I have engineered a knife sharpening hone, made from a cast iron brake disc, a trailer hub and small electric motor. It uses 3 micron diamond paste. This hone is able to take a blade from 120 grit stage to the 3 micron grit stage (4000 grit) in about 30 seconds. Only a microbevel is used. In other words, only a region very close to the edge is ground. But it is very effective and VERY repeatable. The knife goes on the hone at the exact same angle every time.

After grinding on the 3 micron paste, I transfer to 0.5 micron diamond paste on Mylar film, on a glass plate. This stropping requires little work, maybe 10 to 20 strokes. The edge is as sharp as you can get. It will cut through 40wt. rayon thread with 50 grams attached.

In my work on this hone, I found that my local auto parts junkyard did not know how to locate a 5 hole brake disc with a hole spacing diameter of 4.5 inches (114.3mm). This is called a 5x114.3 in the trade (apparently). After MUCH searching on the Internet, I found this site which helps you locate the name and year of the car from which the brake rotor can be extracted. Also, tire shops accumulate brake discs. They should be able to get one for you, given enough time. They should charge you less than $10.

I began by finding a trailer hub at agrisupply.com for about $20, and the approx. 8 inch long axle to go with it, about $8. If you don't have an agrisupply near, search for "trailer parts" on the Web. The bearings come with the hub, but NOT the shaft. Buy the shaft from them too. I mounted the axle vertically in a plywood box, and found a 5x114.3 brake disc to match the hub. Note 114.3 mm = 4.5 inches. You can ask a brake shop to re-surface the disc. They charge anywhere from $8 to the rip-off $60. Also, they have different standards for smoothness and flatness. I am in a wonderful small town, Laurinburg, NC, so working with these folks is a pleasure. Now grease the bearings, mount the disc and bolt it on. Now for the edge treatment of the brake disc, I filled in the air vent holes with, first paper, then body filler. Then sanded the edge smooth. Next I added a rubber sheet over the body filler to provide friction for the motor shaft and to provide some "give."

I found a 1/5 hp motor @ 1050 rpm. I placed a short piece of special black and red rubber hose from Lowe's on the 1/2 inch shaft. The motor is mounted so that this rubber contacts the edge of the cast iron brake rotor. The speed of the rotor is now about 80 rpm, which seems to work pretty well.

Make sure the brake disc surface is smooth with fine SiC paper and apply the 3 micron diamond paste to the iron surface, rubbing in with a piece of hardwood, or even metal.

I will add pics later, hopefully showing how I hold the angle of the blade, plus my very own invention regarding this angle holding jig. You want to only sharpen the very edge, not the whole bevel. I am talking about 1 mm of the edge. If you hold the angle, 15 seconds per side is enough. Well, ok, maybe 20 seconds.

I would appreciate hearing from those of you who have ideas about improving this hone.

Knife Jig

This jig is 73 mm x 43 mm x 8 mm. Made of maple. The screws are nylon. The most important thing to do is to cut the width of the slot to the right size to get a snug fit for the knife. The screw end provides the third leg of the triangle, the other two being the point of the knife and the heel of the blade. To adjust the screws, place the blade edge on a flat surface and adjust the screws so that the very end of the knife is 20mm above the flat surface. This gives a 15 degree elevaton. 15 +15 = 30 degrees total edge bevel for a 6 inch knife.

jig1 jig2 jig4 jig5 This jig is 73 mm x 43 mm x 8 mm. Made of maple. The screws are nylon. The most important thing to do is to cut the width of the slot to the right size to get a snug fit for the knife. The screw end provides the third leg of the triangle, the other two being the point of the knife and the heel of the blade. To adjust the screws, place the blade edge on a flat surface and adjust the screws so that the very end of the knife is 20mm above the flat surface. This gives a 15 degree elevaton. 15 +15 = 30 degrees total edge bevel for a 6 inch knife.

Hone 3micron diamond paste-- left side

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3 micron diamond paste, right side (below)

3micright

0.5 micron diamond paste left (below)

A glass plate is now placed in the center. It has Chrome oxide mylar attached with baby oil. Then 0.5 micron diamond paste is rubbed into it.

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Now 0.5 micron diamond paste on the right side.

05micright

Overview (below)

An oak bar holds the pins to which the end of the knife is attached.

overview

Carl Stross' instructions for preparing a lapping plate: "I have a lapping machine I use from time to time to lap gages - it looks almost exactly like the sharpening machine on this thread , just bigger. This is how I prepare the lapping disk : I saw a disk of DURABAR cast iron some 300mm dia and face it on the lathe with a straight cutter in 2 passes. Cast iron must be absolutely dry when turned - even a fingerprint will cause a measurable bump. A brake disk is perfectly fine as well if reasonably free of carbide inclusions. I spread on the disk a THIN layer of diamond or Borazon paste with my finger ( very thin ! ), 1-2um particle size in my case. One syringe should last a lifetime. Must be an OIL based paste. I use a drop or two of mineral turps to spread it easier. Then I use a small roller bearing with a handle to press HARD the diamond particles INTO the lap disk. I wash the disk with thinners to remove all loose diamond particles then put some kerosene on the disk and rub it with a flat piece of hard steel. Some areas of the lap disk will become matt and some shinny. The shinny ones need more charging with diamond paste. Once the entire surface of the disk is matt and washed of all free particles, the lap is ready. Such a lap lasts a very long time ( years ? ). Diamond is to hard tool steel what steel is to butter. I use either lapping oil for superfinishing ( it's a special oil ) or kerosene. Light machine oils won't do. Kerosene cuts fast and uniform. I use diamond paste for carbide gages and Borazon for steel. Steel dissolves diamond. From time to time I lap a small carbide ring ( conditioning ring ) to keep the surface of the lapping disk dead straight."