Introduction
A CNC sealing face needs drawing requirements for its functional area, surface form and texture. Define the mating component, identify the contact band, specify the required flatness and roughness, and agree how the supplier will inspect the finished part. If the purchase includes a leak test, define the assembly, test conditions and acceptance rule separately.
Consider a circular cover that bolts against a housing. The fastener pattern can fit perfectly while a bowed contact band changes how the seal is compressed. A finish reading from a convenient spot outside that band also tells the buyer little about the texture the seal actually touches. Marking the intended interface on the drawing gives machining, finishing and quality teams the same surface to work from.
The product photographs in this guide show examples of circular faces, shoulders and recessed geometry. The cover and seal arrangements discussed below are engineering examples for preparing an RFQ.
1. Identify the Interface That Does the Sealing
Start with a section view of the assembly. Highlight the surface contacted by the seal, the opposite mating face and any groove or shoulder that establishes its position. A cover can have several concentric steps. Each may serve a different purpose, such as locating the cover, supporting a fastener or providing clearance for the mating body.
The design owner should identify the seal type and relevant service conditions. A static O-ring face seal, a flat gasket and a metal-to-metal interface can demand different contact geometry and finish. For an elastomer face seal, the gland geometry, mating gap and selected seal determine compression together. Service conditions such as temperature, medium and pressure direction inform that selection. The seal manufacturer's design guidance is a starting point for the specific assembly. Parker O-Ring Handbook
When sending an RFQ, include the approved drawing and a model that shows the interface. Identify which dimensions describe the delivered part and which describe the installed assembly. If the seal supplier provides a gland drawing, make its revision available to the design owner and clarify which requirements have been incorporated into the part drawing.
2. Separate Flatness, Texture and Surface Damage
Flatness controls how far a specified surface can depart from a plane. Surface roughness describes finer texture over an evaluated profile. The drawing may also need a separate limit for a scratch, dent, raised burr or other local damage where the seal makes contact. These requirements answer different inspection questions.
| Requirement | What to specify | What the evidence should show |
|---|---|---|
| Flatness | Controlled face or band, tolerance and measurement condition | Form across the designated area |
| Orientation | Relationship of the face to the required datum where applicable | Evaluation in the drawing datum system |
| Surface texture | Parameter, limit, units and any required trace direction | Reading on the designated contact area |
| Surface damage | Location and acceptable condition of scratches, dents or burrs | Visual or other agreed evidence for the affected band |
| Leak test | Assembly, medium, test conditions and acceptance criterion | Result for the defined test configuration |
A finely finished face can still bow across its diameter. A flat face can have a scratch across the contact band. The inspection plan should account for both scales of variation, plus local damage that could affect contact. A surface photo is useful for identifying the geometry and communicating an observed mark. Quantitative acceptance comes from the agreed evaluation.
If orientation matters, identify that separately. Flatness applies to the form of the selected face. A requirement for that face to remain parallel to a mounting plane needs its own datum relationship. Mixing the two in a note such as “flat to the bore” leaves the supplier and receiving inspector to resolve the intent.
3. Specify Roughness on the Surface the Seal Touches
Ra is an arithmetic average of profile deviations over the evaluation length. It is useful when the drawing specifies it and the inspection method is appropriate. The reading represents the selected trace and measurement settings. It does not directly describe every isolated scratch, the overall face form, or the pattern of tool marks across an entire annulus. Mitutoyo surface roughness measurement guide
Call out the parameter and units on the controlled band. If tool lay matters, state the required direction or condition and agree how the trace will cross the machining marks. On a circular part the visible texture may follow a circumferential or spiral path. Whether that pattern is acceptable depends on the interface and the seal design. Describe any restrictions on paths that cross the contact band in terms the shop can inspect.
Narrow faces need a measurement access check before the quotation is final. A stylus may need room to approach and travel along the prescribed trace. Steps, shoulders and adjacent holes can interrupt the available path. Ask the supplier to show where the measurement will be taken and how the agreed settings can be used on the actual surface. If the stated method cannot reach the controlled band, resolve an alternate method with the design owner.
