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GD&T machined octagon centered on shaft

GD&T machined octagon centered on shaft

(OP)
Lets see if you guys can figure this one out,
I'm correcting old designs and drawings for my company, and I'm trying to clearly indicate the concentricity of an octagon shape on a shaft.
This octagon is to be press fit into a pinion (68mm h6).
Datum [A] is the center of the shaft.
The ground cylinders that will mate with bearings are: '[CON|Ø0.030|A]'.

I've looked at using basic dimensions: [3x45.00°] & [SYM|0.000(M)|A] except that symetry does not allow the of MMC.

Do I have to use profile tolerance for this application?

RE: GD&T machined octagon centered on shaft

"Concentricity is is a condition of a surface of revolution in which all points of the surface are equidistant from a common axis."
An octogon does not meet these conditions.  Profile may be the best alternative.

RE: GD&T machined octagon centered on shaft

The application of concentricity is complex and rare.  I would consider uses position, runout or profile before concentricity but in your case I would use profile.

Best Regards,

Heckler
Sr. Mechanical Engineer
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RE: GD&T machined octagon centered on shaft

I agree with Heckler. I very rarely used concentricity. It is more difficult to inspect.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

RE: GD&T machined octagon centered on shaft

Amen to profile. It is the way to go for this application.

RE: GD&T machined octagon centered on shaft

3
My preference would also be to use Profile of a Surface.  There is another alternative, however.

You can use positional tolerace to locate the center plane of opposing faces because they are features-of-size.  This also allows you to use MMC if appropriate.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca

RE: GD&T machined octagon centered on shaft

I as well agreewith heckler. I would go get the Octogonal material from stock, check Central Steel and Wire catalog. This is a stock item.
Regards,
Namdac

RE: GD&T machined octagon centered on shaft

(OP)
Getting closer?


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RE: GD&T machined octagon centered on shaft

3
Cylindricty and the 2 runouts are applicable on round features of size - not applicable here.

The profile of a surface is theoretically applicable since it would control the shape and location relative to the datum but one must have a CMM with scan mode to measure this feature. It would not be measured on the shop floor well unless this equipment is available.

Positonal tolerances may be the best approach and one could use MMC. I would suggest a 0 tolerance beyond MMC since this hex shape has a snug assembly fit. I would also reference my datum(OD) at MMC. I might even place the word "BOUNDARY" below this FCF. The virtual condition is actual the MMC size and shape of the hex.

This would then be conducive to a checking fixture in which there would be a hole of MMC size for the datum and hex at the MMC size and in true position to the datum. One would then place the part datum first into the fixture and rotate the part so that the hex lines up. The part should then be able to be pushed (with the force of 1 finger) all the way down in the fixture. If it goes all the way, presto, the feature is within tolerances.

This feature could then be confirmed on the shop floor by the Operator.

Dave D.

RE: GD&T machined octagon centered on shaft

MechTech, getting there, but not done yet. Pls click on the following link for how I'd tackle this, at least at first glance.

http://www.profileservices.ca/files/tidbits/tidbits_missc.html

Symmetry & Concentricity are too hard to validate, so don't use them ... EVER!  You don't need the 45-degree angles to be shown; they're quite obvious from the graphic.  The toleranced size across the flats shouldn't be used if you're using the surface profile to control the flats.  The radius shouldn't be reference, it should be basic.  The profile tolerance should be .05-ish with a unilateral-inside zone (shown by a heavy chain line inside of the geometry).  You can / should use positional tolerance to locate the center plane generated by the dimension across the two edge radii.  There isn't much contact area on those radiused edges at the vertices of the octagon, so they're not really a good locating surface.  They would be better if the vertices were rounded by a Dia-36.5 cylindrical surface, with reduced flats, providing that the alignment within the pinion is more relevant than the secondary use of the flats.

Make any sense, MechTech?

