[Illustration: FIG. 51.—Elevation of simple reciprocating steam
engine.]
The bed is made of wood, preferably oak, into the parts of which
linseed oil is well rubbed before they are screwed together, to prevent
the entry of water. A longitudinal groove is sawn in the top of the
bed, as indicated by the dotted line in Fig. 51, to give room for the
connecting rod in its lowest position, and a cross groove is scooped in
line with the crank shaft to accommodate the lower part of the crank
disc and the big end of the rod. (If the wing W under the cylinder is
screwed to the side of the bed, instead of passing through it, as
shown, a slight cutting away of the edge will give the necessary
clearance in both cases. )
[Illustration: FIG. 52.—Plan of simple reciprocating steam engine.]
The cylinder and valve tube A should be flattened by filing and rubbing
on emery cloth, so that they may bed snugly against one another and
give a good holding surface for the solder. A steam port, S P, should
next be bored in each, and the “burr” of the edges cleaned off
carefully so as not to obstruct valve or piston in the slightest
degree. “Tin” the contact surfaces thinly, and after laying valve tube
and cylinder in line, with the portholes corresponding exactly, bind
them tightly together with a turn or two of wire, or hold them lightly
in a vice, while the solder is made to run again with the aid of a
spirit lamp. If it seems necessary, run a little extra solder along the
joint, both sides, and at the ends.
The valve, if built up, consists of a central rod, threaded at the rear
end, four washers which fit the tube, and a central spacing-piece. The
forward washer is soldered to the rod. Behind this is placed a felt
packing. Then come in order the central spacing-piece, with a washer
soldered to each end, a second packing, and a fourth washer. The series
is completed by an adjusting nut to squeeze the packings, and a lock
nut to prevent slipping. The back end of the valve must be wide enough
to just more than cover the steam port. If the felt proves difficult to
procure or fit, one may use a ring or two of brass tubing, with an
external packing of asbestos cord.
The cylinder wing W should have the top edge turned over for an eighth
of an inch or so to give a good bearing against the cylinder, and be
held in position by a wire while the soldering is done. It is important
that the line of the wing should be at right angles to a line passing
through the centres of the valve tube and cylinder.
Shaft Bearings.—Take a piece of strip brass half an inch or so wide and
3-1/2 inches long. Bore four holes for screws, and scratch cross lines
an inch from each extremity. Turn up the ends at these lines at right
angles to the central part, stand the piece on some flat surface, and
on the outer faces of the uprights scratch two cross lines at the
height of the centre of the cylinder above the bed. Mark the central
points of these lines.
Next select a piece of brass tubing which fits the rod chosen for the
crank shaft, and bore in the bearing standards two holes to fit this
tubing. Slip the tubing through the standards and solder it to them.
The ends and central parts of the tubing must now be so cut away as to
leave two bearings, BB—that at the fly-wheel end projecting far enough
to allow the fly wheel, when brought up against it, to just clear the
bed; that at the crank end being of the proper length to allow the
eccentric to be in line with the valve rod, and the crank disc to
occupy its proper position relatively to the central line of the
cylinder. Finish off the standards by filing the tops concentrically
with the bearings.
The eccentric may be built up from a metal disc about 3/4 inch diameter
and two slightly larger discs soldered concentrically to the sides. The
width of the middle disc should be the same as that of the eccentric
rod. A careful filer could make a passable eccentric by sinking a
square or semicircular groove in the edge of a wide disc. The centre of
the eccentric must be found carefully, and a point marked at a distance
from it equal to half the travel of the valve. To ascertain this, pull
the valve forward until the steam port is fully exposed, insert a bar
at the rear end of the valve tube, and mark it. Then push the valve
back until a wire pushed through the port from the cylinder side shows
that the port is again fully exposed. Insert and mark the bar again.
The distance between the marks gives you the “travel” required.
Order of Assembly.—The following list of operations in their order may
assist the beginner:
Make the bed.
Cut out cylinder barrel, piston, and valve tube.
Bevel off the ends of the last inside to allow the valve to enter
easily.
Make the valve.
Bore the steam ports, and solder valve tube and cylinder together.
Solder holding-down wing, W, to cylinder.
Finish off the piston.
Solder the bearings in their standards.
Prepare shaft, crank disc, crank pin, and piston rod.
Fix the cylinder to the bed, in which a slot must be cut for the wing
and holding-down bolt.
