Keyways

Straight keys (inch): the key width and height for a shaft diameter follow the ANSI B17.1 size series: the U.S. Army manual TC 9-524 Fundamentals of Machine Tools (1996), Table 8-5, for shafts from 1⁄2 to 2 in, and the 1927 American Standard square and flat stock keys for shafts from 1⁄2 to 6 in. For shafts over 1¼ to 1⅜ in the page uses today's 5⁄16 key (the 1927 table gives 3⁄8). Shafts under 1⁄2 in and over 6 in are not covered. The normal keyseat depth in both the shaft and the hub is half the key height, H/2, measured at the edge of the keyseat, not on the shaft centreline.

Check dimensions: a keyseat depth is awkward to measure directly, so the standards give a dimension you can take with a caliper or micrometer: from the keyseat floor to the opposite side of the shaft, S = D − Y − H/2, and from the hub keyway floor to the opposite side of the bore, T = D − Y + H/2 + C. Y is the chordal height, the small amount the shaft surface falls away between the centreline and the edge of the keyseat: Y = (D − √(D² − W²)) / 2. C is the hub allowance, +0.005 in, which leaves clearance over the key. Taper keys are not covered: cutting their tapered hub keyway needs more than these numbers.

Cutter feed from the top: with an end mill or keyseat cutter on the shaft centreline, touching off on the top of the shaft and feeding down H/2 + Y puts the keyseat floor at the standard depth at its edges. On a hub, the cut from the bore wall at the keyway centre is H/2 + C − Y.

Woodruff keys: TC 9-524 Table 8-4 lists 15 keys, No. 204 to No. 1011, with the shaft range each suits, the height of the key above the shaft and the depth of the keyseat. The S.A.E. Woodruff key standard, under its old numbers (10 to 255), lists 26 keys, the same sizes as today's ANSI B17.2; the page also offers the 11 keys TC 9-524 leaves out (No. 305, 506, 607, 808, 809, 1009, 1010, 1211, 812, 1012 and 1212). Those 11 keys are offered by number only, with no shaft range. The same table gives the limits shown for every key: the keyseat's width and its depth at the side of the keyseat, and the key's width, diameter, height and height above the shaft. The key's length along its flat top is not in the table; it is calculated, because it is the chord of the key's circle at its height. The key number reads directly: the last two digits are the nominal diameter in eighths of an inch and the digits before them the width in thirty-seconds (No. 608 = 6⁄32 × 8⁄8 = 3⁄16 × 1 in). The page gives the largest key listed for the shaft, or any key you choose, and names the others listed for that shaft. With a shaft entered, the list offers only the keys TC 9-524 lists for that shaft; with no shaft, any of the 26 can be looked up (the 11 S.A.E.-only keys only this way). A key that can't be cut in the shaft at all (wider than the shaft, or a keyseat deeper than the shaft is thick) is refused. The keyseat is cut by a Woodruff cutter of the key's nominal diameter and width sunk straight in on the shaft centreline; its depth from the top of the shaft is the listed depth plus the chordal height. TC 9-524's keyseat depth is the top of ANSI B17.2's +0.005/−0.000 band. The hub keyway follows ANSI B17.2 practice: depth = key above shaft + 0.006 in (+0.005/−0.000), width = key width + 0.001 in (+0.002/−0.000).

Metric keys (DIN 6885-1): key b × h, shaft keyway depth t1 and hub keyway depth t2 for shafts 6 to 50 mm come from the bd_warehouse open library's DIN 6885 extract, and for shafts over 50 to 500 mm from GOST 23360-78, Table 2: the Soviet (now interstate) standard for parallel keys, which corresponds to ISO R773 and has the same sizes as DIN 6885-1. DIN depths are measured at the keyway centreline, from the shaft surface and from the bore, so the check dimensions are simply d − t1 and d + t2 with no chordal term, and the cutter feed from the top of the shaft is t1. Both depths are +0.1, +0.2 or +0.3 / 0, by key height (up to 6, over 6 to 18, over 18 to 50 mm). The keyway width fit is an ISO 286 zone on the keyway (key stock is h9): free H9 shaft / D10 hub, normal N9 / JS9, close P9 / P9, computed by this site's ISO fits engine.

Fit (inch straight keys): the tolerances follow the ANSI/ASME B17.1 classes of fit for parallel keys. Class 1 is a relatively free fit for bar stock keys, which are made slightly under size; the keyseat is +0.002 to +0.004 in wide depending on size, so the sides run from a little clearance to metal-to-metal. Class 2 is a relatively tight fit for key stock, which is made slightly over size (+0.001 to +0.003 in); the keyseat is +0.002 in wide, so the sides run from slight clearance to slight interference. In both classes the shaft keyseat depth is +0.015/−0.000 in (S +0.000/−0.015) and the hub keyway depth +0.010/−0.000 in (T +0.010/−0.000). The side and top-and-bottom fits shown are worked out from those limits. Class 2 square key stock up to 1 in is +0.001/−0.000 in on width and height (Federal Specification OO-K-220B, Key, Machine, 1989).

Shop notes: commercial key stock is often made slightly oversize so it can be fitted by hand; measure the key before cutting a close-fit keyway. An end mill of the nominal width usually cuts a few thousandths wide and can wander; for a fitted key use an undersize cutter and finish to width, or a keyseat cutter. In a hub, a broach bushing with the right shim gives the depth directly, and the T dimension lets you check it with a caliper. Keep the keyseat centred on the shaft axis (0.010 in max offset) and parallel to it (0.002 in lead over 4 in) or the key will bind on one side.