Looks like a counterbore drill with the end missing to me.   Counterbores widen out the top of a drilled hole to take the head of a bolt so that it doesn’t protrude above the surface.  Allan bolts are often fitted that way.

The missing part is a short cylinder in front of the teeth that keeps the drill straight in the hole.  Only the four down facing edges on the end of the tool cut, not the sides.

Good buy if any counterboring in that size is needed, not good if an end-mill was intended!

Dave

Chatter is inherent in the design of the endmill due to the straight flutes. Ideally it would need to be used on a rigid mill. The OP doesn’t specify a mill, but for reference I don’t regard my Bridgeport as particularly rigid. I’d guess that the cutter is from the 1960s/70sv when carbide was only common in short lengths, and before solid carbide cutters became the norm.

I have a range of secondhand HSS endmills and slot drills up to around 1″ diameter, but they are only used for special jobs. For general milling on the Bridgeport and CNC mill I use 6mm and 10mm diameter solid carbide cutters. With limited rigidity and power (1.5hp for the Bridgeport) it is better to run smaller cutters at higher rpm than larger cutters at lower rpm. It is still possible to take cuts with the smaller cutters that will take the mills to the limit, and avoids the eye watering cost of large solid carbide cutters.

For large areas of work I prefer to use smaller (50mm diameter) insert cutters.

Andrew

Edit: Chatter might be ameliorated by taking large widths of cut so multiple teeth are always engaged.

The reason the flutes are straight is as Andrew says they are dated to pre solid carbide and it would be one hell of a job to braze helical pieces of carbide to a sultably shaped blank. You do sometime see than with a slight rake similar to a TCT router bit.

Again as with woodworking the lack of the helix stops the cutter lifting the top layer of composite materials in the same way a straight or downwards helix router bit prevents the grain or veneer being lifted.

They don’t actually need a helix to clear swarf as it is an “end mill” it is designed for side cutting not slotting so the swarf simply gets thrown out the side.

I seem to remember reading that originally all milling cutters had straight flutes be that carbon steel and then later HSS and it was Mr Bergstrom of Weldon Tool Co that first came up with the idea of helical flutes but they had a tendency to pull out of the chucks of the time so he came up with the idea of a flat on the side of the cutter to take a grub screw . That is why cutters with a flat on the side are still known as Weldon Shank today.

Your average dovetail cutter benifits from having straight flutes which are a lot easier to grind than if they also followed a helix.

I have some of those, bought before brexit from Germany. Some have helical inlays and some straight. I would tend to use them for squaring up and roughing if the finish is not great. I also have some inlayed dovetail cutters, 45, 55 and 60 degrees.

The screwed shank cutters for Clarkson Autolock, Osborn Titanic and Acramill holders were designed to prevent the pull out experianced when helical fluted cutters became common. Some of my inlayed cutters have the screw thread shanks.