Announcement

Collapse
No announcement yet.

Twisted Bed?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Twisted Bed?

    I have a very nice South Bend 9A with very little wear on the ways. Problem is, I am cutting a slight taper. When I say slight, i am measuring about .005 larger on the headstock end of a workpiece 6" long. This is with the work being held in a four jaw chuck, and using the tailstock for support. If the tailstock were to be slightly misaligned would it cause the taper when holding in a four jaw chuck? Or, am I looking at a possibly twisted bed?

    I had a fellow call me last night that is experimenting with Stirling engines, and we will need to be within .0005". I know I can do the work, but my machine needs a bit of tweaking to keep up with that!
    Arbo & Thor (The Junkyard Dog)

  • #2
    Even with a perfectly aligned lathe, when using tailstock support, tuning may be necessary.

    If the part is small on the tailstock end, adjust away from the cutting tool. Using a dial indicator on the T/S will assist adjustment.

    Having accomplished this, do not move the T/S until the part is completed, as it may not return to the same location.
    Jim H.

    Comment


    • #3
      Yes, a twisted bed will cause a taper. See the link below. It's for Atlas lathes, but the principle is universal.
      Have fun,
      Ed

      http://www.atlas-press.com/tb_bedlevel.htm
      Ed Pacenka

      Comment


      • #4

        Hmmm....this is from the Atlas link. I'm all for proper setup, but I have to wonder if they aren't issuing a 'catch-all' statement to cover themselves.... "Satisfactory performance is impossible if the lathe bed is out of level as little as one thousandth of an inch." That's pretty hard to ever meet, repeatably, across a distance of several feet. I'm surprised they don't mention that a temperature and humidity deviation of more than 1% is disallowed.

        * A properly leveled lathe is the first essential for accurate work and long service life.
        * The built-in accuracy of the lathe can be permanently destroyed by improper leveling.
        * Satisfactory performance is impossible if the lathe bed is out of level as little as one thousandth of an inch.

        AN IMPROPERLY LEVELED LATHE WILL

        * CHATTER
        * TURN TAPER
        * BORE TAPER
        * FACE CONVEX OR CONCAVE
        * SCORE BED AND CARRIAGE WAYS
        * SCORE SPINDLE
        * RUIN SPINDLE BEARINGS
        * MAKE CARRIAGE BIND
        * TWIST HEADSTOCK AND SPINDLE, BED, CARRIAGE AND TAILSTOCK OUT OF ALIGNMENT RESULTING IN EXCESSIVE UNEVEN WEAR.

        Comment


        • #5
          Try turning something parallel that's held only in the chuck and not supported by the tailstock. Make sure it's hefty enough so it won't deflect significantly from the pressure of the tool. If you can turn something parallel that way, your tailstock is off and needs adjusting. (A misaligned tailstock can indeed pull something out of line that's held in a chuck.)

          If the part comes out tapered when held only in the chuck, your lathe bed is twisted (or warped). Raise/lower the right front leg of the lathe with shims, making test cuts, until the lathe turns parallel. A precision level may get you close, but the ultimate test is turning a piece of work held just in the chuck and verifying that the lathe turns parallel.

          As far as making a Stirling engine: no matter how good your lathe is, you may need to lap the bore and piston with an internal and an external lap to get the accuracy you need for good performance.
          ----------
          Try to make a living, not a killing. -- Utah Phillips
          Don't believe everything you know. -- Bumper sticker
          Everybody is ignorant, only on different subjects. -- Will Rogers
          There are lots of people who mistake their imagination for their memory. - Josh Billings
          Law of Logical Argument - Anything is possible if you don't know what you are talking about.
          Don't own anything you have to feed or paint. - Hood River Blackie

          Comment


          • #6
            For bed alignment try Rollie's Dad's method. You don't need to include the tail stock (or worry about its misalignment)

            http://www.John-Wasser.com/NEMES/RDMLatheAlignment.html

            However to set a lathe up properly you need to follow the correct sequence or else you will be chasing your own tail for a week.

            A comprehensive test certificate is one good source of the correct sequence.

            Regards
            Phil

            Comment


            • #7
              I think you guys promoting "twisted bed" problems as the root of all evil in turning operations need to take a step back.

