Introduction: Lapidary Grit Dispenser

About: I ain't no physicist, but I knows what matters.

I designed and built this grit dispenser for a friend who owns a lapidary sphere making machine. He has let me borrow the machine enough times that I gave it to him as a gift (But I get to use it, too. Heh)

When grinding spheres, you need a constant water drip. You must occasionally sprinkle diamond grit onto the stone to ensure the removal of material. With the automatic dispenser, you don't need to constantly monitor the grinding process.

After use, the dispenser can be emptied, disassembled and cleaned, then reloaded with a different grit or compound.

3D models created (by MEEEE!!!) with AutoCAD 2008
Schematic created using Eagle 6.4.0

Step 1: PVC Parts

PVC Parts needed to make the dispenser:
1-1/4" tee with 1/2" FPT port
1-1/4" to 1/2" FPT reducer
1/2" MPT to 3/8" hose barb (nylon)
1-1/4" pipe plug
Not shown: 1/2" MPT plug for the fill hole
PVC Cement

Step 2: Feed Screw

The feed screw is made from a self drilling wallboard anchor. The thread diameter is 1/2", and the original overall length is about 1-5/8".
Cut off both ends as shown, then drill out the center to fit the 1/8" shaft.
The feed screw is a nice fit inside the hose barb.

The shaft is a 1/8" rod about 5" long. Threads are used on one end only. 1/8" all-thread will work OK. Buy a longer piece and trim to fit. (We will trim the length in a later step.)

I'm using a hose barb because I will attach 3/8" vinyl tubing to drop the grit onto the sphere. It keeps the dispenser up and out of the way.

Step 3: Shaft and Sleeve

A sleeve for the shaft is made from 3/16" OD brass tubing, with a 3/16" brass washer soldered as a mounting surface. Length is 3/4".
Make the bottom stub shorter than the thickness of the 1-1/4" pipe plug to keep the shaft from shifting up and down.

Drill a 3/16" clearance hole in the pipe plug to mount the sleeve. Glue is not required.

On the shaft use locknuts to maintain the correct clearance.

File a flat on the threaded end of the shaft for the collet set screw.

Step 4: Trim the Shaft

Insert the hose barb. Finger tight only. Don't use sealant. This fitting will be removable to service and clean the unit.

Measure the distance to the top of the nut. I have 3/8".
If you're old like me, you'd better write that down.

Remove the hose barb and install the shaft assembly. Adjust the nuts to remove any slop and still allow free rotation.
Leave enough length to attach the collet.

Bottom out the shaft into the hose barb and measure again. I have 1-1/4". Sooo, 1-1/4" minus 3/8" means the shaft is 7/8" too long.
Remove an extra 1/4" to leave a cavity under the screw. This will allow the grit to drop freely as the feed screw turns.

Use epoxy to attach the screw onto the shaft. The end of the shaft is flush with the bottom of the screw.
The feed screw will be turning counter-clockwise.

Step 5: The Motor

The motor I am using is a four wire (bi-polar) stepper with 48 steps per revolution. It came from a dead printer.On the motor shaft is a collet made from 5/16" aluminum rod.

A DC gearmotor turning 60 RPM or less will give good results. The motor must turn sloooowly. Two or three revolutions will give a good dose of grit.

Note to the stepper motor novice:
If you choose to use a stepper motor, be prepared to...
   Build (or buy) a stepper controller and driver.
   Spend many hours learning stepper theory and design.
   Research different stepper types.
   Pore over the various circuits used to control and drive steppers.

The time and effort will pay off in a good way. You will become the stepper motor guru among your peers. (Driving stepper motors is a very esoteric subject.)

Hint: Look at the L297/L298 stepper controller and driver ICs.

The motor mount might be anything you have in your scrap bin. I used 3/4" x 1/16" aluminum strap. It's attached to the dispenser with self-tapping panel screws. The mount doesn't need to be extremely strong. There's not a lot of stress on the shaft or motor.

Step 6: Timer Circuit

Here is the Grit Dispenser timer circuit. It uses a 555 timer. A 556 dual timer will also work if you transpose the IC pins. The circuit is simple enough that building it on a proto board should not be a problem.

R3 controls the off time. The range is about 2.5 minutes to 14 minutes.
R4 controls the on time. The range is about 0.5 seconds to 7 seconds.
Pressing pushbutton S1 (N.O. momentary) will manually start the 'on' time between cycles.
V+ can be anywhere between +5 and +15 VDC.

For the supply voltage V+, a wall transformer will work if the output current is high enough to drive the motor. (Google your motor part# and get the specs)

Q1 can be any PNP power transistor that will handle the motor current and voltage. Adjust the resistor values (R5 & R6) to match the transistor specs. Always use an adequate heat sink.

If you are using a stepper motor, the output Q is used as the 'enable' input to the stepper controller (no power transistor). You may need to add a small NPN transistor to invert the output (2N2222 or similar).

Note from author: Please no questions about electronics, 3D solid modeling, my new girlfriend, stepper motors (mmmmaybe steppers), quantum mechanics, losing one sock in the dryer, etc, etc, etc.

Someone you know may be a closet electronic hobbyist. Ask him (or her).

For the technically challenged, bribe a friend to build the circuit. Maybe bake a cake. A steak dinner might work, too.

Step 7: Other Uses

For the aquarist, fill the dispenser with dry fish food and hang it over the aquarium. Use a momentary switch to supply power directly to the motor. When the switch is pressed, the feed screw turns, and your fish will be fed. Happy fishes.

Using this dispenser as a fish feeder may seem just a tad self-indulgent, but...what the heck. Maybe I'm just that kinda guy.

Let's move on, shall we?

Make a half dozen of them and line your kitchen wall with spice dispensers. Shhh, let's watch the Master Chef at work...
"Hmmm, let's see. Push the button for one second to get one teaspoon of parsley. Oh, this is wonderful! Thank you, wotboa. I'm going to bake you a nice cake!"

Make a giant one to feed your cat, dog, or pesky children. You will need to find a larger feed screw.

Make three for dispensing instant coffee, sugar, and creamer. That would look so cool hanging by the breakfast bar.

Make a powdered hand cleaner dispenser for the garage. No more greasy hand prints all over the box of soap.

Fill it with houseplant food pellets and run it on batteries. You can wander around the house giving your plants random doses of food or fertilizer.
What a great way to impress the in-laws! (or convince them that you really are a loonie)

Feel free to add to my list of 'Other Uses'. 

Build and enjoy your new fish feeder spice rack coffee dispenser plant feeder grit dispenser.