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  1. #21
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    By "single part", I meant one ring on the full circle. Not cut in halves.

    And the problem of the threaded hole for the clamping bolt is what I had in mind when I wrote :
    " One should also probably re-think the junction between the ring and the two bolts that tightened the T-nuts in the original design."

    I have a design in mind, with a single part that would be kind of a "traction ring", including the clamping studs.
    Let's allow some more time so see if other ideas pop up.

    PS : and you are right in that the inner ring of the casting also serves as locating bore to center and support the head on the support. Wich makes getting rid of it a bit complicated...

    That kind of repair can be a tricky design exercice !

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    Hi guys,

    not sure if I am being stupid here, but, how were the T-nuts originally inserted in the T slot? I mean, I can see an opening coaxial to the bevel gear that is visible but I am not sure if you have to take the whole head apart in order to fit the Tnuts or whether T-nuts would even fit through there.
    What I am implying is, could it be that the T-slot had an opening just right on the center of the break?

    BR,
    Thanos

    Edit: not very possible, since, as it seems the top T nut more or less coincides with the break (or where I thought there could be an opening) at the horizontal configuration of the head which would not make much sense...

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    Done ! Lockdown has some bright sides... I had some time to play in the shop so at last the Deckel S11 workhead is repaired, and I think the result is nearly as strong as the original setup ! ��
    No soldering, no brazing, no heating of the head casting.

    From the begining, I was convinced that the right solution was not to try to fix the casting itself, but rather to work around the problem and make some extended T-nuts. Ideally, a full circle nut. But how to insert it in the slot ??
    Of course, there was the solution to make the T-nut in several parts bolted together. But bolts require holes that would obviously have been the weak point of that method.

    After some head scratching, I had the idea to replace the original T-nuts with a full-circle "traction ring" and (nine) retaining segments that would screw on the ring, rather than beeing bolted on it.



    The traction ring is narrow enough to plunge in the T-slot of the casting, while the segments are obtained by sawing a second ring made accordingly to the bottom section of the slot (wich of course, is wider).










    The ring and segments are threaded with a 1,25mm buttress thread for better load capacity.
    The segments are inserted individually in the T-slot in the correct order, then the traction ring is gently screwed and tightened et voilà !







    Worked like a charm ! ��

    Of course, there were some other "details" to take care of, such as restoring the main register with a sleeve, re-boring the corresponding bore on the workhead casting so as to make it perfectly round again, or cleaning up the broken edges of the T-slot (hence the need for the rotary table)...



    Last edited by TNB; 04-07-2020 at 03:30 AM.

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  5. #24
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    That is clever. I would have thought that the segments might have a tendency to bind up on the threads as the ring was screwed in.

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    Absolutely not. They screw in just like the two threaded rings did before the second one was divided in segments.

    The buttress profile of the threads doesn't generate any radial effort when tightening.

    Considering the input load comes through two M6 rods (original design), the load on the segments is probably on the very low side plus (and that's something I did think about at first) I think the curved shape of the segments greatly increases their stiffness.

    The weak point could have been the traction ring since it is very narrow and low (less than 5mm wide x 9mm height - approx. 85mm diameter) IF I would have had to drill holes in it.

    That design allowed to preserve its strengh.
    That, plus the two "Mickey Mouse ears" for the draw rods, that help distributing the load on the traction ring.

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    Very nice, thanks for sharing!

    BR,
    Thanos

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    Tien, clever solution, and nicely done!

    The traction ring is threaded on the OD, and the retaining segments are threaded on the ID. I can curious about how you cut this thread. Did you first thread the retaining ring and then cut it up into segments? Or did you cut out the 9 retaining segments, glue or clamp them to a backing plate, and then thread the ID? The problem with first cutting the ID thread and then cutting it up into segments is the small amount removed means that the segments don't quite match up in height on the thread. Though I suppose you could assemble it then grind/mill that part level.

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    Hi Bruce

    I went to the simplest solution and made a full retaining ring (check out the pictures above and you'll see the traction and retaining rings before I finish them).
    Then I cut it after determining the max dimension allowed for the segments (so that I can get them in the slot, that is !).

    I don't really understand the height problem you envision.

    The final "height" is attained when the traction ring bottoms out in the segment, so it will not vary providing the segments are screwed all the way down (or up, depending upon your way of seeing things !) on the ring.

    You could even forget a few segments and obtain exactly the same result.
    Even over-tightening the segments would probably stress the threads, but not alter their height on the ring.

    I was obviously overthinking the thing but at some point, I even thought about threading both sides of the traction ring (ID/OD) and the retaining ring... I quickly gave up because of the impossibility to check thread size and make sure both threads would effectively work together.
    Would have been neat though !

    Imagine a traction ring threaded with the same profile as the root of a turbine blade : how cooool !!!


