A series of hard, Strong, carbide alloys used for metal-cutting tools, particularly for machining steel, as well as other metals, is supplied under the name of Kennametal. The basic ingredient of the steel-cutting grades of this metal is the new substance corresponding to the chemical formula WTiC2.
The discovery of this substance, and methods of producing it, has advanced the metal-cutting art, for this compound produces very fine-grained hard-metal compositions not subject to the grain growth experienced in the manufacture of previous hard-tool compositions. Hard alloys made from it as a basic ingredient do not "crater" in cutting steel. The processes, and the compositions produced with WTiC2 as a basic ingredient, are covered by patents 514033,4 5. Kennametal grades KH, KM and KS are, respectively, the medium and soft grades of this alloy as indicated by the letters H, M and S.
The softest grade is harder than that of any tool steel (hardened high-speed steel having a maximum Rockwell "C" of 66.5) and that the strength in cross-rupture test approaches that of high-speed steel, while its thermal conductivity is about the same.
Choice of Grade
Grade KH is the most widely used for machining steel, as well as Monel metal, malleable iron, brasses, bronze and aluminium. Steel machined with KH includes forgings, bar-stock and steel castings, both in the annealed state and heat-treated to hardnesses up to 550 Brinell. KH is also particularly used for precision boring of steel, such as in connecting rods.Grade KM is employed where intermittent cutting and rough machining over blow holes or bolt holes, lugs or Jar shapes calls for greater strength of the tool edge. Grade KS is used for shaping and planing steel. For cutting steel, where only one grade of tool may be purchased, to do a variety of work, grade KH is generally supplied.
For average work, medium carbon tool-steel shanks are satisfactory, such as .70 carbon steel. The size of the shank should be such that at least twice as much steel as the thickness of the tip will support it, after milling the recess. An end mill of the radius of the standard tip will cut the recess, which must have a flat bottom, although the sides should not fit the tip too snugly, as it is better to leave a cushion of brazing material around the walls.
Brazing the Tips
Brazing with oxy-acetylene torch has been found most satisfactory, as the average shop is well equipped for this work. A non-oxidising flame must be used, flux applied and reasonable care taken to avoid burning or oxidising the tip of the shank.
Tobin bronze, or other naval Brass such as Chamet, is customarily used in torch brazing. Such bronze Is 60 per cent, copper, 1 per cent, tin and 39 per cent, zinc, and melts at 1,650 deg. F.
Silver solder (60 per cent, silver, 25 per cent, copper and 15 per cent, zinc) melts at 1,325 deg. F. and may also be used.
Copper brazing is preferably done in a muffle furnace with borax flux, as described later, and is employed where large numbers of tools arc to be tipped.
The procedure for torch brazing the tip of a steel shank, recess milled with an end null of the same radius as the Kennametal standard tip is as follows:
The shank should be heated at the tip, using borax flux. When the flux runs freely, "tin" the recess with brazing material by touching a 1/10in. rod of bronze to it, allowing it to melt and run freely over the surface of the recess. Next, place the tip in the recess with tongs or pliers, cover with flux and heat the underside of the tool at the recess with the torch until the flux and bronze are again melted. Finally, the torch may be played on top of the tool.
Next, press the tip firmly in place with the tang of an old file and hold it until the bronze is solidified. Allow to cool in a tub of carbon black or annealing compound.
Furnace brazing of Kennametal tips is customary when a larger number of tools must be made. A muffle gas furnace, copper and borax are required, and the operations are as follow. Preheat the recessed shanks to between 1,500 deg. and 1,700 deg. F., with the recesses covered with a copious supply of borax. This may be done in a separate preheat furnace or in a cooler portion of the muffle furnace. Pull out the shanks and scrape off the borax with a wire brush or scraper, while hot. Place the tooltips in the recesses and put a piece of pure copper foil or sheet above the blank. It will melt and run underneath by capillary action. Cover the whole end with plenty of borax. Place in a muffle at 2,150 deg. to 2,200 deg. F. until the copper flows freely.
Pull the tool out of the furnace with tongs and, as it is on the threshold plate of the furnace door, press the tip down with tongs or an old file and hold momentarily until the copper freezes. Place in a tub of carbon black or annealing mixture of mica and carbon to cool overnight, or for several hours. Brazing may also be done in hydrogen furnaces if proper precautions are taken, but if borax flux is used in a gas-fired furnace a better braze is obtained than in the hydrogen furnace. Hydrogen has a decarburising effect.
Grinding Kennametal Tools
The tools should never be ground with the wheels customarily used for steel tools. Use only the special wheels recommended by the grinding-wheel manufacturer. Use soft wheels and light feeds when grinding the harder grades. Use soft wheels for broad contact and slightly harder wheels for narrow contact, and re-grind as soon as the tool shows any evidence of wear. By doing so it is necessary to remove only a few thousandths to restore the cutting edge.
When grinding, use a moderate pressure and medium feed across the entire cutting surface of the wheel. Heavy grinding pressure docs not result in faster stock removal, but merely breaks down the wheel more rapidly, and may result in disastrous overheating. Keep the tool constantly in motion, maintaining the original rake and clearance angles. To avoid chipping, grind towards the cutting edge. Never quench the tool while grinding. If air cooling is not fast enough, immerse the steel shank in the water, but not the tipped end. It is desirable to appoint and train one man (or a special crew) to grind all Kennametal tools. Uninstructed operators should not be allowed to grind these hard-alloy tools.
A number of excellent and relatively low-priced tool grinders have been brought out for the express purpose of grinding single-point carbide tools accurately, rapidly and economically. Some of these are designed for freehand grinding. Adjustable graduated tool rests, provided with key ways to guide the protractor or fixture supporting the tool, ensure accurate angles of rake and clearance. Right-hand and left-hand tools may be ground with equal facility by reversing the grinding-wheel rotation.
All standard Kennametal turning tools have a smaller radius than are usually supplied, exhaustive tests having proved that a radius of .030 in. is to be recommended.
Rigid Machine Grinding
It is advised that only diamond-impregnated bakelite wheels be used for machine grinding of Kennametal. The hardness of WTiG2 the chief ingredient, is greater than that of tungsten carbide and, while free hand grinding is possible with the loose-texture silicon carbide wheels, a tool clamped in a vice or on a magnetic chuck is invariably " flawed" or "cracked" by a silicon carbide wheel of any sort, as these glaze before one passes over the work. This may be proved by etching a tip which has been surface-ground in a rigid fixture, using boiling nitric- and hvdrofluoric-acid mixtures. The cracks will always show up. The work is also heated up by the dulled silicon carbide wheel. On the contrary, with diamond wheels the work stays cool, and etching of a diamond-ground tool shows no cracks. These diamond-impregnated bakelite wheels are far cheaper per tool than loose silicon-carbide wheels.
For roughing, Norton or carborundum 100-grit diamond-impregnated bakelite wheels are recommended, while for finishing, Norton or carborundum 240-grit diamond-impregnated bakelite wheels should be used. The diamond wheels may be mounted on individual arbors and never removed.