nmtb-tool-holders

NMTB tool holders are single-flange, steep-taper holders that connect a cutting tool to the spindle of a machine tool. NMTB stands for the National Machine Tool Builders Association, and the design is one of the older North American tooling standards. The shank seats in the spindle on a 7:24 steep taper and is drawn tight by the machine, the same taper geometry used by CAT and BT holders. This page covers what NMTB tooling is, how the NMTB30, NMTB40, and NMTB50 sizes compare, the verified dimensions for each, and how NMTB differs from CAT and BT.

We also buy used and surplus NMTB tool holders. If you are clearing a tool crib or retiring an older machine, you can sell your used tool holders to us by the lot rather than one at a time. More on that below.

Sell your used NMTB tool holders

Retiring an older machine or clearing the crib of NMTB30, NMTB40, and NMTB50 holders? We buy them by the lot, mixed brands and any condition. Send a few photos and a rough count for a fast quote.

Call 517-514-5196 or fill out the quote form below.

What NMTB Tool Holders Are and How They Work

An NMTB holder is a single-flange tool holder with two flat drive keyways cut into the flange. What sets the original NMTB design apart from later holders is the retention: instead of a retention knob gripped by the spindle drawbar, the early NMTB shank has a threaded extension on the small end of the taper that a threaded drawbar screws into and pulls up into the spindle. The flange diameter, flange thickness, and the distance from the gage line to the outer face of the flange are not tightly toleranced on NMTB holders, so those dimensions can vary between manufacturers.

Many NMTB shanks were later run in Quick Change spindles. Quick Change tooling is used on CNC machines and uses a pull stud on each tool and a claw that locks on the stud and pulls it into the spindle. Most, but not all, NMTB shanks are compatible with the corresponding size of Erickson Quick Change spindle. It depends on the manufacturer, and tool holders made by Erickson that are marked Quick Change will work fine. Other brands will usually work, but confirm the part sizes first, because small differences can keep a holder from seating in your quick change spindle. If you run machinery built in the mid-20th century, NMTB is most likely what you need. The Bridgeport-style R8 collet taper is a different interface and is not an NMTB size.

NMTB30, NMTB40, and NMTB50 at a Glance

Size Gauge (taper big-end) diameter Retention thread Typical use
NMTB30 1.250 in (31.75 mm) 1/2-13 Lighter-duty machines and older mills and drills.
NMTB40 1.750 in (44.45 mm) 5/8-11 The most common size on general-purpose machining centers and mills.
NMTB50 2.750 in (69.85 mm) 1-8 Heavier cuts and larger machines where rigidity matters.

NMTB Tool Holders (Quick Change) Shank

ANSI B5.18, National Machine Tool Builders Association, 1927. The metric equivalent is DIN 2080 / ISO 297.

Also called Quick Change, NMTB, MM, National Standard, or American Standard Machine Taper. Below you can find the table of sizes:

ANSI B5.18 (from Kennametal catalog 7060)
Size D-1 D-2 D-3 D-4 max D-5* L-1 L-2 L-3 L-4 L-5 L-6 G
30 1.250 [31.75] 0.655 [16.24] 0.673 [17.09] 1.380 [35.05] 1.812 [46.02] 2.690 [68.33] 0.810 [20.57] 0.125 [3.18] 0.076 [1.93] 0.344 [8.74] 0.359 [9.12] 1/2-13 thread
40 1.750 [44.45] 0.935 [23.75] 0.984 [25.00] 1.890 [48.0] 2.500 [63.5] 3.690 [93.73] 1.180 [29.97] 0.188 [4.78] 0.076 [1.93] 0.313 [7.95] ? 5/8-11 thread
50 2.750 [69.85] 1.495 [37.97] 1.545 [39.24] 2.760 [68.6] 3.500 [88.9] 5.000 [127.0] 1.000 [25.4] 0.188 [4.78] 0.141 [3.58] 0.453 [11.51] ? 1-8 thread

* These are Kennametal dimensions. D5 varies with different mfgs.

ISO R 297-2583 and DIN 2080
Size D-1 ~ D-3 D-4 max D-5 L-1 L-2 ~ L-4 L-5 L-6 min G
30 31.75 ~ ? ? 50 68.39 ? ~ 1.62 8 ? M12
DO40 44.45 ~ 22.63 50 63 93.40 ? ~ 1.6 10 11.0 M16
45 57.15 ~ ? ? 80 106.8 ? ~ 3.2 12 ? M20
DT50 69.85 ~ 34.25 78 97.5 126.8 25 ~ 3.2 12 16.0 M24

 

Tool holders built to those two families are usually interchangeable in everyday use, but there are small differences in pull stud and flange detail, so confirm the standard before you mix them.

