Luigi Mazzenga
CALIPERS
Calipers were designed to provide accuracy a standard scale or rule could not. There are three types of calipers available, the vernier caliper, dial caliper and digital caliper. However, whichever you choose they will share the same basic construction, a beam, a slide, and a depth gage.
Normally, calipers are made of hardened stainless steel but can be available with carbide tipped jaws (expensive) or made entirely of plastic (cheap). They will have two pairs of jaws, with the lower jaws used to measure an outside diameter (O.D.) and or length of an object while the upper jaws are used to measure an inside diameter (I.D.) and or width of a slot, or pocket. The sliding scale will run along the beam pushed along by a thumb screw and it will have a lock. The vernier will also have a depth rod for measuring deep recesses. Calipers can come in four, six, eight, twelve, and 24 inch lengths with higher end models having a SPC function which is used to chart the dimensional accuracy of a detail or a set of details during a production run.
The vernier caliper is the most economical of the three and has lines engraved along the beam every .025 of an inch with every fourth graduation numbered. The sliding portion has graduations set at .001. In reading the calipers, note the number of tenths (.100) or inches, by using the zero line of the slide. Then add the number of thousandths (.001) indicated by the best line alignment between a number on the beam scale and a number from the slide scale that you can determine.
Unfortunately, using a vernier caliper can be difficult as one gets older and your eyesight diminishes. Otherwise, because it has just three moving parts it is very reliable.
The second type of caliper is the dial version. This caliper is much easier to read because the engravings are set at .100 of an inch with every tenth line numbered and comes with a large dial mounted to the slide that reads from 0 to .100. Simply use the front edge of the slide as your zero and measure your hundredths and or inches. Then read the number that the indicator hand on the dial is pointing to, and add.
One problem with a dial vernier is that the dial has a pinion that runs along a rack screwed into the beam. If a chip gets lodged between the pinion and rack the pinion will jump the rack causing the zero (on the dial) to register other than at the twelve o’clock position. While you can move the dial to set your new zero position it can be awkward to look, at say, the two o’clock position for your zero. If you have a dial vernier it is best to lay it upside down when not in use so chips cannot find their way between the pinion and rack.
The third caliper is the digital version. It is popular today because it has multiple functions available. On- off, inch or millimeters, zero settings, and a LCD display all at the press of a button. It uses a battery to provide power to these functions and you should always have a spare battery available.
Calipers should not be dropped and always kept clean and they should not be trusted to carry out very critical dimensioning. If you have to hold a dimension less than .001 of an inch you should use a micrometer. And for safety’s sake, you should never try to measure an object while it is rotating!
MICROMETERS
The outside micrometer is the industry standard for measuring tools because of its high accuracy and ease of use. They can measure as low as .0001 of an inch and can be had in a digital version with or without SPC. Some economic micrometers will come with an odometer style counter with thimble graduations.
Micrometers are made with a C shaped frame and incorporates an anvil, spindle sleeve (with a scale), and thimble (with a scale). They can be purchased individually and or in sets and can come with special anvil adapters (low cost) or made specifically with the type of anvil and spindle desired such as a disc micrometer. The accuracy of the micrometer is derived by several factors which are: thread accuracy, flatness of the measuring surfaces, and the rigidity of the frame.
The precision ground thread found on a micrometer uses a 40 threads per inch (tpi) spindle, which in one full rotation advances .025 of an inch. The sleeve is graduated every .025 and the thimble every .001 of an inch. Some micrometers will also have an additional vernier scale that allows the user to read to .0001 of an inch. The measuring faces are flat and parallel and normally have carbide faces but can be had with hardened steel to reduce cost. Micrometers should not be dropped or squeezed as if it were a c-clamp and always kept clean. And as with the vernier you should never try to measure an object while it is rotating!
CUTTING TOOL MATERIALS
The two most common materials used for cutting tools in a machine shop is high speed steel and carbide.
High speed steel
High speed steel (HSS) is used for cutting tools, such as drill bits, end mills, lathe tools, and saw blades, to name just a few examples. HSS is preferred over high carbon steel because of its resistance to high temperature, retention of hardness, increased wear resistance. It is a tool steel that with the addition of alloys, such as cobalt, chromium, molybdenum, and tungsten, offers strength to withstand strong cutting forces and performs well in intermittent cutting applications. HSS is also economical especially compared to carbide.
HSS lathe tools are easily ground by hand using a pedestal grinder, however end mills and other specialized tools will need a fixture for proper angle and clearance requirements.
Carbide
Carbide is also used extensively in the metal working industry today because it can withstand more heat than HSS, which results in higher speeds and feeds, and is superior to HSS for cutting exotic and tough materials especially those used in the defense and space industries. Carbide is a composite with carbide acting as the aggregate and a metallic binder serving as the matrix. Carbide cutting tools come in different grades, depending on the material that needs machining and are generally more expensive than HSS, though carbide tipped tools are more affordable. Generally carbide is more cost effective in a production or research and development (R&D) setting.
