Sheet Metal - Fit Tolerance

[SeanFarr]

Hi All,

I have a biggie question here. As I may have posted earlier, I work for a company that is new to CAD and has no other personnel in the "engineering" department than myself. We have no standards or procedures and I have no one to discuss questions or Inventor practices with. (Thanks goodness for this forum!!)

We use quite a bit for sheet metal components that fit into tube frame structures. In my inventor model I have been modeling the parts to fit exactly into these tube frame structures. I have attached a basic example to help clarify my question, the frame.ipt is just an extruded piece to resemble a tube frame, and the sheetmetal.ipt is to sit center inside the frame and have seam welds all around. The frame has a 25 in square opening, now here is where my question comes in, when creating a detail drawing of these parts and assembly, do I leave the sheet metal part as 25 in square or do I take off a 1/32 off all edges, to allow for fitting.

So do i change the actual sheetmetal.ipt to 24.9375 square or do I leave as 25 in and insert a min-max tolerance onto the drawing.

I just find it confusing and when looking at the model on the screen with a bunch of little gaps, looks bad and is more difficult to mate all parts together. But I have run into issues already with a fabricator. They made the frame and it was within tolerance. The width of the frame was 1/16 narrower then the dimension specified on the drawing. The fabricator farmed out the sheet metal components to a laser cutting company, which cut the sheet metal part to the exact specs of the drawing. So once the parts came back to the fabricator, they had to grind/re-cut some edges to allow for fit.

This I think should have been taken into account on my end before they even got the drawings. Since all seams get welded, a small gap is OK, but how do I apply that fit tolerance? I have been through this forum, but find it difficult to search because each person uses their own terminology when posting. Also have been through Google and came up with some examples, for holes and shafts, would this have the same theory as my example? There must be some sheet metal common practices and procedures out there.

I do have an education in this background, but schooling only seems to touch on basic topics, it would seem that work experiences are to fill that gap. Unfortunately I am stuck here at this point. I don't mind purchasing books or reading documents on line, I just can't seem to find any. 

If anyone can help with this example or point me towards some external references, that would be great!!

Thanks,

Sean Farr

Inventor Pro Suite Ultimate 2013

[japike]

You have to communicate to everyone involved how the two pieces fit together. If we want the parts to fit together tightly, we often model the parts to nominal dimensions and apply appropriate tolerances to each part in the drawings. If we want a gap, then we will model the parts that way, but still apply a tolerance on the drawings, so that everyone understands what we are trying to accomplish.

[SeanFarr]

So in my case I should adjust the sheet metal part sizes to allow for fit. I have attached an image below, the frame is identified by black arrows and the red arrows indicate sheet metal components,

Are these sheet metal components to be changed in size to allow for fit. and constrained to be centered in the frame so there are gaps on the ends near the frame faces ( green arrows) . and gaps in between the vertical and horizontal sheet metal components(purple arrows)?

The model/assembly will look wrong or incorrectly modeled, but this is how one allows for fit? I know this has a lot to do with how the assembly is put together. As mentioned above, all seams get welded to fill in all gaps.

To constrain the sheet metal parts in the correct positions, I will need to use more than just the origin planes. So more work-planes and axis' will be needed to accommodate that?

 

[JDMather]

I think this might be the biggest mistake that CAD users make - modeling parts as though we can make exact parts out on the shop floor. 

Include real world clearances assuming min/max tolerances of actual manufacturing processes.

So you have to do a little more work - try running that excuse by the shop floor people who have to fabricate this stuff.  You won't get very far.  Try running this excuse by the boss when vendor parts (or in house parts) made to your specification don't fit together.  You won't get very far.

Model in the needed clearances and you can also add tolerances (Tools>Document Settings) and test min/max combinations of tolerances (except on hole position).

[SeanFarr]

Ok, well that definitely clears up the actual modeling of parts and assemblies. I may have worded my earlier post incorrectly, I was not concerned about the "extra" work to do in regards to constraining parts, it was more of a question as to this is how it is supposed to be done.

The next part of that question is is there such a thing as general gap tolerance for fitting? The tolerance I used for the frame assembly was   -/+ 0.1 in, going back to my opening post, when the frame was 1/16" narrower, it supposed to 47.5" wide and ended up 47.4" , it was within tolerance.

So I should leave at least 0.1" of a gap for mating faces? Or round it to 1/16" per mating face so a 1/8" shorter in total?

Again, is there such a document/text that provides this information for manufacturing processes such as this as well as others?

Thanks again!

Sean Farr

[swalton]

You are getting to the fun part of design.  The gap you need for easy fitup depends on the tolerances of the parts.  The tolerances of the parts depends on the required function of the parts, the size of the parts, the manufacturing process that you pick, the cost you want to pay for the parts, and the skill of the person making/assembling the parts. 

There are no hard and fast rules for proper gap.  Work with your vendors and shop personnel to get a set of reasonable tolerances for your products and processes. Once you know what they are comfortable holding, you can use that to make choices about the proper sizing of components.  Feel free to go outside their comfort zone as long as your product requires it for proper function.   You will pay more for those parts.  If your vendors/shop personnel can't hit the tolerances your design needs, re-evaluate the design.

For a sheetmetal part a few inches across and 16 ga or under, I might allow +- 0.015 across each bend.  If I am making 60' long rail car, I might allow +-0.25" for locating major sub-assembles.  Bearing fits and press fits will be toleranced down to a few ten-thousands of an inch.

In your case, your biggest possible panel needs to be smaller than the smallest possible opening in the frame. So if your smallest opening is 47.4, your largest panel needs to be at most 47.4.  This will give you line-for-line contact when assembling these two parts.  Get your hammer. 

You will also want to allow some extra gap, somewhere between 0.010-0.60 or more depending on how much gap you need for easy assembly.  You also need to look at the gap that you get when your frame is as large as possible and your panel is as small as possible.  If that gap is too big, will it cause your design any problems?

[swalton]

Also, try the "machinery's handbook" for some basic information about what tolerances are reasonable. 

[SeanFarr]

So I should leave gaps based on the manufacturing of the parts. Some of the sheet metal parts are laser cut, which is a very precise method of cutting, so if a piece is to fit into a 25"x25" frame opening, that has a +/- 0.1, then I should make that sheet metal part 24.8 if it had the same tolerance.

To figure out that gap or clearance, I need to talk to my fabricator, find out how each part is going to be made, and base my tolerances off that.

In my example, is it uncommon to note a detail drawing, that some dimensions are should be confirmed off of the frame. In the attached image, if the length of the frame piece (red arrow) is at max toleranced length, and the placement of the center frame member is off to one side, and the sheet metal member (green arrow) is made at its shortest possible length, then there could potentially be over .2 gap at both ends. which is too much to fill weld. (material is only 1/8")

But if I made sure on the drawing for that sheet metal piece had the correct profiles dimensions, but said to verify the length based of the already made frame, is that bad/uncommon practice? 

These are just some questions that come to mind. I like to get as much info possible before I apply any changes or practices to my work.

Thanks!!

Sean Farr