Pattern-making

Front matter

Class T$7Zf O

Book -- /

Copyright N°

Contents

CHAPTER PAGE

I. General Trade Relations and Qualifications of a Pattern-maker 1

II. Foundry and Machine Shop Practice as Determining the

Chapter I General Trade-relations And Qualifications Of A Pattern-maker

1. Effects of Specialization. The pattern-maker is a modern development, — a part of specialized industry — and as such he must be a necessity or he would not be tolerated.

It is highly probable that the original casting was accidental; molten metal cooling in a shape which proved useful. As time went on, the shapes that could be made were brought under control. At first, everything from the conception of the form to the production of the completed casting was the product of one mind.

Specialization has divided this into a series of steps or processes as follows: designing, drafting, pattern-making, foundry work, and machine-shop practice. Each of these is again subdivided until we reach our present-day shop conditions. In spite of the division, the five general processes are dependent on one another to such an extent that each must keep the others in mind.

When work is divided among a group of men, some form of expression must be found which all can understand and which will not require personal contact. Drawing has been selected for this purpose and it must be uniform in order to be intelligible.

2. Relation to Foundry Practice. In considering the work of the pattern-maker, we see at once that the closest connection is with the work of the foundryman, be he molder or core maker. Such frequent reference must be made to foundry conditions, and such careful attention paid to the convenience of the molder and the core maker, that it might almost be said that the best place to learn pattern-making is in the foundry. While it is not necessary for a pattern-maker to be a molder, he should be familiar with general foundry methods and equipment, and should consult frequently with the men who are to use the patterns he makes.

The pattern-maker is merely a tool-maker to the molder, and a pattern may be defined as any form or device by the use of which a mold may be made. This brings the pattern-maker into the general classification of non-producers, in the sense that his product is only a means to an end and not an end in itself.

3. Demand for the Pattern-maker. Other wood workers such as joiners, carpenters, and cabinet-makers produce articles which go into permanent use as made. No matter how much the elimination of the pattern-maker may be considered desirable, he is apt to stay with us for some time to come, and undying fame awaits the man who can produce all castings without patterns. Possibly the use of permanent molds will dispense with some patterns cf comparatively small size, but such molds will never render the pattern-maker unnecessary. The pattern-maker who is resourceful, who can do a job quickly without assistance, and who works in harmony with the drawing room

GENERAL TRADE-RELATIONS AND QUALIFICATIONS 3

on the one hand and with the foundry on the other hand, can always have a job. In this respect he is on a par with the skilled mechanic in any line.

4. Qualifications Necessary. In most trades, the principal qualification of a good workman is to be expert in handling the tools of his craft, but the pattern-maker, like the designer, is more largely concerned with that kind of planning which requires a strong effort of the imagination. Such planning must include not only the construction of the pattern, but also, before this construction can be determined, the method of molding, which must be selected from the alternative plans which suggest themselves to a good pattern-maker. This is what makes pattern-making an art rather than a trade, for while the pattern-maker must be a skilled craftsman, his success does not depend very largely on the mere cutting and fastening together of pieces of wood. As the draftsman has the flat sketch of the designer to make into a flat drawing, so the molder and machinist have a solid body or form from which to make another of somewhat similar shape; the pattern-maker, on the other hand, has only a flat drawing from which to make a solid pattern or form which may bear but little resemblance to the casting which is to be produced. This makes the work of the pattern-maker more like that of the designer than that of any of the other contributors to the finished article.

No book can make a pattern-maker, yet books may be of great assistance to him, and the contempt which many workmen have for " book learning " is not justified in the minds of the best craftsmen. The work is clean, pleasant, varied, and well paid, while the working conditions are usually better than are found in other trades. The intelligence and skill required for such intricate work bring together a superior set of men whose requirements rank with those of the engineer. Among these may be noted an ability to draw, a working knowledge of geometry, a general acquaintance with machine tools, a good sense of proportion, and some idea of the strength of materials. If a man can read a drawing as well as the man who made it, can appreciate the limitations of the foundry, and can also see things from the point of view of the machinist, then such a man may aspire to become a pattern-maker if his judgment is quick and accurate, if he is self-reliant and resourceful, and, most important of all, if he has the gift of imagination to see a mental picture of the pattern, of the way it is to be used in the foundry, and of the finished casting.

Chapter II Foundry And Machine-shop Practice As Deter- Mining The Type Of Pattern

While the pattern-maker does not need to be a molder, he must know the general principles underlying foundry practice, and, above all, if he is to consult with the molder, as he should do on many occasions, he must be familiar with the terms used in the foundry. A very brief summary of foundry operations, while not attempting to cover the practice, will introduce and partially explain the terms ordinarily used.

