Run out on datums 1

[A.Drafter]

Hello,

I was wondering why it is necessary to put a runout with a dual datum on the datum ends?

There is a round part with two datum diameters on each end "A" and "B". The middle diameter is controlled by a runout .010|A-B|.

Why is it necessary to put a control box runout.005|A-B| on each of the datum diameters?

 

[drawoh]

A.Drafter,

Datum features are selected to immobilize and locate your part. It sounds like you have a shaft that will be mounted to a bearing at each end. In ASME Y14.5, the datum[ ]A-B notation explicitly describes that mounting scenario. You can use two V-blocks to simulate it.

An alternate datum scheme would be for one end diameter to be datum[ ]A and the end face to be datum[ ]B. You could specify position or run-out on the opposite end, and your current run-out in the middle. You can fixture this by clamping the datum[ ]A face to a V-Block. Almost certainly, this is a less accurate functional description of what you are seeing, above.

--
JHG

 

[A.Drafter]

Hello drawoh,

Thanks.

I am still confused about why it is necessary to put a runout at each end where the bearing is mounted. There is a run out |runout|.005|A-B| at Datum A and at Datum B. Are we measuring a deviation partly to itself? Is this something that I have to take into account when doing the stackup, that with the V-Block setup each end could vary by .005? Is that an allowable for the inspector?

A.Drafter

 

[3DDave]

This is to orient each end to their common axis. If it isn't there then there is no control of this mutual orientation.

I'd suggest some care on the use of v-blocks for runout if the surfaces are potentially not perfectly round. See

https://www.qualitymag.com/articles/92577-form-101

 

[TheTick]

V-blocks are a very poor substitute for a spindle.

 

[TheTick]

I always wondered why the datums are separate e.g. "A" and "B", and not two zones for the same datum marked with targets, e.g. "A1" for one end and "A2" for the other.

 

[drawoh]

TheTick

I am fine with the [A-B] notation. Do datum targets clearly specify the whole FOS diameter as opposed to a spot on one side?

A run-out specification on the FOS datum features controls roundness. This could matter.

--
JHG

 

[Belanger]

The Tick -- I've always seen datum targets used when only a portion of a feature is meant to create the datum.

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

 

[drawoh]

TheTick,

The V-blocks may be what is available. Can you measure run-out of your datum diameter in a spindle?

--
JHG

 

[Dean Watts]

Why is runout on the datum features a best choice for a coaxiality? Position would work fine too. Instead of runout or position, I think a size tolerance with <CF> (datum features A and B) may be the best approach.

Dean

http://www.validate-3d.com

 

[Kedu]

work fine too. Instead of runout or position, I think a size tolerance with <CF> (datum features A and B) may be the best approach.

Dean,

If the two diameters do not have the same size, do you think CF option would work?

What do you think?

 

[Dean Watts]

Kedu,

If two different diameters then I think position or runout or profile of a surface are the only choices left to control their coaxiality.

I think <CF> is limited to features of the same size or to coplanar surfaces with flatness, parallelism, perpendicularity, or angularity. Coplanar surfaces with profile applied could also have <CF> but in that case the only effect is a change in the measurement data reporting, with a single measured profile reported instead of one for each surface.

Dean

http://www.validate-3d.com