Face Driving Applications:
Dispelling the “Limited Applications” Myths

Neidlein Product Manager
LMC Workholding, Logansport, IN

Mechanical face drivers might be the best way to turn parts compared to other types of traditional chucking methods. They allow turning applications to have increased flexibility to lower cycle times, turn both the smallest and largest of parts and even allow interrupted and heavy cuts. All these benefits combined with extreme low maintenance add to the manufacturer’s bottom line by making their turned part work more profitable.

Over the years, companies performing turning operations have finally realized the versatility of using face drivers. Along the way, they have continued to gain greater acceptance for a multitude of turning applications. The major benefit of a face driver is found in its ability to allow the part to be completely turned from one end to the other in one operation. In a typical turning application, the operator chucks one end of a part and begins cutting. As he nears the chuck area, he has to stop, remove the part from the chuck, turn the part around or “flip it”, re-chuck it and cut the end that was previously chucked.

Two problems are inherent in this situation. First there is an increase in the part’s overall processing cycle time. In high volume part manufacturing especially, this chucking/re-chucking can significantly add to the part’s overall cycle time. Also, the operator (many of whom operate multiple machines) is wasting time chucking and re-chucking parts. Their time could be used more effectively and profitably tending additional machines or performing other operations (IE: programming, quality checking, etc.).

Secondly, any time a part is removed from the turning center’s chuck, it runs the risk of tolerance stack-up with the run-out and concentricity of the finished part no longer being in tolerance after removal and cutting. If the part is not perfectly re-chucked to original alignment, then its dimensions can vary enough to end up being scrapped or needing rework.

Face drivers can easily avoid both of these problems, because they eliminate the need to flip the workpiece during cutting and allow the entire part to be machined in one chucking.

Heavy Cuts…Interrupted Cuts… Range of Workpieces...
Different Applications—All Accepted in Face Driving

Face drivers truly offer few limitations for their use. Manufacturers at one time thought they were primarily just for long and straight cylindrical shafts requiring turning between centers. Now they’ve found that anything that can be placed and turned between centers can be done with a face driver.

Some parts that are routinely cut using face drivers include automotive transmission parts, crankshafts, cam shafts, pinion gears, electric motor shafts and forgings and axle’s up to 36” in diameter. To be safe for turning using a face driver, the rule is a three-to-one ratio drive diameter to rough workpiece diameter, but five-to-one ratios have also been used. This means that the largest outside diameter of the part is five times larger than the drive diameter.

At one time, heavy cuts were thought to be an issue with face drivers because of part slippage. But this is no longer true especially with mechanical face drivers. With proper center pin loading and the use of drive pins (some applications do not require drive pins…IE: Neidlein FDNC Face Drivers are used in gear hobbing applications and include a drive disk system to enable “no slippage” gripping performance), just about any type of part can be turned… and with greater efficiency than a jawed chuck as previously mentioned. One example is turning forged rolls weighing four tons and a drive diameter of 36 inches.

Interrupted cuts were also thought to be an application unacceptable for face drivers, but today these types of cuts can easily be done and with fast feeds and deep cuts, allowing a boost in productivity. Plunge cuts or grooving interrupted cuts into hardened gears and hard threading are routinely done today with face drivers.

Face drivers also offer a unique ability to handle a wide range of workpieces with one size driver. For example, Neidlein’s Size 4 Models (both spring loaded and fixed center versions) can handle workpieces from 1.575 to 7.0 inches with a simple change of drive pins.

Other applications such as hard turning, grinding, gear hobbing and milling are also accepted applications for face driving. For example, gear hobb clearance can be an issue for many types of workholding, but now with the correctly specified and utilized face driver, this application can be very successful. Spring-loaded center pins using a drive disk (instead of drive pins) work extremely well for hobbing. Also the drive disk can be quickly changed for sharpening or replacement because it is easily removed.

