How to Improve Machining Center Accuracy

Accuracy is not just inherent to the machine—it also depends on how the machine is used. Try these techniques to let a new or existing CNC machining center achieve its potential precision.

Article From: 8/26/2009 Modern Machine Shop, ,

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We tend to think of machining center accuracy the way we think of the machine’s travels. That is, the accuracy is a built-in fact of the machine. Some machines are more accurate and some are less accurate, just as some machines have large travels and some have small ones.

This view is partially true.

However, that machining center is also part of a system. The machine is set up a certain way and run a certain way. It exists within an environment in which the temperature, among other factors, is subject to change. And the machine by itself is a system, too—made up of moving components that dynamically affect one another. All of these influences contribute to the machine’s accuracy, so all of them potentially have to be taken into account.

A new video tutorial addresses this very point. How well is your machining center leveled, for example? And how is the spindle tram? “Improving Milling Accuracy” from video contributor Todd Schuett examines questions such as these. The tutorial is one of various videos and other online resources that speak to different steps machining center users can take to realize more of their machines’ precision.

To navigate these resources, read through the considerations below. All of these ideas are explored in more detail through related links. Read the steps below for clues to how you might improve your own machining center accuracy, then go to the suggested resources to see these practices in action and pursue any of these points in more detail.

1. Know The Spindle

Do you change the tool offset when the spindle speed changes?

Machining center users tend not to apply tool offsets this way—they are more likely to treat the tool tip position as static. Yet it doesn’t take much reflection to see the fallacy in this. Different speeds generate different temperatures and different centrifugal forces within the spindle. To some extent, the spindle responds to these changes. Therefore, a logical step for holding tighter tolerances is to get to know the spindle’s behavior.

One straightforward way to do this is to identify changes in tool tip position through experimentation. Measuring a gage pin in the spindle at different rpm values can reveal this information. Tool offsets can then be given appropriate corrections for various speeds.

Resource: The video “Improving Milling Accuracy” describes finding and using speed-related tool offsets in this way.

Bonus resource: A laser tool probe is also useful for measuring the true tool tip position while the spindle is running. Watch the video “Laser Tool Probing for Productivity and Accuracy.”

2. Measure The Process Instead Of The Part

Inspection usually occurs after the work is already done, so that the gage essentially polices the process. An alternative is to measure first—gaging the process instead of the part. In other words, identify the sources of error in the process and correct them before the machining occurs.

Granted, there might be a number of these error sources. A three-axis machining center has 21 degrees of freedom, or 21 potential sources of inaccuracy. That’s before the tooling and workholding are added.

Nevertheless, the number of error sources is finite. Both mechanical error sources and likely human error sources can be identified, and they can be addressed and corrected one by one.

Simply understanding that a process can be improved in this way is often a significant first step toward achieving a process that has less need for gaging after the fact.

Resource: See the article “How To Perfect A Machining Process.”

3. Raise The Bar On Drawbar Attention

One of the most ignored components of a machining center is also one of the most critical. The holding force of the drawbar ought to be inspected frequently and routinely—perhaps as often as every other month. After all, it’s not really the toolholder that holds the tool in the spindle; it’s the drawbar. A reduction in force here affects not just accuracy, but also tool life. The need for periodic drawbar repair should be expected, and recognized as a fact of life for most machining center applications.

Resource: See the article “Don’t Forget The Drawbar.”

Bonus resource: A related problem is toolholder distortion from over-tightened retention knobs. Read “The Knob Problem,” and find a link there to video showing a measurement of the toolholder bulge.

4. Control Chatter

Chatter can be reduced by making the setup and tooling more rigid. In addition, chatter can be overcome by finding a spindle speed at which the rate of cutting edge impacts harmonizes with the natural frequency of the system.

Diagnostic tools are available for finding the harmonic spindle speeds of particular milling passes in this way, but these ideal speeds can be found through test cutting, too. Achieving a quieter cut not only improves the machine’s accuracy for smooth surfaces and delicate features, but also allows the machine to achieve a higher metal removal rate.

Resource: “Chatter Control For The Rest Of Us” describes the procedures a mold shop used to find stable cutting speeds by taking experimental passes.

Bonus resource: Software for identifying optimum milling speeds figures into a video that captures milling passes with and without chatter. See (and hear) the footage here.

5. Inspect With A Reference

Can a machining center equipped with a measurement probe inspect its own performance? The answer is seemingly “no,” because any thermal error affecting the machining pass would affect the machine’s ability to measure. Yet some machining center users get past this limitation.

A proven way to do this is to use a known reference object inside the machining zone. If this calibrated reference object is made of the same material as the machined part and has similar geometry, then any measurement error could be captured using the reference. The error would be the difference between the known value of a reference dimension and the measured value of that same dimension. This difference can then be used as an offset to update and correct the machine’s own measurement of some similar dimension of the actual part.

Doing this can let the machine inspect its own work. Perhaps more significantly, doing this can also let the machine revise its positioning and maintain its accuracy throughout a production run during which part and machine position error would otherwise creep in.

Resources: Renishaw uses this reference-measurement strategy in its own UK-based automated production process. See video of how this works here. This page also includes a link to “Break The Accuracy Barrier,” a detailed article about the reference-comparison technique.

Bonus resource: That automated machining process at Renishaw also uses the machining center for turning—with the workpiece mounted in the spindle and the tool secured on the table (rather than the other way around). The technique doesn’t directly relate to accuracy, but it’s interesting to watch. See it here.

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