General Shop Usage Notes / County College of Morris / Engineering Technology These notes are a useful starting point and should be supplemented by a hands-on demonstration and lecture. :SAFETY: Safety glasses with side shields are mandatory. Normal sun glasses are never acceptable. Face shields and special, darkened welding shields may also be required for some processes. No open toe shoes or “flip flops”. Short sleeves or rolled up sleeves. No coats, hanging jewelry, ties, etc. when running machine tools. Longer hair should be tied back. Be sure you know where all “Master Off” power switches (“Panic Switches”) are in the lab. Be sure you know where all fire extinguishers are in the lab, and review proper operation. Be sure you know where all emergency exits are in lab. Some common lab errors: Never remove and operate machines without safety guards. Never use compressed air to blow dust off yourself, a machine or others. Compressed air is never to be aimed at others under any circumstance. Do not leave chuck key in lathe, drill press or similar machines. Never machine or cut unsupported or unsecured work pieces. Watch for trip hazards related to extension cords or material on floors. Watch for “eye level” hazards such as material in metal storage rack. Never turn on unfamiliar machine prior to operational training. Be sure to block bright arc-light and use proper ventilation when welding. :LAYOUT: Generally, absolute datum with “origin” (0,0,0) in lower left corner is preferred. Become familiar with Cartesian (X,Y and Z) coordinates. Start with a reference corner that is “square” and machined. A saw-cut end is never acceptable for precision metal working. Be familiar with height gauge, dial caliper, digital caliper, micrometer, surface plate, solid machinist square, V-block, parallels, center punch, center drill and related equipment that is used for layout. For layout of features (such as holes), make a table or X & Y locations prior to layout. Always reference X & Y position to center of hole. Never reference from edge of hole. Decide upon practical tolerances. Be sure you use methods that result in consistent measurements such as “stop blocks”. For small number of pieces, you may wish to “line drill” all pieces in a “stacked” layout to ensure consistent dimensions. :Twist Drill (also called “jobber’s drill” or simply, “drill bit”): Uses a 59 degree “point” which is a compromise angle for most metals. Usually made from High Speed Steel (HSS). Some drills are made from improved carbide or coated Ti material for longer life. Imported, low cost, carbon steel drill bits (typically bought in the “Dollar Store”, etc.) are a waste of money. Drills are available in metric, fractional inch, number and letter size. A large chart that compares the various sizes can be very useful. The “point” is actually a flat area called a “spade”. Twist drills create crude holes that are usually rough in surface and oversize. The holes are often out-of-round. A twist drill is not a finish tool; it is more like a chain saw because it is used to do rough, unfinished work. Always use appropriate RPMs and “clear” drill bit. You can never clear the drill too much. This is especially important on blind holes. Water soluble coolant can help but is not required with many non-ferrous metals such as aluminum or brass. Light oil or lard can be used when drilling mild steel (watch fumes). Some specialty drilling fluids are highly carcinogenic; read labels. Be especially careful when “breaking through” a surface on a through-hole with smaller drill bits. If a drill bit breaks off in a work piece, you cannot “drill it out” without special, expensive equipment (ie-EDM machine). If the drill bit is broken above the surface, you may be able to grab it with a pliers and twist it out backwards with a gentle forward/backward motion. If the bit is broken off “flush” or below the surface, the work piece may be ruined. Dull twist drills usually make oversize holes (since they wobble off-center). If you drill a piece of steel with a dull drill bit, you may “work harden” the piece. This may make the piece impossible to drill even with a new, sharp drill bit. You may have to use a grinder to remove the work hardened material (typically about 0.030” deep) before successful drilling can commence. Use cutting speed calculations or refer to chart below for basic setups. Be very careful using large diameter twist drills in hand-held electric drills. They can “catch” can cause serious injury. Similarly, never hand-hold work pieces in a drill press, especially with large diameter twist drills. Drills can be hand sharpened on a pedestal grinder. A hands-on demonstration would be required to explain this. Once you understand the process, practice on old, dull twist drills. Be sure the spade is narrow enough and centered. Use a machinist reference book for specific angles. A “Drill Doctor” semi-automatic sharpener can be a useful tool. Never use Morse Taper shank drill bits in a 3-jaw chuck or collet. When drilling on a drill press, it is often best to keep one finger near the “Off” switch. When the drill “breaks through”, shut off the motor and wait for the quill to stop turning before lifting the drill out. This will prevent “lifting” the work piece which usually spoils the hole. For drilling on a lathe (using the tail stock with 3 jaw chuck) a “half-drill” can be quickly fashioned from a broken drill shank or good quality dowel pin. Approximate Twist Drill RPMs: Drill Diameter Aluminum or Brass Mild Steel 1/8 3000 1500 1/4 1500 750 1/2 750 375 1.0 300 150 Special drill bits for plastic usually have a different angle (typically 90 degrees). Drill bits for wood are often thin, flat-ended “spade” drills that allow lots of room for chips. :REAMERS: A reamer is a finish tool. It cannot drill a hole, but can enlarge an existing, drilled hole. Reamed holes are much more “true” in terms of diameter, straightness and surface finish. For RPMs, use about half the speed recommended for twist drills. Do not run the reamer up-and-down the hole numerous times: this will “oversize” the hole. Feed fairly rapidly and shut off the motor as soon as the reamer “breaks through”. Wait for the quill to stop turning before lifting the reamer out of the hole. In some cases, a reamer turned by hand (in a tap holder) is acceptable. Most reamers are straight-fluted but some specialty reamers have spiral flutes. Do not try to remove too much material with a reamer. They are only intended for light, “finish” cuts. Reamer / Drill size information: If a reamer diameter is 0 to 1/4 “, choose a drill that is about 0.010” undersize to pre-drill the hole. If a reamer diameter is 1/4" to 1/2 “, choose a drill that is about 0.025” undersize to pre-drill the hole. If a reamer diameter is 1/2" to 1.0 “, choose a drill that is about 0.00” undersize to pre-drill the hole. There is some overlap in sizes above, but this can be used as a general, rule-of-thumb for aluminum, brass and mild steel. For example, if a 3/8” reamed hole (0.375”) is desired, drill a hole that is approximately 0.350” in diameter first (this is about 0.025” undersize). There is no actual drill size available in 0.350” so you will have to choose the closest size available. 11/32” (which is 0.344”) would be an acceptable size for pre-drilling. :TAPS, DIES and THREADS: Threads are used fasten mechanical component together. Thread sizes generally give the diameter first and then the thread “pitch”. With Imperial (“inch”) threads the thread pitch is given in “threads-per-inch”. In metric threads, the thread pitch is a measure of the distance from one thread ‘peak” to the next. Typical sizes are: 1/4 –20, 3/8 –16 and 1/2-13 (all Imperial sizes) M8-1.0, M10-1.5 and M5-0.8 (all metric sizes) Threads are often available in “fine” and “coarse” threads. A tap is a hardened cutting tool that can cut a thread in a pre-existing hole. Since the tap is somewhat brittle, it is very important that the pre-existing hole be drilled to the correct diameter. If the hole is too small, the tap will bind and snap off. It is virtually impossible to drill out a tap with normal cutting tools and the work piece may be ruined. Special tap extractor tools are available but they are often unsuccessful. Be sure to use a tap drill chart to select the correct drill bit size prior to tapping. For example a 1/4-20 tap require a pre-drilled hole of 0.201” (which is a #7 drill). If you wanted a 1/4-20 tapped hole, you would never drill a 1/4” hole; the hole would be too big and there would be no material left for the tap to cut. Tapping can be done by hand or machine. When tapping by hand, a hand tap should be turned forward 1/4 turn and then backed up about 1/8 turn (to clear the chips). Then turn forward 1/4 turn and repeat. Taper hand taps are easiest to start but they will not thread to the bottom of a blind hole. A bottoming hand tap can be used, but is difficult to start, so the first few threads can be cut with a taper hand tap. For through holes a machine tap (or “gun” tap) can be used. These do not require “backing up” every 1/4 turn (because they push the chip forward in a through hole). Special tapping fluid can be used to help the process. Some of this fluid is highly toxic so use caution. Typically, soft materials like aluminum work better with “coarse” taps (such as 3/8-16), because there is less chance of stripping. For harder materials such as mild steel, a coarse or fine thread (such as 3/8-24) is acceptable. Fine threads tend to be resistant to vibration and loosening. Dies are for cutting external threads on a shaft. They are used in a similar fashion to taps. A shaft should be reduced in diameter before threading. For example, a 1/4-20 die used on a 1/4" mild steel shaft will work best if the shaft is reduced to a diameter of about 0.230" before threading. Consult text book or tap/die chart for more information. :MILLING AND END MILLS: End mills are not interchangeable with drill bits. They look somewhat similar, but serve a very different function. Although they are called “end” mills, they can cut on their side as well as their end. 2-flute end mills are useful for softer materials such as aluminum. Feed rates on soft material are higher, creating more chips. The extra area between the flutes in a 2-flute end mill provides more space for these chips. With less cutting flutes to “share” the cut, each flute absorbs more heat. But since softer materials cut easy, there is less heat build-up. These end mills can be used to cut mild steel, but feed rates must be reduced to prevent overheating of cutting surface. 