Two Important Face Driving Rules

Two important rules are applicable to using face drivers…

First, the application must have a machine that can deliver sufficient tailstock force to hold the workpiece. However, force depends on the type of workpiece being turned, the raw material it is made from, the number of drive pins used in the face driver and what type of operation being performed (IE: turning, grinding, hard turning, gear hobbing, etc.). If there is doubt as to how much force is needed, you can consult the face driver manufacturer for guidance. Generally most machines have enough tailstock force for a mechanical face driver, manual tailstock grinders and engine lathes being an exception, please consult your face driver supplier to check forces on your machine and if it is enough for your specific application.

Secondly, the first cut should generally be towards the face driver to better set the workpiece and let the drive pins do their work by biting into it. Once the first cut is made towards the face driver, then cuts toward the tailstock can be made. Grinding and hard turn applications may not need this though.

Face Driver Design Components

Face drivers have four main parts: the flange/shank, nose cone/carrier body, center pin and drive pins. Three different mounting positions are also available.

Flange/Shank: a flange mount or shank mount is used to attach the face driver to the turning center.

Center Pin: At the center of the face driver is a center pin that holds the part on centerline to the other center (or tailpiece support) on the opposite side, generally a tailstock/footstock with a live or dead center. A live/dead center is used to center the workpiece and locates its point in the workpiece’s center hole. Center pins can be changed for different types and sizes of workpieces being turned. (IE: Hardened workpieces typically require a carbide center pin or different diameters of center sizes). They center the workpiece to rotate along the machine’s axis and permit the drive pins to engage the part’s face. Widely accepted in industry today is the spring loaded center pin design that includes length positioning from the drive pin or face of the part. Fixed center pins (length measurements are taken from a gauge ball vs. the face of the part) are also available. Fixed center versions also provide even more extremely precise runouts, TIR measurements to .0002” – less than .0004” TIR on spring loaded designs from Neidlein models). In fixed center face drivers, a hydraulic or pneumatic actuator is used to push the drive pins into the part as opposed to spring loaded where the tailstock pushes the part to the drive pins. In cases where you use a fixed center driver (extremely high tolerances needed or where lengths are measured from the center hole using a gauge ball) you must be sure you have the right size actuator and draw-bar, so as to not overpower the tailstock and optimize your machines hydraulic/pneumatic system. Your face driver supplier should be able to provide these accommodations and items. LMC is also a chuck/cylinder manufacturer so this is standard for them to apply.

Drive Pins: To make sure that a part can be turned efficiently, drive pins are used that act as teeth that bite into the part’s face. These are replaceable and are available in sets of three, five or six…depending on the size of the driver and the part they’re turning.

Some applications require drive pins that are coated with materials such as carbide or diamond. Metal parts with a Rockwell C of 48 or more should use coated drive pins. Diamond coated pins are still used for grinding hardened workpieces. Grinding does not submit as much force as turning.

Drive pins can also be micro-serrated and hard-coated from LMC/Neidlein. With these applications, they reduce or eliminate part slippage, dramatically reduce scrapped parts, extend performance life over diamond-coated drive pins and increase turning productivity.

Finishing hardened workpieces using face drivers is gaining greater acceptance with the advanced capabilities of face drivers, machine tools and cutting tools. However, a drawback has been the ability to successfully and accurately clamp hardened workpieces, while at the same time, machining them entirely in a single operation. To insure proper work holding, a suitable driving element is needed which can handle high tolerance demands and still be cost effective to make hard turning possible. To do this, a system of micro-serrated and hard-coated drive pins mounted around the face driver (which has proven successful for many turning applications, but especially hard turning) are available.

Life expectancy for micro-serrated and hard-coated drive pins is six to ten times longer than diamond-coated ones in hard turning and grinding applications. These new pins are also much more reliable than diamond-coated pins that can lose grip or diamond chips off under tough turning conditions such as hardened workpiece turning. Even if a part should slip with serrated and coated drive pins, it won’t remove the serrations or coating as quickly as with diamond-coated products. Also, if slippage occurs, the pins generally can be used again, unlike diamond-coated ones that need replacing. In addition, to handle a greater workpiece variety, these pins can grip on a smaller surface area, than diamond-coated pins.

