From 25% non-conformance to near zero: MSP identifies CNC inaccuracies in minutes with metrology software

About Vert Technologies

Vert Technologies designed and manufactured the patented Conical Rotary Compressor (CRC), which was invented and developed in-house. The compressor was compact and scalable, and it delivered high pressures in a single stage without increasing noise. It was supplied to the refrigeration, heat pump and gas compression markets. Its performance depended on complex conical rotors machined to very tight tolerances on a 5-axis CNC machine. Please note: Vert Technologies has since ceased trading. We have kept this case study as a record of the project and its results. Quotes are as given at the time.

Challenges with the original CNC machining process 

“When you’re dealing with high-tech, high-precision parts, your process needs to be perfect in order to achieve the necessary accuracy,” said Nicol Low, then Chief Operating Officer at Vert Technologies. “One USP of our technology is that it is a quiet compressor, however, the parts we were producing weren’t achieving the low noise levels that we would expect, and flow performance was suffering too. We needed to rectify this fast.”

The process was time-consuming and relied heavily on skilled manual work. Each part required a highly-skilled operator to manually datum the part in the fixture to try and achieve an accurate alignment before machining. After machining, the rotor’s surface would be scanned and compared with the CAD model, and manual checks would confirm if run-out was within tolerance.

“The rotor design is relatively complex and the metrology of it is quite limited,” Nicol explained. “This method worked initially, but the machine eventually drifted and the accuracy of fixture location reduced. This impacted the quality of the machining, resulting in non-conforming parts. There was a lot of time wasted trying to get parts to come off the CNC accurately.”

The company had already tried subcontract machining without success. “We looked to see how we could improve the accuracy and repeatability of our own procedures,” said Nicol. “We were confident that improving the part location and ensuring the accuracy of the machine tool was key to minimising non-conformance, but were unsure of how to best put that into practice.”

Vert invested a large amount of time and money in investigations, with limited success, and the pressure on the machine shop grew.

The engineering team suspected the CNC machine. “We were seeing issues on different parts that were being produced on the same machine, which led us to believe the machine wasn’t working to the accuracy it should be,” said Nicol.

To test this, the team machined simple conical and cylindrical parts without the complex rotor form. The test parts showed an unexplained taper along the machine’s Z axis. Other tests showed run-out drifting and parts being machined off-centre.

“If the machine wasn’t able to produce simple forms accurately, it was never going to produce a more complex 3D-part perfectly,” Nicol said.

The team tried dial indicators, manual updates to the fixture datum, and the OEM’s kinematic check for the 5th-axis location. “It solved our machine performance to some extent, but because we were still having to manually shift datums, it wasn’t a suitable process for the high-precision parts we were machining,” said Nicol. “We knew we couldn’t get the parts to the right specification with the manual methods we were implementing so knew it was time to try something different.”

Solving months of trial and error in under ten minutes

MSP’s first step was to find out exactly what was wrong with the machine. MSP engineers installed NC-Checker, which assesses whether a machine tool’s geometric performance is accurate enough to machine a part to tolerance. The software uses 5-axis probing to complete a series of rotary and linear checks to highlight any errors with the probe, kinematics, or with the mechanical performance of the machine. It produces automatic Benchmark reports that show exactly where the machine is under-performing, so problems can be fixed before cutting begins.

“The benchmarking of the machine functionality was great as it instantly showed us where our machine inaccuracies were, and showed how accurate it could be when the machine was working to its full potential,” said Nicol.

With the machine’s geometry understood and corrected, MSP turned to automating Vert’s manual fixturing and alignment processes with NC-PartLocator. This would ensure skilled staff no longer had to align parts and shift datums by hand, and parts would be machined to the right tolerances. 

NC-PartLocator uses 5-axis probing to measure the part and automatically calculate how the part differs from the nominal CAD. The metrology software updates the controller, compensating for any misalignment, location or distortion errors to ensure the part is machined to tolerance.

“With the introduction of MSP’s 5-axis Best Fit Alignment software, not only have all the manual steps been removed, but we can now check everything before cutting the part. This has resolved many of the problems that previously resulted in non-conforming parts,” said Nicol.

An improved process with significant results

Using NC-Checker and NC-PartLocator as part of Vert’s processes, non-conformance fell from 25% to near zero. Part run-out, which had reached 120µm and was failing final inspection, fell to as little as 25µm, well within tolerance.

“The higher noise levels and lower performance we had been experiencing with the compressors stopped immediately once the MSP software had been implemented,” said Nicol.

Vert had put significant time and effort into machine investigations with limited success. With NC-Checker, any inaccuracies were pinpointed straight away. Nicol said: “Using NC-Checker to diagnose and rectify the issues on our machine tool has meant the machine parameters are the best we’ve ever seen. To have the confidence that our CNC machine tool is accurate and the parts it machines will be within tolerance and repeatable every time is invaluable.”

Significant reductions were made in part alignment, speeding up production. “We spent two months using manual trial and error methods: making a part, moving the datum, checking it and measuring it, and repeating until the part was within spec. Using MSP, this process takes under 10 minutes.”

Other benefits could be realised too. “Having MSP, and the automation it provides in aligning the part, reduces the number of personnel needed on the shopfloor. Senior engineers no longer need to be involved day to day - production can be successfully completed by any of our operators.”

The company was also able to keep rotor production in-house. “We strive to be experts in producing our rotors and by keeping our manufacturing capability in-house, it means we are in control of the learning that goes into making our parts accurate,” said Nicol.