Some tool holders are hardened more than others.

Your insert tool I assume was carbide so if that was not touching it you may have difficulty.

If you want to try a solid cutter then get one designed for the harder steels that says it will cut 60HRC. Similar with inserts look for ones designed for the harder more difficult steels often classed as “group S” rather than just basic general purpose ones.

What milling machine do you have?

It is highly unlikely that the tool body will be HSS, as it is expensive and its ability to cut when red hot isn’t needed.

It is perfectly possible to cut toughened/hardened steel with carbide endmills. There are some important points to note:

• Use a commercial quality endmill that is properly ground and sharp
• Run high speeds, shallow depth of cut and reasonable chip loads
• The ideal is for the shear zone at the cutting tool to be red heat as it will then be soft
• A rigid mill helps but is not essential – I have milled HSS on a hobby grade CNC mill

Julie

Cost of cheap tool £5-20

Cost of carbide cutter £15-30

Is it worth it?

I’m with Phil, angle grinder.

On 3 January 2026 at 16:16 old mart Said:

I have milled dozens of 20mm square lathe tools down to 17.53mm to enable them to get on c/l without bothering to shim. Both lathes use exactly the same height tooling. All have been milled with shell mills using inserts for steel, most with a 50mm Ceratizit 5 insert which uses round inserts which can be indexed up to 8 places. The holders are all tough steel, the cheap Chinese ones take just as much effort to mill as the top brands like Sandvik and Kennametal. You can get inserted endmills down to 8mm diameter and a 2 or 3 insert holder between 12 and 16mm would work well in a small mill.

I had an insert carbide mill (2 insert) anyway so using it to trim down over size tooling is a no brainer.

I too have trimmed down tools to put them permanently on centre height in a four way tool post. Far more rigid on my Mini lathe than a QCTP.

On 3 January 2026 at 07:50 Hollowpoint Said:

Cost of cheap tool £5-20

Cost of carbide cutter £15-30

Is it worth it?

I agree.  Unless there’s a d****d good reason I avoid machining difficult metals.  Too many other things to waste time and money on!

At least no-one has suggested grinding lumps out of the lathe so the holder will fit it.  Been done…

🙂

Dave