Close sliding fits are intended for the accurate location of parts which must assemble without perceptible play.

RC 2

Sliding fits are intended for accurate location, but with greater maximum clearance than class RC 1. Parts made to this fit move and turn easily but are not intended to run freely, and in the larger sizes may seize with small temperature changes.

RC 3

Precision running fits are about the closest fits which can be expected to run freely and are intended for precision work at slow speeds and light journal pressures, but they are not suitable where appreciable temperature differences are likely to be encountered.

RC 4

Close running fits are intended chiefly for running fits on accurate machinery with moderate surface speeds and journal pressures, where accurate location and minimum play are desired.

Basic hole system. Limits are in thousandths of an inch. Limits for hole and shaft are applied algebraically to the basic size to obtain the limits of size for the parts. Data in boldface are in accordance with ABC agreements. Symbols H5, g5, etc., are hole and shaft designations used in ABC System.

Medium running fits are intended for higher running speeds, or heavy journal pressures, or both.

RC 7

Free running fits are intended for use where accuracy is not essential, or where large temperature variations are likely to be encountered, or under both these conditions.

Loose running fits are intended for use where wide commercial tolerances may be necessary, together with an allowance, on the external member.

Locational clearance fits are intended for parts which are normally stationary but which can be freely assembled or disassembled. They run from snug fits for parts requiring accuracy of location, through the medium clearance fits for parts such as spigots, to the looser fastener fits, where freedom of assembly is of prime importance.

Basic hole system. Limits are in thousandths of an inch. Limits for hole and shaft are applied algebraically to the basic size to obtain the limits of size for the parts. Data in boldface are in accordance with ABC agreements. Symbols H6, H5, etc., are hole and shaft designations used in ABC System.

Transition fits are a compromise between clearance and interference fits, for application where accuracy of location is important, but either a small amount of clearance or interference is permissible.

Basic hole system. Limits are in thousandths of an inch. Limits for hole and shaft are applied algebraically to the basic size to obtain the limits of size for the mating parts. Data in boldface are in accordance with ABC agreements. “Fit” represents the maximum interference (minus values) and the maximum clearance (plus values). Symbols H7, js6, etc., are hole and shaft designations used in ABC System.

Locational interference fits are used where accuracy of location is of prime importance and for parts requiring rigidity and alignment with no special requirements for bore pressure. Such fits are not intended for parts designed to transmit frictional loads from one part to another by virtue of the tightness of fit, as these conditions are covered by force fits.

Basic hole system. Limits are in thousandths of an inch. Limits for hole and shaft are applied algebraically to the basic size to obtain the limits of size for the parts. Data in boldface are in accordance with ABC agreements. Symbols H7, p6, etc., are hole and shaft designations used in ABC System.

Light drive fits are those requiring light assembly pressures, and produce more or less permanent assemblies. They are suitable for thin sections or long fits, or in cast-iron external members.

FN 2

Medium drive fits are suitable for ordinary steel parts, or for shrink fits on light sections. They are about the tightest fits that can be used with high-grade cast-iron external members.

FN 3

Heavy drive fits are suitable for heavier steel parts or for shrink fits in medium sections.

Force fits are suitable for parts which can be highly stressed, or for shrink fits where the heavy pressing forces required are impractical.

Basic hole system. Limits are in thousandths of an inch. Limits for hole and shaft are applied algebraically to the basic size to obtain the limits of size for the parts. Data in boldface are in accordance with ABC agreements. Symbols H7, s6, etc., are hole and shaft designations used in ABC System.

Metric Screw Threads.^a

Preferred sizes for commercial threads and fasteners are shown in boldface type.

Metric Drill Sizes. Decimal inch equivalents are for reference only.

^aANSI B18.2.1-1981 (R1992).

^bANSI/ASME B18.2.2.-1987 (R1993).

^cHexagon cap screws and finished hexagon bolts are combined as a single product.

American National Standard Square and Hexagon Bolts and Nuts and Hexagon Cap Screws (continued). See ANSI B18.2.2 for jam nuts, slotted nuts, thick nuts, thick slotted nuts, and castle nuts.

