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Tony Ennis
07-08-2010, 09:03 PM
Which countersink 'included angle' is appropriate for machinists' screws?

I seem to have a choice of 60, 82, 90, and 100 degrees.

Regnar
07-08-2010, 09:17 PM
Stolen from Wikipedia.


Countersink cutters are manufactured with six common angles, which are 60, 82, 90, 100, 110, or 120, with the two most common of those being 82 and 90. Countersunk-head screws that follow the Unified Thread Standard very often have an 82 angle, and screws that follow the ISO standard very often have a 90 angle. Throughout the aerospace industry, countersunk fasteners typically have an angle of 100. The ideal countersink angle for holes tapped with 60 threads, when no countersunk fastener head will sit in the countersunk area, is often 60; but often another angle is used if that is the cutter that is at hand, and the difference usually doesn't matter.

lynnl
07-09-2010, 10:47 AM
Good Grief! After reading that, you're left with the same questions you started with. :rolleyes:

I was looking through a Lee Valley catalog at some countersink stuff, and they made the comment that, while
82deg is most common here in Ntha Amer., much of the European is 90deg. Of course this was dealing with
screws, hinges, etc. primarily for wood.

Just off the top of my head, I can't say as I've ever noticed a difference between wood and metal screws that
I've known. So, I'd guess at 82.

Toolguy
07-09-2010, 11:22 AM
Inch screws - 82*, Metric screws - 90*.

Evan
07-09-2010, 01:54 PM
The angle matters a great deal for flush socket head cap screws. It is the friction between the head and the counter sink that makes the screws self locking. Since there is no other way to lock a flush screw mechanically in a blind threaded hole this is very important. Also, the OD of the countersink tool should match the head of the flush SHCS. In order to seat the screw truly flush the countersink is intended to slightly counterbore the hole. If you use a countersink that is too large for the screw you will have to make a very ugly countersink that is much wider than the screw head.

garagemark
07-09-2010, 02:08 PM
For 99.9% of what most of us U.S. garage dwellers do, 82 degree is sufficient (as well as correct). And since it is usually impractical to have a sink for every size head, I keep three diameter sizes- small, medium, and large. Again, they fit 99% of the head diameters that I work with routinely.

Evan
07-09-2010, 03:58 PM
I failed to mention that the self locking tendency can be extreme in aluminum. It is possible to torque down a well fitted flush screw tight enough that it cannot be removed. I know this for a fact. :rolleyes:

Mike Folks
07-09-2010, 05:56 PM
In Aerospace work due to the thiness of the material, usually 100 degree is used for countersinking.

For most other comercial work 82 degreee is used(at least here in the U.S.).

rohart
07-09-2010, 06:56 PM
My metric screws in the UK are 90 degrees.

Don't buy three bladed countersinks. They easily chatter to a four-sided hole. Countersinks with seven blades chatter less, and the chatter isn't so bad when they do go.

Maybe they'd be alright in a tight mill, but in a common or garden drill-press I find one in five holes get chattering. Holding the workpiece firmly and applying quite a lot of pressure, lubricated, seems the best way of avoiding it, but when it starts there's usually no way back.

CCWKen
07-09-2010, 10:17 PM
Don't buy three bladed countersinks. They easily chatter to a four-sided hole. Countersinks with seven blades chatter less, and the chatter isn't so bad when they do go.

Maybe they'd be alright in a tight mill, but in a common or garden drill-press I find one in five holes get chattering. Holding the workpiece firmly and applying quite a lot of pressure, lubricated, seems the best way of avoiding it, but when it starts there's usually no way back.
Use the lowest speed and make sure you're centered. Having to use a lot of pressure makes me think your bits are dull causing the chatter. Mine will go clean through 3/8" plate in a snap with little more than hand-rest pressure.

dp
07-09-2010, 11:07 PM
I had a request to provide some brass screws for an antique door on a bar down in California. Not much in the way of specifications except slotted oval head, 1/4"x20 thread. I went with 80. They fit perfectly.

http://metalworkingathome.com/?p=40

Tony Ennis
07-10-2010, 12:01 AM
DP, from your link, "Burnishing with a piece of HSS tool blank"

Was the lathe turning when you did that?

dp
07-10-2010, 12:07 AM
DP, from your link, "Burnishing with a piece of HSS tool blank"

Was the lathe turning when you did that?

It was - at about 800 rpm. but I stopped it to take the photo so I wouldn't get hit in the forehead with camera parts.

Evan
07-10-2010, 01:02 AM
Mine will go clean through 3/8" plate in a snap with little more than hand-rest pressure.


Yep, a 5/8 countersink makes a better drill bit than a 5/8 drill bit in aluminum. I use multi flute countersinks, six flute mostly. The secret to no chatter really doesn't have much to do with the type of countersink. It's all about eliminating workpiece movement and using a firm positive cut with some kind of lubricoolant. Of course if your quill has six degrees of freedom it isn't going to do a very good job.

Paul Alciatore
07-10-2010, 01:58 AM
Yep, a 5/8 countersink makes a better drill bit than a 5/8 drill bit in aluminum. I use multi flute countersinks, six flute mostly. The secret to no chatter really doesn't have much to do with the type of countersink. It's all about eliminating workpiece movement and using a firm positive cut with some kind of lubricoolant. Of course if your quill has six degrees of freedom it isn't going to do a very good job.

Not true. The O Flute countersinks are practically chatter free. I won't buy or use anything else. But get good ones, the cheap imports are not properly ground and they are hard to feed.

dp
07-10-2010, 02:51 AM
Not true. The O Flute countersinks are practically chatter free. I won't buy or use anything else. But get good ones, the cheap imports are not properly ground and they are hard to feed.

I have one of those I got at Boeing Surplus when it was around and it works best on my noodle mill at less than 20 RPM. But it works perfectly. And fast!

Evan
07-10-2010, 06:57 AM
The only difference I have seen with those is that they cut too slow. The advice to buy good ones applies to all edged tooling. You can't afford to scrimp on cutting tools. Subtractive machining depends entirely on the quality of the cutters. No matter how good the rest of the equipment is if the cutter is poor then that will be directly evident in the work. This especially applies to operations where the rough cut and the finish cut are the same cut.