Flat end turns melting hot on machining rusted steel

I have rusted steel of some sort that I am trying to machine a hole on using flat end HSS in a circular arc. It's only a few mm deep.

One flat end turned melting red and was already burned out just trying to drill the initial hole on the material. The settings were around 300 RPM, approx 12 mm (half inch) flat end, and a very modest 15 mm / minute feed rate and modest cooling by hand. On a Bridgeport clone...

I have previously had significant trouble working with similar material - I thought it would behave like butter which it did when sawing it. What gives?

Hi CNCAndrew! It sounds like you have some nasty scale on your part, and scale is always tough on tooling as it is harder than the core material. There are a couple of ways to improve tool life when there is nasty scale on your part:

-Conventional Mill instead of Climb Mill. With Climb Milling your End Mill hits the outside of the scale on every cut, causing the tool to wear quickly. With Conventional Milling the End Mill comes into the clean material underneath the scale. This is generally the only time you should choose Conventional Milling over Climb Milling.
-I'm not sure what diameter tool you are running, so it is hard to say if 300 RPM is high or low. But I'd also look at lowering my SFM a little (I'd be at 25-50 SFM should be a good starting point for HSS)
-You also may need to go with Carbide or Powdered Metal over HSS. The HSS Substrate may not be hard enough for the material and a Harder Substrate like Carbide or Powdered Metal will wear less and give you longer life.

Hopefully this helps!

Mike

I agree with the conventional milling approach , scale is tough on the best of cutters .


It helps to know the type of steel . Glowing red is never good and if it's high carbon then you could be causing work hardening and making matters worse . Pour the cutting oil to it , or , if your running coolant then increase the concentrate considerably .

You didn't mention the brand of hss cutter your using and the cutters could be the problem . A lot of them are absolute junk , especially the import cutters . I'd try slowing the rpm a bit more and see what happens , but really the best thing to do is get better cutters for the job .

Hi,
just to be sure....you are using centre cut endmils? The endmill is spinning in the correct direction? What coolant are you using?

Is it not worth a couple of minutes with an angle grinder to knock the rust off, even if its just the location of the holes?

Craig

Where did you get the settings from?

15mm/minute feed rate is very slow. Feed too slow = rubbing = heating and dull tool

There is a huge difference in speeds and feeds depending on the type of steel and heat treat condition. 300rpm should be fine. If that's AR500 plate you are work hardening it instantly. If it's just 1018 then you are aren't too far off but still need to feed at least 2x faster. Also, some Chinese hss end mills have a poor heat treat and are only good for aluminum. If it's any type of work hardening steel and you dwell for just a fraction of a second you will get a hard skin that will kill even an premium hss end mill. This is exactly why mystery metal is frowned upon. If it's a fairly rigid setup and your mill is tight, a carbide endmill will chew through most mystery metal without any complaints.