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Holding .001 with a Collet Stop?

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  • Holding .001 with a Collet Stop?

    I'm working on a 40 pc run of parts needing to hold +/- .001, so I got the idea of using a collet stop to accurately set the part for each step. But like so many of my ideas .... it ain't working!

    It's a cheap ENCO collet stop which consists of noting more than a threaded rod, so I stepped that up with a sleeve over it for a wide flat surface to set the part against. I'm also making sure there's no burrs on the part seating area and giving the pocket a shot of air before setting the part in. I'm using a production closer and tapping with a brass hammer as I put tension on the collet. If there's something else to do seat a part accurately I haven't thought of it yet ....

    The particular's are 7/8" dia Stainless 303 at 1.345" long. I'm only holding a 1/2" in the collet because I'll eventually need to turn .820 of the length down to .625. 1/2" Brazed carbide bit under a mister with suds in it on an 11" Logan ..... and I'm getting a beautiful finish. Just not always where I want it.

    The interface between the ENCO threaded rod and the sleeve I made might be under suspicion. The threaded rod is 3/8" with a roughly beveled end and the whole thing isn't exactly centered. I put the hole in my sleeve with a 1/2" drill bit so we have a rough bevel against the cone the drill left. I dunno.

    For all I know it's not possible to reliably hold .001 with a collet stop. So here I am. Hat in hand once again. Any advice?

    SP

  • #2
    The thing about collet stops is that most of them screw into the back of the collet and position the part in reference to the COLLET, not to the lathe (spindle/headstock). When the collet is tightened, it is being drawn back into the taper in the adapter/closer. The position it stops at will depend on the diameter of the part being held. Any deviation in the diameter of the part will show up as a deviation in the axial location of that part. Even worse, the deviation is amplified: that is, a 0.001" difference in the part's diameter will be increased to several thousanths of difference in the position due to the taper's angle. So, you loose big time. Even if the parts are the same diameter, any difference in the tightening torque on the collet will still translate to a positional difference. Collets are great at centering but poor at axial location.

    If you want good axial positional accuracy, the collet stop would need to be mounted to the spindle itself or a fixed part of the collet closer, not the collet. Otherwise, to hold 0.001" accuracy, you need to zero your position for each part you mount. CNC would make this easy. Mount part, touch, zero, go. I am sure there must be other solutions/workarounds for this problem, but I am not experienced enough to know them all.

    One thing I have done with part runs like this is to use a three jaw with zero set centering ability. Once is is centered on the first part, the rest will generally center to +/- 0.001". Now I position the carriage at a reference point and use the tool tip to position the part while I tighten the three jaw. Oh, I have a QC tool holder which also has good positional accuracy when tools are changed. Besides, it is really the tool more than any part of the lathe, that the part needs to be positioned in reference to.
    Last edited by Paul Alciatore; 12-29-2008, 01:03 AM.
    Paul A.
    SE Texas

    Make it fit.
    You can't win and there is a penalty for trying!

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    • #3
      Another issue is wether you are using a handwheel colllet closer or a lever type. Lever types are more accurate as they are repeatable in drawbar pull. With a handwheel you can never guarantee it will be the same.

      You really need a dead length collet. Hardinge does make a dead length collet set up for 5C.

      Got a surface grinder? You could grind them to length.

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      • #4
        Originally posted by Paul Alciatore
        Mount part, touch, zero, go.
        It's that "touch" part I've been trying to avoid Paul. It's hard thing for a FNG to learn, but if I have to get better at it ... I will.

        And Thank You for the explanation on collet axial challenges. +/- .003 seems to be completely doable but if .001 is beatin' a dead horse it's time to be moving on.

        Lever type on the collet closer Macona and since I'm using 5C, what's a dead length collet? It did occur to me that I could cut a 1/2" depth into an emergency collet, and if I do I'm gonna order 2 so I'll have one for the next time.

        SP

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        • #5
          The dead length collet is a fairly sophisticated rig - worth a Google search to get more info. It will hold close to absolute length with a reasonable difference in diameter.

