We all make mistakes. Making mistakes is part of life, but when we’re working on mission critical parts for our customers at Swiss-Tech we know even small errors or variations in our manufacturing process could have major impacts on those parts. In addition to our robust planning process for onboarding new parts, we also take calculated steps to reduce operator measurement error and have implemented new data recording procedures to help limit opportunities for error.
When making parts, Swiss-Tech operators use various inspection frequencies and techniques to ensure critical dimensions and features meet customer requirements. Our operators use an Acceptance Quality Limit (AQL) of 0.65 percent, which means no more than 0.65 percent of the parts are defective on average over several production runs.
“Swiss-Tech utilizes C=0 sampling plans that are based on the premise of accepting the lot of parts if zero defects are found during the inspection and rejecting the lot if one or more defects are found during the inspection,” Swiss-Tech Quality Manager Dan Hancock said.
Swiss-Tech operators also use the Statistical Process Control (SPC) and 2-Bin methods to limit making any bad parts.
“Part of the set-up information tells operators how often they should be inspecting the parts and what methods they should be using,” Swiss-Tech 2nd Shift Supervisor Quentin Consoer said.
A critical time to measure parts is when it comes back from an outside process at another supplier.
“Parts are inspected after an outside process to make sure the manufacturing process that was just completed didn’t make a dimension go out of tolerance,” Consoer said.
During in-process inspection, operators make both variable and attribute measurements to make sure the part matches the product specifications. Variable measurements are those that give an actual number for the item being inspected, like using a ruler to measure a length. The operator knows exactly how close the part is to the optimum value. Attribute measurements use a gauge that tells the operator whether a part feature is good or bad, but not how bad. The “goodness” of it is left to the operator’s judgement. This type of measurement does not give as accurate a result.
“Attribute measurements are measured as pass/fail, like hole diameter tolerance being tested by a pin gauge. If the pin goes into the hole it passes. If it doesn’t, it doesn’t pass,” Hancock said. “Variable measurements are part features measured to an actual value.”
SPC Method Monitors, Controls
The SPC method is an analytical decision-making tool that allows Swiss-Tech to determine whether the manufacturing process is performing correctly or not.
SPC emphasizes early detection of any variations and prevention of problems rather than correcting problems after they have occurred, which is a big advantage over other inspection methods.
“In addition to reducing waste, SPC can lead to a reduction in the time required to produce the product,” Hancock said.
SPC makes it less likely the finished product will need to be reworked or scrapped because dimensional variations are monitored throughout the manufacturing process instead of just at the end. A dimension starting to go out of specification is caught before the dimension becomes out-of-spec.
Our operators follow the SPC process with tablets while at their machines. “The tablets on the machine make it easier for the operator to put in the data without running all over the shop looking for a computer,” Consoer said. “It also makes it quicker.”
When using the two-bin method, operators use two bins, one with parts confirmed as good and the other with parts still to be checked.
Process of inspection using 2-bin Method:
- Parts coming off the machine are collected into Bin 1 until a certain number of parts have been completed.
- Operators inspect the last part of that batch, if accepted, they move parts from Bin 1 into Bin 2.
- If the parts are rejected, they are set aside and all parts in Bin 1 are checked.
- Only good parts are placed into Bin 2.
When a part’s dimension is moving out of tolerance, operators make an offset on the computer to bring the dimension back toward the mean variation allowed.
“We are trying to change the tooling before it gets so bad that the parts need to be scrapped,” Consoer said.
Operators also verify measurements every time they change tooling to make sure dimensions with the new tool are not out of tolerance or making scrap parts.
Visually inspecting parts
Throughout all of these inspection processes but particularly during the 2-bin method, operators are also looking for visual flaws – dings, dents, burrs – in all of the parts.
A ding could be a chip mark from when the part was clamped down. Dents are usually caused by a handling problem from being bounced around. Burrs are a feather like chip sticking up on a dimension.
Final inspection is when a part has gone through all its manufacturing processes and the entire part is inspected to make sure it fits all of the part specifications. The AQL determines how many randomly selected parts in each lot will be inspected.
“When the final inspection team thinks the parts are out of spec, they put the parts on the Material Review Board cart to be checked by an engineer to see if it is really out of spec or not,” Consoer said.
The MRB is a team represented by quality, manufacturing, engineering and planning to review possibly nonconforming parts.
If the part is good it moves on to shipping, but if they are bad they are sent back with a non-conformance ticket to tell the operators what needs to be done with them. The MRB also initiates any actions to prevent possible recurrence of nonconforming parts in the future.
From introduction to final inspection, our Swiss-Tech team takes every step possible to make sure 100 percent of the parts you receive meet your expectations.