Toolholders Technical Information
A toolholder is the interface between a precision machine spindle and a precision cutting tool. The toolholder has three base elements; the taper connection, theretaining device and the cutting tool connection.
Types of Tapers
The toolholder selected is specific to the machine spindle’s taper design; you cannot interchange the toolholder with a machine tool with a different type of spindle.
R8 : Most manual (Bridgeport type) milling machines use an R8 taper, which is not truly a taper as defined by the Machinery’s Handbook since it is a 3 1�?�16″ long
undercut straight shank held in place by a 7�?�16″-20 threaded drawbar. The only taper is the 16.85° included angle of the 15�?�16″ long cone section.
Morse Taper: Come in eight sizes identified by a number between 0 and 7. This is abbreviated as MT followed by a digit, for example a Morse taper number 4 would beMT4. The MT2 taper is the size most often found in drill presses up to ½″ capacity. Morse tapers can have three types of ends: tang to facilitate removal with a drift,
threaded to be held in place with a drawbar and flat or no tang or threaded section.
The CNC toolholder consists of two sections (taper & toolholder) divided by the gage line (the imaginary line marking the portion of the toolholder that matchesthe bottom edge of the machine spindle). The taper section fits into the spindle allowing the machine tool to transfer rotary motion to the tool which is held inthe toolholder section. The number following the toolholder type (i.e. NMTB 30) is the spindle size number. The larger the spindle size number, the larger themachine. The larger toolholder will be longer and have a greater diameter gage line. Despite all the names, there are actually only two tapers involved with CNCtoolholders. CAT V-Flange, NMTB & BT-Flange all utilize the 3 ½:12 or 7:24 taper (the height of the taper reaches 3 ½�? after a distance of 12�?). They are dividedinto two categories: single flange & dual flange.
CAT, CV or V-Flange Taper (Caterpillar “V-Flange�?):
CAT V-Flange tapers use inch threads for the retention knob and are used to hold either inch-dimensioned ormetric-dimensioned cutting tools. These are dual flange tapers and come in the following sizes: 30, 35, 40, 45, 50. This is the most common type of taper used onCNC machines.
BT Flange Taper (Bonsai Taper):
BT-Flange was developed in Japan and is also a dual flange taper. They are also held in place by retention knobs, but the threads aremetric. BT toolholders are symmetrical about the spindle axis, while CAT toolholders are not. This gives BT toolholders greater stability & balance at higher speeds.BT & CAT look very similar & can easily be confused. The difference is in flange style & thickness.
NMTB:NMTB (National Machine Tool Builders’ Association) is a single flange toolholder. It is held in place by a drawbar. These are often referred to as Quick-Changeand come in sizes 30, 40 & 50.
HSK (Hohl Shaft Kegel, or Hollow Taper Shank):
HSK tooling is becoming increasingly popular for its stability & balance in high speed machining operations. Short for theGerman words meaning Hollow Taper Shank, the HSK taper is much shallower at a ratio of 1:10 (the height of the taper reaches 1�? after a distance of 10�?). This helpsspeed tool changes & is designed to increase grip as spindle speeds increase. It utilizes a drawbar retention system which causes gripper fingers to wedge into aclamping position.
Toolholder Selection for Higher Speeds
The critical criteria for the selection of tooling for higher speeds are Runout Accuracy/Concentricity and Balance.
Holders should be selected based on their ability to hold the tool concentric to spindle rotation. This normally results in tools that arefriction driven, such as collet chucks, shrink fit holders, power milling chucks, etc. When selecting a friction drive holder over a positive drive such as a side lock endmill holder, you are sacrificing positive drive for better runout accuracy and balance.
As spindle speeds are increased in a given application, the more important the balance of the holder becomes. The effect that a lack of balance will have on acutting operation is relative to the type of cutting forces generated in the cut. The higher the cutting forces the less effect balance will have on the overall operation ofthe machine tool. Under most circumstances a pre-balanced holder is all that is required to achieve a balanced machining condition. If you select a highly balancedtoolholder but with poor concentricity, the effective cutting force from the outside cutting edge will be detrimental to the operation.
Types of Toolholders
A collet chuck includes a chuck body, shank and fastening unit. They are very versatile since different collet sizes can be used in one holder allowing fordifferent size tools being held. They come in a variety of shank sizes and types.
Single Angle Collet System:
- ER Collet Chuck: For most drilling and lighter higher speed milling applications. Very good concentricity and balance. Safe operational speed as high as 30,000 rpm.Larger sizes have a limit based on centrifugal forces.
- TG/PG Series: For heavy drilling and some milling applications. The system provides good concentricity and good grip force. The nut system does not lend itself for good balance considerations at higher speeds.
Double Angle Collet System (DA Series):
For smaller drilling application where clearance is necessary. This system is the simplest to use but lacks the characteristics forconcentricity and grip force. Not suitable for precision high speed applications
End Mill Holders:
For heavy machining. Used to hold end mills. End mill holders hold cutting tools with more rigidity than collets. They are available in Weldon type forusing tools with Weldon flats.
Hydraulic toolholders are the easiest holders for most operators to use and should be applied in close tolerance operations or where expensiveround tools are being used and maximizing tool life is important. They minimize tool chatter with vibration damping and offer excellent balance characteristics, whiledelivering high runout accuracy.
For precision in moderate and lower speed milling and nominal drilling applications. Very good concentricity and good side load capability. Movablelarge clamping nut restricts the balance and centrifugal force capability, limiting the speed to 8,000 to 12,000 rpm.
Shell Mill Holders:
For holding milling cutters that utilize the American National Standard mount for Shell Mills. Modern cutters are commonly indexable. Shell mill lockscrew and socket head cap screws are commonly used depending on milling cutter manufacturer’s specification. Either or both are provided with each holderdepending on the size.
Shrink Fit Holders (Heat Shrink):
For nominal size drilling with higher speed and higher feed milling applications. Excellent concentricity and excellent balance. Nomoving features, and thin nose diameters, make it exceptional for high speed machining. Safe operating speeds are extremely high due to little impact from centrifugalforces. A Heat Shrink machine should be purchased to achieve optimal performance
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