Abrasive Tutorial - No Purchase Necessary - CLICK TO READ
Appendix E Abrasive Belt Basics - Excerpt from No Weld Grinder Plans
All rights reserved Do not copy or transmit Copyright 2009 Tracy Mickley
Open Coat vs. Closed Coat Abrasives
You will find abrasives listed as either open or closed coat. Open coat has less abrasive on the surface to reduce clogging from materials. This is highly desirable in specific applications even though you might think you will get more out of a closed coat abrasive. If open coat only has 50% to 70% surface coverage, Closed coat has more than 70% surface coverage is better suited for metal or glass. As a general rule, inexpensive belts, especially AO belts, are open coat.
Abrasive belts use different types of bonding to hold the abrasive material to the backing belt. Some bonding materials are waterproof, some are not. The difference in technology between bonding agents is as significant as the abrasive material.
Belt and sheet backing material varies in stiffness and water resistance. A J-Flex belt is very flexible and is used in the slack belt attachment. The J-Flex belt will roll around contours. A Y or X weight belt is heavy and stiff. There are several different kinds of backing material. Generally cloth backing is used on better quality belts, paper on cheaper. It is good to know if a belt is water resistant or not.
All abrasives wear and break down as they are being used. The measure of this trait is called Friability and is designed into abrasives. You want highly friable (easier to break down) abrasives in wood and other soft materials. When grinding metal, you want materials that dont wear and break down so quickly.
Grit size standards:
To make things even more confusing, there are at least three standards for measuring grit size. They are the US based CAMI, Micron and FEPA scales. Most abrasives manufactured in the U.S. use the CAMI standard or just commonly called grit. Engineered abrasives typically use the Micron basis which generally has a very small tolerance of variation to the grit size. Many abrasives also use FEPA, an international based standard commonly noted by P rating such as P100 grit. None of these standards match up exactly. It is important that you know which standard abrasive you are using when you mix belt types and brands. There are at least a dozen other grit measurements used around the world. You will generally run into just 3 of them in the USA. They are grit, Pxxx grit and Micron. By far the commonly used is plain old grit.
AO or Alumina Oxide: Commonly found in wood working abrasives. It is usually on the very low end of quality and price. AO belts wear out quickly when used for metal but are very appropriate for shaping and sanding handle material or even leather. Usually brown or sand in color and typically highly friable which means it breaks up easily and exposes new sharp edges.
SC or Silicon Carbide: Usually black or gray in color. You will often find this in wet/dry sand paper marketed toward the automotive body repair market. Often used on metal or paint. It is more friable than AO and wears very quickly. SC is most often consumed in sheet form. It is not commonly found in belts.
AZ or Alumina Zirconia: Typically referred to simply as Zirc belts. This is very hard stuff and is used in better or high quality belts. The downside to using belts with abrasive this hard is that you have to use a fair amount of pressure to break the material down to expose fresh, sharp edges. This isnt usually a problem grinding knife blanks, especially with the harder super steels used in todays knives. The low friability is a problem grinding or sanding wood or other natural material. Zirc or Ceramic belts are recommended for knife grinding. They are more expensive but will out last several AO belts.
Cork Belts: These belts have a thick surface of cork bonded to a belt backing. AO is embedded into the cork to provide a grinding surface that has some give to it but will still grind or polish, depending on the grit size.
Ceramic: A man made material just about as hard as diamonds in some cases. Ceramics are top of the line in metal grinding abrasives but most experienced knife grinders consider them worth the expense. Ceramic belts are more than just natural material glued to a belt. These abrasives are designed from the bottom up for friability, uniform grain or grit size, cooler grinding temperatures and very long life. A ceramic belt will out last several AO belts and while they are often 3 or 4 times higher in price than an AO belt, they will almost always out last those 3 or 4 belts and provide better performance. These belts are as hard as they get abrasive wise but also have different levels of friability designed into them for various applications. If you find ceramic belts dont stay sharp as long as you think they should, increase the pressure of your grinding application to break up the ceramic material and expose fresh, new sharp edges. A quality ceramic belt is usually the most expensive belt initially but ultimately the cheapest belt due to its long life and very consistent performance.
Engineered Abrasives: These premium belts are designed for specific applications. They have very uniform abrasive material and can go to very small grit size. Norton Norax belts are an excellent quality belt using engineered abrasives that are as small as 5x microns or about 1200 SAE grit. Norax are exceptionally good metal finishing belts but are nearly worthless on any other material as they release dark gray abrasive dust that tends to stain anything other than metal.
Other types of Belts:
Non-Woven: These belts are used to de-burr metal edges or to provide a uniform scratch pattern. Most people are familiar with 3M brand Scotch Brite pads and essentially that is a non-woven abrasive. These are commonly used by knife makers to put a satin finish on knife blades. These belts are often found in coarse, medium, fine and extra fine. The coarse belts will actually remove steel (albeit very slowly) while the finer grits tend to polish metal. These are for metal use only. These generally cost from $16 to $30 depending on the brand and application. Hint: Dont pay $30.
