# Countersink Question



## tinnerjohn (Apr 13, 2016)

Im the back of mind I seem to recall reading or being told that a for best results you should lightly countersink before tapping a hole. I'm making a list of small tooling I need to order and found at least three angles of countersinks. Is there a preferred or recommended angle? I wish I could remember everything I've read, seen and been taught over the years, but fortunately there are forums like this. John


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## JimDawson (Apr 13, 2016)

82° is the most common, and is the standard for countersinks for Imperial screws, 90° is the standard for metric screws.  Technically, I suppose a 60° would be the correct one to use for a tapped hole because threads are 60°, but I don't own any.

Either would work fine for countersinking a hole before tapping, and yes is does help and makes for a more professional looking job.


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## Andre (Apr 13, 2016)

I notice it helps a little to help the tap grab, but not enough help to justify the extra setup time. Most of the time your countersink angle isn't on the threaded portion anyway.


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## tinnerjohn (Apr 13, 2016)

Thanks Jim. I was thinking 82, but wasn't sure. Its been awhile since I tapped a hole, I've chased  a lot of them over the years when rebuilding or repairing stuff. I have some projects in mind that are going to need threads, so I'm trying to tool back up. John


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## tinnerjohn (Apr 13, 2016)

One thing I remember (I think), is that it makes starting the screw or bolt a lot easier. John


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## TOOLMASTER (Apr 13, 2016)

tapped thousands of holes. if you counter first it leave a little cleaner start thread...but no biggy to do it after either


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## kenscabs (Apr 13, 2016)

82 is the countersink for a flat head screw.  For threading use a 90 and only drill just a bit wider than the major diameter of the thread.


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## Bluedog (Apr 14, 2016)

I think a countersink also going to keep the tap from raising some material around the hole which would be good for parts fitting flush together.


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## neshkoro (Apr 14, 2016)

If you don't want to buy too many tools an 82 degree countersink works well and then it also can be used for countersinking flat head screws.
I've been in the business for 50 years and it was common practice to countersink all threaded holes before tapping. The countersink diameter should be just slightly larger than the diameter of the tap. The countersink should be done before tapping as most of the time the tap will throw up a burr. If you tap first and then countersink you take the chance of pushing that burr into the threads. If the part gets heat treated you might have issues starting the screw as the burr might be in the way. Just my opinion.

Bill


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## JimDawson (Apr 14, 2016)

neshkoro said:


> If you don't want to buy too many tools an 82 degree countersink works well and then it also can be used for countersinking flat head screws.
> I've been in the business for 50 years and it was common practice to countersink all threaded holes before tapping. The countersink diameter should be just slightly larger than the diameter of the tap. The countersink should be done before tapping as most of the time the tap will throw up a burr. If you tap first and then countersink you take the chance of pushing that burr into the threads. If the part gets heat treated you might have issues starting the screw as the burr might be in the way. Just my opinion.
> 
> Bill



:+1:


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## neshkoro (Apr 14, 2016)

Why was my reply copied by Jim?


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## JimDawson (Apr 14, 2016)

Actually it was ''quoted'' by me.  I'm agreeing with you, note the :+1: below.


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## neshkoro (Apr 14, 2016)

Got it. Thanks. Now I understand how that works!


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## Billh50 (Apr 14, 2016)

neshkoro said:


> If you don't want to buy too many tools an 82 degree countersink works well and then it also can be used for countersinking flat head screws.
> I've been in the business for 50 years and it was common practice to countersink all threaded holes before tapping. The countersink diameter should be just slightly larger than the diameter of the tap. The countersink should be done before tapping as most of the time the tap will throw up a burr. If you tap first and then countersink you take the chance of pushing that burr into the threads. If the part gets heat treated you might have issues starting the screw as the burr might be in the way. Just my opinion.
> 
> Bill





Bill is quite right about tapping after counter sinking. The tap will not only push a burr up but can also raise some material right around the hole. Counter sinking slightly larger than the outside diameter of the tap first will lessen the raise spot or eliminate it all together depending on the material being tapped. It is always a good idea to run a file or stone over the hole to make sure there is no raised area.


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## TOOLMASTER (Apr 14, 2016)

it's fairly simple once you get the hang of it..


