Living with HSS, How to Improve Productivity?
a) Use of Cobalt drills and cutters:
Actually, “cobalt drills/ cutters" (which actually refer to HSS tool grades M-15 and M-42 that contain 5% and 8% cobalt) were developed to tackle drilling of tough materials like Stainless steel, Inconel, tool steels, titanium alloys etc.
Where a switchover to carbide tool from HSS is impossible and totally uneconomical, use of these cobalt tools is one option available to process engineers to increase productivity for machining normal steels and other regular materials. This option is also resorted to, when increased tool life for a given cutting speed is the need of the hour.
With cobalt drills, increase in cutting speeds up to about 40% or more over regular HSS drills can be possible, with other conditions remaining similar. Depending on the drill size and minimum order quantity, Cobalt drills can cost from 1.75 to 2.5 times more than normal HSS drills. A right compromise between increased tool life and increased productivity has to be arrived at to effectively balance the increased cost of the tools.
b) Coating of TiN on HSS tools
The next alternative in increasing productivity with HSS tools is to coat the drills and cutters with materials like TiN (Titanium Nitride) by Physical Vapor Deposition (PVD) or Chemical Vapor Deposition process. Advancement in coating technology is bringing in other coatings like TiAN, TiCN, and CrN.
Such coatings considerably increase the wear resistance and lubricity at the cutting point and facilitate use higher speeds and feeds and improve tool life. TiN coated tools can easily be identified by the characteristic golden color coating on HSS tools.
TiN coating can increase the cost of drills by 30 to 50% depending on size and batch quantity and practically about 30-100% increase in drilling speeds can be applied with coated drills. It should be noted that regrinding of drill point is going to take away the coating at the cutting point. If the rake angle is retaining the coating even after regrinding, to that extent, the coating remains useful, though to a reduced extent.
It must be noted here that the quality of coating plays a very vital role in the effectiveness of coating and it can vary considerably from supplier to supplier. As a consequence, the benefits of coated HSS tools may also vary.