US Number and Letter Guage

Number and letter gauge drill bits are still in common use in the U.S.. In the past, they were popular elsewhere, but now have been largely discarded in favour of metric sizes. A complication of using a gauge rather than a standard measurement of length is that the user always has to refer to a table of gauge sizes. For example, to drill a hole which is 10 thou (10 thousandth of an inch = .010) bigger than a 1/8 inch bolt, the user must first work out that 1/8 inch is 125 thou, so the hole size needed is 135 thou, and then must look up in a reference book which gauge drill bit has this diameter - a number 29. Number drill bit gauge sizes are analogous to, but different from, American wire gauge. (See the conversion table below). Number gauge is routinely used from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). Number gauge is actually defined at least down to size 97, but these smaller sizes are rarely encountered. It happens that as the technology for making small drill bits and drilling small holes has become more available, metric measurements have become the norm. Number and letter gauge drill bits are almost always twist drill bits. There is no particular reason why the gauge cannot be used to measure bits of other types, but the gauge covers a size range across which the twist drill bit is the most commonly used. The gauge-to-diameter conversion does not follow a set formula, but rather was defined as a useful and practical measure. The graph shows how gauge diameters change with gauge. Each step along the horizontal axis is one gauge size. The step size between adjacent gauges is smaller for smaller gauges. This is appropriate, because the tolerance of the diameter of drilled holes is closer for smaller drill bits. The increment from one gauge to the next for a number 92 drill bit at 0.2 mm diameter is just 5%, compared to 10% for standard metric sizes.

Metric Drill System

Metric drill bit sizes define the diameter of the bit in terms of standard metric lengths. Standards organizations define sets of sizes that are conventionally manufactured and stocked. For example, British Standard BS 328 defines sizes from 0.2 mm to 25.0 mm. From 0.2 through 0.98 mm, sizes are defined as follows, where N is an integer from 2 through 9: N * 0.1 mm, N * 0.1 + 0.02 mm, N * 0.1 + 0.05 mm, N * 0.1 + 0.08 mm, From 1.0 through 2.95 mm, sizes are defined as follows, where N is an integer from 10 through 29: N * 0.1 mm, N * 0.1 + 0.05 mm, From 3.0 through 13.9 mm, sizes are defined as follows, where N is an integer from 30 through 139: N * 0.1 mm From 14.0 through 25.0 mm, sizes are defined as follows, where M is an integer from 14 through 25: M * 1 mm, M * 1 + 0.25 mm, M * 1 + 0.5 mm, M * 1 + 0.75 mm, In smaller sizes, bits are available in smaller diameter increments. This reflects both the smaller drilled hole diameter tolerance possible on smaller holes, and also the wishes of designers to have drill bit sizes available within at most 10% of an arbitrary size hole. The price and availability of particular size bits does not change uniformly across the size range. Bits at size increments of 1 mm are most commonly available, and lowest price. Sets of bits in 1 mm increments might be found on a market stall. In 0.5 mm increments, any hardware store. In 0.1 mm increments, any engineers' store. Sets are not commonly available in smaller size increments, except for drill bits below 1 mm diameter. Drill bits of the less routinely used sizes, such as 2.55 mm, would have to be ordered from a specialist drill bit supplier. This subsetting of standard sizes is in contrast to general practice with number gauge drill bits, where it is rare to find a set on the market which does not contain every gauge. Metric dimensioning is routinely used for drill bits of all types, although the details of BS328 apply only to twist drill bits. For example, a set of forstner bits may contain 10, 15, 20, 25 and 30 mm diameter cutters.

