Can You Cut or Drill After Plating? Process Sequence and Cut-Edge Protection

한국어 · English · 简体中文 · Tiếng Việt

Innozinc with Assembly in Mind · Part 5

Drilling an extra hole in a plated bracket or cutting a pipe to fit a site dimension exposes fresh metal on the cut face and inside the hole. Even if the part passed its plating inspection before machining, protection for these newly exposed surfaces still needs to be specified.

At the purchasing stage, agree on which operations will be completed before plating, which will remain after plating, and who will protect and inspect the machined areas. This helps prevent repair areas from being overlooked or inspections from being deferred between the fabricator and the assembler. Innozinc’s thin coating, adhesion and formability become useful manufacturing and assembly properties when applied within this clearly defined process sequence.

Consider completing operations with fixed dimensions before plating

If the final length and hole positions are known, first consider cutting, drilling and deburring before plating. This allows surfaces exposed by machining to be included in the surface treatment. After plating, check the final dimensions of holes, threads and mating features, together with coating coverage on the required surfaces.

If length or position must be adjusted at the installation site, some machining may need to remain after plating. In that case, the work instruction should cover more than “cut on site”: include cut-face protection and inspection as part of the same operation. For parts whose inner surfaces will be difficult to reach after assembly, schedule protection before assembly.

Machining before plating
Cut, drill and dress edges → Apply Innozinc plating → Check final dimensions and coverage → Assemble

Machining after plating
Apply Innozinc plating → Machine designated areas → Check exposed surfaces and adjacent coating → Apply required protection and inspect → Assemble

Review both the final assembly drawing and the fabrication drawing when deciding which work to complete before plating. For example, holes and welds with fixed installation positions can be completed during fabrication, leaving only length adjustments that require site measurements for later. Operations on the same part need not all fall on one side of the plating step. They can be divided according to when dimensions become fixed and when the machined surfaces are accessible.

Hole and thread dimensions should account for the coating that will be present after plating. If only the dimensions before plating are considered, enlarging holes or reworking threads during assembly may remove the coating. Including mating parts and gauges in the sample evaluation helps identify, in advance, any machining that dimensional corrections would require.

Mark the locations of post-plating operations on the drawing. Distinguish the areas that may be machined after plating from surfaces that must retain their coating. For mating faces and gasket seating surfaces in particular, also specify the surface condition required for final assembly.

Forming, cutting and drilling, and welding require different inspection areas

Bending and roll grooving deform the substrate and its coating together. Roll grooving is a cold-forming operation that presses a groove into the pipe wall. It differs from cut grooving, which removes metal. After forming, inspect the deformed area and its surroundings for cracking and peeling, and check the final geometry. Victaulic’s explanation of roll and cut grooving

At a bend, examine the outer surface in tension, the inner surface in compression, and the areas directly contacted by tooling separately. An intact coating on a flat area does not establish the condition at a point of concentrated deformation. Prepare samples using the plate thickness, bend radius, bend angle and tooling conditions intended for production, and record both the surface condition immediately after forming and the final geometry. SSAB’s guidance on steel bending conditions

Cutting, drilling and machining remove metal and create new surfaces. Treat a cut end, the wall of a drilled hole and a machined groove as separate inspection locations. If deburring or edge dressing also damages the surrounding coating, include that area in the protection scope.

Welding thermally affects the joint and the surrounding coating. Inspect areas stripped during weld preparation, the weld itself, and adjacent areas affected by heat or subsequent grinding. Keep weld-quality inspection separate from surface-protection inspection, and complete both before releasing the part to the next operation. AGA’s guidance on repairs after welding galvanized steel

Innozinc’s adhesion and formability provide a basis for evaluating post-plating operations

Innozinc is a thin composite plating that combines a zinc layer with an inorganic ceramic film. It provides corrosion resistance while limiting the dimensional allowance added by plating. Its adhesion and ability to retain the coating as the substrate deforms can be used in subsequent fabrication. For parts that undergo bending, grooving or fastening, both dimensions and surface condition can be evaluated after the actual operation is complete. Innozinc structure and post-plating fabrication properties

For parts whose shape has changed, such as bent specimens, examine both the deformed area and its surroundings. Machine samples using the customer’s material, thickness and geometry, and evaluate the integrity of the coating remaining after forming separately from protection of the new surfaces created by cutting.

Details of grooved-pipe applications and impact and deformation testing are available in Fire-Protection Grooved Pipe and Innozinc Impact and Deformation Performance, respectively.

Compare the coating at the bend after bending

Close-up views of bent pipe allow the coating condition in the deformed region to be compared. In the Innozinc specimen below, the silver-gray coating remains continuous; in the hot-dip galvanized specimen, coating cracks are visible at the bend. Whether the coating remains continuous after bending is a criterion for defining surface protection and subsequent repair areas on formed parts.

Inspect the pipe interior for peeling around the groove

Grooving requires checking not only the groove geometry and dimensions but also the coating on the deformed pipe wall. Even if the external groove appears properly formed, lifting or peeling may occur on the inside. Post-grooving inspection should therefore cover both the external groove and the deformed area on the pipe interior.

The grooved specimens below show a difference in the condition of the coating inside the pipe. The ceramic-zinc-plated specimen on the left was recorded as having no coating abnormalities, while the hot-dip galvanized specimen on the right is marked with lifting and peeling around the groove.

Areas where peeling interrupts coverage belong within the post-forming inspection and repair scope. Because the pipe interior becomes harder to reach after assembly, it is preferable to complete inspection and any required protection once grooving is finished. For an Innozinc application, prepare samples using the actual pipe material, wall thickness, groove geometry and processing conditions, and check groove geometry and coating integrity together.

