This month, I’ll talk about machining phenolic plastic, also known as Garolite or Bakelite. The material is hard and strong for a plastic and used for a variety of machinery components. It is reinforced with paper or cotton fiber and is a thermoset material, which means that it doesn’t melt and retains strength at higher temperatures. Being a plastic, the modulus of elasticity is much less than for metals. Compared with steel, phenolic is about 50 times less, meaning that an applied force will cause 50 times more deflection in a part made from phenolic. When clamping or cutting a part, this fact is important.
From time to time, I get a job to modify a Boston Gear QD60, a spur gear made of phenolic. It has a 5" pitch diameter and comes with a 0.75" bore without a hub projection. The gear needs the bore enlarged to 3" (±0.001"), as well as a bolt circle added. To resize the bore, I clamp the gear in a four-jaw chuck on a lathe. Figure 1 shows the setup. Pieces of aluminum stock are placed between the chuck jaws and the gear teeth. Clamping calls for a light touch so the teeth aren’t damaged. These gears are pricey, so I don’t want to turn one into scrap.
After centering on the 0.75" bore, I use a carbide boring tool to make the 3" feature. I like brazed-carbide boring bars because I can make a very sharp edge and set the tool with a positive rake angle. Due to the low modulus, the material will deflect under the edge if it is not sharp enough. When taking a heavy cut, too much material will be removed. With a light cut, too little material will be removed. If you have fussy features, sharpness and a positive rake matter. I cut this bore at 270 sfm or slower.
The material is abrasive and rapidly wears cutting edges. Since the part is clamped lightly in the chuck, I take light cuts, 0.08" on diameter per pass, until the bore is 0.02" or so undersized. Then I check the cutting edge. I dress it with a diamond file if needed and then hit the finished size. Hitting the mark is the art of a machinist.
This material does not make curly chips; it crumbles when cut and makes toxic dust that’s not a good idea to breathe. So if you want to live for a while, take care. I am 75 and don’t plan on checking out soon, so I use a dust mask when doing this lathe work.
The bolt circle is drilled on a CNC milling machine in the usual way. Figure 2 shows the mill setup. Be careful because 6" milling vises can generate a lot of clamping force. I find the center with an indicator. If I make more than one part at a time, I check the center on each one. The bolt hole size is 0.3125". I use a high-speed steel, screw-machine-length drill with a peck drilling cycle and Rustlick water-based flood coolant. The coolant takes care of the dust.
Holes are drilled at 700 rpm and a feed rate of 3 ipm. This produces a nice hole with clean edges and no frayed fiber. The combination of abrasive material with a low modulus causes the holes to drill undersized. I use a 0.315" drill and take care to keep the edges sharp. The finished holes are good at 0.31" to 0.314". I check the hole size with gauge pins. A duller drill drills a smaller hole, which clamps on the side of the drill. A little care prevents that. If I made a lot of these parts, I would go to carbide drills, but that would be a whole different story. Like I have said in the past, high production is not my game.
Related Glossary Terms
- abrasive
abrasive
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
- boring
boring
Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.
- centering
centering
1. Process of locating the center of a workpiece to be mounted on centers. 2. Process of mounting the workpiece concentric to the machine spindle. See centers.
- chuck
chuck
Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- coolant
coolant
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- feed
feed
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- inches per minute ( ipm)
inches per minute ( ipm)
Value that refers to how far the workpiece or cutter advances linearly in 1 minute, defined as: ipm = ipt 5 number of effective teeth 5 rpm. Also known as the table feed or machine feed.
- lathe
lathe
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
- milling
milling
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- milling machine ( mill)2
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- modulus of elasticity
modulus of elasticity
Measure of rigidity or stiffness of a metal, defined as a ratio of stress, below the proportional limit, to the corresponding strain. Also known as Young’s modulus.
- pitch
pitch
1. On a saw blade, the number of teeth per inch. 2. In threading, the number of threads per inch.
- rake
rake
Angle of inclination between the face of the cutting tool and the workpiece. If the face of the tool lies in a plane through the axis of the workpiece, the tool is said to have a neutral, or zero, rake. If the inclination of the tool face makes the cutting edge more acute than when the rake angle is zero, the rake is positive. If the inclination of the tool face makes the cutting edge less acute or more blunt than when the rake angle is zero, the rake is negative.
Author
Brandt Taylor is owner of Berlin, Massachusetts-based Taylor Engineering, a machine shop and manufacturer of lathe chuck jaws.
