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Mid Tolerance Modelling for a naive machine shop


New member
We are a 33 years mature electronics company using SE and with our own milling facility to create machined housings for our high tech products.

We have a conflict between the engineering office wanting to create toleranced 2D drawings and the machine shop needing a mid tolerance IGES to feed the mills.

Now I must say that we are not a traditional milling shop in that we had never seen a mill until 3 years ago and have no historic baggage on 'how things would be done by a proper machine shop'. We regard the mill like a printer equivalent on a computer. The design office we believe should be doing the brain work and a file should appear and the mill cut metal. What we do not want to have to do is apply 'operator knowledge' to play with the IGES and add subjective knowledge to the process.

So are we being naive ? Is there a route from SE to take a 2D dimensioned drawing and reset this to mid tolerance just for the IGES ?

BTW we are getting superb results from an old Bridgeport 560 with Heidenheim and a new Hardinge 480 with Fanuc but it is painful having to manually tweak the 2D and the IGES. We use SprutCam as the post processor.

Probably opened a whole can of worms but we would appreciate your comments !

Solid DNA

New member
Hi Pezula

This debate is as old as we manufacture stuff :)

First If i recall correctly, iges can export in 3d and 2D? which one do you use?

From my machinist experience, i prefer to use STEP or Parasolid for 3D and DWG and DXF for 2D

Second, the way dimensions are place on a drawing indicate the design intent. example 1.000 +.01 +.015.

Indicate that the part have to be at least .01 bigger. If we have a part at 1.017 that may not be a reject part. So the type of dimension (tolerance) will help the machinist make a first evaluated guess. Do we reject the part, do we rework or do we send back the part for a closer evaluation.

Some parts can be store and wait for the next one that have some dimensional difference, when assemble they will work perfectly together.

On the other hand, you talk about a mature company, so their is a good chance, that the machinist is well aware of the function play by the part is making.

One other thing i see, since you are mature business (33 years old), you will probably have machinist that will retire, the newly hire machinist will not have any reference on the product, therefore he's only references will be how dimensions are tolerance to evaluate it's job.

It might be obvious but have you.....

Best advice, first ask the design team do to a refresh on all the dimensions and tolerances they used and WHY, make sure the machinist listen in a religious way.

Next ask the machinist to explain why they need mid tolerance, This time the design team have to listen in a religious way....Make sure the machinist explanation is not a general explanation like (it will be easier for us to measure, everyone want it's life be easier) but ask them to pin point exact situations where mid tolerance would help make better product.

Your company need to understand why the machinist ask for mid surface ( tolerance). You need to found exactly where this request is justify and where this is only as lazy demand.

The mill is not only just a simple equipment, the mill is now part of the process.

In a usual/general way, the CAD file is create at the nominal dimension not the mid one.

well i can go on, but i think it layout the general idea, if need send me mail to digg depther
Last edited:


New member
More of the story if it helps

Thanks for the response which is appreciated and confirms the debate.

We have no milling history being an electronics company that has come to milling only in the past 3 years. We therefore have no baggage and as mentioned in my post we regard the mill somewhat like a printer on a computer, simply a device to produce hard copy for what has been designed by someone with design skills. If that design is mid tolerance then what appears as hardware will meet our needs without having to depend on 'local' knowledge from the machine operator ... in our case there is no accumulated knowledge to get in the way.

I can see your argument that there needs to be tolerance noted on 2D drawings if the operator is applying tweak knowledge to the process but in our case he doesn't. We have Renishaw probing on the mill so providing the operator puts the billet in the right general position he simply has to hit 'run'. The probing checks billet position and tool dimensions and compensates automatically for variances (billet twist, tool wear, tool breakage etc). It is sufficiently automatic that we can do dark milling with confidence.

I agree that the CAD to mill post processor needs some operator knowledge on feed rates etc but this is all done prior to download to the mill. Once the file is downloaded the operator is a spectator and usually is getting on with something else or if dark milling he is at home watching a soap with a web cam if he needs to see the state of play.

Call it naive perhaps, but that is how we are working and we are producing some highly complex housings for demanding end use, some of which we have found the more mature highly experienced external S/C milling shops are declining to quote against. But then they generally seem to want to ignore the 3D file and hard code from the 2D drawing .... why ???

The frustration is the traditional design office CAD entry staff wanting to tolerance everything in 2D and we see no value in this. We just want a dimension that is how it is meant to be when it comes off the mill. It will then be the exactly the same when we run it in 6 months time and if we send the post processed file to an offshore source they will produce something identical.

Not sure that has helped much ...

Solid DNA

New member
Even if you say that you have no background in milling, i found that you are much in front of many companies that use CNC milling.

I was not sure if i should go in that direction base on your initial comment. But base on this second comment where you describe your setup, it seem that you are Paperless and Draftless you are maybe 5 or ten years ahead of your time. :)

Your "dark milling" made me thing about the STEP NC initiative




Since the process is entirely digital, tolerances will be only for inspection to ensure that the milling is properly setup. At some point in time you will need human intervention to validate that the digital settup is properly calibrate.

Not sure what we can add except that CAD Designer should trust the CAM and provide a minimal information to validate the whole process by doing some spot checks or statistical analysys...