BRIGHTSTAR
PROTOTYPE CNC CO., LTD
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May. 28, 2026
Have you ever encountered this situation? A precision CNC machined aluminum part, after hard anodizing, becomes difficult to assemble, threads will not engage, or bearings cannot be pressed in? You machined exactly to the drawing dimensions, so why are things wrong after processing?
This is not a machining error, but a characteristic of hard anodizing: it changes the dimensions of the part. Understanding the reason and magnitude of this change is an essential step when designing aluminum parts.
So, how much do hard anodized part dimensions actually change? Do they get larger or smaller? How should you reserve allowance for anodizing? Which features are most affected? This article will answer these questions in detail.
Before diving into dimensional changes, let us briefly review the basic principles of hard anodizing.
Hard anodizing is an electrochemical surface finishing process. The aluminum part is immersed in a low-temperature electrolyte solution with a relatively high current density, forming a thick, hard ceramic aluminum oxide film on the surface.
Compared to standard anodizing, hard anodizing has several distinctive characteristics:
The oxide film is much thicker. Standard anodizing film thickness is typically 5 to 20 microns, while hard anodizing film thickness can reach 25 to 150 microns, which is 3 to 10 times that of standard anodizing.
The hardness is much higher. Standard anodizing film hardness is about 250 to 350 HV, while hard anodizing film hardness can reach 400 to 600 HV, approaching the hardness of quenched steel.
Wear resistance is excellent. Due to its high hardness and porous structure that retains lubricants, the hard anodized film has outstanding wear resistance.
Corrosion resistance is good. The thick oxide film provides excellent corrosion protection.
Color is limited. Hard anodized films typically appear dark gray, gray-black, or tan, and are difficult to dye into bright colors.
Hard anodizing is commonly used for parts requiring high wear resistance and high corrosion resistance, such as cylinder walls, pistons, guide rails, hydraulic valve spools, and food machinery parts.
This is the core question. The answer is: Hard anodizing increases part dimensions.
Why does this happen? It comes down to how the oxide film grows.
During anodizing, the aluminum part acts as the anode. Under the influence of electric current, aluminum atoms are converted to aluminum oxide. This process involves volume expansion. Aluminum oxide has a lower density than metallic aluminum, so the same number of aluminum atoms occupy a larger volume.
Specifically, the oxide film grows bidirectionally: approximately one-third to one-half of the film thickness grows outward, and the other half grows inward.
This means: If you measure a part before anodizing, after anodizing, due to the outward growth, the external diameter will increase; due to the inward growth, the internal diameter will decrease.
Simple rule of thumb: external dimensions increase, internal dimensions decrease, threaded holes become tighter, and shaft journals become larger.
The dimensional change caused by hard anodizing depends on several factors: film thickness, material grade, electrolyte formulation, current density, etc. However, there is a general estimation method.
Basic estimation formula:
The total dimensional change is approximately one-half to two-thirds of the film thickness.
More precisely: for external surfaces (such as shaft journals, external diameters), the dimension increases by about 50% to 60% of the film thickness. For internal surfaces (such as holes, internal diameters), the dimension decreases by about 50% to 60% of the film thickness.
Example:
Assume you are hard anodizing a part with a film thickness of 50 microns.
For a 20 millimeter diameter external shaft journal, after anodizing, the diameter will increase by approximately 25 to 30 microns, becoming about 20.025 to 20.030 millimeters.
For a 20 millimeter diameter internal hole, after anodizing, the diameter will decrease by approximately 25 to 30 microns, becoming about 19.970 to 19.975 millimeters.
Typical dimensional changes for different film thicknesses:
At 25 microns film thickness, dimensional change is approximately 12 to 15 microns. This is typical for thinner hard anodizing, suitable for precision fitting parts.
At 50 microns film thickness, dimensional change is approximately 25 to 30 microns. This is the most common hard anodizing thickness, suitable for most wear-resistant applications.
At 75 microns film thickness, dimensional change is approximately 38 to 45 microns. Suitable for high-wear environments.
At 100 microns film thickness, dimensional change is approximately 50 to 60 microns. Suitable for extreme wear applications such as cylinder walls.
Important note: The above values are estimates. Actual dimensional change may vary depending on material, process conditions, part shape, and other factors. For critical fitting dimensions, it is recommended to verify through testing or confirm with your anodizing supplier.
Different part features have different sensitivity to dimensional change.
After hard anodizing, the major diameter and pitch diameter of external threads both increase. This causes the threads to become tighter, and thread gauges may not pass.
If the threads need to maintain fit, there are two solutions: one is to machine the threads slightly smaller before anodizing, reserving allowance for the increase; the other is to mask the thread area so it is not anodized. After masking, the threads maintain their original dimensions, but that area does not have anodized protection.
After hard anodizing, the minor diameter and pitch diameter of internal threads both decrease. This causes the threads to become tighter, and bolts may not screw in.
Solutions are similar to external threads: reserve allowance or masking.
Bearing mounting is very sensitive to dimensions. For interference fit bearings, if the shaft journal increases too much after anodizing, the bearing may not be able to be pressed in, or may be damaged during pressing.
For bearing mounting surfaces, the common practice is to mask these areas, or use a thinner oxide film (15 to 25 microns) and reserve allowance during machining.
Dimensional changes on sealing surfaces affect sealing effectiveness. For static seals (gaskets, O-rings), moderate dimensional change is usually acceptable. But for dynamic seals (oil seals, mechanical seals), dimensional change may cause leakage.
For seal grooves, after anodizing, the groove width decreases and the groove bottom diameter changes. These changes need to be considered during design.
Sliding mating surfaces such as guide rails and pistons are sensitive to dimensional change. After anodizing, the clearance becomes smaller. If the original clearance was small, anodizing may cause binding.
