Over the years, companies performing turning operations have come to realize the versatility of using face drivers as a complete workholding solution. 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 held in the chuck. The problem here is that there is crucial cutting time be wasted every time the part is flipped and not cutting chips. Over the course of a year this can prove to me a huge loss in productivity.
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 axles 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 just about any type of part can be turned.
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, 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. This can be done by simply swapping out drive pin sets to achieve a very healthy diameter range for example an FRB45 has a range from roughly 1.5”-5” in capability.
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.
There are two important rules to keep in mind concerning face driving. 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 (e.g. turning, grinding, hard turning, gear hobbing). If there is doubt as to how much force is needed, you can consult the 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 toward 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 toward 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.
Nose Cone: This area of the face driver (attached to flange/shank area) is the housing and guide for the drive pins, center pin, and its compensating mechanism/spring-loading system. The compensating mechanism/spring-loading system allows the drive pins to adjust for any variation in flatness of the workpiece’s end. In FRB’s mechanical design, a convex/concave washer system works with the spring-loaded or fixed center pin to allow the system to better work with irregularly faced workpieces (typically found in unevenly sawed parts or near net shape cast or forged parts — up to 35 degrees of variation from perpendicular is allowed in some FRB models). In addition, its clamping segment design holds the center pin in a fixed position once clamped, which results in maximized TIR performance.
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 (i.e. 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 FRB 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 machine’s 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/FRB. 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 ensure proper work holding, a suitable driving element is needed that 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 chip 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.
- 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.
Other benefits of face drivers are reduced energy costs, and 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 (FRB offers a completely sealed system).
Consider the ‘Support Factors’
Make sure you choose a supplier who will work closely with your company to help you specify the correct face driver for your applications. Some even offer 30-day trials 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.