Thread: When a taper is not a taper

I'll put my hand up as well. I spent hours looking for ISO specs that were different.

Phil

I am the one responsible for causing this confusion. When I bought my mill my concerns were raised when I investigated the availability of ISO30 tooling with an angle of 8 degrees 17 minutes 50 seconds as shown in the cross sectional spindle drawing contained within the service manual.

A Wikipedia search revealed the NMTB angle to be 16.7112 included. And therein lies the .... up. Being mathmatically inept added to the error. An error pointed out by Forrest Addy on the Home Shop Machinist forum

Source for a 16 deg included angle taper reamer? [Archive] - The Home Shop Machinist & Machinist's Workshop Magazine's BBS

Forrest Addy
12-30-2010, 05:25 PM
Are you sure of the angle? I don't know what the standard says but often 7/24 (3 1/2" per ft) total taper is the default figure for machine steep (self-releasing) tapers. This is 8 degrees 17 min 50 sec on the half angle or 16 fegrees 35 min 40 sec total included.

Parenthetically the R8 taper is fairly close but different from MMT 30 taper. They will not seat properly nor can they be used to gage each other.

Incidentally the Wiki entry quoted below from this link is incorrect (observe the taper angle citation marked **)

The taper is variously referred to as NMTB, NMT or NT. Essentially this defines a taper of 3.500 inches per foot or **16.7112 degrees.** All NMTB Tooling has this taper but the tooling comes in different sizes. NMTB-25, 30, 35, 40, 45, 50 and 60. With the 40 taper being the most common by far."

It was this (incorrect) angle that caused me to query the compatability of the ISO and NMTB tapers.

This was before I found out that the information on Wikipedia must be taken with a grain of salt.

BT the author of this time wasting exercise. Sorry Fellas

Guys, it's a learning experience.
We now know first hand that web based information can sometimes be wrong (although not it seems in this case); it's important to ask the right question; that the order of operations in maths is vital and lastly that Bob may have some tooling made by someone who made the same mistake.

On the plus side, we have now confirmed that a 30, 40 and other tapers are the same regardless and can be used without fear of damaging the spindle.

Over all a good outcome (after all, apart from a little embarrassment no one was injured while working on this thread)

Michael

Originally Posted by Anorak Bob

I am the one responsible for causing this confusion.

Oh so it's Bob's Fault! That's alright then.

Originally Posted by Michael G

Over all a good outcome (after all, apart from a little embarrassment no one was injured while working on this thread)

Michael

Thats all right for you to say, I just come back from chasing my scrap man down all over town after throwing all my NMTB out because you said they where different. LOL

No seriously though thanks for your time researching, at least now we all know for sure they are interchangeable., I don't own any NMTB so I have never tried them myself, but from what I have read I have never seen where someone said they didn't fit, and everyone has always said they where all interchangeable other than the draw bar thread.

I was going to buy some cheap NMTB years ago but held off because of having to change the draw bar. I am also going to CNC my mill one day and have a tool changer so that wouldn't work real good.

Dave

My efficient Dutch co-worker, Martine prepared this little drawing clarifying the confusion. I can sleep easy now.

BT

My mill is an earlier version of Bob's. It came with the standard ISO drawbar in the horizontal spindle and an imperial drawbar and tooling for the vertical head. I have never any problems with loose or tight spindle fittings. I keep a piece of copper bar 20 mm dia x 50 long near the mill and a gentle tap on the loosened drawbar with this has always released the the fitting.
Mm.