5. Molding Sand. A mold is usually made by withdrawing a pattern from sand, leaving a cavity into which molten metal may be poured. This molding sand must be able to resist heat and have strength enough to keep its shape. It is composed of over 90% silica and about 6% alumina; the first makes the sand refractory and the last gives it strength, when the sticky quality of the alumina or clay is brought out by moistening. The addition of water to molding sand is called tempering and the result is that molding sand as used is somewhat damp. The sand just mentioned is not prepared in any way except by the tempering referred to and so is called green sand. It is not very strong at best, but it is cheaper than any other method and a large percentage of casting is done in this sand.

When a stronger mold is required, a coarse fire sand, almost pure silica, is mixed with clay water to the consistency of a very stiff dough. After a mold has been made in this sand it must be baked to drive out the water and make the mold strong. This makes what is called a dry sand mold.

For very heavy work, a somewhat similar mixture called loam, with some chopped straw mixed in, is used.

6. Flasks. Very little molding is done in which the top of the mold is left open; usually a cover of sand is placed over the cavity. In order to economize sand and to make

Fig. 1.— Two-part Flask.

it possible to move the molds about, boxes called flasks are used. The common or two-part flask consists of a lower part called the drag or nowel, and an upper part called the cope. (Fig. 1.) The drag has a smooth upper surface with sockets to receive the flask pins which project from the cope. Flasks are sometimes made with more parts and are called three-part, four-part, etc., the intermediate parts between the cope and the drag being called cheeks. The interior parts of the flask are sometimes made in sections which can be drawn out sideways and which are called drawbacks. This is frequently done in dry sand work.

FOUNDRY AlsiD MACHINE-SHOP PRACTICE 7

If the cope is large in area and not very deep proportionally, bars may be fastened edgewise in the cope (Fig. 2) and in addition to this, cast-iron hooks called gaggers are wet in clay wash and laid against the cope bars. These bars and gaggers are used to prevent drop-outs.

7. Preparing the Mold. In plain molding in a two-part flask, the pattern is placed on a pattern or mold board, the drag inverted over it, and molding sand is sifted in through a sieve or riddle to cover the pattern. As this

Fig. 2.— Bars in the Cope.

sand comes in contact with the pattern and forms the face of the mold it is called facing sand. In large work and especially in dry sand molding a stronger sand mixture is used for facing. The drag is then filled with sand which is forced into place with rammers. The ramming must be done carefully, not hard enough to lose the porosity of the sand, which allows the gases to escape, nor so soft that the sand will drop out or be pushed to one side by the pressure of the metal. The top of the drag is then struck off with a straightedge, a bottom board placed over it, and the drag rolled over. To prevent the sand in the cope from sticking to that in the drag, some sand free from clay called parting sand must be sprinkled over the drag. The cope is then placed, facing sand is riddled in and the cope filled up, rammed, and struck off. If it is thought the gases may not pass off through the sand, the mold may be vented by pushing a vent wire in several places down through the cope nearly to the pattern. As the molten metal must pass down through the cope in order to reach the mold cavity, an opening called a sprue down to the level of the drag must be provided. The cope is then lifted, the patterns rapped to loosen them from the sand, and drawn up from the drag without disturbing the sand. Channels in the surface of the drag, called gates, are cut to connect the sprue with the mold cavity. Cores are then set if required and are sometimes supported by wrought- or cast-iron chaplets, tinned to prevent rust, which might cause an explosion. The mold is then closed by replacing the cope, clamped or weighted to resist the pressure of the metal, and poured from a ladle of molten metal.

As the metal is heavier than the sand, all loose sand, entrapped gases, etc., will rise to the top of the mold. Therefore, if a surface must be particularly sound, it must be arranged to cast that surface in the drag.

If a surface is required to be particularly hard, a piece of iron called a chill is used as a part of the mold, either as a section of the flask (Fig. 3), or set in the mold like a core.

When a pattern is frail, or irregular, especially if a large number of castings are wanted, a false cope, match, or odd side, of plaster of Paris or of sand mixed with litharge may be used in place of a plain mold board. (Fig. 4.)

