Staightness of a tapered pin.

[Madhu454]

Hi All,
have one more question on straightness.
How to control the straightness of a Tapered Shaft.

 

[Madhu454]

I mean the staightness of the median line. not the surface elements

 

[dingy2]

If the tapered shaft can be placed in a chuck, go for it and then zero off the other end with a dial indicator and a stand. Once you zero off the opposite end from the chuck, you now have the axis true. Place the indicator in the centre of the feature half way down its length and rotate the part. Record the TIR, let's say it is .010. You now have a straightness of .005. This can be performed at various cross sections along the shaft and one would report the worst condition. Unfortunately, we have roundness included so this method I would consider a 80% method.

On a CMM, take a number of readings on one end of the tapered shaft to arrive at a centre. Take the same number of readings on the other end and then create an axis. We now have eliminated the roundness of the feature. Take a readings perpendicular to the axis in a few cross sections and report the worst case non-straight situation. This is probably a 95% method but we need to take as many readings as practical.

Dave D.

http://www.qmsi.ca

 

[pmarc]

The question is how to specify it on the drawing?

 

[israelkk]

You define a diameter close to one end of the pin as datum A and another diameter close to the other end as datum B and then define straightness of the pin axis with reference to A-B. The checking will be done as dingy2 suggested. It will be easier for checking if you allow/define centering points on both ends of the pin.

 

[pmarc]

israelkk,

The point is that per Y14.5 and ISO standards referencing any datum in straightness FCF is simply illegal.

 

[dingy2]

Madhu454:

To reflect the straightness requirement on a tapered shaft on the drawing, place a feature control frame with a leader line to the OD of the tapered shaft. In the feature control frame place a diametrical tolerance zone (reflecting centre line) and the tolerance. If the feature control frame does not have the diametrical tolerance zone, it would reflect the straightness of linear lines on the OD.

It is not necessary to show one end as datum A and the other end as datum B.

Dave D.

http://www.qmsi.ca

 

[fsincox]

the ISO allows the application of a straightness to the centerline of your tapered pin.
Frank

 

[israelkk]

Do not use straightness symbol at all just use run-out or total run-out along the pin with reference to A-B as defined in my previous post.

 

[powerhound]

You won't be able to use total runout on a tapered pin. The only way you are going to control the axis of a tapered pin is by controlling its surface elements, even though you said you didn't want to do that. What you are trying to achieve is done with either profile of a surface or circular runout. Only use runout if there is an available datum feature from which to measure. A tapered pin is not a feature of size.



Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[Belanger]

Total runout can be used on a tapered surface, as long as the angle of taper is given as a basic dimension. The problem with any kind of runout in the present situation is that there is no other feature to serve as a datum; to have a surface attempt runout against itself is simply form control. But it sounds like the original question was after straightness of the center line.

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

 

[Belanger]

Dave, you mentioned having a feature control frame with the straightness symbol simply pointing to the OD, and then you wrote that the diameter symbol, or lack of a diameter symbol, would be the clue as to whether the intent was surface or FOS straightness.

Isn't there more to it than that? We would usually say straightness should have the FCF tagged with a size dimension in order to communicate that it's axial straightness. Consider Fig. 5-1 in the 2009 standard ... adding a diameter symbol would not instantly transform that callout into axis straightness.



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

 

[powerhound]

I guess since I didn't see an example in the standard of total runout on a tapered diameter AND since every example of a tapered diameter is controlled by profile of a surface, I assumed that was the way it should be specified. Of course I am open to correction if I have just missed a figure. I understand extensions of principle but does total runout on a tapered surface give something that profile of a surface doesn't?

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[Belanger]

Extended principles -- exactly. Paragraph 9.4.2. says that for total runout, the dial indicator is "fixed in orientation normal to and translates along the toleranced surface." So if I give the angle of taper as a basic dimension, that just means that the dial indicator is also rigged at that angle; the definition hasn't been compromised.
(With my luck, however, this will be something that ASME "didn't envision" when they wrote that paragraph!)

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

 

[israelkk]

ASME Y14.5-2009 fig. 2-21 sample defines how to dimension taper pin. If you select in this example the 9.9 to 10.1 dimension and tolerance according to your requirements (for example 10 +/- 0.05 instead of 10 +/- 0.1) you control the radial tolerance which in fact dictates the straightness too.

 

[dingy2]

J-P:

Well, I admit that there is not an example of straightness of the axis on a tapered round pin. I will also agree that it is best to show the straightness of the axis under the dimension but that is not possible here since the taper is made up with 2 dimensions - 2 diameters or a diameter and an angle. We cannot show a centre line with straightness but must shown the feature.

What does that leave us?

We could show the tolerance of a taper as israelkk stated but that is the taper and not the straightness of the axis.

Usually the leader line from a feature control frame to the OD means straightness of the linear line along the length of the feature when straightness is shown without a diametrical tolerance zone. With a diametrical tolerance zone, it does mean axis.

I agree that this is not perfect and might cause confusion out there but I think it is the best option.

Dave D.

http://www.qmsi.ca

 

[israelkk]

dingy2

"the zone" the pin has to be within indirectly defines a centerline.

 

[Belanger]

Dave, I guess I agree that this being a tapered pin leaves us in somewhat of a gray area. With the philosophy of extended principles, your idea of simply pointing to the OD and showing a diameter symbol in the FCF seems to make sense.

However, for a cylindrical part (non-tapered), I will maintain that we can't point to the OD and simply rely on the diameter symbol (or lack thereof) to communicate which variety of straightness is being done (again referring to Fig. 5-1).

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

 

[dingy2]

J-P:

I agree that on a cylindrical feature, one must show the feature control frame with a diametrical tolerance zone on the straightness below the size tolerance.

Dave D.

http://www.qmsi.ca

 

[pmarc]

Guys, don't you have an impression that again (just like in case of location of conical features) profile of a surface is the most reasonable tool for controlling straightness of median line of tapered/conical features? Or at least the most legal one.

This would be of course indirect control but as long as straightness and total runout cause troubles because it is not shown anywhere in Y14.5 how to technically apply them for that kind of situations, profile of surface at the end may be the only way to go.

Extended principles philosophy may work but not necessarily has to.