# How To Make An Elongated Countersunk Hole?



## ric686 (Oct 2, 2015)

I need to make a couple of elongated countersunk holes to allow some lateral adjustment of a mounting plate. The holes are obvious but what's the right way to do the countersinking? I haven't seen any end mills with 82 degree points and I don't know that milling with a regular countersink would work or be safe. I'm mostly working with brass and aluminum, but what's the right way to do it in something like steel?


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## JimDawson (Oct 2, 2015)

Elongating with a countersink works just fine in any material they are designed to cut.  I normally use a single flute countersink for this operation.


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## coolidge (Oct 2, 2015)

Well I would think you would mill one side of the slot and then the other, I think trying to mill both would chatter with a counter sink would chatter. A CNC mill would machine the slot then zip around the outer edge with one of these. http://www.amazon.com/MariTool-Cham..._n_feature_keywords_two_browse-bin:7024453011


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## JimDawson (Oct 2, 2015)

coolidge said:


> Well I would think you would mill one side of the slot and then the other, I think trying to mill both would chatter with a counter sink would chatter. A CNC mill would machine the slot then zip around the outer edge with one of these. http://www.amazon.com/MariTool-Chamfer-Degree-Double-3500--375-82DEG/dp/B00NI0HZ0C/ref=sr_1_2?s=industrial&ie=UTF8&qid=1443804427&sr=1-2&refinements=p_n_feature_keywords_two_browse-bin:7024453011



That's why you use a single flute countersink, no chatter


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## Andre (Oct 2, 2015)

I have made a few elongated countersunk holes in my shop, for adjustable plates on a endmill TC grinder I rigged up, then promptly gave up on 

Used a 1/4" endmill to mill the slot then a 1/2" 82* 6 flute (I think) countersink to mill the slot. No fuss, worked great. Just watch out on the ends you don't do too far, or it can look bad.

I also use countersinks (both single and multiple flute) to mill chamfers. Just make sure you use 90* countersinks for chamfers!


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## coolidge (Oct 2, 2015)

Personally I would machine a square shoulder with a larger end mill vs the 82 degree counter sink and use socket head cap screws.


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## Firestopper (Oct 2, 2015)

coolidge said:


> Personally I would machine a square shoulder with a larger end mill vs the 82 degree counter sink and use socket head cap screws.



That would look a lot cleaner.


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## chips&more (Oct 2, 2015)

+1 everything said. You can grind/modify an end mill yourself. I would use a two flute for this and just give attention to one of the flutes and relieve the other. You can also buy end mills with 82° points. Also while on the subject of countersinks. If you have not tried a “Cross Hole Countersink” you’re missing out…Dave.


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## ric686 (Oct 2, 2015)

coolidge said:


> Personally I would machine a square shoulder with a larger end mill vs the 82 degree counter sink and use socket head cap screws.



Normally I'd agree, but in this case the plate is mounting to a wood guitar body so I'm using the oval head phillips sheet metal screws that the original bridge used.

Thanks for all the tips!


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## Billh50 (Oct 3, 2015)

I have used a bit similar to this one and ground the angle needed on one side and relieved the rest. Making a single flute cutter.


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