8. Deciding upon the Kind of Pattern. When a pattern-maker receives a drawing of an article for which a pattern

FOUNDRY AND MACHINE-SHOP PRACTICE is necessary, his decision as to the kind of pattern to be made will be governed by many considerations. It is very easy to say that the pattern-maker is responsible for the proiron chill

Dry sand core Mold cavity . Green sand cope

Green sand .nowelr-

Fig. 3.— Section of Mold with Chill.

duction of the finished article with the least possible expenditure of time, labor, and materials in the pattern shop, foundry, and machine shop; but it is not as easy to decide

Patterns '

Fig. 4.— Odd Side or False Cope/

which of the many possibilities will prove to be the most economical. While the molder may be consulted as to the best method from the foundry point of view, it must be remembered that a good molder can get a mold from almost any conceivable shape. One enthusiast is on record as saying, " I can mold the pronged horns of an antlered buck from the natural pattern as it grew on the animal's head." The equipment of the machine shop which is to handle the castings must also be kept in mind, but the principal, and many times the governing factors are the metal to be used and the number of castings wanted.

In a "one-casting" pattern, the burden of labor can safely be thrown on the foundry, and such a pattern would be called temporary and ought not to be stored, but destroyed as soon as used. A pattern to be used infrequently, as is the case with a so-called master pattern, must be made so that the retention of its shape will be its principal feature. A pattern to be used regularly for a long period would be called a standard pattern and should be made so that strength to resist use and abuse in the foundry would be its main feature.

When the information given on the drawing has been absorbed, the different molding possibilities should be considered. Note the choice in the case of such simple patterns as the plain gland (Fig. 5) and the solid roller (Fig. 6). Many times there are no precedents to govern the choice, and only a careful consideration of such factors as cost of molding, cost of cores, soundness of faces to be machined, strength and cost of pattern will bring about the combination of greatest efficiency. The modern tendency toward casting in one piece as contrasted with the older practice of bolting on parts tends to make more complicated patterns.

The same considerations govern the~ repair of old patterns, as there is often question as to whether the changes required on an old pattern to bring it up to date are not

FOUNDRY AND MACHINE-SHOP PRACTICE more expensive than the construction of a new one. No good pattern-maker enjoys making over patterns, even those made by himself. It is hard on tools and temper,

Gland Casting

Cope

Side

Line

Line i 1

Fig. 5. — Possible Patterns for a Gland Casting, and the result is never satisfactory from the workman's point of view.

In view of the different governing factors and their relative importance, it is not surprising that, with the same data, several good pattern-makers might each decide upon a different method. As there can be but one best way to i Split pattern or whole pattern bedded in

Coped down

Cover core print

Three-pari?

Cope side Line

Fig. 6. — Possible Patterns for a Solid Roller.

make a pattern, it is evident there should be the freest consultation on an important job of all parties concerned, in order that the plan finally adopted may represent the most advanced ideas to be found in the engineering staff, of which the pattern-maker is not the least important.

Chapter III Allowances And Their Effect On The Pattern

9. Allowances. One of the peculiar things about pattern-making is that certain changes in shape and dimensions from those shown in the drawing are necessary in order to get a casting which finally will be exactly as shown on the drawing. These allowances as they are called are made necessary partly by the nature of the metal to be used, partly by the nature of the mold to be made, and partly by the work to be done on the casting after it leaves the foundry.

The Allowance for Shrinkage is 1 made necessary by the fact that nearly all commercial metals shrink somewhat in changing from the liquid to the solid state. The amount of shrinkage varies with the metal and also to some extent with the shape and weight of the casting. The casting is usually somewhat smaller than the mold in which it was cast, so the pattern must be made a little over the drawing dimensions in order to overcome this change in size. In the case of cast-iron, it should be noted that hard irons shrink more than soft, and that large castings shrink less in proportion than light ones.

Small gray iron castings made in green sand will shrink about J" per foot, while large castings in dry sand or loam will shrink at the rate of tV per foot. What are known as malleable iron castings are cast in a hard white iron which shrinks about ^" per foot. Such castings are usually small and should not be over \" thick. Steel castings, which by the way are usually made in dry sand, must have an allowance for shrinkage at the rate of \" per foot. Castings in brass, an alloy of copper and zinc, or in composition, an alloy of copper, tin, and zinc, shrink about yq" per foot, while castings in bronze, an alloy of copper and tin, shrink about -£2" per foot. Lead shrinks a little over J" per foot, but a 4 to 1 alloy of lead and antimony has no shrinkage and so can be used for making duplicates of the master pattern. Type metal also has no shrinkage.

10. Unequal Shrinkage. If parts of the casting have sand between them in the mold the shrinkage tends to crush this sand when cooling takes place. If this sand happens to be a core which is harder than the rest of the mold, then the shrinkage cannot take place so readily and may thus be less in one direction on the casting than in another. A cylinder with a dry sand core, for example, will shrink less in diameter than in length for the reason that the core checks the shrinkage.

