Outside Processing Vendors: Heat Treat, Plating, Grinding
Outside processing is where many precision machined parts quietly lose their schedule. A shop can hold a bore to tight tolerance all week, then watch a heat treat batch come back with distortion that pushes a critical diameter out of spec. Heat treat, plating, and grinding vendors sit outside your walls, but their results land on your part number and your on-time record. This guide covers the decisions that matter most, from callouts on the print to the vendor scorecard you review every quarter. Here is how we manage outside processing at SCPM.
Table of Contents
Quick Takeaways
Key Insight | Explanation |
|---|---|
Write heat treat callouts as ranges with a test location | "Harden per spec" gives the processor no target and gives you no basis for rejection. State the hardness range, where it is measured, and the case depth method. |
Set grind stock from distortion history | Use the movement you measured on the last several lots of the same part family, not a generic allowance table. |
Call out which plated surfaces are functional | Electroplate builds on edges, threads, and press fits alike. Mark the features that must stay in tolerance after plating, and state whether dimensions are pre-plate or post-plate. |
Verify bake steps on high-hardness plated parts | Hydrogen embrittlement does not show up at receiving. Put the bake time and temperature on the traveler and keep the record with the lot. |
Quote outside time from realistic vendor history | Queue time at a busy heat treat house can add a week without changing the stated cycle time. Quote from observed turnaround, not the sales estimate. |
Keep a second source qualified for critical processes | A backup vendor needs one trial lot a year to stay credible. Without it, a single vendor problem becomes a shipment hold. |
Why Outside Processing Decides Delivery
Most shops treat outside processing as one line on the traveler. In practice, it is where the part changes the most after you stop controlling it. Heat treat moves dimensions, plating adds material, and grinding removes what distortion left behind. Your machining plan has to assume all three.
A typical chain for a hardened steel shaft runs rough machining, stress relief, heat treat, straightening, finish grinding, plating, and final inspection. Each handoff adds days or weeks, and each one can change the number you measured on your own floor. If your quote only includes machining hours, you are underpricing the job and overpromising the date.
The most expensive mistake we see is treating outside operations as a black box. A part comes back, fails a bore on the CMM, and everyone argues about whose fault it was. The fix is simple in concept: know what each vendor does to the part, document it, and inspect against the post-process condition, not the condition you wished you had.
Every outside operation changes the part. An inspection plan that ignores those changes is measuring a part that no longer exists.
Delivery is also affected by how outside processors batch work. Their queues do not care about your customer deadline. We plan outside time as a fixed block in the schedule rather than a floating buffer, because a buffer gets consumed by the first busy month.
Pro tip: Put the outside process sequence on the routing in the same order it happens on the floor. A router that shows heat treat after grinding will send your planner and your vendor in opposite directions.
Heat Treat Specifications That Prevent Rework
The most common heat treat problem we see is not the process, it is the callout. A drawing that says "harden per spec" gives the processor no target and gives you no basis for rejection. Specify the hardness range, the test method and location, the case depth if applicable, and the quench and temper requirements you actually need.
Hardness and Case Depth on the Drawing
Hardness should be a range, not a single number, and it should name the test location. A surface reading on a case hardened part tells you nothing about core hardness, and a core reading tells you nothing about the case. If you need effective case depth, state the test method and the hardness that defines the boundary, because processors use different conventions.
Also name the lot sampling rule. A witness coupon from the same load is inexpensive evidence. Without it, you are accepting a certificate that describes furnace setpoints rather than the parts in your box.
Distortion: Where It Comes From
Distortion is partly material, partly geometry, and partly process. Long thin sections, asymmetric holes, and uneven wall thickness move more than blocky parts. Stress relief before finish machining helps, and so does leaving enough stock for the grind to clean up what the furnace did.
When distortion keeps showing up on the same part number, do not blame the heat treater first. Ask whether rough machining left uneven stock, whether the part was fixtured in a way that trapped stress, and whether the quench method changed. In our experience, the machining sequence causes more repeat distortion than the furnace does, even though the furnace is the easier suspect. A good starting point for root cause is the ASM chapter on problems associated with heat treated parts, listed in the references below.
Plating and Coatings Without Surprises
Plating is the easiest outside operation to ignore on a print and the easiest to regret. Electroplated layers build on every exposed surface, including edges and threads, and that buildup changes fit. A bore that measures comfortably before plating can close up enough to bind after it.
