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How to set two leg type calipers..?

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  • #16
    Originally posted by aboard_epsilon
    i could do with a set of callipers for measuring wall thickness.
    These exist, they are called lock joint calipers. The measurement is taken and the caliper is locked. One leg is attached to a stub, which is locked. The leg is losened from the stub and swung clear of obstructions, then moved back to the stop and relocked in place.

    One of the first "lathehands" I worked with did not own or know how to use a micrometer. He had a selection of stubs of shafting in an assortment of sizes. he would turn to diameter or bore for a bearing using them as standards and nothing more than spring calipers.
    Jim H.


    • #17
      Originally posted by Forrest Addy
      I worked for many years in a section where transferring inside dia readings via inside spring calipers by necessity evolved to an art form. When line-boring split case double suction pumps on an HBM, you can't directly measure a trial cut with the bar in the way. Once the trial cut is checked the tool can be bumped out to desired size.

      Telescrope gages, inside mikes, bore gages etc don't work when line boring on an HBM. The bar extends all the way through the pump. It's a 20 milute job to remove the bar and the evolutions involved in doing so may disturb tailstock settings, tool settings etc. Given the are maybe 20 inside diameters and grooves, the time to rassle the pump cover, bar etc results in a doubling of the time to re-bore.

      Sine inside alipers are relatively inexpensive the man on the machine is free to bend, twist and doctor the caliper legs to gain best access to the bore in question without the need to disturb the boring bar.

      Thus an ancient tool - the inside caliper - is still used in this modern day and age to transer readings from inaccessible inside diameters. To do so the operator has to develop an exquisite sense of feel and trust to blind transfer from first reading to the micrometer. It is possible to work to 0.0005" accuracy. Generations of my peers engaged in split case etc for the last century have done so.

      I can show anyone how to measure ID's with an inside caliper then transfer to an OD mike in a few minutes. A few days of intermittant practice will solidify the technique in your skill base. Here is the trick: consider the barely perceptible graze of the caliper tips. If the graze in the bore and on the micrometere is equal the any minor error in the actual setting is off-setting. So the trick is equal feel. Don't believe me? Practice with a bearing inner race. You will be all over the map at first but time with fetch the skill.

      White hats and techs find it hard to believe such a subjective evolution can result in hard, re-producible readings. All during my career every few years a little cluster of non-believers would spend a few hours at Glen Clarke's machine or Felix Hamilton's and come away head-shaking believers. This little demo was actually a high-stakes event. Skilled use of inside calipers or huge PITA expensive, ineffective high tech solution hinged on convincing un-knowing dweebs having authority of the enduring value of yesteryear's low tech solutions.

      Many gimmicks using the most compact DTI mounted on the modified legs of an inside micrometer but these were less than satisfactory for deeper bores.

      Whenever the topic of line boring somes up I have to trot out a favorite question: For extra points, what is a "heel dragger" tool?
      A very well written and enjoyable description!

      For the uninitiated, 2 (very poor quality, sorry) photos, not of a pump case, but a single cylinder engine being bored by the above described method.



      • #18
        Originally posted by JCHannum
        These exist, they are called lock joint calipers. The measurement is taken and the caliper is locked. One leg is attached to a stub, which is locked. The leg is losened from the stub and swung clear of obstructions, then moved back to the stop and relocked in place.
        Just to illustrate the principle, these are outside calipers with that feature.

        These are Starrett (US made, not UK). I have quite a number of these ranging from this, the smallest to some much larger ones, nearly all Starrett or Brown and Sharpe. They originally belonged to my grandfather and have been used and passed down through my family. I don't use them nearly as much as my grandfather, who like the posts describe above worked to very close limits with them.


        • #19
          will have to look out for them ..there were a pile of them at the car boot sale today ..i didn't really study them they were all "looked" to be duplicates of what i all ready had ..will look more carefully from now the ones youve showed pics of may be lurking amongst them . these were all 50 pence each

          all the best.markj


          • #20
            Look for the better quality ones. Most will probably be just cheap firm joint calipers, OK, but not in the same league as the type above, which have a fine adjustment as well as the lock joint feature. My larger Brown & Sharp calipers have a nice fine screw adjustment that works against a kind of leaf spring. They are particularly nice. It doesn't matter if they are a bit rusty, they will clean up easily. Nobody seems to want this type of tool any more - you should be able to get some bargains with a bit of patience.


            • #21
              Originally posted by The Artful Bodger
              The old lathe books tell of using a basic (two leg type) caliper to test for diameter and this works well for me if I have some reference to set the calipers too. But if I dont have a reference how do I set the caliper accurately?

              Yes, I know about internal micrometers but I am not sure these are what I want, maybe, but I dont have one, yet.
              I use calipers to set my calipers.
              I use the inside jaws on my dial calipers to set the friction? calipers. I'm usually using the friction calipers because the diameter is too large for my dial calipers to get around, usually just using them to check the o.d. while roughing.