Probing questions: CMM Inspection

Courtesy of All photos: D. Nelson

Spindle probe in position to check the surface of a chamfered hole.

While some may question its use, on-machine probing can reduce setup time while boosting in-process dimension control and enabling part verification.

The benefits of using probes on machine tools are not always obvious. Objections such as: “it adds cycle time” and “machine tools should cut, not measure” are common.

However, all shops want to boost productivity and minimize costs. On-machine probing can help accomplish these goals through setup-time reduction, in-process control of critical dimensions and part verification.

On-machine probing systems typically include two probes. A tool setter probe measures the length and diameter of tools while a spindle probe locates part fixtures, workpieces or both. Probes work in conjunction with a machine’s servo encoders and control system to effectively turn the machine tool into a measuring device that automates the process of making offsets.

Tool Protection

Machine shops typically use on-machine probing for several reasons. For example, when Rosene Machine Inc. bought its first horizontal machining center, a Makino a51 equipped with a pallet changer, it also wanted protection against broken tools. “The machine runs longer with less operator intervention than other machines,” said Dennis Rosene, president of the Firth, Neb., job shop. “We wanted some protection against the cascading effect of broken tools. We knew that probing would add a little cycle time, but in the long run it saved money by extending tool life and preventing scrap parts.”

Rosene purchased a Renishaw TS27R tool setter probe, but initially saw no reason to buy a spindle probe to accompany it until a serviceman installed one so the shop could try it. “We quickly figured out just how much help the spindle probe was,” Rosene said. “It made setups far easier since it wasn’t easy to reach in the machine to manually dial in fixtures from one side.”

As a result, the shop had to rethink what tool length offsets should look like. The shop was used to seeing negative numbers for tool lengths in its offset page. When the probe measured tools, it found the actual tool lengths and input them as positive values.

When tools are touched off manually, the tool length offset is a relationship of the tool to the workpiece. Each tool has an offset that represents where the tool touches the part relative to the machine coordinates.

Probing assigns a unique value to a tool assembly. The tool remains independent of the workpiece location and the tool’s offset can stay with the tool, thus eliminating the need to touch off the tool for every setup of a new part run.

Manual setups typically involve touching the tool tips on the top of a workpiece. Manually jogging the tool into a near-collision situation is risky, at best, particularly for delicate tools such as small-diameter drills. Tools can chip or break during this process.

Even a conscientious operator can make mistakes. Whenever manually entering numbers is required, mistakes such as transposing digits or forgetting a decimal point can easily be made and repeated with drastic effects.

Most CNC machining errors do not happen in-process. They occur during setup—the most common manual operation—in subsequent tool changes and during workpiece transfers. If these operations are partially automated, the possibility of error is reduced dramatically.

That was the case at Rosene Machine. The advantages of using a spindle probe became even more evident as the shop searched for ways to reduce machining costs. On one part, the shop cut machining costs by nearly 50 percent, based partly on its ability to dial in every part loaded in the machine as part of the machining process.

Finding the Workpiece

Using probing as part of an automatic process to find a workpiece within the machine envelope also offers the potential of machining parts in ways not normally thought to be profitable.

Because the machine can locate the workpiece, it can compensate for problems that may exist with locating parts consistently in fixtures. For example, many workpieces have size variations that limit machining options. If a casting has part variations that are within 1⁄32 “, it will be impossible to machine true-positioning tolerances of ±0.005 ” without some means of setting the work offsets after loading each workpiece into the fixture.

On-machine spindle probes can check each workpiece prior to machining and reset the work offset. While using a probe in-process adds to the cycle time, it offers several advantages.

Rosene Machine saw the advantage of using the spindle probe after setting up the first production run on its new HMC. As previously mentioned, reaching into a horizontal machine to dial in fixtures is difficult for the operator. The problem comes from having only one place of entry, with climbing into the work envelope as the only means to see the workpiece and reach dial indicators and edge finders. This problem can occur on vertical machines, too.

Jon Alexander of Rosene Machine demonstrates the difficulty of reaching into a horizontal machining center.

Workpieces that are near-net shape make properly locating the workpiece more critical. In the case of Rosene Machine’s first production run on the HMC, the shop had material laser cut by a local shop. The laser cut a critical area, leaving only enough material for a finish pass, thus eliminating the roughing passes present in its previous manufacturing strategy.

Near-net-shape workpieces are typically rough and have drafted edges that offer fixturing challenges. Holding each workpiece in exactly the same location is difficult, making fixtures more expensive and prep work necessary. However, spindle probes offer the option of automatically locating work offsets.

Locationally Challenged

Of course, locating a part feature can be a problem with other types of workpieces as well. For example, molded or cast metal workpieces can present locating challenges, as can molded composite parts.

Royal Plastics Manufacturing Inc., an aerospace job shop in Minden, Neb., machines carbon fiber-reinforced plastic. Most of its products have complex shapes with spline surfaces. One part required chamfered holes with the depths of the chamfers held to ±0.005 “. While the part was held tightly in a molded fixture, problems surfaced in machining the holes and chamfers.