by Roger Bunce


In December 2011 I 'road tested' Eccentric Engineering's

Diamond Tool Holder for MEWS. More recently, Eccentric Engineering introduced Crobalt tool bits for use with their tool holders. The article by Gary Sneesby, together with his video (see below), certainly looked convincing, so I was delighted when the editor of MEWS asked me to do a quick test on Crobalt.

It is not my intention to repeat Gary's excellent description, but merely to describe a couple of quick tests as an adjunct to my original article.


In accordance with Gary's video I only retested two materials: cast iron rod and stainless steel EN303. These are two of the 'tougher' materials used by model engineers. For the test conditions please see my original article Diamond Tool Holder. The important point is that my test conditions for both Crobalt and HSS were identical. Unlike Gary, I did not grind a chip-break on the tool bit when machining the stainless steel. For all my tests manual feed was used in order to 'feel' the cutting action and, in the case of stainless steel, provide a 'stop-start' chip break. The maximum depth of cut was defined as that just before excessive chatter/vibration occurred.


In addition to those shown below, I increased the recommended speeds for both materials. For the stainless steel: from 900 to 1300rpm (the fastest speed of my machine); the maximum depth of cut was reduced to 0.12in. For cast iron: from 250 to 400rpm; the maximum depth of cut remained the same at 0.11in. In all cases, when machining the cast iron, the depth of cut included cutting through the hard outer skin.

I emphasise that the lathe used largely determines the depth of cut.  Mine is nearly 50 years old and nowhere near as robust or powerful as the lathe used by Gary. But my test conditions for HSS and Crobalt were identical, so the test is still a valid comparison of the tool bit materials.


These tests show that, under identical machining conditions, the depths of cut and cutting speeds achievable using Crobalt were considerably greater than these using HSS. Furthermore, no special facilities are needed to grind Crobalt. It is ground using a normal aluminium oxide wheel using the grinding jig provided with the Diamond Tool Holder. Gary's article includes the relative merits of the various tool bit materials, but for most purposes Crobalt would be my material of choice.


Eccentric Engineering

P.O. Box 60

Mount Waverley Melbourne VIC 3149 Australia