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In what situations is it more cost-effective to repair a machine tool than to buy a new one? Will cutting tools of the future be repair-free at all?

Should you repair old knives or buy new ones? This is something the user of any circular tool – such as drills, end mills and reamers – should consider. The wind of savings brought on by the global recession has fueled a growing interest in tool restoration. When thinking about this, there are many variables - probably a lot more than some people think. Tool geometry, usage, substrate and coating all influence the decision to repair a tool. Other factors include: workpiece value, tool life, process complexity, machining location and level of machining technology. In conclusion, while experts in the field hope to establish some general guidelines, there are few hard and fast rules on the subject.

The simpler the CNC tool, the more likely it is to repair it. Carbide drills are the most common regrindable tool, and are the easiest to regrind, explains Robert Goulding, Seco Tool's director of technical design. When the drill bit is blunt, the drill tip can be reground. If the drill bit is chipped or seriously damaged, the damaged drill tip can be removed first, and then reground. As long as the reverse taper portion of the drill is not excessively worn, it can usually be repaired with a simple resharpening. Regrinding has less effect on drill diameter than other tools, making it ideal for repairs. The size control varies with each repair. An ordinary drill bit may be repaired and reused 5 to 6 times before it is scrapped, while a drill bit used for machining high-precision holes may only be repaired and reused 3 times.

In addition to drills, end mills are also the most suitable tools for repair and reuse. The tricky thing about fixing end mills is that resharpening affects their diameter. However, modern CNC machines with adjustable offsets make it possible to use reconditioned end mills, and many CNC machines can automatically set offsets with the help of tool presetting equipment.

Larger or more expensive tools are also candidates for regrind restoration. Bill Sebring, technical director of Niagara Tools, which manufactures solid carbide end mills, drills, thread mills, end mills, grooving saws and other types of milling cutters, points out that typically, solid carbide diameters smaller than 12.7 mm are End mills don't have much resharpening value. He believes that the most worthwhile restorations are Christmas tree cutters and roughing end mills. Christmas tree cutters are often used to machine turbine blades with complex and precise geometries. Advances in CNC grinding technology have made it easier and easier to regrind such complex tools. Niagara Tool Company refers to the regrinding of solid carbide end mills as "remanufacturing," where the remanufactured tools are ground on the same CNC tool grinders that produce new tools. The company also recoats repaired tools using its in-house PVD coating center.

Coatings can also determine whether a tool is worth repairing, Sebring points out — regrinding a CVD diamond-coated or diamond-like-carbon (DLC) tool is often not economically viable. For titanium- or chromium-based PVD-coated tools, remanufacturing is not a problem, as all functional cutting surfaces of such tools can be reground directly without first removing the coating. Like many knife makers, Niagara's standard practice is to only re-sharpen the knives it makes.

Gear hobs and other gear cutters are also expensive and can almost always be reground. Tom Ware, Product Manager at Star SU, points out that while there are some carbide insert gear cutters (usually used for roughing larger module gears), most gear cutters are solid HSS or hardened steel that can be reground for repair. Quality alloy knives. Depending on the machining conditions, the life of the gear tool varies greatly. Some tools can only process 5 workpieces and need to be reground, while some tools can process tens of thousands of workpieces. For gear cutters, restoration is definitely worth the money: repairing a gear cutter can cost as little as 10% to 15% of the price of a new one. Two important technological advances in gear hobs and other gear cutters—the increased use of high-speed steel tools over carbide, and the introduction of newer PVD coatings—enable longer life and easier repair. In particular, AlCrN coatings (produced by Oerlikon Balzers under the trade name "Alcrona") are increasingly used and are very beneficial for the increasing number of high-temperature cutting and tool repairs. In fact, it is easier to remove the AlCrN coating from the tool than it is to remove the AlTiN or TiAlN coating. The chemistries used to remove the AlCrN coating do not cause any damage to the tool base material (especially the cemented carbide), while the chemistries used to remove the TiAl N coating attack the cemented carbide base.

Another important trend, according to Ware, is more outsourcing of gear tool repairs. Repairing gear tools requires a high combination of skilled labor, machine equipment (especially coating equipment) and specialized programming skills, so it is not very attractive for individual users to repair gear tools in their own workshop. Due to the combination of multiple resources, some companies (such as Star SU) have the ability to use CNC machines - mainly driven by CAM programs programmed for the production of gear tools - to efficiently repair gear tools. The complete technical capability makes it possible to coat the tool itself and, by adjusting the grinding program, only remove the worn part of the tool. As the original manufacturer of most of the gear cutters they repair, they know how to guarantee the quality of the regrind. Ware believes that the future trend is to shorten the turnaround time to complete tool repairs as more and more manufacturers abandon their own tool repairs and rely on outsourcing. As competition intensifies, the industry standard turnaround time of typically two weeks is likely to shrink further.

Some businesses place more emphasis on tool repair services than others. Unimerco, for example, started out in North America as a tool repair service, and only later started producing new tools at the request of customers. The company specializes in drills, end mills, reamers and other solid circular tools (including those with PCD inserts). Jim Stead, the company's manager of applications engineering, explains, "In addition to providing our own customers with new custom-designed tools, we have built a system to support tool repair. Most tool makers focus primarily on producing new tools, if they want to Repairing old tools takes up their production time and equipment. Although we now also produce new tools, we continue to maintain a complete machining cell dedicated to tool repair." Like other professional tool repair companies, in order to shorten the turnaround time, Unimerco also Has its own PVD coating equipment.

Not just Unimerco knives, but many other knife brands also offer regrinding services. However, Stead points out that some new knife makers are not always interested in re-sharpening, or willing to do so. He said that the price of repaired knives is 1/4 to 1/2 of new knives. For reconditioned tools, the key is to ensure consistent quality. He explained that every Unimerco factory around the world uses the same machine tool equipment and restoration process. All knives are reground according to restoration specifications that define their geometry. Every tool produced by Unimerco has a unique number that allows access to its technical specifications - product drawings, flute profiles, and electrical discharge machining (EDM) profiles for PCD inserts. “We are more demanding on PCD tools, giving each tool – even PCD tools from other manufacturers – a unique code. Repairing PCD tools requires specialized knowledge and skills. The damage can be so subtle that it needs to be seen under a microscope."