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Pivot Drill Size and hole for Repivot

bobolink
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bobolink Watch Enthusiast

bobolink

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I am doing my first repivot and have a question about drill sizing.  I understand that the drill / hole needs to be .02 - .03 larger than the pivot wire that will go into it.  My question is about the following.  I have an arbor that is .50 and an old pivot that is a comfortable 15 in the pivot gauge.  This tells me I should drill it to .17-.18.  The dilemma is finding a drill that exact size.  All the drills I see seem to be either .15 and smaller or .20 and larger except one that I found that turns out to be .1524mm (.006").

Any thoughts out there on this?  Is the recommendation of .02-.03 oversize correct? I guess I could use this .1524 drill and burnish end of the pivot wire down by .01 or .02 to allow it a better fit.

Thanks,

bob

nevenbekriev Distinguished Member

nevenbekriev

Posted
Posted (edited)

The things are different in reality. And everyone finds his own way in making things to suit his own filling of 'good'.

I will recommend to drill with what drill bit You have. Of course, it must be at least 0.15, as the hole of the bearing is this size. Then, 0.2 will be good, but 0.3 will be problematic to drill in 0.5 and press a pivot in, as the material can easyly crack. Then, when the hole is drilled, You will choose the pivot wire that will be correct for the hole. This usually means that the wire wil not go in, and reducing the diameter (slghtly tapered) will be needed to make it go in tightly.Then I will recommend to use Henkel Loctite 638 'just in case', and wait 1 day or heat the wheel to let loctite harden. In the end, the new pivot should be reduced in diam. to fit the hole and in the same time - to make it centric if the hole was no complitelly centered.

The main problem here is drilling the hole. Usually when drilled to some depth, the drill bit begins to stuck in the hole and rotates harder and harder, then the tip of it breaks and thus ruins all the pinion making it useless. The decision I found is to sharpen the drill bit not symetrically - the 2 cutting edges on the tip to be not equal. Thus the hole is drilled a little bigger than the drill bit diam. The depth of the hole is not needed to be more than 3X hole diam. As You see, I don't use exact sizes of hole or pivot wire and I don't care of it, and don't even have what to measure with...

Now, put this repeater aside for 2-3 years and practice on another movements. You will break probably at least 50 drill bits before get learned how to drill and repivot. Practice is needed... Till then You will learn how to assemble without breaking pivots too.

Edited by nevenbekriev
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Neverenoughwatches Distinguished Member

Neverenoughwatches

Posted
Posted
5 hours ago, bobolink said:

I am doing my first repivot and have a question about drill sizing.  I understand that the drill / hole needs to be .02 - .03 larger than the pivot wire that will go into it.  My question is about the following.  I have an arbor that is .50 and an old pivot that is a comfortable 15 in the pivot gauge.  This tells me I should drill it to .17-.18.  The dilemma is finding a drill that exact size.  All the drills I see seem to be either .15 and smaller or .20 and larger except one that I found that turns out to be .1524mm (.006").

Any thoughts out there on this?  Is the recommendation of .02-.03 oversize correct? I guess I could use this .1524 drill and burnish end of the pivot wire down by .01 or .02 to allow it a better fit.

Thanks,

bob

Hi bob, make the pivot wire to suit the hole not the hole to suit the wire, once fitted the pivot can be burnished down to size. In the case of a balance staff you would want enough on the wire to blend it into the staff cone. 

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bobolink Watch Enthusiast

bobolink

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Posted

Thanks Neven.  It’s not the repeater. I would not dare.  It is a movement I’ve had in a drawer.  The  repeater is resting. I will buy plenty of extra bits. 

Thanks Neverenough.
 

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Neverenoughwatches Distinguished Member

Neverenoughwatches

Posted
Posted
On 2/26/2024 at 3:47 PM, nevenbekriev said:

OK, have to say that I ment the chinese tungsten carbide bits that I use, which are pritty brittle and easy to break.

I can add to that, i haven't managed to drill any amount of depth with one yet without one chipping or snapping in half. Some of them are extremely brittle, practice make perfect.

nickelsilver Distinguished Member

nickelsilver

Posted
Posted

I always use tungsten carbide, but 99% of the time I make the drill. Simple spade drill, with slight back-taper along its length. Being free-hand ground, the lips are never exactly the same, so it tends to drill slightly oversize, and that along with the back taper means in the event of a break it just falls out of the hole. They are also less likely to break than spiral drill, in my experience at least. Spiral drills are generally ground with an extremely slight back taper, but in reality when drilling it's like the whole drill is in contact with the hole.

 

HSS can take quite a bit of misalignment and abuse; but I have never gotten one to drill properly hardened arbors in watches or clocks. Carbide is brittle and fragile, but seems happy drilling hardened and tempered steel.

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nickelsilver Distinguished Member

nickelsilver

Posted
Posted

I usually do it with drill in a pin vice, hand held, but I do put a rod in the tailstock that is a very loose fit in the backside of the pinvice. This keeps it more-or-less lined up.

 

The main reason I do it like this is for very small diameters I freehand grind the drill, so not only are the lips not exactly equal (I do try to get them equal), the drill isn't really centered on the shank. Holding it in a well aligned tailstock just wouldn't work. The upside is you do have a terrific "feel" of what's going on, and also if the drill suddenly hangs up the pinvice can spin in my fingers hopefully saving the drill.

 

For diameters over about 0.40mm I can make the drill on my tool grinder, and these end up well centered and can be used like normal drills, either in a live spindle like a drill press or jig borer, or in the tailstock of the lathe.

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