Dave, verification of the surface profile on the flats is extremely easy without a CMM.  Chuck the workpiece on the feature of size that is generating the datum axis; chuck should be inspection quality and indexable.  Lock the workpiece from rotating once a flat is in a horizontal position.  Zero an indicator at the basic offset of 34 from the axis of the chuck, and check for deviation from there.  Rotate the workpiece basic 45-degrees, and repeat the check.  The controlled alignment diameter can also be checked in the same setup.  It's a very quick method, and yields numeric results rather than go/no-go.
The gage you describe would also work nicely, it's a question of quantities and the need for numeric feedback.  It's always nice to have options.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca

RE: GD&T machined octagon centered on shaft

MechTech2000,

   I looked at your drawing.  

   If your octagon is being pressed into a mating octagon shape, your tolerancing should be very simple, albeit, very accurate.  Just use a profile tolerance all around.  The symmetry and the +/- tolerances do not help you.  Your drawing defines the distance between opposite faces very accurately, but allows adjacent faces very much more error.  

   If it were my drawing, the surface tolerance would be 0.01 all around, and I would use a phantom line to show that this error all went inside from nominal.  See Figure 6-15 in ASME Y14.5M-1994.  I would not bother with the point to point dimension except maybe as a reference.  I would specify the radius sloppily as something like +0.5/-0.  

                         JHG

RE: GD&T machined octagon centered on shaft

If you wanted to try using concentricity in this case don't do it directly with your polygon, instead use an inscribed circle in the polygon. That should eliminate some of your problems in displaying positional and relational feature controls.

RE: GD&T machined octagon centered on shaft

MechNorth - I do agree with you on the method of measuring Profile of a Surface on hex surface. One must make sure that the initial face must be level rotationally and then after that surface is confirmed, rotate the part 45 degrees and go again. One has a +/- .005 using a profile tolerance of .010.

If one goes in this direction, make sure that there is only a small number of parts being produced and all dimensions reflecting the angle and distance would be basic. One does not need the point to point dimensions, only the flats and the angle.

Position at MMC is appropriate for large number of product.

RE: GD&T machined octagon centered on shaft

I was always under the impression that concentricity controlled centerlines between features regardless of the shape of the feature therefore you could call out a round shaft concentric to the end of a square bar or vice-versa. I would have recommended calling the profile of the hex concentric to the shaft, however seeing as most others here seem to be settled on "profile of a surface" I'll have to re-evaluate my understanding of the concentricity callout.

Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: GD&T machined octagon centered on shaft

MechNorth,

I have read many of your responses and agree and am generally impressed with your knowledge of GD and T.
This particular thread is perhaps with more than one good solution however.  

I tried unsuccessfully to open your link to your proposed solution. (I am by no means a computer 'whiz'.)  So I am left without the solution that you proposed for this.
If there is another way to get it please let me know, thanks.

But I still am left with one question, regarding the statement that you made that the 45 degree angle was obvious. I thought it looked more like a 44 degree angle.  Which is to say that I think it needs to be specified.


What do you think?
 

RE: GD&T machined octagon centered on shaft

Ringman, thank you for the compliment.  

I don't like to post things on third-party sites, but if you reach me through my website, I'll send you a PDF of the graphic.  

The 45-degrees is from basic geometric relationships of regular polygons and complementary angles.  I've posted the graphic explanation, but it's basically the same as from Machinery's Handbook.  I started to type out the explanation, but that made it look very complicated.  I'll send it with the dimensioning graphic also.  http://www.profileservices.ca/files/tidbits/tidbits_missc.html

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca

RE: GD&T machined octagon centered on shaft

powerhound,
The quote I posted defining concentricity on 24Oct06 is taken directly from ASME Y14.5-1994.

