centering oblong workpiece in four-jaw chuck
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    Default centering oblong workpiece in four-jaw chuck

    I'm trying to center this piece in a 4-jaw chuck so I can re-drill the hole in the center, but I'm not having much luck. I managed to get it down to around .005 once, but I blew it out, had to start over, and haven't gotten anywhere near that since. The boss in the middle has an OD of .480 and the main body is .642 tall, so there's not a lot of room to work with. The boss is machined and as far as I can tell is actually round (I mic'ed it north/south and east/west and got the same number), and the bottom of the piece is flat and wide enough that I can push it down flush on the faceplate of the chuck to help with mounting. I think the biggest problem is that at some point I end up with the low and high spots with one jaw at 5:00 that I can't get the key in without moving the chuck. When I do that, how do I make sure I only move it one dimension and in the right direction? Is there some better trick than just keep fumbling with it and hope it eventually lines up?

    _20180504_091304.jpg

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    5:00
    ?? Were it me, I would have 2 jaws on opposite sides of the middle bore, and 2 jaws on the outside by the other bolt holes. It may seem sketchy, but for that bore it's not like the part is going anywhere, just take light cuts.

    But really that's a Mill job.

    R

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    I would likely hone the best flat side to see a nice flat witness. Then lay it on a well trammed drill press..put on two hold down clamps.. double check it was flat and drill and ream in the drill press. Yes this only goo if the location is now good.

    Could even drop a pin into the other twe holes (or just one) to be sure to is plumb and straight.

    With finding the two other hole to be flush you might stand the part on their flats and then drill and ream form the back side..

    The outer part body not likely to run true to the bores so very hard to get straight by holding in any chuck holding on the out side...but the other two holes and any machined surfaces are likely to run very true to square..

    You have to locate it from a machined surface. IMHO

    You might even use the two bolt hole and bolt it to a stub held in the 4jaw..moving the stub about would be fast and keep the part bore square. This would be needed if the drill press way would not keep proper location.

    Could be bolted to a flat stock and the flat stock held flat on a face plate so tickle knocking to find center location of the needed bore..
    Last edited by michiganbuck; 05-05-2018 at 09:49 AM. Reason: spellink error

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    Quote Originally Posted by litlerob1 View Post
    ?? Were it me, I would have 2 jaws on opposite sides of the middle bore, and 2 jaws on the outside by the other bolt holes. It may seem sketchy, but for that bore it's not like the part is going anywhere, just take light cuts.

    But really that's a Mill job.

    R
    I should have specified that I had the jaws grabbing on the diagonal, which is how I ended up with jaw positions at 5:00, etc.. I see what you're saying, and I'll give that a try tomorrow. I would rather do this on a mill, but I don't have one and this is a one-off, and it seemed like a better solution than my sketchy drill press that's just an old hand drill mounted on a column.

    _20180504_091325.jpg

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    I agree with litlerob on holding from the two out-ends and middle..

    Still you would be very lucky to get .005 square with holding the part from the sides.
    Might be able to indicate the two bolt hole for square...and tap knock square...
    Guess you could turn it around and put the drill bit in the chuck and the part clamped to an angle plat in your tool holder position..Yes you could indicator sweep the angle plate, but doing that give a hard hand push to see the andle plate is solid to drill forces,

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    I've never seen someone try to use a four jaw chuck this way. It's no wonder you're struggling to get it centered. Is there a reason you're not holding on the narrow and long dimensions, or stated another way, rotated 45°?

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    Normally when you hold something in the 4 jaw chuck, you can move the part on two orthogonal axes by moving pairs of jaws. The way you are holding the part, there is no way to move the part along the axis that intersects the outboard holes. Moving perpendicular to that axis requires moving 4 jaws nearly simultaneously. Rotate the part 45 degrees, and I think you will find it pretty simple to center. There is no issue with having the pairs of jaws with significantly different positions(just be sure they clear the ways). It looks like there are already flats on the end of the long axis.

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    Use a faceplate with clamps
    p1060854.jpg

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    If you orient the part 45* to what you have now the part might drop into the jaw slots. You might need shims behind the part to prevent that. As others have said orient the part with the long dimension parallel with a jaw slot. If there is draft on the part (sides not parallel) it would be best to set the part up on a faceplate. I assume you want to locate the hole perpendicular to the machined face.

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    Quote Originally Posted by Porschefix View Post
    Use a faceplate with clamps
    p1060854.jpg
    Or if you haven't a face plate * - bolt the part to a block on material (alu's fine ** - drill & tap 10mins tops) and grip the block in a 4 jaw.

    * Get one IMHO a job shop lathe without a faceplace is only half tooled at best

    ** I've several if not dozens of blocks etc in the shop I keep for such jobs, .....hell I've even used hard wood with wood screws before now.

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    Quote Originally Posted by Limy Sami View Post
    Or if you haven't a face plate * - bolt the part to a block on material (alu's fine ** - drill & tap 10mins tops) and grip the block in a 4 jaw.

    * Get one IMHO a job shop lathe without a faceplace is only half tooled at best

    ** I've several if not dozens of blocks etc in the shop I keep for such jobs, .....hell I've even used hard wood with wood screws before now.
    Yup, use the reference surface on the part to maintain perpendicularity while you geehaw around with the other 2 directions in the 4 jaw. Trying to control tilting at the same time you try to center something is a fool's errand

    In cases where a faceplate or plate held in the 4 jaw is not an option, like chucking some motor endbells with all kinds of knobs and draft, I'll put the part on the mill and tickle 4 flat spots on each quadrant of the part, with the tool at a fixed height. This makes chucking the part sooo much easier and stable, to boot.

