Thread: When a taper is not a taper

Hi Michael,
Can't help on the ISO as yet but Machinery's Handbook 28 page 939 say "Tolerance on rate of taper to be 0.001 inch per foot applied only in the direction which decreases rate of taper"

Stuart

Michael, in my limited experience of making spindle adaptors for lathes, it doesn't take much of an error to cause problems. If you were going the other way you might be able to grind it or get it done. And I love shimming but it sounds like a headache in this case. I think I would consider selling your NMTB stuff on and getting the right shanks.

Hi Bryan,
Where did 8.29722 degrees come from?
Fitting and machining says ISO-297 is a copy of the NMTB.
Wikki says "The taper is variously referred to as NMTB, NMT or NT. Essentially this defines a taper of 3.500 inches per foot or 16.5943 degrees[2] (also referred to as "7 in 24" or 7/24). All NMTB Tooling has this taper but the tooling comes in different sizes. NMTB-10, 15, 20, 25, 30, 35, 40, 45, 50 and 60. With the 40 taper being the most common by far.
CAT, V Flange,SK, ISO (also known as INT, Inter or International) and BT tooling use this same taper: the difference is in the flanges and pull studs (a male extension from the drawbar thread, used in CNC machines with toolchangers and/or power drawbars"

"2. ^ The angle of the cone is 2 atan(7/48)."

Machine taper - Wikipedia, the free encyclopedia

But my math says that 16.5943 is wrong?

Can't find a copy of iso297 that I dont have to pay for

Styart

Stuart, the 8.29722 degrees was a number that I put up. Here I must admit to an assumption - I haven't been able to find a kosher copy of the ISO standard but I did find a copy of a Schaublin page that has DIN 2080 plastered on it. The assumption is of course that when the ISO standard was created it was lifted pretty much complete from the DIN standard (as quite a number of standards were). After I posted my question, I did wonder whether my assumption was valid - another good reason to find a copy of the standard.

The Wikipedia article seems to be lifted from a series of pages from Tools-n-Gizmos: New and Used Surplus Machine Tooling site. One of the ways he gets his specifications seems to be measuring physical pieces, so while overall it's a useful site the difference between the two angles is so slight that The difference may have been put down to measurement error.
I could get my NMTB holders ground if I was really concerned with the issue - at the moment it's question in my mind. With the difference being so small one of the issues is identifying which is which. On one of those spare days that I have I may have to try bluing the socket and seeing how much contact I get.

Attachment 197180

Michael

Hi Michael
I don't have any NMTB taper holders here but have been lead to believe they are the same as NT30 that I have. I went though looking a few years back and everything I found said they where interchangeable other than the draw bar or the flange on some types like the CAT etc.
Have a look at this page,
Machine Tool Shanks (Tapers)

Dave

Dave, that's the page that I think Wikipedia have based their information on. I'm not 100% confident it is correct. I'd really like to see a page or two from the ISO standard before I can agree that the taper is the same. One of the problems with the Web is that misinformation can be picked up so easily. It may be that the Schaublin page is wrong, but being a published catalogue from a reputable maker I have greater faith in that.
NT is the same as NMTB as far as I know.

Michael

Hi,
It has been discussed over on Practical machinist a lot of times and they all say it's the same other than the flanges like I spoke about above.

I see my mill also getting advertised with a ISO30 taper as well as the NT 30. I know the BT30 and NT30 are the same because all the Chinese sellers confirmed this, but you need a longer draw bar because they are meant for pull studs on a CNC mill.

I will be interested in you findings, because I have always believed they are the same.

Dave

I'm glad you started this thread Michael. Schaublin indicate in their publications that the ISO 30 taper angle is 8 degrees 17 minutes 50 seconds. They also state that it is a NS 1 1/4" - VSM 33931, size 32 - DIN 2079, size 30 taper just to bamboozle us all. Included in their sales pitch promoting the attributes of the 1961 No. 13 mill is the following - "The steep-angle taper bore has the great advantage that the tools can be released without hammering."

Well that may be the case where only ISO 30 tooling has been used in that bore but all my arbors require the application of a blow with a hide mallet on the draw bar to release them. Some a heavier blow than others. The mill is 50 years old, wear and tear would have taken it's toll on the spindle bore and the insertion of non native tapers would not have helped.

Phillip "Metalman" has the same mill. It would be interesting to find out if his tapers are a mixture and if so, how easily they are removed.

BT

Hi Bob,
Even mine from new needs a blow with the 24mm spanner to release it after undoing the draw bar.

Dave

The only difference is the draw bar thread...I have got NTxx ISOxx holders with metric course threads, metric fine threads, whitworth threads and Unified threads.. They are all the same except for the threads..

Originally Posted by .RC.

The only difference is the draw bar thread...I have got NTxx ISOxx holders with metric course threads, metric fine threads, whitworth threads and Unified threads.. They are all the same except for the threads..

Yes, same here. My horizontal mill is a 40 taper and I've a pile of oddball 40 taper tooling, some came with draw studs, some with inch threads, some with metric threads. Taper is the same for all practical purposes (meaning that, if it's not the same, it's so close to it that for my purposes, it is the same).

PDW

The photos below are of a CTC NMTB 30 arbor that has only ever been used in the ISO 30 tapered bore of my mill and a pair of ISO 30 arbors, the unmarked one being the chuck that came with the mill and the other, a French Ebay purchase where the seller stated it was removed from a Schaublin mill. There is condsiderable scuffing towards the wide end of the NMTB taper and a fairly even distribution on the ISOs. Make of it what you will.

I will continue to use both in the 13 because trying to obtain real ISO tooling has proven difficult. The Chinese think there is no difference when a chuck purchased as NMTB rolls up marked the ISO equivalent, SK.

BT

And the answer is - when I make an arithmetic mistake.
I posted the angle question over on PM thinking that with all the Euromill fans over there some one would know the answer.
The solution is that I was taking 1/2 x arctan (7/24) = 8.130102 degrees when I should have been taking arctan (7/24 x 1/2) = arctan (7/48) = 8.297144 degrees, which is within 0.28 of a second of the angle that Schaublin quote.
The end result is then that a 40 taper is the same world wide and if it doesn't fit in the socket properly then it has been machined poorly - either through an inaccurate setting or possibly by some idiot that made the same mistake that I made.
It means that my reasonable quality (but second hand) 40 taper tooling should be fine for use but the "I can't believe that they can make it for that price" stuff needs to be checked before use, perhaps by bluing the taper and checking the contact area in the socket.

Michael
(feeling a little embarassed to have made that error)

At least you're not alone Michael - I look a dill as well.