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What Is Geometric Dimensioning & Tolerancing? ( GD&T )

Geometric Dimensioning and Tolerancing (GD&T) is a language used on mechanical engineering drawings composed of symbols that are used to efficiently and accurately communicate geometry requirements for associated features on components and assemblies. GD&T is, and has been, successfully used for many years in the Automotive, Aerospace, Electronic and the Commercial design and manufacturing industries. In today’s modern and technically advanced design, engineering and manufacturing world, effective and accurate communication is required to ensure successful end products. Currently, ASME Y14.5–2009 is the accepted geometric dimensioning and tolerancing standard superseding ANSI Y14.5M-1994 used within the USA and ISO 1101 is used outside of the USA. Success oriented industries and organizations which, require accurate and common lines of communications between engineering, design, manufacturing and quality should consider geometric dimensioning and tolerancing (GD&T) as their mechanical drawing standard. Some advantages of GD&T (geometric dimensioning and tolerancing) are;

  1. Provides a clear and concise technique for defining a reference coordinate system (datum’s) on a component or assembly to be used throughout the manufacturing and inspection processes.
  2. Proper application of geometric dimensioning closely dovetails accepted and logical mechanical design process and design for manufacturing considerations.
  3. Geometric dimensioning dramatically reduces the need for drawing notes to describe complex geometry requirements on a component or assembly by the use of standard symbology that accurately and quickly defines design, manufacturing and inspection requirements.
  4. GD&T concepts such as MMC (maximum material condition) when applied properly will facilitate and simplify the design of cost saving functional check gages, manufacturing fixtures and jigs.

The following are common reasons one should apply GD&T principles:

  1. Part features are critical to function or interchangeability.
  2. When functional gauging techniques are desired.
  3. When a common reference (origin) or datum is required to ensure communication is consistent between design, manufacturing and inspection.
  4. When a standard interpretation or tolerance is not already implied.
  5. Simplify tolerance analysis.
  6. Replace complex or long geometry requirement description notes with a single geometric symbol.
  1. Productivity and GD&T
  2. To be competitive in this global market, manufacturers must continue to drive increased productivity from all aspects of the machining process. Automated machining has produced tremendous gains in productivity through speed and repeatability.
  3. But to fully leverage those investments, our customers have learned that where and how the part is held will have the greatest impact on whether the parts meet spec, and how fast they can push the machine.
  4. At the end of the day it’s all about making more sell-able parts. This means eliminating defects and compressing the time to setup and process. Fully understanding and applying GD&T principles allows our customers to be more productive. Here is how that works.

 

Perpendicularity
Perpendicularity per-pen-dic-u-lar-i-tee

Definition: The condition of a surface or axis at a right angle to a datum plane or axis.  Perpendicularity tolerance specifies one of the following: a zone defined by two planes perpendicular to a datum plane or axis, or a zone defined by two parallel planes perpendicular to the datum axis.

If the surface to be machined is to be perpendicular with the datum plane facing the chuck;

  • How is datum face controlled to ensure that the perpendicularity results are acceptable?
  • What might explain the challenge in achieving capability?
  • What is the cost of scrap really costing the company?

A qualified surface is required on the chuck face that in turn acts as a datum reference for the work piece. A separate, but stationary “part stop” (or “locator”) performs best in this regard.

The results may still not be acceptable even if this is the course of action taken.  The primary reasons are due to the chuck itself.

  • All conventional sliding jaw chucks experience a measure of jaw “lift.”

    There are (4) primary reasons:

    1. The clearance between the master jaw and that of the body.

      The phrase “sliding jaw” speaks to this engineered clearance.

    2. This same clearance has increased over time due to wear.
    3. Chuck body flex.
    4. Hydraulic pressure is too high for the jaw height being used.

While these effects can be further reviewed, the fact remains that sliding jaw chucks inherently are not appropriate for tightly controlled perpendicularity requirements.

Where does this leave us?

The earlier described part stop (i.e., locator) still offers the qualified surface needed, but the chuck, rather than lifting the part away from the part stop, must instead pull the part back against this surface. Only then can the control surface be trusted and the work piece be in the right location by which to machine the opposite surface and be assured of two perpendicular surfaces.

In low volume environments the operator can ensure this location surface is kept free of chips and contamination via 100% inspection of this surface, but in a high volume environment this isn’t an option.

