die grinder for cutting slots ?

Given the work may be in short bursts, an additional receiver/vessel would help, anything really that can hold a volume of air.
The outlet hose is ideally the same bore as the bore of the fitting to the compressor.
Smaller bore hose will create a pressure drop.
Make sure the outlet filter element is not blocked or filthy, adjust the pressure reg to the lowest pressure acceptable, again conserving air.
Check for leaks.

 

I had to do something similar for my gearbox mounts, I drilled a series of (in my case 10mm) holes for the slot, then using one of those 1mm thick cutting discs in the angle grinder cut the sides of the slot, turn over and repeat. With 3mm steel you can probably get an almost perfect slot that only needs a little filing. There is no real need to have the line of holes except that there is far less metal to remove with a file. For those that haven't used the Dewalt 1mm discs then you will be astounded how precisely you can cut with them. I have tried all the major brands and the Dewalt stands head and shoulders above the rest, Tool Station is normally the cheapest source especially when they are on special offer. A couple of months ago I discovered an old tin of Norton 1mm discs that I probably bought ten or more years ago and couldn't believe how bad these discs were back then.   

When my lad was blacksmithing he referred to those discs as “plasma discs” due to the fineness in their cutting.
I have (miss)used a great many, keeping the ones, worn down from normal cutting, for finer work.
Used with great care & safety equipment, remarkable delicate results can be achieved. A lot of this is due to the changes in cutting speed as the disc reduces in diameter; Pi d & all that.
When cutting sheet metal, I clamp 🗜 on a guide bar & use that as my datum. A venerable piece of “bed iron” served for many years, unit I ran out of edges which could still describe straight.
I have great affection for old steel & non ferrous, you feel you are working parent metal & not a mixture of old toasters, oil tankers, cars & cutlery.
M

I am glad someone else grades their use cutting discs, I have used them until they are almost the same diameter as the locking nut when cutting slots. I just thought I was of Scottish descent. 

😂😂

I have worn discs which are now old friends. I have accrued several angle grinders over the years, battery, mains & air, each has their allocated disc type but my oldest DeWalt caries the specials, those whose full circumference is a memory & are now down to the quick.
I’m glad that others have as higher daftness quotient as me.
M

I prefer the ultra thin 0.8 mm cutting discs for fine work......0.2mm doesn't sound like it'd make much difference, but it really does!
A light touch is required.

The angle grinder is probably the most numerous power and battery tool I have, the damned things are every where, in 3", 4", 41/2 ", 5" 7" and 9" sizes the 5" being the most used, the little ones are great where there is no room to use the others and the big bastards seldom see use ,but sometimes they're the best thing for the job at hand, especailly when a deep cut is needed.
When doing any serious fabrication work I have several grinders lined up on the bench at the same time, each with different types of disc fitted, it saves time and buggerising about changing discs.
It means your never going to be tempted to use the wrong disc for the job because it's just a little bit that you need to do.
So I'll have  one with a cutting disc, one with a flap disc for light to medium grinding and one with a grinding disc for heavy grinding.
I'll often have another with a wire brush wheel too!
Time is money and life's too short to be fart arsing about constantly changing angle grinder wheels!
I'm the same with drills, I'll have one with a small pilot drill bit in it, another with an intermediate sized bit and another with the correct sized bit for the job, often with yet another with a countersinking bit, hole saw or some such if required. All set to the right drill speed.

Learned a lot from this thread.