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Lord Woodesy wants to have a glorious antlered helm! But he has heard that having antlers, horns, or other projections on a helmet can be severely detrimental in conflict since they can be grabbed or violently twist the head if struck. But Lord Woodesey really wants that antlered helmet.

So he came up with an idea, what if he had his helmet smith attach antlers but leave them not so firmly connected so that a blow will break them off instead.

So my question is, would it be possible to make a helmet that has antlers (made of actual deer antlers, a young deer's) that are only loosely connected so that you can wear it into combat and generally "around"? It would also be nice to know if this would work with other things such as horns or metal structures.

Can Lord Woodesey have an antlered helm? Yep, you bet, absolutely. As Kilisi pointed out, lots of impractical/decorative armor existed back in the day, just because the wearers liked how it looked. Does Lord Woodesey really need it to be battle-worthy? Even if he does, allow me to point out that in a fantasy/RPG setting if you say "Lord Woodesey appeared, wearing a badass antlered helm", no one is going to say, "but are the antlers attached loosely enough to prevent brain damage?"

However, since you've asked, and it got my imagination firing, I came up with a few (uneducated) attempts at making "easy-break-less-head-trauma" helmets. Many might be impractical if tested, but it was fun to dream. And even if none of my ideas would actually work, I have no doubt that it is possible to do what you are asking. A good engineer could probably solve the puzzle fairly quickly. Would it ever be quite as safe or efficient as a normal, practical helmet? Unlikely. You have freakin' horns sticking up off your head. So, with that out of the way, here are my best horn-hat catastrophes:

...or we could be boring and just glue it...

Yes, absolutely

Lord Woodesey and I seem to have had the same idea, great minds think alike, so they say. There are several ways to have a deer antler helm without it being detrimental.

The easiest, and by far the simplest, idea is to fix the antlers to the helm and then cut through them near the base, leaving you with two nubs attached to the side of the helmet. Then, you glue the antlers back on to those nubs (glue did exist in Medieval times but you may also be able to use wax if you prefer).

This may seem redundant but, by cutting and gluing the antler, you have created a structural weakness. This means that, when force is applied from someone pulling it or a weapon strike, the antler breaks off where we glued it to the base. This prevents someone being able to use it against you but still allows for you to wear your antlers into battle. The plus side of this is you may also be able to re-glue the antler back on after the battle if you pick it up and its not too damaged. A similar thing can also be done with horns, using the exact same process as described above.

Metal though is a little bit different. Due to its weight, it would be a lot harder to use glue to achieve the same effect with steel or bronze, possible with modern glues but less so with Medieval ones. Instead, what we can do is make these pieces out of a lighter material such as aluminium or tin and weld the pieces together. This means they do not add too much extra weight, can be broken off easily due to the structural weakness and can possibly be repaired or replaced after the fight.

Alternatively

Rather than risk Lord Woodesey damaging the antlers (or whatever else he chooses to decorate his helmet with), we can instead have detachable horns. Essentially we can have something similar to arrow or spear heads. A metal fixture goes onto the side of the helmet and the antler or horn is pushed into that fixture (it has already been filed down to make sure it is the right size and a snug fit).

This way your antlers can easily be pulled out of their fixtures before battle, so they are not a hinderance, and put back in when not in combat. This also prevents them from being damaged or lost in a fight. You could even do a similar thing with metal.

You could do it easily, it still won't be as effective as an unadorned helmet, and I don't see the need to wear it in an actual battle.

There was lots of ornamental armour around back in the day, and the Japanese Samurai armour had all sorts of projections on helmets. So it's not a showstopper. The projections were thin soft metals that would crumple or get chopped off if hit. So something as simple as attaching the antlers to a thin soft metal holder would do the job.