1CBF5d01.pdf

The MUNCASTER

steam-engine models

EDGAR T. WESTBURY is reviewing some classic models of

the past in the light of modern techniques

Continued from 21 February 1957, pages 270-272

R noticed that the drawings of

I must confess that I have not a

great deal of patience with the type

of reader who makes a major issue

out of a missing dimension or a

slight discrepancy in a drawing; the

more complicated drawings become,

the more difficult it is to avoid minor

errors which escape the most careful

checking, and the more likely they are

to deter the timid beginner from

tackling construction.

If the cylinder, A, is made from

stock material, the circular portface

may be made from a piece of 7/8in.

dia. bar, filed or machined to fit

closely to the side of the barrel and

preferably secured by silver soldering.

At the same time, short half-round

pieces can be fitted above and below

this face. It will be seen that a boss

or spigot is provided on the opposite

side of the barrel to receive the point

of a pivot screw, and this should

similarly be fixed, exactly in diametric

alignment with the portface.

The drawing shows a raised beading

in the centre of the cylinder; this is

not a necessity, but is desirable both

from the point of appearance and also

to stiffen the cylinder wall. For

machining the bore and facing one

end flange, I recommend mounting

on an angle plate with a strap bearing

on the spigot, but care should be

taken not to apply so much pressure

as to risk distortion of the barrel.

The other end flange is faced by

mounting on a mandrel.

EADERS will no doubt have

the simple engine described

in the last article were not fully

dimensioned, and just in case there

should be any complaints about

this I will anticipate them by

saying that Muncaster, in com-

mon with many pioneer model

designers, did not consider. it

necessary to give more than a few

leading dimensions on drawings.

I have, however, added a scale

which should be helpful in supplying

the deficiency, in conjunction either

with a simply-made scale rule or a

pair of proportional dividers.

The present-day engineer is accus-

tomed to drawings which have every

dimension, including in many cases

limits and clearances, fully marked,

as this is absolutely necessary in

industrial production, where different

parts have to be made, or even suc-

cessive operations carried out on

single parts, by individual workers

out of direct touch with each other.

2-Double-

acting

oscillating

engines

DOUBLE-ACTING OSCILLATING

Mark off centres

It is absolutely essential that the

centre indentation in the spigot

should be dead in line with the hole

in the centre of the portfaee, and in

order to ensure this, it would be a

sound policy to mark off these centres

with a surface gauge, with the cylinder

resting on the machined flange face

to locate the horizontal line, and

then mounting it on a mandrel resting

in V-blocks for the vertical line. The

intersections on both sides are then

centre-drilled, and the Cylinder

mounted between centres for machin-

ing the portface.

Flanged covers are secured, each

by four screws, to the ends of the

cylinder, and as the engine is double-

acting the upper cover is fitted with a

gland, or “ stuffing-box” as it was

termed by the early engineers. It is

important that this should be exactly

central and truly tapped; these con-

ditions can be ensured by using the

methods which I have described for

engines of my own design.

No exact details are given of the

MODEL ENGINEER

Use initiative

CYLINDER ENGINE

In cases where all the machining

and fitting on a one-off job are in the

hands of a single constructor, how-

ever, meticulous marking of every

essential and non-essential dimension

is by no means so important. While

I should be the last to condone

inaccurate or slipshod work in any

kind of model engineering, I believe

that one can become a slave of the

blueprint, and it is good engineering

practice to work occasionally to what

the professional engineer would con-

sider inadequate drawings or speci-

fications, if only because it helps one

to cultivate a sense of proportion.

Unlike the previous engine, the

construction of this example, illus-

trated in Fig. 7, favours the use of

castings for the main structural parts,

though they can be produced by

cutting from the solid and fabrication.

For instance, the hollow base, D,

could be made by soldering or brazing

a rectangular frame of strip material

to a flat plate, and the bearing sup-

ports or A-frames, F, could be made

by bending strip material to the

profile, as seen in the end view, and

soldering or brazing in the curved

supporting rib or crossbar.

It is most important that these

frames should stand exactly per-

pendicular to the base when fitted

and that the top surfaces which form

the bearing seatings should be exactly

the same height and in alignment.

While it is possible to correct errors

in these respects by packing under the

feet of the frames or the bearings, this

is certainly not a practice to be

encouraged.

In the field of natural history,

reasonably accurate re-constructions

of prehistoric animals were made

from fragments of fossil bones long

before they could be verified by further

evidence; and many model engineers

have produced good work with

nothing more in the way of informa-

tion than a few rough sketches and

possibly a photograph or two.

7 MARCH 1957

337

Muncaster

models . . .

The crankshaft., G, may be either

built up or machined from the solid;

it is not necessary to give details of

either method as they have been

described on innumerable occasions

in connection with other engines. I

may observe, however, that this offers

quite a good opportunity for the

beginner to get some practice in

marking out and machining an

orthodox type of crankshaft from a

piece of flat bar, which should be of

about 1 in. x 3/8 in. section.

