Countersink a hole to a specific dia. help

[jlsmithseven]

Hi guys,

Something I've struggled with since last year has returned. I use it at work all of the time, but for some reason the formula doesn't want to stick with me.

Say I drill a hole with a 3/8" drill and the print calls out for a .425 diameter hole chamfer on it. I understand I need to use a 1/2" countersink, but I struggle with grasping how deep I need to go on the hole.

I also understand how 82,90, 118 included angles all make a difference in the calculation. I know that 90 degrees is the easiest to figure out because you divide it by 2, but then I get lost again. I've had many people explain it to me, but I was wondering if any of you had a quick and easy method to make it stick. I appreciate your help!

82 and 90 deg. are about 99% of what I do so if you could explain both of those it'd be awesome.

 

[higgite]

If you are near a computer when you need the info, or if you have a smart phone with a browser, try this:
http://janproducts.com/CounterSink_Calculator.html

Tom

 

[jlsmithseven]

I have seen that website, but it doesn't help me much.

 

[woodchucker]

I have seen that website, but it doesn't help me much.

The calc Seems to work for me.
.425 82degree = .2445 depth.

edit, it appears to be correct. I tested using a 118, then realized my mistake.
just checked with an 82 degree chamfer bit.

 

[RJSakowski]

The depth of the countersink, as measured from the tip of the countersink is x = d/2*cot( A/2) = (d/2)/tan(A/2) where A is the angle of the countersink and d is the diameter of the countersink. Scientific calculators are available on most, if not all, computer operating systems and on Android systems. For an 82º countersink, cot(82/2)= cot(41) =1.1504. For a 90º countersink, cot(90/2) = cot(45) =1.

This assumes a sharp point on the countersink.

 

[Wreck™Wreck]

Bloody hell pull the other one, a 90 Deg. chamfer is 45 per side or the diagonal of a square, one does not require trig to figure this out, other angles are no more difficult. Measuring such a feature however is a different matter, the math is simple and straightforward. In practice your machine will have errors. A countersink gauge is one method yet I suspect that you will not likely want to spend $300.00 for one tool with a specific purpose. I could be wrong.
https://www.mscdirect.com/product/details/86445178

 

[4GSR]

When ever I need to countersink a hole for a flat head screw, I use the screw for a gage.
I've never in my life has a specification for a countersunk hole to be held to a specific diameter any tighter than +/-.03". Which most of the time, I eyeball and move on to the next hole. And if someone gives me flack over it, I'll pull out my cordless drill, put a countersink in the drill, give it to them and say here you go, fix it your way!

 

[markba633csi]

I got the same result with the online calculator: 82 deg and .425 = .2445 depth
But I always use a screw as a gauge. Too lazy to do the math.
Mark

 

[T Bredehoft]

Use a screw, but you'll quickly find out the screw head isn't the diameter of the c;sunk cavity. Ya gotta go deeper.

 

[Franko]

I would probably put a scrap piece in and nibble at it until it was correct, using the bolt head as a guage. Record how deep the plunge was and use that on the finished piece. I would zero the plunge from where the countersink makes contact with the starting hole.