Who this is for: For machinists and hobby CNC owners dialing in a new material or tool — who want the conservative starting point, not a paid app's 40-field interrogation.
Material and tool in — get RPM, feed rate, chipload and MRR from the classic SFM formula, with carbure notes.
Quick answer: Feeds & speeds: RPM = SFM × 3.82 ÷ tool diameter; feed = RPM × flutes × chipload (chipload ≈ 1% of diameter per flute, ×1.5 aluminum). A 1/4" 2-flute in aluminum: 250 SFM → 3,820 RPM → 28.7 IPM (chipload 0.0038-inch/flute). Carbide runs 3–4× the SFM — start conservative and dial up.
fl
2–4 typical
Spindle speed
3,820 RPM
250 SFM (HSS)
Feed rate
29.03 IPM
RPM × flutes × chipload
Chipload / flute
0.0038"
≈0.002×D (×1.5 Al)
MRR (approx)
0.91 in³/min
0.5×D radial × 1×D axial
Diameter
RPM
Feed (IPM)
8/64" (0.125)
7,640
29.03
12/64" (0.1875)
5,093
28.52
16/64" (0.25)
3,820
29.03
20/64" (0.3125)
3,056
28.73
24/64" (0.375)
2,547
28.53
32/64" (0.5)
1,910
28.65
40/64" (0.625)
1,528
28.73
48/64" (0.75)
1,273
28.77
1"
955
28.65
Chatter fix order: shorten stickout → reduce feed slightly → bump RPM. Straw-brown chips mean heat; golden chips mean you're making money. These are HSS-safe starts — carbide wants more SFM once the rig proves it.
RPM = (SFM × 3.82) ÷ D; feed = RPM × flutes × chipload. HSS starting points — carbide runs 3–4× the SFM. Cut air first, then dial up while chips stay golden.
Core facts
RPM
(SFM × 3.82) ÷ diameter
Feed
RPM × flutes × chipload
SFM (HSS)
Al 250 · brass 200 · steel 90 · SS 60 · Ti 40
Chipload
≈0.01 × D per flute (steel); ×1.5 Al
Example
1/4" 2fl Al: 3,820 RPM · 28.7 IPM
Status
Quotes use standard shop math: machine-hour rate × cycle time + materials + setup amortization. SFM/chipload figures are conservative HSS starting points (carbide runs 3–4× SFM). Compiled September 2026 — estimates for budgeting; real quotes come from the shop that runs the job.
How are feeds and speeds calculated?
Two formulas run the shop. Spindle speed: RPM = (SFM × 3.82) ÷ tool diameter in inches, where SFM is surface feet per minute for the material–tool pair. Feed: IPM = RPM × flute count × chipload, where chipload is the per-flute bite (≈ 1% of diameter per flute for steels, ×1.5 for aluminum at these diameters). This calculator uses conservative HSS SFM starting points — aluminum 250, brass 200, mild steel 90, stainless 60, cast iron 70, titanium 40 — and outputs the RPM, feed, chipload and an approximate MRR. Carbide tools run 3–4× these SFM values, so the output is deliberately the safe start: dial up from here while chips stay golden and the spindle stays quiet.
Common uses
First cuts in a new material on a hobby CNC or manual mill
Sanity-checking a CAM post's guessed speeds before hitting cycle start
Comparing HSS vs carbide starting parameters
Teaching the RPM/IPM relationship without the paid-app wall of fields
Why the calculator stays conservative
Paid feeds-and-speeds apps model tool coating, stickout, coolant and machine rigidity — worth it in production. This calculator gives the HSS-baseline starting point that won't break a hobby machine on day one: cut air first, then take light passes, then push. The gap between 'safe' and 'optimal' is where shops make money — and where hobby machines make scrap. Respect the gap until your spindle tells you otherwise.
Frequently Asked Questions
What is the feeds and speeds formula?
RPM = (SFM × 3.82) ÷ tool diameter (in); feed = RPM × flutes × chipload. SFM depends on material and tool — aluminum ~250 HSS, mild steel ~90, stainless ~60 (HSS conservative; carbide 3–4×). The calculator runs both formulas with per-material chipload estimates.
What SFM should I use for aluminum?
HSS: 250 SFM (up to 300 with good coolant). Carbide: 800–1,500+ depending on rigidity and chip evacuation. Start at the calculator's HSS figure with carbide too — you can always speed up; you can't un-weld an aluminum chip from a slot.
What is chipload and why does it matter?
The thickness of chip each flute cuts per revolution. Too light (sub-0.0005) and you rub instead of cut — work-hardening stainless and killing tools; too heavy and flutes pack or snap. Rule of thumb: 1% of tool diameter per flute (0.0025 inch on a 1/4 inch tool), ×1.5 for aluminum.
Why do my cuts sound different from the calculator's numbers?
Rigidity, runout, depth of cut and holdup all shift the sweet spot. The formulas give the starting point; your ears (chatter = slow RPM or heavy feed) and chip color (straw-brown = hot) tune it. Chatter first fix: reduce feed or increase RPM slightly, and shorten stickout.
How do I convert to metric?
SFM × 0.3048 = m/min; RPM formula in metric: RPM = (vc × 1000) ÷ (π × Dmm) — algebraically identical to the inch version. Feed: mm/min = RPM × flutes × chipload_mm. The math is one conversion away; the starting-point philosophy is identical.