Dry Cutting


Dry Whirling Makes Economic Sense

Dry Cutting

Short, comma shaped chips resulting from the tangential cutting of whirling, help to carry away heat generated by machining. This in combination with the speed and high precision of Leistritz whirling machines enable heavy cutting without coolants or lubricants. Whirling, inherantly a power lean process, outperforms milling by a factor of 3 times. But the primary economic advantage of this young process is the enormous cost savings for the purchase and disposal of coolants and cutting fluids. Additionally one must consider the devastating consequences of pollutants to the environment.

Coolant retrieval and filtration costs can today be as high as $1,500 per ton! So much that the use of coolants can more than double the overall costs involved in some manufacturing processes. But costs are again only half the story, the energy consumption needed to originally produce these coolants, the environmental impact in coolant disposal, and the health hazard to machine operators, are somewhat less tangible but no less important. Every liter of coolants used endangers humanity and the environment.

No company realizes this more than the large manufacturing plants of the automobile industry, where coolant costs can be staggering. So much so that Leistritz has recently sold 5 whirling machines to Ford, USA for the production of steering worms. One of the deciding factors in winning this order is that the Leistritz machines operate 100% dry. Ford has taken the responsibility to lead the industry, by reducing coolant and cutting fluid usage by 80%. To the pessimistic, this may appear to be an unattainable, but with help from Leistritz, Ford is proceeding.

Coolants pollute not only visibly by adhering to the chips or atomizing into the atmosphere, but invisibly in the form of bacteria. Bacteria becomes apparent with an offensive odor, but by then it is to late, the coolant has “turned” and must be filtered and reprocessed. The cause for this “turning” is typically due to sediment, which may have been missed by any machine side filtering. This sediment is the nutritive medium for anaerobic bacteria, which oxygen in the emulsion normally consumes. Oxygen, however, cannot penetrate foreign oil films from machine tool lubrication systems, so the bacteria is free to multiply.

Most machine operators believe coolant to be a harmless water flow. As long as the coolant sits in the tank this is true. But circulating the coolant through the system can cause a dangerous chemical reaction. Interaction between the air, salts from the cutting process, or foreign materials like cigarette butts, forms nitro-amino acid, a suspected carcinogen. So the “harmless water flow” becomes an aggressive, skin allergen.

Coolants can also be blamed for the recent growth in asthma related illnesses. Here the coolant fog, present in factories, plays a major role. The German equivalent to the Surgeon General has observed a striking increase in asthma ailments within industrial, metal working employees. It is not hard to imagine the potential damage these nitro-amino acids can cause to one’s delicate respiratory system, especially considering the effects it has on the relatively hard skin of the hands.

Dry whirling with Leistritz machines has several other technical advantages as well. Whirling is inherently a fast cutting process for machining many types of screw forms, such as, extruder screws, augers, ballscrews, or gear worms. Whirling, with its gentle, tangential cutting action gives excellent surface finishes. So good that subsequent operations like grinding or polishing can very often be eliminated.

Whirling produces a large amount of chips in a short time when rough machining in instances when finish machining will be done. This is the case with Ford, which realized that steering worms with consistent grind stock amounts, are much more accurately ground. Even in roughing operations, the more precise a worm is machined, the more precise that same screw will be finish ground. In addition, when a worm is rough whirled with statistically controlled pitch diameter and lead, the amount of grind stock can be reduced, thus reducing the grinding time and minimizing wheel dressings.

Not only is there an economic advantage to reducing the grinding time, but we come back to the topic of coolant usage. Grinding is a process that requires coolant or worse yet cutting oil, but in instances that call for the best possible surface finish and accuracy, it is the traditional process of choice. Reducing the grinding time, however, also reduces the overall use of coolant emulsion by the same percentage and all its negative aspects.

Statistical studies have discovered that 17% of total machining costs are for coolant, not including possible recycling and disposal costs. This figure is so high that it lies in third place behind capital costs (39%) and overhead (27%). Wages (9%) and tool costs (8%) are a distant 4th and 5th on the cost scale. Yet many manufacturers are unaware of just how expensive coolant really is. Where dry machining has gained momentum lately for both turning and milling operations, Leistritz has already been dry whirling for many years. One reason may be the short comma shaped chips produced by replaceable carbide inserts arranged radially within the whirling ring. Because the whirling time is very short and most of the heat created by the cutting forces and friction is carried away in blue colored chips, the workpiece is hardly warmed thus contributing to the very low pitch error tolerances. The slant bed design of the >Leistritz machines also guarantees that the hot chips fall directly into the chip conveyor and do not lay on machine ledges to warm and distort the machine castings. Furthermore, recent advancements in carbide technology, especially regarding coatings (TiCN, TiAlN) has greatly increased tool life. To further reduce tool costs the Leistritz tool system utilizes regrindable inserts. Each insert has 4.5mm (15-20 regrinds) of carbide for resurfacing in a flat surface grinder to expose a sharp cutting edge.

In the production of ballscrews whirling has revolutionized the manufacturing engineering process. Today hardened material with HRC62 is purchased and hard whirled dry to finished quality using CBN (Cubic Boron Nitride) inserts. In this process surface finishes of RA0.2-0.4 (8-16RMS) can be achieved. Previously, soft material would be rough machined, hardened then ground. This resulted into much higher machining & handling times and of course production costs, not to mention the necessary coolant.

Other advantages of the modern Leistritz whirling concept are quick set-up and tool change times, so small lot production runs become truly efficient. At the Leistritz assembly hall in Pleystein, Germany, 10 minute set-up times can be demonstrated. In today’s manufacturing economic formula, non-productive time must be minimized. The patented, Leistritz tangential tool system permits fast tool changes of the entire whirling ring by simply loosening 4 wedge-clamps. This allows the operator to quickly exchange preset whirling rings in the least amount of time, then while the machine is operating the inserts of the used ring can be exchanged. With the Leistritz system, re-ground inserts can be installed with new inserts in the same ring. When installed, the flat ground top of each insert is used to locate against a positive stop, the geometry of the whirling ring guarantees that each insert will cut to the same diameter and in the same plane. The regrindablity (15-20 times) of these inserts gives manufacturers the opportunity to get more utilization, thus lowering tool costs.

The greatest portion of the cost advantages with Leistritz whirling machines comes from dry whirling. It has been calculated in Germany over 1.1 million tons of coolants and cutting fluids are used annually, at a cost of 1 billion Euro. Not including the reclamation or filtering costs. Not including the costs for employee illnesses due to its use. And not considering the costs for washing and drying the workpieces prior to assembly.