Pedal Powered Apple Grinder: More Details


[A guy from Downeast somewhere near Penobscot Bay wrote and asked for more detail on how to build the grinder. The following is text of my email in response. If I get ambitious I’ll write a real article someday.]

The quickest way to orient yourself to how the grinder is arranged would be to go into a wood shop and look at the spinning part of a jointer – it’s a lot like that. Here’s a description of how we built it. Other ways of accomplishing the same effect would be fine as well. The goal is to have a round drum with four steel cutters embedded in it, protruding slightly such that each blade takes about 1/16″ off of the apples.

To make the shaft, we started with a piece of hexagonal stainless steel bar stock 3/4″ on the flats, purchased from McMaster-Carr (an online industrial supply company). We cut it to length (about a foot), and turned each end down to just under 5/8″ on a metal-cutting lathe. We also faced the ends of the shaft and drilled them with a spot drill for a live center. (If you don’t have access to a metal lathe, you can go to a machine shop or try calling a good metal fab shop. Turning a hex bar down to round is about the simplest operation on a lathe). When we were done, the bar was hexagonal for about 5.5 inches in the center and round on both ends.

The next step was to create a wooden cylinder bonded to the hexagonal shaft. We then got some 1×4 hard maple boards and cut pieces the length of the hexagonal portion. On two of these, we used a tablesaw to make trapezoidal relief cuts, such that when the wooden pieces were stacked they fit snugly around the hexagonal shaft. We then bonded and clamped the whole stack together with gorilla glue, and bonded the shaft in at the same time.

When the glue was dry, we had a square wooden block 3.5″ on a side, with the shaft embedded in the center. We then roughed it to octagonal on the tablesaw, and turned it to round on a wood lathe (we cut a cross pattern in one end with a hacksaw to accept the usual spur center on the drive end, and used a live center in the tailstock).

The trickiest part of the whole operation was to cut four flats on the cylindrical drum to accept the blades. We screwed a square block on the end of the wooden cylinder (with a hole cut in it for the shaft) and used the tablesaw to cut the flats, but it was a bit dodgy, and it would have been a lot safer and more comfortable had we taken the time to make a jig. The flats were not quite even, and the chips of apple were a bit too thick to begin with, so we adjusted them manually with a rabbet plane until they were more even and the chips were thinner. When the drum was complete, we put it in the oven at about 250F in a shallow pan of paraffin, and let the wax soak in to protect it and make it shed apple pulp better.

We then made four stainless cutters. It would have been quicker to use ground stock but we weren’t thinking things through clearly when we bought the stock, so we got hot-rolled 304 stainless strap stock, 1/8″ by 1″. We cut four pieces the length of the cylindrical portion of the drum, and ground, filed, and honed them to a 75 degree angle (this is much blunter than a chisel or knife, but it cuts apples fine and tends to munge them up which makes the juice come easier). It would be much easier to make the blades on a milling machine, but we didn’t have one handy. As it was, we got the angle by clamping all four blanks together in the vice offset such that they formed a 75 degree angle, and using a combination of angle grinder, bastard file, and diamond hone to create a flat surface. Again, it would have been much smarter just to buy ground stainless stock such as – for all I know it would work fine at a 90 degree angle, and it would be much easier to machine to the 75 degree angle because the edges would be crisp instead of rounded and warped, which is how the hot-rolled stuff comes.

To finish the blades, we drilled and countersunk four holes for #10 stainless wood screws at intervals along the length of each one. We then placed them on the drum and spotted down the holes, then piloted the drum and drove the screws home.

Fabricating the drum is the core of the grinder project; the rest is fairly simple. We made a plywood housing/hopper to enclose the grinder, and bolted four-hole flange bearings to carry the shaft. There’s a throat plate made of plywood with a stainless steel facing that is mounted adjacent to the spinning drum to create a pinch point, and it’s adjustable so the gap can be reduced to the minimum possible dimension – this is important because if the gap is too big, large chunks of apple will pass through it and the juice yield will be low. We made a hopper out of plywood to dump the apples in, and a frame to hold the assembly above a large bucket. We used human pedal power to run the grinder. This was accomplished by bolting a bicycle to the grinder assembly via the front fork, and creating a long loop of chain running forward from the large chainring of the bike to a thread-on 7-speed freewheel on the grinder assembly. The freewheel drove a large surplus #40 sprocket on a jackshaft, and the large sprocket drove a smaller sprocket via a short loop of chain. With two stages of speed increase, we achieved up to 11:1 speedup relative to the rate of pedaling in top gear, such that the drum could be made to go about 1000 RPM. The shafts are supported by 5/8″ flange-mounted sealed ball bearings, so the friction was quite low. We also mounted a large surplus flywheel on the shaft to even out the load – this made the pedaling experience much more pleasant.

This last weekend we had a bunch of friends over and cranked out about 80 gallons of cider in about 1.5 days of work. It worked great, and the pedaling is not that strenuous. Our yield was about 65%, which is not quite as good as with the garbage disposal but respectable enough. The most annoying thing is that it doesn’t self-feed – from what I understand this is a common issue with fruit grinding equipment. We didn’t mind too much though, we just had someone with a wooden push stick shove the apples into the spinning wheel one or two at a time. With an energetic cyclist going strong, it can obliterate 1-2 apples per second – lots faster than the garbage disposal.

Happy Cidering!



3 Responses to “Pedal Powered Apple Grinder: More Details”

  1. T. Dean Says:

    Greetings Ben,

    I’m writing a book on human-powered devices, due to be published by New Society Publishers in November. If possible, I’d like to include a description and photo of your pedal-powered apple grinder in the book. Please contact me if you’re interested in contributing.

    Many thanks.

  2. KJMClark Says:

    Hey, where’d you get the flywheel? I’m trying to put together a human-powered tractor but I want to add one or two flywheels. Thanks for any ideas!

  3. Rowan Says:

    Do you have plans for the pedeled equipment. Trying to set up building workshops for homesteading and small farms in my area, mainly new people . this would be a good workshop to do. thank you .

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