Do you accept parts inspected by CNC?

[bxbzq]

I work in a small company and we have suppliers doing machining for us. CMMs are quite fancy equipment for them. They claim inspecting parts using CNC would give comparable, if not the same, (or good enough, using their words), results as CMM. They explain both work the similar, ie using probes travel along part feature or simulator surfaces to take measurements.
I don't know much about CNC machining process or CMM inspection. The explanations make sense to me but I'm still not fully convinced. Can anyone share your knowledge?

Many thanks.

 

[geesamand]

Generally, job shoppers without CMM get easily confused by repeatability and accuracy. Their machines are generally highly repeatable, such that the same numbers come up on the display every time. However the error they make can be happening every time. Anyway that's my existing bias.

Some CNC's have on-board CMM capability. As long as the part has no tendency to spring back when unclamped, this can be effective.

It's quite simple IMHO - ask them to submit a sample of the part to your QC or another shop's CMM with as-found dimensions. If the part dimensions match the CMM results, you have grounds to consider the CNC's measurements as good. The numbers will tell the story - you need not make a judgement call here.

 

[ajack1]

It can certainly be done although I don’t think you get all the features of a full blown CMM with software, or would get the results in the same format, if that matters.

As has been mentioned I would expect to lose some accuracy and not many companies I know keep there CNC machines in a clean room, certainly not those without a CMM anyway. Having said that it might mater if you are measuring to three decimal places but not at all if you are measuring to +/- 0.5mm.

In fact you could put forward a strong case for only needing very minimal inspection on a part with open limits done on a CNC machine.

With the limited information from the OP, I am not sure anyone could give a very definitive answer.

 

[ctopher]

It can be done, I have done it.
But, the cost of tying up a CNC machine for CMM could be high.

Chris, CSWA
SolidWorks 13
ctopher's home
SolidWorks Legion

 

[HGMorgan]

In order to use a CMM as an inspection tool, it has to be certified, and re-certified on a scheduled basis.
A CNC could be used, but for inspection buy-off purposes, it would have to go through a certification process as noted for a CMM.
This was pushed several years ago as a way to catch errors @ manufacture, and to eliminate some inspection activity.
When it was understood that it would take manufacturing machine time away from the CNC (time for certification, time for in process check),
it kinda went mostly away.
A lot of shops use CNC for an in-process check. Since it is not an inspection buy-off, it doesn't require the certification, but then
the feature(s) still have to be checked by inspection with certified tools.
There are good reasons to use a CNC to make checks, but replacing a CMM is not one of them.


Harold G. Morgan
CATIA, QA, CNC & CMM Programmer