Tolerancing in sheetmetal parts when good surface to surface contact for electr conductivity? 1

[var10]

Hey

I have recently started to look into our tolerancing information on drawings for sheetmetal parts and found out that we basically have an impossible to achieve tolerance.

+/- 0.1mm for 2 dec is not achievable by most sheet metal suppliers specially when its Aluminium. I also need a good surface to surface contact while not increasing the tolerances by too much to achieve this goal.

I am not looking for any specific answers but these are Aluminium components for aerospace industry and they have to pretty darn accurate. Any suggestions are most welcome.

Thanks,

 

[MikeHalloran]

Aluminum's natural protective film is not reliably conductive.
Chemical conversion finishes for aluminum are not reliably conductive.
Anodic finishes for aluminum are reliably not conductive.

You could, e.g. nickel plate the faying surfaces. ... but you have to clamp the seam with a lot of fasteners for high frequencies, or insert a nickel mesh or braid and clamp or crush that.
Various toothed strips of e.g. beryllium copper are produced for electrically bonding aluminum flanges. They do scratch the surface, release fine particles, etc.
With some difficulty, you can tin and solder aluminum.
It's probably easier to weld it.

For groundborne electronics, UL requires that each separate piece be equipped with, e.g., a dedicated grounding stud, and the pieces be electrically connected by copper jumpers in order to provide grounding for safety. This is completely ineffective for high frequencies, so if RF is involved, you need the mesh or the toothed strip or something like that.

Conductive epoxy is a waste of time and money.
Conductive paint is a waste of time and money.



Mike Halloran
Pembroke Pines, FL, USA

 

[var10]

Thanks Mike. Can you please relate conductivity with tolerances that can be used. I am more concerned about physical interaction rather than chemical conversion coatings. The parts are all alodined to have good conduction and corrosion resistance.

I would like to stick with alodining but give the parts a higher tolerance so that the sheet metal suppliers can achieve but yet not compromising on surface contact during assembly stages. Because right now we have a very tight tolerances on our drawings which are resulting in a lot components being rejected. I could probably come up with different set of tolerances with trial & error but can not afford the time nor money.

Thanks,

 

[MikeHalloran]

It might be helpful to know exactly why you are trying to achieve electrical bonding between parts that are adjacent or fastened. I.e., for RF, gaps between fasteners in a flange become slot antennas, so the allowable gap length is determined by the highest frequency to be suppressed/contained. You must have a sparky on site who can help you with that. Note that the gap width normal to the faces is essentially irrelevant except at frequencies where it approaches a wavelength. If you are only interested in conductivity at really low frequencies, then a single bolt is adequate.

But I'm guessing about the actual shape of your parts, and your problem may lie there, or be addressable there. I.e. if you're trying to hold a .2mm range over two bends, consider changing the joint details so there's only one bend per part, with say a lap joint where the other bend was (the other bend having been transferred to the complementary part).

Say you're looking at a tubular section that's square. You could make it with two plates and two channels, but the channels have two bends and are therefore somewhat difficult, for purposes of our discussion at least. Suppose in this hypothetical example that the square section instead comprised four angles. One bend each, with lap joints at the corners holding the pieces together by spot welds or fasteners, with dimensions established by fixturing instead of by the parts themselves.

I'm guessing your parts are rather more complex than a square tube, but I think you can see where I'm going.

If you can't change the basic joint design, it might be helpful to go through the process used to make your parts, cut by cut and bend by bend, with an eye toward making the tolerances easier to achieve by changing where the measurements are made. Simplest example is whether you dimension over a pair of flanges or inside them, etc. If you can arrange it, you need to go through the process with the actual person on the floor who has to lay out the part and set the stops on the machinery. It could be a very productive day.



Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

+/- 0.1mm for 2 dec - what does this mean. Typically you can't use the idea of assigning tolerance based on number of decimals with metric dimensions because per ASME Y14.5 metric dims don't have following 0's. Or do you use some other spec?

Sheet metal parts are typically at least slightly flexible so the fasteners will often tend to pull the mating surfaces into contact but this will vary with part geometry.

Like Mike says I think you need to give more detail on your application to get meaningful input from members.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

var10,

Bending tolerances are around ±0.4mm. I would guess that the punching or water jet cutting is more accurate, perhaps ±0.1mm?

You have to make your design work with these tolerances. Can you use slots? Can you use grounding strips as flexible elements?

--
JHG

 

[hendersdc]

We really need to know what you are applying the tolerance to in order to give you usefull feedback. You can certainly hold +/- 0.1mm on a hole diameter or even between holes on the same face but once you add bends it becomes a different story all together.

How is your surface contact affected by the dimension you are tolerancing? A picture or drawing would help clarify things.

 

[HDS]

Your question is difficult to answer because it is hard to understand. You have to design the parts for the tolerances you will get from the process. Where the design requires tolerances tighter than the process you have to change the process or pay for additional tooling.

As for good practices for grounding and sheet metal look at computer cases. The OEMs have put allot of thought into refining them. Lots of overlapping parts, minimal gaps and clever spring fingers.

The EMI gasket suppliers all have good design guides you can read.