Compensating for Thermal Expansion to Maintain Part Accuracy

Moldmakers, who must consistently produce very accurate and precise molds, need to learn how to overcome and work with thermal expansion.

Parts’ overall accuracy is critical to moldmakers. Compensating for the effects of thermal expansion in the machine tool is one of the critical functions in maintaining overall part accuracy. Thus, understanding how to overcome and to work with thermal expansion demands the attention of moldmakers, who must consistently produce very accurate and precise molds—and have more to do than worry about the negative effects which heat within their machine has on their parts.

Our first consideration in dealing with heat is that which is generated in the spindle by the bearings, belts, spindle motor and the slide ways. In a C-frame machine structure this heat migrates into the casting, and we see expansion in the Y-axis and Z-axis. As the Y-axis expands due to heat, and the column of the machine tends to lean forward and down, the Y-axis and Z-axis positions are affected.

Once the machine thermally stabilizes, the column tends to move back close to its original position. As the machine expands, the effects of this thermal expansion force operators to continuously chase tolerances and or mismatches in surfaces. This heating and cooling process could possibly continue throughout the day, as the machine sits idle during setup or lunch breaks.We want to address these thermal expansion issues to eliminate as much heat as possible. This can be accomplished by using a chiller to cool the spindle, allowing it to run as cool as possible. The chiller will maintain a consistent temperature range within the spindle cartridge, even in extreme duty cycles, to within a few degrees of ambient.

Now that we’ve removed a significant amount of heat, let’s turn our attention to spindle lubrication.

Keeping the Spindle Lubricated
A new technology exists that ensures proper grease distribution to the spindle bearings over long duty cycles. This new grease lubrication system applies a shot of grease every 70 hours to each of the spindle bearings to ensure proper lubrication.

What is unique about this new technology is that typically grease-packed spindles are packed for life and never get re-lubricated. It’s quite normal in moldmaking for spindles to be running at top speed for many hours, resulting in the grease being thrown out of the bearings by centrifugal force. Once the grease goes away, the heat begins to build, the bearings begin to fail and your spindle is done. So by having a re-lubrication system, there effectively is no limitation on spindle hours, as the spindle’s always being greased.

The machine is always alive, moving to compensate for heat-driven growth. Other builders address thermal compensation, but not dynamically. They don’t measure all the time (if at all). The theory, “We know this machine grows a certain amount, so we’re going to compensate just for that amount.” This approach is a huge mistake because the assumption is that the machine heats and cools the same way, every time, under all duty cycles.

Dealing with Heat within the Ball Screws
Another source for thermal expansion that needs to be dealt with is the heat generated at the ball screw. As with most moldmaking operations that require long cycle times at generally high feedrates, part accuracy can be affected due to thermal expansion of the ball screw.

Due to the high-speed axis motion, an expansion in the X-axis, the Y-axis and the Z-axis can be seen. Like the expansion in the head, the effects will be inaccuracies in the part (mold cavity, core, etc.), which may lead to the need for re-work or additional polishing.

As each axis of the machine has a motor that drives one end of the ball screw, the motor knows where it is and how fast it’s going from an encoder that’s mounted on the end of the motor. As the motor spins, the encoder spins with it, and that’s how it tracks the position of the table. This is called indirect measurement.

Most shops today utilize indirect measurement systems, which is fine for many applications; however, in moldmaking this may not offer the most accurate and consistent results. When you have growth in the ball screw, the encoder has no way of knowing that. As the encoder is down at the other end of the ball screw, it doesn’t know exactly where the table is.

So with expansion the table may be in a different location than when you started. One may think that this is a relatively small consideration, but on a machine using high-duty cycles and high feedrates, traversing a 40" area, growth could amount to numbers in the thousandths, which in machining terms is huge. This is not insignificant, however; it needs to be dealt with.