I've bought a used DRM Alinox, that turns out to have one of the screws on the lower burr carrier stuck. More force = more destruction of what's left of the screw head. I've read arguments for and against an overnight soak with penetrating oil, hammering (with the burr carrier out of the grinder), and screw extractors. All seem to carry inherent risks.

Now another option occurred to me, and I couldn't find any comments on this. Might a good soak in hot water with an espresso cleaning agent, e.g. Pulycaf, work? Or would that risk a) swelling of any trapped coffee gunk, thereby worsening the problem or b) damaging some of the materials, like the burr metal?

If the carrier is aluminum and the screw is stainless, it's possible the aluminum is corroded. We used Tabasco sauce to remove stainless screws in corroded aluminum masts. Worked every time. Let a couple of drops soak in overnight.

Wow I didn't realise Tabasco had such powers!

This carrier is made of brass, and the screw is stainless. But the burr chamber looks like it's aluminum; don't know whether it may corrode from alittle distance.. The other two screws do look a little blackened along their shafts, but mainly there's a lot of coffee residue in the threading. That's why I was thinking pulycaf, but I fear it could further jam the screw or corrode the burr's edges. Would Tabasco work to get the residue softened too?

edit: added two pictures, one with flash for depth of field and one without flash for better colours. The residue on the two screws that came out is clearly visible. (I replaced both with new screws)

Clean out all the gunk around the screw with a needle first. You really don't have much chance of getting the brass carrier out at this stage so I'd go with a drop of penetrating spray and then find a nice flat bladed screwdriver. As long as you put pressure downwards it should not slip or chew it up any further. Drop of super glue on the end of the screwdriver may help. Get someone else to stop the motor turning by using a spanner on the hex bolt.

Whatever you do don't use an impact driver as the brass will break.

Heat the screw/burr with a heat gun or hair dryer. Get it good & hot & see if the screw breaks loose. The heat will soften any coffee residue.