If parts are being auto-loaded, how do you know that the work piece is against this desired surface? What is it really costing you if this feature of perpendicularity is out of control? If we are addressing a downstream part, it is no longer just the cost of the one operation, nor the raw material cost, but additionally the cost from all previous operations combined. In fact, the closer to the last operation we are, the more important it is to ensure that the part quality is kept intact or else risk the total loss of investment.

How do we protect this investment when facing a stringent perpendicularity requirement in a high volume setting?

Essentially, we need to incorporate a validation system and subsequently receive feedback on its performance. In other words, if the part is not against the part stop when loaded, when do you want to find out? Before or after machining? Of course the answer is before the part is machined, so that you can correct the misloaded part before it becomes scrap. Systems as described typically present ROIs of less than (8-10) weeks.
 

Parallelism

Parallelism par-uh-le-liz-uh m

Definition: Parallelism is the condition of a surface, line or axis which is equidistant at all points from a datum plane or axis.

If the surface to be machined is to be parallel with the datum plane facing the chuck;

  • How is this datum face controlled to ensure that the parallelism results are acceptable?
  • What might explain the challenge in achieving capability?
  • What is the cost of scrap really costing you?

What is needed is a qualified surface on the chuck face that in turn acts as a datum reference for the work piece. A separate, but stationary “part stop” (or “locator”) performs best in this regard.

Why might the results still not be acceptable even if this is the course of action taken?

The primary reasons are due to the chuck itself. All conventional sliding jaw chucks experience a measure of jaw “lift.” There are (4) primary reasons:

  1. The clearance between the master jaw and that of the body. Thus the phrase “sliding jaw” speaks to this engineered clearance.
  2. Due to wear this same clearance has increased over time.
  3. Chuck body flex.
  4. Hydraulic pressure is too high for the jaw height being used.

While these effects can be further reviewed, the fact remains that sliding jaw
chucks inherently are not appropriate for tightly controlled parallelism requirements.

Where does this leave us?

The earlier described part stop (i.e., locator) still offers us the qualified surface needed, but the chuck, rather than lifting the part away from the part stop, must instead pull the part back against this surface. Only then can we trust the control surface of the work piece to be in the right location, by which to machine the opposite surface and be assured of two parallel surfaces.

In low volume environments the operator can ensure this location surface is kept free of chips and contamination via 100% inspection of this surface, but in a high volume environment this isn’t an option.

If parts are being auto-loaded, how do you know that the work piece is against this desired surface? What is it really costing you if this feature of parallelism is out of control? If we are addressing a downstream part, it is no longer just the cost of the one operation, nor the raw material cost, but additionally the cost from all previous operations combined. In fact, the closer to the last operation we are, the more important it is to ensure that the part quality is kept intact or else risk the total loss of investment.

How do we protect this investment when facing a stringent parallelism requirement in a high volume setting?

Essentially, we need to incorporate a validation system and subsequently receive feedback on its performance. In other words, if the part is not against the part stop when loaded, when do you want to find out? Before or after machining? Of course the answer is before the part is machined so that you can correct the misloaded part before it becomes scrap. Systems as described typically present ROIs of less than (8-10) weeks.

 

Concentricity

Concentricity kon-sen-tri-si-tee

Definition: That condition where the median points of all diametrically opposed elements of a figure of revolution (or correspondingly-located elements of two or more radially-disposed features) are congruent with the axis (or center point) of a datum feature.

Since the concentricity definition (i.e., ANSI Y14.5-1994) is a matter of inspection, we will address concentricity in more simpler terms and specifically as it relates to the subject of workholding.

Work holding performs two basic functions.

  1. Positions the part appropriately.
  2. Grips the part sufficiently to resist part movement against cutting forces.

Positioning the part in this context means to center the cylindrical part shape to a given tolerance. If you believe your workholding system is preventing you from realizing the desired concentricity results, what can contribute to this?

  • Work holding accuracy
  • Chuck repeatability
  • Jaw forming technique
  • Jaw lifting
  • Worn chuck

It might be worth reviewing the definition of both “work holding accuracy” and that of “chuck repeatability.” Then we can compare them to each other.

Accuracy is a TIR measurement of the chucking system while gripping a master test bar. The resulting run out would be a composite resultant of the following factors: Chuck condition, soft jaw forming technique, hydraulic pressure setting, mechanical alignment of the machine turret/spindle, etc.

Chuck repeatability is a static (i.e., radial) measurement that isolates the chuck’s unique performance to repeatedly hold the original position as established during the “Accuracy” test.

