THE SHOP

Performance & Hotrod Business - July '15

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PRECISION ENGINE allowed to easily slide on the lathe hori- zontal rails. With the lathe head turning, slide the drill head toward the workpiece until the tap enters the hole to be threaded. The tap threads will begin to cut into the parent material, and will self-feed, (give the drill head a slight push to help it along). Note: this will work only if the work- piece can be properly secured and centered on the lathe head; and if the female hole is open, (allowing the tap to pass through). If the hole is blind, or needs to be only partially threaded, the lathe must feature a quick shut-off. Otherwise, the tap will feed through the hole uncontrolled. When the tap bottoms out, disaster will strike, with the tap breaking, the workpiece breaking, or the workpiece being knocked out of the lathe head. Either way, it won't be pretty. However, you can use a tap on a lathe by working the tap manually, using the lathe only as a fixture. A centering tool can be mounted to a lathe-mounted drill head, which will locate the hole's dead center. With the tap secured in a hand-tap tool, locate the centering tool's point into the small dimple featured on the rear end of the tap. (Note: common hand taps found at retail outlets will normally not feature this centering dimple. If you need dim- pled taps, these must be purchased from an industrial supply house). This will be slow-going, but it'll keep the tap straight and centered. Tap a bit, and push the drill head closer to the work- piece to keep contact between the center- ing tool and the tap, etc. The problem that many people face when trying to tap a hole is getting the tap to begin a straight entry. If the tap enters and begins to cut threads off-plane from center, you'll end up with a hole that has deeper-cut threads on one side of the hole and shallower threads on the opposite side of the hole. Another option is to use a drill press. If you're uncomfortable in allowing the drill press to power the tap, you can tap manually with the workpiece secured in the drill press vise. Once the hole has been drilled, without disturbing the drill press table or vise, chuck a pointed pilot or pointed chamfering bit into the drill chuck, and use this as a centering device, (with the centering bit engaged into the center dimple of the tap or tap wrench), holding the tap in a centered and straight position. Turn the tap wrench with one hand while applying gentle pressure on the drill press up/down arm. Once the tap has entered at a depth at least equal to the bolt diameter, you can remove the tap from the drill press and continue by using a manual tap driver. The idea is to keep the tap at a straight angle relative to the hole. If you begin to tap at an off-angle, you'll create a sloppy, angled threaded hole that will be useless. Take advantage of tapping fluid, (not just any lube, but dedicated tapping fluid), to ease cutting and to avoid galling. This is especially important when cutting threads in aluminum. Assuming the threads will feature a right-hand twist, rotate the tap handle MACHINE SCREW SIZE TAP DRILL SIZE Alum, Brass, Plastics Stainless Steel, Iron No. or Dia. Major Dia. Threads/ Inch Minor Dia. Drill Size Dec. Equiv. Drill Size Dec. Equiv. 0 0.600" 80 0.0447" 3/64" 0.0469" 55 0.0520" 1 0.0730" 72 0.0560" 53 0.0595" 52 0.0635" 1 0.0730" 64 0.0538" 53 0.0595" 1/16" 0.0625" 2 0.0860" 64 0.0668" 50 0.0700" 48 0.0760" 2 0.0860" 56 0.0641" 50 0.0700" 49 0.0730" 3 0.0990" 56 0.0771" 45 0.0820" 43 0.0890" 3 0.0990" 48 0.0734" 47 0.0785" 44 0.0860" 4 0.1120" 40 0.0813" 43 0.0890" 41 0.0960" 4 0.1120" 48 0.0864" 42 0.0935" 40 0.0980" 5 0.1250" 40 0.0943" 38 0.1015" 7/64" 0.1094" 5 0.1250" 44 0.0971" 37 0.1040" 35 0.1100" 6 0.1380" 32 0.0997" 36 0.1065" 32 0.1160" 6 0.1380" 40 0.1073" 33 0.1130" 31 0.1200" 8 0.1640" 32 0.1257" 29 0.1360" 27 0.1440" 8 0.1640" 36 0.1299" 29 0.1360" 26 0.1470" 10 0.1900" 24 0.1389" 25 0.1495" 20 0.1610" 10 0.1900" 32 0.1517" 21 0.1590" 18 0.1695" 12 0.2160" 24 0.1649" 16 0.1770" 12 0.1890" 12 0.2160" 28 0.1722" 14 0.1829" 10 0.1935" 12 0.2160" 32 0.1777" 13 0.1850" 9 0.1960" ¼" 0.2500" 20 0.1887" 7 0.2010" 7/32" 0.2188" ¼" 0.2500" 28 0.2062" 3 0.2130" 1 0.2280" ¼" 0.2500" 32 0.2117" 7/32" 0.2188" 1 0.2280" 5/16" 0.3125" 18 0.2443" F 0.2570" J 0.2770" 5/16" 0.3125" 24 0.2614" I 0.2720" 9/32" 0.2812" 5/16" 0.3125" 32 0.2742" 9/32" 0.2812" L 0.2900" 3/8" 0.3750" 16 0.2983" 5/16" 0.3125" Q 0.3320" 3/8" 0.3750" 24 0.3239" Q 0.3320" S 0.3480" 3/8" 0.3750" 32 0.3367" 11/32" 0.3438" T 0.3580" 7/16" 0.4375" 14 0.3499" U 0.3680" 25/64" 0.3906" 7/16" 0.4375" 20 0.3762" 25/64" 0.3906" 13/32" 0.4062" 7/16" 0.4375" 28 0.3937" Y 0.4040" Z 0.4130" ½" 0.5000" 13 0.4056" 27/64" 0.4219" 29/64" 0.4531" ½" 0.5000" 20 0.4387" 29/64" 0.4531" 15/32" 0.4688" 9/16" 0.5625" 12 0.4603" 31/64" 0.4844" 33/64" 0.5156" 9/16" 0.5625" 18 0.4943" 33/64" 0.5156" 17/32" 0.5312" Tap Drill SizeS (inch FormaT) Use this chart to determine the decimal-inch equivalent represented by an alpha or numeric drill. (Use a caliper to measure the drill O.D. in order to identify the bit.) 6 n PRECISION ENGINE n July 2015

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