tapping stainless steel on laser cut holes

At the ship yard, we oversized the holes slightly, as long as this does not effect your integrity. We did this using a waterjet., you have small holes, in thin material, so this may be a concern.

We also used beeswax as a lubricant to thread.

I do not wish to appear facetious...........although in most cases this does happen.

Surely this is something that would have had to have been researched BEFORE you tackle a job of this size and nature...........or do you take on a job and think "Hell! How am I going to do this job?"

M4 holes in 3mm plate will not give you much strength, why not just oversize the hole and fit threaded inserts.

You can try GANG TAPPING, depending on pitch distance (Center-to-center distance of the holes).Get Ceramic taps and use no lubricant.

Years ago (1974),I had designed and built a tapping machine (tapping at 2000 rpm) for a production rate of 6500 holes per shift ≡≈20,000 holes per day with 99.8% accuracy (using Go/No GO guages inspection) for a sheet metal nuts manufacturer.Total annual production was 48,000,000 nuts for export to the USA markets from India.

Any radial drill machine fitted with the Gang tapper head will give accuracy and productivity on relatively large area type of work piece.The gang tapper will of course have to be custom designed and put together.

Please visit www.artecrobotics.com for background info.

Hope this helps.

Could it be that the, "Heat affected Zone," is part of your problem. I can visualize the wall of the hole being brittle perhaps and not welcoming to a tap.