§1Three shop staples
This page groups three things every machine shop relies on daily — high-volume turning, sawing stock to length, and the fluid that serves nearly every cut. They meet in the workflow rather than in principle.
The screw machine is the automatic lathe that makes small turned parts — screws, pins, fittings — by the thousand from bar stock, unattended. The band saw is the shop’s first operation, cutting raw bar and plate to length before any other machining. And cutting fluid flows over most cutting operations to cool and lubricate them. Each is simple to use but repays understanding: the screw machine by its cycle economics (§2–3), the saw by the three-tooth rule that picks its blade (§4), and the fluid by knowing what it does and which kind to use (§5–6).
Contents§2Automatic screw machines
A screw machine is a fully automatic lathe that feeds bar stock and cycles a set of tools to turn, form and part off a finished small part, then repeats — no operator per cycle.
Fed from a long bar, the machine advances the stock to a stop, then brings a sequence of tools to bear — turning, forming (with the form tools of the tooling section), drilling, threading — and finally parts off the finished piece, before feeding the next length and starting again. Cam-operated machines run a fixed cycle set by cams; CNC “Swiss” machines do the same under program control, with a sliding headstock that supports slender work close to the tool. Either way the point is unattended, repeatable, high-volume production of small turned parts: once set up, the machine runs cycle after cycle to identical parts, and the economics turn entirely on the cycle time (§3). It is the mass-production form of everything the turning pages describe.
Contents§3Parts per hour
A screw machine’s output — and so its cost per part — is set by its cycle time: the seconds to make one piece divide into the hour.
If the machine completes one part every 20 seconds, it makes 3600 ÷ 20 = 180 parts per hour, and the machining cost per part is simply the machine’s hourly rate divided by that figure. Shaving the cycle — a faster feed here, an overlapped tool there — raises output in direct proportion, which is why screw-machine set-up obsesses over the cycle: on a run of a hundred thousand parts, a second saved per cycle is hours of machine time. The same speed-versus-tool-life economics of the econometrics page apply to each cutting step within the cycle, but at the machine level the headline number is parts per hour, and it falls straight out of the cycle time.
§4Band saws and the three-tooth rule
Sawing has one governing rule for choosing the blade: at least three teeth must be in the cut at once. It sets the tooth pitch from the thickness of the material.
If fewer than three teeth span the material, a single tooth takes too big a bite and strips or breaks; if far too many are engaged, the tooth gullets cannot hold the chips and the cut packs and burns. The rule is to keep roughly 3 to 24 teeth in the cut. For a 25 mm-thick section, a 4-teeth-per-inch blade (6.35 mm pitch) gives 25 ÷ 6.35 ≈ 3.9 teeth engaged — just enough; a 6 TPI blade gives about 5.9, comfortably in range, while a fine 10 TPI blade would put nearly 10 teeth in the cut, better for thin-wall tube. Thick, solid material wants a coarse blade for chip room; thin material and tube want a fine blade so at least three teeth always engage. The blade speed, meanwhile, follows the material as any cutting speed does — slow for steel, fast for aluminium.
§5What cutting fluid does
Cutting fluid earns its place by doing four jobs at once — and which matters most depends on the cut.
It cools the tool and work, carrying away the heat of cutting so the edge keeps its hardness and the work does not distort — the dominant need in fast, hot cutting such as high-speed turning and grinding. It lubricates the sliding of chip over tool, cutting friction, heat and built-up edge — the dominant need in slow, heavy, high-pressure cuts such as tapping, broaching and threading. It flushes chips out of the cut, keeping flutes and teeth clear (vital in drilling, sawing and grinding), and it protects the fresh metal and the machine from rust. A given fluid leans toward cooling or toward lubrication by its make-up (§6), so the fluid is chosen for whichever of the four the operation most needs — cooling for speed, lubrication for pressure.
Contents§6Types of cutting fluid
Cutting fluids form a spectrum from pure oil to pure water-based, trading lubrication against cooling.
| Fluid | Nature | Best at |
|---|---|---|
| Straight (neat) oil | mineral oil, undiluted | lubrication — heavy, slow cuts, tapping |
| Soluble oil / emulsion | oil emulsified in water (~1:20) | a balance of both — general machining |
| Semi-synthetic | less oil, more additive | cooling with some lubrication |
| Synthetic | chemical solution, no oil | cooling and flushing — grinding, high speed |
| Water carries heat away far better than oil (its high specific heat, from the materials pages), while oil lubricates far better — so the spectrum runs from neat oil for lubrication-limited cuts to water-clear synthetics for cooling-limited ones, with soluble-oil emulsions, mixed roughly one part concentrate to twenty of water, as the everyday middle ground. Correct dilution and cleanliness matter as much as the choice: too weak an emulsion fails to protect, and stale fluid harbours bacteria. | ||
§7Quick reference
The working core of the page on one card rack.
Screw machine
automatic lathe, high volume
20 s cycle → 180/hour
Saw pitch
3–24 teeth in the cut
25 mm → ~4–6 TPI
Fluid jobs
cool · lubricate · flush · protect
Choose
oil for pressure (tapping)
water-based for speed/grinding
Emulsion
~1:20 concentrate:water