Fractional-Inch Drill System

Fractional inch drill bit sizes are still in common use in the US. In the past, they were popular elsewhere, but now have been largely discarded in favour of metric sizes. ANSI B94.11M-1979 sets size standards for jobber length straight shank twist drill bits from 1/64 inch through 1 inch in 1/64 inch increments. For morse taper shank drill bits, the standard continues in 1/64 inch increments up to 1 3/4 inch, then 1/32 inch increments up to 2 1/4 inch, 1/16 inch increments up to 3 inches, 1/8 inch increments up to 3 1/4 inches, and a single 1/4 inch increment to 3 1/2 inches. One disadvantage of this scheme of sizing is that the size increment between drill bits is very large for the smaller sizes, 100% for the first step. The implication is that number gauge drill bits have to be used to bridge the gaps. Another disadvantage is the convention in labelling the bits. Rather than an integral number of 64ths of an inch, drill bit sizes are written down as irreducible fractions. So, instead of 78/64 inch, or 1 14/64 inch, the size is always written as 1 7/32 inch. This can lead to confusion and mistakes unless great care is taken. Below is a chart providing the decimal-fraction equivalents that are most relevant to fractional-inch drill bit sizes (that is, 0 to 1 by 64ths). (Decimal places for .25, .5, and .75 are shown to thousandths [.250, .500, .750], which is how machinists usually think about them ["two-fifty", "five hundred", "seven-fifty"]. Machinists generally truncate the decimals after thousandths; for example, a 27/64" drill bit may be referred to in shop-floor speech as a "four-twenty-one drill".)

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Introduction to G-code

G-code is the common name for the most widely used computer numerical control (CNC) programming language, which has many implementations. Used mainly in automation, it is part of computer-aided engineering. This general sense of the term, referring to the language overall (using the mass sense of "code"), is imprecise, because it comes metonymically from the literal sense of the term, referring to one letter address among many in the language (G address, for preparatory commands) and to the specific codes (count sense) that can be formed with it (for example, G00, G01, G28). In fact, every letter of the English alphabet is used somewhere in the language, although some letters' use is less common. Nevertheless, the general sense of the term is indelibly established as the common name of the language. G-code is sometimes called G programming language. This usage may be more common outside North America than inside. American industrial CNC users tend to say G-code only.

Standards and Development

The first implementation of numerical control was developed at the MIT Servomechanisms Laboratory in the early 1950s. In the decades since, many implementations have been developed by many (commercial and noncommercial) organizations. G-code has often been used in these implementations. The main standardized version used in the United States was settled by the Electronic Industries Alliance in the early 1960s.[citation needed] A final revision was approved in February 1980 as RS274D. In Europe, the standard ISO 6983 is often used, although in varied states sometimes used other standards, example DIN 66025 or PN-73M-55256, PN-93/M-55251 in Poland. G-code began as a limited type of language that lacked constructs such as loops, conditional operators, and programmer-declared variables with natural-word-including names (or the expressions in which to use them). It was thus unable to encode logic; it was essentially just a way to "connect the dots" where many of the dots' locations were figured out longhand by the programmer. The latest implementations of G-code include such constructs, creating a language somewhat closer to a high-level programming language. The more a programmer can tell the machine what end result is desired, and leave the intermediate calculations to the machine, the more s/he uses the machine's computational power to full advantage.

Programming Enviornments

G-code's programming environments have evolved in parallel with those of general programming - from the earliest environments (e.g., writing a program with a pencil, typing it into a tape puncher) to the latest environments that stack computer-aided design (CAD), computer-aided manufacturing (CAM), and richly featured G-code editors. (G-code editors are analogous to XML editors, using colors and indents semantically [plus other features] to aid the user in ways that basic text editors can't. CAM packages are analogous to IDEs in general programming.) Two high-level paradigm shifts have been (1) abandoning "manual programming" (with nothing but a pencil or text editor and a human mind) for CAM software systems that generate G-code automatically via postprocessors (analogous to the development of visual techniques in general programming), and (2) abandoning hardcoded constructs for parametric ones (analogous to the difference in general programming between hardcoding a constant into an equation versus declaring it a variable and assigning new values to it at will). Macro (parametric) CNC programming uses human-friendly variable names, relational operators, and loop structures much as general programming does, to capture information and logic with machine-readable semantics. Whereas older manual CNC programming could only describe particular instances of parts in numeric form, parametric CAM programming describes abstractions which can be flowed with ease into a wide variety of instances. The difference is analogous to creating text as bitmaps versus using character encoding and glyphs, or to the way that HTML passed through a phase of using content markup for presentation purposes, then matured toward the CSS model. In all of these cases, a higher layer of abstraction was introduced in order to pursue what was missing semantically.

