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Friday, December 12, 2014

Shop Efficiency Part 2 : Face Drivers for CNC Turning

In our second installment in our series on Shop Efficiency we are about to take an in-depth look at a workholding option for CNC turning that very often goes unconsidered. "In the old days", turning between centers was the method of choice for any type of shaft or similar type workpieces. The process began with manually drilling center holes in each end of the material ... then attaching a drive dog on one end that was used to drive the parts rotation ... using the tailstock at the other end for support ... and the cutting began. With the advent of CNC ... turning between centers has for the most part become a lost and forgotten art. The use of soft jaws and the process of turning the part around has become the method of choice. Most machine tailstocks are pushed to the end limit and left there to collect chips or worse ... are not even considered or purchased when the machine is bought.

But with shops looking to increase production ... decrease set-up and set-up times ... and at the same time increase quality ... face drivers are getting a face-lift and are becoming more and more popular for many types of machining. The ability to machine the entire workpiece in one set-up gives shops the ability to maximize their production capabilities with only a minimal expense. So let's take a deeper look at face drivers ... the concept and the design.

Introduction to Face Drivers

As the above illustration shows ... the use of face drivers in conjunction with your CNC machine's tailstock ... allow complete access to the entire workpiece. This not only reduces the number of operations and set-up ... it greatly increases the accuracy of the workpiece machined. With everything machined in the same set-up ... tolerances and concentricity are greatly increased. Although most used for shaft work ... a quick look around the shop would probably reveal a lot of "shaft type" work where face drivers could be considered for the workholding option. Right and left hand turning can be employed as necessary without any restrictions.

How Does a Face Driver Work

The above illustration shows that the face driver consists of (2) main features ... the center and the drivers. The center fits into the spot drill or center drill hole that is pre-machined into the stock. The driver pins are what drives the workpiece in rotation and can either be hydraulic or mechanical ( such as spring loaded ). So as the workpiece is located between centers ... it is also pushed up against the drive pins which dig into the end of the workpiece and cause it to rotate as the spindle rotates.

Attaching the face driver to the spindle can be done with a variety of methods ... the easiest and most common is probably holding it in the chuck jaws. Depending on your spindle face and configuration ... other methods might be Morse Taper or a flange mounted directly to the spindle face.

The "clamping" of the workpiece is two fold ... centering followed by clamping. As the workpiece locates on the center points ... the action of the tailstock forces the workpiece onto the spindle side center and up against the drive pins. As the workpiece is forced deeper onto the center ... the drive pins dig into the face of the workpiece. The drive pins adjust individually to accommodate any irregularities in the face. Under the continued load of the tailstock, the drive pins penetrate and "clamp" the workpiece ... while the centers maintain the axis of rotation.

Points to Consider When Selecting A Face Driver

Here a couple of major points to consider when selecting a face driver ... these may also effect your decision to consider a face driver for your situation :

  1. The diameter of the face driver ... as measured across the driver pins ... should be smaller that the diameter where it will be locating to allow for complete access to the workpiece material.
  2. The diameter of the raw stock should not more than 3 times the diameter as measured across the driver pins.
  3. Drive pins are different for CW and CCW rotation ... consider how you will be machining the workpiece and what direction the spindle will be rotating when selecting a face drive.
One of the top manufacturers of face drivers is Riten Industries Inc ... they can be found on the web along with additional information on face drivers and other CNC turning workholding options by clicking the image below.

Please come back for our next installment in our series on Shop Efficiency.
Until next time ... Happy Chip Making !!

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1 comment:

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