[Illustration:  Fig. 69.—­Model steam turbine, showing vertical cross section (left) and external steam pipe (right).]

The rotor in this instance is enclosed in a case made up of two stout brass discs, D and E, and a 3/4-inch length of brass tubing.  The plates should be 1/2-inch larger in diameter than the ring, if the bolts are to go outside.  The stouter the parts, within reason, the better.  Thick discs are not so liable to cockle as thin ones, and a stout ring will make it possible to get steam-tight joints with brown-paper packing.

The wheel is a disc of brass, say, 1/25 inch thick and 4 inches in diameter; the spindle is 3/16 inch, of silver steel rod; the bearings, brass tubing, making a close fit on the rod.

If you cannot get the ring ends turned up true in a lathe—­a matter of but a few minutes’ work—­rub them down on a piece of emery cloth supported on a true surface, such as a piece of thick glass.

Now mark out accurately the centres of the discs on both sides, and make marks to show which face of each disc is to be outside.

On the outside of both scribe circles of the size of the bearing tubes, and other circles at the proper radius for the bolt hole centres.

On the outside of D scribe two circles of 2-inch and 1-11/16-inch radius, between which the steam pipe will lie.

On the inside of D scribe a circle of 1-27/32-inch radius for the steam ports.

On the outside of E mark a 7/8-inch circle for the exhaust pipe.

On the inside of both mark the circles between which the ring must lie.

Bolt Holes.—­The marks for these, six or twelve in number, are equally spaced on the outside of one plate, and the two plates are clamped or soldered together before the boring is done, to ensure the holes being in line.  If the bolts are to screw into one plate, be careful to make the holes of the tapping size in the first instance, and to enlarge those in D afterwards.  Make guide marks in the plates before separating, between what will be the uppermost holes and the circumference.

Bolts.—­These should be of brass if passed inside the ring.  Nuts are not necessary if E is tapped, but their addition will give a smarter appearance and prevent-the bolts becoming loose.

Bearings.—­Bore central holes in the discs to a good fit for the bearings, and prepare the hole for the exhaust pipe.  This hole is most easily made by drilling a ring of small holes just inside the mark and cutting through the intervening metal.

For A, B, and C cut off pieces of bearing pipe, 1/2, 1/4, and 3/4 inch long respectively, and bevel the ends of B and C as shown, to minimize friction if they rub.  File all other ends square. (Lathe useful here.)

Bore oil holes in B and C, and clear away all the “burr.”  Make scratches on the bearings to show how far they should be pushed through the case.