Tap and Die (Fractional Inch)

Lead pitch are closely related concepts. They can be confused because they are the same for most screws. Lead is the distance along the screw's axis that is covered by one complete rotation of the screw (360 degrees). Pitch is the distance from the crest of one thread to the next. Because the vast majority of screw threadforms are single-start threadforms, their lead and pitch are the same. Single-start means that there is only one "ridge" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360 degrees), it has advanced axially by the width of one ridge. "Double-start" means that there are two "ridges" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360 degrees), it has advanced axially by the width of two ridges. Another way to express this is that lead and pitch are parametrically related, and the parameter that relates them, the number of starts, very often has a value of 1, in which case their relationship becomes equality. In general, lead is equal to S times pitch, in which S is the number of starts. While specifying the pitch of a metric thread form is common, inch-based standards usually use threads per inch (TPI), which is how many threads occur per inch of axial screw length. Pitch and TPI describe the same underlying physical property - merely in different terms. When the inch is used as the unit of measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example, a 1/4-20 thread has 20 TPI, which means that its pitch is 1/20 inch (0.050 in or 1.27 mm).

Course vs. Fine Threads

Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, whereas fine threads have a smaller threadform relative to screw diameter. This distinction is analogous to that between coarse teeth and fine teeth on a saw or file, or between coarse grit and fine grit on sandpaper. The common V-thread standards (ISO 261 and Unified Thread Standard) include a coarse pitch and a fine pitch for each major diameter. For example, 1/2-13 belongs to the UNC series (Unified National Coarse) and 1/2-20 belongs to the UNF series (Unified National Fine). A common misconception among people not familiar with engineering or machining is that the term coarse implies here lower quality and the term fine implies higher quality. The terms when used in reference to screw thread pitch have nothing to do with the tolerances used (degree of precision) or the amount of craftsmanship, quality, or cost. They simply refer to the size of the threads relative to the screw diameter. Coarse threads can be made accurately, or fine threads inaccurately.

Thread depth

Screw threads are almost never made perfectly sharp (no truncation at the crest or root), but instead are truncated, which is known as the thread depth or percentage of thread. The UTS and ISO standards codify the amount of truncation, including tolerance ranges. A perfectly sharp 60 degree V-thread will have a depth of thread ("height" from root to crest) equal to 86.6% of the pitch. This fact is intrinsic to the geometry of an equilateral triangle - a direct result of the basic trigonometric functions. It is independent of measurement units (inch vs mm). The typical depth of UTS and ISO threads with truncation included is around 75% of the pitch. Threads can be (and often are) truncated a bit more, yielding thread depths of 60% to 65%. This makes the thread-cutting easier (yielding shorter cycle times and longer tap and die life) without a large sacrifice in thread strength. For many applications, 60% threads are optimal, and 75% threads are wasteful or "over-engineered" (additional resources were unnecessarily invested in creating them). To truncate the threads further different techniques are used for male and female threads. For male threads, the bar stock is "turned down" somewhat before thread cutting, so that the major diameter is reduced. Likewise, for female threads the stock material is drilled with a slightly larger tap drill, reducing the minor diameter. The pitch diameter is unchanged by these operations which change material dimensions prior to tapping (thread cutting). This balancing of truncation versus thread strength is common to many engineering decisions involving material strength and material thickness, cost, and weight. Engineers use a number called the safety factor to quantify the increased material thicknesses or other dimension beyond the minimum required for the estimated loads on a mechanical part. Increasing the safety factor generally increases the cost of manufacture and decreases the likelihood of a failure. So the safety factor is often the focus of a business management decision when a mechanical product's cost impacts business performance and failure of the product could jeopardize human life or company reputation. For example, aerospace contractors are particularly rigorous in the analysis and implementation of Safety factors in the manufacture of manned space flight equipment or even launch equipment for unmanned satellites. Material thickness affects not only cost, but the weight and the cost to lift that weight into orbit. The cost of failure and the cost of manufacture are both extremely high. Thus the safety factor dramatically impacts company fortunes and is often worth the additional engineering expense required for detailed analysis and implementation. The fate of extremely expensive space hardware often hangs on the thread depth for bolts used in the mounting of the hardware to space

