# How are closed lug holes machined?



## borge-san (Oct 16, 2016)

Hi guys, I'm new to this forum so I hope you allow a newbie question. 

I wonder how closed lug holes are machined. Through-bore lug holes are pretty straightforward, but milling a partial hole without crashing the tool into the case or the other lug strikes me as a bit of a challenge. Particularly if they are straight. If closed lug holes are made by a thin tool cutting a not-quite-but-almost straight hole, I'd love to know where I can get access to such a tool or milling bit.

The reason I'm asking is that I'm desinging a non-watch consumer electronics product where a lid will be attached using pretty much the same parts and measurements as where the strap meets case. In my design the lugs are much, much thicker than in a wrist watch so that the only way to success is to recycle the tooling used for milling closed lug holes. The part will most likely be made from aluminium.

Thanks,

Borge


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## rmcsherry (Sep 20, 2007)

Strangely, I was wondering that yesterday! I found this image, although it's not much clearer, I'll have to ask one of the machinists in my company (precision engineering, not watches) if they have any ideas.










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## Chronopolis (Apr 8, 2009)

I was wondering the same thing.
And, why do they do that? Wouldn't drilled-through lug holes be much easier to make?


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## Ard (Jul 21, 2014)

I'll be waiting with baited breath to know myself, I had never thought much of it until I got hold of a watch with through & through holes. The strap change is sooo easy with drilled lugs and I have no problem with the look.


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## rmcsherry (Sep 20, 2007)

Angled head milling cutters do the same thing, so perhaps it is just a specialist micro version of this tool? Eagerly awaiting an experts input! 

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## borge-san (Oct 16, 2016)

Thanks guys, I'm glad I'm not the only guy wondering about this! 

Personally, I think closed lugs look more elegant. But in my particular application a through bore would have to become >5cm long which is very impractical. 

The tool in post #2 is something I haven't seen before, but something I predicted would exist. I'd love to know more about what is available and how it is properly named so that I can qualify machine shops with this thing available. 

Cheers,
Borge


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## MechaMind (Mar 23, 2014)

You could EDM the holes - as the tool is not necessarily rotating the "drill" could be a simple double kinked copper tube


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## Medusa (Feb 6, 2010)

I think in at least some cases they are drilled at an angle and not at exactly 90 degrees like many of us thought. After doing a little research I came up with this picture that show the lugs being drilled at an angle.










Then I grabbed my Seiko and fitted a precision screwdriver into the lug hole. It does seem that the lug holes are drilled at a slight angle to clear the opposite lug.










There are probably several ways it is done including but not limited to.
*Special 90 degree lug drilling machine ( as pictured in previous post ) 
*Drilling the lugs at a slight angle
*Drilling straight through and then weld/fill and finish the outside of the case
*Modular designs in where the pre-drilled lugs are attached to the case.


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## gangrel (Jun 25, 2015)

Drill thru then back-fill might happen, but that seems like the HARDEST approach. You'd have to ensure the plug didn't go too deep, you'd probably want to do some finishing on the inside as well, and you have to match the exterior finishes. 

rmcsherry's pic looks like the simplest approach for fairly high-volume production. Something like a dentist's drill would be another approach.


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## MechaMind (Mar 23, 2014)

Someting like this would work,- as long as the case material is conductive

http://www.guethner-werkzeugbau.de/images/maschinenpark/oct3525za-b.jpg


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