IT grades

Lower IT number, tighter tolerance. For 25 mm: IT6 is about ±0.008 mm, IT7 ±0.013 mm, IT9 ±0.026 mm.

Fits

• H7/h6 sliding fit for locating shafts.
• H7/k6 light press for bushes.
• H7/p6 interference for bearings and hubs.

Surface finish

Ra 3.2 μm is normal machining. Ra 1.6 for bearing seats and seal faces. Ra 0.8 for sliding surfaces. Below that, expect grinding.

Where to spend it

Every tight tolerance adds slower feeds and more inspection. Tight on bores, seats and datums; general tolerance everywhere else.

High-precision manufacturing means making parts to tight tolerances, typically ±0.01 mm or better on critical features, and doing it repeatably across a batch, with measurement data to prove it.

What makes it possible

• Capable machines: rigid CNC lathes and VMCs, kept in calibration.
• Process control: fixed setups, controlled tooling and in-process checks.
• Metrology: gauges and CMM inspection that measure what the drawing asks for.
• Environment: temperature swings move steel by microns, so measurement conditions matter.

When you need it

Bearing seats, sealing faces, gear and pump fits, hydraulic bores and anything that locates another part. Everywhere else, general tolerances keep cost down.

How to buy it

Ask a supplier for their measurement method, not just their machine list. Precision you cannot measure is a guess.

Buyers often compare machine hour rates. The real price drivers sit elsewhere.

• Setups: each clamping costs setup time. A part needing four setups costs far more than one needing two.
• Tolerance: moving from ±0.05 to ±0.01 mm slows feeds and adds inspection.
• Material: stainless and alloy steels wear tools faster than cast iron or aluminium.
• Batch size: setup and programming are spread across the batch. This is why we quote from 20 pieces.
• Inspection: full CMM reports on every part cost more than first-article plus sampling.
• Raw material form: a near-shape casting needs far less machining than solid bar.

Want a lower price? Relax non-critical tolerances, reduce setups and send annual volumes so we can plan fixtures.

Grey and SG iron start from similar chemistry. The difference is the shape of the graphite. In grey iron it forms flakes; in SG iron, magnesium treatment turns it into spheres. That one change decides how the part behaves.

Choose grey iron (FG200, FG260) when

• The part sees vibration you want absorbed: motor frames, machine bases, pump bodies.
• You machine a lot of it. Flake graphite breaks chips and is kind to tools.
• Loads are mostly compressive and steady.

Choose SG iron (SG400/15 to SG500/7) when

• The part takes shock, bending or internal pressure: valve bodies, pressure housings, brackets.
• You need elongation. SG400/15 stretches 15% before failure; grey iron barely stretches at all.
• You want steel-like strength without fabricating.

The cost side

SG iron costs more per kg because of the magnesium treatment and tighter process control. It also lets you thin sections for the same strength, so the part can end up lighter and similar in price. Send both options for quote if you are unsure.

Every sand casting starts with a pattern, the shape pressed into sand to form the cavity. The pattern is usually the biggest one-time cost, so the route matters.

3D-printed patterns

Printed from your CAD. No patternmaker queue, and a design change costs a reprint. Best for samples and pilot builds in the 1–25 piece range, or when you need poured parts this month.

Wooden patterns

Lowest tooling cost and easy to modify. Wood wears and moves with humidity, so dimensions drift over long runs. Good for short runs and large, simple parts.

Aluminium or metal patterns

Highest upfront cost and longest to make, but they barely wear. Use them once the design is frozen and volumes run into the hundreds or thousands.

The common path

Many of our customers go 3D-printed for samples, then straight to metal once the first article passes. It skips paying for a wooden pattern you will throw away.

Green sand casting is still the most economical way to make iron parts from 1 to 100 kg. The process is old. The control around it is what separates a good foundry from a cheap one.

1. Pattern

A replica of the part, slightly oversize to allow for shrinkage, with draft and core prints. Check: pattern dimensions against the 3D model before the first mould.

2. Core making

Sand cores form internal passages such as pump volute channels. Check: core strength and that it sits positively in its prints.

3. Moulding

Sand is rammed around the pattern in a two-part flask (cope and drag). Check: sand moisture and compactability, the usual root cause of gas porosity.

4. Melting

Iron is melted, typically in an induction furnace, and adjusted to grade. Check: spectro analysis of the bath before pouring.