4-flute or 6-flute end mills are useful for cutting harder material. The increased number of cutting edges “shares” in increased heat load generated in harder materials. They have less space between the flutes, but this does not matter since harder work piece materials cannot be cut at high feed rates, resulting in less chips in a given amount of time. 4-flute ends mills can be used to cut aluminum, but feed rates must be reduced to prevent clogging. Some end mills are “center cutting” and can “plunge” like a drill bit (although it is still preferable to pre-drill with a slightly smaller diameter twist drill). Many end mills are not center cutting, so care must be used not to “load up” the center. End mills are never held in 3 jaw chucks. They must be used with collets. Recommended RPMs for various materials are the same as RPMs for drill bits. Maximum depth of cut on end mills is equal to the radius of the cutter. For students using milling machines in a school lab, 0.050” should generally be considered maximum depth for most work. Always use “forward” milling direction for preliminary, roughing cuts. This will create a rougher finish but is much safer and easier on equipment. When the work piece is close to size, it is acceptable to use very light “climb” milling cuts (no more than 0.010”) with slow feed rates. No one should operate a milling machine until they fully understand the difference between “climb milling” and “forward milling”. Most of the accidents that happen on milling machines are caused by people who do not understand this principle. Be sure that you understand the process of removal and installation of a collet on a Bridgeport-type (knee) milling machine. Incorrect methods can spoil the draw bar or result in dangerously loose cutters. Never leave wrench on draw bar. When moving the mechanical range lever from ‘high” to “low” on a Bridgeport-type (knee) milling machine, be sure gears are fully engaged before turning on. Watch to make sure milling machine spindle is not turning in reverse (a common mistake). Be sure quill lock is tight. Table locks can be left loose for most milling operations. Always compensate for “lash” (or “slop”) in table axis. Be sure vise is square before cutting. Use of parallels is normally a good idea. Be sure to “seat” work against parallels with a soft mallet. When using fly cutters, be sure you have “swing clearance” before turning on machine. Feeding in “from the side” is usually better than “plunging” (even with center cutting end mills). :LATHES: There is no such thing as “lathing”. You are usually “turning” (along the OD) or “facing”. Other operations such as drilling, knurling and trepanning are possible. Keep tool “on center” with slight back rake. Never leave key in chuck, even for a few seconds. Never “stop” a coasting chuck with your hand. Do not reach in with your hand to clear out chips on a running lathe. Even when stopped use caution as chips may be razor sharp. Be sure tool holder and tool bit are tight. Small radius (“sharp”, as viewed from top) tools are best for roughing. Large radius (“rounded” ”, as viewed from top) tools are best for light cuts and smooth finish. When using grooving (or “parting”) tools, be sure to cut slightly wider than tool width or tool may drag and break. Watch clearance as you approach chuck jaws. Always cut towards chuck (cutting away may drag piece out of chuck). Never “over-extend” without tail stock support. Maximum unsupported extension for aluminum is about 3 diameters. In other words, a 1-inch aluminum shaft cannot hang out of the chuck more than about 3 inches without tailstock support. Similar for brass. For steel, you can hang out 4 diameters. Never use auto feed (clutch-driven) function or threading (half-nut) function unless you know how to use each of them and how they differ. It is very dangerous to use threading function in high range. :PEDESTAL GRINDER: Number one rule: If it doesn’t spark, it doesn’t go against a grinder. Non-sparking metals (like aluminum or brass) will clog a grinding wheel. The pieces of aluminum lodged in the wheel can also expand with heat and blow the wheel up. Non-sparking metals can be used on a belt sander. Be sure tool rest is adjusted to a safe clearance distance (about 1/8”). Otherwise, the grinder will be dangerous to use. Watch that sparks don’t shoot towards flammable materials. Never grind against the side of the wheel. You can tap a wheel to see if it sounds “clear” with a bell-like ring. A dull-sounding wheel may have a dangerous crack in it. If steel that you are grinding turns blue or red, slow down. You are using too much pressure. If you are grinding tool bits, keep cooling them in water or grind slowly. Otherwise, if the tool turns red or blue you may have spoiled the tool by destroying its “temper”. You can identify steels with a spark test on the pedestal grinder. Mild steel (low carbon) tends to make longer, yellow sparks. Tool steel (high carbon) tends to make smaller, orange sparks. Mild steels are better for welding and machining but cannot be easily hardened or heat treated. Quenching red-hot mild steel has no useful effect on it. High carbon steel can be softened (annealed), then drilled, machined and then re-hardened for tool purposes. But it is very tricky to weld and can sometimes “air harden” when you are machining it. Other Links That May Be of Interest: http://vintageracer.tripod.com/events.html http://npmccabe.tripod.com/steam.htm