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Datums

Six Degrees of Freedom

Datums

Datum Features

Simulators

Symbol Placement

Datum Precedence

Datum Targets

Datum Guidelines

Virtual Condition

Free State

Additional Information:

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We prefer working with your

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following the course

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Coplanarity

Applications

Position

Composite Position

Implied Conditions

Symmetry

Coaxial Features

A Comparison of

Runout

Concentricity

Fixed and Floating Fasteners

Floating Fastener

Fixed Fastener

Projected Tolerance Zone

Max says -

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Back to GD&T TipsActual Mating Envelope

Calculating Additional Tolerance Permitted when a Position Tolerance is Modified at MMC.

An additional tolerance, often called Bonus tolerance, is permitted when a position tolerance is modified at maximum materialcondition (MMC) and the actual mating envelope of the feature has departed from the MMC.

In these illustrations, the part is black and the actual mating envelope is red.

If a hole, for instance, has the following size and geometric control, and the hole measures .502. It would be incorrect to use abonus tolerance of .003 (.502 - .499(MMC)) if the hole is not perfectly oriented to the Datums. If the hole is out of perpendicular

to datum A by .002, for instance, the bonus that may be used is reduced by that amount. The bonus would be merely .001 andthe allowable position tolerance = .016.

For an internal feature such as a hole or slot:

For an external feature such as a boss or tab:

Note: The Actual Mating Envelope must be oriented relative to the specified Datums. When an inspector merely uses thesize of a feature to calculate the bonus tolerance, out of spec parts may be accepted.

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Back to GD&T TipsAngularity

Use Two Datum References for Angularity!

When setting up a part for the measurement of angularity on a sine bar, it is difficult to orient the part before performing theinspection.

This problem is overcome if a second datum is referenced in the angularity callout

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Angularity Does Not Locate - Use Profile of a SurfaceAnother Angularity Does Not Locate Tip

A common error on drawings is to apply only angularity to features that are notperpendicular or parallel to the datum reference frame. The most important characteristicto control is location. Position locates features of size. Profile of a surface is the onlygeometric tolerance that locates surfaces. The orientation tolerances of perpendicularity,parallelism and angularity never locate features. The angled surfaces on this part, forinstance, need to be located relative to the datum reference frame. If only angularity hadbeen applied, the drawing would be incomplete. Profile of a surface locates the surfaces inaddition to controlling the angularity. If the orientation of these surfaces needs to be betterthan the orientation provided by the profile of a surface tolerance, an angularity tolerance

may be added as a refinement. Locate first-refine second.

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Get Up to Speed with the ASME Y14.5-2009 RevisionPDF of This Tip

By getting up to speed on the 2009 revision of Y14.5 you will learn of the changes which:

Add new concepts and symbols to clarify the meaning of the drawing

Clarify concepts found in the 1994 Standard

Extend principles found in the 1994 Standard

Resolve differences or discrepancies found in the current Standard

Incorporate concepts from other Y14 standards

Reference Y14.41, the standard that nables the tolerancing of 3-D models and reduced dimension drawings

In addition to the 7 new symbols we now have a way to override datum precedence. There are new categories of features such as irregular features of size, continuous and complexfeatures. There are new tolerance zone definitions and new ways to apply old symbols.There is way too much to cover in a Tip. So, my tip to you iscontact Tec-Ease, Inc., to get updated to the new revision of the Y14.5 standard through our seminars, computer basedtraining, on-line training and of course our books, pocket guide and wall chart.

We also offer the new revision of the Standard in an electronic form that keeps the text and graphics together and has extensive link and search capabilities. It is titled The Y14.5 Standard-Ease 2009. You can learn more at http://www.tec-ease.com/09standard.htm.

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Back to GD&T TipsBasic Dimensions

Another Basic Dimensions Tip

Give Production the Dimensions They Need.

When GD&T is used, the geometric tolerances apply to the features - not the dimensions. Therefore when BASIC dimensions areused with geometric tolerances, several different dimensioning schemes may be used without changing the meaning of the drawing.The first drawing below is an example of baseline dimensioning. The other two illustrate chain dimensioning. Since the positiontolerance is related to datums A, B and C, all three drawings have the same meaning even though the dimensioning is different. Thiswould not be true if the dimensions locating the holes were toleranced rather than BASIC. Because the meaning of these drawings isthe same, the designer should consider the needs of those who will read the print when placing dimensions.