After anodizing, through holes become smaller in diameter. If the hole is for a bolt or dowel pin, allowance needs to be reserved. Dowel pin holes are the most sensitive to dimensions and are typically not anodized, or are masked.
Since dimensional change is unavoidable, how can you prepare for it during the design phase?
First, confirm with your anodizing supplier what film thickness they will apply. Different applications require different film thicknesses. Do not arbitrarily specify a value; determine it based on the part's operating environment and wear requirements.
Use the estimation formula mentioned earlier to calculate the expected dimensional change. For external surfaces, dimensions will increase, so the machined dimension should be slightly smaller than the target dimension. For internal surfaces, dimensions will decrease, so the machined dimension should be slightly larger than the target dimension.
For critical fitting dimensions, it is recommended to specify both the machined dimension (before anodizing) and the final required dimension (after anodizing) on the drawing. For example: shaft journal diameter to be machined to 19.970 to 19.980 millimeters, after hard anodizing diameter to increase to 20.000 to 20.015 millimeters.
This way, both the machinist and the anodizing supplier understand your intent.
If this is the first time producing a part, or if you are very sensitive to dimensional change, it is recommended to make a few test samples first. Measure the dimensions before and after anodizing, and adjust the machining allowance based on the measured data.
If certain areas cannot tolerate any dimensional change, they can be masked. Masking materials protect specified areas from being oxidized during the anodizing process. After masking, those areas maintain their original dimensions but also do not have the anodized protective layer.
Common masking methods include using PTFE tape, rubber plugs, dedicated fixtures, or peelable coatings.
Standard anodizing typically has a film thickness of 5 to 20 microns, with dimensional change of about 2.5 to 10 microns.
Hard anodizing typically has a film thickness of 25 to 100 microns, with dimensional change of about 12 to 50 microns.
The dimensional change of hard anodizing is 3 to 10 times that of standard anodizing. For precision fitting parts, if the high wear resistance of hard anodizing is not needed, choosing standard anodizing can reduce the issue of dimensional change.
Selection recommendations:
If the part requires extremely high wear resistance, or if the operating environment involves abrasive wear, choose hard anodizing, but manage the dimensional change.
If the part only needs general appearance protection and corrosion resistance, choose standard anodizing, which has smaller dimensional change and lower cost.
If the part has precision fitting requirements and cannot tolerate dimensional change, consider other surface finishing methods such as chemical conversion coating (conductive oxidation) or painting.
Q: Do hard anodized parts become larger or smaller?
External diameters become larger, internal diameters become smaller. Overall volume increases because the growing oxide film causes volume expansion.
Q: Can the dimensional change of hard anodizing be predicted?
It can be roughly predicted, but precise values need to be determined through testing. Different materials and different supplier processes may have variations.
Q: Can I hard anodize threads?
Yes, but allowance needs to be reserved. The common practice is: machine the thread, hard anodize, then run a tap through again (a process called "re-tapping") to restore the thread dimensions. Alternatively, mask the thread area.
Q: How do I measure the dimensional change after hard anodizing?
The most accurate method is: measure critical dimensions with high-precision instruments before anodizing and record them. After anodizing, measure the same locations with the same instruments and calculate the difference.
Q: Can hard anodized parts be machined again?
Yes, but it is not recommended. The hard anodized film is very hard. Secondary machining requires diamond tools and will damage the protective function of the oxide film. If secondary machining is necessary, only do it on non-critical areas.
Q: Does hard anodizing affect the use of thread gauges?
Yes. After anodizing, thread dimensions change, and the thread go gauge may not pass. If thread gauge inspection is required, it is recommended to mask the thread area, or perform re-tapping after anodizing.
This is a typical case study of hard anodizing application.
Requirement: An aluminum piston slides inside an aluminum cylinder. The piston needs hard anodizing to improve wear resistance. The cylinder bore also needs hard anodizing. The required fit clearance is 30 to 50 microns.
Design steps:
Determine film thickness: based on expected wear, choose a film thickness of 50 microns.
Calculate dimensional change: after anodizing, the piston external diameter increases by 25 to 30 microns. The cylinder internal diameter decreases by 25 to 30 microns.
Calculate total fit clearance change: since both mating surfaces are anodized, the total clearance decreases by 50 to 60 microns.
Design original machined dimensions: assume a target fit clearance of 40 microns. After anodizing, the clearance will decrease by about 55 microns. Therefore, the machined clearance before anodizing should be designed at about 95 microns.
Verification: machined clearance 95 microns before anodizing, after anodizing clearance about 40 microns, meeting requirements.
Without reserving allowance, machining directly to the target clearance would result in the piston seizing in the cylinder after anodizing.
Hard anodizing changes part dimensions. External diameters increase, internal diameters decrease, and the change is approximately one-half to two-thirds of the film thickness.
Understanding this characteristic and reserving allowance during the design phase is key to successfully applying hard anodizing.
For critical fitting dimensions, it is recommended to: determine the required film thickness, calculate expected dimensional change, specify before and after anodizing dimensions on the drawing, make test samples for verification if necessary, and mask extremely sensitive areas.
Brightstar provides professional one-stop services from CNC machining to hard anodizing. Based on your part requirements, we can provide dimensional change estimates and allowance recommendations, ensuring that after anodizing, the parts meet both performance requirements and dimensional requirements.
If you have questions about hard anodizing or need help designing allowance, please feel free to contact us.
Ready to Choose Hard Anodizing for Your Aluminum Parts?
Whether you need high-wear-resistance hard anodizing or need precise control over dimensional change, Brightstar can provide professional services.
Email Amy: amy@brightstarprototype.com
Call or WhatsApp: +86 13750105351
Send us your CAD files and drawings for hard anodizing recommendations and dimensional allowance solutions.
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