PATTERNS with FINS and FLANGESPARTICULARLY in the case of

providing cooling fins is themethod of making patterns

and obtaining castings often super-ior to the alternative of machiningparts from the solid. AU-overmachining of fins is possible forthe cylinder barrels of small aircompressors and four-stroke en-gines, and those of two-strokeengines above the ports. Similarly,in the case of cylinder heads,turning fins from the solid ispracticable for simple types as maybe used on compressors and sometwo-stroke engines.

The method of turning from thesolid,- however, does not cover casesof cooling fins round two-strokeexhaust ports, angular fins on barrels(when these may be inclined and astraight-line flow of air is desired),and where fins on cylinder heads maybe disposed vertically or radiatingfrom a centre line. Then pattern andcasting provide straightforward meansof overcoming the problem-the onlyother solution to which would besome awkward milling operations.Cast fins may also be much improvedby work on them with files, burrsand emerycloth, and are certainlybetter than none at alL

For small engines and compressorsfins may be provided on cylinderbarrels, either by turning simplepatterns from solid wood, or bybuilding up and using separate piecesof flat wood for the fins. For theprocess of turning from the solid,the wood must be hard, close anduniform in the grain. Boxwood isprobably best, though good beechcan be used. The pattern should beturned as at A, with the fins, the plainportion of the barrel and a coreprint each end, Q and R, for the bore.

Spacing of the fins, S, should notbe less than about 9/32 m. or 5/16 in.,their depth not more than aboutfin., and their outside diameter notmore than about 2-1/2 in. These dimen-sions can, however, be modifiedsomewhat. Less deep fins could bespaced slightly closer while widerspacing would admit of their. beingsomewhat deeper. Their thickness atthe bottom should be 3/32 in. to 1/8 in.,joining to the barrel by a 1/16 in. radiuswhile the tip thickness should beabout 1/16 in. rounded.

5 DECEMBER 1957

For a two-stroke engine cylinder,pieces can be fitted into the fins forthe ports, keeping always to theprinciple of avoiding undercuts whichwould prevent clean moulding. Forthe exhaust ports, blocks can befitted, glued and tacked, as at T, andanother block, U, for the inlet port.On the further side of the pattern, theblock for the transfer passage wouldextend from the base flange up to thelevel of the exhaust ports. Radii maybe provided at the joins with plasticwood.

For securing the cylinder head,lugs are required at the top of thebarrel to tap to accept studs. Similarlugs are, therefore, required on thepattern in the space V. These can beprovided either by turning the spaceonly to a depth to give the dimensionsover the comers, then sawing andfiling away the material along theflats; or the space can be turned tonormal depth, then separate lugsfitted in as at B-a sectional view.The base flange W can be made byturning to the corner dimension, thencutting away along the flats. The

LUGS

/\

0B

763

BYGeometer

flange must be in square alignmentwith the top lugs.

Construction of a built-up, angularfin pattern can be as at C, the endscarrying core prints X, Xl. The baseflange Y can be a separate piece fromflat wood. The plain barrel Z can beturned as a short length joined to thetop carrying the core print X1 thencut through at the fin angle. Thefins and spacing pieces can then befrom flat wood, the spacing piecesclamped up and turned before thefins are fitted. Fin spacing will thusbe uniform and precise.

Similar accuracy obtains whenmaking cylinder head patterns, asat D, by cutting the fins from thinflat wood (beech or oak), and gluingand tacking together with spacingblocks between-l to 6. q

CHUCKING PIECE

MODEL ENGINEER