Monday, February 15, 2016

Putting it together

So after a little more than two weeks of work the Model 600 is up and running!  Here is how it looks in it's new home, hooked up to the bolting cabinet.

Gluing the top onto the tun (casing).  The staves actually did not stay true and the entire housing is kind of squashed, although you can't see it without a measuring tape.  I made the tun to be one-piece, meaning that the upper millstone has to slide out the bottom.  I managed a nice fit but perhaps a little too tight.  I'm not sure if the stone would come out in the middle of summer when the wood is swollen with moisture.  The upper millstone has 3 large thread inserts around the circumference, and at least 6 smaller ones on the top, in no particular pattern.  I am using bolts in all of them.

So here is what it looks like without the tun.  The game is that the upper millstone has to be perfectly supported in space so that the lower millstone can be raised up to just barely touch it, in perfect alignment.

This hopper is a bit small but I had already made it for another project so it will do for the time being.  The shoe is so-so and when I get around to building a larger hopper I will try to make a better shoe.

Belt slippage became an issue on the second run of the mill.  The runner stone would start slowing down if the grain was fed too fast.  I built the mill with two-belt pulleys because my free two hp motor came with one already on the shaft, and it seemed about right.  But running with two A-sized belts I am getting slippage.  I looked online and determined that in this circumstance a single belt can only be expected to transfer about .7 hp.  Keep in mind the rpm coming out of the gearbox is 180 rpm.  The driver pulley has an A pitch diameter of 4.4 and a B pitch diameter of 4.8.  The pulley on the spindle is a 5/5.4.  I might do a little better with B belts, but I think a more drastic solution is required.  So for now I just tightened the belt as much as I dared.

I don't want to tighten the belt too much because it starts messing with the stone alignment and pulling on the upper bearing, which needs to allow the spindle to slide through when the runner stone is raised and lowered.  So my long-term solution was to order some three-belts pulleys.  This was easier said than done because sheaves of this type can get expensive.  The ones I finally found were only about $40 with the bushings, but then belt size became a problem.  The new sheaves take a 5V belt, and those don't usually come in sizes less than 50" (my ideal is a 41" belt in this space).  Anyway, I finally found some 45" 5V belts online and I think there is just enough room underneath to slide the power unit over and tighten the belts.  

There is a lot of hemming and hawing in this area.  Large sheaves can get expensive--several hundred dollars is quite common.  The next mill I build will probably not have a gearbox, so reduction will have to be by belt.  The European mills using a 600 mm stone run the stones at 480 rpm, which would take a more than 3:1 reduction with the belt drive.  Furthermore, to get enough torque at that rpm requires a serious motor, in the 10-15 hp range.  5 hp is about the max you can expect to run off of single phase 220 v electricity, so things start to get messy and expensive.  I wanted to stay on single phase 220, and the 2 hp motor I source was really nice because it only draw 13 amps at full load.  I can use No 12 wire and a 20 amp light switch to control it, but any bigger and I will need a special motor start switch.  I wanted to keep this project sane and simple.  

I'm really glad I took the time to weld up a good adjustable bearing base for the lower bearing.  I can really tune up the alignment quickly and accurately and get the stones singing, like they should.

Note the 3/8" rods clamping the tunning down to the table.  The lower stone will be pushing up into the fixed stone, and that stone needs to be able to counteract that force.  This is where this style of mill departs from most stone mills built prior to the 20th Century, where it was normal for the top stone to turn and the bottom to remain stationary.  In the old style, the pressure is limited to the weight of the runner stone, whereas in this style the pressure is created by the bridge tree pushing the runner stone up to the bedstone. 

I never did a budget but I think this cost me around $1500 to build.  The millstones were $1000 shipped to the airport, the gearbox $85, and I spent $75 at the machine shop.  The motor was free and a lot of the wood was also free.  There were bearings and a shaft to purchase, and two bushings that I used to make fittings with.

Saturday, February 6, 2016

Model 600

I have been anxious to build a larger mill ever since the first one got going.  After finding an acceptable source for stones in Denmark, I wired out the money and did not have long to wait.  It turned out that air freight was the cheapest so in a week or so the millstones were waiting for me in a bonded warehouse.  I made purchase terms CPT (cost paid to) Mitchell Field, again to save money, and I also did the customs clearance myself.  It was pretty easy.  The hardest part was the stones themselves--shipping weight was 360 lbs!  I rolled the fixed stone into the house, this one is a little larger than the runner stone and I think it weighs around 200 lbs.  The working area is 600 mm in diameter, and overall it is a bit bigger (27" across the top).

Here is the frame going together.  I used 4 x 4 Douglas Fir legs, 2 x 8 stringers, and some high-quality plywood for the top.  The bridge tree is a solid piece of white oak, and the jack is 3/8" rod that I threaded to 16 tph.  The construction is a little weird since I want to be able to disassemble it next year and get it up the stairs, which are quite narrow.

Here you can see the thrust bearing and the adjustable housing I made for it.  The runner stone is basically sitting on this bearing, and the bridge tree can raise and lower the spindle shaft.  The bearing housing is adjustable so that I can "tram the spindle," ie, align the runner stone to the fixed stone.  The lower bearing is an ordinary farm bearing.  I could not find the thrust specs for it so I will just wait to see how it does.  The shaft is 1 1/2".  The 9.6:1 gearbox is a lucky find from craigslist, otherwise I was looking at a double reduction drive to get the speed I want.  This will drive the mill at around 160 rpm I think.

Here is the spindle coming through the mill table.  The tapered bushing was custom made for me at a local machine shop.  I welded up the other bushing to fit into a keyway in the runner stone and drive the stone. 

The spindle has two flats on it so it can tap on the damsel while running and keep the grain flowing smoothly.

Another shot of the lower thrust bearing.  Bearing and pillow block came from fleet farm, and I had to trim the corners of the pillow block to fit my adjustable housing.

Lowering the runner stone into place for the first time.

The runner stone wobbled on the tapered bushing, which was a surprise.  It thought it would lock on perfectly.  I ended up slotting the bushing so that the taper would lock it onto the shaft better, and I made this additional bushing to drive the millstone into the tapered bushing.  I also had to pick the stone up a couple of times and put some brass shim stock on the side of the taper in order to get the stone square to the spindle shaft.  The black bushing is a weld-type bushing that comes from Farm and Fleet.

Lowering the fixed stone into place.  These type of millstones are designed to have the lower stone moving, not the upper stone.  Having tried it both ways on the mini-mill, I really like the lower stone being the runner stone.  The eye never clogs up and a single grain kernel feeds instantly into the stones for grinding.

The upper stone needs to be suspended on something so I settled on making the housing out of staves.  I had to cut up at least 40 to get all the way around, using up a lot of nice wood in the workshop.

It was kind of exciting getting this thing glued up in a timely fashion.  The masking tape works wonders for gluing up staves.  Tape the outside, flip it so the inside seems open up, then brush the glue into the joints.  When the staves are put in place the tape does an amazing job of clamping everything together.  Cross fingers it cures up and makes a strong housing!