ISO 9001:20152023 金手獎

+886-4-2534-5219 [email protected]

元順利股份有限公司Request for Quote
ysl-cnc surface-finishing-4

How Sandblasting and Polishing Affect CNC Part Appearance and Measurement

Sandblasting and polishing change both the surface texture and critical dimensions of CNC parts, and measured values can shift by more than 0.01 mm relative to the as-machined state. For precision parts, this means the same engineering drawing must specify whether the reference state is "as-machined" or "as-finished," and the measurement locations and inspection procedure must be agreed with the supplier; otherwise, disputes are likely during acceptance. Buyers should specify both the surface treatment and the measurement reference clearly at the RFQ stage.

Key Takeaways

  • Surface treatment shifts dimensions

    Blasting and polishing alter surface texture and critical dimensions; measurements may shift by more than 0.01 mm relative to the as-machined state. Precision parts must specify the reference state.

  • Reference state must be indicated on drawings

    Engineering drawings should specify a "before treatment" or "after treatment" reference, and the measurement positions and inspection process should be agreed with the supplier; otherwise, acceptance disputes are likely.

  • Certain part types are prone to measurement failure

    Thin-walled parts, slender shafts, threaded parts, and deep-hole parts are most likely to experience deformation or dimensional shift after blasting or polishing, leading to measurement failure.

  • RFQ must provide four items of information

    When requesting a quote, you must provide the surface treatment method, measurement reference state, acceptance tolerance and roughness, and inspection process requirements—none of these can be omitted.

Which dimensions does sandblasting change, and why do measured values drift?

Sandblasting uses high-speed abrasive to impact the metal surface, removing burrs and tool marks while also causing slight plastic deformation and material loss. For CNC turned or milled parts with precision tolerances at the 0.007 mm level, actual dimensions after sandblasting can shift by 0.01 to 0.03 mm relative to the as-machined state, depending on abrasive grit size, blasting pressure, and duration. If a buyer only specifies "±0.01 mm" on the drawing without defining the measurement reference, acceptance disputes often arise where "the machine measurement passes but the post-treatment measurement fails." It is recommended to clearly mark the "pre-treatment" or "post-treatment" reference on the engineering drawing and require the supplier to indicate measurement locations in the inspection procedure, such as avoiding direct blasting areas or using a datum surface as the measurement origin. Yuan Shun Li performs in-process inspection and final pre-shipment inspection using 3D CMM coordinate measuring machines, and can measure according to the customer-specified reference state, provided that the reference is clearly defined at the RFQ stage.

What is the impact of polishing on dimensions and surface roughness?

Polishing (mechanical polishing or mirror polishing) removes surface material to reduce roughness, bringing Ra values from 0.8 µm down to below 0.3 µm, but it also slightly reduces outer diameter, inner diameter, and length dimensions. For cosmetic parts or seals, the dimensional change after polishing must be included in tolerance calculations; otherwise, assembly may result in interference or excessive clearance. When specifying an Ra 0.3 µm surface roughness, buyers should also note the "post-polishing" reference and acceptance tolerance on the drawing, and require the supplier to provide measurement records before and after polishing. Yuan Shun Li's CNC machining can achieve an Ra 0.3 µm surface roughness, but polishing is a post-process surface treatment, so the dimensional transition between machining and polishing must be confirmed with the supplier at the RFQ stage.

surface finishing scene 1

Which part types are most likely to fail measurement due to surface treatment?

Thin-walled parts, slender shafts, threaded parts, and deep-hole parts are the most likely to experience measurement failure after sandblasting or polishing. Thin-walled parts (e.g., aluminum alloy housings under 1 mm thick) may deform after sandblasting, causing flatness and parallelism to exceed drawing requirements; slender shafts (e.g., stainless steel shafts with a length-to-diameter ratio exceeding 10:1) may have their diameter reduced after polishing, potentially falling below the lower tolerance limit; threaded parts may retain abrasive residue at the thread root after sandblasting, affecting measurement and subsequent assembly; deep-hole parts (holes deeper than 100 mm) may have reduced hole diameter after polishing, affecting the fit with a shaft. For these part types, buyers should specify at the RFQ stage whether "pre-treatment measurement" or "post-treatment measurement" is required, and require the supplier to indicate which measurement serves as the reference in the inspection procedure. Yuan Shun Li can machine parts up to 250 mm in maximum diameter, 500 mm in length, and 120 mm in depth, but post-treatment deformation of thin-walled parts and slender shafts must be evaluated on a case-by-case basis.

