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Knuckle repair on Bridgeport type mill (head nodder)

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  • Knuckle repair on Bridgeport type mill (head nodder)

    This repair is on an Induma 1-S mill, but the problem can occur in any of the Bridgeport type mills. Lane guided and helped me through this procedure, he says its about the third or fourth head/knuckle hes done - though I have never seem it discussed on any of the forums, so thought it might be of interest. Mind you, it is not something for the faint of heart to tackle, very labor intensive, and with out Lanes help, I would not have attempted it.

    Be aware that while I usually see the Induma mill referred to a "Bridgeport type" this is only true with the spindle head - and even then, theres a fair amount of differences, the rest of the mill is quite different.

    The part the spindle housing connects to is generally referred to as the 'knuckle' (I think..) allowing the spindle to rotate and 'nod'. The spindle is attached to this knuckle with four T bolts running in a circle T slot, that when loosened allows the head to rotate. The knuckle is cast iron and when too heavy a hand is applied to the four T bolts, they will break through the T slot losing the ability to secure the spindle - and, tram the head in.

    This is what happened to this Induma I bought (came in a 'package' deal with a SB 10L, and the Shaper that Lane rebuilt) and didnt discover until I got it to the shop and needed to rotate the head to get it through the shops 7' door - it refused to rotate. Someone came up with the brilliant idea to drill and tap the knuckle and screw in long studs, permanently fixing the head in the one position - forget being able to tram it!! This is it removed from the mill, shows the four tapped holes the studs were in - along side of it is a piece of scrap yard 8" round that will be used to repair this mess.

    This is the knuckle in the vise, milling off the mangled T slot, with the piece of scrap yard round machined up to replace the T slot. Then the 'donut' in the vise cutting the slot - took about 6 hours to get it all plowed out! The piece cut horribly, was probably 4041? After breaking the one proper sized slot cutter we had, we reverted to a woodruff cutter!, yes and believe it or not, it did the entire job with out further problems, except being awfully slow...I bout wore my hands out cranking that super spacer round & round.

    This is the final result - attached the donut to the knuckle with six 3/8" low head socket head screws and ordered up 4 new T bolts from Mcmaster Carr. When re-assembled, shouldnt be able to tell from original.

    If everything seems to be going well, you have obviously overlooked something........

  • #2
    Very good! I have two knuckles here that need repair... very common problem - the underside of the t-slot gets cratered from over tightening and maybe other abuse!

    The last time I bought a good used replacement, but freight is a killer.

    I was going to try to JB weld in a circular pieces made from shim stock to "line" the inside surface of the t-slot, but your repair looks less fiddley and likely better.


    • #3
      Thanks for posting...currently contemplating a similar series of machining steps re: lathe compound/cross slide connection and as a newbie very interesting in the best order to do forming the final "T" slot, did you cut the inner or outer surface first?
      My assumption is you did not do both during just one pass (if so, how did you fit the cutter in as the "square" access point does not appear to be large enough to fit a cutter that would cut both at once, does it not need to be round or a large enough square...or am I missing something or is the photo a bit deceiving...that "port" looks rectangular)

      Anyway, inner or outer surface of the final "T" shape first? and does it make a difference? And is it important that it be a regular milling cut or can you "climb" cut?

      I am trying, with no experience, to relate this to wood working where there is definitely a "grain" but also a better way to do it in terms of chip clearing...

      Edit: just noticed another detail, mid-right edge, air to help clear chips?
      Last edited by RussZHC; 11-20-2011, 09:27 PM.


      • #4
        What's the story with those two ragged, perforated slots through the bottom?

        Is that just where the casting was thin ...down to nothing, when it was originally produced? Or did it meet with an accident at some time?
        Lynn (Huntsville, AL)


        • #5
          The replacement T bolts were 1/2" (originals were 14mm) so we had to cut to accept the head of the 7/8" T and the 1/2"shank of it.

          In cutting the slot we started with a 3/8" roughing end mill to get the needed depth of 1". That piece of scrap yard steel quickly became magnetized when I was cutting it to dimension on the lathe and that, along with its stringiness, plagued us throughout the process. So, when milling that initial 3/8" slot, the deeper it got the more the chips became a problem with sticking in the slot, so yes that is air trying to help clear the chips. We moved the 3/8" EM to each side as we went along widening the slot to approximately 1/2", and as we got that dimension, replaced the 3/8" EM with a 1/2", ending with about .540 for the shank of the bolt. Before the slot cutter broke, we used it to start the T cut (a slot cutter will cut going 'down') - it went about 1/4 of the way cutting the T making lots of chips that were just too much for it and it 'popped' - doing the rest of the T with the key cutter (not the best way, for sure ... but...)

          On finishing the T's cut, took a small end mill, 3/16-1/4"?, and opened the notch to accept the 7/8" head of the T bolt to allow it to drop into the slot.
          If everything seems to be going well, you have obviously overlooked something........


          • #6
            What's the story with those two ragged, perforated slots through the bottom?
            That is where the milling for the geared piece - ?? dont know what its called - that drops in that round hollow went through the casting, it rotates the head. Its not seen on assembly - like in the last pic of it finished.
            If everything seems to be going well, you have obviously overlooked something........