Tap and Die (Fractional Inch)

Lead pitch are closely related concepts. They can be confused because they are the same for most screws. Lead is the distance along the screw's axis that is covered by one complete rotation of the screw (360 degrees). Pitch is the distance from the crest of one thread to the next. Because the vast majority of screw threadforms are single-start threadforms, their lead and pitch are the same. Single-start means that there is only one "ridge" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360 degrees), it has advanced axially by the width of one ridge. "Double-start" means that there are two "ridges" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360 degrees), it has advanced axially by the width of two ridges. Another way to express this is that lead and pitch are parametrically related, and the parameter that relates them, the number of starts, very often has a value of 1, in which case their relationship becomes equality. In general, lead is equal to S times pitch, in which S is the number of starts. While specifying the pitch of a metric thread form is common, inch-based standards usually use threads per inch (TPI), which is how many threads occur per inch of axial screw length. Pitch and TPI describe the same underlying physical property - merely in different terms. When the inch is used as the unit of measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example, a 1/4-20 thread has 20 TPI, which means that its pitch is 1/20 inch (0.050 in or 1.27 mm).

Course vs. Fine Threads

Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, whereas fine threads have a smaller threadform relative to screw diameter. This distinction is analogous to that between coarse teeth and fine teeth on a saw or file, or between coarse grit and fine grit on sandpaper. The common V-thread standards (ISO 261 and Unified Thread Standard) include a coarse pitch and a fine pitch for each major diameter. For example, 1/2-13 belongs to the UNC series (Unified National Coarse) and 1/2-20 belongs to the UNF series (Unified National Fine). A common misconception among people not familiar with engineering or machining is that the term coarse implies here lower quality and the term fine implies higher quality. The terms when used in reference to screw thread pitch have nothing to do with the tolerances used (degree of precision) or the amount of craftsmanship, quality, or cost. They simply refer to the size of the threads relative to the screw diameter. Coarse threads can be made accurately, or fine threads inaccurately.

Thread depth

Screw threads are almost never made perfectly sharp (no truncation at the crest or root), but instead are truncated, which is known as the thread depth or percentage of thread. The UTS and ISO standards codify the amount of truncation, including tolerance ranges. A perfectly sharp 60 degree V-thread will have a depth of thread ("height" from root to crest) equal to 86.6% of the pitch. This fact is intrinsic to the geometry of an equilateral triangle - a direct result of the basic trigonometric functions. It is independent of measurement units (inch vs mm). The typical depth of UTS and ISO threads with truncation included is around 75% of the pitch. Threads can be (and often are) truncated a bit more, yielding thread depths of 60% to 65%. This makes the thread-cutting easier (yielding shorter cycle times and longer tap and die life) without a large sacrifice in thread strength. For many applications, 60% threads are optimal, and 75% threads are wasteful or "over-engineered" (additional resources were unnecessarily invested in creating them). To truncate the threads further different techniques are used for male and female threads. For male threads, the bar stock is "turned down" somewhat before thread cutting, so that the major diameter is reduced. Likewise, for female threads the stock material is drilled with a slightly larger tap drill, reducing the minor diameter. The pitch diameter is unchanged by these operations which change material dimensions prior to tapping (thread cutting). This balancing of truncation versus thread strength is common to many engineering decisions involving material strength and material thickness, cost, and weight. Engineers use a number called the safety factor to quantify the increased material thicknesses or other dimension beyond the minimum required for the estimated loads on a mechanical part. Increasing the safety factor generally increases the cost of manufacture and decreases the likelihood of a failure. So the safety factor is often the focus of a business management decision when a mechanical product's cost impacts business performance and failure of the product could jeopardize human life or company reputation. For example, aerospace contractors are particularly rigorous in the analysis and implementation of Safety factors in the manufacture of manned space flight equipment or even launch equipment for unmanned satellites. Material thickness affects not only cost, but the weight and the cost to lift that weight into orbit. The cost of failure and the cost of manufacture are both extremely high. Thus the safety factor dramatically impacts company fortunes and is often worth the additional engineering expense required for detailed analysis and implementation. The fate of extremely expensive space hardware often hangs on the thread depth for bolts used in the mounting of the hardware to space

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Handedness

The helix of a thread can twist in two possible directions, which is known as handedness. Most threads are oriented so that the threaded item, when seen from a point of view on the axis through the center of the helix, moves away from the viewer when it is turned in a clockwise direction, and moves towards the viewer when it is turned anti-clockwise. This is known as a right-handed (RH) thread, because it follows the right hand grip rule. Threads oriented in the opposite direction are known as left-handed (LH). By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads. Left-handed thread applications include: Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to fretting induced precession. Examples include: The left hand pedal on a bicycle.[3] The left-hand grinding wheel on a bench grinder. The lug nuts on the left side of some automobiles. In combination with right-handed threads in turnbuckles. In some gas supply connections to prevent dangerous misconnections, for example in gas welding the flammable gas supply uses left-handed threads. In a situation where neither threaded pipe end can be rotated to tighten/loosen the joint, e.g. in traditional heating pipes running through multiple rooms in a building. In such a case, the coupling will have one right-handed and one left-handed thread In some instances, for example early ballpoint pens, to provide a "secret" method of disassembly. In mechanisms to give a more intuitive action as: The leadscrew of the cross slide of a lathe to cause the cross slide to move away from the operator when the leadscrew is turned clockwise. The depth of cut screw of a "Stanley" type metal plane (tool) for the blade to move in the direction of a regulating right hand finger. The term chirality comes from the Greek word for "hand" and concerns handedness in many other contexts.

Form

The cross-sectional shape of a thread is often called its form or threadform (also spelled thread form). It may be square, triangular, trapezoidal, or other shapes. The terms form and threadform sometimes refer to all design aspects taken together (cross-sectional shape, pitch, and diameters). Most triangular threadforms are based on an isosceles triangle. These are usually called V-threads or vee-threads because of the shape of the letter V. For 60 degree V-threads, the isosceles triangle is, more specifically, equilateral. For buttress threads, the triangle is scalene. The theoretical triangle is usually truncated to varying degrees (that is, the tip of the triangle is cut short). A V-thread in which there is no truncation (or a minuscule amount considered negligible) is called a sharp V-thread. Truncation occurs (and is codified in standards) for practical reasons: The thread-cutting or thread-forming tool cannot practically have a perfectly sharp point; at some level of magnification, the point is truncated, even if the truncation is very small. Too-small truncation is undesirable anyway, because: The cutting or forming tool's edge will break too easily; The part or fastener's thread crests will have burrs upon cutting, and will be too susceptible to additional future burring resulting from dents (nicks); The roots and crests of mating male and female threads need clearance to ensure that the sloped sides of the V meet properly despite (a) error in pitch diameter and (b) dirt and nick-induced burrs. The point of the threadform adds little strength to the thread. Ball screws, whose male-female pairs involve bearing balls in between, show that other variations of form are possible. Roller screws use conventional thread forms but introduce an interesting twist on the theme.

Pitch Diameter

Pitch diameter, also known as mean diameter, is a diameter in between major and minor. It is the diameter at which each pitch is equally divided between the mating male and female threads. It is important to the fit between male and female threads, because a thread can be cut to various depths in between the major and minor diameters, with the roots and crests of the threadform being variously truncated, but male and female threads will only mate properly if their sloping sides are in contact, and that contact can only happen if the pitch diameters of male and female threads match closely. Another way to think of pitch diameter is "the diameter on which male and female should meet". Thread pitch diameter is analogous to gear pitch diameter, which is related to how two mating gears should meet.