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ASME Y14.41 - 2003 - Learn to deal with Basic Dimensions.

Another ASME Y14.41 - 2003 Tip

There are two types of dimensions-toleranced and basic. Dimensions that are toleranced may be toleranced on the field of thedrawing, in a general note or a supplemental block of the drawing format. Basic dimensions are toleranced indirectly withgeometric tolerances. The geometric tolerance applies to the feature, not the basic dimension. We have Rule #1 in the ASMEY14.5 standard which explains the meaning of a toleranced size dimension. For other than size, toleranced dimensions canhave multiple meanings. For that reason, the ASME Y14.41-2003 standard on digital modeling recommends that directtolerancing only be used to define the size of a feature [3.1.1, item(4)]. Geometric tolerancing is the preferred method.They do allow some direct tolerancing for other than size but in many cases recommend that the dimension origin symbol beused to clarify the meaning.

Here is a drawing made in accordance with the ASME Y14.41-2003 standard. Any dimensions not shown may be determined

by making queries of the digital model.

Digital product data definition, reduced dimension drawings and paperless part definition will continue to grow in popularity.The sooner organizations embrace geometric tolerancing and use toleranced dimensions primarily for size, the soonertechnology in this area will advance.

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Boundaries

Think Boundary, not Bonus

The typical approach to inspecting a feature such as then1.000 hole is to measure the size, calculate a bonus andthen try to locate the axis of the hole to see if it is within theallowable position tolerance. Another approach is to inspectthe size and make measurements to see if the hole violatesthe inner boundary (virtual condition). If a hard gage wereproduced to inspect the position of this hole, it would have apin n.970 (+gage tolerance and wear allowance) located atthe basic location from the datums.

This hard gage may be simulated using a height gage in anopen setup or CMM probe. By adding and subtracting half thevirtual condition from the basic dimensions, the distance fromthe datums that may not be violated may be calculated.These distances may be easily inspected.

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Boundaries

Think Boundary, not Bonus

The typical approach to inspecting a feature such as then1.000 hole is to measure the size, calculate a bonus andthen try to locate the axis of the hole to see if it is within theallowable position tolerance. Another approach is to inspectthe size and make measurements to see if the hole violatesthe inner boundary (virtual condition). If a hard gage wereproduced to inspect the position of this hole, it would have apin n.970 (+gage tolerance and wear allowance) located atthe basic location from the datums.

This hard gage may be simulated using a height gage in anopen setup or CMM probe. By adding and subtracting half thevirtual condition from the basic dimensions, the distance fromthe datums that may not be violated may be calculated.These distances may be easily inspected.

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CenterlinesPDF of This Tip

Dimension to Centerlines When it Makes Sense

Many past Tips have illustrated dimensioning from and to center lines. There continues to be questions about when it isokay to dimension with center lines and when it is not appropriate. Dimensions may be to and from center lines on adrawing if the dimensions are basic and the center line is representing the:

In all of these cases if direct tolerancing is used, confusion may result. There is no reason to have this confusion except tofall back on thats the way weve always done it. If you always do what youve always done, youll always get what youvealways got. In other words, you will never improve. The drawing below shows the common correct application ofdimensions to center lines. In every case the dimensions to and from the center lines are basic.

axis, center point or centerplane of a feature of size

center of a pattern such as a bolt pattern

center of a radius

center of a slot length or width

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Circularity

Size - Controlling Form

Another Circularity Tip

Yup, Circularity Error Can Be as Large as the Total Size Tolerance.

Here is a question that comes up from time to time. "How can the circularity error be as large as the total size to lerance since circularityerror is a radial tolerance?" The reason is that the LMC and MMC size limits of a feature of size are not coaxial values. The total sizetolerance on the inside diameter of 60.71 is 0.06mm. The figure at the bottom illustrates an extreme case where the hole would just accepta 60.68 (MMC) full form check while at some cross sections the LMC size of 60.74 was not violated. Therefore, size is verified. Thecircularity error of this particular feature is 0.06, the total size tolerance, since the circularity tolerance zone is comprised of two concentriccircles. The size of the circles defining the circularity error could be 60.68 and 60.8 as illustrated. If this isn't desireable, a circularitytolerance with a value less than 0.6 Could be added to refine the circularity control provided by the limits of size. In this case, a 0.025circularity tolerance was added.