RE: GD&T machined octagon centered on shaft

ewh,
  I went back to the source that I originally drew that conclusion from and I'm not sure what to make of it. It's on the tec-ease.com website and it's one of the GD&T tips of the month. Look at runout vs. concentricity and see what you think. I may have taken the article out of context. I deal with this callout unnecessarily on almost a weekly basis as people think that concentricity is always the best way to call out concentric features. We do a lot of large tubes here and because tubes are cylindrical in form, everybody calls out cylindricity when it's absolutely unnecessary. I guess it's just something you have to deal with when most draftsmen have only basic GD&T familiarization training and then they're turned loose.
  If you go to the site I talked about, let me know your thoughts. I'm always on the lookout for new views and challenges. That's why I like this place so much.

Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: GD&T machined octagon centered on shaft

Concentricity is one of the most commonly misunderstood and hence misused controls that I've seen (Symmetry is another one).  The most common misconceptions are that (i) concentricity relates to two surfaces which follow each other without respect to a center point or plane, and (ii) concentricity only applies to a nominally circular surface.  Sometimes you have to study & re-study the standard to get the subtleties, and this is one of those times.

Under Y14.5M-1994, 5.12-Concentricity: "Concentricity is 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.", which means that it is not restricted to nominally circular features, but rather to elements of opposed features (i.e. on opposite sides of a center point or center plane), as indicated in some illustrations in the Tec-Ease tip.  

5.12.1, last 3 lines of the first paragraph; "...a concentricity tolerance requires the establishment and verification of the feature's median points."
5.12.1, Italicized paragraph; "...finding the median points of the feature may entail a time-consuming analysis of surface variations. Therefore, unless there is a definite need for the control of the feature's median points, it is recommended that a control be specified in terms of a runout tolerance or a position tolerance."
... in other words, don't do it!!!

The Tec-Ease Tip illustrates several configurations where concentricity can but should not be used, and illustrates that in the particular case of round features, total runout should be used instead.  Unfortunately it doesn't show the preferred controls for the other situations.  I'll drop a note to Don @ Tec-Ease to see if he can amend that Tip.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca

RE: GD&T machined octagon centered on shaft

Thanks Jim,
  I feel a little better now. I always broke concentricity down to this: It only controls centerlines; The form of the features will be controlled by the tolerance unless another form control is applied. From your reply, I think I was correct in my understanding of concentricity and although concentricity is usually reserved for high speed (RPM) applications and such, this might be an exception considering it's not as difficult to check because there are flat surfaces to measure from. The biggest problem I can see is that the flat-to-flat tolerance (opposing flats) may have to be unnecessarily tight to keep the hex looking symmetrical. I guess when that is taken into consideration, profile of a surface may be the way to go here. What do you think?

Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: GD&T machined octagon centered on shaft

MechNorth - I agree with you that concentricity (won't get into symmetry here) should not be used should not be used on a drawing. One should use positional or circular runout.

Most people think that concentricity means centre to centre distance between 2 coaxial features but it doesn't. They also get messed up in the interpretation of the diametrical tolerance zone. This is one of 2 areas where radially tolerances apply.

I am a bit confused when you state a concentricity misconception is that it "only applies to a nominally circular surface". Are you stating that it can apply on flats on a hexigon? Please show me an example in the standard (I have also studied the standard) where concentricty has applied to anything but a round feature of size? Figure 5-57? 5-54?

If concentricity should be replaced with circular runout (you stated total runout), then could circular runout apply to anything other than round features of size that share the same axis?

Dave D (your northern neighbour)

 

RE: GD&T machined octagon centered on shaft

Powerhound, you are correct, concentricity controls the derived centerline only, and the actual surface must be controlled otherwise.  If the surface is circular, Runout/Total Runout is the best solution.  I would use Total Runout for a rotary balance situation, it's a lot easier to verify than concentricity.

