Machine taper

In the design of machine tools there is a need for users to be able to quickly and easily install or remove particular cutting bits or other accessories from the powered rotating spindle of the machine tool. One example is a lathe headstock, which has a rotating spindle to which one may want to mount a spur drive. Another example is a drill press on which one may want to mount a bit, a chuck, etc.

Several options exist: (1) a threaded spindle, into/onto which accessories are screwed, (2) a permanently mounted chuck (as with some drill presses, or (3) a taper mount.

This simple, low-cost, and versatile tool mounting system involves (A) tool bits with shanks that are gradually tapered towards the mounting end, and (B) a matching hollowed-out spindle. Tools are simply slipped into the spindle; the pressure of the spindle against the workpiece drives the tapered shank tightly into the tapered hole. The friction across the entire surface area of the interface provides a surprisingly large amount of torque transmission, so that splines or keys are not required.

This system is known as a machine taper.

Method of specification; overview of families

There are multiple different standard tapers, each differing in (a) the diameter at the small end of the truncated cone ("the minor diameter"); (b) the diameter at the large end of the truncated cone ("the major diameter"); (c) the axial distance between the two ends of the truncated cone.

The standards are grouped into families. The most famous family is the "Morse taper", which comes in 8 varieties, from "Morse 0" at 59.4 mm (2.34 inches) from end to end - to "Morse 7" at 295.3 mm (11.625 inches) from end to end.

It is noteworthy that even though one could design a family of tapers that all have the same included angle (e.g. the Foobar taper, from the 2" long Foobar-2 to the 100" long Foobar-100, all of which have a 5 degree included angle), the actual families of tapers in existence do not exhibit this design feature. Morse tapers, for example, taper at anywhere from 0.04988 inches of diameter per inch of length (for the Morse 1) to 0.05200 inches of diameter per inch of length (for the Morse 7).

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

Details of use

Bits or accessories are inserted into a tapered receptacle and pushed or twisted into place. They are then held by friction.

Tapered shanks "stick" in tapers best when both the shank and the receptacle are clean. Shanks can be wiped clean, but receptacles, being deep and inaccessible, are best cleaned with a specialized taper cleaning tool which is inserted, twisted, and removed.

Tapered shank tools are removed from tapered receptacles either by inserting a drift punch axially from behind, and tapping the tapered shank tool out of the receptacle, or - if the tapered shank tool is so designed - inserting a wedge shaped block of metal called a "drift" into a rectangular shaped cross hole through the shank, and tapped. As the cross section of the drift gets larger when the drift is pushed further in, the result is that the drift, bearing against the foremost edge of the tang receptacle, pulls the tapered tang out.

Families of tapers

Morse

The Morse Taper was invented by Stephen A. Morse (also the inventor of the twist drill) circa 1864. Since then it has evolved to encompass smaller and larger sizes and has been adopted as a standard by numerous organizations including ISO as ISO 296:1991 Machine Tools -- Self-Holding Tapers for Tools Shanks, and DIN as DIN 228-1:1987-05 Morse tapers and metric tapers; taper shanks.

Morse Tapers come in eight sizes identified by number between 0 and 7. Often this is abbreviated as MT followed by a digit, for example a Morse taper number 4 would be MT4. The MT2 taper is the size most often found in drill presses up to 1/2" capacity.

Morse tapers can have two types of ends:

tang (illustrated) to facilitate removal with a drift
threaded to be held in place with a draw bar

The taper itself is roughly 5/8" per foot, but exact ratios and dimensions for the various sizes of tang type tapers are given below.

Morse Taper Dimensions (mm)

Morse Taper	Taper	A	B (max)	C (max)	D (max)	E(max)	F	G	H	J	K
0	19.212:1	9.045	56.5	59.5	10.5	6	4	1	3	3.9	1°29'27"
1	20.047:1	12.065	62	65.5	13.5	8.7	5	1.2	3.5	5.2	1°25'43"
2	20.020:1	17.780	75	80	16	13.5	6	1.6	5	6.3	1°25'50"
3	19.922:1	23.825	94	99	20	18.5	7	2	5	7.9	1°26'16"
4	19.254:1	31.267	117.5	124	24	24.5	8	2.5	6.5	11.9	1°29'15"
5	19.002:1	44.399	149.5	156	29	35.7	10	3	6.5	15.9	1°20'26"
6	19.180:1	63.348	210	218	40	51	13	4	8	19	1°29'36"
7	-	83.058	285.75	294.1	34.9	-	-	19.05	-	19	1°29'25"

Brown and Sharpe

Brown and Sharpe tapers are an alternate 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 is within a close range of .500 per foot.

R8

This taper was designed by Bridgeport Machines Inc. for use in their milling machines. It is used with a drawbar extending up through the spindle to the top of the machine to prevent the collet from falling from the spindle when lateral forces are encountered. The collet, which is inserted into the taper, has a precision hole in one end for holding a cutting tool and is threaded for a drawbar on other end. They are also keyed (see image) to prevent rotation, although for low cutting forces this may not always be needed. The drawbar thread is typically 7/16"-20tpi (UNF).

The cutting tool is placed in the collet, the collet placed into the taper, and the drawbar is tightened into the top of the collet from above the spindle. The collet has a groove to engage a key in the spindle to keep the collet from spinning inside the taper and to aid in the installation and removal of the collet. The angle of the cone is typically 16 degrees and 51 minutes (i.e. 16.85 degrees) with an OD of 1.25" (source, Bridgeport Manufacturer).