machining tolerences - online manufacturing for cnc

MACHINING TOLERENCES

A GUIDE TO GENERAL

MACHINING TOLERANCES & GD&T

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GD&T (Geometric Dimensioning and Tolerancing) is an international, concise and precise

symbolic language developed for the express purpose of documenting Design Intent, to detail

the functional requirements of a manufactured assembly part and to ensure the compatibility and

interchangeability of those parts.

GD&T provides the links between theoretical 3D models, CAD (Computer GD&T is the tool for

precisely and clearly defining functional specifications for size, form, orientation Aided Design), CAM (Computer Aided Manufacturing) and CAI (Computer Aided Inspection) and the real world;

and location of the elements of a manufactured part.

Improves the quality of the design, production and control processes and preserves the

organization’s knowledge base.

Reduces costs by shortening the production-cycle duration and by contracting hidden

costs.

Improves communication between organizational divisions (Design, Production, Control) by leveraging a uniform and agreed-upon common language; uniform understanding

of all organizational functions and those of suppliers and manufacturers reduces, by 50 percent, the need of formal meetings to explain Design Intent.

Allows the manufacturer to focus on functional constraints in the design and the

elimination of tolerance specifications where they are unneeded, , thus streamlining the production and control processes).

Allows the functional inspection of a single item (the geometric requirements reflect the functional interfaces and constraints).

Can reduce the number of changes in the drawings due to a good tolerance analysis, the

production of compatible drawings and an understanding of setting tolerance values.

GD&T Uses:

ONLINE DIGITAL MANUFACTURING 2020

Digital Manufacturing Services Contact: +44(0)2039629020 or email: [email protected]

The latest DIN standard sheet version applies to all parts made to DIN standards. Variations on

dimensions without tolerance values are according to “DIN ISO 2768- mk”.

GENERAL TOLERANCES

DIN ISO 2768

GENERAL TOLERANCES FOR LINEAR AND ANGULAR DIMENSIONS (DIN ISO 2768 T1)

LINEAR DIMENSIONS

EXTERNAL RADIUS AND CHAMFER

EXTERNAL RADIUS AND CHAMFER



GENERAL TOLERANCES FOR FORM AND POSITION (DIN ISO 2768 T2)

STRAIGHTNESS AND FLATNESS

PERPENDICULARITY

SYMMETRY

RUN-OUT

Sharp edges will be broken and deburred by default. Critical edges that must be left sharp

should be noted or specified on a print.



BASIC TOLERANCES: BILARTERAL, UNILATERAL, AND LIMIT

LIMIT

1.50051.5000

Limit dimensions are two dimensional values stacked on

top of each other. The dimensions show the largest and

smallest allowed. Anything in between these values is

acceptable.

UNILATERAL

1.500+0.005- 0.000

A Unilateral Tolerance exists when a target dimension

is given along with a tolerance that allows variation to

occure in only one direction.

BILATERAL

1.500±0.002

A Bilateral Tolerance exists if the variation from a target

dimension is shown occuring in both the positive and

negative directions.



CLEARANCE & INTERFERANCE FITS

CLEARANCE

INTERFERANCE

B

A

B

A

Interferance fits need to be toleranced so: Any condition of A is MORE than any condition of B.

Clearance fits need to be toleranced so: Any condition of A is LESS than any condition of B.

This should NOT require tools to install - (Loose to Tight)

This should require tools to install - (Press fit, Shrink Fits, ...)



GEOMETRIC DIMENSIONING & TOLERANCES EXPLAINED

DATUMS

TRUE POSITION

HOW DATUM FEATURES ARE SHOWN ON A DRAWING



DATUMS are a “universal reference” between designers, machinists and QA.

GD&T: A symbol based system of communication feature-based tolerances. Uses sets “DATUMS” as refference for defining feature tolerances.

Datum features are denoted with a series of capital letters. These letters are in boxes and tied to

the datum feature with a black triangle.

Datum feature “A”

Feature control frame referencing Datum “A”

This letter will also show up in any feature control frame that uses this datum feature as a

reference. A feature control frame can reference multiple datums at once and each one can be

referenced as needed.

True position denoted by the symbol above is typically used for hole locations. It determines how

much variance a feature can have from its corresponding CAD file before it is out-of-spec by referencing datums. True Position is usually used for sets of holes. Measureing true position usually requires equipment like CMM machines and comparators.

DATUMS are typically:

- Planes

- Axes

- Cyliners

FLATNESS - PARALLELISM - PERPENDICULARITY -CONCENTRICITY - PROFILE



Flatness: non-datum call out for the acceptable

variation on a plane.

Parallelism: Call out to refference the acceptable

variation of a surface from a parallel datum plane.

Perpendicularity: Call out to reference the acceptable

variation of a surface from a perpendicular datum

plane.

Concentricity: Call out the acceptable deviation from

a datum axis (coaxiality)

Profile: Call out the acceptable deviation from a

datum axis (coaxiality)

Can be measured with hand tools & gauges



CMM or comparators typically required

CMM or comparators typically required

Use title block to cover general tolerances

Call out tighter tolerances or GD&T on critical features only

GD&T and very tight tolerances may require CMM, Comparators or

non-manual tools.





CAD BEST PRACTICES

DRAWING BEST PRACTICES

Model the part to the nominal tolerance. This avoids machine operators

mistakenly cutting out-of-tolerance on the first pass.

Use hole wizard and other templates to reduce guesswork on nominal dimensions.

Only call out critical tolerances, tight tolerances, GD&T and unilateral

tolerances.

Use title Block tolerances to reduce clutter.

MAKE SURE DRAWINGS AND CAD MATCH

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