Thickness Buildup and Fit
Call out whether each dimension applies before or after plating, and specify the thickness range by surface. A blanket note like "zinc plate per spec" leaves the plater guessing which surfaces to build and which to mask. Masking adds cost and handling, so call it out only where the function needs it.
Threads and press fits need special attention. A common solution is to specify a pre-plate size with a plating allowance, then verify the post-plate gauge. Do not assume the plater knows which features are functional. Mark them on the print with a note and a sketch.
Hydrogen Embrittlement and Baking
Electroplating high hardness steel carries hydrogen risk. If the part is hard enough to be sensitive and the process can charge the steel, a bake is usually required soon after plating. The bake time and temperature depend on material, hardness, and the governing specification, so write them on the traveler rather than leaving them to the plater's habit.
The mistake to avoid is treating the bake as optional because the part looks fine. Embrittlement can appear later, during assembly or in service, and it is a hard problem to explain to a customer. Verify the bake step in the paperwork and keep the record with the lot.
Grinding After Heat Treat
Grinding is the operation that rescues dimensions after heat treat, so it needs stock planned from day one. If rough machining leaves too little material, the grinder cannot clean up distortion, and the part becomes scrap or a deviation request. If it leaves too much, you lose time and risk burning the surface.
Stock Allowance for Post-Heat Grind
Set the stock allowance from your distortion history, not from a generic table. If a part family moves a few thousandths on a diameter, leave enough per side to clean up that movement plus the wheel's removal tolerance. Track the actual stock removed on the first few lots and adjust the rough machining plan when the numbers say so.
Share that allowance with your heat treat and grind vendors. A grinder who knows the pre-grind stock can plan the wheel and dress cycle, and a heat treater who knows the target can adjust fixturing and quench orientation.
Burn and Surface Checks
Grinding burn damages surface metallurgy and can create cracks that stay invisible until the part is loaded. Require a surface check on the grind operation, using a method your customer accepts, on the first article and at a defined frequency. State the method in the process specification, not just in conversation.
Also confirm that the grinder's paperwork includes final dimensions, not only a pass stamp. A certificate that says "conforms" gives you no way to trend stock removal or spot a drifting wheel.
Pro tip: Ask your grinder to record pre-grind and post-grind dimensions on the same sheet. The gap shows how much material the heat treat actually moved, and your rough machining team can use that data next quarter.
Qualifying and Choosing Outside Vendors
Choosing an outside vendor is a qualification exercise, not a price comparison. A processor can offer a good rate and still have no control over the parameters that matter to your part. Start with the process, then the capacity, then the paperwork, and only then the rate.
Certifications and Process Audits
Ask for the accreditation scope, not just a certificate on the wall. A heat treat certification may cover some furnaces and some specifications but not the ones you need. Confirm that the scope lists your material, hardness range, and process specification. If the vendor cannot show that, treat the certificate as a sales document.
A process audit is more useful than a paper audit. Walk the furnace area, look at load documentation, and ask how they handle a nonconforming lot. The answer tells you more about their discipline than any brochure will.
Trial Lots and Witness Coupons
Run a trial lot before you move production. Send a small batch with witness coupons or sacrificial parts, measure everything before and after the process, and compare the results against your own data. Two or three trial lots will expose more than a month of reading certificates.
Coupons are cheap if you plan them. Build a coupon into the fixture, or send a bar from the same heat when possible. A coupon that proves hardness on the same load is worth more than a certificate written after the fact.
The sourcing model you pick determines where problems hide. The table below compares three approaches we see in practice.
Sourcing Model | Where It Works | Where It Breaks |
|---|---|---|
Single prime processor coordinating heat treat, plating, and grinding | Fewer handoffs and one owner for paperwork and scheduling | One vendor failure stops every operation, and sub-processor visibility is often weak |
Direct relationships with a separate heat treater, plater, and grinder | Better pricing per step and direct control over each specification | More traveler handoffs and more places for mismatched specifications to slip through |
In-house grinding with outsourced heat treat and plating | Tight control over critical dimensions and faster rework turnaround | Capital and skill requirements, and plating usually stays outside anyway |
Routing, Traveler, and Lead Time Control
Outside time is the part of your schedule you control least and quote most often wrong. Quotes that show machining days but not heat treat batch cycles, plating queue, and inspection return time set the customer up for a late ship. Build outside time into the quote using the vendor's actual turnaround, not the optimistic number from the sales call.
Building Outside Time Into Quotes
Break outside lead time into three parts: transport to the vendor, queue time at the vendor, and return transport plus your receiving inspection. Queue time moves the most. A heat treat house with a full furnace schedule can add a week without changing its stated cycle time.
Keep a rolling log of vendor turnaround by process and by month. Quote from a conservative point in that history rather than the average, because the average hides the batch that sat for ten days. One bad month is usually enough to change how we quote the following quarter.
Traveler and Paperwork Discipline
The traveler is the only document that follows the part through every outside process. Show each operation, the vendor, the specification revision, the required paperwork, and the return condition. If the traveler is vague at the heat treat step, the vendor will improvise, and improvised processing is how a good part gets scrapped.
Keep one owner for every outside move. When two people can send a part out, it leaves twice, and nobody knows which order it is on. Assign one named person to release outside work and one to receive it back.
Inspection and Documentation at Receiving
Receiving inspection is where outside processing either proves itself or fails quietly. A part can look right and still be wrong in ways that matter, such as a hardness reading taken on the wrong surface or a plating layer that fills a thread. Inspection after outside processing should be designed around what each process can change.
Receiving Inspection for Processed Parts
Start with the paperwork check: specification revision, lot number, process dates, and any deviation notes. Then measure the features the outside process can move. For heat treated parts, that means hardness and critical dimensions after distortion. For plated parts, it means thickness, fit gauges, and bake verification.
Do not rely on a single measurement at receiving to tell the whole story. Sample the lot, record the readings, and trend them. A slow drift in hardness or a creeping increase in plating thickness is easy to miss on any one part and easy to see on a control chart.
Certs, PPAP, and First Article Impacts
When you run PPAP or first article inspection, the outside processes belong in the plan. Include the heat treat and plating specifications, the results from trial lots, and the control method you will use in production. A customer will ask how you know the processed part meets the print, and a vague answer costs more than a detailed one.
Changing a vendor or a process usually triggers a review of the inspection plan. Plan for that before you switch. A new heat treat house may need a fresh trial lot and updated control records, even when the part drawing never changed.
Keeping Vendor Performance Honest
A vendor who is good in month one can slide by month six. Staff changes, equipment problems, and volume spikes all affect outside processing, and most of them show up first as small misses. The job is to see those misses early and act before they become a shipment hold.
Scorecards That Track the Right Numbers
Track on-time return, first-pass acceptance at receiving, and nonconformance count by process. Those three numbers show whether the vendor is hitting the date, the specification, and the paperwork. Resist adding ten metrics you will never review.
Review the scorecard on a fixed schedule with the vendor, not only when something breaks. A monthly call with three charts is more useful than an angry email after a bad lot. Bring specific part numbers and specific readings to the call so the discussion stays on the process.
Escalation and Second Sources
Define what triggers escalation before you need it. A single hardness miss may be a sampling issue, while two misses on the same load point to the furnace. Write the response into the vendor agreement: containment steps, root cause timing, and what happens to parts already shipped.
Qualify a second source for any process that stops your shipments when it fails. The backup does not need the same volume. It needs one trial lot a year so you know the process still works when the primary vendor cannot take the work.
Pro tip: Keep the second source's paperwork current even when they receive no work. A qualification record that is two years old is a delay you will pay for during an emergency.
Frequently Asked Questions
What should I put on a drawing for heat treat?
Specify the hardness range with the test location, the case depth method if applicable, the sampling rule, and the quench and temper requirements. Reference the exact specification revision so the processor and your inspector work from the same document.
How much grind stock should I leave after heat treat?
Base the allowance on the distortion you have measured for that part family, then track the actual stock removed on the first few lots. Adjust the rough machining plan when the data shows the allowance is consistently too large or too small.
Do all plated parts need a hydrogen bake?
No, but high hardness steel plated in an electrolytic process is at real risk. Check the material, hardness, and governing specification, then put the bake requirement on the traveler and verify it in the paperwork before the lot ships.
How many trial lots should I run before moving production to a new vendor?
For most critical parts, two or three trial lots with witness coupons, plus a process audit, is a reasonable starting point. Run more if the part is safety critical or if the first lots show scatter in the results.
How can I shorten outside processing time without cutting corners?
Quote from realistic turnaround history, batch parts with the vendor to reduce queue time, pre-approve paperwork so it does not hold release, and remove unnecessary handoffs. Do not shorten inspection to save time, because inspection is the step that catches the problems outside processing creates.
What outside processing problem has cost your team the most time, and how did you solve it?
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