We have a sweep TIR gage but True Position Call Out 1

[JoeBlock]

Right now the instructions are find the smallest reading, zero the indicator and sweep then record the highest reading. That's TIR right? but the print says True position. Can I get TP using this TIR Gage? The part is sitting on the datums and there is a master.

 

[Belanger]

What you've described is indeed Total Indicator Reading (TIR), but you can't check position with a dial indicator (TIR gage). That's because your gage will feel variations in position but also form (such as out-of-roundness, which is beyond the job of a position tolerance).



John-Paul Belanger
Certified Sr. GD&T Professional

http://www.gdtseminars.com

 

[dingy2]

JoeBlock:
You, indeed, are coming up with the TIR or in some parts of the country called "FIM".

I am going to ASSUME that this TIR is in a hole and not a flat surface so you are actually checking roundness (maybe cylindricity) of the hole. This, again, is assuming that you are setting up on the hole rather than taking a basic dimension to the hole. So many assumptions here.

Position tolerance only applies to features of size such as a hole or pin of any shape and, simultaneously, confirms location, perpendicularity, orientation and shape. If the positional tolerance is shown in MMC, then there is a solid cylindrical plug gauge at its virtual condition size. If the positional tolerance is shown in RFS, then one would use a tapered cone rather in the checking fixture and, somehow, the actual location off its true position (theoretical location) would be shown.

Hope this helps.

Dave D.

http://www.qmsi.ca

 

[axym]

Further to John-Paul's comment, the TIR method checks the runout of the feature and not the Position. Sweeping individual cross-sections of the feature gives a value for Circular Runout, and sweeping the entire feature gives a value for Total Runout.

That said, you can use the Total Runout value as a conservative estimate of the Position value. Because Position ignores form error, the Position value will never be larger than the Total Runout value. If the feature is perfectly cylindrical, the two values will be the same.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[dingy2]

Evan:

How do you know that the datum is co-axial with the feature and both feature and datum are cylindrical? It is stated that "the part is sitting on the datums".

mmmmmmm - A lot of assumptions here.

Dave D.

http://www.qmsi.ca

 

[JoeBlock]

John, David, Even,

Thanks for all the help. Picture a large step cylinder. The datum is half way up the longest bore. The gage chucks up on that datum. Then the indicator drops down through a bushing and checks the TIR of a smaller bore. 1) First we chuck up on the master cylinder. 2) Then we drop the indicator through the bushing and zero it (there is a small 3 micron TIR error in the Master). 3) We remove the master and chuck up on the part's large cylinder. 4) Drop the sweep through into the small cylinder. 5) We spin the indicator until we find the highest reading. 6) Then we zero the indicator. 7) Then we spin the indicator and record the highest reading.

In step 5 when we sweep the part what does the high reading represent? Is that the runout? Then in step 7 what does that reading represent? Is that TIR?

Thanks Again,
Gary

 

[dgallup]

Step 7 is runout. If you moved up & down the bore & checked several locations you would have total runout. The only thing accomplished by zeroing the gage in step 5 is eliminating having to add the plus & minus readings of the gage.

In any case, runout will always be greater than or equal to your position error. Runout is a composite measurement. It includes both error in position and error in form. If your surface is exactly concentric (position error is 0) but egg shaped you will still show runout. Your surface can be perfectly round but off center and you will still show runout. If your runout is less than your positional tolerance than the part is definitely good but if your runout is more than your positional tolerance it may be good or bad.

 

[JoeBlock]

Thanks Dgallop,

I had one further question. When you add both together or zero then you have doubled the rad so there is no need to multiply by two like when you calculate true position correct?

Thanks Again,

Gary

 

[Belanger]

Runout provides a radial tolerance zone, but position is a total diameter (or sometimes a total width).

So if you have zeroed the gage on the highest point, and then sweep to find the new extreme point, your TIR needs to be less than or equal to the runout number shown on the print -- no doubling or halving of anything.

John-Paul Belanger
Certified Sr. GD&T Professional

http://www.gdtseminars.com