Cast a bronze bushing blank by pouring 85% copper, 10% tin, 5% lead bronze at 1,000-1,050°C into a sand mould whose pattern is 1.5% oversize with 2-3 mm of machining stock on each cut face, then bore it to size on a lathe. Spinning an iron mould at 500-1,000 rpm while pouring gives denser metal with the impurities thrown to the inside, where the boring cut removes them.
Part of Bearings and Bushings
Making the rough bronze blank that a lathe will turn into a bushing, by static sand casting or by centrifugal casting.
A Bushing Blank Must Be Cast Oversize Because Bronze Shrinks And Then Gets Machined
A casting is never the finished size. Bronze shrinks by about 1.5% as it cools (15 mm per metre, from the shrinkage table in Sand Casting), and the casting must carry extra metal on every surface the lathe will touch. The cast blank is therefore made bigger than the finished bushing by the sum of both allowances, and the pattern bigger still.
Sand casting is the simple route. Centrifugal casting is the better one: the denser metal is forced outward against the mould wall, and gas bubbles and slag, being lighter, collect at the inside surface. The bore is then cut out of exactly that layer, so the faulty metal ends up in the chips.
What You Need
- A pattern turned on the lathe, oversize as described below, with a draft so it lifts out of the sand (see the pattern draft note in Sand Casting)
- Green sand: 85-90% silica sand, 8-12% clay and 3-5% water by weight, per Sand Casting
- A core for the bore, made from oil-bonded sand
- Bronze: roughly 85% copper, 10% tin and 5% lead, melted in a crucible and poured at 1,000-1,050°C (Bronze Making)
- For the centrifugal route: a steel or cast-iron tube mould and a way of spinning it at 500-1,000 rpm
Method: Sand Casting
Step 1: Turn the pattern. Make it larger than the finished bushing by the machining allowance plus the shrinkage.
Step 2: Ram green sand around the pattern in a two-part flask.
Step 3: Remove the pattern and set the core in the bore position.
Step 4: Pour at 1,000-1,050°C. Pre-pour checks: mould weighted down, vents poked through the cope, and everything dry.
Step 5: Cool slowly, break out and clean.
Method: Centrifugal Casting
Step 1: Fit the tube mould on a spinning shaft and bring it to speed, 500-1,000 rpm.
Step 2: Pour molten bronze into the spinning mould. Centrifugal force pushes the denser metal outward.
Step 3: Stop, cool and extract. Bore out the inner surface to remove the impurities; the result is a dense, defect-free bushing.
Worked Example
A bushing of 60 mm outside diameter for a 50 mm shaft needs 2-3 mm of stock on each machined surface, so 4-6 mm on a diameter. The cast outside diameter is 64-66 mm, and the pattern, 1.5% bigger than that, is 64 x 1.015 = 64.96 mm up to 66 x 1.015 = 66.99 mm. The core box takes the same shrinkage allowance as the pattern; that part is standard foundry practice rather than a figure from this wiki.
For the centrifugal route, the outer metal sits against the mould wall. For a 64 mm bore mould, the radius is 0.032 m. At 500 rpm the angular speed is 2 x 3.1416 x 500 / 60 = 52.4 rad/s, so the acceleration at the wall is 52.4 x 52.4 x 0.032 = 87.7 m/s squared, or 87.7 / 9.81 = 8.9 times gravity. At 1,000 rpm it is 104.7 x 104.7 x 0.032 = 350.8 m/s squared, or 35.8 times gravity. The range spans four times the force. Where in that range a particular mould needs to be is not stated in the source and should be found by trial on scrap.
Common Mistakes
- Forgetting that the lathe will remove 2-3 mm. A blank cast to finished size has nothing left to cut and its skin, the worst metal, stays on the bearing surface.
- Pouring into a damp mould. Water becomes steam and throws metal.
- Applying the shrinkage allowance twice, once to the pattern and again when planning the stock. They are separate allowances and add.
- Leaving the impurity layer in a centrifugal casting. The point of the method is that the inside surface is bad and is then bored away.
Safety
Molten bronze, steam and a spinning mould
At 1,000-1,050°C, bronze burns skin through leather. Any water on the mould, tools or ground can flash to explosive steam. Wear a face shield, leather gloves, long sleeves and closed shoes. Bronze with 5% lead gives off lead fume when poured: work outdoors or with strong cross-ventilation. A centrifugal mould is a rotating machine full of liquid metal: guard it, stand clear of its plane of rotation, and know where the stop is and how long the mould takes to coast down before you pour.
What’s Next
- Learn next: Boring and Finishing - cutting the cast blank to final size
- Combine with: Bronze Bushings - what the finished sleeve does and how to size it