Hand Cranked Grinder: Reassembly and Use

To begin reassembling the grinder I needed to replace the screws that held the large gear in place.  First, I need to know what size screw it is.  I used a thread pitch gauge (see pic) to determine the how many threads per inch (TPI) there are which I can use to find the correct screw.  In the SAE world, there are two primary screw thread  groups: National Coarse and National Fine.  The gauge showed 18 TPI which tell me it’s a NC 5/16″x18 screw.  You could also take the unknown screw to a hardware or home improvement store where they usually have a board with multiple male and female fasteners you can test it in.  With the size of the screw known I was able to goto the store and pickup a set screw (bottom in pic) to use.

Reassembly is the opposite of disassembly so I’ll skip the step by step.  I used wheel bearing grease on the gears and motor oil on the axles.  It occurred to me that the threaded holes on top of the axles are there so that the axles can be oiled periodically.  I also determined that it has a 1:12 gear ratio.  So, for every one complete turn of the handle the wheel turns 12 complete revolutions.

Here are a couple of pictures of it back in one piece.  It’s mounted on a scrap piece of wood clamped to my bench.  I picked up a cheap wheel to test it out with.

When I received the grinder I was missing two screws that held the cover on and one that allows you to oil the crank axle.  I wanted to replace these screws with some slotted ones so that they’d match the others.  Apparently thats not the easiest thing to do.  I stopped by ACE and they didn’t have them in slotted heads, only Phillips.  Since ACE didn’t have them, I knew Lowe’s wouldn’t so I had to take a trip to Pensacola Hardware.  Pensacola hardware is a real hardware store.  They carry just about everything you’d want from a hardware store and have helpful people working there who can find stuff when you ask!  Crazy I know.  They had the slotted, round head, 1/4″x20 screws but in 3/8″ length.  I need 1/4″ length so I used the hacksaw to remove a little bit of the threads.  With that done, I now wanted to make them match the rest of the grinder.  To so this, I used a technique called “Oil Blackening.”   The idea behind it is pretty simple.  You oil the metal and then heat it.  As you do this a coating will develop on the part that darkens depending on how long you’re heating and oiling it.

The first step is to remove the zinc plating from the outside of the screw head.  I used some 150 grit sandpaper to do this.  Next, the surface needs to be cleaned using paint thinner or some other cleaner.  It may seem a odd to clean it since I’m just going to put oil on it but it helps create an even finish.

After a liberal coat of oil (Boiled Linseed Oil in this case) heat is applied with a torch.

Once the head of the screw gets hot enough the color will start to change to a brownish color.

The color can be pushed darker by adding more oil and heat.

Eventually the surface takes on a dark metallic black color.  It looks darker in person than the picture makes it look like.

After doing this a few more times I have three screws ready to go.

The next issue to address is making a cover for the gears.  I made a piece up using some Aluminum flashing that I was going to use as a template.  After putting it on though, I thought it looked pretty good so I decided to stick with it.

Here is another view with two of the new screws pointed out with arrows.

Here is a short video of the grinder in use on a piece of scrap that I captured with my phone.

It works pretty well but you can stall it and yourself out if you press to hard.  The wheel axle has a small amount of wobble in it but I don’t think it’ll cause a problem as I will mainly use it to establish a rough edge on chisels and plane blades before taking them to the sharpening stones.  I’ll also used it to sharpen lawnmower blades and other things like where a perfect edge isn’t required.  Overall, I think it turned out pretty well!

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