Metal Fabrication - Sampling Inspection

All,

I work for a manufacturing company that is divided up into three areas one of which is a Workshop; producing metal parts from plates measuring from 3mm up to 80mm (CNC: Waterjet + Horizontal Boring & Milling).

We are trying to come up with the best sampling scheme for the parts being produced. Some of the parts have characteritics requiring 100% inspection but we also have many smaller parts (ribs/sections needed for welded parts) that are produced in lots (up to 80 at a time) affected significantly (in terms of cutting quality) by the waterjet.

Can the ISO 2859 and 3951 sampling schemes be used for such an operation?(even if parts aren't technically being 'submitted' as a continuing series of lots).

 

Waterjet is my biggest challenge. Abrasive feed (largely pneumatic) and varying water pressure along with hard/soft spots in material being cut, can dramatically affect accuracy, as well as material thickness (kerfing). Even a multiaxis machine can be a challenge. On critical parts we have to inspect 100%, and fallout can average 1-3 percent. Less critical parts can be checked periodically, but that is no guarantee that something didn't change during the cut. We do cut a reference cube before critical parts, and use that for basic settings, but adjustment during cutting operations is still frequently required. Abrasive quality also is a factor, but a more controllable one with frequent sieve tests, although the common problem here is contamination from opening the abrasive bags (cotton pieces, plastic shreds, etc.that are not caught by the screen).

 

Mr. Nichols thank you for the welcome.
No machine capability studies have been done thus far. We are just getting started here with these machines and have not produced very many parts.

 

Not quite sure I'm following your process here. Could you please elaborate a little more?

 

Thank you for the explanation.

@Miner @hogheavenfarm @Andy Nichols After a machine capability study is completed, could a similar inspection plan also be used for watejet cutting operations? For example, a 10mm plate is used to cut 10 separate products of varying quantities (62 parts in total) - could a check be done on the first part and if in spec then on another part at a specified interval? To avoid having to try and inspect a sample from the entire lot produced...
@hogheavenfarm do you follow any in process inspection plan throughout the waterjet cutting?

 

Waterjet is the difficult one as there are so many variables that the operator has little control over. for critical (Like aerospace parts), I actually inspect critical dimensions on each piece before cutting the next, even if they are nested on a big sheet. I find we can go about 10 pcs before an "adjustment" is needed, usually accomplished by a small (0.001) tweak to the offset setting. I keep a spreadsheet for these that tracks maximum kerf offset and we adjust as it approaches the maximum. On 2" thick pieces, the waterjet will typically run about 0.010" kerf top to bottom, due to the declining water pressure at that thickness, and the "spray" from the abrasive. On many parts that means running as close to lower tolerance at the top of the piece to stay within the maximum at the bottom. The jet will tend to "wander" as it penetrates the piece, especially thicknesses greater than 1". With the spreadsheet running, I can tell the operator to back off or add a few thousandths to the offset to keep the part in spec. Even with heavy monitoring, we still lose pieces occasionally, so they have to be factored into the run. Standard machining operations are simpler, as the others have pointed out, a sampling plan generally works fine.