By David P. Monyhan (retired)
Conventional abrasives have kind of taken a back seat to PCD and CBN cutters in the automotive world in the last few years. Since most common engines being rebuilt these days are made from multi-alloy components they are more easily machined with PCD or CBN. But PCD and CBN can’t do everything, so abrasives are still needed in today’s high end machine shops.
The diesel market still depends heavily on abrasives due to the hardness of the iron and steel components being used in diesel power plants. Abrasives are still widely used for surface finishing, porting and polishing, cylinder honing, crankshaft grinding and polishing, flywheel grinding and more.
Before getting into the specific applications, there are some things that you must keep in mind for all abrasives. I call these the Abrasives ABCs.
- Match the abrasive to the material you are working on. Some, or should I say most, companies ask that you select one abrasive that is supposed to be the “Grand Puba” of abrasive material to grind anything and everything. Let me tell you that is certainly not the case at all. If you’re working on a hard material, you want a softer abrasive. Be sure to match your abrasive to the type of material, for example, if you’re machining nickel-chrome valve seats, you’ll want an abrasive designed to cut nickel-chrome.
- Make sure you have a sharp and pointed dressing diamond or star dresser
- Make sure you have clean grinding coolant for those applications that require it. Old, dirty coolant will make the stone work harder and won’t flush the debris away as efficiently.
Okay, let’s take a look at some specific applications.
Here is where we have never quit using a traditional type of abrasive. A valve should be ground and to grind a valve properly we need to match our abrasive to the type of material we are grinding. Valves have changed a lot over the years and we will have materials very different from what we use to grind back in the day. Today we have stainless steel, titanium, Inconel, stellite and of course standard iron and steel.
By incorporating the Abrasive ABCs into your grinding procedure you have the best chance to grind the material without stone load and the finish will be what you require for the application. Keep in mind some valves have a special coating that prevents you from grinding at all. Pay close attention if you see any cautions and or notes from the manufacturer to insure you don’t grind away the coating.
Here again the valve seat has evolved and of course the machine manufacturers have also evolved. Many shops have already migrated to using the multi-angle carbide cutters that cut multiple angles simultaneously. However in my travels around this great country of ours I still a lot of seat grinding going on. Valve seats come in a variety of materials. Nickel chrome, Stellite, cast iron, copper beryllium, and powdered metal. You simply cannot expect one grinding spec to grind all of these varying materials.
Here is where we still see a lot of abrasives being used. I stated earlier that a lot of shops have acquired the newer cutting machines that use CBN and PCD. Some of you have converted your existing grinding machines to also use the CBN or PCD. But as good as CBN and PCD is there are just applications that need to be ground. Diesel cylinder heads with exposed valve seat or pre-cups. Aluminum cylinder heads with exposed valve seat or pre-cups.
Surface grinding machines are designed to run either multiple segments that form the whole grinding wheel that are individually held in place with wedges or single piece wheels mounted directly to the drive spindle of the motor. If you use segments then you have a pretty good selection to choose from. Segments generally come in three different specs.
- Silicon-Carbide abrasive for cast iron and aluminum
- Aluminum Oxide abrasive for steel and hardened metals
- Aluminum Oxide and Silicon-Carbide combination abrasive for aluminum, cast iron, hardens hardened iron and steel.
Always be sure to replace your segments in a group. If your machine uses 10 segments, you should replace all 10 of them at the same time. This helps with machine balance and concentricity.
This is still a very popular method for a quick clean up on the surface or even a final surface for some applications; especially exhaust manifolds. The beauty of a belt surface is there is little to no set up but you still have to match the abrasives and also grit to the material you are grinding. Make sure the belt you use has a waterproof, poly back so it won’t tear or fray during the surfacing operation. Some of these types of belts require a lot of down pressure and other doesn’t. Silicon-carbide 40 grit is generally used for cast iron. Silicon-Carbide 80 grit is for aluminum. Zirconia-Alumina 40 grit for cast iron and steel but this needs a lot of down pressure and most machines running this belt would have an air or hydraulic hold down system incorporated into its design. Aluminum oxide 40 grit is standard on most machines and works with little down pressure. As always, remember the Abrasives ABCs.
Porting and Polishing:
I know you can buy CNC ported cylinder heads but you would be very surprised just how many shops still do it by hand. These little cartridge rolls or tootsie rolls as they are called need to be of the highest quality you can find. They are not all the same. The good ones are fully glued to prevent them from un-rolling during use.
Some key things to remember when porting and polishing are:
- Use a porting a polishing fluid to improve the finish and prolong the cartridge roll
- Never use squashed or reformed cartridge rolls as they may become shrapnel when spun
- Never use a bent mandrel as this will cause vibration and reduce your ability to control the abrasive while in use
- Always wear safety glasses when doing this type of work
Hundreds, maybe even thousands of articles have been written about Cylinder Honing by people smarter than me. I’ll leave the details to the experts from Sunnen, but I will highlight things we all need to know in general.
Honing stones are designed for specific materials and for specific ranges of size and for specific types of honing such as manual honing versus machine honing. Grits range from around 70 grit up 600 grit for most automotive applications. Bore ranges are down to about 1.250” and up to 9.00”.