Given the proper parameters, these drive pins can achieve roundness tolerances of less than 0.002 mm (.000008”) in long production runs.

Other Application “Considerations”

Mechanical vs. Hydraulic Face Drivers: While both types are widely used, mechanical face drivers provide stronger reasoning in selection. Mechanical models have compensating drive pins and minimal center runout due to extended guide length on center, the center pin and its locking mechanism (in spring loaded models only). The center point is mechanically locked when it’s in the clamped position. These face drivers offer accuracy less than 0.0004” TIR. Part diameters that can be driven vary from as small as 0.240” to as large as 36” in diameter on standard face drivers. (Smaller and even larger diameters have been machined with special application face drivers from LMC/Neidlein). Parts weighing up to 8,000 lbs can be used with mechanical styles from manufacturers such as Neidlein (as compared to typical hydraulic face drivers being limited to parts at half that weight). And longer part turning may require the use of a steady rest (also available from LMC) for addition workpiece support.

Other limitations to hydraulic designs include:

• tolerance and repeatability concerns due to differences in hydraulic fluid temperatures (this changes your Z axis positioning in the machine and reduces length accuracies and repeatability)
• higher maintenance costs, continually changing hydraulic fluid and seals
• concerns with potential fluid leaks and seal problems, rusting of internal components
• 50% of the tailstock force is used to collapse the center pin in hydraulic face drivers because they have no center pin locking mechanism, only 5-10% of tailstock force is used to collapse the center of a Neidlein mechanical face driver leaving 90-95% of tailstock force for the drive pins to bite into the workpiece, thus requiring less tailstock force. This is because in Neidlein spring loaded mechanical systems the drive pins act in two ways. Not only do they bite into the part, but they also clamp the center pin. This is the most efficient use of tailstock force. This is in spring loaded center versions only.

Mounting Styles: Flange Mount— Using a spindle adapter for the turning center, this style of mounting attaches directly to the machine spindle. This is the most rigid type of mounting available. Shank Mount—When a machine spindle uses a Morse-taper or straight shank mount, this face driver is used. Soft Jaw Mount—Using a chuck with soft jaws, the face driver’s shank and nose cone are placed inside the chuck and gripped using the jaws. Chuck Mount---A special face plate is used to mount right to the face of your specific chuck once the jaws are removed.

Cost Savings:Other benefits of face drivers are reduced energy costs, less wear and tear on the spindle and the machine’s internal components, because of the moment of inertia to turn heavy chucks. It takes a lot more energy to spin a large chuck mounted on the machine than it might with a small face driver. Mechanical face drivers also offer significantly reduced maintenance costs. A simple grease application when changing drive pins is usually all that is required. While companies typically don’t give this much thought, saving energy and potentially saving on maintenance costs could total up to a substantial savings over time.

On Live/Dead Centers: Your face driver is typically only as good as your tailstock center. Carbide tipped centers work well in hard turning and grinding applications. Choose a design for optimized performance: maximized TIR, withstanding high axial and radial load performance and maintenance-free performance (Neidlein offers a completely sealed system).

Consider the “Support Factors”:Make sure you choose a supplier that will work closely with your company to help you specify the correct face driver for your applications. Some even offer 30-day trial offers for application try-outs or will perform try-out work at their own facilities. Choose a supplier with standard face drivers and drive pins that are ready for shipment as stocked off-the-shelf items. In many cases, the need for specially-built face drivers (per customer specs) are required for unique applications (or even specially-engineered drive pins) and these needs should be matched with your workholding partner’s capabilities. Replacement parts for special face drivers and drive pins should be made available within several days.