[myhäje]

Hello.

We are using several slabs of steel (about 200x200x10 mm) as fixtures and pressure plates to attach each workpiece to a pallet. It works nicely, but they are heavy (over 10kg) and when changing workpieces it is really strainful on the shoulders. We could machine out excess material, but my boss fears that it will make them weaker and they will eventually start to bend. If I start drilling holes to them (about 8mm deep, for example), is there some way to estimate a safe distance between drill holes and drilling pattern so that the integrity won't be compromised in any way?

[txcncman]

Your boss is correct. Even drilling one hole in the plate will make it weaker. But will drilling holes in the plate make them not usable is the question. A design engineer is someone that should address the question with you. You should hire one.

[Geof]

It depends on how the load is applied and how the plates are bolted or clamped down. For instance if you had a plate that was clamped around the edges with the part bolted on like in the picture a hole in the center will have very little effect on the rigidity. But it does not remove much weight.

[Christian Knüll]

Hi,

why not just take one, drill some holes, test it... drill some more holes, test it again... ... ... until it is too weak?

Of course depending an how it is used there are better and worse positions to drill holes - e.g. keep the holes away from the contact surfaces where load is applied...

[dmalicky]

I've found it's very time consuming to remove significant weight by drilling holes, especially in steel. The easier solution is to switch to an aluminum plate, if that material will work for the application. Alum has a lower modulus (E) than steel, so for equivalent bending stiffness (E*I), increase the plate thickness. I=(B*H^3)/12 (B=width, H=height or thickness). Same bending stiffness given by Es*Is = Ea*Ia (s=steel, a=alum). Es=3*Ea. B is the same. So that boils down to 3*Hs^3 = Ha^3. So if Hs=10mm, Ha needs to be 14.4mm for the same stiffness. The density of alum is 1/3 that of steel. So a 15mm alum plate will weigh 1/2 as much as the 10mm steel plate, yet be slightly stiffer.

[Flateric]

Deep anodizing will also dramatically stiffen the aluminium, I believe anyways.

[Geof]

I had never heard the idea that anodizing could alter the mechanical properties of aluminum so I did some Googling. This was all I found in one link:

Frequently Asked Questions

[I]Does anodizing affect the mechanical properties of aluminum?

Mechanical properties of aluminum, such as ultimate strength and yield strength, along with percent elongation, reportedly have been tested before and after anodizing without exhibiting any difference in the strength of the aluminum. Aluminum is a very soft metal and if not protected can be abraded away. The anodic oxide is much harder that the aluminum and the anodize process is used on parts like pistons so that the parts will last much longer. /I]

I do not think any faith should be put in claims that anodizing makes aluminum (alloys) stiffer. Harder on the surface and more abrasion resistant yes but even thick anodizing is only one or two thousandths of an inch thick and does not give much protection from heavy point loads. The best analogy I have heard about anodizing is that it is similar to a thin crust of ice on powder snow; you can lie on it and slide but if you stand up your feet break through.

[tjamscad]

Have you decided what your going to do with this problem? There are several options to remove weight and not drastically effect strength depending on what you are doing with these plates.