Attach the piston rod to the piston, and insert piston in cylinder.
Bore hole for shaft in centre of crank disc, and another, 9/16 inch
away (centre to centre), for crank pin.
Solder in crank pin squarely to disc.
Pass shaft through bearings and slip on the crank disc.
Pass front end of piston rod over the crank pin.
Lay bearing standard on bed squarely to the centre line of the
cylinder, turn crank fully back, and move the standard about till the
back end of the piston clears the back end of the cylinder by about
1/32 inch.
Get standard quite square, and adjust sideways till connecting rod is
in line with axis of cylinder.
Mark off and screw down the standard.
Make the eccentric, eccentric rod, and strap. Slip eccentric on shaft.
Put valve in position and draw it forward till the port is exposed.
Turn the eccentric forward, and mark the rod opposite centre of valve
pin.
Bore hole for pin, and insert pin.
Hold the crank shaft firmly, and revolve eccentric till the port just
begins to open on its forward stroke. Rotate crank disc on shaft till
the crank pin is full forward.
Solder eccentric and disc to shaft.
Solder steam pipe to cylinder, and a brass disc to the rear end of the
cylinder.
Fit a fly wheel of metal or wood. This must be fairly heavy, as it has
to overcome all friction during the return or exhaust stroke.
Action of Engine.—During the forward motion of the piston the valve is
pushed back by the eccentric until the steam port is fully opened, and
is then drawn forward, covering the port. At the end of the power
stroke the port has begun to open to the air, to allow the steam to
escape throughout the exhaust stroke, in the course of which the valve
is pushed back until, just at the end of the stroke, the steam port
begins to open again.
Notes.— (l.) The connecting rod may be made shorter than shown in Figs.
51 and 52; but in that case the piston also must be shortened to allow
for the greater obliquity of the rod at half-stroke.
(2.) If two opposed cylinders are made to operate the one crank, a
double-acting engine is obtained. Both valves may be operated by a
single eccentric, the connecting rod of one being pivoted to a small
lug projecting from the eccentric strap. If three cylinders are set 120
degrees apart round the crank shaft, a continuous turning effect is
given. This type will be found useful for running small dynamos.
(3.) If it is desired to use the exhaust steam to promote a draught in
the boiler furnace, it should be led away by a small pipe from the rear
end of the valve tube.
XV.
A HORIZONTAL SLIDE-VALVE ENGINE.
The reader who has succeeded in putting together the simple engine
described in the preceding chapter may wish to try his hand on
something more ambitious in the same line. The engine illustrated in
Figs. 53 to 66 will give sufficient scope for energy and handiness with
drill and soldering iron. The writer made an engine of the same kind,
differing only from that shown in the design of the crosshead guides,
without the assistance of a lathe, except for turning the piston and
fly wheel—the last bought in the rough. Files, drills, taps, a hack
saw, and a soldering iron did all the rest of the work.
Solder plays so important a part in the assembling of the many pieces
of the engine that, if the machine fell into the fire, a rapid
disintegration would follow. But in actual use the engine has proved
very satisfactory; and if not such as the highly-skilled model-maker
with a well-equipped workshop at his command would prefer to expend his
time on, it will afford a useful lesson in the use of the simpler
tools. Under 50 lbs. of steam it develops sufficient power to run a
small electric-lighting installation, or to do other useful work on a
moderate scale.
[Illustration: Fig. 53.—Elevation of a large horizontal engine.]
The principal dimensions of the engine are as follows:
Bedplate (sheet zinc), 13-1/2 inches long; 4-1/2 inches wide; 1/8 inch
thick.
Support of bedplate (1/20 inch zinc), 3 inches high from wooden base to
underside of bedplate.
Cylinder (mandrel-drawn brass tubing), 1-1/2 inches internal diameter;
2-13/16 inches long over all.
Piston, 1-1/2 inches diameter; 1/2 inch long.
Stroke of piston, 2-1/4 inches.
Connecting rod, 5 inches long between centres; 5/16 inch diameter.
Piston rod, 5-1/8 inches long; 1/4 inch diameter.
Valve rod, 4-1/8 inches long; 3/16 inch diameter.
Crank shaft, 5 inches long; 1/2 inch diameter.
Centre line of piston rod, 1-1/4 inches laterally from near edge of
bed; 1-5/8 inches from valve-rod centre line; 1-5/8 inches vertically
above bed.