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Concentricity and Position - Coaxiality Controls

Another Coaxiality Control Tip

There are Five Ways to Control Coaxiality of Features. They are:

With so many options it is no wonder that many people find the differences hard to understand. Each control has its place,although some are more difficult and time consuming than others. By comparing sample inspection methods, the differencesmay become clearer. This month's tip will illustrate the differences between position and concentricity. Next month's tip willcontinue with the runout tolerances and profile.

Position may use the MMC and LMC modifiers on the tolerance and Datum references. The illustration below shows apossible gage (ignoring gage tolerance and wear allowance) that could be used when MMC is the modifier. If the sizes arewithin the size tolerances but the part doesn't fit the gage, the features position (coaxiality) is out of spec. This type of controlworks well when the main concern is the assembly of this part to another. The gage may usually be thought of as representingthe worst mating part.


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According to the ASME Y14.5M-1994 Standard, when no modifiers are present, the implied condition is regardless of featuresize as shown below. In this case, the axis of the datum feature and the feature being controlled must be determined to findthe error in coaxiality. Although this control may be applied to bearings and dynamic balance applications, the job can usually

be accomplished at a lower overall cost by using one of the runout controls.

All of the coaxiality controls are intended to control concentric features. For that reason, many designers and engineerschoose concentricity. Unfortunately, concentricity ignores the size, roundness and cylindricity of the feature. It requires thatthe inspector derive a median line (see the June 1997 Tip-of-the-Month). In a situation where you don't care about the size,roundness or cylindricity of the feature, concentricity may be specified. In thirty plus years of reviewing mechanical designs, Ihave never found a design where this is truly the case. The closest application, perhaps, is when dynamic balance is needed.In such a case, measuring a part statically does not assure dynamic balance if the material is not homogeneous. If dynamicbalance is required, a dynamic balancing note is probably in order rather than concentricity. For that reason, I often use theline, "When in Doubt, Use Runout."




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Composite Tolerancing

Another Composite Tolerancing Tip

When datum references are repeated in the second segment of a composite or single segment control, the meaning is

different.

The tolerance in the upper segment of a composite tolerance is located by all applicable basic dimensions. On the drawingabove, the red tolerance is located by the red dimensions.

The lower segment of a composite tolerance does not use the basic dimensions which originate at the datums. Only the basic

dimensions within the pattern are applicable. If a datum is repeated, it indicates that the orientation of the pattern must be heldto the tighter tolerance. In this case, the perpendicularity to datum A must be within 0.2 and the pattern of two holes may nottilt more than 0.2 relative to datum B.


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You will now notice that there are two position symbols being used. This callout is not composite. It is called two singlesegments. The upper segment has the same meaning as the upper segment of the composite callout shown earlier. Thelower segment, however, improves the location as well as the orientationof the feature(s) relative to the datums referenced inthe second sement. Notice that the 19mm dimension from datum B is shown in red. The pattern must be positioned at the19mm dimension from B within 0.2 total even though the pattern may be out of position as much as 0.6 total relative to datum

C.




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Concentricity and Position - Coaxiality Controls

Another Coaxiality Control Tip

There are Five Ways to Control Coaxiality of Features. They are:

With so many options it is no wonder that many people find the differences hard to understand. Each control has its place,although some are more difficult and time consuming than others. By comparing sample inspection methods, the differencesmay become clearer. This month's tip will illustrate the differences between position and concentricity. Next month's tip willcontinue with the runout tolerances and profile.

Position may use the MMC and LMC modifiers on the tolerance and Datum references. The illustration below shows apossible gage (ignoring gage tolerance and wear allowance) that could be used when MMC is the modifier. If the sizes arewithin the size tolerances but the part doesn't fit the gage, the features position (coaxiality) is out of spec. This type of controlworks well when the main concern is the assembly of this part to another. The gage may usually be thought of as representingthe worst mating part.





