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A machine taper is a system for securing cutting bits and other accessories to a machine tool's spindle.
Machine tool operators must be able to install or remove tool bits quickly and easily. A lathe, for example, has a rotating spindle in its headstock, to which one may want to mount a spur drive or work in a collet. Another example is a drill press, to which an operator may want to mount a bit directly, or using a drill chuck.
The machine taper is a simple, low-cost, highly repeatable, and versatile tool mounting system that uses tool bits (or holders) with gradually tapered shanks, and a matching hollowed-out spindle.
For light loads (such as encountered by a lathe tailstock), tools are simply slipped onto or 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 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 matching drawbar. The drawbar is then tightened, drawing the shank firmly into the spindle.
Tools with a tapered shank are inserted into a matching tapered socket and pushed or twisted into place. They are then retained by friction. In some cases, the friction fit needs to be made stronger, as with the use of a drawbar, essentially a long bolt that holds the tool into the socket with more force than is possible by other means.
Caution needs to be exercised in the usual drilling machine or lathe situation, which provides no drawbar to pull the taper into engagement, if a tool is used requiring a high torque but providing little axial resistance. An example would be the use of a large diameter drill to slightly enlarge an existing hole. In this situation, there may be considerable rotary loading. In contrast, the cutting action will require very little thrust or feed force. Thrust helps to keep the taper seated and provides essential frictional coupling.
The tang is not engineered to withstand twisting forces which are sufficient to cause the taper to slip, and will frequently break off in this situation. This will allow the tool to spin in the female taper, which is likely to damage it. Morse taper reamers are available to alleviate minor damage.
Tapered shanks "stick" in a socket best when both the shank and the socket are clean. Shanks can be wiped clean, but sockets, 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 a socket using different approaches, depending on the design of the socket. In drill presses and similar tools, the tool is removed by inserting a wedge shaped block of metal called a "drift" into a rectangular shaped cross hole through the socket and tapping it. As the cross section of the drift gets larger when the drift is driven further in, the result is that the drift, bearing against the foremost edge of the tang, pushes the tool out. In many lathe tailstocks, the tool is removed by fully withdrawing the quill into the tailstock, which brings the tool up against the end of the leadscrew or an internal stud, separating the taper and releasing the tool. Where the tool is retained by a drawbar, as in some mill spindles, the drawbar is partially unthreaded with a wrench and then tapped with a hammer, which separates the taper, at which point the tool can be further unthreaded and removed. Some mill spindles have a captive drawbar which ejects the tool when actively unscrewed past the loose stage; these do not require tapping. For simple sockets with open access to the back end, a drift punch is inserted axially from behind and the tool tapped out.
There are multiple standard tapers, which differ based on the following:
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 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)|
The Jacobs Taper (abbreviated JT) is commonly used to secure drill press chucks to an arbor.
|Taper||Small End||Big End||Length|
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.
|Taper||Large end||Small end||Length||Taper/|
The Morse Taper was invented by Stephen A. Morse in the mid-1860s. Since then, it has evolved to encompass smaller and larger sizes and has been adopted as a standard by numerous organizations, including the International Organization for Standardization (ISO) as ISO 296 and the German Institute for Standardization (DIN) as DIN 228-1. It is one of the most widely used types, and is particularly common on the shank of taper-shank twist drills and machine reamers, in the spindles of industrial drill presses, and in the tailstocks of lathes.
Morse Tapers come in eight sizes identified by whole numbers between 0 and 7, and one half-size (4 1/2 - very rarely found, and not shown in the table). Often the designation 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 ½" capacity. Stub (short) versions, the same taper angle but a little over half the usual length, are occasionally encountered for the whole number sizes from 1 through 5. There are standards for these, which among other things are sometimes used in lathe headstocks to preserve a larger spindle through-hole.
Morse tapers are of the self-holding variety, and can have three types of ends:
Self holding tapers rely on a heavy preponderance of axial load over radial load to transmit high torques. Problems may arise using large drills in relation to the shank, if the pilot hole is too large. The threaded style is essential for any sideloading, particularly milling. The only exception is that such unfavourable situations can be simulated to remove a jammed shank. Permitting chatter will help release the grip. The acute (narrow) taper angle can result in such jamming with heavy axial loads, or over long periods.
End-Milling cutters with a Morse taper shank with a tang are occasionally seen: for security these must be used with a C-collar or similar, fitting into the neck between cutter and shank, and pulling back against the large end of the taper
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 number||Taper||A||B (max)||C (max)||D (max)||E (max)||F||G||H||J||K|
|0||1:19.212||9.045||56.5||59.5||10.5||6||4||1||3||3.9||1° 29' 26"|
|1||1:20.047||12.065||62||65.5||13.5||8.7||5||1.2||3.5||5.2||1° 25' 43"|
|2||1:20.020||17.780||75||80||16||13.5||6||1.6||5||6.3||1° 25' 50"|
|3||1:19.922||23.825||94||99||20||18.5||7||2||5||7.9||1° 26' 16"|
|4||1:19.254||31.267||117.5||124||24||24.5||8||2.5||6.5||11.9||1° 29' 15"|
|5||1:19.002||44.399||149.5||156||29||35.7||10||3||6.5||15.9||1° 30' 26"|
|6||1:19.180||63.348||210||218||40||51||13||4||8||19||1° 29' 36"|
|7||1:19.231||83.058||285.75||294.1||34.9||-||-||19.05||-||19||1° 29' 22"|
The National Machine Tool Builders Association (now called the Association for Manufacturing Technology) in the US laid down standards for machine tool design, among other things: the taper used on CNC (Computer Numerically Controlled) milling machines.
