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Piston type toolpost repeatability?

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  • Piston type toolpost repeatability?

    I searched the archives (sorting through 239 posts) for toolpost information and read many posts about the wedge type being superior to the piston type as far as repeatability. No actual data about about how many .xxx's a particular type repeated to though.Anbody actually check/test one to see how well they repeat? I know this will vary with brands. We had both (CXA) at work for umpteen years and I never found a difference.(ignorance is bliss) I'm leaning toward the Phase II piston type as I feel it is as repeatable as my chinese lathe.

  • #2
    I have just looked through a couple of catalogs and an Aloris brochure, and I see no numbers as to the repeatability of either type of toolpost.

    The only time it would come into play in a home shop is when you are making multiple, identical parts that would require tool changes between operations. Otherwise, it is not that much of a factor.

    I recommend the piston type, and invest the money saved in more toolholders, you can never have too many of them.
    Jim H.


    • #3
      Aloris claims accuracy within millionths of an inch in their full tool catalog.

      For their new indexable posts, they provide a number: 0.00005" The dovetail mechanism is the same so I would expect a non indexable to be the same or better.

      The wedge vs piston should make no difference and Aloris shows both on the same page without differentiating.

      What will affect it is how clean the mating dovetails are, dry or lightly lubed and how fast you lock in place. Your repeatability comes from the holder following the dovetail and stopping at the lock nut.

      added - I have both Aloris and Phase II wedge type TPs and the Phase II is also very good. The most notable difference is that the Phase II has a "softer" feel when locking the holder in place, possibly due to a little more slop in the wedges and cam. The Asian setscrews also have a tendency to "bite" with time. There is usually a hard "snap' when loosening setscrews that have been tightened for a while. I attribute this to the quality and finish of the materials used. A box of decent quality (but imported) setscrews will only set you back the price of one more (imported) toolholder

      Last edited by nheng; 09-09-2006, 09:36 AM.


      • #4

        Originally posted by nheng
        What will affect it is how clean the mating dovetails are, dry or lightly lubed and how fast you lock in place. Your repeatability comes from the holder following the dovetail and stopping at the lock nut. Den
        I wonder if the amount of "umph" you pull on the handle would affect repeatability?
        Paying Attention Is Not That Expensive.


        • #5
          Quick Change is pretty much what it means. Repeatibility within millionths is a joke. Too many determining factors, just manufacturing hype. Think about hand locking pressure. Everyone is different.
          Last edited by Millman; 09-09-2006, 10:05 AM.


          • #6
            Unless you were using lasers for contact free measurement and a test fixture to hold it, I think you'd be pretty hard pressed to verify their claims.

            Even then, as ERB mentions, the "umph" you put into it probably changes everything

            Virgil, I don't think you'll see much difference between the Aloris, Phase II, piston or wedge. The rear dovetail that's taking the brunt of the cutting force is solid with the toolpost and isn't going to move on any of them.

            If you put extreme side pressure on the toolholder, the piston end face has to take the force while in the wedge type, both dovetails (one solid, one wedged, effectively solid) carry it.

            If Forrest is around, perhaps he could weigh in on this, especially after his once again excellent HSM article on toolholders.
            Last edited by nheng; 09-09-2006, 11:31 AM.


            • #7
              Not out of experiance but just in theory i believe the wedge to be a little better for repeat and also rigidity, i just bought the wedge for these reasons although its probably overkill, here's the reasoning for repeatability; the wedge pulls the tool holder down to a point on its limit while tightening and this means you at least wont have the catastrophic screw up of the tool holder hanging up on a little crumb of crud or even being kept away from the limit buy cutting fluid, true maybe that this means it can be manipulated with more pressure but then i think this would be the operators fault due to not having a feel for the elatisicity of materials plus i think that after the limit gets involved and loaded then the wedge will not keep progressing the tool holder down because the non wedge dove side of the tool holder wants to stay put because its under great load also, as for the rigidity i think the wedge leaves no room for error, the wedge grid locks the tool holder with an inclined plane basically making it a solid piece that cannot be deviated under load, the piston type puts a direct load on the tool holder outward and forces it to center by the two outer doves however this piece can still be rocked under extreme conditions, not beating on the piston type but there is a diff.

              One thing i would do if i owned piston is make sure i had a little pressure downward (by one hand) while tightening the quick change lever with the other hand, i wouldnt just leave it up to gravity.


              • #8
                Reading the Aloris literature, the 0.00005" accuracy quoted is for the indexable toolposts, not the standard QC toolpost. I suspect that refers to the repeatability of the indexing, not removing and replacing the individual holders.

                A catalog I have lists a Super Precision head that they claim 0.0001" repeatability for. The standard wedge is touted for it's accuracy and rigidity, while the piston is only praised for it's rigidity. No numbers are given in any case, which would lead me to believe the difference is minor.
                Jim H.


                • #9
                  I think the accuracy they quote is bogus also since the "umph" will most likey move the carrage/topslide a small amount greater than what they quote.

                  However I can see where the wedge type would be more repeatable.The front of the dovetail is fixed and rigid while the back is the moveable part.When you engage it the toolholder pulls back against and tight to the front fixed side of the dovetail which provides positive location.

                  The piston type on the otherhand has the holder being pushed out against both sides of a fixed dovetail,but because of the clearance needed to facilitate changing holders there is a possiblity of the holder cocking slightly askew,but only by a few thou.

                  I don't think this will matter a great deal thou.
                  I just need one more tool,just one!


                  • #10
                    Thanks for the replies. Decisions used to be so easy when it was the companys money. Lathe? No problem new Leblond 15", Mill? No problem.New 3 axis proto trak. Inserted tooling? Call the Sandvik rep. Scrounging to tool up my home shop is a new ball game.