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?

Reply to
Scott Proctor
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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

Reply to
Sean-Michael Adams

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

Reply to
Scott Proctor

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

Reply to
Jerry Steiger

One would think that a few rejections of the things at inspection along with shipping it back by air freight collect and refusing to pay any invoices would get the idea across, sooner or later.

A 3D CAD/CAM model sort of forces them into using a CAM system & CNC to make things .... more accurate results than 2D drawings & hand work I'd hope. Juat add tolerances, no dimensions, to the "drawings" .

I'd wager ANSI Y14.5 might come in really handy here.

Hint about the CMM that will inspect to the model .... using

14.5 from the drawing(s) ...
Reply to
Cliff Huprich

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

Reply to
Sean-Michael Adams

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

Reply to
Jerry Steiger

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