Taps and Dies - Cont.

The floating action of radial-floating tap holders is generated in several ways, including a parallel floating drive, a ball drive and a hex drive.

The first of these, the parallel floating drive, is a variation of the Oldham coupling and features two pairs of driving pins. The shank pair of driving pins is parallel to the tool axis while the nosepiece pair is perpendicular to it.

With this design, the holder can drive both large and small-diameter taps and reamers. The holder has both a short overall length and a small body and can provide from 0.062 in. to 0.094 in. of radial float without the use of balls or rollers.

One of the primary advantages of the parallel floating drive is that it prevents any tendency to box float -- an amount of float in each of the four different axes. Another benefit is that these holders permit coolant-through at up to 1,000 psi. This is because the design of the centering toolholder mechanism allows the inlet flow to be controlled, reducing pressure on the mechanism. Shops also can adjust centering pressure.

Well suited for heavy reaming applications, the parallel floating drive tap holder has an antifriction design that extends tool life and ensures cutting performance.

One problem with these holders is that they lack angular float. Therefore, extended length taps and reamers -- more than six times diameter in length from the face of the toolholder -- without a bushing can cause the mechanism to bind and prevent floating.

The ball-drive design uses steel ball bearings to float the tool and absorb machining thrusts. Two tangent points on the radius of the ball are the only points of contact. This design provides a full, unrestricted 360-degree float movement, up to 0.200-in. of radial float and a truly round float rather than a box float.

While these tap holders work well in light reaming and tapping applications, they cannot take heavy-reaming type loads. The reason for this is that the only point of contact is the tangential surfaces of the balls. Heavy loads exacerbate the formation of small depressions in the floating-member surface, which are caused as the balls work back and forth in a small area. These depressions lead to radial and axial looseness, tool chatter, bell-mouthed holes and short toolholder life.

The hex-drive design uses a center hex nut with a spring-loaded ring or rubber donut to equalize the forces around the center. Although this rugged device offers coolant-through capability and prevents box float, its large-diameter mechanism housing impedes close-center applications. The design of the mechanism also allows some angular float, which can cause bell-mouthed holes. In addition, the unit does not truly center itself and has a tendency to whip under high rpm.

Another disadvantage is that the hex-drive's centering mechanism has a tendency to wear prematurely, causing reduced tool life in the cutting tool and toolholder. And, as it wears, it sets in the direction of the wear.

Tap holder considerations

Contaminants such as grease, dirt, coolant, chips and grime can gum up a floating holder, so shops should look for ones with Teflon-molded seals. These seals exert constant force against the ground front face of the housing while the Teflon's lubricating characteristics offer a minimum resistance to motion.

Another thing to consider is the placement of the coolant inducer. These attachments typically are used with older equipment when coolant-through spindles are not available. With most floating holders, the inducer is attached to the nosepiece of the holder, which creates an undesirable size and weight condition that doesn't permit the holder to float true. However, some designs permit the rotary inducer to be placed behind the centering mechanism, which allows the coolant to flow through the holder without restricting float.

Most floating tap holders come in a variety of models and sizes and, through the use of interchangeable components, satisfy a complete range of user needs. These configurations include all popular Morse taper, straight, automotive and metric shanks as well as collet and quick-change tap adapter nosepieces.

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