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How A Face Driver Works
The driving head is composed of a locating center, drive pins and a compensating medium. Typically the center is spring loaded. The center point locates in the center hole in the workpiece to establish the axis on which the part rotates. The center point then retracts under the tailstock force, allowing the drive pins to contract the part face.
The compensating medium can be either hydraulic, a spherical washer or an elastomer ring. This compensation enables the drive pins to penetrate the face of the part equally even if the part face is not square.
This is the initial penetration of the drive pins into the work, approximately 0.010 - 0.015 in., depending on the material hardness and tailstock force. The completion of the clamping occurs when the cutting tool engages the workpiece cutting toward the face driver and increasing the penetration to about 0.030 in. After the drive pins are seated properly, machining can be done away from the face driver.
Tailstock pressure is the key to face driving of a part. With the newer machines on today's market, the tailstock forces available are usually very light. This problem can be easily addressed with a face driver that utilizes different drive pin styles. By simply using a narrower driving edge, the drive pin penetration can be greatly improved even when the tailstock pressure is low. Another positive result of this change of pin style can be less wear on the machine spindle bearings when applying heavy tailstock forces.
Unlike a chuck, face drivers permit machining the entire O.D. of the workpiece in one clamping. This is not only eliminates a second operation, but produces a better quality part, since the part is machined between centers. If the part is finish ground between centers, the face driver allows you to leave minimum grind stock, reducing time for that operation. Since face drivers locate on the end of the part they offer unparalleled cutter clearance.
Despite misconceptions in the field, face drivers can be used for heavy machining. In some cases, stock removal can exceed 0.375 in. on diameter without part slippage, allowing the rough and finish machining to be done in one operation.
Face driving centers need little maintenance. The center points and drive pins can be easily replaced. Drive pins can be resharpened, as much as 0.100 in., extending their life.
Special face drivers can be designed for unusual applications. When a fixed location is required for centering the part, the drive pins can be activated using the machine drawbar. Where the production runs are short and the parts vary in diameters, special multi-range face drivers can accept a wide range of part sizes just by moving the drive pins to different driving diameters. In hobbing applications, face drivers with solid drive rings can provide adequate support with no side movement.
With today's focus on tooling that is inexpensive, reduces operations, improves accuracies and decreases maintenance, the face driver will continue to be an effective alternative when processing turned parts.
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