Meaning of "Equally Spaced" when no GD&T applied 1

[randy64]

There is a hole callout that states there are 36 holes equally spaced on a bolt circle, there is no GD&T called out, and the title block states that unless otherwise noted all angular dimensions are +/-2 degrees.

Does the term "equally spaced" imply a basic angular dimension for the location of the holes, i.e. the location of the 36 holes is exactly 10 degrees from each other and each hole can very +/-2 degrees from that location?

OR, does this type of callout leave open the possibility that the 2 degrees could stack up from hole to hole and you could end up with the last hole being off from where you really want it by 72 degrees?

 

[MintJulep]

Exactly.

 

[randy64]

Exactly what, MintJulep?

 

[Belanger]

Aye, there's the rub... I think MintJulep is reiterating that it is ambiguous.

But of the choices you give, I would go with the first one: even that last hole has to maintain its 10º ± 2º in the final swee

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[drawoh]

randy64,

Here are my notes on calculating hole tolerances. Among other things, I worked out the tolerances needed to make plus/minus tolerances work on a pitch circle. Note how I assumed absolute angles. I would apply your tolerance to each and every dimension.

--
JHG

 

[tbuelna]

randy- Based on what you describe, your interpretation is correct. The angular location of each hole center to its adjacent one should be within +/-2deg at the pitch diameter. And there is no stack up of tolerance. The max angular limit between two adjacent hole centers would be 12deg and the min angular limit would be 8deg. The cumulative total of the angular hole spacing would obviously be 360deg.

 

[randy64]

Thanks everyone for your input.

tbuelna, I didn't claim an interpretation, just threw out two possibilities.

If forced to choose between the two, I would go with the second one (possible stack up) simply because there is no implied basic angular dimension. If we want a basic angle, we must put it on the drawing.

I use "equally spaced", but only in conjunction with a positional tolerance called out in a FCF. I think this is a legal way to imply that the angles between the holes are basic.

I asked the question because I am using a go-by drawing that has "equally spaced" with no basic/GD&T callouts in a few places. I was wondering if this was "legal." My conclusion is that it is not, based on the fact that it is too ambiguous - too many different ways it could be interpreted as evidenced by the replies on this thread.

Thanks!

 

[drawoh]

randy64,

GD&T is mostly not a matter of legal versus illegal. Standards like ASME[ ]Y14.5 explain that things mean, including notes on the drawing. You might be surprised!

When you claim that holes are equally spaced, you have the option of specifying a tolerance. If you crunch the numbers as per my notes, the tolerances get very tiny as your pitch circle increases in size. If you do not specify an angle tolerance, the angle tolerance from your title block applies, as you noted. [±]2[°] is very unlikely to be accurate enough.

The specification is legal. All sorts of non-functional parts also are legal.

--
JHG

 

[JNieman]

My practical-mind says that there /is/ a basic angle on there, by calling out "equally spaced" you have the angle. If there are 4 objects, the basic angle is 90d between each instance. If there are 6 objects, there is 60d between objects, etc. I believe it does not need to be stated that a complete circle has 360 degrees, and that "equal spaces" is easily calculated by the obvious number of instances in the pattern. Thus the basic angular quantity is there.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[KENAT]

It's ambiguous as the first couple of responders commented.

Arguments could be made for this situation violating at least somoe of the fundamental rules:

ASME Y14.5m-1994 section 1.4(a) "Each dimension shall have a tolerance,..."

ASME Y14.5m-1994 section 1.4(b) "Dimensioning and tolerancing shall be complete so there is full understanding of the characteristics for each feature...."

ASME Y14.5m-1994 section 1.4(c) "Each necessary dimension of an end product shall be shown...."

ASME Y14.5m-1994 section 1.4(i) "A 90° angle applies where center lines and lines depicting features are shown on a drawing at right angles and not angle is specified...."



When not used with basic dimensions 'equally spaced' implies conflicting tolerances in situations like this, feature to feature V overall stack up V "implied 90°".

Now if a drawing standard isn't referenced some how by the drawing then it's difficult to say it's wrong because what yardstick do you measure to.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

KENAT,

Your angles have tolerances because you added a note stating that, unless otherwise specified, angles are [±]2[°]. The rule is being followed. If you specify six holes equally spaced, then each angle must be 62/58[°]. If an angle falls outside this, the part may be rejected by the customer.

I interpret "equally spaced" as separate, stacking tolerances, as opposed to absolute, non-stacking tolerances. The calculations on my website assume absolute, making the OP's drawing even more hopeless.

If a positional tolerance is applied to the holes, then the "equally spaced" specification becomes basic, whether or not there is a box around it somehow, or a note stating this. All of this is way less clear than a diameter applied to the pitch circle, and an angle and quantity, all boxed. One of the thing I like about boxed dimensions is that I apply them after I apply the feature control frame. Any dimension on my drawings that is not toleranced or boxed, is not controlled, and the drawing is not complete.

--
JHG

 

[KENAT]

drawoh

I was thinking along the lines of "if it's not clear what the effective tolerance is then arguably that's equivalent to not having a tolerance".

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[3DDave]

JNieman,

The problem with taking the angles as basic because of 'equally spaced' is there is no allowance for the amount of variation from that basic value. If the hole aren't perfectly located the part would be subject to rejection and there's no way they will be perfect. I think there is no mention of 'equally spaced' in Y14.5, so there is no common interpretation for it.

Kenat,

Some people I've had the displeasure of working with insist that a complete definition is a hindrance. After all, they have grinders and pistol drills on the assembly line for a reason. By miracle, the parts always fit and they saved money on inspection as well.

 

[tbuelna]

First of all, any basic dimension by definition has no tolerance on its own. The basic dimension must be associated with some GD&T notation.

There are very good reasons the GD&T system was adopted. And the hole pattern case described in the OP is a perfect example of why GD&T is so helpful. The issue of how different interpretations of the "36 equal spaces" angular spacing callout can lead to wide variation in hole positions is only one part of the problem. There is the issue of the tolerance and location of the pitch circle itself relative to the part to consider. And also the tolerance of the hole to add into the mix. For example, the center-to-center distance between two adjacent holes in a radial pattern with a given angular spacing can vary depending on their radial location relative to the nominal pitch circle diameter. The nice thing about a proper GD&T scheme is that it takes all of the various combinations of tolerance errors into consideration and allows the maximum flexibility. While a conventional dimensioning scheme does not.