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Grinding My Jaws

Discussion in 'MACHINE RESTORATION & WAY SCRAPING' started by RJSakowski, Oct 1, 2016.

  1. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    I recently took a critical look at my Grizzly 6 -1/2" 4 jaw chuck and found the jaw grind to be seriously deficient and I decided to try to regrind my jaws.

    I have a Craftsman 1/4" die grinder that I would press into service but I needed to come up with a stable mount for the grinder. My design utilized the AXA style QCTP and a Shars 5/8" tool holder. This design permitted me to adjust the angle of the die grinder spindle axis to the lathe spindle axis as well as the height of the die grinder relative to the lathe spindle axis

    In order to grind the jaws of a lathe chuck in situ, some means of preloading the jaws mimicking conditions of use is required. For 2 piece jaws, a ring which utilizes one of the jaw mounting screws can be used but some other method must be used for single piece jaws. My approach was to use the 45º bevels on the jaws so that the side force is balanced by the adjacent jaws.

    I made the preload plate from a piece of 1/4" HR steel. The cutout was machined on my Tormach although it could have been made on a manual mill with a RT. A 1/4" carbide EM was used. The 135º inside corner sharp was finished with a small file. The OD was roughed out on the4 band saw and turned on the lathe.
    Lathe 4 Jaw Spacer.JPG
    Die Grinder Mounting Block.JPG
    The die grinder mount was made from a 2 x 2 x3" block of HR steel. The exterior was milled first and pilot hole followed by a 1/2" drill. The block was then mounted in the 4 jaw and a 1/2" pin was inserted in the hole for the purpose of centering the hole in the block to the lathe axis.
    The hole was drilled out to 7/8" and then bored to 1.375". The Craftsman die grinder has a tapered nose so the compound was rotated to match the taper of .0175"/1.90" and the final cut was made using the compound feed.

    The mount was then transferred to the mill where the holes for the clamping screws were drilled and tapped and slot for the clamp was cut with an 1/8" end mill. Because the EM wasn't long enough to cut the full slot, I finished cutting through with the band saw and cleaned the slot with a flat file.
    4 Jaw Grind 3 .JPG
    The faces of the chuck jaws were blued so the grinding progress could be monitored. The preload plate was mounted on the chuck and the jaws were lightly snugged up. The jaws were then gradually tightened to ensure a balanced load on each jaw. A fairly heavy load was used.
    A 3/4" cylindrical point was used for grinding. The grinder was centered vertically and adjusted so its axis was parallel to the lathe axis. The lathe was set for the lowest speed (150 rpm) and the feed was set at .007"/rev.
    4 Jaw Grind 1 .JPG

    4 Jaw Grind 2 .JPG
    Before commenting on the lack of protection for the ways, the photo was set up after the fact. Way protection was used.

    The grinder and the lathe were turned on and the grinder move out to just engage the chuck jaw. It was then fed in to a point where the far end of the grinding point was about 1/2" past the inside face of the jaw. The feed was then reversed to bring the grinding point clear of the jaw. The grinder was then moved out by .0015" and the process repeated.

    One jaw extended considerably more than the other three so quite a few passes were required before the other three jaws were engaged.

    The taper of the original grind was about .007" over length of the jaw. In addition, the original grind had a radius of curvature of about 1.1" whereas the maximum radius that I could grind in situ is about .75". Because of this, I had to grind an additional .004" to carry the grind to the edge of the jaw face. All in all, I had to remove about .011" after intial contact was made with the last jaw.

    After grinding, the jaws were checked for taper. With a moderate load on the jaws, the diameter at the rear of the jaws was .0015 larger than the front. A 1"inch bar was mounted in the chuck and adjusted for zero runout. The indicator was moved to a distance 9" from the chuck and the bar was checked for stability. Whereas with the original grind, the bar could be set at any position within a .014" radius, now when the bar was deflected it returned to its original position within .0001"

    Success!

    While I don't anticipate having to regrind the jaws, I numbered the jaws so they can be inserted in the same position each time and I also indexed the spacer so I can load it in the same position as the first grind.
     

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    mksj, MozamPete, WesPete66 and 10 others like this.
  2. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    RJ,

    That's the proper way to grind your jaws!!!

    Ken
     
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  3. tpic402

    tpic402 United States Active User H-M Supporter-Premium

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    I did mine very similar , except I preloaded the jaws with a ring and extended length bolt, replacing one of the jaw bolts. It took the runout from .007 to below .oo1 I was thrilled as I am sure you will be.
     
  4. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    To do this the proper way, As RJ has shown, you have to preload the jaws out on the ends. This way, you grind the "sprung" out of the jaw so you get even gripping along the length of the jaw.
    On a new chuck this is generally not the problem. T.I.R. is the issue if it's a new 3-jaw chuck. Then you can preload the jaws as tpic402 has mentioned and grind them to run true.
    Ken
     
  5. Christian Poulsen

    Christian Poulsen United States Active Member Active Member

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  6. JR49

    JR49 United States Active User H-M Supporter-Premium

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    Great write up RJ, but there is one thing I'm not following about the sentence I quoted. Considering this is an independent 4 jaw, how do you know that the one jaw that was extended more than the other three wasn't just turned in (tightened) a little more than the other three? Hope to learn more about this as I have a 4 jaw with bellmouthed jaws, and have been trying to figure out a way to preload them perfectly cylindrical so that I can grind the same amount off the inside end of each jaw to correct the bellmouthed problem. Thanks, JR49
     
  7. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    Hey JR, Sorry for the long response time. I haven't been following this thread and just saw your question today.

    When I mounted the preload ring, I tried to sneak up on tightening the jaws. Overtightening one jaw would tend to push the adjacent jaws to the side. The fit of the ring is fairly close so there was not much room for side movement. There is a possibility that the grind on one of the jaws could be slightly off axis but it shouldn't be enough to affect the use. I did check to verify that the jaw was the problem and not my mounting procedure. If one jaw seemed to be extending more than the others, you could try to loosen it and readjust the other three jaws. An indicator will tell you if there is a problem.

    Grizzly sells replacement jaws and the OEM jaws are not marked in any way to match them to the chuck body so I assume that they are most likely ground in an external fixture rather than mounted on the chuck. It is entirely likely that the one outsized jaw could have been made on a different day, month or year.

    In any event, the grinding process has improved the instability and angular runout issues. Were this a three jaw scrolling chuck, I would be more concerned.
     
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