CHM machining explained with process steps, uses and limitations

What CHM machining means

CHM machining is commonly used as an abbreviation for chemical machining, a nontraditional subtractive process that removes material through controlled chemical dissolution. Instead of pushing a cutting edge through metal, the process protects selected areas with a maskant and exposes the remaining areas to an etchant. In manufacturing, CHM is most often discussed alongside chemical milling, chemical blanking and photochemical machining. It belongs to the wider family of machining processes, but its value is different from turning, milling or grinding: it can machine thin, delicate or hard-to-cut metal sections without cutting forces, tool wear, burr formation or a heat-affected zone. The trade-off is that dimensional control depends heavily on chemistry, masking, time, temperature and undercut management.

For engineers comparing process options, the practical question is not whether CHM can remove metal. It can. The better question is whether the part geometry, material, tolerance, production volume and environmental controls make chemical machining the right route compared with CNC machining, laser cutting, EDM, ECM, stamping or waterjet cutting.

How the chemical machining process works

Equipment details vary by shop and material, but the basic CHM sequence is consistent in manufacturing handbooks such as ASM Handbook Volume 16 and in photochemical machining references. The process converts a drawing or pattern into protected and exposed surface areas, then allows the exposed areas to dissolve at a controlled rate.

1. Cleaning and surface preparation

The workpiece must be cleaned before masking. Oils, oxides, fingerprints and shop contamination can prevent maskant adhesion or cause uneven etching. Poor cleaning may lead to local over-etching, weak edge definition or unexpected pinholes. This step is less visible than the etch itself, but it is one reason chemical machining quality depends so much on process discipline.

2. Maskant or photoresist application

A maskant is a chemically resistant coating that protects surfaces that should remain unchanged. For simpler chemical milling work, maskants may be applied and selectively removed by a template, scribing or peel operation. In photochemical machining, a photoresist is exposed through artwork or direct imaging so fine patterns can be developed with repeatable detail. The maskant and etchant must be compatible; otherwise, the chemical that attacks the metal may also attack the protective layer.

3. Etching by controlled chemical dissolution

The exposed metal is brought into contact with an etchant, commonly an acid or alkaline solution selected for the alloy. Common industrial examples include ferric chloride for several steels, copper alloys and nickel alloys, with other chemistry used for aluminum, magnesium or titanium. These examples are not processing instructions. Actual chemistry depends on alloy grade, shop controls, safety rules and environmental permits.

Etch rate is influenced by concentration, temperature, agitation, exposure time, dissolved metal loading and surface condition. A 2026 review in Canadian Metallurgical Quarterly describes these variables as central to material removal rate, roughness, edge deviation and dimensional accuracy in photochemical machining. In short, CHM is not simply dipping metal into chemicals; it is a controlled manufacturing process.

4. Rinsing, stripping and inspection

After the target depth or cut-through is reached, the part is rinsed, the maskant is stripped, and the surface is cleaned. Inspection may include dimensional checks, edge assessment, surface finish review and confirmation that critical features have not been over-etched. Because many CHM defects originate before etching, inspection often feeds back into cleaning, artwork compensation and etch-time control.

Common forms of CHM machining

The term CHM machining can refer to several related processes. They all use chemical removal, but they differ in geometry, tooling approach and part intent.

• Chemical milling: Used to reduce thickness or create shallow pockets, tapers or weight-reduction features. It is historically associated with aerospace structures where removing material without mechanical distortion can be useful.
• Photochemical machining: Also called PCM, photo etching or chemical blanking. It is typically used for thin sheet-metal components with fine, repeatable patterns, such as screens, shims, gaskets, springs, diaphragms, lead frames and precision foils.
• Chemical engraving: Used for shallow marking, texturing or decorative features rather than deep material removal.

The distinction matters. A buyer asking for chemical milling on a thick structural panel may need different process controls than an engineer specifying photo-etched stainless steel shims. Both may fall under chemical machining, but their cost drivers and tolerance risks are not the same.

Where CHM machining fits best

CHM is strongest when the geometry is relatively thin, flat or shallow and the part benefits from low mechanical stress. It is especially attractive when a pattern would be slow or tool-wearing to produce mechanically, or when a sheet component needs many internal features without punch tooling.

Typical fit areas include:

• Thin sheet and foil parts: Fine screens, filters, meshes, encoder discs, shims and precision gaskets can be good candidates.
• Delicate components: Springs, diaphragms and thin webs may be damaged by clamping or cutting forces in conventional machining.
• Hard or work-hardening alloys: Since chemical removal is not based on cutting force, hardness is not the same barrier it is in milling or drilling. The material must still react predictably with an available etchant.
• Parts requiring burr-free edges: Chemical machining can avoid the burrs created by shearing, punching or mechanical cutting, although edge taper and undercut still need attention.
• Prototypes with changing flat patterns: For some thin-sheet parts, changing artwork can be faster than manufacturing new hard tooling.

CHM is usually a weaker fit for thick blocks, deep cavities, high-aspect-ratio vertical walls, threaded features, precision 3D surfaces or parts where chemistry compatibility is uncertain. It can also be a poor choice if the supplier cannot demonstrate strong control of masking, etchant chemistry, waste handling and dimensional compensation.

Advantages and limitations that affect process selection

Chemical machining is often described as stress-free and burr-free, but those benefits should be weighed against practical limits. The most important technical limitation is undercut.

Key advantages

• No cutting forces: Thin or fragile sections are less likely to bend from machining loads.
• No tool wear in the conventional sense: The pattern and chemistry perform the removal, so there is no cutting edge to dull.
• No heat-affected zone: Unlike laser cutting or thermal processes, wet chemical etching does not rely on localized melting.
• Burr-free material removal: Properly controlled CHM avoids mechanical burrs, which can reduce secondary deburring on thin parts.
• Pattern flexibility: In photochemical machining, complex flat patterns can often be revised through artwork changes rather than hard-tool changes.

