Practical experience validating CNC Mills

Chrisx

Quite Involved in Discussions
I have seen a few posts on the forum regarding validation of CNC mills. Most discuss the general aspects of process validation, which is not particularly helpful. I think that most likely the critical process parameters are tool feed and speed. These parameters are different for every cutting tool and come from the tool manufacturer's recommendations. There might be 50 different tools, so this alone is no small task. Possibly, I can use retrospective data to satisfy the PQ. We should have data from parts that were cut with each tool that may show good process capability. However an OQ will be more challenging. Establishing process range for feed and speed on every cutting tool and for every mill will take the mills down for a signficant time. It's not likely that this is feasible. Possibly, I can establish a machine equivalence so that full validation is only required on one mill and an abbreviated validation on the other 20 mills. Still, the job seems daunting. Anyone else have any practical experience actually performing a CNC mill validation in a regulated medical device environment?
 

Funboi

On Holiday
My experience of machining tells me that its the repeatability of the positioning of the tool/workpiece since that can change due to wear, over time. This is why Renishaw offer products to perform validation checks. These dont check feed/speed, but geometric accuracy.
 

Chrisx

Quite Involved in Discussions
I'm a little familiar with the Renishaw ballbar. No doubt it is a valuable tool and I was considering this as part of the validation, but I wasn't sure wher it would fit in. Have you had any experience with FDA accepting this as process validation? Is it considered an OQ or a PQ? It does not seem like it would establish process control limits (OQ) or establish that the process consistently process a product that meets predetermined requirements (PQ). Can you explain how it meets the requirements?
 

Chrisx

Quite Involved in Discussions
Ballbar testing seems more like an IQ activity. It does not seem like an OQ or PQ. It seems like it is more of a verification of the mill calibration.
 

Chrisx

Quite Involved in Discussions
FDA recognizes the IMDRF guidance for process validation. The guidance defines OQ as "establishing by objective evidence process control limits and action levels which result in product that meets all predetermined requirements". Typically, this is performed by producing product at the low end and high end of the process limits to demonstrate that product meets specifications over the entire range of process control limits. That is why I brought up feed and speed in my initial posting. It seems like these might be the process control limits.

What process control limits does the ballbar test establish? How does it demonstrate that product meets specifications? The ballbar test calibrates that the spindle will move to the programmed location within a circle. It's certainly a good thing to calibrate that the mill accurately positions the spindle in the correct location, however I'm not certain I could explain how this meets the requirements of an OQ or a PQ.
 

Bev D

Heretical Statistician
Leader
Super Moderator
Umm. I’m a bit confused here. IQ, OQ and PQ validation is required for production equipment where the final result of the part cannot be verified by measurement. (In old-day language: a special process). What are you doing with a CNC mill in medical devices that constitutes a special process? Or has your Customer applied a blanket IQ/OQ/PQ validation for everything?

Another point: while feeds and speeds are the basic important factors in CNC machining, there are many others that can be critical and these vary from part to part and program to program. Angle of approach, type of material/material variations, fixtures, number of passes, how tools are adjusted during use, etc. can all create nonconforming parts…so if you are making parts where a characteristic can’t be verified then you are in dangerous territory.
 

Tidge

Trusted Information Resource
My $0.02

The ballbar test appears to me to be a (very likely) powerful risk control for certain failure modes for the CNC machine. I would expect that it be done as part of an IQ; I can imagine that the expected schedule of use for the CNC could benefit from regularly scheduled executions of the ballbar test outside of any process validation activities... similar to how folks who do inspections may regularly check their calipers against a set of gage blocks. The latter is not what would strictly be called "calibration", such an activity is more of a verification that the calibrated instrument has not become obviously uncalibrated.
 

Chrisx

Quite Involved in Discussions
Agreed, ballbar is a verification. If the result is out of specification, additional adjustments and possibly repairs would need to be made to the mill. Agreed, it is a valuable. In fact, one concern I have is how much value I will gain from validating a CNC mill to the IMDRF guidance. Unfortunately, I happen to know of at least one contract manufacturer that received a warning letter from not validating mills. They were also struggling to convince FDA that the ballbar test would be an adequate validation. I'm not sure if they won out in the end. I was hoping some other options might come up from the forum.
 
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