So an interesting debate which seems to have little concrete answers available..


Lets say I want to drill and tap for an M3 bolt in the edge of some alloy plate.

What is the minimum thickness that the alloy plate can be? IE, how much distance is required between the threaded hole and the edge of the plate?

Could I use a 5mm plate? Or would I need a 6mm or perhaps an 8mm?

[Edited on 11/11/17 by tegwin]

I do something similar with vesa adjustable mounts, I use a M8 bolt in a 10mm thick mild steel plate in my case, the hole drilled is 6.8mm and then threaded. In my case I end up with 1mm per side and I really load the bolts that go through it.

I can't see you having any problems with using a 5mm plate and an M3 thread, this assumes that you accurately drill the 2.3mm hole centrally

David

Thanks for the reply David,

One of the concerns I have is that (assuming the hole is dead centre) in a soft alloy there is a risk of upsetting the walls outwards rather than actually cutting the thread material out resulting in a bulging face, not a massive issue but something I need to design around.

There is some slight distortion that happens even when using mild steel, in my case it's so slight and not a concern for me

If I was using a soft alloy I would probably support it either side using engineers steel in a clamp arrangement

David

Whats it for? If its not taking any load, (which being an m3 i assume it wone be doing much) then a couple of mm should be about fine. An m3 nut isnt going to be much thicker than 2.5-3mm, albeit in steel. If working loose is an issue then combine with some locktite or similar.

Ege margin:

http://www.wanttaja.com/shopsheets/EDGE%20MARGIN.JPG

Have you cut the sheet yet? If not, you could drill & tap the hole, then cut the piece out.

2D on loaded parts, 1.5 on cosmetic/unloaded.

Hi
I was always taught as an engineer that in like for like material
ie: steel bolt - steel plate to achieve max strength 1.5 X the diameter of the thread is correct. ( standard metric thread )
this gives maximum strength for pull out.
now I'm not sure on say steel bolt in alloy. at a good guess id say 2 to 3 times diameter.
so:. this in thinner material it may only be possible so to achieve "grip" but not full strength then I would say at least 3 x full threads minimum
ie: m6 x 1mm pitch std thread min 3mm thick. more if you can.
fine thread may be better as you are not going to achieve full strength anyway , this give better grip in thinner than 1.5 thickness.
most engineering solutions in thin metal or ally involve the use of insert , (nut serts ) and a few more .
these give full strength on thin materials.

it also depends what you are trying to bolt together , nut and bolt is good , if stuck glue the nut to the back !

Hmm thanks for the input gents.

Looks like 2 or 1.5D is a sensible figure. Makes the alloy quite thick/heavy for my purposes. Might have to reconsider the design.

Very simply what I am trying to do is join carbon plates together at right angles using aluminium brackets. Effectively creating a box that has a number of small screws holding the plates on and thus creating quite a stiff structure overall.



[Edited on 11/11/17 by tegwin]

Have you given any thoughts to do the job with adhesives? Some of the modern adhesives used for fitting body panels might be suitable?

I had not, that is an interesting thought! I do need to be able to remove the lid of the box but adhesive elsewhere could work. I am looking at making several hundred of these boxes so any process that takes too long or is "skilled" goes out the window

quote:

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Originally posted by tegwin
Hmm thanks for the input gents.

Looks like 2 or 1.5D is a sensible figure. Makes the alloy quite thick/heavy for my purposes. Might have to reconsider the design.

Very simply what I am trying to do is join carbon plates together at right angles using aluminium brackets. Effectively creating a box that has a number of small screws holding the plates on and thus creating quite a stiff structure overall.



[Edited on 11/11/17 by tegwin]

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quote:

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Originally posted by coyoteboy

quote:

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Originally posted by tegwin
Hmm thanks for the input gents.

Looks like 2 or 1.5D is a sensible figure. Makes the alloy quite thick/heavy for my purposes. Might have to reconsider the design.

Very simply what I am trying to do is join carbon plates together at right angles using aluminium brackets. Effectively creating a box that has a number of small screws holding the plates on and thus creating quite a stiff structure overall.



[Edited on 11/11/17 by tegwin]

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Thick? Did I miss a point - by D I was talking hole diameters distance from the edge - thickness having decreasing importance as you go above 0.75mm-ish

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How does having a thicker plate give you more clearance? Do you mean wider?

Approximate rules of thumb...

Assuming light load...
You need min 1.5 bolt diams to the edge of the plate (2D if heavily loaded), for the plate to not tear out/deform.
You need min 1 bolt diams plate thickness for the thread to be strong enough, (2D if heavily loaded).

This is where it's best to use a (captive) nut to compress the plate, not thread the plate.

If you like a bit of theory...here's the European Space Agency's threaded fasteners Handbook I'm almost certain there's a design strategy for what you're looking for in there

http://www.ecss.nl/wp-content/uploads/handbooks/ecss-e-hb/ECSS-E-HB-32-23A16April2010.pdf

If your that thin I'd be looking to add a return / flange on the edge to give you more metal for the connection without increasing the overall weight


as above the general rule of thumb is 1.5 or 2.0 x the bolt diameter (from the centre to any edge) and at least 1.5 x the diameter deep for a tapped hole

if your tapping into the alloy the strength you'll get would likely be about the same as if you used an alloy bolt of a similar grade to the part your tapping


the following is the BS 8118 version (doesn't cover tapped holes specifically)

alloy bolts tensile strength (in N) is = 0.6 * Pf * At * K1 / Ym

alloy bolts tensile strength (in N) is = 0.6 * Pf * At / Ym (N.B. alloy bolts are not recommended for tension)


At (tensile stress area) for M3 = 5.0308 mm˛

Pf (bolt stress limit) for 6082-T6 [<M6] = 165 N/mm˛
for 6061-T8 [<M12] = 175 N/mm˛
for 5056A-H24 [<M12] = 175 N/mm˛

K1 = 0.95 for close fitting holes or 0.85 for normal clearance holes

Ym (safety factor) typically = 1.2


so as an example: an M3 bolt in to 6082-T6 could possibly have a strength of 415N tensile or 394N in shear - both of these would assume the above edge distances have been met....
obviously the steel bolts should be stronger (at least if you get decent ones and not ones from B&Q or other cheap Chinese rubbish )
but the stronger bolt would likely just mean that the thread in the alloy would get stripped first...


[Edited on 15/11/2017 by mcerd1]

If its not majorly structural then the across flats dimension of a standard nut would seem about right to me.