CNC cutting basics: what happens before the spindle starts

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  How a CNC saw removes material
• 2
  Blade choice follows material, not habit
• 3
  Feed and speed windows for common stock
• 4
  Why the cut blank sets up the 5-axis job
• 5
  Where sawing stops being the right first step
• 6
  Saw or mill for the first cut

A CNC saw holds the workpiece and the blade in the same rigid frame, then drives the blade through the material at a controlled rate. On a band saw, the blade is a continuous loop running over two wheels. On a circular saw, it is a toothed disc. Both cut with the same action: each tooth takes a small chip, and the machine decides how fast that chip is fed.

The control system owns three variables. Blade speed is surface speed at the tooth tip, given in m/min. Feed rate is how fast the blade advances into the cut, in mm/min. Cutting pressure follows from the two. Program the wrong pair and you get either a glazed, work-hardened surface or a snapped blade.

Chip load per tooth is the number that matters. It equals feed rate divided by blade speed and tooth count. Too small, and the tooth rubs instead of cutting. Too large, and the tooth breaks off. Most band saw trouble traces back to this one figure.

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  Chip loadFeed per tooth; the controlling number on any saw
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  Surface speedTooth-tip speed in m/min; set by material hardness
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  Blade tensionToo loose causes wander and a curved cut

Blade selection is dictated by the material and the finish you need. Diameter or band width, tooth material, tooth count, tooth geometry and set all change with the workpiece. A blade that runs well in 6061 aluminium will destroy itself in 17-4PH stainless within a few cuts.

Tooth count is the first decision. The rule is simple: keep at least three teeth in the cut at all times. Cutting a thin wall with a coarse blade lets teeth straddle the section, catch, and strip. Cutting thick bar with a fine blade packs the gullets, and the chips cannot clear.

For aluminium, a coarse pitch with a positive rake and a large gullet works. Stainless needs a finer pitch and lower surface speed, because it work-hardens under the tooth. Titanium sits in between but punishes heat. Carbide-tipped blades cost more and hold size longer in hard or abrasive stock.

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  AluminiumCoarse pitch, positive rake, fast surface speed
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  StainlessFine pitch, low speed, steady feed to stay under the work-hardened layer
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  TitaniumModerate pitch, flood coolant, no dwell in the cut

Feed and speed are a window, not a single number. Start at the low end of surface speed and the recommended chip load, then listen. A healthy cut sounds steady and produces curled chips. A squeal means the blade is rubbing. A knock means a tooth is catching.

Coolant matters more than most shops admit. Flood coolant carries heat away from the tooth tip, flushes chips out of the gullet, and lubricates the band. On stainless and titanium, coolant is not optional. On aluminium, a mist is often enough, but chip evacuation still has to work.

Blade tension and guide clearance are the quiet variables. A blade that wanders cuts a curved face, and the mill downstream has to remove that error before it can hold a tolerance. Set tension per the machine manual and check guide clearance every shift.

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  Start lowBegin at the low surface speed and work up
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  ListenSteady sound and curled chips mean the window is right
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  RecheckGuide clearance drifts with wear; check it each shift

A saw cut is the first operation in a longer chain, and its errors travel downstream. If the blank is out of square, the first face milling pass removes an uneven layer, and residual stress releases unevenly. The part then moves during finishing, and the last dimension goes out of tolerance.

A clean, square, stress-relieved blank gives the 5-axis machine a predictable starting point. Allow 0.5–1.0 mm of stock on faces that will be milled, and more on faces that will be ground. Under 0.3 mm is risky on a sawn face because saw marks run deeper than the average depth.

Sawing also saves spindle time. Removing a 20 mm cut-off on a band saw takes seconds. Removing the same material with an end mill takes minutes and wears tools. On 10,000-part runs that difference shows up in the price of every part.

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  SquarenessOut-of-square blanks push error into the first milling pass
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  Stock allowance0.5–1.0 mm on milled faces; more if ground
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  Spindle timeSaw off the bulk; mill only the finish

Sawing is a straight-line process. It cuts bar, tube, plate and extrusions into shorter pieces. It does not produce a pocket, a thread, a radius or a bore. If your first operation needs any of those, you are already in milling territory.

Thin-wall tube and small-diameter bar are awkward. The section flexes under the blade, so the cut closes behind the tooth and pinches. A support fixture or a filled tube solves it, but the setup cost may exceed the saving over a mill.

Very hard or abrasive stock changes the economics too. Carbide blades cut it, but the cost per cut rises, and the finish may still need milling. For one-off parts in tool steel, it is often faster to mill the cut-off than to set up a saw.

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  Straight cuts onlyNo pockets, threads or radii from a saw
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  Thin sectionsSupport the work or the cut pinches
• 3
  Hard stockBlade cost may beat the saving over milling