Carbide is difficult to sharpen and can chip or crack easily. As a result care must be taken when sharpening and determining speeds and feeds.
Tools you will use to machine the abacus:
CUTTING TOOLS
Hacksaw
A hacksaw consists of three parts, the frame, handle, and blade. Frames can be solid or adjustable and are usually found in lengths of 8, 10, or 12 inches long. Most hacksaws will use HSS blades with the teeth facing away from the handle and will cut on the forward stroke. Use the full length of the blade in slow and steady strokes applying pressure on the forward stroke. Cutting on the back stroke will dull the blade and depending on the material that needs cutting different saw blade pitches can be found.
Center drills
Made of HSS or carbide, center drills are often used to position holes accurately for further drilling operations. Its short drill tip will not deflect thereby controlling the position. Center drills are normally held in drill chucks, but can be held in a collet, if runout is a concern. Center drills come in a range of body sizes from 1/8th to ¾ inches and are classified 00 to 8 and can be single sided or double sided.
Generally they will have a 60 degree taper following the drill tip for live center work on a lathe, but may come in 82 or 90 degree angles for center drilling and counter-sinking unified national fine (unf), unified national coarse (unc), or metric flat head screws in one operation. Note that, 00 tips tend to break easily as a result of material hardness or poor lubrication which can cause galling, so caution is advised. Because it is HSS, which is hard, you will not be able to drill the broken tip out. Electrical discharge machining (EDM) or a carbide tool must be used.
Drills
Drills are generally made out of HSS, but can also be carbide tipped or made from solid carbide. Twist drills are the most commonly used type of drill found in a machine shop. They can come with a straight shank (called jobber length drills) which has a relatively short length to diameter ratio, which helps maintain rigidity. Twist drills can also come with a Morse taper, which has a tapered shank and a tang at the end which prevents slipping, and provides more driving power.
Drills will come with two flutes and a host of different helixes depending on the depth of cut and material hardness. For normal operations a 118 degree drill tip angle is used, but a 135 degree tip angle is available for harder materials. They can also be made with a split point, which reduces the pressure needed to machine away material from the center web.
Drills are generally sharpened by hand or with a special grinding fixture using a pedestal grinder. Care must be given not to overheat the tips, which can result in annealing. Attention to equal tip angles is also important since an unequal tip will cause one flute to do all the cutting, which could lead to an oversized and out of round hole. Angle gages are available to help. To avoid drills from grabbing in soft material low helix drills should be used with a flat ground on the leading edge of the lip, while high helix drills should be used for deep holes where chip extraction could be a problem. Drills come in numbered, fractional, letter and metric sizes.
Counter-bore
Counter-bores are cutting tools used to enlarge a previously drilled hole and provide a flat bottom to recess a cap head screw below the material surface. They are guided into the hole with a pilot to ensure concentricity. They are generally made from HSS and come in two or more flutes with two flutes providing the most chip clearance. The diameter of the counter-bore is usually 1/32nd larger than a cap head screw but can be purchased larger if needed. Interchangeable pilot counter-bores are available, when counter-bore pilot hole sizes vary. The interchangeable pilot is held in with a set screw.
Taps
Taps are made of HSS or carbide with special coatings applied to increase wear resistance if necessary. Taps are used to cut internal threads in a hole. The active cutting part of the tap is the chamfer while the fluted portion of the tap provides space for chips to accumulate and for the passage of cutting oil. They are found in two, three, and four flutes for normal applications.
The three most common taps are called taper, plug, and bottoming taps. A taper tap, usually has 8 to 10 threads undersized, and is good for starting a tap straight because its long chamfer will cause the tap to engage deeper in the hole before cutting. The plug tap is the most commonly used tap and has a much smaller taper, usually 3 to 5 threads, while bottoming taps have the smallest taper, 1 to 2 threads, and is used to produce a full thread closer to the bottom of a blind hole than a taper or plug tap can.
Tap wrenches, whether standard or T-handled, are used to drive the tap into the hole. A hand tap fixture that holds the tap in a bushing and is driven by a spindle, or a tapping block, is the best way to ensure a tap cuts squarely in a hole. However, one should learn how to tap by hand, as there will be instances when fixtures cannot be used.
The major diameter of a tap is the outside of the tool as measured over the thread crests. The pitch of the thread is the distance from one thread crest to next thread crest. So that, a ¼ – 20 tap would mean that the diameter is .250 and the pitch is 20 threads per inch with a distance of .050 between threads and there is a tolerance range to choose from concerning the pitch diameter. Hole sizes for taps can be easily found by using reference materials and will normally give you the size for a 75 percent thread
The quality of the hole can affect the thread. A hole that is out of round, bell mouthed or oversized will give you a poor fitting thread. Since the taps are hard, broken taps cannot be drilled out. EDM, carbide or special extracting tools must be used.
