Machining Monday

The NMTB Taper: The Standard Three Rivals Patented Before CAT

You have pulled a toolholder out of a spindle and never thought twice about the number on it. 30, 40, 50. That number, and the steep angle behind it, started with three companies that normally fought each other for every sale. In 1927 they filed a patent together, and the taper they agreed on is still in your spindle.

Before 1927, Every Taper Was a Little Different

Before 1927, milling machine spindle tapers were shallow, and they varied. The 1927 patent puts it plainly: earlier tapers "are generally on the order of one-half inch, up to three quarters of an inch per foot," and "Tapers previously commonly employed vary somewhat."

A shallow taper holds itself in. That is fine for a drill chuck. It is a problem on a milling machine, where an arbor or cutter has to come out, go in and come out again all day. A shallow taper sticks, and a stuck arbor means a hammer, a drift and lost time.

Three Rivals, One Patent

On March 25, 1927, four engineers filed a patent titled Milling-Machine Spindle and Tool: Joseph B. Armitage and Edward J. Kearney of Milwaukee, Benjamin P. Graves of Providence and Lester F. Nenninger of Cincinnati. It was assigned to three companies at once: Kearney and Trecker of West Allis, Wisconsin, Brown and Sharpe of Providence, Rhode Island, and Cincinnati Milling Machine of Cincinnati, Ohio.

These were three major American milling machine builders, and they competed for the same customers. They filed jointly anyway. US Patent 1,794,361 was issued on March 3, 1931.

Why 3.5 Inches per Foot

The patent specifies "a taper of three and one-half inches per foot of length," which it says "has been found to answer all the desired conditions for milling machines." Three and a half inches per foot is the same as 7 in 24, the 7/24 taper still printed in toolholder catalogs.

The reason for the angle is in the patent too. The steep taper is "primarily for the purpose of preventing sticking or binding of the tool when the tool is to be removed." But the inventors knew there was a limit: "too steep a taper is not desirable for accurate alignment." Seven in 24 was the compromise between a tool that releases and a tool that runs true.

The Taper Does Not Hold the Tool

Here's the part most machinists never think about: a 7/24 taper cannot hold a tool by itself, and it was never meant to. The patent says it outright: "any taper which will not stick will not of itself safely hold an overhanging tool in position."

So the job was split up. The taper locates the tool and centers it. Keys on the spindle flange drive it, fitted into slots on the arbor flange to transmit rotation "in either direction of rotation." And a draw-in rod through the spindle threads into the back of the arbor and pulls it home. The patent even says the keys should fit "both at the sides and bottom" so the arbor cannot drop out at certain positions.

Locate with the taper, drive with the keys, hold with the drawbar. Every steep-taper toolholder since has followed that division of labor.

The Taper Named After the Builders

The taper took its name from the National Machine Tool Builders' Association, founded in 1902. HAIMER describes the NMTB taper as developed by the association and designed to be changed by hand, with a straight extension at the back that threads directly into the machine's drawbar. The same association launched the first National Machine Tool Builders' Exposition in 1927, the show that became IMTS, and it is known today as the Association for Manufacturing Technology.

Common sizes are listed by one seller of NMTB holders as:

The same seller notes that the metric equivalent is DIN 2080 and that many NMTB shanks were later adapted to Erickson-style quick-change spindles, though not every brand is compatible.

Then the Robot Needed Something to Grab

A threaded shank and a hand-turned drawbar work fine when a machinist changes the tool. They do not work for an automatic tool changer. The changer needs something to grip, and the spindle needs to pull the holder in without anyone turning a wrench.

That is what the CAT taper added. According to BIG DAISHOWA, Caterpillar engineers developed the CAT toolholder in the 1970s. It kept the 7/24 taper and added a V-groove in the flange for the changer arm to grab and a pull stud that the spindle's drawbar clamps automatically. HAIMER notes that Caterpillar championed the standardization of steep-taper holders in North America for machines with automatic tool changers. Japan's BT standard uses the same taper with metric pull-stud threads and a thicker flange.

The taper never changed. Only the way the tool gets held and handled did.

A 1927 Agreement in Your Spindle

NMTB holders still turn up on manual mills and older machines, and some shops still keep a drawer of them. But even if you never touch one, the angle in every CAT 40 or BT 40 holder in your shop traces back to that 1927 patent.

The number on your toolholder is a size. The angle behind it is a truce between competitors that has held for almost a hundred years.

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