Ideas for workholding / fixture please!
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    Default Ideas for workholding / fixture please!

    Hi all,

    First post for me, so Hi again!


    I'm running out of ideas for a fixture/workholding of a part.

    In this part I need to mill three 'bayonet lockings'(?), 120 degrees apart (at the circumference).
    The part dimensions are Ø170mm (6.7") by 85mm (3,4"), I added a picture also.

    Now the question is what do you think is the best fixture for this part, I need to make 50 of them.


    Hope to hear from you,

    Coert
    Attached Thumbnails Attached Thumbnails whatsapp-image-2021-09-16-08.48.28.jpg  

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    Is it hollow? 3 jaw chuck internal grip mounted on rotary table. Bolt to mill table so chuck is horizontal.

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    Thanks for the reply.

    Yes it's hollow with a wall thickness of 2mm.

    Your option is one of the options, but the minimal wall thickness is a bit of a problem I think. A 6-jaw chuck would be better, but unfortunately we don't have that.

    An extra option with this solution is to use a tailstock with a delrin ring.

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    Make a spider that fits inside, put in three jaw chuck clamping on the spider so the chuck does not collapse the part. Machine the bayonets between the jaws.

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    Vacuum Chuck suggests itself,
    Mark

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    Quote Originally Posted by CoertHFT View Post
    Thanks for the reply.

    Yes it's hollow with a wall thickness of 2mm.

    Your option is one of the options, but the minimal wall thickness is a bit of a problem I think. A 6-jaw chuck would be better, but unfortunately we don't have that.

    An extra option with this solution is to use a tailstock with a delrin ring.
    Punch it then. That's how this sort of goods are usually done for volume production.

    Simply support the hollow rim on a curved arc of a shoe, mating pair of arced shoes....or even rollers.

    The section of it you need to work on is functionally the same as a 2 mm wall tube, the closed "spinner" part of the shape is not involved.

    Or mill it, but same deal. You don't NEED to grip the entire part and spin it.
    Only grip and reinforce the rim where the cutting force is to be applied.

    Shouldn't be rocket science to index off the first slot cut?

    "Worried?" Go to the delron form (or shiney-wood) concept - and rig it for internal expander WITH an external "cinch" band, too. All you need is a narrow cylinder. External band needs cut-outs, Internal should have clearance pockets as well.

    Mating clamping so you neither stretch nor shrink it.

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    Quote Originally Posted by gregormarwick View Post
    Make a spider that fits inside, put in three jaw chuck clamping on the spider so the chuck does not collapse the part. Machine the bayonets between the jaws.
    That's a possibility for sure. Thank you!

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    Quote Originally Posted by boslab View Post
    Vacuum Chuck suggests itself,
    Mark
    Thanks for your suggestion. We have a vacuum chuck already but the part does not stay perpendicular to the machine axis. Already tried to correct it with a corner plate, but it doesn't work.

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    Quote Originally Posted by thermite View Post
    Punch it then. That's how this sort of goods are usually done for volume production.

    Simply support the hollow rim on a curved arc of a shoe, mating pair of arced shoes....or even rollers.

    The section of it you need to work on is functionally the same as a 2 mm wall tube, the closed "spinner" part of the shape is not involved.

    Or mill it, but same deal. You don't NEED to grip the entire part and spin it.
    Only grip and reinforce the rim where the cutting force is to be applied.

    Shouldn't be rocket science to index off the first slot cut?

    "Worried?" Go to the delron form (or shiney-wood) concept - and rig it for internal expander WITH an external "cinch" band, too. All you need is a narrow cylinder. External band needs cut-outs, Internal should have clearance pockets as well.

    Mating clamping so you neither stretch nor shrink it.

    Thanks for your idea. Punching isn't one of the possibility's since we don't have punching machines.

    I don't really get your last idea, it is pretty stiff to compress

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    Make an acetal or ply disk 1" thick that fits the ID. 3 jaw chuck from the outside on a rotary table?

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    Quote Originally Posted by NewGunPlumber View Post
    Make an acetal or ply disk 1" thick that fits the ID. 3 jaw chuck from the outside on a rotary table?
    Quote Originally Posted by gregormarwick View Post
    Make a spider that fits inside, put in three jaw chuck clamping on the spider so the chuck does not collapse the part. Machine the bayonets between the jaws.

    I think you both mean the same. That's what I'm going to try now. Thank you very much!

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    Quote Originally Posted by CoertHFT View Post
    Thanks for your idea. Punching isn't one of the possibility's since we don't have punching machines.

    I don't really get your last idea, it is pretty stiff to compress
    You don't want to distort it. You want to grip it with balanced force.

    If the only workholder you have is a 3-Jaw?

    You first make an internal plug for the jaws to work against.

    And rigid, not resilient. Adjustable with a skosh of expansion, even.

    This is not a new challenge.

    Put yourself in the shoes of an ancient armorer making helms and riveted features for battles when "high tech weaponry" was still Bronze swords and a whole army had to be equipped, faster and better than the other side.

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    Quote Originally Posted by thermite View Post
    This is not a new challenge.

    Put yourself in the shoes of an ancient armorer making helms and riveted features for battles when "high tech weaponry" was still Bronze swords and a whole army had to be equipped, faster and better than the other side.
    Sounds like you're recommending a hammer and chisel. 😜

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    Dividing head, with ring, foot stock with ball, do it between centres, you might have to plug the middle with timber or uhw to stop chattery resonance, assuming vertical mill to cut the bayonet slots, reminds me of a Harley pan head headlight I once repaired ( rust ) that had the same bayonet slots, if it was original I don’t know, the metal spinning my mother in law worked used similar “ cup and ball” fixturing on round bowls
    Mark

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    Quote Originally Posted by fciron View Post
    Sounds like you're recommending a hammer and chisel. ��
    'Observing", rather than recommending. But they work just fine if you know your s**t.

    https://www.metmuseum.org/toah/hd/deca/hd_deca.htm

    Hydraulic nibbler is faster.


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    Is the part a stamping/pressing (ie hollow)?

    If so, I'm thinking sacrificial soft jaws on a chuck, turned to the ID, on a simple indexing fixture. Mill slightly into the jaws with a larger diameter first, then mount the parts and mill them.

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    Am I the only one curious about what kind of machine he's got to do the work? If it's a vertical mill, you could get away with making a triangle plate that has a round protrusion that fit inside the part. Set up an angle plate on the mill, clamp the part/fixture plate against the angle plate with the fixture sitting on one of the three triangle flats and mill your feature. Turn it to the next flat for the next feature. Repeat until done.

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    You don't tell us what machines you have to work with. In my shop I would use my Bridgeport right angle adapter and a rotary table.

    To fixture it on the rotary table I would use a sacrificial round piece of aluminum plate slightly larger in diameter than the open end of the work piece. To that piece attach another sacrificial round piece of aluminum plate exactly the inside diameter of the work piece. Then you can clamp down from the bullet nose end of the work piece. The bottom aluminum plate would have to be very thick or spaced high enough above the top of the rotary table to avoid interference with the right angle head.


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