Dave, the standard is an imperfect and incomplete document.  There are a number of sections that illustrate only a couple of the possible situations that are described in the text, and you often have to extend the application described into new situations.  Flats on a hex are one such situation.  The key here is the radially opposed points.  Check the sketches I posted on my site.  http://www.profileservices.ca/files/tidbits/tidbits_missc.html
One problem that I have with using concentricity for this application is that you would need the stylus/probe to be inline with the diametral line to get the correct reading, rather than normal to the surface ... as a result, the radius of the tip of the probe/stylus increases the error of the measurements.

By the way, where are you located, northern neighbour?  

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca

RE: GD&T machined octagon centered on shaft

MechNorth - I think that you meant "diametrically opposed" and I hope that you don't mind but I disagree with your thought. I am using the standard and there is nothing that would indicate anything other than a round feature of size anywhere. Let me know of anywhere in the standard where it reflects, states or implies your thought.

You are saying that one could use circular runout on flats on a hexagon?? Sorry but I absolutely disagree with that and I hope the Designers here never place that on a drawing. Profile of a Surface, right on, but concentricity or circular runout - no way.

I will let you have the last word on this subject.

RE: GD&T machined octagon centered on shaft

Hi Dave,
Rereading my posts, I haven't said to use total runout on the flat of a hex..."If the surface is circular, Runout/Total Runout is the best solution."  Perhaps I should have said cylindrical instead of circular.  I fully agree with you that you can't use runout/total runout on the flats of a polygonal object.  Also, the standard itself uses diametrically opposed and radially opposed interchangeably {...(or correspondingly-located elements of two or more radially-disposed features)...}, see above post.

Some progressive uses of GD&T require the extension of principles from the standard into new applications.  The standard doesn't indicate the use of three-level composite surface profiles, but does illustrate a two-level usage...does that mean you can't use 3-levels?  No, and it is a very useful control.

The standard does not indicate anywhere in the definition of concentricity that it is only applicable to a Feature of Size; it speaks only of diametrically/radially opposed elements.  The standard is quite consistent in indicating a Feature of Size where that is the intended limitation but it does not specify a FOS here.  To flip the table, why do you feel that it only applies to FOS?

I would never use concentricity or symmetry, much less in this application.  I rather like Profile of a Surface...it's robust and inclusive of other controls.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca

RE: GD&T machined octagon centered on shaft

MechNorth - I stated "round feature of size", not "feature of size" and I should have said that the part must be small enough to be placed in a divider head or chuck.

The standard states that "runouts" are recommended in place of concentricity. One would never place a total or circular runout on a hexagon shape or any other shape than a cylindrical feature of size. It just does not happen.

I hope that the Designers here do NOT place concentricity on anything else but a round feature of size. Better yet, don't use it all. Roundness is a factor in concentricity and trying to find the median point is rather difficult. Circular runout should be used in place of conentricity. It is easy and we all can understand it.

Shop Floor - If you see concentricty with a diametrical tolerance zone - tip - just perform a ciruclar runout using the value in the FCF as the circular runout criteria. One does not have to divide the value by 2 and all that stuff. Circular runout does take into consideration the roundness and also off-centre condition.
 

RE: GD&T machined octagon centered on shaft

Wow, what a direction this thread has taken. Although it hasn't been hijacked, it has led to a productive discussion on the application of misunderstood callouts. I've almost abandoned my contention of using concentricity but I'm still hung up on the idea that using diametrically opposed elements to establish a centerline might be a good idea. I know in practice, most people who understand concentricity rarely use it, but that needn't be the rule across the board. Maybe most people avoid it just because they're used to avoiding it. The callout has to be good for something besides a point to argue against using. Just as concentricity is usually reserved for high speed applications, maybe the other reservation is to center a polygon on a round shaft. It's just a thought.

Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: GD&T machined octagon centered on shaft

Dave, I got burned by omitting the first part of the 5.12.1 italicized paragraph.  My apologies.  Here is the relevant material from that paragraph; "For example, a nominally cylindrical surface of revolution may be bowed or out of round in addition to being displaced from its datum axis.  In such circumstances, finding the median points of the feature may entail a time-consuming analysis of surface variations. Therefore, unless there is a definite need for the control of the feature's median points, it is recommended that a control be specified in terms of a runout tolerance or a position tolerance."