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## ChipsAlloy! (Apr 14, 2016)

Bill explained it exactly the way I see it!  When a guy at work brings me a piece he made with threaded holes that have not been countersunk ( slightly) first I can tell cause it doesnt have that "finished" looks to it


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## neshkoro (Apr 14, 2016)

I used to work for a Tool and Die shop that did a lot of work for a high end electronics company building does for them. They demanded that all the die components  that. had screw or dowel pin holes be counter sunk. The screw holes also had to have the threaded holes counter drilled 1/64" larger than the major diameter Han the major diameter of the thread.  The counter bore would be about 1/8" deep. A lot of work but they wanted quality. 


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## Billh50 (Apr 14, 2016)

neshkoro,

Many places did that so the first thread would not break out if a bolt was over tightened. Or at least that was the excuse I kept getting for years of machining.


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## neshkoro (Apr 14, 2016)

That's the reason. 


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## TOOLMASTER (Apr 14, 2016)

i use my old aircraft countersink a lot...the old offset design spinner...


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## John Hasler (Apr 14, 2016)

neshkoro said:


> I used to work for a Tool and Die shop that did a lot of work for a high end electronics company building does for them. They demanded that all the die components  that. had screw or dowel pin holes be counter sunk. The screw holes also had to have the threaded holes counter drilled 1/64" larger than the major diameter Han the major diameter of the thread.  The counter bore would be about 1/8" deep. A lot of work but they wanted quality.
> 
> 
> Sent from my iPhone using Tapatalk


When the section being clamped is thin compared to the bolt diameter I want the hole counterbored so that some of the bolt is free to stretch.


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## RIMSPOKE (Apr 14, 2016)

THE REASON TO USE AN 82° COUNTERSINK ON ENGLISH THREADS & 90° FOR METRIC 
IS THAT THOSE ANGLES MATCH THE HEADS ON E Vs M FLATHEAD SCREWS .  

I ALWAYS LIKED THE SIMPLICITY OF THE MATH FOR THE METRIC . BECAUSE OF THE 90° 
ANGLE , THE DIAMETER OF THE COUNTERSINK WILL BE TWICE THE DIAMETER OF THE THREAD . 
( 5mm FLATHEAD USES A 10mm COUNTERSINK . 8mm SCREWS USE A 16mm & SO ON )

UNLESS THE SCREW HEAD WILL SINK BELOW THE SURFACE OF THE TAPPED PART , IT DOESN'T MATTER . 

AS FOR COUNTERSINKING THE HOLE BEFOREHAND , THAT JUST PREVENTS 
A BURR STICKING UP WHERE THE THREAD STARTS , & IT LOOKS NICE .


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## astroracer (Apr 15, 2016)

Doing Product and Fixture design at GM all of the threaded holes are c'sunk 90* with the same diameter as the major diameter of the thread. Whether SAE or Metric, the same note is used.
 At home I c'sink the same way. After drilling with the tap size drill, I use a regular drill, sized to the major diameter of the thread (the bolt size), and c'sink just enough to see the edge of the drill come to size on the surface of the material. Not worrying to much about "exactness" or varying depth of cut between holes. The "chamfer" helps in starting the tap and keeping the tap from raising a burr as it works.
Mark


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## tinnerjohn (Apr 15, 2016)

Thanks for all the info. I've noticed that most of the engines and old tools I've worked on looked like they had been countersunk, and it seems the older I get the more I want to "make it look like someone who knew something did it" to quote my Dad. John


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## Wreck™Wreck (Apr 15, 2016)

If you want to save time by not countersinking as a second operation start the hole with a spotting drill which have a 90° included angle, spot larger then the major diameter then drill and tap. This does 2 jobs at the same time, accurately starts the hole and deburrs in one operation.

Spotting drills are not center drills, spotting drills are used for spotting accurate positions for the twist drill to follow, center drills are used for forming lathe centers.


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## Ebel440 (Apr 22, 2016)

I agree with wreck the best way is spot the hole first for accuracy and appearance. Occasionally for the best fit with holes for flat heads you may also need to recountersink after tapping to remove a small burr at the bottom of the countersink caused by the tap.


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## neshkoro (Apr 24, 2016)

Why would you want to put a flat head screw Ina threaded hole without something in between the screw and the piece with the threaded hole? I can't think of an occasion to do that. 


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## T Bredehoft (Apr 24, 2016)

TOOLMASTER said:


> it's fairly simple once you get the hang of it..



The tap is the part with the water coming from it.


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