CUTTING FACES ON THE INNER MOULD

CUTTING FACES ON THE INNER MOULD

 

FLAT FACE

These two faces will create a thicker casting around the mounting studs.

Magnet rotor moulds

The PMG needs two magnet rotors. Only one mould is needed, but production is easier if there are two moulds, so that two rotors can be produced at one time.

The outer mould (diagram 22) is similar to the stator outer mould, but simpler

Use the PCD jig to drill four holes to match the holes in the magnet disks.

Each magnet rotor also needs an inner disk mould (diagram 23), with the same pattern of four holes.

23 MAGNET ROTOR INNER DISK

7deg
И---------- 140

LLJ

All moulds are sanded down to a very smooth surface, and finished with polyurethane varnish and wax polish. Do not use ordinary paint on the moulds. The heat of the resin process will cause the paint to wrinkle and spoil the appearance of the casting.

 

Jigs for the stator

Stator studs jig (see diagram 24)

The stator needs four 8 mm supporting studs cast into it. These studs need a jig to hold them in place, until the resin is set. This jig is made from wood 380 x 50 x 25mm. It must be made precisely, or the studs will not fit the spine later.

• Make a punch mark at the exact centre of the largest face (see diagram 24).

• Use dividers or compasses to mark arcs at a radius of 1 78 mm from this mark.

• Punch four marks on these arcs, 30 mm apart and 1 0mm from the edge.

• Drill through with an 8mm drill (using a smaller size first to be accurate). Use a
drill press, to drill the holes truly square.

• Remove some of the underside of the ends of the piece of wood, so as to prevent
contact with the fibreglass resin.

24. STATOR STUDS JIG

DRILL HOLES WITH A PILLAR DRILL FOR ACCURATE ALIGNMENT

30

30

320

 

Paper templates (see diagram 25)

Fibreglass 'chopped strand mat' (CSM) is to be used in the stator. Make some paper templates for cutting out the pieces of CSM. Later you can lay the templates on the sheet of CSM, draw around them with a felt pen and then cut the pieces out.

25. PAPER TEMPLATES FOR GLASSFIBRE CSM

380mm

MAKE THIS TEMPLATE BY LAYING PAPER AROUND THE WALL OF THE MOULD AND MARKING THE EDGES.

75mm

Stator construction

This section tells how to make a stator, using the jigs and moulds from section 3. It is a good idea to wind a coil before making the stator moulds, so that the mould can be checked for correct fit.

Winding the coils

• Mount the reel of winding wire on an axle behind you, in line with the coil former. The wire should form an 'S' bend as it winds onto the coil (diagram 26).

REEL (CORRECT)

26. CORRECT WIRE FEED

COIL FORMER

REEL (INCORRECT)

• Bend the tail of the wire 90 degrees, at a point 1 00mm from the end. Do not
handle the bend any other part of the wire; leave it straight. Bent wire does not
make a compact coil.

• Place this bend in the notch, so that the tail hangs out.

• Twist the tail loosely around one of the butterfly bolts.

• Grip the wire between the reel and the winder in a piece of rag to keep it tight.

• Wind the handle of the crankshaft.

The first turn lies against the cheek piece on the side where the tail comes out. The other turns lie against each other neatly, without crossing over. Build the coil up in even layers. Count the number of turns carefully. Normally there will be 1 00 turns.

• When the coil is complete, pass a piece of sticky tape under the coil on both sides
and bind it tightly. Do not cut off the winding wire until this is done, or the coil
will spring out, and loosen. Cut the tail of wire 1 00mm away from the coil.

• Remove the coil from the former, and wind five more coils in exactly the same
way.

• Place the coils on a table (so that they are all exactly the same way up (diagram
27) Check that the starting tail is on the upper surface, and not hidden under the
coil.

• Number the coils 1 -6, writing on the masking tape.

27. THE COILS MUST ALL BE THE SAME WAY UP

STARTING TAIL HIDDEN UNDER

Scrape the enamel off the last 20mm of each tail of enamelled wire, until it is all bright copper. (A hacksaw blade makes a very good scraper, when the edge has been sharpened with a grinder.) Solder on tails of flexible wire (diagram 28).

28. SOLDERING ON TAILS OF FLEX

 

Suggested lengths of flexible tails: coils 1 and 6 - 800 mm flex coils 2 and 5 - 600 mm flex coils 3 and 4 - 400 mm flex

• Cover the soldered joints with sleeving. Leave no bare copper showing.

• Label the tails with the coil number and the letter A or B.