Metric hexagon bolts, hexagon cap screws, hexagon structural bolts, and hexagon nuts.

American National Standard machine screws.

Length of Thread: On screws 2″ long and shorter, the threads extend to within two threads of the head and closer if practical; longer screws have minimum thread length of ″.

Points: Machine screws are regularly made with plain sheared ends, not chamfered.

Threads: Either Coarse or Fine Thread Series, Class 2 fit.

Recessed Heads: Two styles of cross recesses are available on all screws except hexagon head.

Metric machine screws.^a

Length of Thread: On screws 36 mm long or shorter, the threads extend to within one thread of the head: on longer screws the thread extends to within two threads of the head.

Points: Machine screws are regularly made with sheared ends, not chamfered.

Threads: Coarse (general purpose) threads series are given.

Recessed Heads: Two styles of cross-recesses are available on all screws except hexagon head.

Maximum length of slot is 4”+ W. Note that key is sunk two-thirds into shaft in all cases.

For parts lists, etc., give inside diameter, outside diameter, and the thickness; for example, .344 × .688 × .065 Type A Plain Washer. Preferred Sizes of Type A Plain Washers.^b

For parts lists, etc., give nominal size and series; for example, regular lock washer (preferred series).

To find small diameter of pin, multiply the length by .02083 and subtract the result from the larger diameter. All dimensions are given in inches. Standard reamers are available for pins given above the heavy line.

All dimensions are given in inches.

Dimensions of 90° and 45° elbows, tees, and crosses (straight sizes). All dimensions given in inches. Fittings having right- and left-hand threads shall have four or more ribs or the letter “L” cast on the band at end with left-hand thread.

Dimensions of 90° and 45° elbows, tees, and crosses (straight sizes). All dimensions given in inches. The 250-lb standard for screwed fittings covers only the straight sizes of 90° and 45° elbows, tees, and crosses.

Dimensions of elbows, double branch elbows, tees, crosses, laterals, true Y’s (straight sizes), and reducers. All dimensions in inches.

Dimensions of elbows, tees, and reducers. All dimensions are given in inches.

Scales are usually made of plastic or boxwood. The better wood scales have white plastic edges. Scales can be either triangular or flat. The triangular scales combine several scales on one stick by using each of the triangle’s three sides. A scale guard, shown in Figure A51.1f, can save time and prevent errors by indicating the side of the scale currently in use.

Several scales that are based on the inch-foot system of measurement continue in domestic use today, along with the metric system of measurement, which is accepted worldwide for science, technology, and international trade.

The remaining four scales on this triangular metric scale include the typical scale ratios of 1:5, 1:25, and 1:75 (Figures A51.2b and c). These ratios may also be enlarged or reduced by multiplying or dividing by a factor of 10. Metric scales are also available with other scale ratios for specific drawing purposes.

Metric scales are also used in map drawing and in drawing force diagrams or other graphical constructions that involve such scales as 1 mm = 1 kg and 1 mm = 500 kg.

Because the engineers’ scale divides inches into decimal units, it is convenient in machine drawing to set off inch dimensions expressed in decimals. For example, to set off 1.650″ full size, use the 10 scale and simply set off one main division plus 6-1/2 subdivisions (Figure A51.3). To set off the same dimension half size, use the 20 scale, since the 20 scale is exactly half the size of the 10 scale. Similarly, to set off the dimension quarter size, use the 40 scale.

An engineers’ scale is also used in drawing stress diagrams or other graphical constructions to such scales as 1″ = 20 lb and 1″ = 4000 lb.

These scales are useful in dividing dimensions. For example, to draw a 3.6″-diameter circle full size, it is necessary to use half of 3.6″ as the radius. Instead of using math to find half of 3.6″, it is easier to set off 3.6″ on the half-size scale.

Given axes AB and CD,

Step 1. Draw line AC. With O as center and OA as radius, draw the arc AE. With C as center and CE as radius, draw the arc EF.

