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| 3M Abrasives |
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| Finest abrasives. | ||
| Microbevels front and back. | ||
| Use a jig. | ||
| Copyright (c) 2002-08, Brent Beach |
Where am I
This page discusses the abrasives I use for sharpening hand tools (plane blades, chisels, ...). There are much more detailed pages on honing and grinding using either a 1" belt sander or a bench stone.Why Microfinishing?
There are lots manufacturers of sheet abrasives out there. I have tried many different makes and grades and have found that 3M Microfinishing Film is by far the best. This particular group of abrasives is not commonly found in Hardware stores, or even Autobody stores.Microfinishing abrasives are much different from standard abrasives used in woodworking. According to 3M advertising literature the abrasive used is much more finely graded (fewer large pieces of grit to cut deep scratches into your tools), with a bonding to the backing material strong enough to use for honing tool steels.
You can spend a little less if you buy regular sheets rather than the PSA (pressure sensitive adhesive) backed papers. The problems attaching it to the glass eat up any saving quickly. Get the PSA backed papers.
I have spent some time looking for suppliers. Googling 3M Microfinishing Film produces lots of references within the 3M company sites, but the information there appears to be out of date. For example, the listing for 3M Product ID 468L(PSA), lists 30 and 40 micron grits. I have 15 and 40 micron grits with this designation. Dealing directly with 3M will require persistence - they may not know what they sell. It comes in rolls, sheets, discs and disc rolls. If buying in bulk, the rolls might be an economical alternative. Getting someone to give you a price on this may be difficult. Please let me know if you have any luck on this.
Where to get it?
I buy these abrasives at Tools for Working Wood (search for 3M PSA) which has good prices and a bit more variety than other mail order companies.In Canada, Lee Valley has a somewhat smaller selection at somewhat higher prices.
If any reader of this page outside North America finds finds a distributor local to them, I would certainly include a link on this page.
How do I apply the PSA stuff to glass?
The adhesive on the back of the 3M abrasives is very sticky - it grabs and holds. This makes it hard to get a piece smoothly adhered to a glass plate. The secret to applying the paper is a water spritzer. Just spritz the surface and slide the adhesive into place. I have put a video - well three actually - onto youtube showing me applying a half sheet of 15 micron 3M microfinishing abrasive to a sheet of glass.My Grit Selection
My sharpening system has four steps:| 1. | Primary bevel | For many years I used 120 Grit Aluminum Oxide (AlO) belt on a 1" belt sander at 25 degrees. AlO is a moderately hard abrasive that will remove even very hard tool steels quickly. I try to grind the primary up to but not quite touching the edge.
Recently I switched to using a coarse silicon carbide bench stone. |
| 2. | First microbevel | 15 micron Silicon Carbide (SiC) sheet abrasive on a glass sheet at 29 degrees. I use Pressure Sensitive Adhesive backed paper.
15 micron paper cuts quickly provided the microbevel is not too wide. Up to about 1/16" is no problem, by 1/8" it starts taking quite a while. At this point I refresh the primary bevel. Silicon Carbide is a very hard abrasive - hard enough to work even the hardest tool steels. |
| 3. | Second microbevel | 5 micron Silicon Carbide (SiC) sheet abrasive on sheet glass at about 31 degrees. Since the jig system ensures I start sharpening at the edge, this bevel is always about the same width and always takes about the same time. This abrasive sheet lasts much longer than the 15 micron sheet since it has so little metal to remove. |
| 4. | Third microbevel | 0.5 micron Chromium Oxide (CrO) sheet abrasive on sheet glass at about 32 degrees. Again, the jig system ensures I start at the edge, so the bevel is also always about the same width and takes about the same time. This sheet lasts longest of all - many cycles of the 15 micron paper for a single piece of 0.5 micron paper.
Chromium Oxide is also a very hard abrasive. It has no problems with even High Speed Steel, cutting quickly and wearing well. |
Newer Grits
The dark green 0.5 micron Chromium Oxide based PSA sheets are no longer generally available.
| 4. | Third microbevel | 1 micron Chromium Oxide (CrO) sheet abrasive on sheet glass at about 32 degrees.
The abrasive is a very pale green colour rather than a deep forest green. Like the original 0.5 micron paper, it feels smooth to the touch. It appears to cut faster than the 0.5 micron paper, but the scratches it leaves are also not visible using the QX3 microscope. It is probably a good alternative, but I have not yet done any edge durability testing with this abrasive. |
| 4. | Third microbevel | 0.3 micron Aluminum Oxide (AlO) sheet abrasive on sheet glass at about 32 degrees.
