Countersink gage

[mklotz]

There are many different countersink angles. Here is a short list of the more common ones with their included angle in degrees and their usage...

82 - Woodscrews, UNC, UNF, Robertson
90 - metric, BA, BSW, BSF
100 - aircraft fasteners

There may be others as well but those are the only ones I have encountered so far.

Exactly matching the countersink angle to the fastener doesn't matter that much for woodwork since the medium is compressible but, for machine screws, it's important.

I have a lot of difficulty telling an 82 from a 90. In the picture below we have an 82 deg woodscrew on the left and a 90 deg metric machine screw on the right. Even if your vision isn't 3/4 of a century old, you may have difficulty telling the angles apart.

I decided to make myself a gage for distinguishing 82 from 90 fasteners. A small block of aluminum scrap was squared up and V-grooves were carved down the middle two opposing sides using a drill-mill (one of the handier tools in the arsenal). Then one corner of the block was cut off at a 41 deg (half of 82) angle and the other at a 45 deg angle. The former side was stamped 82 and the latter 90 for easy identification.

This picture shows how the gage is employed. Hold the fastener against the gage using the V-groove to orient and stabilize it. Then slide it down until the conical part of the fastener hits the angled portion of the gage. If no light shows through the gage, the fastener requires a countersink of the angle stamped on that side of the gage.

I didn't bother to add a facility for identifying 100 deg fasteners since I don't encounter those often and, when I do, I can generally identify them by sight. If needs must, there are still two unused corners on the gage.

[rgsparber]

Nicely done! My kind of tool.

Thanks,

Rick

[tonyfoale]

Marv, Nice one. I love simplicity.

[DIYer]

Thanks mklotz! We've added your Countersink Gage to our Measuring and Marking category,
as well as to your builder page: mklotz's Homemade Tools. Your receipt:

Countersink Gage by mklotz

tags: measurement

[Moby Duck]

Thanks. I just hope that the Chinese manufacturers of Robertson wood screws know this, and have made them accordingly. More seriously, I don't think that I have ever seen a countersink bit marked with a specific angle and I don't remember anyone saying "change out all the countersink bits", when we changed from Imperial to Metric. You really have to wonder why a committee chose 82 degrees when they created the newer, now obsolete in most of the world, Unified system.

[mklotz]

Thanks. I just hope that the Chinese manufacturers of Robertson wood screws know this, and have made them accordingly. More seriously, I don't think that I have ever seen a countersink bit marked with a specific angle and I don't remember anyone saying "change out all the countersink bits", when we changed from Imperial to Metric. You really have to wonder why a committee chose 82 degrees when they created the newer, now obsolete in most of the world, Unified system.

I asked the question of the Canadians on the HSM forum and they all agree that Robertson is 82 deg. One of them cited the Robertson webpage and says it's confirmed there. You can read the thread here...

Question for our Canadian cousins

Not all countersinks may have the angle stamped on them but they are definitely advertised by included angle. Here are just two examples...

6 Flute Chatterless Countersinks

https://www.mscdirect.com/browse/tn/...navid=12106118

Yes, the choice of 82 degrees is just another of the idiocies of the inferial system. Like number and letter drills and a plethora of upside-down wire and sheet "standards", it typifies the Usonian reluctance to adopt sensible, world-wide standardizations. Perhaps it's all for the best. If Usonians adopted the metric system they'd have two systems they don't understand.

[rgsparber]

Why 82 degrees? Great question. No answer found via Google. I did go to my Machinery's Handbook 23 Revised Edition page 1364 and it shows that for American national Standard Slotted Binding Head and Slotted Undercut Oval Countersunk Head machine screws the included angle is 80 to 82 degrees. If this screw dropped into a countersunk hole with an included angle less than 82 degrees, it would be proud and hang up at the opening. If the countersink was exactly 82 degrees, the screw would always drop into the hole although not have perfect contact except at the 82 degree extreme of the screw. I did not find a spec for countersinks but wonder if they are 82 to 84 degrees. Could be tighter because you only buy one countersink but lots of screws so can afford more precision in the cutter.

In looking at various screw heads in MHB, I see they all have an angular tolerance of +/- 1 degree. So I am willing to accept that this is a spec that manufacturers can easily meet.

From Wikipedia, I see that the standard countersinks are 60, 82, 90, 100, 110, and 120. This just raises another question. Why not 70?

Of course, this doesn't really answer the question of why 82 degrees. Maybe it really isn't that critical and the factory tooled for 82 degrees had the most clout.

Just be glad we are not talking about plumbing!

Rick

[mklotz]

I'm surprised they're labeled with their actual angle. Given the precedent of letter drills, I would have guessed they would be labeled with the letters of the Greek or Hebrew alphabet just to keep things interesting.

Had I been on the committee, I would have voted for 83 degrees just because 83 is prime. :-) Hey, it makes as much sense as anything else in the inferial system.

[j.bickley]

mklotz, I like it. Every now and then, I would have had to countersink some aluminum piecework and go on blind faith that the countersink angle is correct. :P
---ortho

[Moby Duck]

Thanks for the links, the Canadians have got me thinking, and I note that your link to 6 Flute Chatterless Countersinks lists lots of 60 degree countersinks. I can only wonder who might be using those, but perhaps they are designed for wood workers to allow for some wood compression and reduce the chances of splitting.
I personally like the Robertson type self tapping wood screws because they stay on the driver, but I have sheared many off at the head, the very second that the screw snugs up in hardwood, and I have stripped/rounded many a square drive hole on screws that didn't shear off. Getting them out again is almost impossible. In fairness to the Robertson company the screws I have been using are branded as Surefix Square Drive and not Robertson which I have never seen here in NZ. The depth of the hole in the screw head creates a weak point where the head joins the screw, and the metal is soft. One of the links mentions using an impact driver drill for Robertson screws which results in seating the driver tip firmly in the hole. I will have to try that.