Saturday, November 28, 2009

Working On A 3-Axis CNC Turning Center

Operating lathes requires exceptional skills. Years of hands-on work hones the superior machinist. I am myself a neophyte - although I admit I did use sturdy old hand-operated lathes in Mechanical Engineering lab back in my  University days to cut, trim, polish, drill and tap various prepared pieces of metal. So I'm not trying to give advice on CNC Machining here. I'm posting this so as to document what I'm picking up before I  forget it all.

A 3-Axis CNC turning center is a combination lathe and computer that gives the user freedom of working in 3-Dimensions, X, Y and Z.  The diagram below illustrates the coordinates system as related to the bed of the lathe. Being a lathe, it has a headstock with a spindle and a tailstock so that the job can be clamped securely and held that way and then spun at selected speeds to enable cutting operations. For small jobs, you can simply clamp the job to the headstock spindle. You can see a small AL 6061 job clamped to the headstock spindle below. You can also see the toolhead with multiple tools  that the machine can switch on the fly for continuous operation.

CNC (Numerical Control) allows us to control the lathe using programming statements and commands. Take a look at the picture below to see the typical interface of one such 3-Axis Turning Center. The machine is operated by a combination of entering commands into the screen and by operating the knobs and levers.

Don't let the formidable looking interface fool you. Learning it is the easy part. By far, the harder part of machining is the set up. With the aim of minimizing the number of cuts you have to make, you have to figure out the exact series of operations you need to make. Simultaneously you have figure out the set up involved in each of those operations. If you want to minimize the time you spend on it (and trust me, it always takes longer that it should, its in the nature of the beast), then you getting the set ups figured out is the most important factor that counts. A major mistake such as overestimating the clearances of the job and the reach of the tool bit can cause a huge headache. You might have to drill with a drill bit thats too long and that will cause vibration issues. Planning the order of the operations is also fairly important, especially if they build on each other and remove material that is critical for certain other operations.

Many thanks to Bob, the Master Machinist!

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