Turning hard steel

[Allen Smithee]

I'm making a replacement cutter shaft for the flying club's lawnmower. The material is an old piece of steel given by another club member, ~250mm long and ~32mm dia - it's quite hard (a file corner tends to skitter across it rather than cut) and I think it was originally ground with a couple of flats on it. It's now a bit rusty, but the rust pattern is more like silver steel than mild steel - areas of shiny bit between defined areas of rusty bit, so it's probably a medium carbon steel of some kind. I rotated it slowly it in the chuck, carefully tapping it with a soft hammer until it was running true and then used a carbide centre drill to put centre hole in the end. Using a revolving centre and the chuck I can run it up to the 2400rpm my setup (Myford Super7B with 1hp 3-phase VFD) can achieve without vibrations.

So I've started skimming down the OD to clean the rust off. The job needs to take it down to 27mm with some features down to 24mm. I'm taking light cuts (0.2mm) using a carbide insert tool, using my normal approach (run it flat out and let the carbide insert take the temperature until it complains). It's cutting very cleanly with lovely surface finish and the swarf is coming off as a continuous piece like fine wire wool. The swarf is getting very hot - often red hot and occasionally catching fire briefly. Does this mean:

1. I am being a hooligan turning this at 2400rpm and I must back off the speed before it wrecks the tool or the machine

2. I am being a hooligan and I must back off the speed before I set fire to the machine, the workshop and risking a melt-down that will go down to the core of the earth and end all life on the planet

3. I am a machining god who is forcing the material to bend the metal to my will but getting the best out of my equipment...

 

Essentially given that the machine seems happy to take the load and the insert seems happy to take the temperature I'm asking if there is any reason why I shouldn't be doing this?

AS

[steamer]

3

      Hard steel needs to be shown who's boss!    Up the speed and use carbide, and yea it will turn red and sparkly!   If the lathe is happy all is good...
Watch out for part temperature.    It's easy to over shoot and cut undersize.

Try not to set yourself or the shed on fire......

Dave

[Elam Works]

That is what I get when I face ball bearing races down with carbide, glowing hot filaments. It pretty much has to come off red hot to get the steel into the plastic range. I'm doing it in a CNC lathe, so it is somewhat contained. What I do notice is that it pushes back very hard, requiring very rigid workpiece and tool holding. Even with light cuts (I think I was doing 0.1 or 0.2mm DOC) you could see the spindle (15hp) and axis load meters getting a workout. I could have used a CBN tipped tool and that would have cut with much less force than the molded edge of the carbide insert. But they chip so darn easy and cost way more than the carbide insert. For facing down the bearing 2.5mm per side it was more practical to use the carbide and then a light face on the grinder to make sure it was flat. Most of the pyrotechnics in the attached image is the oil in the modeling clay packed it to try and keep some of the swarf out of the bearing cage.

-Doug

[Jasonb]

Although making the insert work is the best way if you look at the makers cutting data for inserts the speed does tend to reduce as the hardness goes up

Somewhere around the 2000rpm would be in the right ball park as the hardness is not actually known. The shallow DOC is also more likely to see the "wire" burning so if the machine will allow a deeper cut that may just see the swarf turning blue so no need for any blue flashing lights.

[Allen Smithee]

But I LIKE the flashy, sparkly lights - they're pretty!

Elam's photo is very much like what I was getting.

AS

[propforward]

Holy moly!

Kind of makes me want to try it.

[jadge]

The key to machining hardened steel is a high surface speed, shallow DOC and high feedrate. The swarf should be coming off red hot. The aim is to get the metal red hot in the shear zone so it is easy to shear. For the parameters stated, spindle speed is higher than needed, DOC about right and no feedrate given. The swarf should be fine, like steel wool, and brittle. A CBN insert would be better than carbide. Here's a link to some trials I did a while back with quantative surface roughness and hardness measurements:

https://www.model-engineer.co.uk/forums/postings.asp?th=51900&p=3

Andrew