Wednesday, January 15, 2020

Grain Mill Part 2 - Rollers

My selection of roller size was decided by the fact I had two identical scrap pieces of 3" diameter steel sitting around.  I thought it would finally be a good use for them.  What I failed to consider was that a larger roller would result in more force required to mill grain.  This is due to two factors:  The shorter lever arm between the end of the handle and milling surface, and the larger contact area from the increased radius of curvature.  This I wouldn't realize until the testing phase.
Design wise, one roller is fixed, and the other is movable to allow variable spacing for different sizes of grain, nuts, and so forth.  The fixed roller would require a longer shaft for attaching the driving handle.  For this reason, I decided to make the fixed roller in two pieces, while the movable roller has the shaft built in.  The fixed roller is secured to the bearings with set screw collars, while the movable roller is held by two 3/8"-16 bolts through either end.  This was another design flaw that I wouldn't realize until later when I found out the bearings for the movable roller weren't actual ball bearings and would seize up when tightened down.
We begin with the movable roller.  First turning one end, and roughing the diameter for the bearing.
Then tapping the 3/8" hole in the end.
Then, the part was flipped around, rough turned and tapped.  Here the bearing diameter was finished to size.
Test fitting the bearing.
After that, the part was flipped around again, this time chucking on the bearing diameter that was just machined.  Now the roller diameter and other bearing diameter are finish turned in the same setup to ensure the best concentricity.
Now for the fixed roller, rough turning the first end.
Then flipping it around, roughing the other end, and rough drilling the center hole.
Next, the center hole was bored to an accurate dimension ~.740".
I then began turning the shaft for the fixed roller from some scrap 3/4" round stock.  First, finish turning the bearing diameter for the end where the handle will be attached.
Then the part was flipped around, and chucking on the bearing diameter, the outer diameter was machined to match the bore of the roller.  Upon taking a test cut, it was apparent the tailstock wasn't aligned with the spindle, and needed to be adjusted in order for the shaft to be straight.
Here, adjusting the tailstock alignment with an Allen wrench.
After the tailstock was corrected, the diameter was finish machined, as well as the bearing diameter.
Checking that the rollers fit within the housing.  I made sure there was at least .030" clearance on either side.
For attaching the fixed roller to its shaft, I didn't want to drill set screw holes through the roller face fearing they'd get grain stuck in them.  I didn't want to weld it either.  What I decided on was diagonal set screws through the ends of the roller.  It's not something I wanted to resort to, but it seemed like my only option.  Using this diagram that looks like a problem from a trigonometry textbook, I calculated the location and angle that wouldn't ruin anything.
I started by setting up the roller in the mill vise with a 45° triangle, and plunge cutting the location with a 1/4" end mill.
I then center drilled, drilled, and tapped it 1/4"-20.
Here the set screw holes are finished with the shaft secured.
I then noted where the set screw contacted with the shaft.
And milled diagonal set screw flats at the corresponding locations.  These make it possible to adjust the centering of the roller on the shaft if needed, and make it more secure.
After test fitting, it appeared I needed to mill some more.
Then I milled flats for the set screw collars on either end of the shaft.
Now that the roller was secured to the shaft, I could finally finish turn and knurl both rollers.
Knurling the roller.
At first, I was having problems with the knurl since only one of the knurling wheels was contacting my piece.  I found the solution was to loosen the tool post so the knurling tool could float up and down as needed.  After that, the knurls were perfect!
Looking good!
One more final step, wire brushing all the little bits of metal that the knurling left behind.  Don't want any metal bits in my rye!

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