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According to the ASME Y14.5M-1994 Standard, when no modifiers are present, the implied condition is regardless of featuresize as shown below. In this case, the axis of the datum feature and the feature being controlled must be determined to findthe error in coaxiality. Although this control may be applied to bearings and dynamic balance applications, the job can usually

be accomplished at a lower overall cost by using one of the runout controls.

All of the coaxiality controls are intended to control concentric features. For that reason, many designers and engineerschoose concentricity. Unfortunately, concentricity ignores the size, roundness and cylindricity of the feature. It requires thatthe inspector derive a median line (see the June 1997 Tip-of-the-Month). In a situation where you don't care about the size,roundness or cylindricity of the feature, concentricity may be specified. In thirty plus years of reviewing mechanical designs, Ihave never found a design where this is truly the case. The closest application, perhaps, is when dynamic balance is needed.In such a case, measuring a part statically does not assure dynamic balance if the material is not homogeneous. If dynamicbalance is required, a dynamic balancing note is probably in order rather than concentricity. For that reason, I often use theline, "When in Doubt, Use Runout."




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Cpk

Another Cpk Tip

Calculating Cpk for Geometric Tolerances at MMC or LMC

The question of calculating Cpk on geometric tolerances modified at MMC and LMC continues to come up. Last month's Tipshowed how to inspect features of size where the geometric tolerance is modified at MMC or LMC without having to find theaxis or center plane. The geometric tolerance in these cases is a single limit control. Once that is realized, it is easy tocalculate a Cpk index on such geometric tolerances. In this case, polar measurements are made to assure that the surface ofthe hole does not violate the inner boundary. The inner boundary is 20 - 0.1 - 0.6 = 19.3. The surface of the hole may not becloser to the basic location than R9.65mm which is the radius of the inner boundary. Recording the distribution of the closestpoint relative to the inner boundary of corresponding holes on multiple parts, provides the data necessary to calculate Cpk ofthe process for the position tolerance of each hole. Any $15 calculator with statistical functions may be used to calculate the

average of the data, a standard deviation and the resulting Cpk.

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Back to GD&T TipsCylindricity Inspection

When Inspecting Cylindricity, There Is No Datum

Cylindricity is one of the more challenging tolerances to inspect. It requires isolating the feature from the rest of the part since therecan never be a datum referenced with cylindricity. In this example, the cylindricity has been applied to 5 features. Cylindricity is anindividual control. Therefore, each feature is inspected independent of the others. An electronic probe gathers many points on thesurface. A computer then evaluates the points to fit them between two concentric cylinders that may not have a radial separationgreater than 0.02. The actual size, location and orientation of the feature are ignored. Cylindricity is a composite of circularity,surface straightness and taper.

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Add a Note to Expand a Pattern for Composite TolerancingPDF of This Tip

Some folks think that composite tolerancing cannot be applied to a pattern of holes thathave different sizes. When they have a group of holes of different sizes to control as apattern it is not uncommon to locate one hole, make it a datum feature and control theother holes relative to that new datum feature. In this case, to guarantee a close fit, arefinement of perpendicularity is also required. Technically, this approach is OK. However,it does require a new set-up to inspect the part as well as the additional perpendicularitytolerance. The first drawing shows this approach

Another approach is shown on the second drawing. Here all three holes are made a single pattern by the note, 3 HOLES. Thisapproach gives the desired fit in the assembly while eliminating a set-up to establish an auxiliary datum reference frame andan additional inspection of the perpendicularity refinement on the larger hole.


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Datum Features Must Be Referenced to Be Used.Another Control of Datum Features Tip

There continues to be the misconception that if a feature is labeled as a datum feature, itsuse is somehow implied. Take the drawing shown. The position tolerance on the holereferences datum features A and B but not C. Therefore the hole must be perpendicular tothe plane established by datum feature A and 20 mm from the axis established by datumfeature B. Datum feature C has nothing to do with the position of the hole. Yes, the hole isshown on a centerline that is perpendicular to datum feature C. So, we know what the goalis. But, since datum feature C was not referenced in the position tolerance, there is no limitto how far the hole can be off the 90angle. A couple of possible, acceptable parts are

shown.