Centre line of crank shaft, 10-3/8 inches from cross centre line of
cylinder.
Bearings, 1 inch long.
Eccentric, 9/32-inch throw.
Fly wheel, diameter, 7-1/2 inches; width, 1 inch; weight, 6 lbs.
Pump, 3/8-inch bore; 3/8-inch stroke; plunger, 2 inches long.
[Illustration: Fig. 54.—Plan of a large horizontal engine.]
Other dimensions will be gathered from the various diagrams of details.
The reader will, of course, suit his own fancy in following these
dimensions, or in working to them on a reduced scale, or in modifying
details where he considers he can effect his object in a simpler
manner.
The diagrams are sufficiently explicit to render it unnecessary to
describe the making of the engine from start to finish, so remarks will
be limited to those points which require most careful construction and
adjustment.
[Illustration: Fig. 55.—Standards of Bedplate.]
The Bedplate.—This should be accurately squared and mounted on its four
arch-like supports. (For dimensions, consult Fig. 55.) Half an inch is
allowed top and bottom for the turnovers by which the supports are
screwed to the bedplate and base. The ends of the longer supports are
turned back so as to lie in front of the end supports, to which they
may be attached by screws or solder, after all four parts have been
screwed to the bed. Care must be taken that the parts all have the same
height. Drill all holes in the turnovers before bending. Use 1/8-inch
screws. Turn the bed bottom upwards, and stand the four supports,
temporarily assembled, on it upside down and in their correct
positions, and mark off for the 3/32-inch holes to be drilled in the
bed. A hole 3/4 inch in diameter should be cut in the bedplate for the
exhaust pipe, round a centre 2 inches from the end and 1-5/8 inches
from the edge on the fly-wheel side, and two more holes for the pump.
Making the Cylinder Slide and Valve.—The cylinder barrel must be
perfectly cylindrical and free from any dents. Mandrel-drawn brass
tubing, 1/16-inch thick, may be selected. If you cannot get this turned
off at the ends in a lathe, mark the lines round it for working to with
the aid of a perfectly straight edged strip of paper, 2-13/16 inches
wide, rolled twice round the tube. The coils must lie exactly under one
another. Make plain scratches at each end of the paper with a sharp
steel point. Cut off at a distance of 1/16-inch from the lines, and
work up to the lines with a file, finishing by rubbing the ends on a
piece of emery cloth resting on a hard, true surface.
[Illustration: FIG. 56.-Cylinder standard before being bent.]
A square-cornered notch 1/8 inch deep and 7/8 inch wide must now be cut
in each end of the barrel, the two notches being exactly in line with
one another. These are to admit steam from the steam ways into the
cylinder.
Cylinder Standards.-Use 5/64 or 3/32 inch brass plate for these. Two
pieces of the dimensions shown in Fig. 56 are needed. Scratch a line
exactly down the middle of each, and a cross line 1/2 inch from one
end. The other end should be marked, cut, and filed to a semicircle.
Drill three 3/16-inch holes in the turnover for the holding-down
screws. The two standards should now be soldered temporarily together
at the round ends and trued up to match each other exactly. Place them
in the vice with the bending lines exactly level with the jaws, split
the turnovers apart, and hammer them over at right angles to the main
parts. Whether this has been done correctly may be tested by placing
the standards on a flat surface. Take the standards apart, and scratch
a cross line on each 1-5/8 inch from the lower surface of the foot on
the side away from the foot. Make a punch mark where the line crosses
the vertical line previously drawn, and with this as centre describe a
circle of the diameter of the outside of the barrel. Cut out the inside
and file carefully up to the circle, stopping when the barrel makes a
tight fit. On the inside of the hole file a nick 1/8 inch deep, as
shown in Fig. 56. Remember that this nick must be on the left of one
standard and on the right of the other, so that they shall pair off
properly.
Standards and barrel must now be cleaned for soldering. Screw one
standard down to a wood base; slip one end of the barrel into it; pass
the other standard over the other end of the barrel, and adjust
everything so that the barrel ends are flush with the, outer surfaces
of the standard, and the nicks of the barrel in line with the standard
nicks. Then screw the other standard to the base. Solder must be run
well into the joints, as these will have to stand all the longitudinal
working strain.
