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Fastener Sorting

You might be surprised!  Is 100% laser inspected good enough for you?  It sure sounds good! Reality may be somewhat different from perception.  There really are a lot of different sorting techniques and machine styles resulting in a wide variation of risk based on how comprehensive the 100% sort really is.  After all 100% inspection is simply a risk management tool.  The degree of risk can be measured objectively by weighing a number of factors including part simplicity, degree of process control in the manufacturing process and most importantly sort criteria.


Part simplicity
The more features that a part (rivet, screw, stud, etc.) has, the higher the risk of out of specification dimensions or tool wear or breakage causing assembly affecting deformations.  Part simplicity may also be affected by tolerancing on the print.  A very simple straight pin is not a simple straight pin if it has a .002mm diameter tolerance over a 50mm length!  The same pin with a .2mm diameter tolerance is a much simpler part.  A less simple part may require a less simple sort or inspection process. 


Degree of process control
Process control can be done through in process inspection, spc documentation, and/or electronic process monitors.  If none of these steps are being taken, the risk of bad product getting into the good parts bin is fairly substantial.   That is why the majority of today's cold heading machines are equipped with short feed detection, and electronic process controllers.  Even the controllers themselves depending on the make and model can pick up the slightest defect if properly set.  Having a quality process monitor and short feed detection reduces risk and, depending on the part, may reduce the complexity required at sort/inspection.


Sort Criteria

Many customers demand 100% inspection or 0 defect.  The question that needs to be answered is "What are you sorting for?"  A single camera machine can sort dimensionally for numerous defects but can only see one side of the part.  A four camera machine can sort 360 degrees for numerous dimensions but if you did not set it up for length under the head it will still accept under and over length parts.


We have discussed some of the things that we know from a customer print might determine sort technique or machine required.  What about information that you don't always get from the customer print.  For instance, the degree of safety built in to the assembly (safety critical usually shows up on the final rev pre-production), the feed technique used in assembly or the fail safes built into the customer’s assembly.


Degree of safety built into the assembly
If the assembly holds a child safety restraint and the part requires heat treat to grade 10.9 a simple pin point laser inspection may not be suitable to ensure that every part is the right hardness.  Failure here leads to possible loss of human life and legal exposure is extreme.  Eddy current sorting should always be built into safety critical components requiring hardening.  It is critical to gather information such as this at the apqp (advance product quality planning) meeting prior to establishing the sort portion of the bill of materials.  Safety critical components require extra attention when determining the sort/inspection process.


Feed technique used in assembly
While this is a lesser point in determining sort technique it does merit consideration.  Product that is bowl fed vs. hand fed (at assembly) is typically more susceptible to foreign material issues.  1 foreign part in 1000 pcs on a bowl fed machine will cause frequent jam ups and downtime instantly raising red flags.  1 foreign part in 1000 pcs on a hand feed assembly is likely to be felt and discarded instantly without disruption to the line and less urgency for action. 


Fail safes built into customer assembly
Believe it or not your customer's management of risk in assembly reduces or increases your level of risk depending on how good of a job they do.  If they put into place fail safes that catch a potential defect from you, your exposure does not go beyond their plant.  If, however, your bad part gets to their customer the **** hits the fan big time!  While the process FMEA is designed to address this, it is rarely (probably never!) done in conjunction with the supplier as a team working to anticipate any potential problem.  You should take some time to see how your customer is working to reduce your risk.


Finally, while a determination of risk will help in selecting the right sort/inspection process, the simple geometry of the part will often limit the choices  by the nature of how well or stable the part is introduced to the sorting station. Very short parts with head diameters close to their overall length will not slide well down an inclined v-track machine without tumbling.  They must be run on glass rotating tables, indexing tables or magnetic belt sorters.  Similarly, studs that have a large washer in the middle often wobble down the v-tack causing instability and a high good part rejection rate.  These parts can't rest on their head for a glass table top and are typically unstable on a magnetic belt so they must run on an indexing type table machine.


Conclusions for fastener manufacturers
You have probably figured it out already if you are in this business!  One sorting machine isn't enough and no matter how many machines you do have there is always one or two or a series or parts that you can't do a proper sort on.  The tendency is to take the "that's the best we can do" approach.  Remember when you take that approach to measure the risk!  Find a good outside sorter with complimenting capabilities and pay the piper!  Once your volume of a specific type of parts warrants bringing the sort inside, it will be very easily cost justified and your capabilities will be broadened.


Conclusions for the OEM's and assembler's
I know that I have been talking for the most part about the fastener manufacturer managing risk but let's face it.  The more risk your fastener manufacturer takes on, the more risk you take on!  Get together with your fastener manufacturer (as a team not a grumpy dictator!) and do a part by part risk assessment.  If it seem’s  like your fastener manufacturer is trying to pull the wool over your eyes then you should be the grumpy dictator.  Real big job?  Then start with the parts that are most important to you and your customers.  If you sweep the dirt under the rug someone will come along eventually and pull the rug right out from under you!  

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Tim Brennan
December 11, 2017
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