Workholding Insights

While cruising LinkedIn last week, I came across a post with a photo that showed a vocational classroom full of conventional knee mills and lathes. In the post the author was criticizing the use of conventional machines to train modern students. The author believes that students should learn machining on ‘modern’ machines like 5 axis machining centers while using CAD/CAM software. His belief is that students will not be able to function in modern manufacturing by learning ‘outdated’ methods and therefore these old machines and methods do not have a place in the curriculum.

Of course, students need to be exposed to CNC programming, 5-axis machining, and CAD/CAM software as part of the machine shop (tool making) curriculums, I don’t think anyone would argue. However, every student should start out on the conventional machines and demonstrate competence using them before they transition to CNC machining and CAD/ CAM programming.Educational Benefits of Conventional Machines

Setting up and using conventional machine tools are critical to success when a student transitions to more complex processes like CNC machining. Workholding, cutting speeds and the creation of the correct part geometry are best learned while standing in front of conventional machine tools and cranking on handles.

A competent and willing student can learn to write usable CNC code in three days or less. I have witnessed it. Teaching a person how to secure an odd-shaped part to the mill table or in a lathe chuck takes much longer. Workholding in a real machine shop is not always as simple as putting a rectilinear block into a vise or a cylinder into a lathe chuck. Students need to be exposed to parts that require creative approaches like machining a special set of jaws to hold an odd-shaped part or using a four-jaw chuck to create eccentric shapes.

Proper use of cutting tools is best learned through the feedback of a machine tool handle. Cutting tools impact four of the five senses. And when something is wrong, one or more of the four senses will provide an early warning signal. CNC machines insulate the machinist from sensory feedback. If a student is not allowed to feel how a dull endmill behaves or hear the sound of a squealing drill that is turning too fast, they are going to struggle to properly use and maintain tools in a CNC machine.

CNC machines arrive in perfect alignment and, except for major crashes, do not need adjustment on a regular basis to maintain proper alignment. Conventional machines are quite different. Machinists must routinely adjust the alignment of various components so that they can produce geometrically correct parts. In some cases, making the part on a conventional machine requires the head of a knee mill to be angled and/or the tailstock of a lathe shifted off center. These are experiences that drive the understanding of how part geometry and machine tool geometry are linked.

Problem solving outside the box

Machinists who haven’t spent time in front of conventional machines will often make adjustments to fixture offsets, tool offsets and programs trying to correct a problem that is a result of mechanical issues or mis-aligned components. I have witnessed multiple occasions where a machinist has tried to program away dimensional issues on a part without success only to find out that some element, usually the workholding device, was misaligned.

When I was in the automotive industry, I witnessed this firsthand. We machined a valve body for power steering using a collet on a rotary table. The rotary table had been worked on and the collet chuck not aligned after reassembly. The eccentric rotation caused dimensional issues and that the machinist was not able to correct with offsets and program changes. The manufacturing engineer who finally solved the issue had been a machinist in a shop full of conventional machines before coming to our plant.

CNC machining and CAM software encourage non-value-added creativity. Programmers can become, with good intent, consumed with creating solutions that are elegant in the simulation but do not aide profitability, which is the ultimate goal in a shop. Using multiaxis interpolation to create a radius when an endmill with a corner radius will suffice, or putting multiple tools in a hole when a custom drill is a better choice, are some examples.

Customers don’t pay for elegant programs; they pay for good parts. Conventional machinists are naturally prone to short cuts. When machinists have to turn handles all day long, they become driven to seek ways to reduce movement, and that ultimately improves shop efficiency. These traits remain undeveloped in the machinist or programmer who only knows CNC machining so, they are more likely to be accepting of inefficient processes.

More versatility

There are other less tangible benefits gained from learning on conventional machines. A good machinist who can jump off the CNC machine and onto the conventional machine is worth a lot more money. Oil and gas, aerospace and power generation are industries that often need machinists who can go back and forth. These machinists get paid exceptionally well and the demand is only growing.

There is no question that machining students need to learn modern machining techniques. However, skipping the foundational elements that come by learning on conventional machines is a step backward. Students who are not exposed to conventional machining are not going to be immediately efficient in the CNC realm, and will struggle to solve problems that result outside of machine tool code. While conventional machining techniques may not always be directly transferrable to CNC machines, the critical thinking skills machinists acquire by using conventional machines will serve them well when the time comes.