Machining HSS?

I’ve turned and milled HSS a couple of times using carbide tools. It was slow going but doable. I have another project in mind that requires milling some HSS but I’ve had to order a new milling cutter for the job. I couldn’t help notice that the stated hardness for most of the cutters I looked at is lower than that quoted for HSS!? Have I just been lucky with some of my existing cutters? The items I’ve machined work ok and seem hard so I’m guessing the HSS stock is ok.

Most HSS is milled in the softer state, then hardened and ground. Turning is easy with hard HSS if you use a CBN tipped insert, but I don't know of milling inserts with this feature. APT do list endmills rated to 65HRC.

Edited By old mart on 27/06/2023 14:36:25

It is straightforward to mill hardened HSS with carbide cutters. As shown below; milling an involute form on the end of a HSS toolbit for use in shaping an internal gear:

The question of hardness for a carbide cutter is a moot point, as the cutters are not homogenous. Instead they are made from fine tungsten carbide powder in a metal (usually cobalt) matrix.

Andrew

Cutwel [amongst others] do a range of milling cutters which should cut hardened HSS.

Tony

I have turned HSS with CCGT inserts.

Martin C

Can you give examples Vic?

Pure Tungsten Carbide is about 30 times harder than HSS, so in theory no contest! Compared with Carbide, HSS is Cheesium. A few possibilities:

• carbide tooling is often a percentage of Tungsten Carbide powder inside a softer matrix. This reduces hardness in favour of some other desirable property such as strength, elasticity, toughness, sharpness or cheapness etc. (Only a BF buys £££ solid carbide when a cheaper cutter does exactly the same job on soft materials.) Possibly the carbide tooling looked at isn't the hardest possible.
• Maybe not comparing like with like? There are several different scales and sub-scales: Vickers, Rockwell, Brinell, MOHS etc.

In my experience HSS is no match for carbide, though it tries to put up a fight. Still good stuff: HSS can be ground into different shapes, including very sharp, and I find it less fussy than carbide for getting a good finish on difficult materials.

Dave

Sorry Dave, I don’t know what you mean by examples? I’ve turned down some HSS rod on my lathe with a carbide insert tool and I’ve also milled a short groove in some HSS flat with a carbide bull nose end mill.

I have another project coming up that needs a 6mm hole in some 5mm thick HSS. Will a solid carbide drill bit do the job?