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Thread: My new Sheldon!

  1. #31
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    Quote Originally Posted by danlb View Post
    I believe this was discussed before.

    On J Tiers' machine, the belts would slip, no damage done other than to the sacrificial part.
    ......
    Don't be so sure.

    With the thing in the best belt position, and in back gear for threading, I can tell you that it made a very valiant attempt to cut a full depth worm. The belt did not slip until the tool was already broken, and the rough edge remaining was being asked to cut a partial groove. That stopped the machine.

    It was not a case of a sheared pin, there is no pin. It was simply my mistake.

    I have not repeated the experiment.

    A very similar design of machine, under a different model number, uses v-belts, which will NOT slip at the same limits as the flat belts.
    1601

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    Hashim Khan

  2. #32
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    A change of topic:- That toolholder is a disaster waiting to happen. Its so overhung that chatter is pretty much inevitable, and at the first dig-in, the leverage is so great that it will probably break the slot in the top slide. Put the old 4 tool turret on, if its not too big. Try to keep as little overhang as possible when setting cutting tools. I don't even like QCTs much on older fairly lightly built lathes for that reason.
    The Boxford lathes in our training workshop had lantern toolposts (known as 'American toolposts' here in the UK) and I didn't like them much because of the overhang.
    'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

  3. #33
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    Quote Originally Posted by Richard P Wilson View Post
    A change of topic:- That toolholder is a disaster waiting to happen. .....
    Yow....

    I had not really looked at that.... Yes, unbolt that thing and toss it out in the woods without looking.... Just LOSE it. I have only seen one or two things that bad or worse.

    It may actually not break out the t-slot, due to the support against the front, but it is going to try hard to break SOMETHING. LOSE it.
    1601

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    Hashim Khan

  4. #34
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    Quote Originally Posted by J Tiers View Post
    I just do not think there is any advantage in stopping the threading feed. It does NOT lower the forces
    Stopping the threading feed does take all forces off the threading feed. In the event of a jam it will probably damage or destroy the tooling and/or the workpiece, but so what?

    In the event of an apocalyptical jam such as I experienced, something is going to be badly damaged. I gladly accepted the loss of an inexpensive, easily replaced gear as I would an easily replaced workpiece in exchange for more expensive damage to the rest of the geartrain. In addition to being costly, that could well take the lathe out of operation for an extended period of time.

    That particular gear is the tumbler gear, and is duplicated for the reverse position. Swapping out the two gears puts you back in operation in a matter of minutes.

    The Sheldon is a belt drive machine, single belt motor to jackshaft, double B belts jackshaft to spindle. They don't slip.
    Last edited by JCHannum; 02-13-2018 at 08:10 AM.
    Jim H.

  5. #35
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    Quote Originally Posted by JCHannum View Post
    Stopping the threading feed does take all forces off the threading feed. In the event of a jam it will probably damage or destroy the tooling and/or the workpiece, but so what?

    .....
    Maybe YOU do not mind damaging the lathe, but I do. Damage to the workpiece is not the only possible outcome.

    As you are aware, stopping the feed takes the load off the feed system ONLY. Everything else is still open to breakage, and in fact, when threading, can INCREASE the loads a great deal.

    OF COURSE it is better not to have the problem at all, because any "crash" is bad. There is no "best" way to handle a crash, aside from not having one in the first place. Once you have one, then it is a question of the least bad mode of failure. In many cases, it may be better to keep feeding, although there are cases where that is the worst alternative. One size does NOT fit all.

    My point is that the shear pins, or potentially, weak plastic gears, can be the WORST solution, if they break in threading conditions which are tough, but not disastrous. In such a case, a bad situation can be made truly damaging. That may be the reason one machine was mentioned above as having a shear pin for feeds, but NOT for threading.

    Good micarta gears are not weak. But many machines now have plastic gears of materials which become brittle over time. They may (and have) broken when they should not have. Same with shear pins.

    Not every "safety" system is worth the potential downside it may have.
    Last edited by J Tiers; 02-13-2018 at 02:30 PM.
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  6. #36
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    You're right as usual Jerry. It is much more sensible to destroy the end gears, reduce the QCGB to dust and strip the half nuts along with potential additional damage to the apron workings than provide an inexpensive, easily replaced gear that will shed its teeth in the event of a cataclysmic jam, all the while providing more than enough strength to accommodate any forces normally encountered in the use of the machine.

    What could I have possibly been thinking?
    Jim H.

  7. #37
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    Jerry, stopping the feed does more than unloading the geartrain, it also stops or nearly stops the cutting force after just one revolution. A groove is done in a second or two, but an ongoing spiral may 'never' end.

  8. #38
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    You assume it CAN cut that big groove in one cut... if not, well, something is gonna give................

    If it CAN cut the groove, there is not a lot of problem, you have time to react and shut it off.

    Quote Originally Posted by JCHannum View Post
    You're right as usual Jerry. It is much more sensible to destroy the end gears, reduce the QCGB to dust and strip the half nuts along with potential additional damage to the apron workings than provide an inexpensive, easily replaced gear that will shed its teeth in the event of a cataclysmic jam, all the while providing more than enough strength to accommodate any forces normally encountered in the use of the machine.

    What could I have possibly been thinking?
    Of course, one could do many things to avoid that cataclysmic vision...... most of which is far beyond reality.... "reduce the QCB to dust"? Really?

    One thing is avoid the screwup to begin with.

    However, it seems that a lot of manufacturers do not agree with you, since they did not provide any relief. Logan had an extra cost OPTION for a feed slip clutch, but did not provide any inexpensive shear pin. I suppose one could regard any of a fair number of parts in an Atlas as sacrificial parts, but I don't think they made all the machines with intentional "mechanical fuses" (some had a break-off bearing for the leadscrew).

    Any "mechanical fuse" setup has to be such as to release ONLY in the event of a real damaging fault. I really question the idea of that gear as an intentional "fuse". While it may have worked that way in your case, I rather suspect it was for noise reduction in most if not all places such were used including your machine. Some shaper manufacturers made micarta bull gears. If they wanted an easily replaceable fuse part, a micarta pinion would make more sense.

    Shear pins have some of the same iissues, but may be, if made precisely the way the OEM part was made, pretty controllable as to shear force.

    Overall, it is pretty likely that something is going to break if there is a "crash". You think it will be one thing, and I suspect, based on my own experience, that it will be something else.

    And, in any individual case, we may both be wrong.... the tool may tear the toolpost out of the compound when it takes a huge cut, either because of the original crash, or because of the huge cut taken after the feed stops in a coarse threading cut.

    My suggestion is to avoid the problem to begin with by being careful. These machines are not very forgiving, that's just a fact of life, and attempts to make them more so may have unintended consequences.
    Last edited by J Tiers; 02-13-2018 at 08:31 PM.
    1601

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    Hashim Khan

  9. #39
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    I think you're both right. JCHannum is the winner with lathes of significant power. Jerry on smaller hobby size equipment. (Yes, the Sheldons have sufficient power and torque to trash the gearbox, and their dual belt drive usually doesn't slip when properly adjusted.)

  10. #40
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    Quote Originally Posted by JCHannum View Post
    That particular gear is the tumbler gear, and is duplicated for the reverse position. Swapping out the two gears puts you back in operation in a matter of minutes.

    Not quite... The reverse gear is only 28T to clear the lower gear while in forward. If you try to use the reverse gear for forward drive it would not engage properly.

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