Countersink Question

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
Click to expand...
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.
 
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 .
 
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
 
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
 
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.
 
Last edited:
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.
 
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.


Sent from my iPhone using Tapatalk
 
You must log in or register to reply here.
Back
Top