CNC Sees the Masters: Sawing Technology Explained

• 1
  How CNC Sees the Masters: Reading a Cut by Force
• 2
  What the Saw Owes the Mill
• 3
  Why Aluminium, Stainless and Titanium Cut Differently
• 4
  Nesting, Yield and the Cost of a Wrong Blank
• 5
  When the Saw Is the Wrong Tool
• 6
  Saw Cut Versus Milled Face: What to Expect

A saw does not cut by shape. It cuts by force, and the force tells it what is happening. A band running through 4140 at the wrong feed pushes the blade sideways, and the blade answers with heat, noise, and a bowed cut face. Modern controls watch spindle load, servo current or hydraulic pressure and adjust feed rate while the cut is running. On a 250 mm round bar of 4140, a light-pass strategy may start near 70 m/min and settle lower once the load curve flattens.

Blade speed and feed are one decision, not two. Surface speed in m/min sets the cutting temperature; feed in mm/min sets the chip thickness. Chips that are too thin rub instead of cut, which hardens stainless and burns the tips. Chips that are too thick overload the teeth and stall the band. The control finds the middle by watching load, and that is how CNC sees the masters of the cut: it reacts to metal, not to a fixed recipe.

Workholding decides whether any of this matters. A bar clamped on one side of the vise can lift on the other side as the blade enters. On a 4,000 mm bundle, one loose clamp lets the whole stack shift and the cut goes out of square by 0.5 mm or more. Shims, a stop block, and a second clamp cost seconds. They save the blank.

Cut length is the other half of the job. A stop gauge repeats to roughly ±0.05 mm on short lengths and drifts further out as the bar heats and the stop wears. On a 3 m cut, a 0.2 mm growth from heat is normal and needs a warm-up piece before the run starts.

• 1
  Watch load, not the clockFeed rate follows cutting force, not a fixed table.
• 2
  Clamp both sidesLift on the free end is the most common cause of out-of-square cuts.
• 3
  Warm up the stopRun one scrap piece before the first good blank on long cuts.

A saw cut is not a finished face, and nobody should treat it as one. A band-sawn face on 6061 typically lands between Ra 3.2 and Ra 6.3 μm, with a slight bow along the length. That face is a datum for the first milling op, not a sealing surface. If the drawing calls for Ra 1.6 μm on that face, the mill removes it.

The useful number is stock allowance. A blank cut 0.3 mm oversize per side gives the mill enough material to clean up a bowed face and still hit size. A blank cut to 0.05 mm oversize often forces a re-cut, because the first pass cannot clear the bow. On thin plate, the bow is worse: a 6 mm plate can curl 0.5 mm across a 500 mm cut when the blade exits.

Squareness matters more than finish for most parts. A blank that is square within 0.1 mm per 100 mm locates cleanly in the vise. A blank that is out by 0.5 mm per 100 mm needs a shimmed setup, and the operator spends the time the saw saved.

Heat is the quiet variable. Cutting 304 stainless at high surface speed work-hardens the cut face, and the first milling pass then fights a hard skin. Slower surface speed with a heavier chip keeps the face soft and machinable.

• 1
  Allow 0.3 mm per sideEnough stock to clean up bow and still hold size.
• 2
  Square beats smooth±0.1 mm per 100 mm locates without shims.
• 3
  Do not burn stainlessHigh surface speed hardens the face the mill must cut next.

Aluminium 6061 and 7075 cut fast and cool. The band can run at high surface speed, and chip clearance is the main limit. Soft 6061 tends to gall on the blade if feed is too light, so a heavier chip is safer than a gentle one. Extruded 6063 cuts even faster but grabs the blade when the clamp pressure is high.

Stainless 304 and 316 work-harden at the cut. Light feed rubs the surface, raises hardness, and the next tooth cuts a harder skin. The fix is a heavier chip and a slower surface speed, plus steady coolant. 17-4PH in the H900 condition is harder again and calls for carbide teeth and a rigid setup.

Titanium Ti-6Al-4V behaves like stainless with a lower heat limit. Heat stays at the cut instead of leaving with the chip, so the blade tip runs hot. Low surface speed, heavy feed, and flood coolant keep the tip alive. Cutting titanium dry is the fastest way to destroy a band.

Inconel 625 and 718 push all of this further. They are the materials where a saw's control loop earns its cost, because a fixed feed recipe either stalls or burns. Magnesium AZ31B cuts easily but the chips burn, so chip handling matters more than cutting parameters.

• 1
  Aluminium likes a heavy chipLight feed galls the blade on soft 6061.
• 2
  Stainless needs a slow, heavy cutLight feed work-hardens the face.
• 3
  Titanium needs flood coolantHeat stays in the tip without it.

Bar stock is bought by weight and paid for by the part. A nesting plan that leaves 12 mm between parts instead of 25 mm returns real money over a 10,000 part run. Software does this well, but the saw has to hold the cut length it was promised, or the extra parts do not fit.

Cut length error compounds. If the saw runs 0.15 mm long on every piece, a 500 piece order wastes 75 mm of bar, which is one extra part lost. That sounds small until the material is Inconel 718 at 4,000 mm per bar. Yield is a tolerance problem before it is a software problem.

Kerf is the other silent cost. A 1.5 mm blade removes 1.5 mm of material on every cut. On a 4,000 mm bar cut into 50 mm blanks, kerf eats about 3 percent of the bar. A thinner blade saves material but deflects more, so the choice depends on the material and the length.

The saw also decides scrap timing. Cutting a blank that turns out 0.4 mm short wastes the cut, the handling, and the material. Cutting it 0.4 mm long wastes one milling pass. When in doubt, leave stock.

• 1
  Kerf is real materialA 1.5 mm blade removes about 3 percent on short blanks.
• 2
  Length error adds up0.15 mm long across 500 pieces loses one blank per bar.
• 3
  Long blanks are cheaper to fixExtra stock costs one pass; a short blank costs the part.

A saw cuts straight lines. If the part needs a curved profile, a pocket, or a bore, the saw only makes the blank. Trying to save a milling setup by sawing close to a contour usually costs more, because the sawn face is not a locating surface and the mill still has to clean it.

Very thin material is a poor fit. Below about 3 mm, plate tends to curl as the blade exits and the cut length wanders. Laser or waterjet holds thin plate flatter, and the saw is better used on the bar that feeds them.

Hardened material above roughly 45 HRC is another limit. Carbide teeth will cut it, but the blade life drops fast and the cost per cut rises. Wire EDM or grinding is often cheaper on small hardened features.

Very short blanks have their own problem. Below about 10 mm, the clamp cannot hold the piece on both sides of the cut, and the last piece often flies. A mill with a bar feeder handles short pieces better, because the part stays supported through the cut.

• 1
  Straight lines onlyContours and pockets belong on the mill.
• 2
  Thin plate curlsBelow 3 mm, laser or waterjet holds flatness better.
• 3
  Above 45 HRC, look elsewhereBlade life drops and cost per cut climbs.