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    Looking at the photos of the installed segmented ring is a bit confusing. Clearly shows different spacing between the segments (gap between each)
    Seems this would be a problem as some segments would in effect be advanced relative to the mating thread and some retarded...making some segments carry more of the load than others, yes?....

    Don't think the connection of turbine blades is a thread...there is no circular pitch there the profile is straight or slightly tapered not curved....But i am betting you already knew this.

    Cheers Ross

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    Ross

    No, and there's no reason why the problem you describe would occur.

    Imagine the retaining and traction rings assembled and tightened.
    Now imagine there would be a way to divide (saw or cut) the retaining ring only, without disturbing the traction ring... Would it change anything in the height of the segments relatively to the traction ring ?
    One could even cut uneven segments, that would be the same.
    And the fact that I had to remove the traction ring from the retaining ring to saw the latter doesn't change anyting to the geometry of both parts.

    The segments you can see on the pictures are just freely insterted in the slot (but in the right order).

    So the unequal spacing between them on the picture is just due to the fact that there's nothing to hold them yet (the traction ring).

    When the traction ring bottoms out in the segments, everything gets back in order because each segment reach its own tightening point at the same place as when it was part of the retaining ring. The holes in the segments allow to complete their tightening with a pin inserted from the other side of the T-slot when the traction ring is mounted.

    I hope it makes sense, because it works !
    Last edited by TNB; 04-08-2020 at 03:30 PM.

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    Tien,
    I think you might have missed what Ross was getting at. If, as he notes, some of the segments are retarded or advanced as compared to their original as machined position (in relation to one another)- this would result in some of the segments tightening before others. Since there is space between the segments some of them might never truly tighten and, as Ross notes, the load could be carried by just a couple of them.

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  17. #32
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    Ok I think I get what you mean.

    So in fact, the uneven spacing of the segments when you insert them in the T-slot doesn't matter, providing they are in the right order.
    They are free and of course, they will start in random positions one relatively to each others.

    But you can't hope to tighten all the segments in one go and be done with it anyway ... (the sole solution would to put some kind of spacers between each segment to compensate for the lost width of the bandsaw blade).

    So when you screw the traction ring on, you come to a point when it will become somewhat harder to tighten it.
    At this point, the hole on the back of the segments allow me to insert a pin through an small opening in the casting from the other side of the T-slot.

    I can then use that hole and the pin to immobilize the segment, and tighten it a little more by carrying on the rotation of the traction ring.

    Doing it one segment at a time and by small increments allows me to even their tightening and to bring them to their final position (where of course, they have an even spacing between each other again).

    Takes much more time to explain than to do actually !

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    Quote Originally Posted by TNB View Post
    Ok I think I get what you mean.

    So in fact, the uneven spacing of the segments when you insert them in the T-slot doesn't matter, providing they are in the right order.
    They are free and of course, they will start in random positions one relatively to each others.

    But you can't hope to tighten all the segments in one go and be done with it anyway ... (the sole solution would to put some kind of spacers between each segment to compensate for the lost width of the bandsaw blade).

    So when you screw the traction ring on, you come to a point when it will become somewhat harder to tighten it.
    At this point, the hole on the back of the segments allow me to insert a pin through an small opening in the casting from the other side of the T-slot.

    I can then use that hole and the pin to immobilize the segment, and tighten it a little more by carrying on the rotation of the traction ring.

    Doing it one segment at a time and by small increments allows me to even their tightening and to bring them to their final position (where of course, they have an even spacing between each other again).

    Takes much more time to explain than to do actually !
    Prolly could have cut it with a wire or cut it first and held it in pie jaws to get the final diameter and then thread.

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    Hi Tien,

    OK, thank you, that answers my question above. Yes, I was concerned about those thin "kerf" gaps left by the bandsaw blade, because I thought that the segments fitted tightly against each other. But what you say makes sense. When the segments are in the final position, they are separated by those same small gaps where the bandsaw removed material. In fact the gaps are clearly visible in the "assembled" photos.

    A second question: what retains the segments on the traction ring? When the thread which binds them together is fully tightened, is it the frictional force between the bottom face of the traction ring and the top face of the segments? If you have any worries about these parts becoming "loose" or separated in use, you could provide additional "retainer" screws (setscrews or grubscrews). There would be nine of these, one per segment, and you could screw them in from above after the parts are assembled in place as currently. The screws would pass through the traction ring, which had a threaded hole centered over each segment. Those screws would then pass partly or completely through the existing "pin holes" in each of the segments, blocking the segments from "unscrewing" from the traction ring.

    Cheers,
    Bruce

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    Bruce

    Once the segments properly tightened on the traction ring, the risk of seeing them becoming loose is almost zero for different reasons.