Avoid leaving a note such as “finish 32” without a parameter or units. Metric and inch units are both used in purchase drawings, and Ra and RMS refer to different calculations. The drawing and purchase scope should identify the intended quantity clearly. Mitutoyo surface roughness measurement guide
4. Define the Gland, Contact Band and Mating Gap
A face seal can involve an annular groove with its own width, depth, edge radii and location. Identify the reference surface for depth and show how the groove relates to the contact band. Make the boundaries clear when only part of a broad flange carries the finish or flatness requirement.
Review the groove and mating part together. A groove dimension from the part drawing may be correct while a step on the mating component changes the final gap. If the assembly relies on a metal stop, the stop surfaces and their tolerances contribute to the installed condition. The seal design owner should evaluate the full stack and select limits for the actual seal and application. Parker O-Ring Handbook
The same review helps clarify edge requirements. A groove entrance may need an edge condition that permits assembly without damaging the seal. A wide edge break at a neighboring step could reduce the usable contact area. Mark where the edge requirement applies and how much surface must remain available. Use the cross-hole and deburring guide for related drawing examples.
Include the Mating View in the RFQ
A section view can resolve an ambiguity that a tolerance alone cannot. For example, a cover may have a narrow inner land and a broad outer flange. The gasket may contact only the inner land while the outer flange seats against the housing. If the roughness symbol points to the broad flange, the shop may evaluate a surface that never touches the gasket. Mark both areas by function and show where the measurement path is available.
When multiple cover sizes share a drawing, identify which seal and mating revision goes with each size. Confirm whether the same face requirement applies across the family or whether the contact band changes. That gives the supplier a concrete basis for quoting inspection effort across variants.
Send Your Sealing Face Drawing for Review
Share the approved drawing, mating interface, material, quantity and finish with YILIU. Highlight the actual contact band, groove, flatness and roughness callouts. Include the required inspection stage and any leak test method so the quotation can cover the intended acceptance scope.
Share the drawing, mating interface, material, quantity, finish and required inspection records. Mark the seal contact band and any test scope.
Open the full RFQ form →5. Review Workholding and the Order of Operations
The chosen setup needs to support the part while preserving access to the controlled surface. Thin covers or recessed housings may move when material is removed or when the fixture is released. A broad face can be easy to reach yet sensitive to support locations and clamping loads. Ask how the supplier will support the part and at what stage the final face will be evaluated.
For a hypothetical cover with a deep back pocket, machining the pocket and the front face can change the stiffness of the part through the sequence. The supplier may plan stock allowance for finishing, adjust support or verify the face after release. The suitable route depends on material condition, geometry and the specified acceptance condition. The thin-wall workholding guide explains why clamping and free-state measurement should be considered together.
Review what happens after the final cutting operation. Handling, cleaning, deburring and treatment can affect the face before it reaches inspection. For example, a support pad on a finished contact band might leave a witness mark when the part is transferred for a secondary operation. An agreed protection plan helps the shop preserve the surface it has already produced.
Where the drawing requires measurement in a restrained condition, define how that condition is created. Include support points and the applicable restraint procedure when they affect the result. For a free-state requirement, agree an inspection setup that supports the part without pulling it into an artificially flat condition.
6. Set the Acceptance Stage After Surface Treatment
Identify the final finish and the surfaces to which it applies. If a treatment affects the gland or mating fit, establish which dimensions must be met on the delivered part. Where masking is required, identify its boundary on the drawing and consider where the transition falls relative to the seal contact area.
A machined face may be measured before finishing for process control and again at final acceptance if the contract requires it. Keep the two stages clear in the inspection record. A pre-finish result is useful for understanding the machining process, while the final requirement applies at the stage stated on the drawing or purchase order.