A user might observe the accuracy (i.e., TIR) to be .005″, but the repeatability (radial measure) to be .0004″ (or .0008″ when converted to TIR). We could conclude that everything in excess of the repeatability value (x2) would be from variables beyond that of the chuck itself. We can also conclude that if these surrounding issues were completely corrected, the eventual shaft run-out would drop to .0008” as determined via the repeatability test.

 

Total Runout 

Total Runout toht-l • ruhn-out

Definition: A composite control of all surface elements. The tolerance applied simultaneously to circular and longitudinal elements as the part is rotated 360 degrees. Total runout controls cumulative variation of circularity, straightness, coaxiality, angularity, taper, and profile when it is applied to surfaces constructed around a datum axis. When it is applied to surfaces constructed at right angles to a datum axis, it controls variations of flatness and perpendicularity.

As stated here above, total run-out is a composite control. As such, the result is the accumulation of:

  • Circularity
  • Straightness
  • Coaxiality
  • Angularity
  • Taper
  • Profile

It should be noted that while the definition includes all of these, it does not necessarily mean that every scrap part is the cause of more than just one. Whether it is determined that you are struggling with one or all six, work holding has a direct impact towards getting total runout under control.

Work holding performs two basic functions:

  1. Positions the part appropriately
  2. Grips the part sufficiently to resist part movement against cutting forces.

If point #1 is performed appropriately, then it can be argued that the source of continued error would need to be located elsewhere (i.e., head stock alignment, turret alignment, etc.).

If point #1 is not appropriate to the application, then how might work holding be contributing to the error? How might the product be to blame? What principles need to be considered?

If we were to assume the work piece were a round cylinder, then we would want to be sure that this same cylinder’s axis perfectly shared the same axis as that of rotation (i.e., lathe application). If this basic requirement wasn’t satisfied, then the last 5 controls above would be in put in jeopardy.

In practical terms, what can cause this to happen?

  • Jaw forming techniques
  • Jaw “lifting”
  • Worn chuck
  • The run-out tolerance is tighter than the chuck accuracy
  • Short gripping length relative to diameter ratio
  • Maintenance

Jaw Forming
Without going into the fundamentals of how soft jaws are formed (i.e., machined), we can describe some of the more popular effects when done so improperly. These would include: jaws having positive or negative tapers from front to back (or back to front), poor surface finish altering the jaws centerline, the centerline being offset by not pre-loading the jaws correctly or in the correct direction.

Short gripping length to diameter ratio (L:D)
A steel washer might be an excellent example of this. Its length (thickness) is just too short for a work holding devise to establish its centerline by. If we were to grip it 10 times, the chuck would attempt to establish a unique centerline with each attempt, and they would still all be incorrect.

What is the solution?
If one of its faces were perpendicular to the outer diameter in such a way as to be used as a surrogate for the outer diameter, we could then use the face rather than the outer diameter (OD). Since it would be imperative to keep this face back against this surface, “jaw lift” would need to be taken out of the equation.

Jaw Lift – as it relates to gripping poor L:D (ratio) parts
All conventional sliding jaw chucks experience a measure of jaw “lift.” There are 4 primary reasons:

    1. The clearance between the master jaw and that of the body. Thus the phrase “sliding jaw” speaks to this engineered clearance.
    2. Due to wear this same clearance has increased over time.
    3. Chuck body flex
    4. Hydraulic pressure is too high for the jaw height being used.

Where does this leave us?

The earlier described part stop (i.e., locator) still offers us the qualified surface needed, but the chuck, rather than lifting the part away from the part stop, must instead pull the part back against this surface. Only then have we built robustness into the process via the work holding.

 

Circularity

Circularity Cir – cu – ler · i · tee

Definition: A condition where all points of a surface of revolution, at any section perpendicular to a common axis, are equidistant from that axis. The circularity control is a geometric tolerance that specifies that each circular element must lie within a tolerance zone of two coaxial circles.

Circularity, also known more commonly as “roundness” is a form control that is applied to any part feature with a diametrical (round) cross section.

With the context of “circularity” confined to round parts, we are left to ask ourselves a series of questions as to where such roundness error stems from, and where workholding participates in such.

  • Do the parts experience a specific roundness shape that of a “tri-lobing”?
  • How could the actual process be affecting the results?
  • How should thin walled parts be processed?
  • Is the gripping diameter itself not round?
  • Is tool pressure adding to the problem?
  • Is the chuck cylinder pressure already as low as possible?
  • Could an increase in surface contact with the part in fact distribute the gripping force better?
  • What impact does reducing the grip force have?
  • What relationship is there between surface finish, tool chatter and work holding ?
  • How might increased friction benefit roundness performance?

