JOINTING STONES: SELECTION AND USE
Important information regarding how to select the correct jointing stone for your application and equipment.

There are two basic types of jointing stones available in today's woodworking. They include the older style hard stone and the newer soft jointing stone. The difference between the stones is in the way they are used. As a general rule, the hard stone is used for high speed steel knives at 3,600 rpm. These stones must be precisely made to fit the knife. If they are mismade the result will be a tool that is changed to match the stone. It is normal to grind the stone to shape and then to hand file it to fit the profile exactly.

With the soft jointing stone, the stone is shaped to match the tool, but can then be chipped in to fit the knife. This is done by hand using the knife. The head is not in operation during this process. Setup stands can make this job faster and easier. The soft stone does not normally create a noticeable change to the profile design. The soft stone is not designed to remove nicks in tools. In some cases, small nicks may be removed, other times the nick will be left in the jointing stone.

When selecting the grit of jointing stone to use, the end result must be considered. The finer the finish needs to be, the finer the grit should be. This is a general rule of thumb. For high quality work, the use of a 500 or 600 grit stone is most often recommended. The use of impregnated stones are used for training of new operators or when the jointing process requires some lubrication. The impregnated stone provides this lubrication which will produce a very fine jointed edge on the tool. This stone reduces the heat created during the jointing process. In the shaping process, the impregnated stone does not create a dust. In an enclosed area, this should be considered because this reduces the inhalation hazard to the operators shaping the stone.

If the hazard of fire is a concern, the use of the non-sparking stones is recommended. This stone was developed by Moulder Services Field Technicians to eliminate a known hazard. The stone is made so that the grain breaks away at the proper rate and does not allow for the heat build up to occur. This heat build up, especially when over jointing, is what causes the sparks that can lead to fires in the dust system. This stone holds up well against the wood chips.

Another key thing to know, is which type of tool material is being used. If you are going to joint high speed steel (HSS), you must use the correct type of jointing stone. These stones range from the impregnated to the original type of soft stone that is still in wide use today. If you will be jointing carbide, the use of a different stone is required. For carbide, the stones range from a specially blended stone to the new super soft non-sparking stone. The specially blended stone works well for many grades of carbide. The non-sparking stone is recommended for applications when excessive jointing occurs. One of the old stand by stones for carbide is the orange medium grit stone. This stone is much harder than the super soft stone, but not quite as hard as the hard stones used for 3600 rpm on HSS.

Once you have found a stone that fits your production and safety requirements, stay with that abrasive. Production depends on consistent stones and tooling over experimentation can cost a large amount of production time.

Grit Size:
The finer the grit, the better the finish. The coarse grit stones hold up better against the wood chips.

Vitrified Bond:
Holds the profile better and longer than resin bond and cuts cooler.

Resin Bond:
Provides a better finish quality, but does not hold its profile as long as a vitrified bond.

SHAPING JOINTING STONES

When preparing the jointing stone to be used, it is recommended that the leading edge of the stone be reduced in size so that the chipping in process is easier. In the case of a straight stone, it is best to bevel off the edges. This reduces the amount of time it takes to set and prepare the stone for use. After the stone has been chipped in to the head being used, it is not necessary to rebevel the stone. The stone may need to be beveled again when installing a new cutter head. The straight stones used are normally harder than those used for profile jointing. To clean the straight jointing stone, use another softer stone and rub across the face of the used stone. This will pull the metal particles from the harder stone to the softer stone. A dremel tool with a wire brush can also be used to clean the straight stones.

When shaping the profile stones, the profile must be opposite of the knife. The stone is ground to shape. The use of a thin diamond wheel is best, but a clean vitrified wheel can be used. If using a grinding wheel that is also used for grinding steel, the wheel must be cleaned before shaping the jointing stone. Shaping the stone with a contaminated wheel can cause the jointing stone to glaze or become contaminated with the steel particles. After the stone is profiled, bevel the stone at about a 45 degree angle to a reduced width of about 1/8 on top of the stone.. Then use the cutter head to chip the stone in. This will make the jointing stone exact to the profile of the knife.

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