" It's not clear to me what tool I have would make a straight clean groove in the bottom of a door. "

A router with a 3-wing cutter of the desired groove width.

The shank normally has a ball bearing pilot that sets the depth of the groove.

The router rides on the face of the door with the base hanging half way off.

The biggest problem is making sure the router does NOT tip at any point during the cutting pass.

I have a router but admit I rarely have used it. I guess I can ask for a 3 wing cutter bit to match the thickness of angle iron I purchase.

thanks!

You can stack 3-wing cutters on their arbor to obtain the desired groove width.

I use aluminum angle since it is thinner than steel.

Be sure to round the edges that will be rubbing in the groove.

To expand on what Brick suggested, this is the kind of router bit you need:

This particular bit is a set that allows you to cut different thickness grooves but you can also but cutters that just create a fixed width.

Although this picture isn't a door it should give you a good idea of how you are going to use the router:

{{!gwi}}

You might want to practice on some scraps before starting on your door. Hope this helps!

Be sure to stop the groove before the exposed edge of the door so it does not show on the exposed edge of the door.

Thank you, I bought the slotting bit and I see this will be a simple job as long as I am holding the router flush to the side of the door. Now I have to figure out why my router is completely dead, although it has been ~10 yr since I last used it. Hope it is only the switch.

"I see this will be a simple job as long as I am holding the router flush to the side of the door."

That is indeed the trick.

The router must NOT be allowed to tip at any point during the cut.