The next step is the fitting of the cylinder covers. If you can obtain
two stout brass discs 2-1/8 inches in diameter, some trouble will be
saved; otherwise you must cut them out of 3/32-inch plate. The centre
of each should be marked, and four lines 45 degrees apart be scratched
through it from side to side. A circle of 15/16-inch radius is now
drawn to cut the lines, and punch marks are made at the eight points of
intersection. Solder the covers lightly to the foot side of their
standards, marked sides outwards, and drill 1/8-inch holes through
cover and standard at the punch marks. Make matching marks on the
edges. Unsolder the covers, enlarge the holes in them to take 5/32-inch
screws; and tap the holes in the standards. This method will ensure the
holes being in line, besides avoiding the trouble of marking off the
standards separately.
Bore a 1/4-inch hole in the centre of one cover—be sure that it is the
right one—for the piston rod.
You can now proceed to the making of the piston-rod gland (Fig. 54,
G1). Fig. 57 shows how this is built up of pieces of tubing and brass
lugs for the screws. If possible, get the tubular parts trued in a
lathe.
[Illustration: FIG. 57.—Vertical section of cylinder.]
Before the gland is soldered to the cover, the cover should be put in
place, the piston rod attached to the piston, and the parts of the
gland assembled. Push the piston rod through the cover until the piston
is hard up against the back of the cover. Slip the gland over the rod,
turn it so that the screws are parallel to the foot of the standard,
and make the solder joint. This is the best way of getting the gland
exactly concentric with the cylinder so that the piston rod shall move
without undue friction. But you must be careful not to unsolder the
cylinder from its standard or the parts of the gland. Blacken the
piston rod in a candle flame to prevent solder adhering.
Steam Chest.—The walls of the steam chest are best made in one piece
out of 1/2-inch brass by cutting out to the dimension given in Fig. 58.
A sharp fret saw will remove the inside rectangle. Get both inside and
outside surfaces as square as possible in all directions, and rub down
the two contact faces on emery cloth supported by an old looking-glass.
[Illustration: FIG. 68.-Wall-piece for steam chest, with gland and
valve rod in position.]
Two perfectly flat plates of 1/8-inch brass are cut to the size given
in Fig. 59, or a little longer both ways, to allow for working down to
the same area as the wall-piece. This operation should be carried out
after soldering the three pieces together. File and rub the sides until
no projections are visible. Then drill twelve 3/32-inch holes right
through the three parts. After separating them, the holes in the walls
and what will be the cover must be enlarged to an easy fit for 1/8-inch
bolts, and the valve plate tapped.
Now drill 3/16-inch holes centrally through the ends of the walls for
the valve rod. If the first hole is drilled accurately, the second hole
should be made without removing the drill, as this will ensure the two
holes being in line. If, however, luck is against you, enlarge the
holes and get the rod into its correct position by screwing and
soldering small drilled plates to the outside of the chest. Also drill
and tap a hole for the lubricator. The attachment of the gland (Fig.
54, G2) is similar to that of the cylinder gland, and therefore need
not be detailed.
The Valve Plate (Fig. 59).—Three ports must be cut in this—a central
one, 7/8 by 3/32 inch, for the exhaust; and two inlets, 7/8 by 3/32
inch, 1/8 inch away from the exhaust. These are easily opened out if a
series of holes be drilled along their axes.
[Illustration: FIG. 69.—Valve plate.]
The Steam Ways.—The formation of the steam ways between valve plate and
cylinder is the most ticklish bit of work to be done on the engine as
it entails the making of a number of solder joints close together.
[Illustration: FIG. 60.—Piece for steam ways.]
We begin by cutting out of 1/20-inch sheet brass a piece shaped as in
Fig. 60. Parallel to the long edges, and 3/8 inch away, scribe bending
lines. Join these by lines 5/8 inch from the short edges, and join
these again by lines 1/4 inch from the bending lines. Cuts must now be
made along the lines shown double in Fig. 60. Bend parts CC down and
parts BB upwards, so that they are at right angles to parts AA. The
positions of these parts, when the piece is applied to the cylinder,
are shown in Fig. 62.
[Illustration: FIG. 61.—Valve plate and steam ways in section.]
One must now make the bridge pieces (Fig. 61, a, a) to separate the
inlet passages from the exhaust. Their width is the distance between
the bent-down pieces CC of Fig. 60, and their bottom edges are shaped
to the curvature of the cylinder barrel. Finally, make the pieces bb
(Fig. 61), which form part of the top of the steam ways.
