How to choose hole sizes when building a sorting sieve?

Question

I'm planning to build a sorting sieve out of a set of plastic containers with holes drilled in the bottom. The container with the largest holes will be on top and the one with the smallest holes will be on the bottom. This will allow items to filter down until they can't fall through the holes anymore. It should effectively sort items by size when I pour the items in the top and shake the entire stack.

Since I haven't found a reasonably priced set of sorting sieves, I'm planning to build my own. I'm planning to use Snapware containers that lock together and drill the holes with spade bits.

In general how should I choose the hole size? Obviously I want it to be between the sizes of objects I'm sorting. I fear:

• That if the holes are almost as large as the large objects, the larger objects will get stuck in the holes and block them
• If the holes are too close to the size of the smaller objects, it will take too much shaking for them to fall through.

I think my choices are:

1. Split the difference and use a hole size as close to halfway inbetween the object sizes
2. Use a hole size closer to the size of the larger object, but small enough that they won't get stuck in the hole. How much smaller (percentage wise) would that need to be?

I have several application of this in mind. I want to try sorting screws, bolts, washers, Lego bricks, and US coins. Let me illustrate with the sizes of US coins. Here are the coin sizes and possible drill bit sizes listed inbetween:

• 0.955" Quarter
  • 24mm (0.945")
  • 15/16" (0.938")
  • 23mm (0.906")
  • 7/8" (0.875")
  • 22mm (0.866")
• 0.835" Nickel
  • 22mm (0.866")
  • 21mm (0.827")
  • 13/16" (0.813")
  • 20mm (0.717")
• 0.750" Penny
  • 18mm (0.748")
  • 17mm (0.709")
• 0.705" Dime

I'm leaning towards 15/16" for sorting nickels from quarters and for 13/16" for sorting pennies from nickels. However between pennies and dimes I have a lot less choice. Is the 18mm too large a hole and the pennies would get stuck? Is the 17mm too small a hole and the dimes wouldn't fall through easily? They also sell 18.5mm drill bits, but they are comparatively expensive items compared to less specialty sizes.

Answer 1

I have no idea how to choose your hole sizes, it sounds like you have thought it through, and I am sure some trial and error will be involved. I can only suggest tools to make the holes.

In the smaller sizes, drill bits graduated in 1/32" or even 1/64" increments are readily available. No need for super quality drill bits if you're just drilling plastic.

For larger sizes, there are adjustable drill bits

These have been around a long long time, and they work very well. Just look carefully when you're shopping for the right shank. A lot will have the shank made for old fashioned bit and brace, which won't work well in a regular modern three jaw chuck.

There are step bits that will drill holes of many sizes, you just have to pay close attention to stop at the right step:

There are also bits I have seen sold as "stepless bits" which will cut holes of any size, but to drill many holes of the same size quickly you'd need a drill press set to stop at the same depth on each hole. They also make a slightly tapered hole, which you may find is a problem, or who knows, may turn out tobe an advantage. It might be better drilling from the bottom rather than the top with stepless bits.