Step 2. Draw perpendicular bisector GH of the line AF; the points K and J, where they intersect the axes, are centers of the required arcs.

Step 3. Find centers M and L by setting off OL = OK and OM = OJ. Using centers K, L, M, and J, draw circular arcs as shown. The points of tangency T are at the junctures of the arcs on the lines joining the centers.

A parabola is generated by a point moving so that its distances from a fixed point, the focus, and from a fixed line, the directrix, remain equal.

Focus F and directrix AB are given. A parabola may be generated by a pencil guided by a string (Figure A52.3a). Fasten the string at F and C; its length is GC. The point C is selected at random; its distance from G depends on the desired extent of the curve. Keep the string taut and the pencil against the T-square, as shown.

Given focus F and directrix AB, draw a line DE parallel to the directrix and at any distance CZ from it (Figure A52.3b). With center at F and radius CZ, draw arcs to intersect line DE at the points Q and R, which are points on the parabola. Determine as many additional points as are necessary to draw the parabola accurately, by drawing additional lines parallel to line AB and proceeding in the same manner.

A tangent to the parabola at any point G bisects the angle formed by the focal line FG and the line SG perpendicular to the directrix.

Given the rise and span of the parabola (Figure A52.3c), divide AO into any number of equal parts, and divide AD into a number of equal parts amounting to the square of that number. From line AB, each point on the parabola is offset by a number of units equal to the square of the number of units from point O. For example, point 3 projects 9 units (the square of 3). This method is generally used for drawing parabolic arches.

To find the focus, F, given points P, R, and V of a parabola (Figure A52.3d), draw a tangent at P, making a = b. Draw the perpendicular bisector of AP, which intersects the axis at F, the focus of the parabola.

Draw a parabola given rectangle or parallelogram ABCD (Figure A52.4a and b). Divide BC into any even number of equal parts, divide the sides AB and DC each into half as many parts, and draw lines as shown. The intersections of like-numbered lines are points on the parabola.

These parabolic curves are more pleasing in appearance than circular arcs and are useful in machine design. If the tangents OX and OY are equal, the axis of the parabola will bisect the angle between them.

Let F and F′ be the foci and AB the transverse axis (Figure A52.6a). The curve may be generated by a pencil guided by a string, as shown. Fasten a string at F′ and C; its length is FC minus AB. The point C is chosen at random; its distance from F depends on the desired extent of the curve.

Fasten the straightedge at F. If it is revolved about F, with the pencil point moving against it and with the string taut, the hyperbola may be drawn as shown.

To construct the curve geometrically, select any point X on the transverse axis produced (Figure A52.6b). With centers at F and F′ and BX as radius, draw the arcs DE. With the same centers, F and F′, and AX as radius, draw arcs to intersect the arcs first drawn through the points Q, R, S, and T, which are points of the required hyperbola. Find as many additional points as are necessary to draw the curves accurately by selecting other points similar to point X along the transverse axis and proceeding as described for point X.

To draw the tangent to a hyperbola at a given point P, bisect the angle between the focal radii FP and F′P. The bisector is the required tangent.

To draw the asymptotes HCH of the hyperbola, draw a circle with the diameter FF′ and draw perpendiculars to the transverse axis at points A and B to intersect the circle at points H. The lines HCH are the required asymptotes.

In an equilateral hyperbola, the asymptotes, which are at right angles to each other, may be used as the axes to which the curve is referred. If a chord of the hyperbola is extended to intersect the axes, the intercepts between the curve and the axes are equal (Figure A52.7a). For example, a chord through given point P intersects the axes at points 1 and 2, intercepts P–1 and 2–3 are equal, and point 3 is a point on the hyperbola. Likewise, another chord through P provides equal intercepts P–1′ and 3′–2′, and point 3′ is a point on the curve. Not all chords need be drawn through given point P, but as new points are established on the curve, chords may be drawn through them to obtain more points. After finding enough points to ensure an accurate curve, draw the hyperbola with the aid of an irregular curve.

