Any experienced machinist will tell you it takes a lifetime to thoroughly learn their craft, and that even after a lifetime they still haven't got it all. That is because the machine tool landscape is changing and the profession of machinist is evolving as well.
Today's machine tools operate at faster speeds, are numerically controlled and perform more accurately than in the past. They use longer tools and offer more versatility in the number of operations they can perform in sequence. Cutting tools of modern materials have been designed to meet these challenges. Workpieces have also changed, with industry now favoring parts of new alloys, lighter in weight, and of more complex geometries and with thinner walls. The formulation of cutting fluids has changed to facilitate and meet these challenges.
Like many professions, that of machinist requires both art and science. A machinist who understands the physics of machining becomes an expert machinist when he gets the "feel" of applying various machining processes to various workpieces.
To help machinists gain expertise and get that "feel" for their work, we offer a review of some of the most common machining problems and a few suggestions.
Chatter This is the granddaddy of machining problems. Any relative movement between the workpiece and the cutting tool will cause vibration that will affect the work. With the advent of high-speed machines and machining processes, and the trend toward thin-walled components, chatter may be the most limiting factor in pushing the physical limits of machining.
Try the following to minimize chatter:
Keep the tool, workpiece and machine as rigid as possible.
Choose the proper tool and run the job at speeds and feeds that excite as little vibration as possible.
Apply the proper cutting fluid in the proper amount.
If possible, use software modeling tools to help determine the optimal parameters for the job. They are not foolproof, but provide a good starting point.
Use your ears. If the process starts to sound "off", it probably is.
Tool Life Cutting tools wear through normal operation. As this happens, changes in optimal cutting conditions occur as cutting forces and temperatures increase, which affects the quality of the workpiece and increases machining time. Here are some things you should know to help extend tool life and improve the overall quality of your product.
Flank wear occurs on the portion of the tool in contact with the work piece. Rapid flank wear can cause dimensional inaccuracy and poor work piece finish. This can happen if the cutting speed is too high or if the tool material is of insufficient hardness. Reduce the cutting speed and select a more wear-resistant tool material (perhaps a coated grade).
Crater wear (cratering) is caused by contact with chips that erodes the tool face. It is a normal occurrence and does not cause problems until the tool is degraded enough to fail. Cratering can be caused by too high a cutting speed, excessive feed rate, insufficient coolant and other factors. Check these to minimize this effect. A coated tool grade may be part of the solution. Tool rake angle is also a factor -- try a positive rake if cratering is a problem.
Built-up edge (BUE) is the condition in which material from the workpiece builds up (or becomes fused) on the cutting edge of the tool. This occurs when cutting softer metals with a lower melting point. If this is a problem, try increasing the cutting speed and make sure you are using sufficient lubricant to cool the workpiece interface. Cutting geometry may also affect this, so if you are using a negative geometry, change to a positive one.
Using the right cutting fluid in the proper amount and concentration can effectively lubricate and cool the tool/workpiece interface. This will increase tool life.
If you have a persistent machining problem, let Acculube assist. Our technicians are experts at problem-solving, and can offer suggestions - and often new technologies - that will give you the best possible results, and greatest overall shop efficiency.
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