5. Pouring

Metal goes in through a gating system designed to fill smoothly and feed heavy sections. Check: pour temperature and pour time.

6. Knock-out and fettling

Castings are shaken out, risers removed, then ground and shot-blasted. Check: visual inspection for cracks, cold shuts and sand inclusions.

7. Inspection and machining

Dimensional checks, hardness and, where required, machining of functional faces. Check: first-article inspection on machined features.

If your supplier cannot tell you what they check at each step, assume they do not.

Sand casting

• Lowest tooling cost; patterns can be wood, metal or 3D-printed.
• Handles larger, heavier parts easily.
• Rougher surface and wider tolerances, so functional faces are machined.
• The default for grey and SG iron housings, casings and bodies.

Investment (lost-wax) casting

• Fine detail, thin walls and smooth surfaces, often near net shape.
• Needs a wax injection die, so tooling is costlier and slower.
• Best for small, complex parts, usually in steel or stainless.

How to decide

If the part is iron, over a few kilos, and will be machined on its critical faces anyway, sand casting almost always costs less overall. Choose investment casting when the part is small, intricate and you want to avoid machining most surfaces.

Lost-foam casting sits between the two: no cores, good detail, moderate tooling. We plan to add it in 2027.

Turning

If the part is defined by rotation about one axis (shafts, bushes, flanges), turn it. Faster and cheaper than milling the same shape.

Milling

Flat faces, pockets, slots, hole patterns and off-axis features belong on the VMC.

Both

A shaft with a keyway, a housing with a precision bore. Each re-clamping can add 0.02–0.05 mm of positional error, so design for the fewest setups and put related features in one setup.

Form

Flatness limits how much a surface can deviate from a perfect plane. Cylindricity does the same for a cylinder.

Orientation

Perpendicularity and parallelism control a feature relative to a datum, for example a bore square to a mounting face.

Location

True position controls where a hole sits relative to datums. It gives a round tolerance zone, which is larger and more useful than the square zone of ± dimensions.

Runout

Circular and total runout control wobble as a part rotates about its axis. Critical on shafts and pulleys.

How it is checked

Most GD&T with datum references needs a CMM or a dedicated fixture gauge. Ask your supplier which they will use before you approve the drawing.

• 3D model as STEP, plus a 2D PDF drawing.
• Material and grade, with any heat treatment.
• Tolerances on critical features only, plus a general tolerance note.
• Surface finish (Ra) where it matters.
• Quantity per batch and expected annual volume.
• Coating or plating requirements.
• Inspection needed: first-article, dimensional report, CMM.
• Target date and delivery location.

Use our quote form: it asks for exactly these, and an engineer reviews your drawing for manufacturability before pricing it.

1. Shrinkage cavities

Iron shrinks as it solidifies. Heavy sections freeze last and pull metal from around them, leaving voids. Fix: even wall thickness, cored-out bosses, and room for risers on heavy sections.

2. Gas porosity

Small round holes from trapped gas or moisture. Mostly a process issue (sand moisture, venting), but deep blind pockets in the design make venting harder.

3. Cold shuts

Two streams of metal meet but do not fuse. Usually long thin walls far from the gate. Fix: respect minimum wall thickness and avoid long unsupported thin webs.

4. Misruns

The cavity does not fill. Same root cause as cold shuts, often worse. If you need very thin walls, tell us early; it changes gating and pouring temperature.

5. Core shift

Internal passages end up off-centre because the core moved. Fix: proper core prints at both ends and features that let the core sit positively.

Bottom line: send your model before the design freezes. A 20-minute review is cheaper than a pattern rework.

Machining allowance is the extra metal left on faces that will be machined. It has to cover casting tolerance, surface scale and any distortion, while leaving the finished face in sound metal.

Typical green-sand values

• Small parts under ~10 kg: about 2–3 mm per face.
• Medium parts 10–80 kg: about 3–4 mm.
• Large parts and the cope (top) side: more, because inclusions float upward.

Why one supplier helps

When foundry and machine shop are separate, each pads the allowance to protect itself. When the same team does both, allowance is set from real data on that part, which cuts weight, cycle time and cost.

ISO 8062-3 groups casting accuracy into dimensional casting tolerance grades (DCTG). Lower numbers are tighter. Hand-moulded green sand iron typically lands in the coarser grades, often around DCTG 11 to 13 depending on size and method. Machine moulding does better.

