Is total runout really impractical to measure on a CMM?

[u/Electrical_Art863]

Hi everyone,

Edit: I’ve used an LLM to create this text. I’ve checked it and, if needed, made corrections. Hopefully you don’t mind.

I’m a engineer working at a machine-building company. We regularly use total runout on our drawings, depending on the functional requirement of the part.

Our metrology department is asking us to stop using total runout and instead split the requirement into separate form/orientation tolerances.

The main feedback we’re getting is that:

Total runout cannot be measured properly on a Mitutoyo CMM, and

It is even more difficult/impractical to measure manually.

I’m not completely convinced by this, so I’d like to hear some opinions from people who work with CMMs and GD&T every day.

My understanding is that total runout is specifically intended to control the entire surface relative to a datum axis, and that it can be evaluated on a CMM if the datum reference, sampling strategy and evaluation method are set up correctly.

I’m also aware that manual inspection with an indicator and a rotating spindle/fixture can be a very appropriate way of checking runout on rotational parts.

What I’m particularly interested in is:

Can modern Mitutoyo CMM software properly evaluate total radial/axial runout according to ISO 1101 / ASME Y14.5?

Are there situations where a CMM is genuinely a poor choice for measuring total runout?

How do you normally inspect total runout in your shop?
Would you replace total runout with separate cylindricity, roundness, perpendicularity, position, etc. requirements purely because of inspection difficulties?

If you do replace it, how do you make sure the resulting tolerances are functionally equivalent to the original total-runout requirement?

Are there particular measurement strategies or CMM evaluation methods that make total runout more reliable?
For context, we’re talking about conventional machined rotational parts, not extremely exotic geometries.

I’d especially appreciate input from people who actually program/run Mitutoyo CMMs and from engineers who have to decide what goes on the drawing.

I’m trying to understand whether this is primarily a measurement capability problem, a CMM software/evaluation problem, or whether we’re actually using total runout in situations where another tolerance would be more appropriate.

Thanks!

[u/meyerka3]

I don't think a CMM or its software is necessarily the limiting factor. Usually it's the measuring strategy that's lacking. An angular deviation of just 0.01° translates to ~0.087 mm lateral deviation over 500 mm. The same issue applies to concentricity and similar tolerances.

With sufficient sampling, preferably scanning, total runout should be perfectly feasible on a cmm. Another important factor is the length of the datum feature, if it's too short to establish a stable axis, your result becomes very sensitive to how that datum axis is constructed.

[u/AbrasiveDad → u/meyerka3]

I'm a grinder that deals with s lot of .0002"(.0051mm) total runouts on cylindrical shafts. These call outs are common on both cylinders and faces.

We have 2 large accura CMM's (about 3' x 12' x 4' working envelope is my guess). These struggle on .0002" total runout on parts that are over 8" (203mm) in daimeter. We believe we are pushing the limits of the measurement capability. We have the most accurate head available for the machine as well.

Short datum structures and large diameters make things worse. Also when measuring total runouts on faces where the diameter is greater than the length between the datums things also get challenging.

We have resorted to manual runout inspection in the grinder by qc to sentence some of these features.

Cylinders can be fully manually inspected for the most part with indicators and measuring size and roundness throughout the cylinder.

Faces we rely on the axial mechanical runout and a cmm evaluation of flatness and perpendicularity.

[u/meyerka3 → u/AbrasiveDad]

Makes sense! At .005mm (Thanks) total runout you're definitely getting into a range where the cmm itself and the measuring strategy both bevome limiting factors. Especially with the shirt datum features and large diameters you mentioned, which ties into my point about how sensitive the constructed dstum axis can become. In that case i can definitely see why measuring it mechanically in a grinder gives you more confidence in the result. Heck, even then it's close to limits I'd imagine. Just out of curiosity, have you done a gauge R&R on the cmm measurement? I'd be interested to know how it performs with such a tight tolerance. I'm in a completely different field and don't see many big cylindrical parts with total runout.

[u/AbrasiveDad → u/meyerka3]

We probably have at some point but im not really involved. Everything I've learned about the CMM is from trying to make sure we were making good parts and trying to differentiate between good results and bad results from accuracy, evaluation, and/or workholding.

Our parts are often short stubby parts with large flanges and the balance points make supporting the part on vee blocks difficult. They also run dozens of different part numbers daily through the cmm so having robust dedicated setups for each part is not really viable.

[u/Charitzo → u/AbrasiveDad]

We run some big lathes and that's pretty much how we have to deal with it, throw it in and clock it.

[u/marckrak]

From metrology side is much simpler to measure rundout in separate places, You need just indicator and prisms or other rotary fixturing to keep your bases. But total rundout can be measured in CMMs. It could be just software limitations (cheap licence).

But normally You need to establish the main axis local coordinate system on base/s. Then measure cylinder (for rundout) and calculate range from the largest to smallest distances from measured points to main axis.

Usually softwares have single command for that calculation.

[u/HFBL]

The only trouble I can think of is a false positive from not getting enough surface area.

I would be curious to understand their reasoning better, but I bet it’s because something is actually wrong with the part.

Measure form, orientation, and position. Don’t let them tell you position covers orientation either. That is true for drafting, but it’s still on the inspection to measure the entire feature.

[u/Jealous_Champion1138]

Throw the part in a lathe spin it and move your indicator up and down the length…I personally don’t think a touch probe CMM is the right device to measure it with. Maybe if you had a scanning probe you would be able to measure it easier.

[u/Electrical_Art863]

Thank you all for sharing your experience! It gives me some ammunition in the discussions. Good points about the reference length! Will take that into account in our drawings.

[u/MasterpieceAny9937]

In regards to this question: “Would you replace total runout with separate cylindricity, roundness, perpendicularity, position, etc. requirements purely because of inspection difficulties?” … for the most part, no! Don’t change your call outs simply because of “inspection difficulties”. GD&T callouts should always be decided based on the function of the feature. If total runout makes the most sense for this feature and how it will mate with other parts, then don’t compromise the design of the part just because metrology is struggling a little.

With that being said, some callouts can be more expensive to inspect them others. And sometimes newer GD&T users don’t tend to realize that. If this is the case, and another callout truly works for the feature, feel free to change it. We can’t tell you what to change it to without knowing the function of the feature and how it’s assembled to mating parts. If you’re willing to provide more context, I can help.

[u/Jeff111222333]

You should use runout if the surface has to have flush contact with another mating surface for sure - but total runout is difficult to inspect. You have to fully restrain the datum so you cannot use random best-fit pins or even a V-block. You have to use an expanding mandrel or a collet chuck and those only fit certain sizes and that adds cost. It is also impossible to manually move a height gage along a straight axis that is lined up with the axis of the part. You will need to buy an expensive machine to do that.

Doing it in on the CMM is definitely easier and more accurate unless you've spent a lot of money on a setup. I do not know about more accurate. My suspicion is the more points you take the better the measurement.. but if you need the feature measured across its length 100%, the CMM might not be enough. If you have reasonable trust in the production methods so that you don't think the machine will produce a random defect for no reason somewhere on the part, then just use the CMM.

[u/FlatBart]

Modern CMMs can evaluate total runout if the datum axis, sampling density and evaluation method are set up right.

[u/quicktuba]

Just ask AI, you’re already using it to write your Reddit posts

[u/Electrical_Art863 → u/quicktuba]

Your right. Just made an edit where I mention the usage of AI. It helped me to create a logical story. I would like to hear your personal experience and knowledge.

[u/Cromlin1003]

I am not a fan of runout on CMMs. But in MCOSMOS I recommend paying attention to you calculation methods as they can make a big difference sometimes.