# Machining Hardened Steel On Mini Mill - Speed And Feed Advise Please



## edojan (Mar 4, 2016)

hello folks, I need to machine an SKS bolt (a fellow who did a Rockwell test on it claims that hardness varies depending on the specific area - from ~ 40 to ~ 5o ) It may also vary depending on where / when the gun was manufactured.  But for all practical purposes I think it is reasonable to assume the hardness between  40 and 50.  So I have a hitorque mini mill from LMS (about 1/2 HP and spindle speed range from zero to 2500). 
Also got a 3/8" carbide end mill (two flute).  The question is the optimal speeds, feeds and the depth of cut.  I  have read about carbide bits to be usable at much greater speeds. I have experimented on a broken bolt with different settings. The cuts are possible up to 1 mm in depth using the maxium speed of 2500. The 0.4 mm (0.016") depth at 2500 rpm are seem to be reasonably possible, even though the vibrations are much stronger than when I cut aluminum. I was able to go as deep as 1 mm (0.04") with the same RPM but much lower feed. At this extreme settings the mill vibrated like crazy but with very low feed level it still did remove the metalg. I was  impressed, but not for long. When I closely examined the carbide end mill I have noticed that it its cutting edge got some damage. Tiny pieces of carbide got broken off (not much just in a couple of place but still - paying the price for my foolish experimenting) So what are some basic rules when cutting *hardened steel *on a *mini mill *using *carbide tools? *And what must be done differently (if anything) if using HSS?  
Oh, also I  almost forgot to mention using TapMagic cutting fluid which created some smoke but also reduced the vibrations.  After that I read some warnings against using cutting fluids with carbide tools allegedly because they get too hot and uneven cooling because of the cutting fluid can produce micro-cracks in the tool.  Not sure if this is the reason for the chipping of the tool's edge or is it because I had a couple of "hard" startings - that is not sure how to properly start the cut without the tool being slammed into the work peace the hard way. I hope I express my self clearly here.


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## T Bredehoft (Mar 4, 2016)

I operate on a PM25, I consider it a small mill. If I were doing what  you are attempting, I would use maybe 300 rpm and cautious hand feed, DOC no more that .020 or .5mm. I would hope to accomplish my goal at the expense of the carbide mill. The vibration of the mill is what caused the chipping.


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## 4GSR (Mar 4, 2016)

Throw out your 2-flute end mill and use a 4-flute end mill.  Run it at about 140-240 SFM or 1500 to 2500 RPM. Or what ever  you are comfortable with. Just don't exceed 240 SFM.  Follow what Tom said on depth of cut and feed slowly.  Flood the cutter with a good cutting oil like Mobil Mobilmet 766, not Tap Magic.

Two flute end mills do not work good on harden steels, +45 HRC.  Two flute endmills can create chatter and the corners of the flutes chip more easier than a 4-flute endmill.  If available to you, find some endmills with a .010" corner radius.  The corner radius helps reduce tooth chipping.

Ken


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## edojan (Mar 4, 2016)

Ken, are you referring to 4F carbide or HSS? Any difference in technique between the two?


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## 4GSR (Mar 4, 2016)

Carbide 4 flute

Sent from my SAMSUNG-SM-G920A using Tapatalk

Edit: HHS will not cut at all on anything above 45 HRC.


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## edojan (Mar 4, 2016)

How about cobalt? I have a "premium" set of 4 flute end mills from LMS will they do?


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## Chipper5783 (Mar 5, 2016)

Cobalt HSS is still HSS.  Carbide is a lot harder than HSS (and much more brittle).  I suspect if you try that "premium" set, it will very, very soon be destroyed.


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## TommyD (Mar 5, 2016)

I agree with a 4 flute carbide, not such a big of a whack when the cutting edge comes round to take a chip.


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## dirty tools (Mar 5, 2016)

I use SME mobile app on my iPhone for information on recommended cutting speeds and feed rates,
it is very easy to use


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## chips&more (Mar 5, 2016)

Carbide definitely has its place in the industry and also in the HM. But, when to use it or not use it in the HM, that is the question?  Carbide has one weakness, it’s hard as hell, but will break like glass. And it likes to be worked, or it does not perform very well. That said, ideally, you would need a rigid machine/set-up and the HP to perform the cutting speed and some coolant. I have a Bridgeport mill and have used carbide cutters with it. You ask? Do I notice any premature failure in the carbide…YES. I do not know about other Bridgeports, but mine is not rigid enough for some of the carbide cutter operations that I have tried. You ask again? Where am I going with this? Well, any mill with less mass and stiffness, like a small bench mill, will be up for a real challenge using carbide…Dave.


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## T Bredehoft (Mar 5, 2016)

Back to basics, two flute vs four flute:

Two flute will cut a slot to size, because only one  flute at a time is cutting
Four flute will cut a slot oversize because while one flute is cutting ahead of the center, one is also cutting on the side.  
For non-slotting functions, a four flute generally has more than one  flute cutting at a time, thus reducing vibration. 
Of course, this is not limited to Carbide, but is true of all materials.


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## edojan (Mar 5, 2016)

Thanks guys. Any advise on how to start the cut (i.e. bringing the end mill into the work piece) to reduce the chance of carbide breakage (all other things being equal)?


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## countryguy (May 18, 2016)

Just wondering how this turned out?   I have cut 1045 on my 3:1.  Carbide 4 flute with a corner radius.   Nothing bigger than 3/8.    I also learned about annealing and how to soften it.   Do what you need done.   Then run a heat treat again.   Recipes are very specific.  Need to also know the steel type.  No guessing there.   Just a note on something my son and I tried as a learning project.    Worked out great. The 1045 was so soft I could bend a 2" wide 1/4 gauge over my knee.  Rc value was much better than as purchased when we did the harden run.       Anyway.  Just noting that with the small mills you can get things done in different ways.       I'd you want to see a great post on HT and DIY search on a post from Ray C  and heat treat.      My 3cents.


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## edojan (May 18, 2016)

countryguy said:


> Just wondering how this turned out?


I ended up buying the carbide end mill (actually I have also bought a 4 flute 3/8)  and taking light cuts of about .4-.5 mm at a relatively low feed rate. I could not possibly use annealing because it would be a terrible idea for a rifle bolt.  On my next project, however I am planning to spot anneal areas of a hardened angle plate in which I need to machine some groves and drill some holes. I would appreciate any tips on spot annealing from the members.


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## T Bredehoft (May 19, 2016)

I worked as a Tool and Die maker for nearly 30 years. never heard of selective annealing.  If this is case hardened, there is a layer of high carbon steel on the surface. To anneal this you heat it cherry red and let it cool slowly, packed in ashes or lime, or some such. Not something you can do in one place.


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## John Hasler (May 19, 2016)

Here's a discussion on another forum: https://thefiringline.com/forums/archive/index.php?t-247057.html


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## T Bredehoft (May 19, 2016)

Thanks, John, spot annealing seems to be done often.  My loss.  I hope I never have to tap hardened steel with a 6-48 tap. 4-40 in aluminum is scary enough.


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