At the risk of sounding stupid, I have often wondered how it is possible to drill into a tank's armour plating to accept bolts for additional items. If the armour is designed to withstand artillery shells (particularly on modern vehicles such as Challenger 2 and Abrams), why does it not allow penetration from a drill? ... Perhaps I am missing something quite obvious here!

Typing error in my last sentence ... I should have said 'why does it allow penetration from a drill?'

On modern tanks the outer shell is just a cover for the armour packages that are encased underneath. Therefore typically it would be a mild steel sheet with a Chobham/Dorchester package (insert other countries armour) beneath. This is what is tapped/welded for things such as mounts for baskets, smoke dischargers etc. Explosive Reactive Armour is the same, a thin sheet encasing the explosive charge which is then mounted on a vehicle using welded mounts.
Older armour would be cast and/or face hardened plate armour and I believe it can still be machined/welded for mounts even after treatment.

Now I understand ... very clear explanation, thanks.

Speaking for German heavy tanks of WW2, I can say that they simply didn't do what you suggest. They welded little blocks with threaded holes to the armour. They were very, very reluctant to make holes in the armour itself. Holes are weak points that can be the beginning of cracks on impact.



David

Further to what David writes above:
Sweden rebuilt a number of Strv m/41 (licence built Pz 38(t)) to personell carriers (PBV 301) and this rebuild required drilling holes in armoured plates which caused problems with cracks.

As for the actual drilling: There is a difference between the slow grinding action of a drill bit and trying to punch a hole straight through.
As an example: I can drill a hole in the bottom of a Coke-bottle using a hand drill, a matchstick (as drill bit) and a small amount of fine sand. I have "drilled" holes in small magnets using similar methods. If there is enough time it is possible to drill holes in anything (even diamonds ...)
/ Robin

Machining is different than a shell penetration. In drilling there are two knife-like edges that are shaving off a minutely thick piece of metal. At any point in the process only small amounts of metal are being worked. In penetration nearly the entire hole volume is being deformed instantaneously.

Homogeneous armor is still softer than a high speed steel drill bit (one of the tungsten, molybdenum, or cobalt alloys) so it is machinable. Even the older face-hardened armor could be machined by today's ceramic tools, or by the tools of the time if the hole was started from the soft side.

In WW II the Germans were critically short of machine tools and cutters. It made more sense weld on soft steel lugs and machine those rather than working with difficult-to-machine situations. At the time (and into the 1950s) hand-powered drills were common and motorized hand tools rare, so the process was difficult regardless of the cutters that were available.

KL