Cutting tool glossary and machining formulas
The catalog quotes fz, the machine wants F. The manual says κr, you call it the approach angle. This page gathers the symbols and formulas you meet in CNC turning, milling, drilling and threading in one place. Type the symbol or the plain-English name in the search box.
Cutting tool and CNC terms
-Machining formulas
-What gets mixed up most often
These are the places that cause the most confusion on the shop floor. A look here answers most questions before you even reach the glossary.
Cutting speed (vc) and spindle speed (n)
Spindle speed is how many times the machine turns; you enter it with S. Cutting speed is how many meters the cutting edge travels across the material. They are not the same thing: at the same 1000 rpm a Ø100 part is being cut five times faster than a Ø20 one.
The insert catalog quotes cutting speed, because that is what the edge has to survive. Working out the rpm is your job.
fn and fz
fn is used in turning and drilling: how many mm per revolution. fz is used in milling: how much material one tooth takes per pass. You cannot type the catalog fz straight into the machine; multiply it by the number of teeth and the rpm to get vf first.
ap and the change in diameter
In turning ap is how deep the tool is engaged, not how much the diameter changes. Going from Ø50 to Ø46 the diameter drops by 4 mm but ap is 2 mm. Confusing the two doubles your pass.
G98 and G99
On a lathe G99 reads the feed as mm/rev and G98 as mm/min. Write F0.2 while the program is in G98 and the machine advances 0.2 mm per minute — the tool looks like it has stopped. The other way round, the feed jumps by a factor of hundreds. Check which mode you are in at the start of the program.
Chip thickness and feed
Feed and chip thickness are not the same number. At a 90° entering angle they are roughly equal, but at 45° the chip is only about 70% as thick as the feed. So changing the entering angle changes the load even though the feed has not moved.
Frequently asked questions
What do the letters in an insert code mean?
The first letter of codes like CNMG, DCMT and VBMT gives the shape of the insert: C rhombic 80°, D rhombic 55°, S square, T triangle, V rhombic 35°, W trigon, R round. The larger the nose angle the stronger the insert, but the less it reaches into tight corners. C suits roughing, V narrow profiles and copying.
How do I know whether my machine has the power?
Use the Pc calculation on the formulas tab to find what the job demands. But note: there are three separate formulas, one each for turning, milling and drilling. Compare the result with the spindle power of your machine and leave yourself a margin; a machine cannot deliver its rating plate figure continuously.
I am running at low rpm, the power is there but the tool struggles.
The limit may be torque rather than power. Below a certain speed a spindle cannot deliver its rated power and torque stays flat instead. It comes up often with large diameter cutters and drills — the Mc formula covers it.
Where do I find the kc value?
kc depends on the material and is listed by material group in the tool manufacturer's catalog. It is not a fixed number either: as the chip thins, kc rises. For a rough estimate use the average kc the catalog gives for that material group; for an exact answer use the manufacturer's own calculation tool.
Can I split threading passes equally?
Do not. As the thread deepens, the length of edge in contact grows, so the same depth of pass removes more material each time. Split them equally and the last passes overload the insert and break it. The Δapx calculation on the formulas tab reduces the passes progressively.
The term I am looking for does not appear.
First set the operation filter back to All — with a filter active only the cards for that operation are shown. Then try the plain-English name instead of the symbol: "nose radius" instead of "rε", "entering angle" instead of "κr". The search ignores case.