For drilled holes, inspect the bore wall, not just the opening

Drilling a hole in steel plate penetrates the existing plated surface and exposes a cylindrical wall of steel. Even if the opening is clean and the surrounding plating remains, the bore wall is a separate surface. If the hole is elongated into a slot or its edges are chamfered, mark these newly machined surfaces as part of the protection scope.

Consider a 12 mm diameter through-hole drilled perpendicular to a 4 mm thick steel plate. The area of the bore wall is calculated as π × diameter × plate thickness, giving approximately 151 mm². This is a geometric illustration and excludes surrounding exposed areas created by deburring or chamfering. The newly formed bore wall should therefore be considered for protection independently of the plating condition around the hole.

Inspect both hole openings and the bore wall. Also check whether an application tool can reach the area and whether the dimensions needed for assembly will remain after the protective material dries. Surfaces that will be concealed by nuts or washers should be checked before assembly. Mark both the protection scope and the inspection stage on the drawing so that surfaces are not omitted simply because they will no longer be visible after fastening.

Specify protection for cut faces and welds

Zinc provides sacrificial protection by corroding preferentially to steel. However, the exposed area that surrounding zinc can protect depends on the width of exposure and the moisture and electrolyte conditions. Rather than omitting protection simply because a cut face is small, specify protection for the machined area according to the service environment and required corrosion resistance. AGA’s explanation of protection at exposed areas of galvanized steel

Inspection of a cut face should not end with the newly cut cross-section. Contact with cutting tools, edge dressing and grinding can affect the adjacent plating, so inspect the boundary between the exposed surface and the remaining coating. Remove lifted coating and residues that could impair adhesion, then follow the preparation and application requirements of the selected repair material.

For welds, establish whether repairing only the weld bead is sufficient. Define the protection scope to include surfaces stripped during weld preparation, areas affected by welding heat, and surfaces ground afterward. Remove slag, spatter and other residues that could interfere with subsequent coating, and carry out weld inspection and surface treatment in the required sequence. Once weld integrity has been confirmed, apply the repair coating to provide continuous protection across the joint and adjacent areas.

Compare the welding operation together with the appearance of the bead

In the comparative field evaluation below, the Innozinc specimen was welded without removing the plating, and good arc stability and weld-bead appearance were observed. For parts that require welding after plating, this demonstrated applicability provides a basis for evaluating welding conditions and the work sequence. After welding, also define the surface-protection scope for the joint and heat-affected zone.

For cut faces and weld areas where steel is exposed, a repair specification using G·SAVE or another material that forms a corrosion-protection layer after surface preparation can be considered. Specify the application area, dry-film thickness, and drying and curing conditions, and evaluate the protection required at the machined area. A repaired surface should not be judged equivalent to the original plating solely because its appearance has become similar.

For pipe openings or surfaces concealed after assembly, being able to reach an area with a tool does not necessarily mean that a protective material can be applied uniformly. Schedule repairs while the operator can access the area. Also check that the protective material will not alter required dimensions on functional surfaces such as gasket seats and threads.

Evaluate samples after the actual fabrication and protection work is complete

Evaluation of an Innozinc sample should extend beyond the flat plated surface. Carry out the bending, drilling and welding required for production in the actual sequence, and submit the part for approval after protection of the processed areas is complete. This connects performance immediately after plating with the condition of the finished part delivered to the customer.

First, check the processed geometry and dimensions. Then examine the deformed areas for cracking and peeling, the exposed areas on cut faces and bore walls, and damage around welds. For the repair coating, verify coverage and thickness over the specified area, together with its drying and curing condition. Do not use a thickness reading from a flat area as a substitute for the condition at a narrow edge or inside a hole; define suitable inspection locations and methods for the geometry in advance.

For parts requiring corrosion testing, identify the areas to be assessed on specimens that have completed fabrication and repair. Recording results separately for flat areas, formed areas, cut faces and welds makes it easier to determine which processing conditions need improvement. Record any treatment or masking of specimen ends alongside the actual protection specification, so that excluded areas are not inadvertently treated as evidence of finished-part performance.

Keep these four items in the purchase order and work handover

Processing scope
Record the drawing number and revision, locations of operations before and after plating, processing methods and final dimensions.

Protection specification
Record newly exposed surfaces and adjacent damaged areas, the protective material, coating specification and drying conditions.

Inspection results
Confirm processed dimensions, cracks, peeling and exposed surfaces, repair-coating continuity, and the results of specified inspections.

Work handover
Record who performs the processing, who applies protection, who verifies inspection, whether the work is complete before assembly, and supporting photographs.

For example, when enlarging a hole in a plated bracket on site, specify how to protect the bore wall and adjacent edges along with the revised hole dimensions. Recording machining completion and protection completion separately makes it clear what condition the assembler is accepting.

Economics can be compared in the same sequence. Both routes include fabrication, plating and inspection costs. For the route that leaves work until after plating, add the costs of additional machining, repair, reinspection and transport, then compare against the same final quality criteria. Agreeing on the protection method before placing the order reduces the need to renegotiate repair feasibility or off-site reprocessing once machining is complete.

When on-site work remains, consider access and preparation time as well as the price of the protective material. The process burden depends on whether parts must be disassembled, whether protection can be applied before assembly, and whether other work can continue while the material dries. Recording these items during sample preparation helps distinguish the reasons for retaining post-plating operations from work that can be completed during fabrication.

Send a drawing marked with the post-plating operations

On a part drawing or photograph, mark where cutting, drilling or welding will be carried out after plating, and send it with the service environment and current plating specification. KEMP will review the Innozinc specification, the surfaces to inspect after processing, and the required protection scope with you. Discuss Innozinc processing and protection specifications