How do all of these factors inter-relate in order to improve roundness results?

With so many possible scenarios and applications, we might just review one concept here. The inter-connection between; thin walled parts, tri-lobing (ie. shape of the deformation) and hydraulic cylinder size.

It is a common observation that a tri-lobed part is the effect of excessively high grip force (for the given application) of a 3-jaw chuck. Assuming the part is still safely held, we might conclude the chuck is to blame, but is it? Does our cause and effect analysis take us far enough?

We might review the source of the grip force to determine what “causes” the grip force value to be.

Yes, it is actually the hydraulic cylinder actuating the chuck that ultimately determines the grip force of the chuck. In fact we have a new situation; Cause-Effect-Cause. Knowing this, can we further reduce the cylinder pressure to minimize the grip force.

hydraulic pressure > chuck grip force > circularity error (deformation)

If through this continued pressure reducing effort you find the pressure at its lowest setting, yet the negative effects of deformation continue to produce unacceptable circularity results, are there any other options?

Yes, and there are quite a few.

  • Review of cylinder size
  • Increasing the number of jaws
  • Increased jaw friction and reduction of grip force
  • Gripping diameter not being round must be addressed

But we also come to a point where the uniqueness of the application and expectations of the customer uniquely drives the direction of appropriateness.

If you have attempted various approaches to improving the circularity (i.e., roundness) of your own
parts and wish to accelerate the rate of improvement, contact us to review the specific details of your
application. Depending on what is currently unknown about the actual root cause, we might recommend
a scoping program that includes a root cause analysis, written report and action plan.

Flatness

Flatness flat ness

Definition: All the points on a surface are in one plane, the tolerance specified by a zone formed by two parallel planes. Like straightness, flatness is referenced to itself and so is applied without need for or reference to a datum.

If flatness control continues to allude you, then like that of circularity control, an in-depth investigation into its root cause would be the next appropriate step. By asking ourselves “where to begin?”, we are led to reviewing the evidence at hand, and that would be the scrap parts themselves.

By visually understand what its defective shape looks like, then we can begin to theorize as to the cause of this shape. Here again, we can look to the effect, to speculate as to what caused this. Let’s consider a few of the more typical shapes and the possible causes:

  • If the face of a round part looked like that of a three lobed potato chip, we might begin by speculating that the workholding or even tool pressure might have contributed to this. This is especially the case when a 3-jaw chuck is being used.
  • If the face was convex (or concaved), we might question the alignment of the machine or program (i.e., taper).
  • If after evaluating several parts we observe a randomness in the shapes and orientation of the same, then there might be reason to consider stress relieving and the processes leading up to this operation.

Let’s take a closer look at the first scenario above

If the part does resemble that of a three lobed potato chip, we next need to determine how this shape came to exist. How could a part that was turned on a lathe come to have alternating high and low surfaces? The answer simply is that the cutting tool removed more material from some of this surface than from others. We are now led to question how this would have come about and there are two likely explanations.

  1. Assuming the part is thin walled, we could theorize that the three jaws properly supported the part every 120° while the balance of the part was unsupported. This would have permitted the tool to push the part surface backward between the jaws, leading to an uneven depth of cust and explain the shape.
  2. Another possibility is that the grip force in conjunction with “jaw lift” distorted the part and presented this now distorted potato chip shape to the cutting tool. Upon unclamping the part, the relaxed part will now reflect this same potato chip shape.
  3. The last consideration is that caused exclusively by pull back chucks and can be described as being the exact opposite of point #2 above. With every pull back chuck used, a separate stationary part stop is utilized to pull the work piece back against. It is this pull back motion/force that can bend the part back over this surface that simply changes the high surface presented to the cutting insert, but the outcome is the same.

Where does this leave us?

Since there are quite a few contributors for flatness error, resist the tendency to guess at the cause. Even worse, to throw money at it. Let’s list here the most probable causes:

  • Work holding
  • Tool pressure
  • Machine condition (i.e., alignment, backlash, etc.)
  • Surface finish
  • Stress relieving
  • Thermal growth (work piece)
  • Part rigidity (A subset of the above)

If “flatness” continues to cost your company money, and you are looking to resolve the matter more quickly, contact us to review the details of your application. Depending on what is known/unknown about the root cause, we might recommend a scoping program that includes root cause analysis, written report and action plan.

Kitagawa Chuck Clamp Force Meter demonstration: