Somewhat depends on the weight of items placed on the shelf/bench but 3mm may suffice.
A central bar in the 1200mm length would provide better support.

Emgee

My big steel bench is 6 ft by 32 inches, somewhat larger than yours will be. The top is 8 gauge, approximately 4 mm. Frame is 2 1/2 inch equal angle with lots of weld holding the top on. There are two cross braces dividing the top into 2 ft by 32 inch bays. Horizontal bracing bars around 6 inches up on side and rear but it's open at the front. I've filled the space underneath with drawer sets on tiny rollers so they can be pulled out. Drawers and sliders stripped from redundant Civil Service filing cabinets.

I find mine solid with minimal flex when something heavy is dumped on it. Unlike ones of lighter construction.

From my experience I'd say that a 4 mm top with a cross brace across the middle will be fine providing its well tied down to a solid frame.

The man who made mine knew his stuff when it comes to welding because it sits flat and true on the frame with no distortion. Not a trivial matter to achieve.

That said Ady1 makes good point about using kitchen worktop to provide stiffness with a thinner top. But I'd still not go below 2mm for the top.

However you make it don't weld or do hot jobs on it the sheet will move.

For hot work get yourself a vermiculite sheet to protect the steel. Its OIK to hold stuff in vice for welding but do think of splatter.

Clive

The top of my hydraulic lifting trolley is about 3ftx2ft and about 2mm thick It is well bowed by the previous, warehouse?, use.

Sheet metal, being thin, is weak wne subjected to a load on the dlat surfaces.

Not surprising, since the stiffness is (B x D^3 ) / 12  With D being only a few mm the sfiffness has to be very low.

Using something like kitchen worktop with thin steel as a protective layer is much better.. Although, unless you are moving sharp edged metal axcross it frequently, using the worktop without the steel protective sheet may not be necessary.

My shelving (Wood, ex house bargeboarding ), survives having a metal box containg a HV6 Rotary Table and accessories, (Probably 30 Kg ) )being pushed on and off without beingcoming scored

If you do use just the sheet steel, it will need support, in the form of angle iron, or box section, to divide the unsupported parts into small areas. For any great weight to be carried, I woulds suggest no more than 150mm square. Even then concentrated heavy loads will permanently bend the thin steel.

And then you need to think about the strength of the framework that carries the shelves!

You don't want the framework carrying heavily loaded shelves to collapse!

Howard

Folding the all edges to make a flange, say 25 mm deep, will stiffen the shelf, but not prevent it bowing in the middle where it is unsupported, causing the outer flanges to bend outwards..

Edited By Howard Lewis on 25/05/2023 06:59:42

I have a commercially made steel workbench. As I remember, the top is about 1/8" thick. It was not rigid enough, especially where my bench vice mounted. I cut down an old solid core wood door to fit under the bench top. It was about 1 3/4" thick. Rigidity problem solved.