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    On the above mentioned bolted to a block in the 4jaw...
    Drill and tap then bolt the part to a slug(block or round) have a north Sharpie mark on the part and the slug.
    Adjust the assembly held in the 4jaw to the tail stock eyeball to center likely be .005 or so close,but may not be square..
    Take part off and skim face the holding slug so it is true flat.
    Put the part back on with the part to the same north mark, to holding slug and 4jaw adjust to locate the bore where you want it..
    If needing to move the bore you might bore a starting bore with a tool bit or a boring bar.

    Note: likely a drill will follow where the hole is and that may not be where you want it..a starting bore so the Od of the drill will be in the hole will help locate it in the right place, A little of a drill's OD in the hole acts like a bushing to keep the better on track to less wonder off.
    Last edited by michiganbuck; 05-05-2018 at 12:16 PM.

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    More on the block method:
    Turn a pin that is a close hand fit to the hole you want to enlarge and long enough to be held in the tailstock. Mount the part loosely on your block using what ever seems stout enough to hold it but loose enough you can adjust it. Chuck the block in the head stock and hold the part up on it while you move the pin in the tail stock into the hole to be enlarged. Tighten your hold downs holding the part to the block. Pull the tailstock pin out. Drill or bore the hole. Boring would be better as it will not follow the messed up hole, but make a nice new straight hole to the size you choose.

    When clamping the part you may need to shim underneath to maintain the alignment of the part to the pin. When its clamped tight and you can move the pin out without binding its good.

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    The 'wings' on that look to be too far offset from the center to allow a decent grip - to a 4 jaw it's going to be like gripping an olive to pop the pip out.

    I'd be tempted to fixture the part in some Woods metal - you can use other material like plaster and such, but the Wood's metal allows a pretty good grip in a 4 jaw. I'd poke something through the hole you're working on and use that to hold it vertical and pour around it. Search for "fixturing alloy" to find it online and examples.

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    Quote Originally Posted by Limy Sami View Post
    Or if you haven't a face plate * - bolt the part to a block on material (alu's fine ** - drill & tap 10mins tops) and grip the block in a 4 jaw.
    It seems to me that Samy's idea is efficient and the way to go. Depends on how many you need to do, I guess. But I'd drill and tap two holes in a block of Al or steel, and bolt the part to the block and then center with a four jaw, or on a faceplate. If you know the exact geometry (that is, the relation between the three holes) you could also drill and ream a precision hole that aligns with the hole you need to bore/ream out. Loose interference fit for a long pin. Put a precision pin in, use it to center the block, put your part on the pin and bolt it down to the block, and pull out the pin. This would allow you to do a lot of parts, centering only once (using the pin, not the hole that is trashed and is of dubious centrality).

    If you used precision shoulder bolts as hold-downs I bet you'd get that 0.005 or better pretty easily. This is, after all, how they made watch plates back in the day.
    Last edited by bosleyjr; 05-05-2018 at 01:38 PM.

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    Pretty much the same as Limy Sami suggested, if you don't have a metal block mount a square of wood (preferably hardwood) in the 4-jaw and use a drill bit in the tail stock chuck to get close to center. Drive a couple of wood screws with flat washers through the other two holes. Leave the screws just snug until you get it zeroed (by tapping it in the direction you want it to go). The washers are to prevent the screws from shifting the part as they are tightened.

    After everything is tight drill to finished size in small increments so you are not putting a lot of torque on the part that could shift it.

    The above advice is based on my guess that it's a home shop and lightly equipped.

    PS: Once you are sure the part is centered a bit of autobody filler or similar where the part meets the wood will help keep it from shifting. You could even use quick setting epoxy if you lightly oil the part first so it fits to rather than adheres.

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    Pretty much the same as Limy Sami and Motion suggested is the way to go...

    Still don't know what the part might be..looks like a bearing cap but what is the problem bore for? Likely it may have to be square with the part and to a close size..how close to size must it be?..Thinking if now screwed up a press or loctight bushing might do for repair for a try again...

    If can't do on the lathe then a diameter turned to the caps inside radius size, with a centered pilot hole might be clamped in the part and then drilled through that pilot hole to go through the part from he inside to outside.. The round drill-pilot might be made on the lathe in a 4jaw.

    This drilling through a jig guide might be done even in a bench vise with a hand held drill motor..with the guid in the vise and a couple C clamps over a parallel. and knocking to make measurement for width-way center.

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    that is a cheap-ass die casting. grab a cordless drill.

    why would you use a proper machine tool to do anything with that?

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    Quote Originally Posted by cyanidekid View Post
    that is a cheap-ass die casting. grab a cordless drill.

    why would you use a proper machine tool to do anything with that?
    Which is probably what some twats done before - and f'kd up.

    P.S. FWIW that cheap ass die casting looks rather like the cam bearing cap off an OHC engine, the centre hole of which often holds down the valve cover - which has a bolt or syud hole that HAS to line up with said cap, and the cover be oil tight.

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    Some ideas.
    Bolt it to anything rectangular from the other 2 holes, adjust as needed.

    Drop it in a box, use cerrobend to fix it in place.
    Tin or lead would work, but lead might not be allowed legally.

    Glue it to a box, drill/tap 4 holding screws on each side.
    Hold the box.

    Use an ice chuck.

    Cast a false female mold using the cheap silicon stuff.

    I would probably put == 20 mm wood blocks on the 4 sides and just grab it in a 12" 4 jaw.
    The wood crushes and conforms nicely, does not distort the workpiece, and is plenty good enough.


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