Cutting Key Slot in Hard Shaft

[pb57]

I have attempted cutting a 3/8 key slot in a shaft on a new gear box off a farm grain cart. The shaft is hard, I attempted a HSS mill first and it wouldn't touch it. I ran a file across it and it just skated across the shaft. I ordered a new carbide 2 flute mill from Mcmaster and ended up breaking the tips off. That was probably my fault as I started the cut going into the end of the shaft very slowly and the tips broke off. I probably should have plunged down first and backed out the end of the shaft? I was running at 250 rpm with cutting oil. Should I buy another carbide mill and try again (not traveling directly into the end of the shaft) or is there a better way or cutter to be using. I was using my new Acer mill and the setup is rigid. thanks Paul

[Arcane]

There's some info here about milling hard steel, up to 60+ HRC.

https://www.mmsonline.com/articles/hard-milling-by-the-numbers

[Doozer]

Creep feed grind it.

-Doozer

[pb57]

Doozer are you saying the key slot may need to be ground?

[JRouche]

I ordered a new carbide 2 flute mill from Mcmaster and ended up breaking the tips off. That was probably my fault thanks Paul

Naw. Not even your fault, stuff happens. I know Every member here breaks tooling. Just the way it is.

The two flute carbide IMO should be an aluminum only bit, specially in that size.

What size carbide bit do you need? Four flute? Round nose (center cutting)?

The folks here are going to tell you what will work best.

If you come to a conclusion post it and Ill see if I have one. Free. Should be a stamp or two and an envelope. I have those. JR

[Doozer]

Buying carbide end mills from McMaster Carr?
They are probably Garr end mills.
They are very hard, but also brittle.
If you run them in a CNC they last a long time,
but manual mills tend to destroy Garr end mills.
Try a softer grade of carbide.
Typically Micro 100 has softer carbide.

Yes grind it.
Side dress the wheel on both sides, thinner than the slot.
Also relief out the sides after you dress them, you can hand relief them if it is easier.
Then conventional feed grind the slot. Do not climb the slot. Bad jew jew happens.
Keep the side wheeling to a minimum. Just a few thou at the end is OK.

-Doozer

[754]

What are you holding it with ? Collet I hope ? I made the mistake of trying to mill across a TI shaft with threads, holding endmillnin drill chuck.
it was a front spindle for a dragster and needed a groove for the washer with the tab to stop it from turning, next time I would use a grinder.

is the shaft maybe just surface hardened ? Getting through some of that with a grinder would help.

[epicfail48]

You can mill hardened steel just fine with a carbide end mill, long as your setup is rigid enough to keep the end mill from dying a rapid death. Keep to conservative cuts, dont let the tool rub, and personally id say avoid using any sort of oil or coolant during the cut. Far as the cutter goes, id go for a 4 or greater flute, and if it works with the final result a bull nose or corner-shamfer end mill will probably last longer cutting hardened material. End result may require sharp corners, but even then if you can use a bull nose end mill to take out the bulk, then a standard mill to finish the corners, youll probably have a better time of it

[dian]

"running at 250 rpm"? did a zero get lost? besides every tool has a specification as to the material hardness it is intended for. a regular carbide endmill will will be good up to 40-50 hrc only.

[Doozer]

"running at 250 rpm"? did a zero get lost? besides every tool has a specification as to the material hardness it is intended for. a regular carbide endmill will will be good up to 40-50 hrc only.

The harder the material, the slower you need to cut it.
250 rpm seems fine to me.
Why on earth would you run 2500 rpm milling hard steel?

-D

[dian]

so 8 m/min is the right cutting speed for carbide?

[Doozer]

so 8 m/min is the right cutting speed for carbide?

Look at the color of the chips.
Throw your calculator away.
Blow up your TV.

-Doozer

[JoeLee]

I would try a carbide tipped slotting cutter, perhaps a staggered tooth one. More rigid than a small dia. end mill.

JL...............

[MattiJ]

so 8 m/min is the right cutting speed for carbide?

about factor of 10 too slow.
Niagra suggests 230sfm and 2340rpm for slotting with 3/8" endmill in up to HRC62 hardness tool steels.

My guess OP exceeded the maximum chip load per tooth by feeding too fast at snail spindle speed.
"sweet spot" between not rubbing and not chipping the cutting edge is not big.

Forces are high in hard milling. You need rigid machine with no slop in movements. Not really a ideal job for old worn machine or noodle like Bridgeport.

[pb57]

thanks for all the replies gentlemen. I ended up sending it out to have ground. I hate to do that but it happens. thanks again Paul