sat, Aug 31, 2013 5:58 PM PDT Dennis Shelgren wrote:

          >I should clarify, the entire top of the bed is worn nearest the headstock.
          >The front and rear edges of the bed are still o.k. In a way the front to
          >back accuracy is o.k. but the toolpost effectively "sinks" as it hits the
          >worn area. The tool, therefore, is cutting lower on the work piece and also
          >farther away. It isn't something I measured other than to lay a test piece
          >on a straight edge and and I can see light on both sides of a short piece.
          >It is so slight that I still used the lathe until the headstock itself
          >became unaligned.
          >If I wanted to use this machine for high accuracy work I would just buy a
          >replacement bed.
        • Dennis Shelgren
          The repeatability is very high, all the parts will have the same error. The accuracy is off by an unacceptable amount on small parts. I only detected the
          Message 4 of 9 , Aug 31, 2013
          • 0 Attachment
            The repeatability is very high, all the parts will have the same error. The accuracy is off by an unacceptable amount on small parts.
            I only detected the problem when a smaller shaft I cut fit it's hole oddly, and I realized(and mic'd) it had to be barrel shaped. I finished the part with a file on the lathe to correct it and moved on with my life. For thin parts or tubes I could cheat with a follower rest which bends the part in line with the worn bed.
            Even on a much worse worn lathe the bed wear/error is divided by the geometry of the part and tool setup. In other words on larger parts the height change of the tool is less with relationship to the part.
            This is probably why it's viable to build a lathe bed with components of less than ideal accuracy.
            As long as you know where the "bad" spots are you can compensate to a degree.
            Dave is right in that it's more a matter of the man and not the machine.
            I have a very old(1890) Sheppard lathe and it's bed is probably wavy, twisted, and worn. I'm still going to clean it up and put a motor on it, and it'll be used for larger parts that don't go on it's smaller cousins.
            As long as I measure as a I work and don't expect perfection, I imagine my time fixing it will be well spent.

          Your message has been successfully submitted and would be delivered to recipients shortly.