Announcement

Collapse
No announcement yet.

Why separate lead from feed screw on a lathe?

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

  • Why separate lead from feed screw on a lathe?

    I can see that using the lead *screw* for feed is bad for wear and accuracy when you NEED a lead screw for accurate threads. But since most all smallish lathes drive the feed via the lead keyway (driving a worm, which drives the longitudinal feed pinion on the rack), what other difference does it make? I can maybe see wear in the gear box? And I can see avoiding wear on the rack which would then be used only for the hand wheel. Or wear on the lead bushings? And maybe a convenience thing of having separate controls (on those that do) so you can just leave it set for your threads as you do another facing or turning op between threading jobs. But is there a “really big dealâ€‌ I’m missing that affects HSM or light production use?
    Russ
    Master Floor Sweeper

  • #2
    Originally posted by BadDog
    But since most all smallish lathes drive the feed via the lead keyway

    Halfnuts.

    The wear comes into play when you talk about driving the carriage with the halfnuts engaged. That would cause undue wear to the treads of the lead screw and halfnuts. Driving from the keyway is not gonna wear the threads of the lead screw. JRouche
    My old yahoo group. Bridgeport Mill Group

    https://groups.yahoo.com/neo/groups/...port_mill/info

    Comment


    • #3
      You're both right!

      Comment


      • #4
        Well it's a simple question,which would wear the halfnut faster?

        A leadscrew with a full length keyway?

        Or a leadscrew with no keyway?

        That's one reason,the other is on larger longer lathes the leadscrew is fairly substaintial,our big lathe at work is a 24x120,it's leadscrew is right at 2-1/4" od,while the feed rod is only 1-3/8".The larger leadscrew less keyway allows for less torsional stresses in the screw which would effect accuracy.Also most leadscrews are supported by plain bearings.Either neked cast iron bores with lube,or bronze bearings and a ball thrust bearing for end loading.You wouldn't want the thing spinning constantly while the lathe is spun up to 1500 rpm at a high feed rate,that would add up to wear fairly quick.
        I just need one more tool,just one!

        Comment


        • #5
          That's what I'm getting at. The very cheapest of the small lathes drive all carriage movement with the half nuts on the threads. I can easily see why this is bad for precision movement like threading.

          But what exactly makes a multi-rod lathe better? I keep seeing comments by people to the effect that the first thing they look for is to "count the rods", or "make sure you get one with multiple rods" (see other post currently on first page), and so on. As I said in my first post, other than the imagined reasons I stated, why would anyone care about the number of rods as long as it's not always driving from the threads?
          Russ
          Master Floor Sweeper

          Comment


          • #6
            Ah, WS's post crossed with mine.

            Now that's kinda what I was looking for. I guess that if you did a LOT of threading, extra wear from the keyway on the half nuts would matter. And I already mentioned the wear on the lead screw components (bushings or gears) being split off with separate drive rods.

            So far, it seems to me that it is exactly as I thought. For the smaller light lathes, as long as it drives from the keyway rather than lead threads/half-nuts, it probably doesn't make a hill of beans how many rods it has when used in HSM or light production. And for a real production machine running day in and out, the division of wear across multiple separate systems along with convenience (as described in my first post) would justify the desire for multiple control rods.

            Summary: For HSM and light production/prototype use, it’s really just about a sort of “bragging rights” and “professional grade” appearance, but really has no significant effect...
            Russ
            Master Floor Sweeper

            Comment


            • #7
              Originally posted by BadDog
              Summary: For HSM and light production/prototype use, it’s really just about a sort of “bragging rightsâ€‌ and “professional gradeâ€‌ appearance, but really has no significant effect...
              I may be the "other" post you're referring too? Maybe not.

              I don't so much care about how many rods it has except that 2 rods probably means I can feed both directions (cross and long.) and still thread. My main objection to the small lathes is not the single rod but leaving out the reverse tumblers and not powering the cross-slide.

              The two lathes I've owned have both been single rod and threads got cut accurate enough for my HSM jobs.

              Paul

              Comment


              • #8
                I don't think so, but don't recall specifically who said what, and it’s not important anyway. I just recall that someone said something to the effect of "make sure to get one with more than just the lead screw", and another said something like "when looking at a lathe, the first thing I do is count the rods". I’ve heard much the same over and over since I got involved in HSM stuff at the first of this year and I was just curious why.

