Machine Base Materials: Grades, Properties, Pros and Cons

Very common

Grey cast iron

EN-GJL-200 to 300, GG20 to GG30, ASTM A48 Class 30 to 45, HT200 to HT300

Pros

• Flake graphite damps vibration well, several times better than steel
• Casts into ribbed, closed, complex shapes that would need dozens of welds
• Machines cleanly and wears well as a sliding surface
• Cheapest per part once a pattern exists and volume repeats
• Compressive strength of three to four times its tensile strength suits a base in compression

Cons

• Brittle: no yield, fractures without warning under tension or impact
• A pattern costs time and money, so the first part is slow and a design change is expensive
• Properties vary with section thickness, thick walls are weaker than the test bar
• Casting defects (porosity, shrinkage, chill) can sit hidden until machining
• Hard to weld or repair

Guide 1 · Guide 2 · Guide 3

Occasional

Ductile (spheroidal graphite) iron

EN-GJS-400-15, EN-GJS-500-7, ASTM A536 65-45-12

Pros

• Yields and stretches before it breaks, far more tolerant of impact and tension
• Higher strength than grey iron at the same section
• Higher modulus than grey iron, about 169 GPa
• Same casting freedom as grey iron

Cons

• Damps noticeably less than grey iron, which is why it is rarely the first choice for a bed
• Harder to cast sound, more demanding on the foundry (magnesium treatment, shrinkage control)
• Pattern cost and lead time are the same as grey iron
• Costs more than grey iron per kilogram

Guide 1

Specialist

Meehanite and other controlled irons

Meehanite GA, GC, GE and similar licensed grades

Pros

• Foundry process and metallurgy controlled and recorded to a licensed method
• More uniform properties through varying section thickness
• Good wear and machinability for ways and guideways

Cons

• Available only from licensed foundries, so supply is narrow
• Costs more than an ordinary grade, and you pay for the control, not a different alloy
• Properties do not beat a well-run grey iron enough to justify it on most machines

Guide 1

Very common

Welded structural steel

S235, S275, S355 (EN 10025); A36, A572 Gr 50 (ASTM); Q235, Q355 (GB); SS400 (JIS)

Pros

• No pattern, so the first part is fastest and a design change is a cut and a weld
• Highest modulus of the common options, about 210 GPa, so the thinnest walls for a given stiffness
• Tough: yields and warns before failure, can be repaired and modified after delivery
• Practically no size limit, and plate and section are available everywhere
• Cost per part stays flat, which suits one-offs and small runs

Cons

• Lowest damping of the set, a bare steel box rings and can chatter
• Welding leaves residual stress and distortion, so stress relief and machining allowance are required
• Quality depends on the weld procedure and the shop, not only the plate grade
• Per-unit cost does not fall much with volume
• Strength grade rarely matters: bases are limited by stiffness, so S355 over S275 buys little

Guide 1 · Guide 2 · Guide 3 · Guide 4

Uncommon

Concrete-filled or mineral-filled weldment

Steel shell to the grades above with a concrete, polymer or mineral core

Pros

• Adds mass and damping to a weldment without a pattern or mould
• Keeps the steel shell as the load path, so strength and toughness stay
• Design changes on the shell stay cheap

Cons

• The fill is the supplier’s own formulation, with no grade standard to point at
• Core cannot be repaired, and a poor bond leaves voids that damp nothing
• Adds weight and a cure period
• Reduced thermal conduction can make heat distribute unevenly

Guide 1

Common in precision, rare elsewhere

Natural granite

Fine-grained black gabbro or diabase, sold as black granite; ISO 8512-2 for surface plate grades

Pros

• Low thermal expansion and low conduction, holds its shape as the shop warms and cools
• No rust, no residual stress, non-magnetic
• Very good damping
• Lapped to flatness that a metal base can only reach with extensive scraping

Cons

• Brittle and heavy, with little tolerance for impact or point loads
• Properties depend on the quarry, so ask for the stone name and test data
• Thread inserts must be bonded in, and changes after the stone is cut are impractical
• