Your threads fit before anodizing — but seize once parts come back from coating. The supplier blames “design tolerance,” you lose days re-tapping or scrapping parts. In reality, most of these failures happen because the shop didn’t plan for coating thickness or skipped masking entirely.
Anodizing adds 0.002–0.005 mm per side, enough to close class-2 thread fits.
When suppliers don’t compensate or mask properly, internal threads shrink and externals gall. The problem isn’t the anodizing — it’s missing process control during prep.
Learn who’s at fault for anodizing fit issues, how to spot skipped masking, and how Okdor prevents thread problems with coating-ready process planning.
Table of Contents
Why Threads Seize After Anodizing — and Who’s at Fault?
Threads seize after anodizing because the oxide layer adds 4–10 µm total thickness that closes class-2 thread clearance — not because the design is wrong. The real fault lies in missing coating compensation and masking during supplier prep.
Few things sting more than getting parts back from anodizing and realizing the threads won’t fit. You lose days re-tapping or scrapping a batch while the supplier insists “the coating changed the tolerance.” In truth, thread seizure almost always starts before coating — when shops skip pre-coating adjustments and let the oxide layer close the clearance between mating threads. What looks like a design flaw is usually a planning failure — no masking, no oversize compensation, no verification after coating.
Many shops send parts to anodizers without noting class-of-fit or expected coating thickness. Type II anodizing typically adds about 0.002–0.003 mm per side, while Type III adds 0.004–0.005 mm. That 4–10 µm total growth is enough to seize a Class 2 thread or distort the minor diameter of fine-pitch threads. Once assembled, the oxide fractures and galls, making re-tapping risky.
We prevent this by integrating coating prep into the machining plan — applying pre-compensation offsets based on coating type and fit class, and defining masking zones in CAD/CAM to protect functional threads. Each batch is verified with calibrated gauges before shipment to confirm post-finish assembly.
Sourcing Tip: If a supplier blames anodizing for seized threads, the real cause is usually missing process control. Ask how they compensate for coating growth or confirm thread class after finishing — it’s the quickest way to separate capable shops from careless ones.
Why Suppliers Ignore Thread Allowance During Anodizing Prep?
Suppliers ignore thread allowance because it adds quoting time and setup cost, even though failing to plan for 0.002–0.005 mm per-side coating growth guarantees seized fits.
Many shops skip thread allowance because it slows quoting and adds setup complexity. Adjusting diameters or modeling oxide buildup requires coating data that most don’t track. Instead, they machine to nominal spec and rely on luck — hoping threads still fit after anodizing.
Job shops operating on thin margins avoid masking or offset machining since both reduce hourly throughput. Without a defined coating-allowance workflow, they can’t control post-finish fit or guarantee assembly torque values. The result is the same pattern: tight fits, seized fasteners, and re-tapping that damages the coating.
We build coating allowance directly into our process. Each anodized job includes a thickness compensation chart defining pre-machined diameters by alloy, anodize type, and fit class. We coordinate with anodizing partners on bath parameters and verify that coating growth stays within the planned range.
Sourcing Tip: Before placing an anodized-part order, ask your supplier one question — “What coating thickness do you compensate for in thread machining?” If the answer sounds uncertain, expect rework later. A reliable supplier measures and plans before sending parts to anodize, not after problems appear.
How Much Does Anodizing Really Add to Thread Size?
Anodizing increases thread dimensions by roughly 4–10 µm total — enough to tighten or seize a class-2 fit if not compensated. The oxide layer grows outward and inward equally, reducing internal clearance and swelling external threads.
Suppliers that treat anodizing as a “finish only” step overlook that even a few microns of growth exceed the clearance in fine-pitch or tight-fit threads. On small diameters, Type III hardcoat can close 10 % of total allowance — enough to turn a free-fit assembly into a seized one after coating. A single miscalculation here often means a 5- to 7-day delay for stripping and re-anodizing.
We measure every coating process and apply numeric offsets based on alloy and bath type before machining. Internal threads are cut slightly oversize; externals are undersized to maintain functional clearance after oxide growth.
Each anodized batch is confirmed with post-finish gauging to verify coating buildup against target thickness — not assumptions.
Sourcing Tip: Ask your supplier how much thickness they expect from their anodizer and how they verify it. If they “don’t track coating growth,” expect re-anodizing delays and scrap risk.
Supplier blamed anodizing for seized threads?
Get a second-opinion quote within 24 hours—upload your drawing and see what controlled coating prep really looks like.
Are the Threads Wrong — or Did the Supplier Skip Masking?
When anodized threads seize, the design is rarely wrong — it’s usually because the supplier skipped masking on functional surfaces. Unmasked threads trap oxide, tightening the fit and tearing when assembled.
Masking takes time and precision, which is why many low-cost shops avoid it. They treat thread masking as optional, even though any coating on functional threads changes pitch diameter by several microns. Once anodized, removing oxide from those areas destroys the surface and forces a remake — typically adding a week and up to 30 % cost.
Our standard workflow isolates threaded and mating areas with high-temperature silicone plugs or precision masking tape, recorded in each job’s process sheet. Masking boundaries come directly from the 3D model to prevent ambiguity at the anodizer.
Each masked part is visually inspected before shipment to ensure coating coverage matches drawing requirements.
Sourcing Tip: If a supplier blames your drawing for seized threads, ask for photos of masking before anodizing. Missing photos usually mean masking never happened — and recovery will cost another coating cycle.
Why Cheap Shops Skip Thread Masking (And Charge You for Rework)?
Low-cost suppliers skip thread masking to save minutes per part — but the time saved up front turns into a week of rework after anodizing.
Masking requires skilled labor, plug kits, and inspection. Job shops chasing volume often skip it to keep cycle times short. When coated threads seize, they bill you for “re-tapping” or scrap the batch — costing 30–40 % more than proper masking.
We include masking and allowance planning as part of normal process control, not an add-on. Threads that must remain bare are protected with dedicated fixtures or plugs, and coating buildup is logged for traceability.
By planning masking early, we eliminate both post-anodize galling and the risk of damaged coatings from re-cutting.
Sourcing Tip: If your quote looks unusually low, check whether masking is included. Hidden shortcuts often lead to lost weeks, re-anodizing bills, and schedule slips you can’t afford.
🧾 Supplier Comparison: Thread-Fit Reliability After Anodizing
Step | Typical Job Shop | Professional Supplier |
Thread prep | Nominal dimensions only | Pre-machined with coating allowance |
Masking | Optional / skipped | Mandatory on all functional threads |
Coating verification | None or visual | Measured ±2 µm gauge confirmation |
Post-finish fit check | Not performed | 100 % gauge inspection before shipment |
Result | Frequent seizure and rework (5–7 days lost) | Guaranteed fit, zero re-anodize delay |
Can You Save a Part With Seized Threads — Or Is It Scrap?
A seized anodized thread can sometimes be repaired, but re-tapping removes the oxide layer that gives anodized aluminum its corrosion resistance and wear strength. Once that coating is cut away, the surface behaves like raw metal—soft, reactive, and easily damaged in service.
Many engineers attempt to chase the thread with a tap after discovering a seized fit. The problem is that anodizing grows both inward and outward. Removing buildup on the flanks also cuts into the hardened oxide base, exposing aluminum and weakening every thread crest. What looks like a clean fix can actually create micro-fractures that propagate under torque or vibration.
A stripped thread can occasionally be salvaged by chemical de-anodizing and re-coating, but the process adds at least 5–7 days and often changes dimensions by several microns. On close-tolerance assemblies, re-anodizing usually shifts fit beyond drawing limits. For aerospace, medical, or sealing surfaces, most suppliers scrap these parts rather than risk latent failure in the field.
We’ve seen projects lose an entire week because shops tried to “save” seized threads instead of addressing the root cause—poor coating allowance and masking.
Sourcing Tip: If your threads seize post-anodize, pause before any rework. Measure coating thickness first; if it’s within spec, the problem lies in supplier planning, not design. Use that data to justify finding a shop that compensates for coating growth from the start.
3 Questions That Reveal If a Supplier Plans for Anodizing
Before sending drawings, three quick questions can reveal whether a supplier truly plans for anodizing or simply hopes coating won’t affect fit. Asking them up front prevents costly re-quotes or seized assemblies later.
- “What coating thickness do you compensate for in thread machining?”
A capable shop answers with specific numbers—usually 0.002–0.003 mm for Type II and 0.004–0.005 mm for Type III—along with how they model that offset in CAM. Vague replies like “not much” mean they don’t track coating growth. - “Do you provide masking photos or process sheets?”
Masking verification proves the supplier controls the finish. Professional shops document protected areas with plug photos or drawings before sending parts to anodize. - “How do you verify post-finish fit?”
Reliable manufacturers use class-specific plug gauges or coordinate-measurement data. “We test one part by hand” is not a process.
Suppliers that can’t answer these within a minute usually lack an internal coating-allowance workflow. That gap explains why they misquote anodized work or deliver assemblies that bind.
Asking these three questions takes less than a minute but can prevent a week of re-anodizing or scrapped parts.
Sourcing Tip: Add these questions to every RFQ checklist. They turn anodizing from a risk variable into a measurable competency—and make poor suppliers reveal themselves before you lose time or budget.
When Re-Tapping Damages the Anodized Layer (and the Part)?
Re-tapping anodized threads restores fit temporarily but destroys the hard, 40–60 HRC oxide film that protects the surface. Each cutting pass shears the brittle coating, leaving exposed aluminum prone to corrosion and galling.
Once the oxide is compromised, the part’s wear life drops sharply. In precision assemblies, retapped threads lose dimensional consistency because the tap follows the damaged profile rather than the original geometry. The result is uneven engagement, higher friction, and accelerated wear under load.
Technically, there are niche repair methods—thread-forming taps with minimal cutting or post-re-seal treatments—but results vary and rarely meet dimensional tolerance again. In high-spec projects, even one retap can create surface fractures invisible to the naked eye yet disastrous during torque testing. A single “fix” can turn a $200 component into a $2,000 field failure.
When suppliers suggest re-tapping seized threads, it often signals they lack preventive coating control. Instead of managing thickness and masking upfront, they rely on post-process repair that hides the root cause.
Our policy is simple: prevent the issue entirely through coating-aware machining, thread gauging before finish, and documented masking plans.
Sourcing Tip: Treat any suggestion to “just re-tap” as a red flag. It means the supplier is correcting symptoms, not systems. Switching to a shop that plans coating thickness from day one costs less than repairing damage after it happens.
Oversize Threads — or Work With a Shop That Plans for Coating?
Oversizing threads before anodizing can help, but without data-driven coating control it’s guesswork that often causes more rework than it saves.
Many engineers instruct suppliers to “cut threads slightly oversize” to offset anodizing buildup. That rule only works if the supplier knows the actual oxide thickness their anodizer produces. Most job shops don’t — they estimate, machine to arbitrary oversize values, and end up with loose fits after coating. What began as a prevention step becomes another tolerance problem.
True control means pairing oversize machining with measured coating growth and consistent masking practice. The offset must match both coating type and class of fit — a difference of just 2 µm can shift from Class 2B to Class 3. Without that precision, you trade seizure for backlash.
Our approach calculates oversize dimensions using recorded data from prior anodizing runs. Thread allowance tables are stored per alloy, bath type, and coating vendor, ensuring the same fit every time. Verification gauges confirm post-finish pitch diameter within ±2 µm of target — eliminating trial-and-error oversizing altogether.
Sourcing Tip: If a supplier claims to “just cut threads bigger,” ask what reference data they use. If they can’t cite numbers, they’re guessing. Working with a shop that models coating growth from past production ensures fit accuracy without repeated prototypes or schedule loss.
How We Prevent Thread Seizure — Before Anodizing, Not After?
Thread seizure prevention starts in CAD/CAM, not at the anodizer. The only reliable fix is a controlled workflow that predicts coating impact, protects threads, and validates fit before shipping.
Our process begins with a coating-aware machining plan. Each drawing is reviewed for surface class, tolerance callouts, and finish requirements. Thread features are labeled for masking or offset machining depending on coating thickness and fit category. This planning happens before the first toolpath is generated — so no surprises after finish.
During production, inspection gauges confirm pre-coating pitch diameters, and masking boundaries are documented in the traveler sheet. After anodizing, each lot undergoes post-finish gauge verification to ensure class-of-fit within ±2 µm. This two-stage inspection catches deviations early and removes guesswork from final assembly.
For urgent projects, our anodizing partners provide real-time bath thickness logs, allowing adjustments within hours. That control means threads remain functional on the first try — no re-tapping, no stripping, no schedule resets.
Sourcing Tip: The best way to avoid thread seizure is to choose a supplier that verifies fit before and after coating, not one that fixes parts after failure. Ask to see their coating-allowance workflow; if it doesn’t exist, your parts are already at risk.
Conclusion
Thread seizure after anodizing isn’t a design flaw—it’s a supplier control problem. We prevent it through coating-aware machining, verified masking, and post-finish gauging. Upload your rejected drawings today for immediate assessment—get a revised, anodizing-ready quote within 24 hours and keep your project on schedule.
Frequently Asked Questions
Not for precision parts. Re-tapping removes the 40–60 HRC oxide layer and exposes raw aluminum, cutting corrosion life by up to 80 %. The correct fix is coating-aware machining and masking that prevents seizure in the first place.
Ask for photos or traveler documentation showing masked threads before anodizing. Missing proof usually means masking wasn’t done. We record every protected area and verify coating boundaries prior to finish, ensuring no oxide forms inside functional threads.
Yes. Type II anodizing adds about 0.002–0.003 mm per side and Type III up to 0.005 mm. That 4–10 µm total buildup can tighten a class-2 fit enough to seize. Controlled shops plan for this thickness before machining so coated parts assemble smoothly without re-tapping or re-anodizing.
Absolutely. Upload your drawing through our portal. Our engineers perform a manufacturability and coating-allowance review, identify the root cause of the rejection, and return a revised quote and feasible machining plan—usually within one business day.
Every batch is gauge-checked within 24 hours of anodizing return. Pitch diameter must measure within ±2 µm of target before shipment—no visual guessing, no delays. This ensures coated threads meet fit class on the first attempt.