The passage is using a cylindrical feature as an illustration, and bases the replacement of concentricity with runout on it being a cylindrical feature.

Powerhound ... DON'T DO IT!!!  Concentricity, if nothing else, is a very difficult control to verify.  Go towards the light!

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca

RE: GD&T machined octagon centered on shaft

I...I...I feel faint...must get air...awwww, forget it. Adios concentricity...I know you're good for something, but what that "something" is, I guess I'll never know.
  Okay Dave, I've been clean and sober from concentricity for a full 5 minutes and I already feel better. What was I thinking?

Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: GD&T machined octagon centered on shaft

I think that both MechNorth and dingy2 deserve a star for delving more deeply into this subject.

RE: GD&T machined octagon centered on shaft

I believe I have a use for concentricity.

We make inspection gauges for holes drilled in multiple parts.  The holes are tightly toleranced and must be concentric.  The gauges are frequently stepped to different diameters.  The different diameters must be cylindric and concentric.  We check runout, but the runout of any individual diameter alone is not enough to ensure the accuracy of the hole we are checking.  We normally inspect these manually, not on a cmm.

Responses and comments welcome.

RE: GD&T machined octagon centered on shaft

The only use I have had for concentricity is similar, used on the bore of a servo-valve (several separate surfaces).

RE: GD&T machined octagon centered on shaft

The stepped gauges reflect a positional tolerance. One cannot have an attribute gauge for concentricity.

For the gauge to be used, the positional tolerance must have MMC and also the reference datum must also be in MMC.

Dave D.

PS - I am now finished on concentricity for good!!!!

RE: GD&T machined octagon centered on shaft

I agree with ewh in the fact that MechNorth and dingy2 deserve a star for thier indepth input. Bravo!! The rest need to read the GDT Manual or take a class. From what I read a class is in order. Check with your local Community Colleges should carry the class. If not let me know and I will give you the ssbn# and you can check Ebay for the Manual.

This was going good until the train left the track.

Regards,
Namdac

RE: GD&T machined octagon centered on shaft

I don't think I effectively conveyed what I meant.  The stepped gauge is what we manufacture.  It is the form of a pin with a handle, with multiple diameters.  The pin must be inserted into the holes in the mating parts.  This pin verifies that the holes are drilled to spec.  The cylinders in the gauge pin need to lie on the same axis.  This is a GO/NO GO gauge to detemine the positional tolerance of the actual holes in the mating parts.  A NO GO result using this particular gauge could result from either a positional error of one of the holes relative to the other or a hole diameter that is too small.  This gauge is next to useless as a check of hole size, since the only thing it can concretely tell you about hole size is if the smallest hole is too small.

Dave, if I were to use a positional tolerance on two holes which have the same nominal center, wouldn't I be defining concentricity?

In summary, this gauge does not measure concentricity.  It must be machined such that the different working diameters on the gauge are concentric.

RE: GD&T machined octagon centered on shaft

In my previous post, where I said "This is a GO/NO GO gauge to detemine the positional tolerance...", please amend that to "This is a GO/NO GO gauge to assess the relative position..."

RE: GD&T machined octagon centered on shaft

Ok - I am back on this one.

Some people call these types of gauges "concentricity gauges" but they are really positional gauges.

Tolerance on gauges is 10% of the part tolerance and to check the gauge, one should apply a circular runout on the steps relative to the datum.

For sure, I am out of here!!!

RE: GD&T machined octagon centered on shaft

"one should apply a circular runout on the steps relative to the datum."

Runout relative to a datum axis would yield concentricity, if the profile is circular, right?

I was interpreting most of this thread as calling for runout instead of concentricity, but runout by itself only gives you deviation from a circular profile, correct?  

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