Machining steel math

So I’ve been talking with the odd person about machining CPM Magnacut on the S5P

The endmill recommended comes with the following specs from the manufacturer
Tool Steels > 36Rc (A2, D2, S7) SFM min:250 max:350 Feed per tooth 0.0017
It’s a 1/4" 4 flute flat endmill
you get the 20 inches per minute from the IPT (Chip Load) (Chip Load) = IPM / (No. of Teeth x RPM)
so the mill has 4 teeth at 3000 rpm we get 20ipm or 508mm/m
508mm * DoC * 6.35(diameter) should equal less than the maximum materal removal per HP (1cubic inch per HP per minute or 16,387.064 cubic mm)
16,387.064/ (508 * 6.35)[3225.8] = 5.08mm DoC at 1hp
at those feeds and speeds we get a DoC of 4.8mm for a 1hp spindle BUT I figured this may be too good to be true and it appears that the math behind it is anything over 1800rpm is linear decline at the shaft on a VFD spindle:
1 HP=(RPM * T) / 5252 which is like 1hp = (18000.3) /5252
which at 3000 rpm is .171hp = (3000*0.3)/5252
unless I’m mistaken somewhere 5.08mm * .171 = .86mm max and .69mm slotting DoC to get the required speeds and feeds for semi optimal longevity on the endmill (for this material optimal longevity on a semi rigid machine would be 500 minutes of use but on this I’d be happy with 120)

Then there’s the question of the rigidity of the machine. Would I want a lower to the bed low profile fixture or would I want a raised but well supported heavier fixture that would bring it closer to the gantry to reduce chatter.

Obviously airblast is a must to prevent chip welding which I’ve been told since its annealed won’t be as bad as hardened but still a must

I’m unsure if I’m missing any factors, miscalculated anything or if this is just a stupid idea but would love some input and knowledge from those more qualified than myself.

Usual preface, I’m with PreciseBits so while I try to only post general information take everything I say with the understanding that I have a bias.

The RPM/feed is correct for the chipload if you are using 200 SFM, the spec you listed is 250-350 though. Unless you have THIS tool in THIS material HP calculations will always be off as tool geometry and things like the steel components change it. Not to mention resonance, runout, deflection, etc.

Not sure where you are getting the RPM to power from. If this is a VFD spindle then it’s dependent on if you are using a constant power or constant torque spindle and the VFD and spindle being properly listed for usable power instead of peak consumption. You also need to check what your minimum RPM for your spindle is as that varies and can damage the spindle if run too low.

Before I get into anything else. Throw out all the data about tool life with any of these “super steels”. Almost the entire Crucible knife steel lines (and their competitors) contain high vanadium carbide. You basically have a slab of steel with tiny pieces of vanadium carbide distributed through it, which is harder than tungsten carbide. Magnacut specifically tries to increase these carbides (compared to other CPM steels) by reducing the chromium. They also added niobium which produces a carbide similar to the hardness of tungsten carbide. (Source for carbides and steel content).

Yes, I know that A2 and D2 have vanadium in them. Not even close to these amounts (~4x D2, 8x A2) or in the process that CPM uses. These are more or less sintered steels to give a more consistent structure and more carbides.

In terms of cutting, you are probably not going to be able to easily math this. You will be limited by the rigidity of the machine regardless of which setup you use and need to keep enough chipload and air/coolant on it to keep from hardening in the cut.

If I were going to try this I’d probably try around 105-200 SFM, with at least a 30% reduction in the tools recommended tool steel chipload (not less than 0.0005"), and very shallow passes. You’ll also need to have near 0 runout or you’ll be fighting variable per flute chipload the whole time. Again, this is just a quick stab at it and really depends on the tool geometry.

Other things that could help would be a true diamond like coating (e.g. Ta-C, at 50% or greater sp3). But you’ll have to watch the temp even more. Better if you can find a high hardness nanocomposite Si, Cr, or B coating. A corner radius on the endmill would help too.

Hope that’s marginally useful. Let me know if there’s something I can help with.

3 Likes