Leather Belts: These are just what you think they are. They are loop of leather in the form of a belt. They are used by some people as power leather strops after they have had some very fine abrasive compound added to the surface. These are hard to find already made. Ive seen them go for as much as $60. Consider buying a 2x72 strip of leather and making your own for $20.
Felt belts: These are belts made from the same material as felt. They are fairly thick at approximately ¼ thick. These almost always have buffing compound applied to them and are used for polishing metal. These are expensive and hard to find. This money is better spent on an actual buffer and buffing wheels.
Now the comment/question is: Thats all great but what should I use?
The answer is, it depends.
I grind knives from thicker than average stock (in the .180 to .2+ range) so I start with a big grit and take off 80% using it. I begin with a 40grit Blaze belt to do the heavy lifting. I grind at full speed initially. Mistakes are easy to fix here and I remove as much metal as I can. When I have 80% to 90% of the metal removed, I switch to a 120grit Blaze belt and slow the grinder down a bit. I shape the blade to 90% to 95% of what I want it to end up being. I make sure and remove every bit of 40 grit grinding marks as I usually take blades to a high mirror polish. Even if you dont grind to a high polish, you always want to remove every bit of the last grit marks.
When Im done with the 120 Blaze, I switch to 100x Norax belts. The Saint-Gobain (Norton Abrasives) engineers say this belt is equal to a 120 grit. From my experience it works out to a 180 to 200 grit. I make sure and grind away every bit of the 120 Blaze grit marks and finalize all metal removal with this belt. I want all the grind lines in place and edge thickness of the blade right where it needs to be when I am done with this belt. The remaining belts I follow this 100x with are all just to improve the finish. The next Norax belt I use is a 65x and again a bit slower on the grinder. Mistakes here are frustrating and a slower grinder makes smaller mistakes. All previous grit marks have to be gone. You will not spend a lot of time on the remaining belts but skipping one will add a lot of time to get a nice finish. I am not trying to remove a lot of metal, I am improving the finish at with this grit and smaller. If I want a nice satin finish, I stop with this belt. My next belts are: 45x, 16x and finally a 5x. These work out to a 240, 600, 800 and 1300 grit respectively. I finish with just a minute or two under a sewn cotton buff wheel with green chrome to give the blade a little color. If you pay attention to the details, you will have a high mirror finish at this point.
If you are grinding .140 to .120, consider starting with a 60grit belt or even an 80 grit belt. If you are grinding under .10, 80 grit will go a little slower but you will have more control. Sharp belts will take metal off quickly and you dont need much bigger than 80 grit when you are grinding thin blade stock. You will probably find 120 grit is the belt you tend to use the most of. 120 is just a good all around grit to use for touch ups and gives you control when shaping your blade.
The next question is: What should I order?
It depends on your budget. You will want metal removal belts and then belts for wood or manmade materials such as Micarta® to grind and shape knife handle material. I even use my grinder to shape leather and kydex sheaths. One thing to keep in mind is you cant do everything on your grinder. Expect to do some hand sanding. The better the finish I want on my knives, the more I find the need to hand sand them. Nothing beats old world hand craftsmanship! Budget some funds for at least a small assortment of 9x11 sheets of abrasive.
Expect to use up 2 belts per knife blade. In real use, you will find high quality belts will last for more than one knife but on the average, its best to budget that you will use up a couple of belts per knife. Most experienced knife makers will tell you to use belts like they are free. A fresh sharp belt is safer to use, cuts much faster, cooler and gives you a better grind. A dull belt will grab at the knife, heats the blade up too fast while grinding (burning your fingers) and makes it hard to get a clean, straight grind. When you find you are struggling with a grind, change to a fresh belt and see if it doesnt get better. Most new knife makers are surprised at how many belts are needed and cost of them. It might be easier to think of it this way, expect to spend $7 to $15 on abrasive belts per knife and it is just part of the cost. Some belts, like the Norax belts seem to last forever. Some belts like a small grit AO Jflex may wear out after one short session.
Lets create a sample first order. Add or subtract quantity or types depending on your budget.
4ea 40 or 60 grit good quality metal belts such as Zirc belt. Make this a couple of 60 and 80 grit belts if you are grinding thinner stock as noted above.
2ea 80 grit Zirc
4ea 120 grit Zirc
2ea Norax belts in 100x, 65x, 45x, 16x and 5x
Double to triple those amounts if you can afford it as you will use these up in good time. Cut the quantities in half if you are on a tight budget. Most experienced knife makers tend to order in 10 or 20 quantities of each grit at a time. Like everything else, belt prices go up in price every year and they eventually get used. Expect to $3 to $6 apiece for Zircs.
When you get some practice under your belt (bad pun), give strong consideration to stepping up to a premium quality belt such as Nortons Blaze or Ceramic line of belts. They will cut faster, cleaner and cooler while outlasting everything else out there. Expect to pay $6 to $10 each for these but keep in mind they will outlast others by 2 to 4 times.
