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Still no 8tpi

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  • Still no 8tpi

    I hate to say it but here's my latest attempt at an 8tpi thread.

    This was with the new boring block and life was wonderful until the compound depth reached .080. I thought we had it this time but as you can see.

    The only thing left is to tear into the machine and see if the dummy who assembled it did anything wrong. I did my best to figure things out as I put it back together but it ain't like I knew what I was doing ......

    Here's a a pic of the rear saddle gib.

    This seems self evident. The screw in the center holds the gib down. You adjust the setscrews at the ends for suitable contact, lock the nuts, and you're done.

    The front gib on the saddle is more questionable in my mind.

    The 2 slotted heads, one's hiding behind the allen, pull up on a bar and if I tighten them up too far the carriage will lock up. So I run 'em just loose enough to get carriage movement and that's all I know about that.

    Incidentally, I use the allen in that pic to keep the compound tied down with it's gib screws whenever I'm not moving it. It's so loose and grabby I've never trusted it fer nuthin'.

    That leaves the cross. It's worn on the operators side some but I keep the gibs tuned to where I'm working. The 2 gib screws that are farthest away typically need the most attention as they hit the unworn area first. I run it as stiff as I can and still pull out of the cut when threading.

    New ABEC-1's in the spindle which is the way it was originally designed and delivered. Chinese bearings of course but that's what Scott Logan sells for them these days too. I got mine from Kaman for a lot less money but that's beside the point.

    Pre-load is accomplished by burying the take up nut against a spacer. Hard to mess that up. I have a spare spacer and have thought about shaving it a bit to up the pre-load, but I have no idea what I'm doing on something like this. More grasping at straws ....

    And then there may be the fact that this poor old worn out thing just ain't capable of that coarse of a thread no more. I dunno. If that's the case I'll just buy a pre-threaded backplate and get on down the damn road! It'll still make lots of other useful stuff.

    So whaddaya think guys? Give up? Try something else?


  • #2
    What size boring bar are you using? It should be at least an inch, larger would be eaven better. Don't let it overhang more than you need to. How is your cutter sharpened? Do you have it set on center. The chatter in your threads may look bad but the threads will still work. Gary P. Hansen
    In memory of Marine Engineer Paul Miller who gave his life for his country 7-19-2010 Helmand Province, Afghanistan. Freedom is not free, it is paid for with blood.


    • #3

      No, don't give up! Let's first start with the tool bit...

      What is the clearance on your bit? The image below is for a square thread but the angle I'm referring to is explained in fig 150 below - this applies to ALL threads. Your bit should have that clearance or it will chatter.

      Are you using thread cutting oil? If not, you can get some dark heavy thread cutting oil at Home Depot, Lowes, True Value Hardware stores, etc. Use lots of it; that will make a big difference.

      If the above is okay, then we'll go to the next step...
      Last edited by Mike Burdick; 06-25-2007, 01:03 AM.


      • #4
        Just to get you further along with what you have, can you put a machinist's jack under the boring bar to help stabilize things? Either at the front or the rear of the tool post should pre-tension things a bit.

        I've used a rubber bungie cord to help relieve the slop in my combo mill/lath on occasion to help fight the chatter.


        • #5
          Face it, you have a light duty machine....

          8 tpi threads are not easily cut in a light machine.

          You might try opening the minor diameter way up.


          • #6
            Try changing rpm while threading. Crank it up as high as you can while still maintaining control. As a general rule you get the best finishes on steel when the chips are smoking hot.
            Free software for calculating bolt circles and similar: Click Here


            • #7
              Well thanx for the advice guys. This isn't a BBS, it's a support group for a buncha junkies!

              I couldn't leave it alone. Went back out there and saddled up the carriage for another try. Got the saddle gibs tighter. Started locking the compound down with 2 screws. Had a sharpen run at the bit. After boring the old threads out I picked and picked at a new set of threads with more spring passes than I could count, and got this.

              Still got a little vibration goin' on but I think we're getting in the acceptable range now. This is annealed 4140 I'm practicing on and I didn't realize how tough that stuff is until I wrecked a hacksaw blade cutting a piece of 1/2" hex the other day.

              Cast iron for a backplate should be a breeze after this stuff. I sure hope so!

              Thanx again.



              • #8
                That is entirely acceptable.
                Free software for calculating bolt circles and similar: Click Here


                • #9
                  I just recently threaded a backplate on my 10in Atlas. It was looking like the picture above. Even .0005" cuts chattered. When I was within 3 or 4 thou. I started pulling the chuck around by hand. Disengaging the back gears made it easier. At first the chatter could be felt and heard even that way. After several passes things smoothed out and I got the nicest threads yet. I put a little "blue" on the shoulder and screwed it on the spindle. It seats evenly all around on spindle shoulder. I used a 3/4" boring bar and a 1/8" bit. Compound set 29 1/2 deg.
                  I think setting the compound as close to 30 deg. as possible helps to keep the cutting forces down and therefore less chatter. Gotta' baby these old machines.