The abrasive is pure white - it is very hard to tell the abrasive sheet from a piece of paper. The grit is so fine that roughness is no guide. On initial testing, scratches left by this abrasive were move visible that those left by either the 1 micron or the 0.5 micron Chromium Oxide abrasives. The problem is that there are a few scratches visible - not general visible scratching. It could be that sizing is not as good on this abrasive. Perhaps after some use these larger scratches will stop appearing. |
Grinding the Primary
First, you have to have a primary bevel.While most of the emphasis in these web pages is on honed microbevels, you cannot even start honing until you have a primary bevel and a reasonably good edge. In fact, if you grind the primary poorly, you probably cannot hone a good final edge.
In my view, the best way to grind a primary bevel is by hand, using a coarse Silicon Carbide bench stone.
Second best is a 1 inch belt sander.
In either case, I grind almost to but do not actually grind the edge. That is, I leave the edge previously obtained by honing. (Except when I get a new blade with bad chips, or want to change the shape of the edge.) Pictures of primary bevels taken during belt sander grinding are on this page. Images of edges obtained using the Norton SiC bench stone are on this page. Be sure to double click on the images. It is possible to get so close to the edge while hand grinding, you might not be able to see the remnant of the old edge in the thumbnails.
Grinding does not shorten the blade! Use and honing shorten the blade. Both operations remove metal on the order of a few thousands of an inch. A grinder can remove 10 times that in a split second.
I have tried two types of sheet abrasive on glass: 40 grit 3M Microfinishing Film and 4 different grits of Norton 3X. Only the 60 grit Norton 3X removed metal fast enough to be useful, but it fractured the edge.
Grit Images
These images of various abrasives were taken using a standalone microscope and a digital camera. The large images are about 150 times magnification, so each 5" image represents about 0.05" of abrasive. (Digitally reduced 3 times from the originals.) The smaller images to the right are slices of 450 times originals.
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Norton Coarse Silicon Carbide bench stone
A little hard to see individual grits, but clearly smaller than the Norton 3X 60 grit and larger than the 3M 40 micron. Could be about the size of the 200 grit, but there is less space between the particles. |
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40 micron, 3M Microfinishing Film
Totally different size and grit count. It is easy to see the individual grit particles. The yellow colour is from a yellowish light shining through the clear backing, the glints from halogen light shining from above. |
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15 micron, 3M Microfinishing Film
The grits are pretty clear, much smaller. The yellow colour is from a yellowish light shining through the clear backing. |
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5 micron, 3M Microfinishing Film
The abrasive has clear backing, but there appear to be enough grits and glue that the paper is opaque. |
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| 0.5 micron, 3M Microfinishing Film |
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60 grit, Norton 3X
Average grit size 254 microns. Just looking at these 4 pieces of abrasive, it is pretty clear that the profile of the grit hitting the plane iron would be quite different depending on how the abrasive was glued to the backing paper. |
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100 grit, Norton 3X
Average grit size 122 microns. Not sure why these particles are a different colour. Norton says they add no colouring to the abrasive - to prevent transfer to the object being sanded. Yet the four grits here are three different colours. |
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150 grit, Norton 3X
Average grit size 89 microns. |
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220 grit, Norton 3X
Average grit size 60 microns. More abrasive particles, but still nothing like the number of particles in the higher grits. Are there enough particles to reduce the chance of edge problems, but not so many that the grinding speed is too slow? |
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Grit Size
The importance of grit size is one of the first things any woodworker learns. Most first use abrasives for finishing their projects. In most cases a woodworker works through a series of abrasives in the 100 to 300 grit range. For sharpening though we need much finer abrasives.While there are many selections of abrasives recommended in various web sites, I have found that for the microbevels just 3 grits are required. I have tested about 40 different plane irons now, some many times. I use the same procedure for sharpening the blades each time, taking pictures with the QX3 often after each sharpening grit.
An analysis of the geometry -- microbevel angles and widths -- has convinced me that each grit is taking off a depth of metal sufficient to remove all of the scratches left by the previous abrasive. There is no need for intermediate abrasive grits.
The grits of the 3 abrasives are:
- 15 micron -- 600 grit -- roughly comparable to Japanese water stone rated 1000
- 5 micron -- 1200 grit -- between 4000 and 5000 Japanese water stone
- 0.5 micron -- 2000+? -- 8000+ Japanese water stone
Abrasive Hardness
While most woodworkers understand grit size, most don't worry about abrasive hardness. Fortunately, it is not a problem when working wood. It is possible to work wood with very soft abrasives. For example, Rottenstone is very soft. The standard abrasive for wood finishing, Garnet paper, is hard enough to work most woods.Stepping up to tool steel though, harder abrasives are needed.
Natural stones may not be hard enough. Oil stones, based on Novaculite or Silicon Dioxide, and water stones, based on Alumina or Aluminum Oxide, are not hard enough for some of the harder tool steels. Both Silicon Carbide and Chromium Oxide are much harder than most AlO and SiO2, plenty hard enough for all tool steels used in plane blades. There is much more on abrasive hardness in the Jig FAQ.