In an equilateral hyperbola, the coordinates are related, so their products remain constant. Through given point P, draw lines 1–P–Y and 2–P–Z parallel, respectively, to the axes (Figure A52.7b). From coordinate O, draw any diagonal intersecting these two lines at points 3 and X. At these points draw lines parallel to the axes, intersecting at point 4, a point on the curve. Likewise, another diagonal from O intersects the two lines through P at points 8 and Y, and lines through these points parallel to the axes intersect at point 9, another point on the curve. A third diagonal similarly produces point 10 on the curve, and so on. Find as many points as necessary for a smooth curve, and draw the hyperbola with the aid of an irregular curve. From the similar triangles O–X–5 and O–3–2, it is evident that lines P–1 × P–2 = 4–5 × 4–6.

The equilateral hyperbola can be used to represent varying pressure of a gas as the volume varies, because the pressure varies inversely with the volume; that is, pressure × volume is constant.

If the cylindrical surface on which a helix is generated is rolled out onto a plane, the helix becomes a straight line (Figure A52.9a). The portion below the helix becomes a right triangle, the altitude of which is equal to the lead of the helix; the length of the base is equal to the circumference of the cylinder. Such a helix can be defined as the shortest line that can be drawn on the surface of a cylinder connecting two points not on the same element.

To draw the helix, draw two views of the cylinder on which the helix is generated (Figure A52.9b). Divide the circle of the base into any number of equal parts. On the rectangular view of the cylinder, set off the lead and divide it into the same number of equal parts as the base. Number the divisions as shown (in this case 16). When the generating point has moved one sixteenth of the distance around the cylinder, it will have risen one sixteenth of the lead; when it has moved halfway around the cylinder, it will have risen half the lead; and so on. Points on the helix are found by projecting up from point 1 in the circular view to line 1 in the rectangular view, from point 2 in the circular view to line 2 in the rectangular view, and so on.

Figure A52.9b is a right-handed helix. In a left-handed helix (Figure A52.9c), the visible portions of the curve are inclined in the opposite direction—that is, downward to the right. The helix shown in Figure A52.9b can be converted into a left-handed helix by interchanging the visible and hidden lines.

The helix is used in industry, as in screw threads, worm gears, conveyors, spiral stairways, and so on. The stripes of a barber pole are helical in form.

The construction for a right-handed conical helix is shown in Figure A52.9d.

An involute can be generated by a point on a straight line that is rolled on a fixed circle (Figure A52.10e). Points on the required curve may be determined by setting off equal distances 0–1, 1–2, 2–3, and so on, along the circumference, drawing a tangent at each division point, and proceeding as explained for Figure A52.10d.

The involute of a circle is used in the construction of involute gear teeth. In this system, the involute forms the face and a part of the flank of the teeth of gear wheels; the outlines of the teeth of racks are straight lines.

Given the generating circle and the straight line AB tangent to it, make both distances CA and CB equal to the semicircumference of the circle (see Figure A52.11). Divide these distances and the semicircumference into the same number of equal parts (six, for instance) and number them consecutively, as shown. Suppose the circle rolls to the left; when point 1 of the circle reaches point 1′ of the line, the center of the circle will be at D, point 7 will be the highest point of the circle, and the generating point 6 will be at the same distance from line AB as point 5 is when the circle is in its central position. To find point P′, draw a line through point 5 parallel to AB and intersect it with an arc drawn from the center D with a radius equal to that of the circle. To find point P″, draw a line through point 4 parallel to AB, and intersect it with an arc drawn from the center E, with a radius equal to that of the circle. Points J, K, and L are found in a similar way.

Another method is shown in the right half of Figure A52.11. With the center at 11′ and the chord 11–6 as radius, draw an arc. With 10′ as center and the chord 10–6 as radius, draw an arc. Continue similarly with centers 9′, 8′, and 7′. Draw the required cycloid tangent to these arcs.

Either method may be used; however, the second is shorter. The line joining the generating point and the point of contact for the generating circle is a normal of the cycloid. The lines 1′–P′ and 2′–P″, for instance, are normals; this property makes the cycloid suitable for the outlines of gear teeth.