SIMPLE PATTERNMAKING A BILITY to produce simple vertically the sand close to the web patterns in wood from which would inevitably be disturbed on castings can be obtained lifting it out. A simple pattern, frees a constructor from continued therefore, must in at least one attitude acceptance of commercial designs be free of "undercuts" and the limitations and work Where machining on a casting will involved in built-up or fabricated be necessary, an allowance must be constructions. made on the pattern, and where a casting will be of substantial size and By Geometer It need not be overlooked, of course, that early efforts may not be so good as comparable commercial designs, not that a particular component might not have been equally well made by a skilled operator of welding equip- ment. But granted these, it then remains virtually indisputable that castings do, as a rule, provide the best means of construction, with freedom-when of one’s own design to express ideas mechanically. Woods employed for small patterns should be the hard, close-grained variety-boxwood, beech, oak , ash- that can be turned in a lathe reason- ably easily without tearing. Rotational speed should be as high as possible, and tools should possess ample rake and clearance. Complicated shapes can be built up by gluing, screwing, tacking and dowelling pieces of wood together, interstices being filled and fillets formed with plastic wood or putty, the surfaces finally smoothed with sandpaper and painted with a filler paint-followed by further rub- bing down and painting if required. An important feature of any simple pattern (which resembles the casting to be produced) is that it must leave the mould cleanly, and to ensure this fact is not overlooked, it is necessary to keep in mind the principle on which such a pattern is moulded. At A and B is shown in section and plan the pattern for a flywheel, which could be cast in cast iron. The moulding box at A is in halves, dowelled together. The pattern is pressed into the sand in the lower half to form its shape, while positioning the upper half of the box causes the shape to be similarly formed in the sand of the upper half. Taking off the upper half of the box, removing the pattern and replacing the upper half, then provides the mould into which the metal can be poured through a channel. As the pattern is relieved to form a central web on the flywheel,, the pattern must be positioned horizontally as shown, for if the pattern were placed 14 NOVEMBER 1957 not necessarily machined all over an allowance must be made for metal shrinkage. Per foot of length, this allowance is 1/10 in. to 1/8 in. for cast iron; 1/8 in. to 3/16in. for aluminium and l/5 in. to 1/4in. for brass. If desir ed “contractio n ” rules can be used on which all dimensions are these proportions overlength. On small patterns the machining allowance will easily accommodate shrinkage on the castings. For general purposes, an allowance of 1/16 in. is satisfactory. If one is sure of good small castings reduction to l/32 in. is possible; where there is doubt an increase to, say, 1/8 in. is advisable. The allowance must be added to all surfaces as at C. On wheels, where the “runne r ” in the 669 PIECE / 0 D HOLLOW 0 I= mould is to the periphery, extra allowance counteracts a possible ad- jacent hollow. A simple pattern for casting a circular cover, as at D, may be pro- vided with a shank as a chucking piece. Machining the casting the shaped end of the cover and outside would be turned holding the chucking piece; this would then be sawn off, and the cover held by the outside for finishing the step and end. Features of any depth to draw from a mould should be tapered on each face about two deg. as shown. Bosses, as at E, may be dowelled and glued in, and fillets made with plastic wood, while a pattern with a hollow can be built up from two pieces glued, as at F. q MODEL ENGINEER

2947 Simple Patternmaking

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SIMPLE PATTERNMAKINGA B I L I T Y t o p roduce s imp le vertically the sand close to the web

patterns in wood from which would inevitably be disturbed oncastings can be obtained lifting it out. A simple pattern,

frees a constructor from continued therefore, must in at least one attitudeacceptance of commercial designs be free of "undercuts"

and the l imitat ions and work Where machining on a casting will

involved in built-up or fabricatedbe necessary, an allowance must be

c o n s t r u c t i o n s . made on the pattern, and where acasting will be of substantial size and

By Geometer

It need not be overlooked, of course,that early efforts may not be so goodas comparable commercial designs,not that a particular component mightnot have been equally well made bya skilled operator of welding equip-ment. But granted these, it thenremains virtually indisputable thatcastings do, as a rule, provide thebest means of construction, withfreedom-when of one’s own designto express ideas mechanically.

Woods employed for small patternsshould be the hard, close-grainedvariety-boxwood, beech, oak , ash-that can be turned in a lathe reason-ably easily without tearing. Rotationalspeed should be as high as possible,and tools should possess ample rakeand clearance. Complicated shapescan be built up by gluing, screwing,tacking and dowelling pieces of woodtogether, interstices being filled andfillets formed with plastic wood orputty, the surfaces finally smoothedwith sandpaper and painted with afiller paint-followed by further rub-bing down and painting if required.

An important feature of any simplepattern (which resembles the castingto be produced) is that it must leavethe mould cleanly, and to ensurethis fact is not overlooked, it isnecessary to keep in mind the principleon which such a pattern is moulded.At A and B is shown in section andplan the pattern for a flywheel, whichcould be cast in cast iron. Themoulding box at A is in halves,dowelled together.

The pattern is pressed into the sandin the lower half to form its shape,while positioning the upper half ofthe box causes the shape to be similarlyformed in the sand of the upper half.Taking off the upper half of the box,removing the pattern and replacing theupper half, then provides the mouldinto which the metal can be pouredthrough a channel.

As the pattern is relieved to form acentral web on the flywheel,, the patternmust be positioned hor izontally asshown, for if the pattern were placed

14 NOVEMBER 1957

not necessarily machined all over anallowance must be made for metalshrinkage. Per foot of length, thisallowance is 1/10 in. to 1/8 in. for castiron; 1/8 in. to 3/16in. for aluminiumand l/5 in. to 1/4in. for brass. Ifdesir ed “contractio n ” rules can beused on which all dimensions arethese proportions overlength.

On small patterns the machiningallowance will easily accommodateshrinkage on the castings. Forgeneral purposes, an allowance of1/16 in. is satisfactory. If one is sureof good small castings reduction tol/32 in. is possible; where there isdoubt an increase to, say, 1/8 in. isadvisable. The allowance must beadded to all surfaces as at C. Onwheels, where the “runne r ” in the

669

PIECE/

HOLLOW

0I=

mould is to the periphery, extraallowance counteracts a possible ad-jacent hollow.

A simple pattern for casting acircular cover, as at D, may be pro-vided with a shank as a chuckingpiece. Machining the casting theshaped end of the cover and outsidewould be turned holding the chuckingpiece; this would then be sawn off,and the cover held by the outside forfinishing the step and end.

Features of any depth to draw froma mould should be tapered on eachface about two deg. as shown. Bosses,as at E, may be dowelled and gluedin, and fillets made with plastic wood,while a pattern with a hollow can bebuilt up from two pieces glued, asat F. q

MODEL ENGINEER