In nearly 20 years I’ve had chance working in different industries such as tool and die, stampings, castings, machining and contract inspections, so I’ve really seen a lot on how cmm is used to measure different kind of parts. And I know creating a good DCC program sometimes is quite challenging especially for novice cmm guys. Since I’ve gained good experience on training cmm operators I’d like to share some of the important steps in my book when creating programs.

People dealing with sheet metals or automotive plastic parts will mostly get a checking fixture to hold parts, which really saves the cmm guy big effort for setting the part up on cmm table. But if you work with castings, mouldings or machining then you probably have to figure out your own way to set the part up. And this is what “open layout” refers to and also the first step I want to talk about:

Step One, Set up

Maybe you work in a “fast-paced” production environment and you think you don’t have time to think about that so you just put the part on some blocks making sure it’s not rocking or falling then start programming. Then you’re wrong. I myself sometimes spend up to 1 hour just to test different set-ups. Think about this: your set up will determine how many different probe angles (or different configurations for Zeiss ST probes) and how much accessibilities on features, so if you find out that it’s not a good setup at the end then you’ve already wasted too much time. I always determine the setup based on the following few thins:

1, Datum features visibility and accessibility

Part should be set up in such way that all datum features are clearly visible (ideally) and easily accessible. Establishing a correct part alignment to drawing is critical to cmm measurement if your part alignment is wrong then all your measurement data is garbage. To ensure that, one thing you do is to make sure you can physically see your datum features to detect any burrs, surface irregularities or anything suspicious and also you should be able to access these features without any shanking risk. When I program and debug part alignment, I always watch every single probing on datum targets to make sure it’s good because I know once the program done I will rarely go back to check. This kind of setup also helps a lot when you run your program later on and get weird measurement results you can quickly double check you datum features and alignment without moving the part.

2, Major features exposure

Let’s say your datum features are really open and easily accessible then next thing you do is quickly browse the drawing and identify major or critical features. If there’re deep bores then you should try to square up the part to cmm axis so your probe won’t shank when measuring those features. Also if it’s a casting which definitely needs a second setup then a good practice is to make the first setup check as many features as possible so in case you’re under a gun for quick inspection then you don’t have to run the second program.

3, Secure

Try push your part with fingers and it should not be moved easily because you don’t want to accidentally move your part just a little bit without any awareness and end up getting wrong data. In case you’re suspicious then go back to run your alignment to check the datum features (now you see the importance of rule 1). So set up some blocks or clamps to help secure the part, but, be careful not to twist or over constrain the part especially when the part is not that strong!

4, Repeatability

If it’s possible you should always use 3-2-1 alignment theory to make your set up repeatable so that you only need to run the manual alignment for the first piece and just press the button for the rest. If you can’t use any more blocks (e.g. part is too small) then even use markers marking lines on table or even eyeball certain features (e.g. holes) you still should be able to make it manual-run free. Just be creative, I always have lots of fun figuring out a creative setup for my parts.