OT - Releasing CTF only Drawings and IGES for part manufacturer

I know this has been hashed over before but...

We are doing allot more these days with fully cored and drafted parts for
casting and injection molding. To 100% detail these is quite some work and
not always clear. We are adopting the practice of releasing a CTF drawing
and IGES for part manufacturer. The issue I have is what tolerance applies
to the non-ctf geometry? We have tried a few drawing notes, but was
interested to see what other are doing to cover themselves?

If I have an issue with a part feature that doesn't have an actual
dimensional value on a drawing, does the CAD file really cover it? Has
anyone had to go to a supplier and have tooling modified or parts remade at
the vendors expense because the end part didn't match the CAD file?

 

Hi Scott,

A basic one liner: "If it interfaces with air, who cares?"

Ideally, you would make a general note that is based on the process
capability. Your vendor can help you here by suggesting what is
pactical to hold for the given process. For your molded parts, things
like material shrinkage, overall part size, mold registration and
number of parting lines might be the consideration. Ask the vendor to
help you define reasonable tolerances.

Yes, absolutely - we presume that the tools and personnel are
available to derive the right size. BUT, if I were you, I would
seriously consider validating the vendor's CAD/CAM/INSPECTION
capability before doing anything this way. Questions to ask:

1) What format is your ideal format to recieve? Native or iges?

2) Does my output (for example iges) "reconstruct" correctly when they
opein it? Do I need to adjust my flavor of iges for things to flow
smoothly?

3) What presumptions do you and the vendor hold common? I.e. Color
coded faces are critical and is the color translation robust (for
example).

4) What impact does sharing data have on non-recurring egineering
costs either positive or negative? Most likely, without models, the
vendor might be charging more for the initial engineering.

5) Does the Vendor have any Model based inspection capability (like
PC-DMIS, SILMA, CMM-Works) and the like? Is it implemented into a
system that they use as a "standard"? Ask to see it work.

6) Can the vendor work with your native data? And if so, can they (and
do you want them to) integrate their manufacturing features into the
part so that you may be aware of the manufacturing-friendly
alterations?

I have in a past life been the vendor in cases like these and in our
particluar case, we accepted 100 per cent responsibility for any error
where we did not correctly implement the part according to the model,
including tooling. We recieved a large cross section of data types
(sw native, iges, dxf 2d & wire, acad, parasolid, etc) Often times as
well, we were able to find errors in part print dimensioning because
we had the model.

I think that this can be very successful but you need to confirm that
your vendor can and will operate this way.

Regards-

SMA

 

Thanks guys, great feedback. One thing I didn't mention is allot of this
work is going to China. We are still struggling with the communication /
language / culture barrier. The level of collaboration we had become used
to with our local supplier base has been quite elusive so far. Thanks
again.

Scott

 

Oh, sh*t. The whole concept of tolerances seems to be completely foreign to
the Chinese. Even if you do a fully dimensioned drawing with explicit
tolerances on every dimension, they don't seem to think about what those
numbers after the +/- symbol mean. (It's not that they can't and don't build
good tools, but they don't seem to change the way they build them if you
call out particularly tight tolerances in certain areas.)

Actually, you are probably better off with just a few dimensions with
tolerances. There is a chance, if you talk to them about those tolerances
very explicitly and very early and then reinforce the conversation as often
as you can, that they will focus on making them come out right.

Jerry Steiger

 

Jerry,

In line with your thinking, I would also add that is seems smart to
keep the drawing and model in pariity by avoiding things like
bi-lateral tolerances, dimensional rounding and things of that ilk.

When model-drawing parity is degraded it creates a conflict as to
which one is the master and can sometimes cause feature drift,
particularly when the drawing dictates a state that is geometrically
not possible for the model.

For example, a 10" wide cover, if dimensioned at 10.000 +.020/-.000,
actually has a middle value of 10.010, while the model is 10.000.


Regards,

SMA

 

I agree whole-heartedly. These are just like changing the text of a
dimension. The only time I ever use bi-lateral tolerances is when I update a
drawing to match the actual molded parts. I often keep the nominal dimension
and stretch the tolerances to allow for the under or over size parts.

Jerry Steiger