In order to avoid this, all you have to do is add datum feature C to the position tolerance.

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Datum Reference Frame

Another Datum Reference Frame Tip

Datums Are Mutually Perpendicular - Even When the Datum Features Are Not.

It isn't always possible or practical to select datum features that are mutually perpendicular to one another when establishing adatum reference frame. Notice that datum feature C is not nominally perpendicular to datum feature B. The datum featuresimulator for C would be made at 35to the datum feature simulator for B (shown here in red). The actual datum planes(shown in blue), which comprise the datum reference framework, would however be mutually perpendicular to one another asis illustrated in the last figure. The deviation of the hole from the 55mm BASIC location would be measured from the thirddatum plane-not from the sharp point on the actual part.

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Datum Shift

Another Datum Shift Tip

Datum Features can Give You a Shift - not a Bonus.

When datum features are modified at MMC or LMC, the datum feature and the features being controlled may be able to shift relative to the datuaxis or centerplane. On the drawing shown below, datum feature B, in the position callout of the four hole pattern, is modified at MMC. Thismeans the datum simulator for datum feature B would have a theoretical design size of 19.6.

Since the actual datum feature could be produced as large as 20.2, the datumfeature could shift as much as 0.6 total. This means the four-hole pattern mayshift out of position in one direction while the datum feature shifts in the otherdirection as shown. A very common error made at inspection is to use thisallowed datum shift as a bonus on the features being controlled. This would allowthe features to be out of position to each other more as the datum feature

departs from MMC. This practice does not agree with the drawing or a hardgage, should one be produced.





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A Datum Feature Is Not a DatumPDF of This Tip

Note: This Tip is in accordance with the ASME Y14.5M-1994 standard.

First, it is not consideredgood practice to havesuch a large flatnesstolerance on the datumfeature and smallertolerances on the otherconsidered features.However, it is technically

correct and this approachwas taken on thisdrawing to make a point.The parallelism is relative to the datum plane established bydatum feature A not to datum feature A.

The question I get is How can the datum be bowed andthere be a tight parallelism tolerance relative to the datum?Well, the answer is, It Cant! The problem is that peopleare constantly confusing the datum with the datum feature.On this part the bottom surface is the datum feature. Nomatter how bowed it is, it is used to establish a datumplane which in theory is flat. According to the ASMEY14.5M-1994 standard in Section 4.2.1, sincemeasurements cannot be made from theoretical planes, the

processing equipment serves as a simulated datum plane for inspection purposes. One way to inspect this parallelism is toplace the datum feature on a surface plate. The surface plate is used as the simulated datum. The travel of the indicatormeasuring the parallelism error is parallel to thesurface platenot the datum feature.

So, as is shown in the last figure, the datum featuremay be out of flat as much as 0.5 but the top surfacemight be parallel (which also controls flatness andstraightness) within 0.1. You might ask what youshould do if datum feature A is bowed the other way.That will be explained next month and will be indexedon the website under Datum, Candidate Datum Set.

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Orienting Datum PlanesPDF of This Tip

Where a primary orsecondary datum axisis established, it isusually necessary toorient the mutuallyperpendicular datumplanes that intersectat the datum axis.Although most folksunderstand that the

datum planes need tobe oriented, theyoften do not agree on how this should be accomplished.

On this drawing, datum feature A is primary and appliesRFS. This establishes a datum axis which is theintersection of two mutually perpendicular datum planes.In order to orient these datum planes, datum feature B isadded to the position tolerance feature control frame on

the 8 hole.

There are two approaches used to orient these datum planes usingthe simulator for datum feature B.

One way is to locate te center plane of parallel planes at the 14

basic location. These parallel planes expand to fill the slot.

The second way is to f irst fill the slot with parallel planes and then

rotate the center plane of these planes to orient the datum planes.

This second method is popular with many CMM operators.Unfortunately it is incorrect. The datum simulator for datum feature Bmust be perfect in form, orientation and location. So, the first method

is correct.

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Page 1 of 1GD&T Tip - Keeping Features In-Line Requires a Geometric Tolerance

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