Another CM Question

[gb2835]

Operating as “parts don’t have revisions,” how does one deal with a part that has a requested change that doesn’t affect FFF, but the part model itself will be modified? I’ve thought of three approaches:

1. Don’t actually change the model but revise the drawing with the changes. Drawing won’t be pictorially accurate. Shop would need to update model (STEP file).

2. Change the model and have configurations for each revision. I can provide shop the final STEP file. This could cause problems with parts that already have 100+ configurations for lengths, diameters, etc.

3. Change the model and save the STEP file, then remove changes and return model to Rev “-“ (we use dash for initial release).

We are trying to find best practices for this so any help is appreciated. Please let me know if more clarification is needed.

 

[BrianPetersen]

Dumb questions follow. Answers might depend upon what industry you are in and what industry conventions are, which might not always be the same.

If the requested change doesn't affect FFF, but requires changes to the manufacturing process, how does a change of manufacturing process not affect FFF? If FFF doesn't change then why does mfg need to change? If mfg needs to change due to some physical change, how does that not affect FFF? (explain yourself ...)

If the change is JUST to the way the CAD model is constructed but not to the physical part, why does the shop need to know?

In the automotive industry, even if a change doesn't affect FFF, a change to the manufacturing process could potentially have to be PPAP'ed or equivalent, and that means a revision number change to the part number and to every document or model affected by the change. How to note that the new revision denotes an interchangeable part, is up to you (and the company's policies for handling such changes - which happen routinely).

"Parts don't have revisions" - part numbers might ...

Undocumented changes to parts or to manufacturing processes can get you into a whole lot of trouble, if there are unforeseen consequences.

 

[gb2835]

Thanks Brian, I will address your questions below.

Q1) If the requested change doesn't affect FFF… (explain yourself ...)

A1) We outsource MFG, and different shops want to make changes to better fit their capabilities. Examples being relaxed tolerances, changes to lead in chamfers, etc. Some shops ask for changes, others can make as is. These changes don’t affect FFF, but I have to update the models.

Q2) If the change is JUST to the way the CAD model is constructed but not to the physical part, why does the shop need to know?

A2) Not sure I follow here, can you explain further?

 

[3DDave]

The problem you face is that you don't need a model drawing, you need an interface control drawing.

In the interface control drawing you dimension very particularly all the surfaces of the part/assembly that make contact in your use of the item. Particularly - flange thickness to assure sufficient thread engagement, hole patterns, dimensions between mating surfaces, orientation from one surface to the next.

Anything else is a do-not-exceed volume that provides enough clearance in your assembly or use of the part to allow access to fasteners, room for fingers to hold nuts or screws, distances from hot surfaces as required.

Then your suppliers get to do whatever they want as long as it fits the interfaces and is within the envelope. Each supplier might be required to supply a model for evaluation to conformance with the envelope and a drawing showing variations they expect though they may want some monetary consideration as blackmail to get that data. It might be worth it in the event that at some future date some other change is desired that changes your acceptable envelope in order to avoid encroaching on one of the possibly myriad designs that had previously been accepted.

 

[3DDave]

One other thing - if there is a possibility this equipment can fail in such a way that causes a massive dollar value or loss-of-life lawsuit, be prepared to have some means in place to record exactly who made each variation of the part per which assembly that part went into so that every one of whatever individual design can be pulled from the market as rapidly as possible. This is one of the reasons for making part number changes even if the F-F-F is essentially identical - liability control.

 

[MintJulep]

Changing tolerances affects form and fit.

 

[gb2835]

@MintJulep

That's not always true. We design a lot of sheet metal parts, and each shop has different standards for tolerances based on multiple bends, bends to holes, bends to edges, etc. In the end all of these parts would be interchangeable, with slight variations in gap between mating surfaces.

@3DDave

Thank you for the input, and the interface drawing is something I am not familiar with; but will look into.

 

[BrianPetersen]

If this were the automotive industry ...

I agree with the interface drawing concept. But then ...

(1) You need a master part number (and perhaps, master design model) with a subcategory assigned to it for each supplier incorporating their own particular revisions, and those revisions have to be sent back to whoever is responsible for the design of the master part whose master part number it is assigned to, and they need to approve and PPAP that supplier's (a) design changes and (b) manufacturing process and its associated quality-assurance measures. ("PPAP" = "pre-production approval process")

(Tolerance changes that affect an interface to another part are affecting FFF ...)

(2) If it's potentially a critical part, then you need some way of tracking which production vehicle (e.g. VIN) ends up with which part including the subcategory for which supplier it came from. When an end user (e.g. me) shows up at the parts counter looking for part number X, the parts counter needs to be able to type in my VIN (that's why they ask for it) and find that this vehicle was originally built with part number X supplier code C, or whatever. If this replacement is under warranty, the department that tracks warranty repairs will be interested in exactly which parts are responsible for warranty claims, and it will be tracked down to the supplier in case they are to be held responsible. Heaven forbid this part being eventually responsible for a recall ... but it happens from time to time. Now the vehicle manufacturer can identify by VIN exactly which vehicles have that part built by that supplier in them without having to recall ALL of them to find out by inspection.

Maybe your industry or that part doesn't require that level of diligence ... or maybe it does.

I would at a bare minimum do paragraph (1) above, in case it is acceptable for the end product to not have trackback to the supplier (e.g. if it is acceptable for a technician later on to establish it by inspection) - and if it is to be done later on by physical inspection, make sure there IS a way to do it by physical inspection (e.g. by having the manufacturer put their own identification markings on it and make sure those markings are in a place that is visible). I've seen a fair number of parts with QR codes imprinted on them.

 

[gb2835]

Great info Brian, and yes maybe it is beneficial I describe the type of work we do.

I started a small business in February offering design and FEM services. Over summer I added a grad student at an entry level position, so only 2 people managing files. We categorize our clients in three types:

Client A: Established business with existing design team outsourcing more specialized design work to us. These clients have their own PDM and workflow so we just follow their procedures.

Client B: Entrepreneurs who have a product idea and we handle the design and source manufacturing. Each client (so far) has one top level assembly, and less than 20 unique parts (not including anything OTS). We use our own CM approach which is why I started this thread, looking at improving our own processes. We don't stock any parts, so no bins or anything.

Client C: A medical device company that was my first client. I've been using their CM approach, but it is *very* bad. I'm currently the only engineer, and we've had discussions where they agree they need to have better processes. I think at some point they will ask me to create CM procedures and migrate their designs (luckily they only have a handful of devices). They do stock, and since these are medical devices there are higher risks that 3DDave mentioned.

Through some searches I was able to find snippets from Y14.24 on interface drawings. I will look over this more as well as Brian's latest post to get up to speed. If the readers will entertain me, I may post ideas for implementing where any and all criticism is encouraged

 

[drawoh]

gb2835,

If your change does not affect form, fit and function, update the model and the drawing.

Do you understand the problem?

I am assembling something that requires part[ ]13282874. I call the warehouse. They go to the shelf and they pull out part[ ]13282874. It works, or else! Any change to your part that could make it not work, is unacceptable. Changes in size and/or tolerances require new part numbers. Adding features to the part also is nasty. I request the part. The warehouse sends me parts that were fabricated to your original drawings, and I need the new ones.

--
JHG