If the buyer wants to protect a machined face during blasting or another operation, identify the protected zone and any permitted changes outside it. Allow the supplier to review whether the chosen process and masking can reach the geometry. The anodizing tolerances guide covers additional considerations when the drawing specifically calls for anodized aluminum.
7. Agree Inspection Evidence and Leak Test Scope
A useful first-article report links each result to the drawing revision and feature. Record the applicable setup, evaluation area and method where these details affect interpretation. A roughness report should identify the measured surface and parameter. A form report should make clear what area was sampled and under what support condition. Discuss coverage on a broad annulus so the result represents the specified band.
A leak test concerns a defined test configuration. If it is part of the purchase, agree which mating hardware and seal are used, how the assembly is restrained, the test medium, pressure or vacuum condition, duration and acceptance criterion. Identify whether the test applies to each unit or a defined sample. State the expected record and what the supplier should do with a failed result. Test procedures and safe limits must come from the approved design and test instructions.
A passing dimensional inspection addresses the features measured. An agreed leak test provides evidence about the assembly under the stated conditions. A buyer can require both when each addresses a different risk. Use the first-article inspection guide to organize the drawing, measured characteristics and disposition.
8. Protect the Interface Through Delivery and Installation
Define the receiving condition in the quotation. Cover how contact faces are separated or protected, how clean components are identified and how records are tied to the shipment. For a critical contact band, agree a packing arrangement that avoids face-to-face rubbing and protects it from loose hardware. If protective material contacts the part, review compatibility with the required cleanliness and surface condition.
Before installation, inspect the contact band and groove for handling damage and debris using the approved procedure. Confirm the correct seal and mating revision. Where assembly preload and sequence matter, use the design owner's installation instructions. A good receiving record can distinguish a delivered surface issue from damage during unpacking or assembly.
For repeat orders, retain the accepted drawing, finish scope and verification plan with the purchase records. Review changes in finish, fixture or measurement method when they affect the specified interface. Compare quotations on the same set of deliverables, including any special packing or test evidence, so the quoted prices reflect a comparable scope.
FAQ
Is Ra enough to specify a CNC sealing face?
Ra describes the roughness of the evaluated profile. Define the contact area, flatness, texture and relevant damage criteria separately. Add groove and mating requirements where they control the seal.
Can a smooth sealing face fail a flatness check?
Yes. Fine texture and overall surface form describe different characteristics. A polished face can still bow. Use the specified form evaluation for the controlled area and a separate method for roughness.
What should I do if a stylus cannot reach the seal band?
Show the exact band and its neighboring geometry on the drawing. Ask the supplier to review access, trace location and settings. Agree an appropriate method with the design owner before placing the order.
Do seal groove dimensions apply before or after coating?
The controlling drawing and finish specification should identify the acceptance stage. If the dimensions affect installed compression or seating, state the delivered condition and any masking boundary explicitly.
Should every machined cover be leak tested?
The application and purchase requirements determine the scope. Specify the test configuration, conditions, acceptance criterion and sampling when leak testing is required.
Conclusion
Specify the area the seal contacts, then define form, texture, groove geometry and final condition in terms the supplier can verify. Pair the drawing with a clear inspection plan and add an approved leak test when the assembly requires one. This gives buyers a sound basis for comparing machining quotes and checking delivered parts.
Request a Quote for CNC Sealing Faces
Send your model and drawing with material, quantity, mating details, finish and required records. Mark the contact band and include the acceptance stage for each critical feature. Send your requirements.
Start with your requirements.
Share the drawing, mating interface, material, quantity, finish and required inspection records. Mark the seal contact band and any test scope.
helen.han@yiliuprecision.comManufacturing evidence you can review
Equipment and material examples refer to the supplied inventory, company profile and manufacturer literature. General guidance and illustrative images are not customer delivery case studies or a substitute for drawing review. Agree on inspection, reporting and delivery scope for each project.
Need to verify a capability or request a correction? Contact the YILIU team.
The photographs show original machined geometry. Seal function, material and acceptance requirements follow the approved drawing and assembly specification.