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Concentric bore

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  • dian
    replied
    what goes into the hole in the end? a shaft on bearings?

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  • BCRider
    replied
    I'd start with a brand new proper name brand drill bit that is a little under 1/8". And peck at the hole and clear chips frequently. I've had good luck in the past with even small drills over that length drilling true to within a few thou. A very few thou in fact.

    But do check that your tail stock ram is centered both side to side and up and down and is not angled to the axis as well. If the lathe isn't true in these respects conical or drifting holes are more than possible.

    If that is the case I think I'd drill the center hole in the stock in the mill or even in a drill press. I'd center drill, drill through undersize, ream then flip over and center drill again. Then mount it up between centers using a drive dog and turn the OD details. I'd give the yourself the added length needed to fit a drive dog.

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  • darryl
    replied
    Drill and ream the part. Then chuck a piece of say .75 diameter stock and turn a 1/8 diameter + stub on it. Press your part onto the stub and face it. Remove, turn the stub down to 1/8 exactly and push the part onto it with the faced side facing the chuck. Apply pressure from the tailstock to hold the part against the .75 diameter shoulder, and machine the pulley.

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  • elf
    replied
    The material will be steel. Bearings will ride on the hub. Ideally, there should be 0 runout at .75". I don't know at this point what the max acceptable runout will be. A round poly belt will be used and it should not touch the bottom of the groove, so its concentricity won't be as critical. I only need one good one, hopefully it won't take too many tries

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  • tom_d
    replied
    Does everything need to be exactly concentric, or just the ID and the 162* V profile? Rough in one side and drill a little over half way through leaving a few thou under size. Flip the part and rough the profile, then drill and finish the ID to size with a reamer. Put a piece of shafting in the ID and check how the hole runs. If it is concentric enough for your specs then finish that side along with the V profile, flip the part and finish the non critical diameters. If the ID isn't running how you would like then hold the shaft concentric and slide the piece on. Provide some tail stock support. The part can be gripped on the ID with some epoxy. Finish the diameters concentric to the ID and shaft, then heat the epoxy to remove the shaft. You may need to clean up the ID with the reamer by hand to remove any residual adhesive.

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  • JoeCB
    replied
    The bore needs to be conc with the outside of the HUB, no mention of the pulley part? In any case, do all in one set-up. Rough turn all OD--- drill and finish bore the ID ( yes a small bar)--- then come back and do a very light clean up on the critical OD surfaces -- cut off. ALSO , no mention of the material or quantity? these will affect how one processes the part

    Joe B

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  • sarge41
    replied
    I would chuck up the stock in a four jaw and rough in the outside shape along with the groove. Then drill undersize and ream the finish hole. Then fit the hole with a snug fitting pin and re-indicate the finished bore. Then finish the outside profile and it's a good job.
    Sarge41
    Last edited by sarge41; 02-25-2021, 09:48 PM. Reason: thick fingers

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  • epicfail48
    replied
    Any reason you couldnt single-point the bore then do the rest of the turning on a mandrel? Sure, need a small boring bar for that, but mirco100 bars arent prohibitively expensive

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  • Paul Alciatore
    replied
    Yes, I would avoid using a center drill for starting the bore. I would use a short spotting bit and you can use one that is larger then the bore (3/16" or 1/4") and go just deep enough to make a crater that is 1/8" in diameter or LESS. I keep a small collection of spotting bits just for things like this.



    Originally posted by MattiJ View Post
    Gun drilling would be safe bet but even ordinary twist drills work often suprisingly good when you make a good starting hole. Dont use center drills, single-point turn the starting hole instead.
    https://www.practicalmachinist.com/v...0/#post3044390

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  • Paul Alciatore
    replied
    YES! I would do the bore first. If you are worried about a drill bit going sideways, use a gun drill of a slightly smaller diameter and then a 1/8" reamer. Add a generous, 60 degree chamfer at the outer end of the bore for a tail stock center for supporting the outer end.

    Using that center in the tail stock, machine the pulley except for the rear face, working from the outer end towards the chuck holding the stock. Then part off.

    If your parting operation did not leave a nice enough face on the end of the pulley, turn it around in the chuck, face it, and chamfer the edges. That facing cut should be quite accurate even if your part is not perfectly centered. The chamfers may be a bit off center, but that will probably not matter.



    Originally posted by Mcgyver View Post
    have it stick out from the chuck and do all the turning and boring at one setting. Or swap it around, ideally with a collet. What aspect is concerning you?

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  • Mcgyver
    replied
    Originally posted by elf View Post

    Simply drilling the bore may or may not be concentric as drill bits are prone to wander.
    good point.....I've posted this so many times I'm bored of doing so lol, but its guaranteed to work. i put a 1/8 hole through 3 inches and it was within a tho out....apparently it was 11 years ago...post 18 here

    https://bbs.homeshopmachinist.net/fo...-hole-drilling

    Last edited by Mcgyver; 02-25-2021, 08:25 PM.

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  • Bented
    replied
    What is the concentricity tolerance?
    It will often look like so on the drawing.

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  • MattiJ
    replied
    Originally posted by elf View Post

    Simply drilling the bore may or may not be concentric as drill bits are prone to wander. The walls of the hub are fairly thin, so not a lot of force can be used to hold it between dead centers.
    Gun drilling would be safe bet but even ordinary twist drills work often suprisingly good when you make a good starting hole. Dont use center drills, single-point turn the starting hole instead.
    https://www.practicalmachinist.com/v...0/#post3044390

    Leave a comment:


  • elf
    replied
    Originally posted by Mcgyver View Post
    have it stick out from the chuck and do all the turning and boring at one setting. Or swap it around, ideally with a collet. What aspect is concerning you?
    Simply drilling the bore may or may not be concentric as drill bits are prone to wander. The walls of the hub are fairly thin, so not a lot of force can be used to hold it between dead centers.

    Leave a comment:


  • Mcgyver
    replied
    have it stick out from the chuck and do all the turning and boring at one setting. Or swap it around, ideally with a collet. What aspect is concerning you?

    Leave a comment:

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