In the assembling of these parts a blowpipe spirit lamp or a little
“Tinol” soldering lamp will prove very helpful.
The following order should be observed:
(1.) Solder the piece shown in Fig. 60 to the cylinder barrel by the
long edges, and to the cylinder supports at the ends. This piece must,
of course, cover the steam ports in the cylinder.
(2.) Put pieces aa (Fig. 61) in position, with their tops quite flush
with the tops of BB (Fig. 62), and solder them to the cylinder barrel
and sides of the steam-way piece.
(3.) Solder the valve plate centrally to BB, and to the tops of aa,
which must lie between the central and outside ports. Take great care
to make steam-tight joints here, and to have the plate parallel to the
standards in one direction and to the cylinder in the other.
(4.) Solder in pieces bb. These should be a tight fit, as it is
difficult to hold them in place while soldering is done.
(5.) Bore a 5/16-inch hole in the lower side of the central division
and solder on the exhaust pipe.
Slide Valve.—The contact part of this is cut out of flat sheet brass
(Fig. 63), and to one side is soldered a cap made by turning down the
edges of a cross with very short arms. The little lugs aa are soldered
to this, and slotted with a jeweller’s file to engage with notches cut
in the valve rod (see Figs. 58 and 62).
[Illustration: FIG. 63.-Parts of slide valve.]
The Crank and Crank Shaft.—The next thing to take in hand is the fixing
of the crank shaft. This is a piece of 3/8 or 1/2 inch steel rod 5
inches long.
The bearings for this may be pieces of brass tubing, fitting the rod
fairly tight. By making them of good length—1 inch—the wear is reduced
to almost nothing if the lubricating can is used as often as it should
be.
Each bearing is shown with two standards. The doubling increases
rigidity, and enables an oil cup to be fixed centrally.
The shape of the standards will be gathered from Fig. 53, their outline
being dotted in behind the crank.
Cut out and bend the standards—after drilling the holes for the foot
screws—before measuring off for the centres of the holes; in fact,
follow the course laid down with regard to the cylinder standards.
Make a bold scratch across the bedplate to show where the centre line
of the shaft should be, and another along the bed for the piston-rod
centre line. (Position given on p. 138.)
Bore holes in the bearings for the oil cups, which may be merely forced
in after the engine is complete.
The crank boss may be made out of a brass disc 2-3/4 inches diameter
and 3/16 inch thick, from which two curved pieces are cut to reduce the
crank to the shape shown in Fig. 53. The heavier portion, on the side
of the shaft away from the crank pin, helps to counterbalance the
weight of the connecting and piston rods. In Fig. 54 (plan of engine)
you will see that extra weight in this part has been obtained by fixing
a piece of suitably curved metal to the back of the boss.
The mounting of the crank boss on the shaft and the insertion of the
crank pin into the boss might well be entrusted to an expert mechanic,
as absolute “squareness” is essential for satisfactory working.
Screw-thread attachments should be used, and the crankshaft should
project sufficiently to allow room for a flat lock nut. The crank pin
will be rendered immovable by a small lock screw penetrating the boss
edgeways and engaging with a nick in the pin.
Fixing the Standards and Bearings.—Place the two bearings in their
standards and slip the crank shaft through them. Place standards on the
bed, with their centre lines on the crank-shaft centre line. The face
of the crank should be about 3/8 inch away from the piston rod centre
line. Bring the nearer bearing up against the back of the disc, and
arrange the standards equidistantly from the ends of the bearing. The
other bearing should overlap the edge of the bed by about 1/8 inch. Get
all standards square to the edge of the bed, and mark off the positions
of screw holes in bed. Remove the standards, drill and tap the
bed-plate holes, and replace parts as before, taking care that the
lubricating holes in the bearings point vertically upwards. Then solder
bearings to standards.
If any difficulty is experienced in getting all four standards to bed
properly, make the bearing holes in the two inner ones a rather easy
fit. The presence of the crank-shaft will assure the bearings being in
line when the soldering is completed.
The standards and bed should have matching marks made on them.
The Eccentric.—This can be formed by soldering two thin brass discs
1-15/16-inch diameter concentrically to the sides of a disc of
1-15/16-inch diameter and 5/16 inch thick. The centre of the shaft hole
must be exactly 9/32 inch from the centre of the eccentric to give the
proper valve-travel. Drill and tap the eccentric edgeways for a lock
screw.