What that means in practice

On a 200 mm dimension, a coarse sand-casting grade allows several millimetres of variation. That is normal and fine for as-cast surfaces. It is not fine for a bearing bore.

The right way to specify

• Call out a general casting tolerance grade on the drawing, not ±0.1 mm everywhere.
• Mark functional faces as machined and give their tolerance separately.
• Show datums the machinist will use, ideally on features cast in one half of the mould.
• Specify machining allowance or let the supplier propose it.

This is exactly why buying cast and machined from one supplier helps: the person setting allowance also owns the machined result.

Gujarat is one of India's densest foundry regions. Price rarely separates suppliers; control does. Ask these before you send a PO.

• Which grades do you pour every week? A foundry that mostly pours FG200 may struggle with SG500/7.
• How is chemistry checked? Spectro per heat is the standard you want.
• What is your weight sweet spot? A shop built for 5 kg parts will not love your 80 kg housing.
• Who makes and stores patterns? Ask about pattern ownership and storage.
• What reports come with the batch? Chemistry, hardness and dimensional, at minimum.
• Can they machine, or do they hand off? Every hand-off is a place for blame to hide.
• How do they handle rejections? Get the policy in writing.
• Can you visit? A foundry that will not show you the floor is telling you something.

Fulcrum answers all eight on the first call. Ask us.

• Setups: design so the part machines in one or two clampings.
• Internal corners: radius at least about a third of pocket depth so a sensible cutter can reach.
• Standard sizes: slots and holes at standard tool sizes avoid special tooling.
• Deep holes: keep drilled depth under about 3 × diameter.
• Thin walls: chatter, distortion, scrap. Add a rib or thicken.

Send the model before freezing it. Our DFM review comes with every quote.

The Sanand, Changodar, Vatva and Odhav clusters give Ahmedabad buyers a huge pool of CNC and VMC job work capacity. The challenge is consistency, not availability.

What to look for

• Inspection beyond a vernier: bore gauges, height gauges, CMM access.
• Written process sheets for repeat parts.
• First-article reports before batch release.
• Clear ownership when a part is rejected.

How Fulcrum works

Today we produce through qualified partner machine shops, with Fulcrum owning process planning, inspection and metrology on every batch. Our own plant at Sanand GIDC-II is nearing completion and will add in-house CNC turning, VMC and CMM.

Machine from solid

Best for 1–20 pieces of simple geometry. No tooling, fastest start, but most of the bar ends up as chips.

Fabricate (welded plate)

Good for large, boxy, low-volume structures. Welding distortion and stress relief add cost, and it does not suit curved flow passages.

Cast

Wins once quantity passes a few dozen, or when the part has curved internal passages, varying wall thickness or ribs. Pump volutes and motor frames are classic cast parts for exactly this reason.

Rule of thumb: if you can draw it more easily than you can weld it, and you need more than about 30, get a casting quote.

Grade

FG260 is the workhorse for water pump casings and volutes. Move to SG iron when pressure, shock or thinner walls demand it.

Volute surface

Rough internal passages cost efficiency. Specify core sand and coating expectations, and whether the volute can be dressed.

Pressure tightness

If casings are hydro-tested, say so, and state the test pressure. It changes how the foundry feeds heavy sections to avoid shrinkage porosity.

Machining datums

Suction and delivery flanges, the bearing housing bore and the mounting feet must relate to each other. Agree datum features with the supplier before the pattern is made.

Reports

Ask for chemistry and hardness per heat, plus dimensional reports on machined features. For new patterns, insist on first-article approval before the batch.

A coordinate measuring machine probes many points to evaluate form and position: flatness, true position, perpendicularity, cylindricity. A micrometer cannot.

Specify CMM for

• GD&T callouts with a datum reference frame.
• Bolt circles and bore positions that drive assembly.
• First-article inspection on a new part.

Skip it for

Simple sizes and lengths. Gauges and micrometers are faster and cheaper. Use first-article CMM plus sampling for production batches.

Grey iron

Flake graphite breaks chips and lubricates the cut. Usually machined dry. Watch for the hard outer skin on the first pass; take the first cut below it.

SG iron

Tougher, with longer chips and more tool wear. Feeds and speeds sit between grey iron and mild steel.

Why it matters for you

When our foundry knows the casting will be machined, it controls skin hardness and sets allowance so the first cut runs clean. That is the real advantage of buying cast and machined from one place.