What should buyers specify about surface treatment and measurement datums at the RFQ stage?

The most common mistake at the RFQ stage is submitting only an engineering drawing without specifying the relationship between surface treatment and measurement datums. Buyers should proactively provide four items when requesting a quote: first, the surface treatment method (blasting, polishing, anodizing, plating, etc.) and the processing sequence; second, the measurement datum state (before or after treatment); third, the corresponding requirements for acceptance tolerance and surface roughness; and fourth, the inspection process requirements (incoming, in-process, pre-shipment). All four items are essential; otherwise, the supplier can only quote and produce based on the "as-machined" state, and dimensional deviations are only discovered at the acceptance stage. Yuan Shun Li's inspection process covers incoming inspection, in-process inspection, and final pre-shipment inspection, and is equipped with 3D CMM and MSA/GR&R/SPC quality control methods to support customer-specified measurement datums, provided the datums are clearly documented at the RFQ stage.

RFQ Preparation Process

  1. 1

    Confirm the surface treatment method

    Clearly indicate the treatment methods such as blasting, polishing, anodizing, or plating, and their sequence, to prevent the supplier from producing according to a default process.

  2. 2

    Specify the measurement reference state

    Indicate whether the reference is "before treatment" or "after treatment," and specify which critical dimensions apply to which reference.

  3. 3

    Indicate tolerances and roughness

    Simultaneously specify dimensional tolerances and Ra requirements, and note the relationship between them, for example, "after polishing Ra 0.3 µm, ±0.02 mm."

  4. 4

    Set inspection process requirements

    Specify the inspection items and frequency at the incoming, in-process, and pre-shipment stages, and require the supplier to provide inspection reports.

  5. 5

    Provide appearance and material information

    Provide reference images or samples of the surface texture, and specify the material grade and order quantity, as these affect the evaluation of post-treatment deformation.

surface finishing scene 2

Why should drawings specify a "post-treatment" datum?

If an engineering drawing only specifies dimensions and tolerances without indicating the datum state, the supplier and buyer may have completely different definitions of "acceptable." A common dispute in practice is: the supplier measures in the "as-machined" state and issues an inspection report, but the buyer measures in the "post-treatment" state upon receipt and finds the dimensions out of tolerance, leading to a disagreement. To avoid this, buyers should clearly state on the drawing either "all dimensions are based on post-treatment datum" or "critical dimensions are based on pre-treatment datum, others are post-treatment," and require the supplier to indicate the measurement timing on the inspection report. Yuan Shun Li can perform CMM measurement during in-process inspection and final pre-shipment inspection, but if the customer does not specify a datum, the default is to measure in the "as-machined" state. Buyers should confirm this at the RFQ stage.

How do blasting and polishing affect appearance inspection?

Appearance inspection (visual, texture consistency, color difference) and dimensional measurement are two independent inspection routes, but blasting and polishing affect both. After blasting, the surface shows a uniform matte finish, but if the abrasive particle size or blasting pressure is inconsistent, uneven texture may occur; after polishing, the surface shows a mirror or semi-mirror finish, but if the polishing direction is inconsistent, scratches or light spots may appear. When specifying appearance requirements, buyers should provide samples or texture reference images and require the supplier to perform appearance comparison during final pre-shipment inspection. Yuan Shun Li's inspection process covers final pre-shipment inspection and can perform comparison according to customer-specified appearance standards, but the appearance standards must be provided at the RFQ stage.

Six items buyers should prepare at the RFQ stage

  • Surface treatment method and sequence

    Clearly specify the treatment methods (blasting, polishing, anodizing, plating, etc.) and the order of operations to prevent the supplier from producing according to a default process.

  • Measurement datum state

    Indicate whether the datum is "pre-treatment" or "post-treatment" and specify which critical dimensions apply to which datum.

