Measure the parallelism of flat bar

[Egges]

Hi all, I'very new in the geometrical tolerancing area, I would like to ask you
- How to measure the parallelism of the flat bar. The customer's require is 0.05 mm. The problem is this flat bar is not totally flat. my boss told me to I used the dial indicator(the bar was put on the granite plate)to check. But I feel that this is to check the straightness of the bar , not the parallelism. I think that because the bar is not really flat, could I only use the micrometer to measure it's thickness throughout the bar to represent its parallelism.

 

[powerhound]

The parallelism callout should be with respect to a datum. A datum in this case should be a theoretically perfect plane. The granite plate is an appropriate datum simulator so you are correct to lay the bar on the granite plate and run an indicator along it. In GD&T, the parallelism is not with respect to the other surface, it is with respect to a datum generated by the high points of that surface. This is what the granite plate does.

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

 

[ctopher]

I agree with Powerhound. I use this process all the time.

Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion

 

[Belanger]

The boss is right. Parallelism inherently includes flatness (unless there is a circled T modifier). So if its flatness error is more than 0.05, it fails parallelism.

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

 

[Egges]

Thanks for all of your answers ... one more question pls,
As Belanger posted, the parallelism inherently includes flatness .. But as reference to the standard (jis, as ..) for the steel bar (ss400), it states that the straightness torelance is appx 2mm/metre long ... Therefore, I understand that the flat bar should be allowed to have a little distortion along the bar (no matter the ends are up or down). So that's why I think that the way I measured (run the dial indicator along the bar placed on the granite plate) is to measure the staightness.
So, now as I measured, the parallelism (as the way I said before) is 0.35 mm because of the distortion at the bar's ends. Then, what I should do is to bring these bars to do a surface grinding, isn't it?

 

[ptruitt]

Is the parallel requirement a part design specification or are you inspecting raw material before it is fabricated?

Peter Truitt
Minnesota

 

[dingy2]

Powerhound said it best and he is following the ASME standard on this.

But

There is a bit of a trick to measuring parallelism if the bottom surface, which is the datum, is not relatively flat. If there is a high point on the datum surface (pimple in the center) the part will teeter or wobble so it will be more than difficult to achieve a solid FIM or TIR. What should you do? I would then set up a 3 solid jacks towards the perimeter 120 degrees apart to create the datum surface. This may not be quite shown in the standard but trying to achieve a FIM or TIR when the part wobbles on the granite table is more than difficult. I couldn't measure the feature if it is wobbling.

Dave D.

http://www.qmsi.ca

 

[ak762]

I would stay with GD&T and specify datum with double precision flatness vs. parallelism for opposite feature instead to follow inspection tip&tricks

 

[Belanger]

Dave, the Y14.5 standard addresses that very problem in paragraph 4.11.2. (However, I don't have a copy of Y14.5.1 handy to see exactly what the stabilization procedure is; maybe it's similar to what you describe). I recall in the 1994 standard that we were told that you could rock the part either way, whichever way made the part acceptable. Gotta love that!

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

 

[dingy2]

Belanger:

4.10.1 (a) - 2009 does cover the rocking on a primary datum in a rocking situation.

It states "This primary datum feature contacts the datum feature simulator on a minimum of three points(see para. 4.11.2 for discussion rocking or unstable datum features).

Looks like the 3 point set up on a rocking situation is covered by the current standard.

Dave D.

http://www.qmsi.ca

 

[Belanger]

Not sure what your last post is saying... Yes, para. 4.10.1(a) mentions the 3-point setup, but it doesn't address a solution to the rocking situation. That situation is deferred to para. 4.11.2, which then defers to the Y14.5.1 standard.

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

 

[Egges]

To ptruitt, the parallelism torelance is specified in the customer's drawing. It's flat bar with appx 1800 x 100 mm dimension. Quite long bar so that's why I think that it's quite difficult to achieve to requirement. The surface smoothness is also specified but only 25 --> seems that it's quite rough surface. Then I don't think that the machining/grinding process is required. Only the parallelism torelance is a big problem for us now.

Could anyone pls enlighten me why I can't use the thickness to represent the parallelism. Thank you

 

[powerhound]

You can't use the thickness to represent parallelism per the ASME standard because parallelism is specified with respect to a theoretically perfect FLAT plane.

It is entirely possible that whoever specified the parallelism on the print is thinking the same way you are, like the train tracks that roll around mountains yet are parallel to each other. This just isn't how ASME defines parallelism though.

Is ASME Y14.5 specified on the print?

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

 

[Egges]

There is no "ASME Y14.5" specified on the dwg. BTW, I'm now take a rough scan on ASME Y14.5 and there are some questions would like to ark.
In the ASME Y14.5-2009 page 100, describing about the parallelism torelance meaning. In the fig. 6.2, showing the one block sized 26.6-26.9 (this should be the min/max dimension of this block), then parallelism is specified 0.12 with the datum A -> the bottom of the block. Could this means that the acceptance boundary is 26.9 + 0.12 = 27.02 ? I afraid that I could confuse with the flatness (page 95) or straightness (page 93).

Thank you

 

[pmarc]

No, it means that parallelism tolerance zone 0.12 can float within 0.3 size tolerance zone. Parallelism is not additive to size limits, therefore maximum height of the block can be 26.9 and minimum 26.6.

 

[ptruitt]

Egges,

Perhaps you should request that the customer specify the Standard that defines the drawing. You had pointed out that the Standard that controls the raw material conflicts with the parallelism requirement. The parallelism call-out takes precedence over the raw material Standard. It is good that you are aware that parallelism will be difficult to achieve, given the raw material. You might need to source raw material using special purchase requirements or straighten the raw material get thicker material and grind it flat, or scrap some of it. So your quote to the customer should reflect the costs of your approach.

Peter Truitt
Minnesota

 

[Egges]

Thanks for all of your answers and suggestions ... I already informed my boss to aware this problem and pls discuss to the customer. Just like you said, Peter, it concerns the products cost to reach the customer requirements.