              It's tempting to invoke a mystery to account for a problem. The first thing I'd look at if my tailstock supported work was tapered would be the tailstock offset. Unclamp it (you can't force the tailstock over if the clamp is holding the two castings together) and dial over half the taper and you're there.

              If the machine is in otherwise fair shape, the bed will have to have quite a twist to show 0.005" taper in 6". In South Bend lathes there's a couple of set screws on the tailstock end pedistal to make removing the twist easy.

              They bear against a rocker assembly. To adjust them, run the carrage close to the head stock, back off both adjustments to relx the bed, make sure the screws turn freely, finger tighten them until they just bear and alternately snug them until they're in firm contact (NOT cheater tight). Re-level as needed.

              Just becuase something authoratative sounding appears on a website doesn't make true. There are parts of the list Greg posted that I'd be skeptical of attributing to bed twist.

              "Satisfactory performance is impossible if the lathe bed is out of level as little as one thousandth of an inch." Funny, I've run lathes teetering on wood blocks and got good performance after a little fiddling.

              AN IMPROPERLY LEVELED LATHE WILL

              "CHATTER" BS. Chatter is a tool/work interaction producing a self-excited mechanical oscillation and bed twist has nothing to do with it.

              "TURN TAPER, BORE TAPER, FACE CONVEX OR CONCAVE" Yes, in rough proportio to twist but lathe beds are very stiff. A lathe would almost have to be teetering on opposite corners to cause the ptoblems I've seen here.

              "SCORE BED AND CARRIAGE WAYS" Possibly over time in cases where lube was neglected.

              "SCORE SPINDLE" BS. How would that happen?

              "RUIN SPINDLE BEARINGS" total BS.

              "MAKE CARRIAGE BIND" Certainly in extreme cases particularly if the lathe was in near new condition and tight in consequence.

              "TWIST HEADSTOCK AND SPINDLE, BED, CARRIAGE AND TAILSTOCK OUT OF ALIGNMENT RESULTING IN EXCESSIVE UNEVEN WEAR? Certainly in proportion to twist and the machine's sensitivity.

              "Bed twist" "spindle slop" and other dire sounding machine tool ailments produce more needless concern than actual problems.

              Some new guys can be subject to a kind of "machine hypchondria." The least little pronouncement makes them jittery. Many a time I've looked over a new guy's proud new war production S/B (or whatever) and casually stooped to look at the underdrive, cycle the belt lever, or something. Doing so compresses my belly sometimes forcing a grunt. Over my shoulder would come a voice quivering with angst "What's wrong? What did you find?" "Nothing, looks good here."

              You guys gotta relax and work through the problem through the optons before you decide "bed twist" has come to call.

              Don't get me wrong, machine tools have to be in accurate alignment and leveling is the simplest way to go about it. Just don't obsess about the wrong things. Plant the machine, then level it. If you have to move it, level it again. Once a year or after an earthquake go around and check the leveling.

              Here's a general rule: If your lathe is turning tapers look at tailstock offset, part deflection, and tool wear first.

              (fixed stupid typos)

              [This message has been edited by Forrest Addy (edited 08-24-2005).]

              Comment


              • #8
                Forrest:

                Good reply. That list of "problems" sounds like it was written by a philosophy major.

                My experience has agreed with all of your comments; however, I am not a machinist and didn't feel qualified to reply.

                It was good to hear it from you.

                Pete

                Comment


                • #9
                  <font face="Verdana, Arial" size="2">Originally posted by Forrest Addy:
                  I think you guys promoting "twisted bed" problems as the root of all evil in truing operations need to take a step back.
                  Etc etc etc... (edited down for simpicity's sake.
                  </font>
                  I agree 100%, Good call.

                  jeff

                  Comment


                  • #10
                    Simply stated, I prefer a leveled lathe, without a twisted bed…

                    A 15 x 36 Clausing Colchester, was re-located to a new spot in the shop. Evidently, the machine was never leveled after being moved. The concrete floor was uneven, after the machine was jockeyed into place, a johnson bar was used to swing the tailstock end, about the headstock, to angle the lathe. This act imparted a twist in the bed. The first time it was used, it cut a .004 taper in roughly 5â€‌ Multiple attempts at the cut yielded the same results. After discovering the problem and leveling (“Untwistingâ€‌ the bed) the lathe, with the same job in the chuck, same tool, and taking the same cut the taper was reduced to .0007â€‌

                    This is not “I thinkâ€‌ or “one would guessâ€‌ this is an actual event.