A change in the thickness of the casting may make one part cool faster than another and so throw a heavy shrinkage strain on a weak portion. As thin sections cool more quickly in the mold than heavy ones, it is sometimes necessary to uncover heavy sections soon after pouring so as to make the shrinkage more uniform. In the case of a pulley, the rim being thin, sets first, then the spokes, and finally the hub, which tends to draw the spokes inward. This may be overcome to a large extent by uncovering the hub almost as soon as pouring is completed. If the shrinkage

ALLOWANCES AND THEIR EFFECT 15

strain is greater than the elastic limit of the metal, a broken casting will be the result, and even though not broken in the mold, a casting may be left with such a strain that it may fail in service with a load or shock well below the ordinary limit of safety.

This unequal shrinkage often causes warping of a casting which, if not possible to overcome by uncovering the mold, may be avoided by making the pattern with an opposite warp or camber. In general, the heavy side of a casting will be hollow: for example, the bottom of a planer table or the ornamented side of a thin flat plate. (Fig. 7.) The amount of camber necessary can only be determined

-^Convex side

Fig. 7.— Thin Flat Plates, Unequal Shrinkage.

by trial, although the necessity for it and the approximate amount may be closely estimated.

11. Draft. If, as sometimes defined, a pattern is a form into whose impression in sand molten metal is poured to form a casting, it follows that to obtain a perfect casting there must be a perfect impression. If the sand is disturbed when the pattern is drawn, it makes patching necessary and sometimes leaves sand so insecure that it washes into the mold when the metal is poured. A pattern must be so made that the large end comes out of the sand first and no part must disturb the sand after the pattern starts. To accomplish this result a certain amcrant of taper or draft is added to the pattern required for the correct size. (Fig. 8.) The amount of draft necessary varies with the depth of the draw and with the intricacy of the work. As it is an extra and adds to the weight of the casting, and possibly to the machine work, it is made as small as is consistent with easy and rapid molding. When part of the pattern is molded in the cope, that part should have rather more draft than a corresponding part in the drag, for the lifting of the cope usually takes place

Patttng

Fig. 8.— Taper Added for Draft.

before any very effective rapping can be done, and yet the lifting should have no tendency to raise the pattern.

The amount of draft on small, light patterns may be about -£2" per inch while on larger work it may be as small as §" or even ■£%" per foot. On large work the taper introduced by the draft becomes noticeable and is often objectionable, while the added weight may increase the cost appreciably. In general it may be said that on unfinished surfaces as much draft as possible should be added without interfering with the design.

12. Allowance for Finish. If the drawing indicates

ALLOWANCES AND THEIR EFFECT that a surface or surfaces are to be finished, it means that the outer part of the casting is to be cut away to a definite size so as to expose clean sound metal, and an allowance equal to the amount to be cut away is added to those surfaces and is called finish. (Fig. 9.) On small castings which are cast true to the pattern, and which do not warp or shrink appreciably, the amount added for finish may not be more than ^", but xe" is more common for iron

Extra finish necessary on cope side

Fig. 9. — Finish where Required Combined with Draft where Necessary.

castings up to 25 pounds made in green sand. Heavy green sand work and castings made in dry sand usually have an allowance of from f " to |" due to the greater tendency to swell and warp as well as to have a rougher surface. Large dry sand castings and more especially castings made in loam will vary in size even when made from the same patterns. The pressure of the metal tends to strain the mold causing swells to appear on the casting. Uneven ramming is a common cause, while the slight difference between sharp and dull iron may be enough to make a change in size. The surfaces on these large castings are not as smooth as is the case on smaller work and all these factors combine to make necessary an allowance for finish of |" to J"-

Surfaces to be finished should if possible be cast in the drag as the metal will be sounder and freer from blowholes. If necessary to finish a surface cast in the cope, the allowance for finish should be greater so as to get rid of the dirt, etc., which floats on the molten metal and so gathers in the cope.

13. Effect of Rapping. When castings are to fit together without machining, the amount of rapping which the pattern gets becomes of great importance. In small patterns, especially if they are thin and have a short draw, the rapping may be enough to more than overcome the shrinkage and so produce a casting which is actually larger than the pattern. In such a case, there is also no need for any draft, as the mold is so much larger than the pattern that no great care is needed in drawing.

