Crobalt cutting tools

Crobalt® Cast Alloy Cutting Tools.

We now stock 1/4" and 5/16" round and square Crobalt tool bits as an alternative to HSS for using on tougher materials.

Crobalt has been manufactured in the USA since the 1920s and is a cast alloy made from 50% Cobalt, with the other 50% being Tungsten, Chromium, their carbides and some other additives. It is available in a multitude of different sizes and shapes from the manufacturer, including square, round, rectangular, and several shapes of parting tool blades to suit different manufacturers holders.

View a comparison test between M42 HSS and Crobalt cutting tips, turning cast iron and stainless at high speed.


Advantages over brazed and indexable carbide tools.

  • Carbide tools (brazed and indexable) work best on large rigid lathes with plenty of horsepower, they tend to bulldoze the material off. HSS and cast alloy can take a sharper cutting edge as they are less brittle, this means they can take a bigger cut with less stress on the smaller type lathes.
  • Crobalt can be  ground using a standard aluminium oxide grinding wheel, as supplied with most bench grinders. Brazed carbide however will need a special green silicon carbide wheel.

  • One piece of Crobalt will outlast an indexable tip many times over. If you make a couple of mistakes using a carbide tip it will be in the bin. With a Crobalt tool bit, a quick grind and it's ready to go again.

  • Higher edge strength than most cemented carbide compositions, meaning higher feed rates when cutting tough materials and less breakage on interrupted cuts.
     
  • Cast Alloy is ideally suited for turning tougher materials such as various grades of stainless, cast iron, high carbon steels, and high tensile steels, but unlike carbide where you would need another grade of tip it is also fine on non ferrous materials such as titanium,  aluminium, and also brass and bronze if a small flat is honed on the top of the point of the tool bit.

  • Less pressure on the cutting tip than carbide due to the increased clearance angles, meaning less deflection on slender work pieces and less stress on the lathe under heavy cutting.

  • As with the usual HSS Diamond Tool Holder tips, the front corner can be honed with an oil stone or diamond sharpener to give whatever radii is required. From a sharp point for getting right into a corner, to a sizeable radius for increased tool life or better surface finish, once the required radius is put on it never needs re-setting.

  • Both HSS and Crobalt are better than Carbide when taking very light finishing cuts. Because of the slightly blunt edge on most carbide tips the tool tends to rub on the surface rather than cut cleanly.

 
Advantages over HSS and other general properties

  • Higher operating speeds and feeds than HSS. If HSS is already working efficiently, speed can be increased by 25-50% and the feed doubled.

  • No coolant needed on most materials.

  • Crobalt will retain its hardness better than HSS at elevated cutting temperatures.
  • Crobalt will return to its original hardness even if heated to 1500ºF, as this is its natural state as cast. This also makes it excellent for brazing or silver soldering to a mild steel shank, perhaps for making a specialist tool when the Crobalt becomes too short for the Diamond Tool Holder.

  • Longer working life and less re-sharpening required than HSS on all materials but especially when used on stainless.

  • The best cutting properties with cast alloy cutting tools are located at the surface of the casting. Because the Diamond Tool Holder tool bit is only ground on the end the tool is always using the surface to cut with, however many times it is sharpened.

  • All cast alloys are non magnetic as they contain no iron. If they get mixed up with your other HSS tools just use a magnet to sort them out.

 

Shortcomings of Cast Alloy cutting tools

  • Crobalt and other cast alloys are more prone to thermal shock than HSS. After grinding Crobalt it should be left to air cool, don't quench the tool bit in water as this can cause microscopic cracks in the cutting edge. When turning you should either flood the cutting edge with coolant or use none at all. Don't drip coolant onto the workpiece or use a brush as this too may damage the cutting edge.

  • As mentioned previously, the best cutting properties are on the surface of this tool bit, which suits the Diamond Tool Holder well.  Don't grind away the tool bit and use the centre of the casting as the cutting edge. For example a thin grooving tool where a lot of material is removed from either side.

  • For turning very hard materials such as heat treated steel, Tungsten Carbide is the tool bit of choice. On the Rockwell hardness scale, Crobalt is about 62HRc, M42 HSS about 65HRc, and Tungsten Carbide in the 80+HRc range.
    Even though M42 HSS is slightly harder at room temperature, its hardness drops at a quicker rate than Crobalt as it heats up, so would be softer at elevated working temperatures.

  • Crobalt is easier to chip than HSS when used in thin sections, so for threadcutting with a Diamond Tool Holder it is better to use CoHSS or HSS bits.

  

 

Sharpening Crobalt Tool Bits

Crobalt is best sharpened with a normal Aluminium Oxide wheel, it does not need a green silicon carbide grinding wheel.

After grinding, allow the tool bit to air cool. Do not quench in water as this can cause cracking on the cutting edge.

The same clearance and cutting angles used on HSS in the Diamond Tool Holder are also fine with the Crobalt bits.

For increased tool life and better surface finish on both Crobalt and HSS it is recommended to put a small radius down the front corner, using an oilstone or diamond impregnated sharpener.

Crobalt will return to its original hardness even after being heated to 1500ºF, so overheating while grinding will not be a problem.