How To Make An Elongated Countersunk Hole?

[ric686]

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?

 

[JimDawson]

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.

 

[coolidge]

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

 

[JimDawson]

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

 

[Andre]

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!

 

[coolidge]

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

 

[Firestopper]

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.

 

[chips&more]

+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.

 

[ric686]

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!

 

[Billh50]

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.