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?
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
An octogon does not meet these conditions. Profile may be the best alternative.
RE: GD&T machined octagon centered on shaft
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RE: GD&T machined octagon centered on shaft
Chris
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RE: GD&T machined octagon centered on shaft
RE: GD&T machined octagon centered on shaft
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
Regards,
Namdac
RE: GD&T machined octagon centered on shaft
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RE: GD&T machined octagon centered on shaft
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
ht
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
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
RE: GD&T machined octagon centered on shaft
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
Powerhound
Production Supervisor
Inventor 11
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Taji, Iraq OIF II
RE: GD&T machined octagon centered on shaft
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
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. ht
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
The quote I posted defining concentricity on 24Oct06 is taken directly from ASME Y14.5-1994.
RE: GD&T machined octagon centered on shaft
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
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RE: GD&T machined octagon centered on shaft
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
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
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
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. ht
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
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
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
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
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
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
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
RE: GD&T machined octagon centered on shaft
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
RE: GD&T machined octagon centered on shaft
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
This was going good until the train left the track.
Regards,
Namdac
RE: GD&T machined octagon centered on shaft
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
RE: GD&T machined octagon centered on shaft
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
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?