Main limitations

• Undercut and edge taper: Wet chemical etching is generally isotropic, meaning it removes material sideways as well as downward. This makes perfectly vertical walls difficult, especially as thickness increases.
• Depth control is chemistry-dependent: Concentration, temperature, agitation and dissolved metal content can change the etch rate.
• Masking defects can become part defects: Pinholes, poor adhesion or registration errors may transfer directly to the workpiece.
• Material compatibility is not universal: Some materials require aggressive etchants, and some combinations create safety, surface quality or disposal challenges.
• Environmental management is part of the process: Spent etchant, rinse water and dissolved metals must be handled responsibly.

Open University Manupedia notes that photochemical machining can struggle to produce parallel-sided holes because of undercut. That point is central to design for CHM: the artwork may need compensation, and the finished edge geometry should be specified realistically rather than assumed to match an ideal CAD wall. See also: CNC Machining.

CHM machining compared with nearby processes

The following comparison is a practical selection guide, not a universal ranking. The best process depends on thickness, alloy, tolerance, edge quality, volume and post-processing requirements.

Process	Where it is strong	Where CHM may be better	Where CHM may be worse
CNC milling	3D surfaces, pockets, threads, rigid workholding and tight machined features	Thin sheets, burr-sensitive parts and delicate webs	Deep precise features, vertical walls and complex 3D geometry
Laser cutting	Fast profiling, thick-to-medium sheet and automated nesting	Parts where heat-affected zones or recast edges are unacceptable	Thicker parts where laser speed and edge control are already proven
Wire EDM	High accuracy in conductive hard metals and thick profiles	Very thin flat parts with many repeated openings	High-precision thick profiles and features needing very accurate straight walls
Stamping or punching	High-volume production after tooling is justified	Low-to-medium volumes, changing designs and very fine flat patterns	Very high volumes where hard tooling cost is amortized
Electrochemical machining	Conductive metals, no tool contact and complex shaped electrodes	Thin flat sheet patterns and photo-defined features	3D cavities and shapes better made with a shaped cathode

In process selection, CHM should be treated as a precision sheet and surface-removal method, not as a direct replacement for every cutting process. It wins when its lack of force, lack of burrs and pattern economy matter more than perfectly square walls or deep 3D accuracy.

Quality, safety and environmental factors

Good CHM results depend on process control. Engineers and buyers should ask how the supplier controls artwork compensation, maskant adhesion, bath chemistry, bath temperature, agitation, dissolved metal content, etch time and inspection. For repeat work, the supplier should be able to explain which variables are monitored and how process drift is corrected.

Safety and environmental controls are not secondary details. OSHA guidance for wet chemical etching highlights potential worker exposure to acids in etching operations, while EPA metal-finishing guidance treats chemical etching and related wastewater streams as regulated industrial concerns in relevant U.S. contexts. The exact obligations depend on location, chemistry, discharge route and facility operations, so manufacturers should rely on safety data sheets, local permits and qualified environmental guidance.

From a manufacturing economics standpoint, chemical management also affects cost. Research on ferric chloride use in industrial photochemical machining has shown that etchant regeneration can significantly improve chemical utilization compared with disposal-and-replacement practices. For a buyer, this does not mean every CHM supplier must use the same recovery system, but it does mean chemical control and waste strategy can influence price, consistency and sustainability claims.

Design checklist before choosing CHM machining

Before specifying CHM, review the part against the process limits rather than treating chemical machining as a generic cutting method.

• Is the part primarily flat, thin or shallow enough for controlled etching?
• Does the alloy have a known, controllable etchant system?
• Can the design tolerate edge taper or compensated undercut?
• Are critical dimensions realistic for the material thickness and feature spacing?
• Will both sides be etched, or is one-sided material removal required?
• Are burr-free edges, low stress and no heat-affected zone more important than square walls?
• Can the supplier document cleaning, masking, bath control and inspection methods?
• Are waste treatment, chemical handling and operator safety controls appropriate for the chemistry?

If several of these answers are uncertain, CHM may still be possible, but it should be validated with trials, coupons or supplier design review before production assumptions are made.

Frequently asked questions

Is CHM machining the same as chemical milling?

Not exactly. Chemical milling is one form of chemical machining, usually associated with controlled depth removal over selected areas. CHM is the broader term and can also include photochemical machining, chemical blanking and chemical engraving.

Does CHM machining work on all metals?

No process window should be assumed for all metals. Many metals and alloys can be chemically machined, but the etchant, maskant, surface preparation and waste controls must be matched to the material. Some alloys require aggressive chemistry or tighter controls than others.

Why is undercut important in chemical machining?

Undercut occurs because the etchant removes material laterally beneath the mask as well as vertically into the workpiece. It affects hole size, wall shape, feature spacing and tolerance. The thicker the material or deeper the etch, the more important undercut compensation becomes.

Is CHM better than laser cutting?

It depends on the part. CHM can be better for thin, delicate, burr-sensitive parts where thermal effects are undesirable. Laser cutting can be better for speed, thicker sheet, automated profiling and applications where a heat-affected edge is acceptable or can be finished afterward.

What information should be included in an RFQ for CHM machining?

Include material grade, thickness, required tolerances, critical dimensions, desired edge condition, finish requirements, annual volume, inspection requirements and any restrictions on chemistry or residual contamination. If the part has fine openings or close spacing, provide the CAD data and identify which features are functionally critical.