Two questions regarding Pitch Circle Diameter in Gear

[Siva Mareesan]

According to ASME standard

1. Can a PCD be used as a datum ?
2. Can we apply geometric controls to PCD of a gear tooth.

Don't get confused of what kind of gear & its application. Its a general question.

 

[3DDave]

1) Sure, if you can figure out how to perform a meaningful inspection.
2) Gear makers never do.

I'm sure you'll work things out.

 

[drawoh]

...
1. Can a PCD be used as a datum ?

Can you fixture to the pitch circle.

You can locate the bore from the pitch circle, or you can locate the pitch circle from the bore. This is why I don't like modifying off-the-shelf gears. Custom gears, with the bores used as datum features, actually are not all that expensive.

--
JHG

 

[Siva Mareesan]

hi Dave,

Since PCD is a imaginary surface, Do ASME supports assigning PCD as a datum Feature.

 

[CheckerHater]

The common approach would be measuring in relation to bore:

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Belanger]

Siva -- yes, according to ASME. Here's a quote from the 2018 version of Y14.5 (paragraph 5.11, with my emphasis added):

"Each tolerance of orientation or position and each datum reference specified for features other than screw threads, such as gears and splines, shall designate the specific feature of the gear or spline to which each applies (such as 'MAJOR DIA,' 'PITCH DIA,' or 'MINOR DIA')."

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

 

[drawoh]

Siva Mareesan,

You can inspect gears over pins. You use three or four accurate pins which contact opposite gear teeth, contacting at the pitch circle. This allows you to measure the pitch circle. If your datum feature is an accurate bore, you can use pins to measure the pitch radius, tooth by tooth.

If you call up your pitch circle as a datum feature, you must locate that circle so that you can measure all the other features from it. This sounds to me like one hell of a challenge.

--
JHG

 

[drawoh]

Siva Mareesan,

If you are absolutely determined to use a gear pitch circle as your datum, how about this?

Datum targets.

You would use pins to pick up the gear tooth faces at the pitch circle diameter. You need to pick up an additional pair of teeth to do X/Y centreing (not shown). Note how the orientation hole defines which teeth to measure from. You can provide the inspection pin diameter as a reference dimension.

--
JHG

 

[Siva Mareesan]

Thank you all for your advise. Thank you Belanger for your ASME reference which I can have as a proof to show PCD as a datum feature & also to apply geometric controls to it.

It was a useful conversation . Thank you all once again.

Regards
Siva Mareesan

 

[mfgenggear]

It is not standard practice to use a gear on an engineering drawing as a datum.

Reason: a gear must be rolled with a master gear or analytically inspected from a Bearing surface by a gear checker
For Tooth to tooth pitch error, total comulativee pitch error.
Lead error, and profile error.
Pitch diameter run out.

So no making the gear as a datum is not standard practice.
Pick a bearing diameter as a datum.
For manufacturing if the gear has centers it can be use as a datum because that will be used to manufacture and inspect the gear.