Machine Tapers

The machine taper is a simple, low-cost, highly repeatable, and versatile tool mounting system. It provides indexability, as tools can be quickly changed but are precisely located both concentrically and axially by the taper. It also allows high power transmission across the interface, which is needed for milling.

Machine tapers can be grouped into self-holding and self-releasing classes.

Self-holding Tapers: The male and female wedge together and bind to each other to the extent that the forces of drilling can be resisted without a drawbar, and the tool will stay in the spindle when idle. It is driven out with a wedge when a tool change is needed. Morse and Jacobs tapers are an example of the self-holding variety.

Self-releasing Tapers: The male will not stick in the female without a drawbar holding it there. However, with good drawbar force, it is very solidly immobile. NMTB/CAT and HSK are examples of the self-releasing variety.

For light loads (such as encountered by a lathe tailstock or a drill press), tools with self-holding tapers are simply slipped onto or into the spindle; the pressure of the spindle against the work piece drives the tapered shank tightly into the tapered hole. The friction across the entire surface area of the interface provides a large amount of torque transmission, so that splines or keys are not required.

For use with heavy loads (such as encountered by a milling machine spindle), there is usually a key to prevent rotation and/or a threaded section, which is engaged by a drawbar that engages either the threads or the head of a pull stud that is screwed into them. The drawbar is then tightened, drawing the shank firmly into the spindle. The draw-bar is important on milling machines as the transverse force component would otherwise cause the tool to wobble out of the taper.

All machine tapers are sensitive to chips, nicks (dents), and dirt. They will not locate accurately, and the self-holding variety will not hold reliably, if such problems interfere with the seating of the male into the female with firm contact over the whole conical surface. Machinists are trained on keeping tapers clean and handling them in ways that prevent them from being nicked by other tools. CNC tool-changing cycles usually include a compressed-air blast while one tool holder is being swapped with the next. The air blast tends to blow away chips that might otherwise end up interfering between the tool holder and spindle.

There are multiple standard tapers, which differ based on the following:

the diameter at the small end of the truncated cone ("the minor diameter")

the diameter at the large end of the truncated cone ("the major diameter") and

the axial distance between the two ends of the truncated cone.

The standards are grouped into families. Though a family of tapers could be designed that all taper at the same angle, existing families all differ.

One of the first uses of tapers was to mount drill bits directly to machine tools, such as in the tailstock of a lathe, although later drill chucks were invented that mounted to machine tools and in turn held non-tapered drill bits.

Brown & Sharpe Brown & Sharpe tapers, standardized by the eponymous company, are an alternative to the more-commonly seen Morse taper. Like the Morse, these have a series of sizes, from 1 to 18, with 7, 9 and 11 being the most common. Actual taper on these lies within a narrow range close to .500 inches per foot.

Size Lg. Dia. Sm. Dia. Length Taper (in/ft)

1 0.2392 0.2000 0.94 0.5020 2 0.2997 0.2500 1.19 0.5020 3 0.3753 0.3125 1.50 0.5020 4 0.4207 0.3500 1.69 0.5024 5 0.5388 0.4500 2.13 0.5016 6 0.5996 0.5000 2.38 0.5033 7 0.7201 0.6000 2.88 0.5010 8 0.8987 0.7500 3.56 0.5010 9 1.0775 0.9001 4.25 0.5009

10 1.2597 1.0447 5.00 0.5161 11 1.4978 1.2500 5.94 0.5010 12 1.7968 1.5001 7.13 0.4997 13 2.0731 1.7501 7.75 0.5002 14 2.3438 2.0000 8.25 0.5000 15 2.6146 2.2500 8.75 0.5000 16 2.8854 2.5000 9.25 0.5000 17 3.1563 2.7500 9.75 0.5000 18 3.4271 3.0000 10.25 0.5000

Jacobs The Jacobs Taper (abbreviated JT) is commonly used to secure drill press chucks to an arbor. The taper angles are not consistent varying from 1.41 per side for #0 (and the obsolete #2 12) to 2.33 per side for #2 (and #2 short).

There are also several sizes between #2 and #3: #2 short, #6 and #33.

Taper Small End Big End Length mm inch mm inch mm inch

#0 5.80 0.2284 6.35 0.2500 11.11 0.4375 #1 8.47 0.3334 9.75 0.3840 16.67 0.6563 #2 12.39 0.4876 14.20 0.5590 22.23 0.8750

#2 Short 12.39 0.4876 13.94 0.5488 19.05 0.7500 #2 12 15.88 0.625 17.20 0.677 26.80 1.055 #3 18.95 0.7461 20.60 0.8110 30.96 1.2188 #4 26.34 1.0372 28.55 1.1240 42.07 1.6563 #5 33.43 1.3161 35.89 1.4130 47.63 1.8750 #6 15.85 0.6241 17.17 0.6760 25.40 1.0000 #33 14.23 0.5604 15.85 0.6240 25.40 1.0000

Jarno Jarno tapers use a greatly simplified scheme. The rate of taper is 1:20 on diameter, in other words 0.600" on diameter per foot, .050" on diameter per inch. Tapers range from a Number 2 to a Number 20. The diameter of the big end in inches is always the taper size divided by 8, the small end is always the taper size divided by 10 and the length is the taper size divided by 2. For example a Jarno #7 measures 0.875" (7/8) across the big end. The small end measures 0.700" (7/10) and the length is 3.5" (7/2).

The system was invented by Oscar J. Beale of Brown & Sharpe.

Jarno tapers Taper Large end Small end Length Taper/

ft Taper/

in Angle from

center/ #2 0.2500 0.2000 1.00 .6000 .0500 1.4321 #3 0.3750 0.3000 1.50 .6000 .0500 1.4321 #4 0.5000 0.4000 2.00 .6000 .0500 1.4321 #5 0.6250 0.5000 2.50 .6000 .0500 1.4321 #6 0.7500 0.6000 3.00 .6000 .0500 1.4321 #7 0.8750 0.7000 3.50 .6000 .0500 1.4321 #8 1.0000 0.8000 4.00 .6000 .0500 1.4321 #9 1.1250 0.9000 4.50 .6000 .0500 1.4321 #10 1.2500 1.0000 5.00 .6000 .0500 1.4321 #11 1.3750 1.1000 5.50 .6000 .0500 1.4321 #12 1.5000 1.2000 6.00 .6000 .0500 1.4321 #13 1.6250 1.3000 6.50 .6000 .0500 1.4321 #14 1.7500 1.4000 7.00 .6000 .0500 1.4321 #15 1.8750 1.5000 7.50 .6000 .0500 1.4321 #16 2.0000 1.6000 8.00 .6000 .0500 1.4321 #17 2.1250 1.7000 8.50 .6000 .0500 1.4321 #18 2.2500 1.8000 9.00 .6000 .0500 1.4321 #19 2.3750 1.9000 9.50 .6000 .0500 1.4321 #20 2.5000 2.0000 10.00 .6000 .0500 1.4321