Keep in mind certain honing stones are for specific types of honing. If you have a hone that has felt wipers then these are designed for dry honing only and are generally used in hand held drills for very light stock removal. Traditional vitrified stone sets with aluminum wipers are used in both hand held drills as well as machines and must be used with a good quality honing oil. Never used tranny fluid or a concoction of “marvel mystery oil and solvent” or any other handed down home recipe, as this will attack most bonding agents that hold the grain together and cause premature failure of the honing stones. Always match the honing stone to the material you are honing, just like all the other abrasives we have been discussing.
Honing Stones, if used properly will wear consistently. Good quality honing oil will keep stones free cutting and prevent loading up allowing the stones to do the job. New stones will sometimes need to have the corners radiused to conform to the round cylinder bore, and in the event your stones do become loaded up you can dress them with a diamond file or even use a stone against a stone to assist in unloading the built up material.
In extreme cases of taper, the stones will actually be worn at severe angles. When is happens you will need to use a truing sleeve to get them back trued up before you can expect any result during the honing process. In general automotive honing we need to be aware of cast iron, hard sleeves, short cylinders and, of course, blind holes. We need to insure we have the correct stones for these different types of cylinders before we start the honing process. Remember the harder the component the softer the stone.
Cam grinding wheels:
Cams are finished ground after they go through the manufacturing process. As you know camshafts are made in both steel and cast iron. Remember that depending on the application, some camshafts are harder than others even when made from the same material. Most cam wheels are in the 100 grit range and are available in diameters from 18” to 20” and widths are 3/4” up to about a 1-1/2”. Premium wheels are for harder material above the 55 Rockwell spec and those will also be made from a Green Silicon-Carbide and have grit around 70. Cam wheels require a lot of clean coolant and must have a very sharp diamond for periodic dressing to prevent loading and therefore burning of the lobe during the grinding process.
Crank grinding wheels:
These big rascals are the biggest grinding wheels or abrasive used in the Engine Rebuilding business. They range from 18” in diameter up to 40” inches in diameter. They come in widths from ¾” up to 3-1/2” in width. Just as with every other abrasive, one wheel cannot be expected to do everything that is automotive or diesel. You must match the wheel to the material you are grinding. If you are grinding automotive and the journal is less than 35 Rockwell then a traditional medium grade wheel will do the job. If you are grinding harder diesel or performance crankshafts (over 35 Rockwell) then you need a wheel designed specifically for these types of applications. Grinding wheels will load up so they need a constant flow of clean, high quality coolant and lots of it. A very sharp diamond is an important player to maintain the proper dress to grind.
Polishing belts are just what the name says: they are for polishing……PERIOD!
I have seen too many shops attempt to correct a poorly ground crankshaft with polishing belts. If the crank is ground correctly then the polishing belt will do its job and improve the finish as well as remove minute imperfections in the journal surface. Polishing belts are available in widths from a narrow as ¾” and as wide as up to 2”. Grits are from 240 grit up to 800 grit. I like to see the use of polishing rouge in the mix to improve the finish to nice luster.
Some key things to remember about crankshaft polishing belts:
- Scalloped edge belts are for polishing the radii on the crankshaft.
- Match the width of the belt to the journal width
- Use polishing rouge for maximum shine
- Chamfer oil holes prior to polishing
- Polishing belts are more prone to moisture-damage than wheels. Store belts in a clean, dry area at room temperature. I heard of one shop that keeps them in the refrigerator and says it protect the belts from the humidity.
Lots to talk about when it comes to flywheel grinding.
Yes there are general purpose stones that do a pretty good job on most cast iron flywheels. However not all flywheels are made of cast iron; they are also made from cast steel, billet steel as well as aluminum with steel wear inserts.
Flywheels are becoming popular again due to stick-shift cars getting better gas mileage. Big over the road trucks all have clutches and that means flywheel grinding. I like to suggest if your are grinding flywheels and don’t know where to start or which wheel is best for what application to use the middle wheel and go up or down the scale from there. If your supplier only offers one type of wheels to grind all, then find a different supplier. Remember soft stones for hard flywheels and hard stones for soft flywheels.
- Always maintain clean fresh coolant
- Always inspect your dresser, especially if it is a star dresser. The star dresser must rotate.
- Diamond dressers need even closer inspection as sometime the diamond can be knocked off and the operator will not know it and you could grenade a flywheel stone during dressing.
- Always wear eye protection.
Grinding wheels and abrasive belts are a part of everyday business in the machine shop. How we handle and store these is the challenge for all of us. Heat and cold and yes heavy humidity can wreak havoc on any grinding wheel and especially for abrasive belts. I have a few final thoughts about abrasives for you:
- Blotters are not just a place for the company’s logo. Never mount a grinding wheel without a blotters….period
- Do not over tighten. Tighten just enough to hold wheel firmly
- Ring test wheels prior too mounting by knuckle knocking the wheel and listening closely. It should ring like a bell and not like a clunk
- Inspect wheel dressers periodically
- Insure wheel safety guards are in place at all times
- Never exceed maximum RPM of wheel
- Visually inspect each wheel before use. If in doubt throw it out. Nobody wants to be a pirate every Halloween.
Treat your grinding wheels and all of your abrasives with care and they will perform the job you need to do when you need to do it.
You may have noticed that I have skipped over Flex-Hones and Ultra-Finish Plateau Hones. While these are abrasives commonly used in the engine building process, they are kind of a special abrasive family that I’ll try to talk about another time.
See ya in the shop,