Grain crusher project, Grade 8: how to make a grain crusher, step by step

The grain crusher is the Grade 8 Technology project about mechanical systems. Learners design and make a machine that crushes grain, such as maize, and explain how it turns the turning of a handle into the movement that does the work. The Slotbok kit builds a crank-driven trip hammer: turn the handle, and a crank, a connecting rod and a lever lift a hammer and bring it down on the grain.

This guide covers what a grain crusher is for, how the mechanism works, the build in ten steps, how to test it, how to write the design brief and specifications, and the advantages and disadvantages.

Watch the whole build first

The 3D animation that comes with the Slotbok kit: every part and bolt going on in order, then the hammer at work. About two and a half minutes.

What is a grain crusher?

A grain crusher breaks dry grain, such as maize, wheat or sorghum, into small pieces or a coarse meal, so that it can be cooked or fed to animals. Big mills use electric motors. For hundreds of years, villages used trip hammers worked by a water wheel instead, and where there is no electricity, grain is still crushed by hand. That is the setting of most Grade 8 grain crusher tasks: a machine that people can work by hand.

How the grain crusher works

1. Turn the handle. This is the input: your hand does the work.
2. The crank pin goes round. The handle turns the crank, and the pin on the crank moves in a circle.
3. The rod pushes the beam. The connecting rod joins the crank pin to the beam.
4. The beam rocks on its pivot. As the pin goes round, the rod pushes the beam up and pulls it down again.
5. The hammer lifts and drops. The hammer hangs from the far end of the beam, and lifts 43 mm on every turn.
6. The grain is crushed in the bowl. This is the output.

Round and round becomes up and down

The handle and the crank turn round and round: rotary motion. The connecting rod turns that into the beam rocking up and down: reciprocating motion, a movement that goes one way and then back. A car engine uses the same crank and rod the other way round, turning the up-and-down of its pistons into turning wheels.

The beam is a lever

The beam turns on a pivot at the top of the frame, which is its fulcrum. The rod pushes on the beam between the pivot and the hammer, so the hammer end, further from the pivot, moves further than the point the rod pushes. That gives the hammer a bigger lift than the rod makes. The trade is force: the rod has to push harder than the weight it lifts. A lever with the effort between the fulcrum and the load is called a third-class lever.

Why the frame is full of triangles

The towers and the base are a frame structure braced with triangles, and a triangle cannot change shape unless one of its sides bends or breaks. That keeps the frame rigid while the beam rocks and the hammer lands.

Bolts, T-slots and nyloc nuts

The kit is bolted, not glued. A nut sits in a T-shaped slot in one part, and a bolt goes through the next part into it, which makes a strong joint that can be taken apart again. The moving joints use nyloc nuts, which have a nylon ring that grips the bolt so the nut does not work loose as the joint turns.

What is in the kit

The Slotbok grain crusher kit is cut by laser from 3 mm MDF and builds a model 324 mm long, 92 mm wide and 240 mm tall:

• 40 laser-cut parts on three 300 × 200 mm MDF sheets, every part labelled
• 20 M4 bolts in 5 lengths: 12, 20, 30, 35 and 65 mm
• 19 M4 nuts and 5 nyloc nuts
• Two MDF spanners and a ruler that doubles as a bolt gauge

For testing, add a little rice or maize of your own.

How to make a grain crusher, step by step

The build follows the kit's 3D animation, in ten steps. Before you start, press the parts out of the three sheets, sand any rough edges, and sort the bolts by length with the ruler gauge.

1. Build the grain bowl

Push a nut into each hexagon of the two foot rings K, stack the rings under the round end of deck H3, put the two wall rings L on top, and bolt it all down with 2 M4 × 12 bolts.

2. Decks into the front frame

Lay front frame A engraved side out and push decks H1 and H2 into its slots. Put a nut into each T-slot and bolt from the outside with 3 M4 × 12 bolts.

3. Fit the back frame

Fit back frame B over the other ends of the decks, put a nut into each T-slot, and bolt from the outside with 3 M4 × 12 bolts.

4. Build the beam

Fit the two cross plates J into one beam side C, put nuts into the T-slots, lay the second beam side C on top, and bolt both sides with 4 M4 × 12 bolts.

5. Make the hammer

Stack the 4 hammer layers E and bolt them together with 2 M4 × 20 bolts and nuts.

6. Hang the hammer

Put a spacer ring on each side of the hammer and push an M4 × 35 bolt through the beam, ring, hammer, ring and beam. Fit a nyloc nut: snug, then back it off a little so the hammer swings freely.

7. Mount the beam on the towers

Lower the beam between the towers with 4 spacer rings on each side, push the M4 × 65 bolt through the towers, the rings and the beam, and fit a nyloc nut on the back.

8. Build the crank

Stack crank webs F1 and F2. Fit the crank pin, an M4 × 20 bolt, from the F1 side with a nut and a ring, and the axle, an M4 × 30 bolt, from the F2 side with a nut on F1.