End mills
End mills are the most frequently used tool with a vertical milling machine. Usually held in an R-8 collet or special tool holder (never a drill chuck), end mills can be made of HSS, solid carbide, or carbide inserts screwed into a purpose built tool-holder with special coatings available, if needed, to increase wear resistance. Generally end mills will be two fluted (used on softer materials such as brass and aluminum), four fluted (used on harder materials such as CRS and tool steels), or three fluted (which is a hybrid of a two and is ideal for slot work).
End mills will cut on the bottom and side and is suited for surface milling, profiling, and slotting. Two flute end mills can be used for plunging and starting their own hole. Four fluted end mills may also center cut however, if their center has been center drilled or gashed it will not cut its own hole. A hole will have to be drilled first in order for the tool to be lowered and its ends used. Two flute end mills have a large chip capacity but a four flute is stronger and permits faster feed rates and generally provides better finishes. Unfortunately, four flute end mills have a tendency to gall in softer materials such as aluminum. If used to cut soft material attention should be given to proper lubrication.
Special care must be given when using the side of the end mill when cutting material to length or in pockets. For instance, when the cutter is rotating (normally clockwise) you will want to feed the material “against” its rotation for heavy cuts. This is called conventional milling. If you feed your work in the same direction as the rotation it is called climb cutting, and this should only be used for light finishing passes. A heavy climb cut can potentially cause the end mill to “bury” itself into your work, damaging the cutter, your part and scaring the “bejesus” out of you!
End mills can be found with different helix angles, and can be single ended or double ended. They can also be straight toothed, and available with a ball nose, or as a corner rounding cutting tool among other shapes.
– Lathe cutting tools
Cutting tools used in a lathe can be made out of HSS, brazed carbide, or carbide inserts that are screwed into a purpose built tool-holder. Generally, we use brazed carbide in the student shop because even though it is more expensive than HSS it is more durable and more economical compared to inserts.
There are a number of cutting tools used in a lathe from threading tools to boring bars, however the most often used tools are used to turn a diameter or face a part and they are designated AR and LR.
AR tools are meant to cut along the Z axis that is from the tailstock towards the spindle or right to left and is designed to reduce a diameter. The LR tool is meant for facing or moving along the X axis, left to right if looking at the spindle from the tail stock.
Cemented carbide can be sharpened by hand using a pedestal grinder but needs special care and two wheels should be used. First an aluminum oxide wheel to grind the shank and then a silicon carbide or green wheel for the carbide portion. If the carbide portion of the tool gets hot, it should not be quenched in water; allow the tool to air cool or dip the shank portion carefully in water to avoid fracturing the carbide.
HSS tools can also be ground using a pedestal grinder (using an aluminum oxide wheel) but should be cooled periodically by quenching it in water before getting red hot. Both types are held in a tool-holder using set screws and the tool-holder is locked in a tool post. The tool-holder is adjustable (up and down) as is the tool post (left to right) in order to center the tool and give it the proper clearance needed.
Tools for part-holding:
A lathe and milling machine will be used for machining parts for the abacus. In the lathe the part will be held in a collet (rotating) while the cutting tools will be held in a tool holder (fixed), which is mounted to the top slide of a lathe.
The most commonly used collet found on a lathe is the 5C collet, which has a twenty degree included angle and is ‘drawn’ into the spindle using a draw bar. The collet will tighten around a part as the collet taper and spindle taper meet. 5C collets can accommodate diameters from 1/64 to 1-1/16 in 1/64 increments with non-standard sizes available also.
Cutting tools for the milling machine will be held in an R8 collet (rotating) which has an included angle just shy of seventeen degrees and as with the lathe is drawn into a spindle with a draw bar. Parts will be held in a six inch vise (fixed). The six inch vise will normally have jaws that are hard and will fix your work firmly and safely. Care should be given to prevent cutting tools coming in contact with anything other than your work. If a tool comes in contact with the vise jaws the tool and or jaw will be damaged. Avoid drilling or milling into the vice and or mill table also.
Most work held in a vise will be supported underneath by parallels. These parallels generally come in pairs 1/8 inch thick, and run from ½ to 1-5/8 inches high. They are also hard and should be avoided by cutting tools. The main purpose of the parallels is to keep work off the bottom of a vise and to position the work perpendicular to the spindle. It also supports work that will encounter heavy pressure in the z axis which can cause the parts to bend or move. Thin parallels (1/32nd) are available for machining operations that could interfere with a standard parallel.