          Cheers,

          Frank Ford
          HomeShopTech

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          • #6
            If money is no object here is a very cool collet nose that holds to collet stationary and pushes the nose forward to clamp.
            http://www.royalprod.com/product.cfm?catID=2&ID=15
            Mark Hockett

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            • #7
              Very educational post, Paul, thanks.

              "CNC would make this easy. Mount part, touch, zero, go."

              Maybe a good excuse to get a DRO?

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              • #8
                DRO or CNC is not going to help you unless you can chuck the material consistently.

                With a deal length collet the part of the collet that holds the work rests against the face of the collet closer. The draw tube pulls the collet body back against the collet chuck squeezing the inner portion of the collet. Since it is resting against the face of the chuck there is no lateral movement of the actual workholding portion of the collet.

                I see these NOS from hardinge quite often on ebay. They are sold in different parts. A master body that attaches to the drawtube and interchangeable jaw sections.

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                • #9
                  If you play it right, a DRO could certainly help. Just get it close, then as Noah said, touch-off on the end (or better yet, face it) and set that as "zero". Work all your features from that established zero and go...

                  Without a DRO, I would do similarly. Set each piece in place using the existing stop, which should get you within a few thou anyway. Then just face each piece to your established "zero" (probably with a DI on the bed?). Than again, just work from that zero once you get there. You could even do the same thing with a hard stop set and using the compound for Z. Just gotta work within your tools...
                  Russ
                  Master Floor Sweeper

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                  • #10
                    I would machine a set of soft jaws to hold the part.Done correctly you will achieve the tolerance easily.
                    Mark.

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                    • #11
                      Good point. That's a prefect solution that should be dead on every time. And with that many to do, the soft jaw overhead is offset, particularly if this is a second op or something that can't be faced after insertion. But facing after setting is generally how I pick up the zero...
                      Russ
                      Master Floor Sweeper

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                      • #12
                        Soft jaws

                        Soft jaws are the way to go. They have been around since for-ever and were used when collets might not have been available for what-ever reason. Further, they can extend out to diameters that are impossible with normal collets.

                        They can handle ANY size within their limits and DON'T have the size restrictions of R8 and C5 and similar collets. And its a true "one size fits all" "gripper" that is as true and concentric to very high orders of accuracy. And you can counter-bore it for use as a stop. But for a good accurate "stop" a good quality chuck with no wear in the jaw "ways" is required.

                        BUT to ensure that the soft jaws do their job with optimum accuracy, the machining should be done with a bit of "right-sized" material gripped at the back of the jaws so constrain any unwanted movement during "forming" that will detract from the finished job.

                        The bored hole in the soft jaws should be a neat sliding fit on the job to be machined. The jaws need to be (re?) machined each time they are mounted.

                        Machining one end of a job with or without soft jaws is up to the machinist. But if the job is the be "end-for-ended" and held in soft jaws the concentricity between the two ends tuned separately will be very accurate indeed as will the depth setting in the counter-bore (if needed/used).

                        I am surprised that many more don't use soft jaws. It has only been mentioned very occasionally until very recently.

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                        • #13
                          I had thought soft jaws as well. But we dont know if he even has a chuck with 2 piece jaws.

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                          • #14
                            More than that

                            It would have to have been cheaper than the collet closer.

                            Given that it is for a "paying/paid" job, it might be a question of not what it costs to buy, but rather what it might cost not to buy it.

                            Anyone who is stuck with the limitations of R8 and C5 collets (ER are OK) will soon appreciate having a soft-collet chuck (but keep it for soft jaw work). But beyond the limits of any collet the "soft-jaw" option for extent to the limits of the chuck as well as its ability to grip very accurately both on the outside as well as inside diameters makes them hard to beat. They are superb for "disk" work as well.

                            Perhaps its too "old school" and not "hi-tech" enough like collets.

                            Yeah - well.

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                            • #15
                              You guys are as always ... amazing. I DO have some soft jaws. I DON'T know if there's enough of them left to run a 7/8" part, , but I'll be finding out as soon as I can.

                              I did do some work on my "touch" and went 2 fer 2, but it's very time consuming. Just plugging in a part would be a lot faster.

                              Thanx again.

                              SP

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