9. Fit the crank and handle

Slide the axle out through frame A and put a nut on the outside. Build the handle from arms G1 and G2 with the knob bolt, a nut, 2 rings and a nyloc nut, then fit it onto the axle and lock it with a nut.

10. Connect the rod

Fit connecting rod D onto the crank pin with a nyloc nut, then fix its top end to the beam with an M4 × 12 bolt and a nyloc nut.

Test: try it out

Pour a little rice or maize into the bowl and turn the handle. The hammer lifts 43 mm and pounds the grain.

If it does not work properly

• The handle is stiff — a moving joint is too tight. Back its nyloc nut off a little.
• A joint wobbles — check that its nut sits fully in the T-slot, and tighten it.
• A bolt will not reach its nut — it is the wrong length. Check it against the ruler gauge.

The design brief and specifications

A design brief is a short statement of what you will design and make, who it is for, and why. Write it in your own words, from your own scenario. A typical one has this shape:

Design and make a hand-operated grain crusher for a small farming community without electricity, that one person can use safely to crush a cup of maize at a time.

The specifications list what the grain crusher must do and be. Good questions to answer:

• What must it do? Crush grain by hand, with no electricity.
• Who will use it, and where? One person, at home or on a farm, on a table or a flat surface.
• What forces must it withstand? The hammer landing over and over, the push on the handle, and its own weight, without the frame bending or the joints working loose.
• How will it be safe? No sharp edges, and the moving parts kept away from fingers.
• What size and materials? Small enough for a table, made from materials that are easy to find and repair.

Then list your constraints: the time, the money, the materials and the tools you have.

Advantages and disadvantages of a grain crusher

Advantages of a hand-operated grain crusher:

• It needs no electricity or fuel.
• It is simple, so it is cheap to make and easy to repair.
• It can be built from materials that are easy to find.
• It saves the hard work of pounding grain with a stick.

Disadvantages:

• It is slow, and tiring for large amounts of grain.
• It needs someone to keep turning the handle.
• Grain can jump out of the bowl.
• Moving parts can pinch fingers if the user is careless.

Making one from cardboard instead

If your teacher asks for a model from household materials, a typical list is thick cardboard for the frame and the beam, kebab sticks or dowels for the axle and pivots, a bottle lid or a short tube for the bowl, a cotton reel or a cardboard disc for the crank, and glue. Brace the frame with triangles, and make the pivots loose enough to turn.

What to write down while you build

• A systems diagram: input (turning the handle), process (the crank, the rod and the lever) and output (the hammer lifting and dropping).
• Which parts show rotary motion and which show reciprocating motion.
• Where the fulcrum, the effort and the load are on the beam.
• How far the hammer lifts for one turn of the handle.
• Your design brief, specifications and constraints, in your own words.
• Anything that went wrong and what you did about it.

Worth saying plainly: copying someone else's answers is the one way to lose marks you had already earned. Everything here is so you can explain your own model in your own words.

Frequently asked questions

What is a grain crusher used for?

To break dry grain, such as maize, wheat or sorghum, into smaller pieces or meal, for cooking or for animal feed.

How does a grain crusher work?

In the Slotbok model, the handle turns a crank, a connecting rod turns that into an up-and-down push on a beam, and the beam, a lever, lifts a hammer and brings it down on the grain in the bowl.

What materials are used to make a grain crusher?

The kit uses 3 mm MDF for every part, with M4 bolts and nuts at the joints. A model from scratch is usually cardboard, kebab sticks or dowels, a lid for the bowl, and glue.

Which grade and subject is the grain crusher project for?

Grade 8 Technology, in Mechanical Systems and Control. It is a common choice for that term's practical assessment task, the PAT.

Where can I buy a grain crusher project kit?

The Slotbok grain crusher kit is cut in our workshop in Gauteng and delivered across South Africa, with every part, bolt and nut in the box.

Afrikaans terme

If you are doing this project in Afrikaans: Graad 8 Tegnologie (Grade 8 Technology) · graanbreker (grain crusher) · meganiese stelsels en beheer (mechanical systems and control) · slinger (crank handle) · verbindingstang (connecting rod) · hefboom (lever) · steunpunt (fulcrum) · hamer (hammer) · roterende beweging (rotary motion) · ontwerpopdrag (design brief) · spesifikasies (specifications) · beperkings (constraints).

Get the kit

The kit is cut in our own workshop in Gauteng, and every design is built once by hand before it is packed, so the parts are known to fit.

• Grain Crusher School Project Kit — Grade 8
• All Grade 8 Technology projects

Other build guides: mine headgear kit, Grade 8 · jaws of life kit, Grade 7, more levers and linkages · electromagnetic crane kit, Grade 7.

Questions about a kit, a class set or delivery to your town: message us on WhatsApp.