ISO20/NMTB tool holder taper angle?!?!

OK, I give up and hope y'all have a better answer(s) than I got. I am building a tightening fixture for my various iso20 tools and want to have the guy cut a taper in the base plate with the same angle as the tool holder. According to Machinery's Handbook and every other legit source, the taper is 3.5 inches per foot, and known as 7/24 for 7 inches taper in 24. Unfortunately, when I trig out that triangle, I come up with anything from 16.26 to 18.43, both of which are too steep. Is it maybe half that angle for some reason? I used MH formulas and Engineer's Edge online calculator and got diff results. I figure the rate should be .291667 per inch based on 3.5 inches per foot. Thanks for any help!

3.5:12 aka 7:24 taper on diameter has a 16.2602047083 degree (16 degree 15 minute 36.7 second) included angle. The angle per side is half the included angle. So your first number was correct.

Are you sure you are dealing with ISO20 ? IIRC, Moore jig borers used a taper that looks like a miniature NT/NMTB/ISO taper, but it has a slightly smaller included angle (15 degree 53 minute 52 second according to one source).

sfriedberg said:

3.5:12 aka 7:24 taper on diameter has a 16.2602047083 degree (16 degree 15 minute 36.7 second) included angle. The angle per side is half the included angle. So your first number was correct.

Are you sure you are dealing with ISO20 ? IIRC, Moore jig borers used a taper that looks like a miniature NT/NMTB/ISO taper, but it has a slightly smaller included angle (15 degree 53 minute 52 second according to one source).

Click to expand...

Thank you sfriedberg.
Finally got a CAD dwg. from the manufacturer Techniks and it shows 8deg. 17min. 50sec. or 8.29722deg., so about half of the included angle we both came up with, but not exactly. Why? I don't know, but if that is what they build to that is what I will build to. Thanks for the reply.

I used to worry over the correct taper until I realized it doesn't matter. Bore a hole the size of the large end. That keeps it centered. It's the keys that do the work.

Quite true Herr Kopf, and that was the original design, but when I ran into such a number of discrepancies in the stated taper I decided to nail it down and cut the taper just for grins. Thanks for the input.

ISO 20 NMTB tool holder angle???

It is infact 3.500 inches per foot... However that is the included angle.....
Therefore, if you were to draw a picture of the angle on a piece of paper that was 12 inches long and zero on one end and then 3.50 inches on the other end and it was centered over the centerline , you would have a picture of the taper angle.

For ease of set up, I set up a sine bar along Z axis so it is Zero at one end and 6 inches away the angle provides a zero that is 6 inches away from your first zero on Z axis and now here you go.....
3.500 divided by 2 equals 1.75 inches. That would be the proper dimension on X axis in 12 inches. Then take 1.7500 divided by 2 equals .8750. Therefore Set a zero on the indicator then set a zero on X and Z axies. Move the Z axis 6.0000 inches and then move X axis .8750 and the indicator should again be at zero.

That represents the angle you need to cut to establish the proper angle provided you are cutting on the centerline of the piece part.

If your sine bar is not the full 6 inches, you can take the .8750 and divide it by 6 to get .8750 divided by 6 equals .1458333. That is the distance the angle is in 1 inch. .1458333 times 5 equals .729165 which equals the distance you need to move X axis to find your zero in 5 inches along Z Axis.

If the part is spinning, you are only cutting on one side.....

Thats my story and I'm sticking wiith it.
Regards Walt.....

Or you could save a whole lot of time and money and just use a 3/8 torque wrench and the tools i have in the attached picture. My dad always told me "KISS" Keep It Simple Stupid!

I hear ya on the KISS idea and have the same pile of wrenches and a 3/8 torque wrench, BUT, these tools are wicked small and I have arthritis and tendinitis that makes my mitts do crazy crap so the more stable the better with the pricey cutters and dodgy work surface. Thanks for the input, have a great day.

Can't have a shite work surface. That will never do. I'm sure you could pick up something manageable for a couple crisp 100's. Have a good weekend.