A piece to which the eccentric strap, eccentric rod, and pump rod are
attached is cut out of 5/16-inch brass. Its shape is indicated in Fig.
53. The side next the eccentric must be shaped as accurately as
possible to the radius of the eccentric. The strap, of strip brass, is
fastened to the piece by four screws, the eccentric rod by two screws.
Crosshead and Guides.—The crosshead (Figs. 53 and 54) is built up by
soldering together a flat foot of steel, a brass upright, and a tubular
top fitting the piston rod. The guides, which consist of a bed, covers,
and distance-pieces united by screws (Fig. 64), have to withstand a lot
of wear, and should preferably be of steel. The importance of having
them quite flat and straight is, of course, obvious.
[Illustration: FIG. 64.—Cross section of crosshead and guide.]
The last 1-3/8 inches of the piston rod has a screw thread cut on it to
engage with a threaded hole in the fork (cut out of thick brass plate),
to which the rear end of the connecting rod is pinned, and to take the
lock nut which presses the crosshead against this fork.
Assuming that all the parts mentioned have been prepared, the cylinder
should be arranged in its proper place on the bed, the piston rod
centrally over its centre line. Mark and drill the screw holes in the
bed.
The Valve Gear.—We may now attend to the valve gear. A fork must be
made for the end of the valve rod, and soldered to it with its slot at
right angles to the slots which engage with the valve lugs. Slip the
rod into the steam chest, put the valve on the rod, and attach the
chest (without the cover) to the valve plate by a bolt at each corner.
Pull the valve forward till the rear port is just uncovered, and turn
the eccentric full forward. You will now be able to measure off exactly
the distance between the centres of the valve-rod fork pin and the rear
screw of the eccentric. The valve connecting rod (Fig. 53, VCR) should
now be made and placed in position. If the two forward holes are filed
somewhat slot-shaped, any necessary adjustment of the valve is made
easier. If the adjustment of VCR and the throw of the eccentric are
correct, the valve will just expose both end ports alternately when the
crank is revolved. If one port is more exposed than the other, adjust
by means of the eccentric screws till a balance is obtained. Should the
ports still not be fully uncovered, the throw of the eccentric is too
small, and you must either make a new eccentric or reduce the width of
the valve. (The second course has the disadvantage of reducing the
expansive working of the steam.) Excess movement, on the other hand,
implies too great an eccentric throw.
Setting the Eccentric.—Turn the crank full forward, so that a line
through the crank pin and shaft centres is parallel to the bed. Holding
it in this position, revolve the eccentric (the screw of which should
be slackened off sufficiently to allow the eccentric to move stiffly)
round the shaft in a clockwise direction, until it is in that position
below the shaft at which the front steam port just begins to show. Then
tighten up the eccentric lock screw.[1]
[Footnote 1: The reader is referred to an excellent little treatise,
entitled “The Slide Valve” (Messrs. Percival Marshall and Co., 26
Poppin’s Court, Fleet Street, E.C. Price 6d.), for a full explanation
of the scientific principles of the slide valve.]
The Connecting Rod.—The length of this from centre to centre of the
pins on which it works should be established as follows:—Slip over the
piston rod a disc of card 1/32 inch thick. Then pass the rod through
the gland and assemble the crosshead and fork on its end, and assemble
the guides round the crosshead foot. Turn the crank pin full forward,
pull the piston rod out as far as it will come, measure the distance
between pin centres very carefully, and transfer it to a piece of
paper.
The rod consists of a straight central bar and two rectangular halved
ends. The ends should be cut out of brass and carefully squared.
Through their exact centres drill 1/8-inch holes, and cut the pieces
squarely in two across these holes. The sawed faces should be filed
down to a good fit and soldered together. Now drill holes of the size
of the pins, using what remains of the holes first made to guide the
drill. The bolt holes are drilled next, and finally the holes for
lubrication and those to take the rods. Then lay the two ends down on
the piece of paper, so that their pinholes are centred on the centre
marks, and the holes for the rod are turned towards one another. Cut
off a piece of steel rod of the proper length and unsolder the ends.
The rod pieces must then be assembled on the rod, and with it be
centred on the paper and held in position while the parts are soldered
together.