Running carbide end mill slower than specified because of spindle speed limit ?

Hi again,

I'm making some metal parts for a roof frame in some C45 steel (1.0503 (EN) Medium-carbon Steel)
For the moment I'm doing the first operation (exterior contouring) on a kind of M14 tie rod part using a 12mm LMT-FETTE DHC Premium end Mill with Ae = 2mm and Ap = 20mm.

The specs for these mill for C45 is 230 to 275 m/min cutting speed (so using 12 mm diameter and 4 flutes we get a RPM range of 6101 to 7295) and a chip load per tooth of 0.14 mm/tooth which gives us a feed range of 3417 to 4085 mm/min (135 to 160 IPM)

I'm running at 6000 RPM (my HAAS mini mill 2 maximum spindle speed, alas....) at 1500 mm/min (60 IPM)
Yes the Feed is SLOW... barely 47 cc/min (2.9 cubic inch/min) chip load per tooth is a mere 0.05 mm/tooth, I'm running way slower because I'm trying to keep my spindle load meter around 85% (7.5 kW HAAS spindle.... :bs: :D ) and I'm not in any hurry (cycle time is 4 minutes, it takes longer just to set up each part in the vice...) and after 1 hour of cutting time the spindle load has only gone up 2% so wear seem to be very low

The thing is in operation step 3 I'm going to have to slot these parts making a 11 mm wide 39 mm deep slot, I plan on doing this with a first "old style" 10mm wide slotting operation using a 10mm LMT-FETTE DHC Premium end Mill, taking 3 mm off at each pass... and a final full depth contouring to 11 mm wide with a carbide TIALN end mill problem being that using my maximum spindle speed of 6000 RPM I can only get 189 m/min on the cutting speed... (ie 80% of the minimum recommended speed)

How does this affect the performance , should I dial down on the chip load ?

I would just work out from your chip load what you feed needs to be with your slower spindle speed. I always use the same feed per tooth whatever spindle speed I’m able to achieve.


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Do the math on a .010" endmill at 12,000 RPM. Far from ideal in any material, but we do it on a regular basis with good success. Sometimes in extreme cases you have to back the feedrate down a little because such low SFM compromises the shearing action of the tool to the point that it will fail at "normal" chip loads. You are nowhere near that.