Machinist's Guide to Toolholder Maintenance: Part One

General Overview

Modern CNC machines feature high-capacity tool changers that automatically swap toolholders in and out of the spindle as needed, by means of a high speed swing arm or a rotary carousel.

Periodically, toolholders should be examined for wear and if necessary replaced to maintain cutting performance. New operators should be taught how to properly evaluate toolholders so they can recognize when toolholders need to be replaced to prevent premature cutting tool failure, or even expensive damage to the spindle.

Many operators do not know why it is necessary to replace their tooling, or have the experience to tell when it is time to do so. Determining if toolholder components need to be replaced is not a difficult task, but does require that the operator knows what to look for.

This article will cover the criteria used to evaluate collet and nut style tool holders, describe when and why it is necessary to replace them and the implications of not replacing them

A typical CAT40 ER 32 Collet Chuck with a ER32 Collet and a Collet Nut

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Sizing Toolholders

A typical size description of a toolholder is CAT40 x ER 32. The “CAT” refers to the flange type, “40” is the taper size, and “ER 32” is the type and size of collet that fits into the pocket.

The other dimension to be aware of is the “Gauge Length”. This refers to the distance the toolholder extends from the face of the spindle (see diagram.)

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Parts of Toolholder

A general understanding of collet toolholder components and their functions is important.

There are four main parts to a toolholder, which can also be called a a collet holder or collet chuck.

1. Retention Knob/Pull Stud
   
2. Taper
3. Flange
   
4. Collet Pocket

On toolholders that are end mill holders, shell mill holders or drill chucks the those portions below the flange are different than depicted here.

Parts of a Steep Taper Rotary Toolholder

Pull Studs / Retention Knob

Retention Knobs, which are also called Pull Studs, are extremely important because they keep the toolholder in the spindle. Using worn pull studs or using the wrong pullstud for your machine may cause the toolholder to suddenly fly out of the spindle during operation, causing an unsafe situation for the machine operator

The retention knob screws into the top of the taper of the tool-holder. Some pull studs are hollow, to permit coolant to flow thru the toolholder.

When in use, the retention knob is held by the clamping set inside the spindle which pulls the holder up into the spindle mouth. A spring-loaded draw bar pulls the holder into place.

Taper

The taper is the conical shaped area of the toolholder that enters the spindle when changing the tool. An 8 degree taper automatically centers the tool into the spindle. The taper is accurately ground to a tolerance of .0002” for both the taper tolerance and outside diameter tolerance.

There is a measurement, AT, of toolholder tapers that designates AT1 through AT8. Most all manufacturers specify an AT3 Taper tolerance as most spindles at made to an AT2 tolerance. Higher speed toolholders will hold a tighter, (AT2, AT1) tolerance.

Some toolholders like HSK have a shorter taper than BT or CAT style.

V- Flange

The v-flange is the part of the toolholder that the automatic tool changer locks onto when moving the tool from the tool changer to the spindle and back again. The flange is visually identified as the “V” groove found on the outer most diameter of the toolholder. Cutouts in the flange help orientate the holder in the spindle.

Note how HSK taper (right) is a dual-contact taper. Meaning that it is flush with the gauge line of the spindle face, creating dual contact between the flange of the holder and the spindle face, and the taper itself and the spindle mouth. Dual contact increases tool-holder rigidity for improved performance especially at extended gauge lengths. Techniks DualDRIVE toolholders provide dual contact on V-flange (BT, CAT) spindles

Collet Pocket

The last part of the toolholder is the collet pocket, into which the collet is inserted before being secured by various types of collet nuts. The collet pocket, internal 8 degree taper,  should hold the same tolerance as the taper as they work together to control runout