CAT, BT, and NMTB Tool Holders

They are NOT the same. The taper matches, but the flanging and key lugs differ.

NMTB, CAT, and BT holders all share the 7:24 steep taper, so the shank angle is identical, but the flange, drive keyways, and retention are different on each. Sometimes people rework a CAT spindle to hold BT holders or the reverse, depending on what tooling is available. If you change the holders you also have to change the tool holder fingers in the ATC. For a deeper taper comparison, see our overview of the difference between BT and HSK tool holders.

Sell Your Used NMTB Tool Holders

Shops that retire an NMTB machine or consolidate operations usually end up with crates of NMTB30, NMTB40, and NMTB50 holders that no longer have a spindle to run in. Rather than list them one at a time, you can sell the whole lot to us. We buy used and surplus NMTB holders, mixed brands and mixed conditions, and we also take surplus carbide tooling and carbide tool holders in the same transaction. Send photos and a rough count, get one quote for the lot, and get paid.

Get a quote on your NMTB tool holders

Send a few photos of the shelves and a rough idea of what is there. We buy NMTB30, NMTB40, and NMTB50 holders by the lot, turn quotes around fast, and arrange the freight.

Call 517-514-5196 or fill out the form below for a quote.


Bottom Line

NMTB tooling is an older single-flange, steep-taper standard that still serves reliably on the machines built for it. If you are not certain which NMTB holder you need, or you have a shelf of them to move, get in touch and our team will help you find the right tooling or quote your lot.

Shop Superior Machine & Tool’s selection of tool holders

NMTB Tool Holders FAQ

What is an NMTB tool holder?
NMTB stands for the National Machine Tool Builders Association steep-taper interface. An NMTB holder is a single-flange tool holder on a 7:24 taper with two drive keyways, standardized in the United States as ANSI B5.18 and in metric form as DIN 2080 / ISO 297. It is largely a legacy interface found on mid-20th-century machines, though many NMTB holders still run on modern machines with the right spindle.
What is the NMTB taper angle?
NMTB uses a 7:24 steep, self-releasing machine taper, which works out to 3.5 inches of taper per foot. That is the same taper angle as CAT and BT holders. Because the taper is shared, an NMTB30, NMTB40, and NMTB50 share the same gauge diameters as CAT, but the flange, keyways, and retention are different, so they are not interchangeable.
What is the difference between NMTB, CAT, and BT tool holders?
All three sit on the same 7:24 steep taper, so the shank angle matches, but they are not interchangeable. NMTB is the older single-flange standard with two flat keyways and was originally pulled in by a threaded drawbar. CAT uses a thicker flange with an Imperial retention knob, and BT is a Japanese standard with a symmetric flange and a metric retention knob. The flange, keyways, and retention differ on each, so a holder built for one will not run correctly in another spindle.
What sizes do NMTB tool holders come in?
The common Imperial sizes are NMTB30, NMTB40, and NMTB50, and the number designates the taper size: the larger the number, the larger the taper. Because NMTB shares the 7:24 taper, the gauge (taper big-end) diameters match CAT: NMTB30 is 1.250 in, NMTB40 is 1.750 in, and NMTB50 is 2.750 in. The verified dimensions for each size are in the reference grids on this page.
Is NMTB the same as Quick Change tooling?
Not exactly. Original NMTB tooling is retained by a threaded drawbar that screws into the back of the holder. Many NMTB shanks were later adapted to Erickson-style Quick Change spindles, which use a pull stud and a claw that grips the stud and pulls it into the spindle. Most NMTB shanks are compatible with the matching size of Quick Change spindle, but it varies by manufacturer, so confirm the part details before mixing brands.
Can I sell used NMTB tool holders?
Yes. We buy used and surplus NMTB30, NMTB40, and NMTB50 holders by the lot, including mixed brands and mixed conditions. Shops retiring older machines usually have crates of NMTB holders that no longer have a spindle to run in, and selling the whole lot at once beats listing them one by one. Send a few photos and a rough count and we will send a quote for the lot.

For how NMTB compares to CAT, BT and HSK, see our Tool Holders 101 guide.