                    Ed
                    Ed Pacenka

                    Comment


                    • #11
                      <font face="Verdana, Arial" size="2">Originally posted by egpace:
                      Simply stated, I prefer a leveled lathe, without a twisted bed…

                      A 15 x 36 Clausing Colchester, was re-located to a new spot in the shop. Evidently, the machine was never leveled after being moved. The concrete floor was uneven, after the machine was jockeyed into place, a johnson bar was used to swing the tailstock end, about the headstock, to angle the lathe. This act imparted a twist in the bed. The first time it was used, it cut a .004 taper in roughly 5â€‌ Multiple attempts at the cut yielded the same results. After discovering the problem and leveling (“Untwistingâ€‌ the bed) the lathe, with the same job in the chuck, same tool, and taking the same cut the taper was reduced to .0007â€‌

                      This is not “I thinkâ€‌ or “one would guessâ€‌ this is an actual event.

                      Ed
                      </font>
                      I'm not disagreeing with you that leveling your lathe is a good Idea and highly reccomended. But... a lot of people will chase this to no end. When in fact it isn't the problem at all. Eliminate the easy stuff first. Also did you ever think the reason the bed was twisted was because you swung it around by the tail stock? I no when moving machinery you can't always do it by the book, but swing the tail around with a bar while leaving the headstock on the ground will put a good amount of force on the bed.

                      This post is not a "I think" post either. I've moved probably 25 - 30 lathes small to very large.

                      jeff

                      Comment


                      • #12
                        Jeff,

                        "did you ever think the reason the bed was twisted was because you swung it around by the tail stock?"

                        Re-read my post, that was my point, moving the lathe by the tailstock end caused the problem. I didn't move the lathe, I was told they humped the tailstock end around to angle it.

                        As far as eliminating the easy stuff first, there is nothing easier than to place a level on the bed in a number of places for a relative relationship of the bubble. My lathes were leveled 10 to 20 years ago. I've haven't had to re-level them since. If I noticed, one day, for no apparent reason one started cutting a taper. I would take the 2 minutes to check the bed for level.
                        Ed
                        Ed Pacenka

                        Comment


                        • #13
                          Sorry about the hiccup.

                          [This message has been edited by egpace (edited 08-24-2005).]
                          Ed Pacenka

                          Comment


                          • #14
                            <font face="Verdana, Arial" size="2">Originally posted by egpace:
                            Jeff,

                            "did you ever think the reason the bed was twisted was because you swung it around by the tail stock?"

                            Re-read my post, that was my point, moving the lathe by the tailstock end caused the problem. I didn't move the lathe, I was told they humped the tailstock end around to angle it.

                            As far as eliminating the easy stuff first, there is nothing easier than to place a level on the bed in a number of places for a relative relationship of the bubble. My lathes were leveled 10 to 20 years ago. I've haven't had to re-level them since. If I noticed, one day, for no apparent reason one started cutting a taper. I would take the 2 minutes to check the bed for level.
                            Ed
                            </font>
                            It's cool, I just misunderstood or can't read well.

                            All I was trying to say was that some people will chase the leveling till they go mad. My Howa isn't level. I set it up with a slight bit of drop towards the tailstock.

                            It is level from front to back (or side to side) just not from headstock to tailstock and I've never had a problem with taper.

                            When i set a machine I always use my "precision" level, but I've seen people who just keep chasing it and chasing it even though to me it looks fine.

                            jeff

                            Comment


                            • #15
                              Jeff,
                              I agree, I don't run through the streets yelling make sure your lathe beds are level. To me the issue is like the first question in an appliance trouble shooting instructions...

                              "Make sure your toaster is plug in to 110 volt wall outlet"

                              You really never should have to ask that question, but the original post by arbo said...

                              "Or, am I looking at a possibly twisted bed?"

                              To me, that meant I haven't check to see if my bed was twisted. The level would give him a good indication.

                              Regards,
                              Ed

                              Ed Pacenka

                              Comment

                              Working...
                              X