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 (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).
This is a "self releasing" or "fast" taper. Unlike the more acute self holding tapers above, such tapers are not designed to transmit torque. This turning effort is carried by driving keys engaging slots on the flange. The purpose is to allow a quick and easy change between different tools (either automatically or by hand) while ensuring the tool or toolholder will be tightly and rigidly connected to the spindle, and accurately coaxial with it. The larger end adjacent to the tool makes for more rigidity than is possible with Morse or RT tapers fitted to comparable machines.
The spindle on the machine tool is built with a female taper and drawbar. Each individual tool must be fitted with a male taper and the proper adapter for the drawbar.
HSK toolholders were developed in the early 1990s. HSK stands for Hohlschaftkegel; German for "hollow shank taper".
The HSK design was developed as a nonproprietary standard. The working group that produced the HSK standard consisted of representatives from academia, the Association of German Tool Manufacturing and a group of international companies and end users. The results were the German DIN standards 69063 for the spindle and 69893 for the shank. The HSK working group did not adopt a specific product design, but rather a set of standards that defined HSK toolholders for different applications. The group defined a total of six HSK shanks. These shank styles are designated by the letters A through F.
Each style is also identified by the diameter of the shank’s flange in millimeters. These diameters are taken from the R10′ series of preferred numbers, from 25 to 160 mm. Styles A, B, C and D are for low-speed applications. Styles E and F are for high speeds. The main differences between the styles are the positions of the drive slots, gripper-locating slots, coolant holes and the area of the flange. The shank itself is made as a hollow taper with a ratio of 1:10, and is about half as long as other machine tapers. The surface inside the shank is cut with a 30° chamfer, making it possible to clamp the toolholder from the inside. The wall of the shank is designed to be thin enough to flex slightly. On the outer surface of the shank flange is a traditional toolchanger V-groove and slots for locating and orienting an automatic toolchanger’s (ATC) gripper. The principal difference between styles A and B is the size of the taper. The B-style shank has a taper one size smaller than an A-style shank with a flange of the same size. D and F shanks have tapers one size smaller than C and E shanks with the same flange diameter as well. Styles C and D were designed exclusively for manual use with the elimination of features to accommodate ATCs. To handle extremely high speeds and machining of light materials, styles E and F are totally symmetrical. Their symmetry minimizes unbalance, which can be a significant problem at high speeds.
An HSK connection depends on a combination of axial clamping forces and taper-shank interference. All these forces are generated and controlled by the mating components’ design parameters. The shank and spindle both must have precisely mating tapers and faces that are square to the taper’s axis. There are several HSK clamping methods. All use some mechanism to amplify the clamping action of equally spaced collet segments. When the toolholder is clamped into the spindle, the drawbar force produces a firm metal-to-metal contact between the shank and the ID of the clamping unit. An additional application of drawbar force positively locks the two elements together into a joint with a high level of radial and axial rigidity. As the collet segments rotate, the clamping mechanism gains centrifugal force. The HSK design actually harnesses centrifugal force to increase joint strength. Centrifugal force also causes the thin walls of the shank to deflect radially at a faster rate than the walls of the spindle. This contributes to a secure connection by guaranteeing strong contact between the shank and the spindle. The automotive and aerospace industries are the largest users of HSK toolholders. Another industry that is seeing increasing use is the mold and die industry.
This taper was designed by Bridgeport Machines, Inc. for use in milling machines. Tools with integral taper are inserted directly into the machine taper, which is not usual with other systems, except Morse. Collets may also be fitted allowing the use of round shank tooling. R8 tapers require a drawbar extending up through the spindle to the top of the machine to prevent loosening when lateral forces are encountered. Collets have a precision bore with axial compression slots for holding cutting tools and are threaded at one end for a drawbar. They are also keyed (see image) to prevent rotation during insertion and removal. However, cutting torques are transferred through friction at the taper, not through the key. The drawbar thread is typically 7⁄16″–20 tpi (UNF).
The R8 system is commonly used with collets ranging in size from ⅛″ to ¾″ in diameter or tool holders with the same or slightly larger diameters. The collets or tool holders are placed directly into the tapered bore of the spindle and the drawbar is tightened into the top of the collet or tool holder from above the spindle. Other tools such as drill chucks, fly cutters, indexable insert cutters, etc. may have an R-8 taper shank built into or added to the tool. The angle of the cone is 16°51′ (16.85°) with an OD of 1.25″ and a length of 15⁄16″. (source, Bridgeport Manufacturer) The resultant inner diameter is slightly over 31⁄32″.
The R8 taper is commonly encountered on Bridgeport and similar turret mills from the USA, or on (very common) copies of these mills from elsewhere. The popularity is due in large part to the success of Bridgeport and other mills that were closely modeled after it and produced throughout much of the 20th century.