                And I absolutely agree on the lead screw reverse which my Griz does not have, as well as wanting power cross feed. But I don't know why that would need 2 rods. In fact, my Rockwell as well as the Logans, SBs, and so on all have only the "lead screw" (in smaller HSM type models anyway). They all drive the power longitudinal feed and power cross via. a keyway driven worm. The threads and half nut are only used for threading (and the like). And they have a tumbler to reverse the lead screw relative to the spindle. I can see no reason why this would not be just as good as a 2 rod (3 rod third typically being saddle controls, needed for really long lathes and only a convenience on short beds, so irrelevant for this discussion) except for the points I laid out initially.
                Russ
                Master Floor Sweeper

                Comment


                • #9
                  One good reason is when that sad crash is heard, the key in a drive rod doesn't damage the leadscrew. A little "upseting" of the thread ends in the keyway turns the leadscrew into a cutter, gouging out the halfut on the first pass.

                  Of course, that is of no concern to anyone here, as we never crash!!

                  Comment


                  • #10
                    Originally posted by uute
                    One good reason is when that sad crash is heard, the key in a drive rod doesn't damage the leadscrew.
                    Most leadscrew-drive machines have a shear-pin for exactly that reason.

                    In fact, the original Clausing user's manual comes with a little packet stapled that has a handful of shear pins
                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                    Comment


                    • #11
                      Originally posted by BadDog

                      And I absolutely agree on the lead screw reverse which my Griz does not have, as well as wanting power cross feed. But I don't know why that would need 2 rods. In fact, my Rockwell as well as the Logans, SBs, and so on all have only the "lead screw" (in smaller HSM type models anyway). They all drive the power longitudinal feed and power cross via. a keyway driven worm. The threads and half nut are only used for threading (and the like). And they have a tumbler to reverse the lead screw relative to the spindle. I can see no reason why this would not be just as good as a 2 rod (3 rod third typically being saddle controls, needed for really long lathes and only a convenience on short beds, so irrelevant for this discussion) except for the points I laid out initially.
                      At work our 1940's model Hendey 14x54 has dual feed worms driven off the keyed leadscrew.Both have independant clutches and both can be engaged at the same time turning a perfect 45*.It has a traveling clutch lever,only engage/dis-engage,no FWD/REV function.It also has a traveling tumbler fwd/n/rev lever on the carrage.It only does SAE threads.It was not a cheap lathe when new,wieghing in around 4100lbs it is well built.

                      The other 24x120 is Italian built,has seperate feed and lead,traveling FWD/N/REV clutch,but no traveling tumbler,it's stuck on the headstock.The machine has a universal QC,does SAE,Metric,Whitworth and Module threads with the flip of a lever or two.You can only use one feed at a time,chaging envolves stoping the feed clutch and switching the selector on the apron It has an electric clutch on the feed and rapid traverse on the carrage.This machine wieghs around 11,000lbs and was a borderline cheap lathe when new.

                      Moral of the story,a mfg can pack as many features as they want to into a lathe,question is will they?

                      Me I would do like Evan and others have done and just add an electric feed motor to the crosslide.As cheap as DC gearmotos and controls have gotten to be there is no reason not to.
                      I just need one more tool,just one!

                      Comment


                      • #12
                        Since were talking about crashing and shear pins, do most lathes have clutches in the apron gear box? I ask since someone mentioned a while back about breaking gears on a high end lathe due to a crash.
                        John

                        Comment


                        • #13
                          Originally posted by JPR
                          Since were talking about crashing and shear pins, do most lathes have clutches in the apron gear box? I ask since someone mentioned a while back about breaking gears on a high end lathe due to a crash.
                          Both at work do,the Hendey has cone clutches and the Lansing has a disc clutch behind the magnet that pulls it in.I think either machine has the potential to break if they crash.The Hendey does have two dogs on the tumbler reverse lever that kick the tumbler into neutral at either limit.

                          Dunno about the smaller lathes,My old Seneca Falls lathe has cone clutches in it,but I am not familiar with Clausing,SB etc.
                          I just need one more tool,just one!

                          Comment


                          • #14
                            Even my Rockwell has a clutch in the apron, I just got repaired it a recently.

                            WS: Not sure what your point was about the electric cross feed. Without it tied to the lead, you would never manage a true 45*. And my new lathe has power cross driven from the apron. Or were you talking about the Griz? Or just a general comment? If your talking about the Griz, maybe the next owner will want to do something like that, but I don't have room to keep both, as much as I would like to keep it...


                            To restate, I'm specifically interested in why a lathe with separate feed drive rod and lead screw has any relevance for HSM, light production, or prototype work. It’s such a prevalent sentiment, I figured there must be something to it. Maybe not...
                            Russ
                            Master Floor Sweeper

                            Comment


                            • #15
                              Originally posted by BadDog
                              To restate, I'm specifically interested in why a lathe with separate feed drive rod and lead screw has any relevance for HSM, light production, or prototype work.

                              Specifically, in my case, my Monarch 10EE just would not run right without the "extra" rods JRouche
                              My old yahoo group. Bridgeport Mill Group

                              https://groups.yahoo.com/neo/groups/...port_mill/info

                              Comment

                              Working...
                              X