  • Acceptance tolerance and surface roughness

    Specify dimensional tolerances and Ra value requirements together, and note the relationship between them, e.g., "Ra 0.3 µm after polishing, ±0.02 mm."

  • Inspection process requirements

    Specify the inspection items and frequency for incoming, in-process, and pre-shipment stages, and require the supplier to provide inspection reports.

  • Appearance standards and samples

    Provide texture reference images or samples, and specify the acceptable visual inspection range and comparison method.

  • Material and batch information

    Specify the material grade (iron, carbon steel, aluminum, copper, FRP, various alloys) and order batch size (prototype or mass production), as these affect the evaluation of post-treatment deformation.

What support does Yuan Shun Li provide in bridging surface treatment and measurement?

Yuan Shun Li's CNC machining covers turning, milling, five-axis and multi-process machining, and Swiss-type sliding headstock, achieving precision tolerances of 0.007 mm (turning/milling), surface roughness Ra 0.3 µm, maximum machining diameter of 250 mm, length of 500 mm, and depth of 120 mm. Inspection is equipped with 3D CMM coordinate measuring machines, and quality control methods include MSA/GR&R/SPC. The inspection process is divided into three stages: incoming inspection, in-process inspection, and final pre-shipment inspection. Surface treatments (such as blasting, polishing, anodizing, and electroplating) are part of the downstream processes. Yuan Shun Li can coordinate with treatment facilities based on customer requirements, or customers may designate their own treatment facility. However, the measurement benchmarks and inspection procedures after treatment must be clearly defined during the RFQ stage. In terms of equipment, the facility is equipped with 20 Japanese BROTHER CNC machining centers, 13 Swiss Star CNC automatic lathes (handling five-axis machining), 8 CNC milling machines, and 7 Japanese TAKISAWA horizontal lathes, totaling 48 machines, capable of supporting batch requirements from prototyping to mass production. The ISO 9001:2015 quality system has been established (certificate number 20/16013), but its validity has expired. The new certificate is pending provision by the owner, and the validity period is not currently disclosed.

FAQ

Why do measurement values drift after blasting?

Blasting uses high-speed abrasives to impact the metal surface, removing burrs and tool marks while also causing slight plastic deformation and material loss. For parts with precision tolerances at the 0.007 mm level, dimensions may shift by 0.01 to 0.03 mm after blasting, depending on abrasive grain size, blasting pressure, and duration.

Does polishing make dimensions smaller?

Yes. Polishing removes surface material to reduce roughness; Ra can drop from 0.8 µm to below 0.3 µm, but it also slightly reduces outer diameter, inner diameter, and length dimensions. This must be included in tolerance calculations, otherwise assembly may result in interference or excessive clearance.

Which part types are most prone to measurement failure?

Thin-walled parts, slender shafts, threaded parts, and deep-hole parts are most likely to experience measurement failure after blasting or polishing. Thin-walled parts may deform after blasting, slender shafts may have reduced diameters after polishing, threaded parts may retain abrasive at the thread root after blasting, and deep-hole parts may have reduced hole diameters after polishing—each requires case-by-case evaluation.

What information should be provided at the RFQ stage?

Four items should be provided: the surface treatment method and sequence, the measurement reference state (before or after treatment), the corresponding requirements for acceptance tolerance and surface roughness, and the inspection process requirements. None of these can be omitted; otherwise, the supplier can only quote and produce based on the "as-machined" state.

Why should drawings specify an "after treatment" reference?

If only dimensions and tolerances are specified without noting the reference state, the supplier and buyer may have completely different definitions of "acceptable." If the supplier measures in the "as-machined" state and the buyer measures in the "after treatment" state, disputes over out-of-tolerance dimensions can arise.

Include surface treatment and measurement benchmarks in the RFQ to reduce acceptance disputes

If you are evaluating surface treatment options for CNC parts, you are welcome to send your engineering drawings and surface treatment requirements to [email protected], or call +886-4-2534-5219 to contact Yuan Shun Li's sales team. We will provide an evaluation based on your part type, material, and batch size.

← How to Choose the Right Surface Finish for CNC Parts?