All Shop Floor Operations References

Brazing and Sharpening Carbide Form Tools

Tuesday January 27, 1998
Prepared by the PMPA Training Committee for a Presentation at the

1987 National Technical Conference



Background

Most cracking problems on carbide form tools are caused by improper brazing, rapid wire EDM cutting and poor tool sharpening. Improper brazing is by far the most serious problem. In order to fully understand the brazing techniques discussed later in this paper, it is important to first mention the basics.

In recent years, carbide manufacturers have been developing their products for CNC equipment and concentrating on carbides that are useful in clamp-on, insert-type tooling. In developing these carbides, titanium and tantalum have been added to improve shock resistance and tool life. From a brazing standpoint, these additives are detrimental because they inhibit the bonding capabilities of silver solder. Therefore, in the brazing process steps must be taken to remove these additives from the surface of the carbide.

The second problem inherent in brazing carbide to steel tools is the coefficient of expansion. Carbide is very much like concrete. It will take a tremendous amount of compression but cannot tolerate tension. When tension is applied to carbide which will not flex, it cracks. That's where the coefficient of expansion comes into play. Steel expands at the rate of 3 to 1 relative to carbide. When brazing temperatures reach 1,500° Fahrenheit, the steel portion of a fairly long cutting tool will expand as much as .006 to .008 per inch. At the same time carbide has expanded only about .002 per inch. As the tool begins to cool to 1,200° and the silver solder solidifies, the steel portion is about .005 to .006 over size and the carbide is .001 over size. As the tool continues to cool, the steel further contracts causing compression and tension on the carbide. Microscopic cracks begin to develop in the carbide because it is unable to flex with the contracting steel. In extreme cases, the carbide will break off the base to which it is brazed.

Brazing Steps

To overcome the above noted problems, the following steps should be taken for the proper brazing of carbide tools:

 

  1. Heat the carbide to about 1,200° Fahrenheit. Allow it to cool. A brownish/green colored layer will develop on the carbide. This is the titanium and tantalum that you have "cooked" out of the carbide surface.
  2.  

  3. Then glass bead blast the carbide. Do not use aluminum oxide because aluminum inhibits bonding with silver solder.
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  5. After blasting, wash the carbide surface with acetone. Do not touch it with your fingers. It should be handled with tweezers or pliers because any oil is detrimental to the brazing process.
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  7. A copper shield between the carbide and steel portion of the tool will be necessary to compensate for the coefficient of expansion between steel and carbide.
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  9. The copper shield will need to be varied, increasing in thickness as the carbide is increased in size and/or thickness. As a starting point, for a piece of carbide 1/16 x ¼ x ¼, .005 of copper or .010 trimetal will work. (for a piece of carbide ¼ x ½ x 1, you might look to use .020 copper or .030 trimetal.)
  10.  

  11. When using copper (not trimetal) the shield should be glass blasted before using to remove oxidation. Both copper and trimetal can be washed in acetone and handled with tweezers or pliers.
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  13. When setting the flame on the oxygen acetylene torch, it should burn white and not bright blue. While the white flame does not burn as hot, it does a much better job in brazing carbide.
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  15. After the tool has cooled enough that the silver solder has solidified (1,000° Fahrenheit), place the tool in a metal box and cover with pulverized mica or some other insulating material and let it slowly cool overnight. This does little for the carbide, but will allow the base metal to remain machinable.

Wire EDM in Carbide

The most common problem with cutting carbides on EDM is that the feed rate and amperage are too high and are melting the carbide matrix. In order to fully understand this problem, imagine the following. The carbide matrix is like ping pong balls connected by rubber bands. Further imagine the EDM as a super heated knife cutting through this material. Not only does it melt the rubber bands which it touches, but it also damages and weakens the rubber bands nearby.

In other words, by cutting at a fast feed rate and high amperage, the adjacent carbide is being damaged. The damage is below the surface and frequently turns up as cratering or cracking problems. Therefore, it is essential to slow down the EDM cutting process. How slow? About one-fourth the speed used for cutting high speed steel. One shop reports that its feed rate is equivalent to .001 to .002 per minute.

Tool Sharpening

There are two elements that are essential to insure proper tool cutting when grinding carbide tools. They are:

 

  1. A dead sharp edge

  2. A superfine finish

In terms of the edge, not much can be said except that it is essential the edge is dead sharp and there is absolutely no radius whatsoever. The edge could be checked with a 10 power loop.

In regards to finish, it is necessary to lap or hone ground surfaces to a mirror finish. The mirror finish should be on the cutting edge and the rake angle. While grinding and lapping are time consuming steps in tool sharpening, longer tool life, as a result, will more than compensate for the extra effort.

A special word of thanks to John L. Detterbeck of Lester G. Detterbeck Co. for reviewing and updating this article, January, 1998.

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