ANOTHER LOOK AT GRINDING VS. HARD TURNING: PCBN HARD TURNING FINISHES STRONG

Hard turning has clearly gained momentum over conventional finishing methods and become recognized as a step up in both productivity and cost-effectiveness. For shops that embrace PCBN hard turning technology, here are the reasons why their economic benefits continue to add up.

Hard turning with tools made from polycrystalline cubic boron nitride (PCBN) is more productive, cost-effective and can produce better results than grinding for finishing hardened (45 to 64 HRc) steel parts. This statement isn’t lost on job shops and contract manufacturers.

More and more shops have adopted PCBN cutting tool material and have long-abandoned the notion that the high cost of PCBN tools is a loss leader. They are discovering that the costs just don’t add up; in fact, they actually lead to more productivity because data show that PCBN inserts can produce equal or better surface finishes than grinding and that PCBN tools make turning more economic sense due to higher efficiencies in terms of productivity and tool life.

The high price of PCBN tools (10X to 20X more than conventional tools) is more than outweighed by their up to 10X to 300X more efficiency in terms of overall productivity and tool life. When calculating and measuring this productivity/cost relationship, it is important to assess the number of parts produced per corner and to calculate the reduction in cycle time due to increased machining speeds and feeds.

For example, the PCBN product line from Seco Tools Inc. (Troy, MI) includes full-faced inserts with a center lock hole and solid inserts with or without a cylindrical hole. Both have been designed to eliminate the depth-of-cut restriction usually associated with tipped products. Other choices include tipped inserts, which, with modern tooling technology, can be available in multiple corners, and solid PCBN.

Depending on the application, any combination of these can be appropriate in terms of cost and productivity.

HARD TURNING ECONOMICS
There are several factors that lead to the economic benefits of hard turning with PCBN tools compared to grinding. Consider these facts:
• The upfront cost of the lathe is only about one-third to one-half the price of a grinding machine.
• PCBN tools have low maintenance requirements.
• PCBN inserts can produce an as-good or even better surface than grinding, while at the same time having a higher removal rate. Results greatly depend on the right combination of feed rate, insert nose radii and work piece material.
• As far as machine cost, a grinder is almost twice as expensive as a CNC lathe. Add to that other benefits that include flexibility in terms of machining capabilities and tool changes that can be made in less than two minutes, without production time losses experienced with a wheel change. This results in fast, cost-effective production, even in small batches of parts.
• CNC lathes also take up less floor space than grinders and do not require flume systems. Further, the structure of many PCBN grades permits for productive machining in difficult conditions including interrupted cuts, and typically does not require the use of coolant, which helps to keep costs down while eliminating the environmental concern associated with coolant use.
• Dry machining also reduces the time and money spent on government-regulated chip disposal and reclamation processes.
• Tool maintenance is also simplified as PCBN tools may be quickly indexed to a new edge or removed and replaced with new inserts, and do not require truing or dressing to maintain the cutting profile.

FINISHING ON TOP
Although grinding is known to produce good surface finishes at relatively high feed rates, hard turning can produce as good or better finishes at significantly higher material removal rates. The turning process consists of small depths of cut, yet estimates of reduced machining time are as high as 60 percent for conventional hard turning and 90 percent for the latest plunge turning techniques.

For increased productivity in conventional side turning and profiling, cutting tool manufacturers have engineered PCBN inserts with a wiper geometry that permit increased feed rates with no sacrifice to surface integrity. Studies have shown that by using the right combination of insert nose radii, feed rate or the new wiper insert technology, hard turning can produce a better surface finish than grinding.

The fact that multiple hard turning operations may be performed in a single chucking, as opposed to multiple grinding setups, also contributes to high accuracy.

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