Wood, plastic, leather:
5ea 40grit cheap AO X or Y weight (stiff) backing
5ea 60 or 80 grit cheap AO X or Y weight (stiff) backing
10ea 120 grit cheap AO X or Y weight (stiff) backing
5ea 240 grit cheap AO X or Y weight (stiff) backing
5ea 400 grit cheap AO X or Y weight (stiff) backing
These will grind metal but you will go through 2 or 3 at each grit level and you will get frustrated using them for that purpose. These are better for anything but steel. Again, adjust up or down depending on your budget but expect to pay around 2 bucks a belt for these. These are open bonded with less grit so they dont clog (as fast) as typical metal belts do when sanding gummy plastic or wood. These are generally the cheapest belts you can buy. Expect to pay $2 to $4 each.
Wood, Plastic, Leather using a Slack belt Attachment:
3ea 80 Jflex AO
5ea 120 Jflex AO
3ea 240 Jflex AO
3ea 400 Jflex AO
These Jflex belts are very flexible and bend into and around contours in handles or bolsters. These are used to give shape most often to handles and are rarely used on blades other than defining a plunge cut. Slower speed is often better than faster speed as they tend to burn the wood or plastic. They have a limited life as an AO belt but there just is not a great selection of these on the market and you have to take what you can get. Expect to pay little over $3 apiece for these.
Metal Finishing Belts:
If you want a satin finish on a blade, consider getting a medium or coarse non-woven belt. These will last for several years of hard use. Just get one and see how you like it. Purchase other grits if you like the finish they give.
Cork belts are a specialty use belt and are used in polishing a blade with a technique developed in Ron Lovelass shop and made popular by SR Johnson. Use these belts in combination with green buffing compound to bring a mirror polish after using a 120grit belt. One belt will last for years. Break them in by running them at high speed for 10 minutes and putting a scrap piece of steel to it for that time. This knocks off the high spots and conditions the cork/abrasive material. Apply green buff compound at a speed where it wont throw it right back off and then polish your knife. This technique smears any scratches and gives a high mirror finish in a fairly quick amount of time. The downside is you get covered in green buff compound. That buff compound is not healthy to breath in, wear a respirator.
How you grind matters a little:
One of the unexpected things about getting satisfactory results from an abrasive grinding belt is how you actually use it. Belts are designed to perform best at a certain speed and amount of pressure being applied to them. Obtaining this information from the manufacture or supply house is a bit of a task and generally doesnt mean a lot. Id tell you dont bother with it. Just know there are differences in designs for every belt. What will tell you the most is hands on use. If you grind metal at a high speed with a lot of pressure (time is money you know if you are a pro!) you might find one type of brand/model belt performs for you much better than another brand/model of belt. Conversely, if you grind at slower speeds with very little pressure you may have to try several different belts before you find one that works well for you. If a respected knife maker recommends a particular brand of belt and you try it and dont get his results, it doesnt mean he was wrong. It may be you just have different grinding styles. Dont get too wrapped up in all that, especially when you are just starting out. Keep in mind most every belt is going to be just fine and you wont notice those subtle differences until you get a lot of time and practice behind the grinder.
Getting more use from a dull belt:
Occasionally, you can get a bit more use from a dull belt. The technique is to take advantage of the friability of abrasive material. Run the belt at full speed and push a piece of scrap metal into the belt with quite a bit of pressure. Push with more pressure than you would normally grind with. The idea is to break down the abrasive material and expose, fresh sharp edges. You do not want to press down so hard you shave off all the abrasive media. You also do not want to do this so long you wear out what little abrasive media there is left. You want to fracture the dull grit into sharper pieces giving you more life from an otherwise dead belt. In practice I have found Norax belts especially benefit from this technique. Not every belt benefits from this technique as most simply wear down evenly and there really isnt life left. Its just a technique to be aware of try if you have a belt that appears like it should have some life left but it has gone dull. Dont expect to double belt life using this technique but it might help you out in a pinch some day.
Belts throw abrasive grit and dust along with whatever they are grinding. Always wear safety glasses and a respirator. If you grind long enough you will have a belt break and slap you. I promise it will scare the ever loving bejeebers out of you. When it happens, take a break or take the rest of the day off. You wont grind all that well with the shakes, trust me. If you are really lucky you will only get slapped once and it will be a small grit and at slow speed. That usually isnt how it happens though. It always seems to be at high speed with a big grit belt that can scratch you up pretty good as it slaps you a couple times. Ive had several belts break over the years Ive been doing this. You need to know it is going to happen and be ready for it. I always wear safety glasses and a respirator so that has deflected the broken belts from slapping me in the face very hard. If you have a damaged belt, dont use it. The few dollars you save by using a damaged belt is not worth the scare (or pain) it will give you if it breaks on you. How often will this happen to you? Not all that often. Maybe 1 in a 1000 belts break but when that first one pops on you, it will definitely have your attention and respect after that.