The hardest part of using carbide tooling may be getting started, especially when it relates to grinding the tooling. There is plenty of technique to the whole process of cutting or grinding carbide. At first it may seem overwhelming, but given a few pointers and a little guidance most grinding room personnel can grind carbide with great results in the same time frame it took to learn high-speed steel grinding.
Grinding carbide or any tool steel has health risks. The greatest hazard is lung exposure to the elements cobalt and chromium. Regardless of grinding wet or dry, tool rooms need adequate ventilation to capture airborne particles. A face mask and respirators should be used as well.
In addition, wash your hands and face before eating or smoking to guard against ingestion of toxic metals. Latex gloves work well to keep hands clean and dry.
Types of grinding
When grinding profile tools such as moulding patterns, two types of grinding can be performed. For brazed-on tooling (that is a dedicated carbide profile brazed to a steel body), we generally face grind to resharpen the edge.
On two-piece knife systems such as Knife-Lock, resharpening is done with a profile grinder in the conventional profile grinding method.
To maintain the edge on brazed-on tooling, the carbide face of the knife is ground.
When sharpening these types of cutters, it is best to have the grinding wheel cutting into the leading edge of the knife. If you think of carbide as powder metal, you can understand that by grinding away from the leading edge you risk small amounts of chip out.
The grinding wheel should be run at high rpm (3,000), and the material removal should not exceed .003 inch per pass. Use plenty of flood coolant on the knife to avoid heat. Excessive heat can crack the carbide or allow it to delaminate. Feed the knife into the grinding wheel slowly to allow the grinding wheel to remove the material completely in several passes.
Carbide inlay is generally used in planer knives or moulder knives, most often as a straight knife for planing or surfacing.
Finding a good diamond wheel that will cut both the carbide and the soft backer is difficult. Because the soft backer steel tends to wear down the diamond wheel at a rapid rate, many people prefer to make two cuts when cutting knife segments.
The first cut is made through the carbide with a diamond blade on a tile saw. The second cut is made with a metal cutting chop saw or "hot saw" using a reinforced cut-off wheel.
Grinding carbide inlay
If you are profiling carbide inlay, be sure to use a diamond grinding wheel that is designed to cut both the carbide and the soft steel backer. Many wheels used for this application are labeled CDX. The diamond wheel has small amounts of boron carbide and ceramic in the bond that helps cut the soft steel backer. The diamond, of course, is needed to cut the carbide. The longevity of the wheel is shortened due to the mix of carbide and soft steel. A light touch during the grinding process will help. We recommend avoiding plunge cuts and run the grinding wheel at maximum speed of 3,000 rpm.
Profile grinding is a two-step grinding process. The first step is a "rough grind." That is hogging out the profile shape with a coarse wheel. Rough grinding is done with an 80 or 100 grit CDX grinding wheel.
When the rough grinding is complete, the second process is the finish grind. This process involves changing the grinding wheel to a higher or smoother grit. Many people prefer 150 or 200 grit for finish grinding. When finish grinding you need to travel the profile very slowly to avoid chip out on the leading edge of the knife.
Some grinders have a reversing switch on the motor to reverse the direction of the wheel. This would give you the same effect as face grinding, feeding the knife into the wheel with the wheel grinding into the leading edge of the knife rather than away from it. (Caution: The grinder must be configured from the manufacturer with a reversing switch.)
Regardless of your technique, the key factor is light, slow grinding to allow the wheel to finish or polish the edge of the knife.
Two-piece knife system
Two-piece knife systems such as Knife-Lock consist of a solid piece of carbide that locks into a high-speed steel backer plate. The backer plate supports the carbide and is ground with the same profile. The backer is a one-time purchase. The carbide indexes up the backer plate as it is resharpened.
The backer plate
The backer plate is ground with conventional vitrified grinding wheels. A coarse grit such as 54 or 60 works best. Keep in mind that the backer plate only supports the carbide, there is no need for finish grinds or side relief angles. The backer plate does not come in contact with the wood. The fact is that the leading edge of the carbide is often set 1/8 inch above the backer plate.
It is critical to keep the backer plate free from debris or burrs from grinding. If the carbide does not seat into the backer plate correctly there is a good chance of knife breakage.
The tracing pin used on the profile grinder will often be a smaller size than the grinding wheel in an effort to "widen" the pattern on the backer plate. Typically, on a 4mm grinding wheel, the operator would use a 3mm tracing pin.
The solid carbide knife is ground with diamond grinding wheels in a two-step process. The profile is rough ground with 80 or 100 grit diamond wheels, and the finish grind is performed with 150 or 200 grit material. Plunge cuts or aggressive grinding are not recommended. Allow the grinding wheel to do the work with plenty of flood coolant.
Side relief on steep-sided profiles such as rabbet or tongue-and-groove knives is often set at 5 degrees. Back relief is set for 15 to 20 degrees; this is less than conventional high-speed steel.
Solid carbide will not leave burrs on the leading knife edge. There is no need to use a slip stone as with high-speed steel.
Carbide does not gain the razor sharp edge of high-speed steel so the surface being cut will not have the same smooth finish by comparison. But it is normally close to the same and is widely accepted in the woodworking industry.
(Keep in mind the more refined the carbide, the better the finish cut. C4 will be noticeably better than C3. The tradeoff is the ability to take shock and the length of run between sharpening.)
In regard to run time and footage, carbide will often outlast high-speed steel by 10 times. So the typical mill with a 5,000 lineal foot run between sharpening can often run up to 50,000 lineal feet.
Problems with hardwoods such as teak, hard maple and hickory are a thing of the past when using carbide. Manmade materials such as MDF, particleboard, plywood and edge-glue stock are allowed to run without a catch.
High-speed moulders with the ability to joint the knives are running miles of material everyday with feed speeds over 300 feet per minute.
If your mill has problems with certain production runs due to knife life, give carbide a try. You can start small with perhaps one bar of carbide inlay and one grinding wheel.
Get some numbers on grinding time and material footages for comparison to the current knife stock you're using. Then compare the number when you use the carbide and factor in the added cost of the grinding wheel and tool steel.
Most mills are running carbide with great results and they wouldn't dream of going back to conventional high-speed steel for certain runs.
Other mills are forced to use carbide due to the material they run through the moulder.
Now it's just a matter of getting started.
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