Definition

The phrase speeds and feeds (or feeds and speeds) refers to two separate velocities in machine tool practice, cutting speed and feed rate. They are often considered as a pair because of their combined effect on the cutting process. Each, however, can also be considered and analyzed in its own right. Cutting speed (also called surface speed or simply speed) is the speed difference (relative velocity) between the cutting tool and the surface of the workpiece it is operating on. It is expressed in units of distance along the workpiece surface per time, typically surface feet per minute (sfm) or meters per minute (m/min).[1] Feed rate (also often styled as a solid compound, feedrate, or called simply feed) is the relative velocity at which the cutter is advanced along the workpiece; its vector is perpendicular to the vector of cutting speed. Feed rate units depend on the motion of the tool and workpiece; in rotating systems (e.g., turning and boring), the units are almost always distance per spindle revolution (inches per revolution [in/rev or ipr] or millimeters per revolution [mm/rev]).[2] In linear systems (e.g., milling), the units are typically distance per time (inches per minute [in/min or ipm] or millimeters per minute [mm/min]), although distance per revolution or per cutter tooth are also sometimes used.[2] If variables such as cutter geometry and the rigidity of the machine tool and its tooling setup could be ideally maximized (and reduced to negligible constants), then only a lack of power (that is, kilowatts or horsepower) available to the spindle would prevent the use of the maximum possible speeds and feeds for any given workpiece material and cutter material. Of course, in reality those other variables are dynamic and not negligible; but there is still a correlation between power available and feeds and speeds employed. In practice, lack of rigidity is usually the limiting constraint..

Cutting Speed

Cutting speed may be defined as the rate (or speed) that the material moves past the cutting edge of the tool , irrespective of the machining operation used - the surface speed. A cutting speed for mild steel, of 100 ft/min (or approx 30 meters/min) is the same whether it is the speed of the (stationary) cutter passing over the (moving) workpiece, such as in a turning operation, or the speed of the (rotating) cutter moving past a (stationary) workpiece, such as in a milling operation. What will affect the value of this surface speed for mild steel, is the cutting conditions: For a given material there will be an optimum cutting speed for a certain set of machining conditions, and from this speed the spindle speed (RPM) can be calculated. Factors affecting the calculation of cutting speed are: The material being machined (steel, brass, tool steel, plastic, wood), The material the cutter is made from (Carbon steel, high speed steel (HSS), carbide, ceramics), The economical life of the cutter (the cost to regrind or purchase new, compared to the quantity of parts produced) Cutting speeds are calculated on the assumption that optimum cutting conditions exist, these include: Metal removal rate (finishing cuts that remove a small amount of material may be run at increased speeds), Full and constant flow of cutting fluid (adequate cooling and chip flushing), Rigidity of the machine and tooling setup (reduction in vibration or chatter), Continuity of cut (as compared to an interrupted cut, such as machining square section material in a lathe), Condition of material (mill scale, hard spots due to white cast iron forming in castings),

Spindle Speed

The spindle speed is the rotational frequency of the spindle of the machine, measured in revolutions per minute (RPM). The preferred speed is determined by working backward from the desired surface speed (sfm or m/min) and incorporating the diameter (of workpiece or cutter). Excessive spindle speed will cause premature tool wear, breakages, and can cause tool chatter, all of which can lead to potentially dangerous conditions. Using the correct spindle speed for the material and tools will greatly enhance tool life and the quality of the surface finish. For a given machining operation, the cutting speed will remain constant for most situations; therefore the spindle speed will also remain constant. However, facing, forming, parting off, and recess operations on a lathe or screw machine involve the machining of a constantly changing diameter. Ideally this means changing the spindle speed as the cut advances across the face of the workpiece, producing constant surface speed (CSS). Mechanical arrangements to effect CSS have existed for centuries, but they were never applied commonly to machine tool control. In the pre-CNC era, the ideal of CSS was ignored for most work. For unusual work that demanded it, special pains were taken to achieve it. The introduction of CNC-controlled lathes has provided a practical, everyday solution via automated CSS. By means of the machine's software and variable speed electric motors, the lathe can increase the RPM of the spindle as the cutter gets closer to the center of the part.

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