Reference
How a Machine Base Actually Gets Made
The construction comparison covers what a finished base is. This page covers how it got there: the process behind each construction, the standard governing each stage, and where the tradeoff actually bites.
The processes side by side
| Process | Tooling | First part | Best volume | Size ceiling | Cost of a design change |
|---|---|---|---|---|---|
| Sand casting | Pattern, wood, resin or metal | Months | Repeat series | Foundry pour weight and flask size | High, pattern rework |
| Weld fabrication | Fixtures only | Weeks, fastest | One-offs and small runs | Practically none, set by furnace and machine tool | Low, cut and re-weld |
| Filled weldment | Fixtures only | Weeks plus cure | One-offs and small runs | Shell size and fill handling | Low on the shell |
| Mineral casting | Mould, steel or GFRP | Mould build plus cure | Series | Mould and mixing capacity | Very high, the mould is the design |
| Natural granite | Saw and lapping plant | Weeks to months | One-offs and small runs | Quarry block size | Not practical after cutting |
| Aluminium plate machining | Fixtures only | Days to weeks | One-offs to series | Plate and machine envelope | Low, re-machine |
Step by step
Grey cast iron / Ductile iron
Sand casting
- Pattern built in wood, resin or metal, sized to allow for shrinkage
- Mould formed around the pattern in green sand or a chemically bonded sand
- Molten iron poured and left to solidify and cool in the mould
- Shakeout, then fettling to remove gates, risers and flash
- Stress relief or ageing before any machining begins
Tradeoff: The pattern is the tooling. It is the slowest and most expensive route to a first part, and the cheapest per part once a pattern exists and volume repeats.
Standards: EN 1561, EN 1563, EN 10204
Welded steel / Filled weldment (shell)
Weld fabrication
- Plate and section cut to size, typically by laser, plasma or waterjet
- Fit-up and tack welding to a jig or fixture
- Welding to a qualified procedure (WPS), by welders qualified to ISO 9606-1
- Stress relief, thermal or vibratory, to release welding residual stress
- Machining of datum faces, way mounting surfaces and bolt patterns
Tradeoff: No tooling beyond fixtures, so it is the fastest route to a first part and the easiest to revise. Per-unit cost barely falls with volume.
Standards: ISO 13920, ISO 5817, ISO 3834, ISO 15614-1
Filled weldment
Filled weldment
- Steel shell built exactly as a plain weldment, above
- Cavity filled with a damping compound: mineral aggregate, polymer or concrete, depending on supplier
- Fill cured, in some formulations for several weeks
- Final machining of the shell’s reference faces
Tradeoff: The fill compound is proprietary to the supplier, not covered by a casting or concrete standard, so ask for its cured properties directly rather than assuming a grade.
Standards: ISO 13920, ISO 5817 (shell only)
Mineral cast
Mineral casting
- Mould built, usually in steel or GFRP, to the finished part geometry
- Steel inserts and levelling elements positioned in the mould at exact coordinates
- Graded mineral aggregate mixed with epoxy resin and cast cold into the mould
- Cured, typically over several days, then demoulded
- Only the insert faces receive machining; the cast surface itself is not cut
Tradeoff: There is no widely adopted grading standard for mineral cast, unlike iron or steel: the mix is the supplier’s own formulation. The mould is the design, so a late change is the most expensive of any construction here.
Welded steel / Grey cast iron / Ductile iron
Stress relief and ageing
- Thermal route: part heated slowly in a furnace, held at temperature, cooled slowly so residual stress relaxes
- Vibratory route: the structure is shaken near resonance so locked-in stress relieves, usable on parts too large for a furnace
- Natural ageing: castings left outdoors for months, the older route, still used on high-end bed castings
- On work that matters the sequence is weld, relieve, rough machine, relieve again, finish machine, because rough cutting releases stress the first cycle left in
Tradeoff: Thermal relief is the most reliable and the most limited by furnace size. Vibratory relief is faster and cheaper but harder to prove, so ask what record the shop keeps.
Natural granite
Natural granite production
- Block quarried and selected for even colour, grain and the absence of veins and cracks
- Block sawn to slab and rough ground to size
- Threaded inserts and reference holes drilled and bonded in with epoxy
- Working faces lapped to the specified grade, ISO 8512-2 for surface plates
- Flatness verified on a reference or with an autocollimator before release
Tradeoff: Almost all of the cost sits in lapping and handling. The stone cannot be adjusted after cutting, so inserts and holes must be right in the drawing.
Standards: ISO 8512-2
Welded steel / Grey cast iron / Ductile iron
Surface preparation and coating
- Blast clean to a stated grade: Sa 2.5 is the common requirement under ISO 8501-1
- Prime promptly, within hours, before rust forms on the bare blasted steel
- Apply the coat system by film thickness and environment, ISO 12944
- Mask or protect machined faces, and rustproof them separately for transit
Tradeoff: "Shot blasted and painted" means whatever the shop has time for. Naming the grade and the dry film thickness removes the argument.
Standards: ISO 8501-1, ISO 12944
Read more
Grey cast iron / Welded steel
Hand scraping
- Surface plate or master straightedge coated with a thin marking compound
- Part moved against it, the high spots mark, and the scraper removes them
- Cycle repeated until the bearing contact is even over the working surface
- Contact verified by points per inch, to the machine builder’s criterion
Tradeoff: Slow and skilled, so expensive. It gives a flat surface that holds oil and bears evenly, and is still specified for sliding ways on precision machines. Ground or milled and bonded linear guides need none of it.
All constructions
Finish machining and inspection
- Rough stock set up on its intended datums, per the drawing’s GD&T scheme
- Way mounting surfaces, bolt patterns and reference faces milled or ground
- In-process flatness and parallelism checked against the callouts
- Final inspection recorded on an inspection and test plan before release
Tradeoff: This stage is where a missing or ambiguous datum scheme costs the most: a fabricator without one will pick a reasonable-looking surface and may pick differently than the designer intended.
Standards: ISO 1101, ASME Y14.5, ISO 2768
Why the process matters as much as the material
Two grey iron bases from two foundries are not the same part if one skipped the ageing step before machining. The material standard tells you what the alloy is; it says nothing about whether the process that turned it into your part was followed. That is what the standards index and an inspection and test plan are for.
It is also why a quote that only names a material grade is an incomplete quote. Ask which of the steps above the shop actually performs in-house, and which it subcontracts.
Related
- Material properties: grades, mechanical properties, pros and cons
- Construction comparison: the properties each process produces
- Standards index: every standard named above, explained
- Glossary: terms used on this page, defined
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