    Remember this is a tool and cutter grinder workead, so the efforts it will endure will always be very limited (in normal use, that is.... Of course, the broken casting is the proof that there will always be exceptionnal circumstances...)

    There are no efforts that would have a tendency to loosen the segments, except during the orientation of the workhead but then the traction ring is loosened so the friction on the segments is marginal. Plus the need for orientating the workhead is limited in real life. No more than 90° and only from time to time.

    The thread pitch is 1,25 for a diameter of ca.85 mm wich makes it a very fine pitch, less prone to loosen than coarser threads.

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    Hi Tien,

    Yes, that makes sense, though if it was mine I would probably add the retaining screws out of paranoia.

    It's a very clever and pretty repair. To prevent dirt and dust from reaching that area in the future, it would make sense to use some two-component putty to replace the missing cast iron. This will have no structural purpose, but will have a functional one: it will keep grease in and grinding grit out of that area.

    Cheers,
    Bruce

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    Amazing repair, makes my head hurt just looking at it. Respect, would of never thought of it. I would of EDM'ed out a pocket with a cnc'ed electrode and then turned up a corresponding ring, screw and glue in a pie section of that. En tout cas beau travail !

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    Merci Laurentian !

    Quote Originally Posted by ballen View Post

    Yes, that makes sense, though if it was mine I would probably add the retaining screws out of paranoia.

    It's a very clever and pretty repair. To prevent dirt and dust from reaching that area in the future, it would make sense to use some two-component putty to replace the missing cast iron. This will have no structural purpose, but will have a functional one: it will keep grease in and grinding grit out of that area.
    Bruce

    If it was yours, you'd realize, you'd *feel* how un-necessary retaining screws are. That's for sure !
    Every time I venture in the design field for my tinkerings, I find there's always a significant difference between the way I imagine things, and the way I "feel" them when parts begin to take shape for real...

    The idea of rebuilding the missing portion of the OD wall of the T-slot to prevent any swarf ingression is a good one.
    In fact, even as is, there's almost no room for dust to get in since the mating part of the workhead casting features a small "lip" that comes just a tad on the engraved ring so it already covers the broken zone.
    But indeed, having "belt and suspenders" can't hurt !

    One of the challenging (for me at least) aspects of that repair was the inside threading of the retaining ring.
    The T-slot was only 6mm deep and its entry just 5mm wide. Leaves very little room to get a 1,25mm pitch buttress threading tool in and back it off at the end of the pass.
    Of course, beeing lazy as I am, my first move was to take the first random tool with a vaguely appropriate shape that was laying around, finish grind it, and go go go !
    Of course, that was a bad idea...
    To tell you the truth, it almost worked. Very well at the beginning, but clearly worsening as the cut load raised... And of course BAM, the tool broke.

    So after insulting myself as wildly as I desserved it, I took a blank, a *real* blank, and ground a tool of the max. suitable size on the S11 (first pic below is one of the initial modified tool).









    I must say that the microscope mount I had just completed for the grinder prooved itself invaluable in helping me to make the tool just the right dimension.
    i.e. small enough to work in the slot and go as deep as possible next to its bottom, yet as big as possible to preserve stiffness.
    It helped me a lot to check the root of the triangle at the tip of the tool was sharp, and to measure the width of the cutting edge so that I could be sure it would cut the full thread but was not un-necessarily big.
    And of course with the proper tool for the task, it worked like a charm !




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    Quote Originally Posted by TNB View Post
    ...
    One of the challenging (for me at least) aspects of that repair was the inside threading of the retaining ring.
    ...
    Hey Tien,

    off subject, but how did you actually cut that thread? (I was wondering from the beginning)

    If I had to do it on the Colchester it would be low rpm and half nut releasing with the help of the DRO, which is, for me, always a stressfull experience. Never had a real crash but always takes years off my life...
    If I had to do it on the Hardinge, things are taken care of by the machine.

    I assume you did this on the Schaublin? Are there any tricks for such tasks there? Or plain skill and a responsive machine?

    BR,
    Thanos

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    I cut the inner thread with the Schaublin 125, using the built-in electrical stops.

    Together with the brake motor, they provide a reliable and precise stop wich enabled to set the travel end of the tool 3/10mm from the bottom of the slot. And that without particular skills, that I can tell ya !

    I also cut a relief groove nearing the bottom of the slot to allow the exit of the tool at the end of each pass, without risking to break the tip.

    Due to the particular profile of the buttress thread, I think the thread would have been much more difficult to cut in reverse, starting from the slot toward the tailstock. The blind slot would have been no problem then, but that direction implied the tool would have worked mostly with its right -non slanted- edge.
    I think it would have been difficult to grind an tool to work efficiently in that direction.

    The Schaublin 150 also features stops acting on the brake motor but the system is mechanically much more complicated, thus less repetable and reliable (in this case, read : dangerous) if the machine is not in good condition.

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