A is for the start of the coil, B is for the finish. Do not mix them up. Or use two colours: black flex for the starts and white for the finishes.

• Lay the coils out in the outer mould.

• Check that they will fit comfortably, and that the tails are long enough to remain
within the mould until the exit point between coils 3 and 4.

It is important to lay all the coils the same way up.

29. THE COILS IN THE MOULD

Preparations for stator casting_____

The stator casting will contain:-

six coils

polyester resin and talcum powder (and perhaps pigment)

fibreglass mat (CSM)

four studs of 8mm x 1 00mm threaded rod

Also, be sure to have the moulds prepared properly. Sand them, seal them, polish them. If PVA release agent can be got, then use it.

 

Cut out pieces of fibreglass CSM, using the templates. There will be 2 circular disks for laying flat in the outer mould. You also need enough curved strip pieces to cover the inside wall of the outer mould in a double thickness of CSM. Overlap 25mm between pieces.

When you are sure that you have everything to hand, start the resin casting process. It is a good idea to read through the procedure first, and check that you understand it all before you start. There are notes on polyester resins in section 8.

The stator casting procedure

Diagram 30 shows the procedure for weighing out the resin and the talcum powder. The talcum powder is only used for bulk mixes (not thin layers with CSM), to prevent overheating, and to thicken the mix. Different mixes use different weights - follow the step by step instructions below. Diagram 31 shows all the parts coming together.

30. MIXING POLYESTER RESIN

Mix resin with catalyst thoroughly but slowly to avoid churning in air bubbles. Add any talcum powder only after the catalyst is mixed. When the resin is mixed, use it at once. After a few minutes in the mixing bucket, it will heat up, and begin to set.

Use exactly the right amount of catalyst. Resin casting needs less catalyst than normal fibreglass work (about half the time). When the workshop is hot, put in less catalyst. Casting thick layers of resin, put in less catalyst. If in doubt, make some trial mixes of resin, to find out the correct amount of catalyst.

If there is no PVA 'release agent', then take care not to wipe the polish off the mould with brush strokes. Apply the resin with a 'prodding' action.

Place the outer mould on some newspaper on a workbench.

Mix 200g of resin, with 3cc of catalyst (and 15-30cc of pigment for colour, if

required). Use no talcum powder in the first two mixes.

Paint this resin all over the inside of the outer mould. Do not paint it on top of the

island in the centre.

Apply one layer of fibreglass mat (CSM), and paint more resin over it again, with a

poking motion to remove bubbles. Work the resin into the CSM.

Apply a second layer of CSM to the wall, but keep one disk for later.

Put the coils into the mould. The wire tails all come out in one place, between

coils 3 and 4.

Mix another 100 g of resin with 2cc catalyst. Pour this over the wires of the coils

so that it soaks in. Avoid making 'pools' of resin.

Mix another 600g of resin with 9cc catalyst and 600g of talcum powder. Pour this

mix into the spaces between the coils. The resin should fill the outer mould until

it is level with the island at the centre.

Shake the mould vigorously, to remove air bubbles. Rotary motion and vibration

will help the resin to settle, and help any air bubbles to rise .

Mix another 200g resin with 3cc catalyst and only 1 00g of talc. Put the second

CSM disk over the coils and paint it with this mix. Thoroughly wash out the paint

brush with thinners.

Put the inner mould down inside the outer mould, and fit the 1 2 mm bolt though the

centre of both. Tuck the wiring neatly into the space between the moulds. One

flat spot on the inner mould sits over the part where the wires come out of the

stator. The resin will rise up the sides. Some resin may spill out.

If necessary, pour resin gently into the gap between the moulds until it rises to

near the top of the female mould. You may need to mix another 1 00g of resin with

1.5cc of catalyst to do this. Keep notes of the amounts of resin used, for next

time.

Place the jig (for the studs - diagram 24) over the inner mould, with one end over

the wire tails. Tighten the 1 2mm bolt with a nut. Insert the four 8mm studs into

the holes, with nuts on top. The studs should be immersed in resin for about half

of their length.

Six stages of the rotor casting procedure

 

31. STATOR CASTING ELEMENTS

The casting is now complete. It should become slightly warm, and harden within hours. If it does not begin to set within a few hours, then put it in a warm place to speed up the process.

When the resin is fully hard, remove the casting from the mould. Be patient and gentle if possible. Remove the jig from the studs. Tap the two moulds apart, using a bolt in each of the holes around the central hole. Knock the casting out of the outer mould by turning it over and knocking the edge of the mould gently against the floor.