how to decide a sheet metal thickness for a machine safe guard or protective guard

Guards come in many varieties. Several factors to consider (from a practical standpoint):

-Equipment used in area
-Allowable movement of the guard
-Fabrication technique
-What is being guarded

For heavy duty applications where motorized equipment could damage the guards will require much heavier material than lighter duty, where only keeping body parts clear is required.

How large is the guard? If the span is large and material is light, you might consider stiffening either by formed beads in the material or attachment of a rib. If the material is heavy gauge, you might consider panels of expanded metal (especially good if there are moving parts inside that should be monitored, but not touched.)

Lighter gauge metals are typically spot welded, formed, or cold fastened (rivets, screws, etc.) Heavier gauge metals are usually welded along the edges.

Always consider a guard as something to keep the most idiotic person you can imagine (then make them even more idiotic) away from danger. Even if it is obvious you should not put your finger or hand there, this person would; just to see what happens. This being said, make sure if you use expanded sheet, the opening should be small enough to keep fingers out (or far enough away they could not get caught--but smaller is better as it keeps finger sized tools out too.)

Typically, I would make guards out of 10-12 GA. Steel because of the industrial environment I deal with and the guards are normally self-supporting. If the guard was smaller and in better environment, you could probably use steel in the 14-16 GA. range. Lighter gauges might work if the guard is stamped metal (integrally strong) or the guard is smaller/well supported.

If in doubt, make it HEAVY. The less chance of someone getting hurt, the less chance of you getting in trouble for them getting hurt (read wrongful injury lawsuits.) There are some OSHA regulations for machine guarding that might be worth reading, just to get an idea what is acceptable and required in the US.

http://www.osha.gov/SLTC/etools/machineguarding/generalrequirements.html

A machine guard needs to be generally robust.
To guard against people accidentally putting a hand into a machine, I’d use 1.5mm steel. If the metal is much thinner than this it may be difficult to weld or may distort in use. If the metal is much thicker than this it may be too heavy and unnecessarily expensive.
If the guard is large, it may need thicker metal as it may distort as it is moved. I’d fold the edges and weld the corners.
If the guard is to protect against flying parts caused by machine failure, a thicker metal would be needed.

thanks for your answers.
but what are the stresses are acting in sheet metal guard