Custom solenoid development: from specification to control plan

A custom solenoid project usually goes wrong in the order of its documents rather than in its magnetics. The control plan has to be an output of the process flow and the failure mode analysis. When it is written from the characteristic list instead, it ends up controlling the 30 dimensions that are easy to measure and missing the one that decides whether the unit pulls 4 N at 3.5 mm.

The sequence that holds is short: functional specification, process flow, failure modes, special characteristics, control plan, capability. Each document answers a question the next one needs, and running them out of order produces a plan that is complete on paper and blind where it matters.

Why this happens

Each document answers a different question. The specification says what the part must do: 4 N at 3.5 mm of stroke, at 12 V minus 10 %, after 10 minutes at 85 °C. The process flow says how it is made, in what order, and which operation sets which dimension. The failure mode analysis says how each operation can go wrong and what the customer would see. The control plan says, for each of those failure modes, what is measured, how often, with what instrument, and what happens when it fails.

Reverse the order and the plan inherits the wrong subject. Start from the drawing and you get every dimension that a calliper or a CMM can reach, which for a tubular solenoid is typically 30 to 40 features. Start from the flow and you get a much shorter list, because only four to six of those features can actually cause a functional failure. The gap is the clearest example: it is not a dimension on any part. It is created at the staking station, when the washer is pressed into the housing, and it varies with press force and seating depth. A plan built from the drawing will inspect the housing depth and the washer thickness and never measure the result.

The physics sets how tight each control has to be. Force falls roughly with the square of the working gap, so 0.05 mm of extra gap costs about 27 % of the force at a 0.30 mm nominal. That single number ranks every characteristic in the plan. A bore diameter tolerance of 0.02 mm affects friction by a few percent; a staking depth tolerance of 0.05 mm affects force by 27 %. The control plan should show that ranking, and a plan that gives both the same inspection frequency has not done the analysis.

Capability is a property of the pair, process and tolerance. A process with a standard deviation of 0.015 mm holds a 0.10 mm tolerance at a Cpk near 1.1 and the same process holds a 0.15 mm tolerance at 1.7. Nothing changed on the floor. This is why a capability index below 1.33 is a statement about the tolerance as much as about the process, and why the honest response is sometimes to widen the drawing rather than to tighten the process.

Check these in order

1. Write the functional specification with four conditions attached to every number. Force at a stated stroke, a stated winding temperature, a stated supply voltage and a stated duty. A force figure without those four cannot be verified, and a supplier who quotes one without them is quoting a peak value measured at contact.

2. Draw the process flow before the drawing is released. List every operation in sequence, including the ones that look trivial: winding, taping, potting, staking, magnetisation, adjustment, test. Mark beside each operation which functional characteristic it sets. The staking operation usually sets two, and the adjustment operation sets one that nothing else can.

3. Run the failure mode analysis on the flow, not on the part. For each operation, ask what can vary, what the effect is at the function, and how it would be detected. A staking depth that is 0.06 mm shallow produces a gap 0.06 mm wide, which costs 30 % of the force, and the only detection before end-of-line test is a press curve that nobody records.

4. Convert only the high-severity, low-detectability items into special characteristics. In a typical tubular design that is four to six items: working gap, staking depth, spring preload, turns count, concentricity, and sometimes the plunger surface finish where friction scatter matters. Everything else is a normal inspection item.

5. Put the control at the operation that creates the characteristic. Staking depth is measured at the press, on 100 % of units, with a gauge resolved to 0.01 mm and a reaction limit at 0.03 mm from nominal. Measuring the same feature at final test, after potting, measures a result you can no longer adjust.

6. Prove the gauge before you trust the plan. Run a repeatability and reproducibility study on 10 parts, 3 operators, 3 trials. Below 10 % of the tolerance the gauge is usable, between 10 % and 30 % it is conditional and needs a defined operator, and above 30 % the number it produces is not a measurement of the part.

7. Establish capability on 30 consecutive units, not on the good ones. Plot the individual values and the range. A mean of 4.6 N against a 4.0 N requirement looks comfortable until the standard deviation puts the lower tail at 3.7 N. Design and control against the lower tail, and remember that the hot-state derating applies on top of it.

8. Verify the plan against a life test that measures force, not just function. A test that records pass or fail at 100,000 cycles tells you when the unit died. A test that records force at 0, 10,000 and 50,000 cycles tells you the drift rate, which is what the control plan needs to set an alert limit rather than a reject limit.

What actually to change

Cut the plan down to the characteristics that can cause a functional failure. A 40-item plan gets audited once and then ignored, because no line can inspect 40 features at the takt time. A six-item plan with a defined frequency, a named gauge and a written reaction plan is followed, and it is also the document a customer auditor will actually read.

Move inspection upstream to the operation that sets the value. End-of-line testing finds the defect after the cost has been added. A staking press with a load cell and a displacement sensor catches the same defect at the station that created it, on 100 % of units, with no additional labour. The press curve costs less than the scrap it prevents at 5,000 units a year.

Write the reaction plan with a quantity, not an instruction. “Stop and notify supervisor” is not a reaction plan. “Three consecutive units outside 0.03 mm, stop the press, quarantine everything since the last good check, re-verify the press with a master” is one, and it can be audited.

Widen a tolerance when the process cannot hold it and the function allows it. A 0.10 mm gap tolerance on a process with 0.015 mm standard deviation gives 1.1 Cpk and endless sorting. A 0.15 mm tolerance on the same process gives 1.7, and if the force margin at 0.15 mm still clears the requirement by 20 %, the wider tolerance is the better engineering decision.

Treat the control plan as a revision-controlled document. Every process change, every new press, every supplier change for the spring or the plunger is a reason to review it. A plan that has not been revised in two years is describing a process that no longer exists, which is the finding that turns a customer audit into a corrective action request.

When it IS the harder problem

The characteristic cannot be measured at the operation that creates it. A staking depth inside a potted housing is the standard case. There are two honest options: monitor a proxy at the press, such as the force-displacement curve at a fixed stroke, and correlate it with a destructive measurement on 5 units every 2 hours; or move the feature so it can be measured. A proxy with a correlation study behind it survives an audit. A proxy without one is a guess with a graph attached.

The drawing carries no special characteristics at all. Then the customer has delegated the entire functional risk to the process, and the control plan becomes the only place where the failure modes are recorded. In that situation the plan should carry the analysis, with the severity and detection rationale written beside each control, because the drawing will not defend the decision later.

The design is capable and the pilot lot is not. A 1.33 index on 30 units has a confidence interval wide enough to include 0.9. Before concluding that the process is incapable, check whether the pilot lot came from one cavity, one shift and one wire spool. Three lots of 100 units, taken across cavities and shifts, separate a process problem from a sampling problem.

The volume is 200 units a year. A full production part approval process costs more than the annual margin. The proportionate answer is a first article inspection on 5 units with every characteristic recorded, 100 % functional test at the minimum supply voltage, and a control plan limited to the four characteristics that the test cannot see. That is defensible, and it is honest about what is not being controlled.

FAQ

Does the control plan have to cover every dimension on the drawing? No. A plan that does is a dimension list. A custom solenoid typically has 30 to 40 dimensions and four to six characteristics that decide function, and only those need a frequency, a gauge and a reaction plan.

When is a Cpk of 1.67 required rather than 1.33? When the failure is safety-related or nothing downstream would detect it. A 1.33 index leaves about 63 parts per million outside the limits, which a 100 % end-of-line test will catch. Where nothing catches it, 1.67 is the usual requirement.

Can the control plan be written after the first production run? It can, but it will describe the process that happened to exist. The steps that created the variation will be missing, because nobody was watching them when the plan was written.

What is the minimum sample for a capability study? 30 consecutive units from one process stream, with a gauge study below 10 % of the tolerance. With fewer units the interval around the index is wider than the difference you are trying to detect.