I've been working on a balance beam scale and powder dispenser. I use a arduino board from dfrobot that has PWM outputs for two DC motors. These motors control the speed of two brass tubes, one large and one small, to dispense the gun powder into a balance beam style scale. I use two IR sensors and a push button to operate the motors. If both IR sensors are blocked, then the scale is down and is ready to be filled. Once the button is pressed, tt will fast fill based on a "learned" time. For the first few refills it will overshoot and learn what amount of time it needs not to overshoot. Both motors run during this time. I have two pots that I can adjust the speeds. Once the time expires or the first switch is seen, then it will stop the large tube motor, slow down the small tube motor and trickle in the remaining amount.
I didn't come up with the entirely on my own, another guy on youtube does this with just a few transistors and two IR sensors. If you get the correct scale that has a "approach to weight" feature, or you modify you own scales and add this feature then you can just hard wire the system as long as you get motors that have the correct speed. Basically you need the Ohaus 1010 scale or the RCBS version of the same scale. With the "learn/time" method you can use any old balance beam scale although it will occasionally overshoot.
What I would like to do next, is add a linear encoder to the scale. The two sensors do work very well, however, occasionally it will undershoot. It undershoots because of the free flight of the powder. Normally the scale is going to go past the IR sensor just because the powder that has already left the small tube is already committed to going into the pan even though the motor has stopped. Sometimes the amount of free flight material can be vary and be less. Currently I press the button to top off the remaining amount. Once the scales are empty and both IR sensors a blocked again it knows that it is ready to fill again.
So I could add a linear encoder strip and glue it to the beam and add some ballast to the pan to offset the added weight. The problem is most of the linear encoder sensors need a very close gap. A balance beam must be free to travel without anything touching it but the knife bearings. Normally at least a 1/4 inch or more gap would be needed to avoid touching the beam. to be within 1/10 of a grain of weight, this would be about 1mm increments on the scale gauge, so the encoder would need to be at least better than 1mm accuracy. My guess is I will need to make my encoder using two IR sensors, but at that level of gap I'm not sure it can be done. Anyone have any other thoughts?
I'm doing this more as a hobby, I realize that RCBS and PACT both make good units, digital scales are slowly getting better although they still need tared and checked throughout the reloading process. The old balance beam doesn't screw up as often and a good digital scale can be costly.
The small tube is a 1/4" brass thickwall and about 4 1/2" in length. The large tube is 5/16" brass thinwall and the same length. To make the powder hopper, go to Lowes and get a 1.5" PVC coupling, they make a short version of this coupling which is the one needed. Get about a 8" piece of 1.5" PVC pipe. Also get two 1.5" pressure test caps (these are white flat discs used to pressure test pipe). This is all fairly inexpensive stuff.
Get a piece of wood or ABS/PVC sheet about 1/2" thick 3.5" x 8". Cut about 1.5" piece off the 8" piece of pipe. Put the test cap on this 1.5" piece then place the coupling on top of that trapping the test cap. Then put the remaining piece of PVC pipe on the other side of the coupling. The 2nd test cap is the lid/dust cover for the top.
Next the the hard part, you really need a drill press and I ended up trying 3 times to get it right even with a drill press. You have to drill two holes for the tubes just above the test cap on the coupling. You will need a small round file to get the holes to except the brass tubes. Next use a 1.5" hole saw on the wood, ABS or PVC sheet, use a round file to open the hole up so 1.5" pipe sticking out the bottom of the coupling can be inserted, this will be the base for the unit. Servo city has some small motors with 3mm shafts, they make a black plastic case for the motors also. I used a 90 RPM motor for the small tube and a 270 RPM for the large tube. Servo city also has a 1/4" to 1/4" ridged coupling and a 5/16" to 1/4" ridged coupling. Also get two 1/4" D shaft about 1" long. On Ebay I found 3mm to 1/4" flex couplings. I came up with some brackets to secure the motors. The last thing is to use the edge of a flat file to cut slots in the brass tubes, this is where the powder will enter the tubes. Assemble all the gear and you got yourself a powder dispenser.
The guy that made his on youtube is better than my setup, but you need a lathe to make the one he has. To see his search for "Auto Charge Powder dispenser Balance beam based." on youtube. His does not use an Arduino, just a few simple transistors for latching the circuit. I started out trying the old way of soldering up circuits, however, the Arduino offers so much more flexibility for experimenting. You have to use a Ohaus or RCBS 1010 scale otherwise it will not work.
Another thing to look at is the PACT powder dispenser. Find some reviews on it that show some of the detail of the unit. Basically your trying to make the same thing.
You do not need any screw to drive the powder. The turning of the tube will do this, this is how most commercial manual powder tricklers work. One more important thing is the angle to tilt the entire assemble, this will speed up the system. Most of my charges can take under 20 seconds to dispense.