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Indicating pre-drilled hole at 12 O'clock in mill vise.

cdhthethird

Plastic
Joined
Nov 26, 2020
Hello,

I have a gunsmithing project we are working on right now and I can't figure something out. I have a round part (barrel) with no flat on it and it has a single drilled and tapped screw hole at lets say 12 o'clock. I need to drill another hole inline with it six inches away. How would I go about setting that up in a mill vise so that the original already existing drilled and tapped hole is at 12 o'clock in the vise? I need both holes to be aligned as close as possible in a linear fashion to one another.

Thanks,
Chuck.
 
If you're lucky the tap shank will fit an existing endmill holder. If not turn threads on a piece of precision stock that does fit an end mill holder.

Thread this piece into the existing hole, use the endmill holder to position/line up vertically on that, release and drop the table to step off exactly 6 inches with the hand wheel.
 
or put a bolt in the hole and put an indicator on it if its a long bolt with a smooth shaft
or keep the shaft loose, put a drill chuck in and tighten down on bolt shaft once aligned in the center of vice jaws.
 
or put a bolt in the hole and put an indicator on it if its a long bolt with a smooth shaft
or keep the shaft loose, put a drill chuck in and tighten down on bolt shaft once aligned in the center of vice jaws.

A good gun bolt/screw might be better than a tap because of the tapered end of most taps.

Watch that the tap or bolt does not swing off-center/straight when it bottoms the hole.

Consider the barrel Od taper.

One can count the turns to the thread pitch to think about not bottoming out and breaking a tap.
A 10-32 travels .031 with each turn, a 10-24 travels .042.

Held stud and a sliding over tap wrench can keep a tap straight to the bore. ) a tap wrench the free travels on a straight stud.)

If you drill into the breach the bullet case may expand into the hole and then the bullet may be stuck, to repair that a screw of precision length to just make the case OD has been used but then breach is then not a safe.
 
Last edited:
You posted the same question in “Gunsmithing”. Unnecessary.

I did so I could get more than one opinion on it, I have met a lot of gunsmiths who are “machinists” and I’ve met a lot of machinists.

For everyone else I appreciate your input and I think I have enough info to tackle this.

Thanks,
Chuck
 
You did not say what size the existing threads are, but here goes. I also assume your barrel is round or at least has a symmetric curve in this area.

I would look for a screw that fits the threads. I would want one with a round or oval head: a smaller radius curve would be better and a straight slot head will allow the indicator to run alongside that slot. Turn one if necessary. I would put it in the hole with a flat washer under it to aid in seating correctly and for the vee thread to do the centering.

Mount the barrel in one or, better yet, two milling vises, shimming it level, and use the curve of the barrel and an indicator in the spindle or some other means to locate it's center line with the quill axis. Now move to the area with your screw and loosen the vises just enough to allow the barrel to rotate. Rotate it with an indicator in the spindle to get the maximum reading on the indicator and tighten the vises. That should be it if the barrel is round.

If the barrel is not completely round, you may have do the above twice to be sure. If you use two milling vises or in any case for that matter, be sure the barrel is parallel to the long axis of the mill.
 
And one can put the action in a vise, micrometer size the barrel in two places where two V blocks hold the barrel straight long ways and to the side.. Touch the barrel with a center drill shank and dial halfway across to center drill spot each hole. Having a very good long travel accurate mill one can dial the distance. with not treusting adding the part at the one existing hole and spot through the other hole.

Or screw on the device with the existing tapped hole, trave down to see the part is straight, spot drill the 2nd location with a drill that fits the hole in the part.

I don't trust a twist drill point to spot very close. A good quality never sharpened center drill spots much better.

Good to have good V blocks that can be accurately secured to a mill table. Stepped bottom ones are good because on can build then higher and hold down on two sides.

If I was just starting out I would manufacture gunsmith fixtures. I see a lot of opportunities there making simple devices that would sell. Mostly tool maker fixtures but with a little added to make then good for smithing.
 








 
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