I am very well versed in your process and have reviewed at least 500 PPAPs that are exactly that: HPDC+Machining+Assembly in some form.
Some folks put EVERY dimension in the
FMEA & Control Plan, some do not.
My preference is to NOT put every dimension in as a failure mode. Here are the reasons:
1) If you put every dimension in, your are burning too much time making sure every dimension is in there. And not enough time focusing on the actual failure modes.
2) You will be constantly redoing your PPAPs every time any dimension changes. Even if the dimension doesn't change, but you find statistically that at some point after SOP you change your guard bands, well, your control plan changes and you have to PPAP again.
3) You will hamstring yourself with executing proper read across / lessons learned. What I mean is if you learn through a corrective action that you had a
PFMEA problem, you will have a mountain of work to change paperwork where a more generic PFMEA is easier to change. I'll honestly take a shop with a robust lessons learned program over a shop with fully detailed PFMEAs any day of the week as a customer.
I could go on, but those should be enough.
Take your machining center, any machining center, and the failure modes are generally:
1) Problems caused by clamping (deforming the part)
2) Problems caused by tool wear
3) Problems caused by tool breakage
4) Problems caused by misloading
5) Problems caused by debris on the fixture
6) Temperature problems (coolant temperature changing, changing the temp of the part)
7) Uncontrolled changes to program
8) Casting problem: bent / missing clamping points or datums
9) Casting problem: heat check / flash on the datums cocking the part
10) Casting problem: missing cast material resulting in non-cleanup
11) Improper tool loading
12) Improper setup
These are the failure modes. Not "this ID is out of spec" then "that ID is out of spec." If you sent me a part with "this ID" out of spec and your root cause was "I made the ID out of spec," I would reject your
8D. If your root cause was "The chiller went down for a shift and the coolant got to warm." THAT's a root cause.
Come up with your own list of how the machining operation fails, not the dimension, and PFMEA how you will check for and control THIS.
Will you need a job specific dimensional sheet with dimensions, limits and gaurd bands? Yes. Your PFMEA should refer to this being a thing that exists, but that's your process specific sheet which may be changed as you run your process. This is in order to protect MY print. Folks tend to forget that you CAN adjust your process to keep the dimensions centered and in control. Don't tie your hands to do this by specifically putting all this in the PFMEA.
On to the Key Characteristics: THESE should be specifically called out in your PFMEA because I have told you they are critical to my unit functioning.
The smart strategy is:
1) Make a generic master PFMEA and Control Plan "block" for your HPDC, your machining and your assembly.
2) Now you make a process for my part. Let's say it takes 3 machining operations. You point to your casting block once, then your machining block 3 times. (After all, your list of machining failure modes should be universal.)
3) You then add in where appropriate my Key Characteristics at the points where they are made AND checked.
Now, you make an error and in your corrective action you need to change your master FMEA. If it's a worthwhile change, you change your MASTER block, and it cascades down to all your other blocks.
A strategy like this will let you focus on what's different from the norm in any new job, instead of burning daylight doing repetitive paperwork on the things that are standard.