When castings must be uniform and true to pattern, methods of molding are followed which do not require any rapping. This usually takes the form of the stripping plate which holds the sand while the pattern is drawn. By this method also the necessity for draft is removed and a pattern can be made with perfectly straight sides. This method is somewhat expensive, is usually employed with metal patterns, and has its principal use where large numbers of castings are wanted which must fit without machining.

14. Allowances for Machining. Sometimes allowances are made which do not appear on the drawing and which are solely for the benefit of the machinist. Projections

ALLOWANCES AND THEIR EFFECT 19

may be cast on for holding, driving,' or for preventing distortion while the casting is being machined. The feet on piston ring castings and the brace across the crank opening of a crank shaft are examples, as well as the centering lug on a tee rest casting or on a drill press frame.

Chapter IV Tools— Bench, Machine And Special

As patterns are usually made in wood, the bench tools used by the pattern-maker are largely those of the wood-worker, or cabinet-maker, while the machine tools used in the preparation of stock are similar to those used in a good cabinet-shop. The peculiar character of pattern work is responsible for many special tools which do not as a rule have a use in any other trade. Only these special tools will be illustrated, together with some special rigs used on common tools.

15. Pattern-maker's Tools. The first advice of any good mechanic to a man just entering his craft is to get standard, high-grade tools and then keep them in the best of order. As applied to pattern-makers, this results in a good man usually having a kit of personal tools worth from one hundred to two hundred dollars. The amount of time spent in keeping tools in order, together with the personal character of the tools themselves, makes it almost a crime to be considered a chronic borrower.

A man employed on work of fair size needs a bench about 8' long by 30" wide and high enough to suit the man's stature. The front of the bench should be of maple or birch, 2\" to 3" thick, and preferably laid up in strips. Such a top can be planed off from time to time and so kept in good condition. The back may be £" thick and slightly lower than the front. This bench should be furnished with two drawers and a vise of the quick-action variety or better yet a pattern-maker's vise (Fig. 10). A cast-iron table about 4' by 2' for truing up and assembling work is a great convenience. Extra large work can best be handled on the open floor and supported on horses or trestles of a height to suit the man and the work. These

Fig. 10. — Pattern-maker's Vise.

supports should be so carefully levelled that a plumb bob could be used.

The pattern-maker has little use for the ordinary standard rule but uses almost exclusively a shrink rule suited to the shrinkage of the metal to be used for the casting. When master patterns are made it is necessary to allow two shrinks. If the working pattern and the casting are to be of the same metal, then it becomes double shrinkage for that metal, but where, for example, the working pattern is to be brass and the casting iron, it means one brass shrink plus one iron shrink. These combinations make necessary a wide variety in shrink rules which can be had in tV

16 7

1 //

TO ,

Iff 5 // and J" per foot. The best

Fig. 11. — Pattern-maker's Knife.

rules are of steel 12" or 24" long. Wooden shrink rules are usually inaccurate and the graduations are coarse. In as simple a tool as the knife, the pattern-maker uses an unusual shape. (Fig. 11.) The large amount of laying out done and the fine whittling, especially in such

Fig. 12.— The Edge Center.

situations as spokes of small gears, etc., makes the small point shown a necessity, while the shearing cut given by the slope of the blade is highly desirable. This awkward whittling is also responsible for the knife with a bent blade seen in some kits of tools.

When it becomes necessary to strike an arc from a center which is located on the edge of a piece of stock, some means must be found to hold the point of the dividers cr trammels. Clamping another piece of wood against the first is often resorted to. The edge center shown in Fig. 12 is much handier and is usually more accurate. It should be made of brass and the centers carefully located on the lines as shown.

Fig. 13. — Trammel Points.

There seems to be no excuse for making a center square in the sizes that are obtainable in the market, but large circular work, especially large gear work, often needs a center square larger than can be bought. In such a case a shop-made tool, if carefully constructed, meets all needs. It may be well to point out that the angle between the two sides of the head need not be 90° as in the commercial article; it is only necessary that the edge of the blade should bisect the angle of the head, whatever that may be.

Fig. 14. — Coping Trammel with Cradle.

Trammel points should be made to fasten on the side of a light wooden bar, and one of the points should have a fine adjustment. (Fig. 13.)

A very useful tool for use where circular portions are to intersect is called a coping trammel. (Fig. 14.) It consists essentially of a base with a fixed vertical spindle carrying an adjustable rotating head with a sliding pencil or scriber. As an aid in holding either round or parted work while using the coping trammel or during other laying out, cradles or V blocks as shown in Fig. 14, are very desirable. They should be made of metal, cast-iron preferred, and laid out very accurately. As they are always used in pairs, they should be planed up in long lengths and cut into sec-