                  • #10
                    That chatter looks just like I was getting with a parting tool mounted upside down with the lathe running in reverse. Once I locked down the compound, it went away. It drove me nuts until I found what the cause was.


                    • #11
                      About that back gib.......... and other stuff

                      You have a Logan 11", right?

                      1) the back gib seems WAY incorrect...... Is that per the manual for the 11"?

                      (you can and should get the manual from Logan)

                      The 10" and 11" are similar enough that I'd be really surprised if teh gibs were different, let alone like THAT "thing". I have parts from an 11" on my 10", and they fit and work, including a t-slot crosslide that I sometimes use.

                      What you show is WAY different from the 10", and also makes no sense. On the 10", the rear gib is held by all three screws, with tension set by the tightness. The extra piece is not present

                      With only the center one, the piece is unstable, not to mention the incredibly silly design that would represent.

                      Should likely look like this from backside

                      2) you are missing the carriage lock, it goes in the open hole at right end of carriage front.

                      3) the front gibs you DO tighten until they give minimum play and reasonable friction.

                      All the above would be clear if you had the manual from Logan. Cost you $25 I think.

                      At a guess, I would say that there may be more silliness involved in your machine, if the rear gib has been replaced with that useless POS make-shift. I would not doubt that the spindle preload might be wrong, the spacer might have been replaced by the previous owner.....almost anything could be messed up if the previous owner was that silly. Do they both measure the same?

                      You need the manual, and a good look through to make sure that at a minimum, the rest of the parts are present in correct form (other than carriage lock and rear gib, which we know are missing/wrong).

                      If you were closer, I'd take that "poor old worn-out thing" off your hands........ At a substantial discount, of course, as you have already described it as "worn out", and it has missing parts and substitute bearings............. betcha I could cut your thread on it....... not that you'd need it then.

                      I think you first need to get it to where it at least has the right parts in it......... THEN maybe you could complain if it doesn't do what you want.
                      Last edited by J Tiers; 06-25-2007, 10:48 AM.
                      CNC machines only go through the motions.

                      Ideas expressed may be mine, or from anyone else in the universe.
                      Not responsible for clerical errors. Or those made by lay people either.
                      Number formats and units may be chosen at random depending on what day it is.
                      I reserve the right to use a number system with any integer base without prior notice.
                      Generalizations are understood to be "often" true, but not true in every case.


                      • #12
                        Try it with some hot roll or better yet some 1140 stressproof.

                        That 4140 would work fine on a machine that is more ridged.


                        • #13
                          Will that plate screw onto your spindle?


                          • #14
                            4140 is tough material and an 8 thread is a deep heavy thread.
                            With those two of those things together on the size of machine your using, the results your getting aren't uncommon.

                            90% of the parts I make at work are made from 4140 stock. Even on a CNC that feeds progressively down the 60 degree angle of the thread with each cut, an 8 thread will chatter sometimes. Difference there is once a CNC is in a threading process there isn't a thing you can do to intervene.

                            Cast iron should cut like chalk for you, but it is very abrasive on tooling.
                            I cut the M39-4 thread for a new cast backplate for my little Grizzly lathe.
                            4MM pitch is .157 lead so it's a little longer lead than your 8 thread.
                            That little machine isn't nearly as rigid as yours and it cut clean threads in the cast iron.
                            Home Model Engine Machinist


                            • #15
                              JTiers, you're right. It's an 11' Logan.

                              And gosh, getting a manual was about the 1st thing I did. I took this whole machine apart and put it back together again and believe me, I'm not that smart! The manual was a necessity.

                              My particular manual has 2 Saddle Assy's in it. The LA-1097 is as you pictured, and then there's the LA-1060 which is what I got. That rear gib pic is kinda fuzzy, maybe I can explain. The manual calls the bottom piece a rear gib and the piece on top a gib bar. The rear gib bolts to the saddle from below like yours, but doesn't set the friction. The gib bar on top of it does that.

                              The gib bar is mounted in the center with a flat head countersink. The ends of the gib bar get forced up into contact with the lathe bed by means of setscrews at either end of the gib. You get 2 points of contact, both approx 1/2" wide, that you can set the tension on. That's my best guess anyway and that's why I put the pic up. I honestly don't see any other way to make it work. Just wanted to make sure I wasn't missing anything.

                              Good eye on the carriage lock too. You know your Logan's and I do have one. It just happened to be off at the time because I was studying the saddle gibs one more time. Would you believe I found washers stuck in there for a carriage lock? Those bent and mutilated things didn't have a chance of holding the carriage still.

                              Topct, that poor piece of metal has been my hands long enough that it will no longer fit anything else in the entire world! It started at 2.25 x 8 for my spindle. Wrecked that. Consulted the HB and stepped out to 2.5 x 8 for practice. Got better but lost again. This last attempt I just bored it smooth and got the threading bit out. If it fits anything it was an accident.

                              It's still in the chuck and I'm gonna keep fooling with it. A handslide like Jim Hubbell suggests could a good idea. I'll be trying that. I'm also partial to Evan's suggestion of a higher speed cowboy cut. Pure ignorance on my part but I've been known to do that .....

                              Thanx Guys.