I would, however, suggest a slight

modification to this component, as

drawn; it would appear that the only

end location of the shaft is that

provided by the flywheel on one side

and the driving pulley on the other.

I do not consider this to be very good

practice, as although it serves its

purpose, in the event of either fitting

being shifted, accidentally or other-

wise, it would be possible for the

crankhead to be forced out of align-

ment.

A much better method would be to

provide positive end location on the

inside of the main bearings, either by

enlarging the idle portion of the

journals to form locating collars, or

better still, extending the bearing

surface inwards up to the webs of the

crank. Incidentally, I do not under-

stand why it should be necessary to

place the bearing frames so far apart

in this engine as it is a cardinal

precept in any engine design to support

a crankshaft as close up to the webs

as possible.

piston, but from other engine dimen-

sions it is clear that this should be

5/16 in. wide, and a groove 3/16 in. wide

x 1/8 in. deep may be turned in it to

take graphited packing. The rod is

5/32 in. dia., and the crankhead

bearing, which is screwed to the

upper end, must be split as shown in

Fig. 8, to enable it to be assembled

on the crankshaft.

The cylinder is mounted between

the fixed portblock, B, and the pivot

block, C, both of which are screwed

to the base between the feet of the

A-frames, and it is most essential that

the centres of these should be dead

in line and that the face of the block,

B, is squarely located so that the

portface of the cylinder beds truly

against it while being quite free to

swing when the pivot screw is properly

adjusted.

This is perhaps a somewhat difficult

condition to ensure, at least for the

beginner,and I suggest that one

method of doing so is to fix the blocks

together temporarily by sweating, with

an aligning dowel in the pivot holes

(leave the tapping of the pivot block

pro tem), and facing off the base

surface by machining or filing exactly

square with the face of the portblock.

Even this, however, will be of no

avail if the mounting surface of the

base is not dead flat and true; it is also

necessary to locate the two blocks

correctly in relation to each other

when they are screwed in position.

Aligning mandrel

To do this, an aligning mandrel is

made from a dead straight piece of

5/32 in. silver-steel rod, long enough

to extend across the base and pass

through both blocks. After squaring

up their positions, the portblock

should be fixed first by its two screws

and then the pivot block; in each case

it will be possible to spot the positions

of the tapping holes in the base from

those in the blocks to avoid the risk

of error.

Small clamps are useful for holding

the parts in place during these opera-

tions and if not already available they

should be made or obtained right

away, as they will certainly be needed

for innumerable subsequent jobs.

When fitted to complete satis-

faction, the hole in the pivot block

may be tapped and a locking screw

fitted to the top (though I should

personally consider that a locknut on.

the screw would be more satisfactory);

the portblock is also equipped with a

centre stud to fit the hole in the

centre of the cylinder portface.

MODEL ENGINEER

FIG.7

Inches 8

-8

Fig, 7 : General arrangement of double-acting

oscillating engine, and details of cylinder

338

7 MARCH 1957

t.++. .+++tt 7 ‘I

Extreme right, Fig. 8:

Details of the split bear-

ing for piston rod

Right, Fig. 9: Portblock,

showing formation of the

inlet and outlet passages

on both sides (to suit

the twin-cylinder engine)

Left, Fig. 10: Arrange-

ment of complete plant

with vertical boiler, show-

ing cylinder portblock,

reversing valve, etc.

FIG.6

each end of the cylinder in turn and

releasing it to exhaust, on alternate

strokes of the piston. As in the case

of the single-acting engine, optimum

timing is obtained when the “ stop ”

of the groove lines up exactly with the

cylinder port at dead centre and is

exactly the same width as the port.

The ports in the cylinder and the

oblique drilled passages should be

1/8 in. dia., and the entry at the mouth

of the cylinder in each case should

be unobstructed, which calls for

chamfering the edge of the cylinder

and the spigot of the cover at this

point. The portfaces should be care-

fully lapped and may with advantage

be relieved in the centre as recom-

mended for the previous engine. Thin

paper gaskets should be fitted to the

covers to ensure steam-tight joints.

I I

FIG. 10

The main bearings, E, are of the

plummer block type, and correct

practice dictates that they should be

split, though this is not a practical

necessity in a small engine as solid

bearings can be threaded on from the

two ends of the crankshaft. They

must, of course, be correctly aligned

but this is easily ensured if they are

made and fitted properly, and any

small discrepancy which may cause

the shaft to run tight when finally

fitted can be corrected by running a

reamer or lap through both bearings

when they are screwed down in situ.

(This, of course, pre-supposes that

the shaft journals are properly mach-

ined or assembled in exact alignment).

CYLINDER PORT LOCATION

In drilling the steam ports in the

cylinder and portblock, it is not

practicable to use the simple method

of ensuring their correct location

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