Screw Threads

point from which a geometric position is to be determined or from which a distance is to be measured. Any letter of the alphabet except I, O, and Q may be used as a datum identifying symbol. A feature control symbol is made of geometric symbols and tolerances. Figure 4-3 shows how a feature control symbol may include datum references. Fillets and Rounds Fillets are concave metal corner (inside) surfaces. In a cast, a fillet normally increases the strength of a metal corner because a rounded corner cools more evenly than a sharp corner, thereby reducing the possibility of a break. Rounds or radii are edges or outside corners that have been rounded to prevent chipping and to avoid sharp cutting edges. Figure 4-4 shows fillets and rounds. Slots and Slides Slots and slides are used to mate two specially shaped pieces of material and securely hold them together, yet allow them to move or slide. Figure 4-5 shows two types: the tee slot, and the dovetail slot. For examples, a tee slot arrangement is used on a milling machine table, and a dovetail is used on the cross slide assembly of an engine lathe. Keys, Keyseats, and Keyways A key is a small wedge or rectangular piece of metal inserted in a slot or groove between a shaft and a hub to prevent slippage. Figure 4-6 shows three types of keys. Figure 4-3.—Feature control frame indicating a datum reference. Figure 4-4.—Fillets and rounds Figure 4-5.—Slots and slides. Figure 4-6.—Three types of keys. Figure 4-7 shows a keyseat and keyway. View A shows a keyseat, which is a slot or groove on the outside of a part into which the key fits. View B shows a keyway, which is a slot or groove within a cylinder, tube, or pipe. A key fitted into a keyseat will slide into the keyway and prevent movement of the parts. SCREW THREADS Draftsmen use different methods to show thread on drawings. Figures 4-8 through 4-11 show several of Figure 4-7.—A keyseat and keyway. 4-2