Difference between Surface Finish (Ra) and Flatness (GD&T)

stm08007

Starting to get Involved
#1
These both seem like they would be very similar measures.. Can anyone shed some light- preferably with a drawing example on the difference between them?

Is it an obvious difference or are these terms used somewhat incorrectly in practice?
 
Elsmar Forum Sponsor
#2
There's a big difference. Surface finish can have a number of parameters applied to it since the finish may be cosmetic (for example) or functional - a sealing surface comes to mind. Therefore while an Ra value maybe what's called up, the "lay" of the finish may be important. The flatness feature may or may not have a surface finish characteristic or anything more than being a datum, for example.

As for what's practiced, they are all subject to "misuse" if the designer/detail draughter doesn't understand the use.
 

Miner

Forum Moderator
Staff member
Admin
#3
It is also a matter of scale. Flatness is a macro-level measurement while surface finish is a micro-level measurement. Flatness may have a large impact on fit and function, while surface finish is more typically function than fit. For example, if two surfaces must move in relation to each other, certain surface finishes are better at holding the lubricant between the surfaces.
 

Michael_M

Trusted Information Resource
#4
Surface finish can be defined by the texture of the part, running a fingernail across different surfaces (say your desk and a wall) will give you different feeling depending on the surface finish.

Flatness can be defined by looking at a flat plane (think of your desktop) Your desktop is flat. Now image lifting one side giving the desk a curve. The difference between the highest point and the lowest point is flatness.

Continue using the desk, the surface finish probably will not change although the flatness really has.
 

stm08007

Starting to get Involved
#5
Surface finish can be defined by the texture of the part, running a fingernail across different surfaces (say your desk and a wall) will give you different feeling depending on the surface finish.

Flatness can be defined by looking at a flat plane (think of your desktop) Your desktop is flat. Now image lifting one side giving the desk a curve. The difference between the highest point and the lowest point is flatness.

Continue using the desk, the surface finish probably will not change although the flatness really has.
Makes sense, but flatness would also be impacted by surface finish right? i.e. a very rough surface would cause flatness to decrease (and surface finish as well)
 
Thread starter Similar threads Forum Replies Date
I The difference between Rz and Rz3 Surface Roughness Inspection, Prints (Drawings), Testing, Sampling and Related Topics 6
R Surface Roughness - What is difference between Ra, Rmax & Rt Inspection, Prints (Drawings), Testing, Sampling and Related Topics 6
T Difference between a subcontractor and a supplier ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 21
H Difference between Stainless Steel 316 ASTM F899 and ASTM A276 Other Medical Device Related Standards 1
A Exact terms for a plating failure and difference between rejection rate and failure rate Manufacturing and Related Processes 9
M Difference between MSA and MSE? General Measurement Device and Calibration Topics 1
G What is the difference between discrete and continuous variables? Problem Solving, Root Cause Fault and Failure Analysis 5
JoCam Difference between Approval and Registration - ISO 13485:2016 ISO 13485:2016 - Medical Device Quality Management Systems 2
S Difference between EU-MDR Annex IX and the Annex-combo X&XI EU Medical Device Regulations 2
T ISO 17025:2017 Clause 4.2.2 - The difference between "be notified" and "be informed" ISO 17025 related Discussions 4
Jimmy123 What is the difference between Error Proofing and Controls? ISO/IATF 16949 - Control Plans FMEA and Control Plans 16
C IEC 60601-1-8, difference between table 4 and annex D IEC 60601 - Medical Electrical Equipment Safety Standards Series 2
A What is the difference between Basic UDI-DI and UDI-DI? EU Medical Device Regulations 5
Q What is the difference between AS9100D 9.3.2.f and 9.3.3.a AS9100, IAQG 9100, Nadcap and related Aerospace Standards and Requirements 7
M Aluminum - What is the difference between 6061-T6 and 6061-T651 (both per ASTM B211)? Manufacturing and Related Processes 4
P What is the exact difference between Risk and Opportunity in context of ISO 27001? IEC 27001 - Information Security Management Systems (ISMS) 7
O Air Flow - Which is the operational difference between LAF (vertical and horizontal) and RLAF? Manufacturing and Related Processes 2
S What the difference is between Stub Acme & Acme thread? Oil and Gas Industry Standards and Regulations 1
S DO 178B - What is the difference between review and verification? Federal Aviation Administration (FAA) Standards and Requirements 1
T The difference between SOP and Kaizen Standardization Lean in Manufacturing and Service Industries 2
K Difference between intended purpose and intended use of the device EU Medical Device Regulations 9
Q What is the difference between normal and licensed internal auditor? VDA Standards - Germany's Automotive Standards 9
J What is the difference between Process Variation and Tolerance? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4
O Difference Between PFMEA & Control Plan FMEA and Control Plans 3
S Difference between an Advisory Notice (ISO 13485) and a Field Safety Notice? ISO 13485:2016 - Medical Device Quality Management Systems 3
T Difference between "data analysis" and "management review" ISO 13485:2016 - Medical Device Quality Management Systems 4
Q What is the difference between 7.4.1 (2008) and 8.4.1 (2015)? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 6
D Difference between uncertainty and expanded uncertainty of measurement General Measurement Device and Calibration Topics 1
S EASA Part 145 - The difference between non-certifying staff and Certifying staff Federal Aviation Administration (FAA) Standards and Requirements 2
S The difference between a Medical Device Accessory and Component Canada Medical Device Regulations 2
S What is the difference between a service request and a complaint? ISO 13485:2016 - Medical Device Quality Management Systems 2
I What is the difference between OOS/OOT Imported Legacy Blogs 3
A Definition Difference between Quality System Procedure and Standard Operating Procedure (SOP) Definitions, Acronyms, Abbreviations and Interpretations Listed Alphabetically 3
Q Difference between Monitor & Measurement ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 4
T QA vs. RA - The difference between QA and RA Coffee Break and Water Cooler Discussions 6
A Difference between Discrimination and Least Count IATF 16949 - Automotive Quality Systems Standard 2
K Difference between Medical Electrical Equipment and Medical Equipment System IEC 60601 - Medical Electrical Equipment Safety Standards Series 2
M Usability Standard - The difference between IEC 60601-1-6 to IEC 62336 Human Factors and Ergonomics in Engineering 1
S What is the difference between Verification and Validation of Test Reports? AS9100, IAQG 9100, Nadcap and related Aerospace Standards and Requirements 5
A Difference between ISO 9001:2015 Clauses 8.1 and 8.5.1 ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 8
M IATF 16949 Cl. 8.4.2.2 vs 8.6.5 - Is there any difference between these clauses? IATF 16949 - Automotive Quality Systems Standard 7
C What is the difference between "Overall Risk" and "Risk"? (ISO 14971) ISO 14971 - Medical Device Risk Management 10
C The difference between a Critical Dimension vs. Critical Characteristic Misc. Quality Assurance and Business Systems Related Topics 2
A ISO 9001:2015 - The difference between 6.1.1b and 6.1.1d - IMHO, d) implies b) ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 1
A Difference between requirement for 4.4.1a (inputs) and 4.4.1d (resources needed)? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 3
R The difference between Feasibility Assessment and Contract Review Contract Review Process 2
V Difference between Non-Conformance, Variance and Deviation ISO 13485:2016 - Medical Device Quality Management Systems 7
A Difference between Environmental Impact and Risk ISO 14001:2015 Specific Discussions 7
A Difference Between Complaints and Feedbacks in terms of Follow-Up Actions Needed Customer Complaints 3
M What is the difference between ISO 9000 and ISO 9001? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 2
Similar threads


















































Top Bottom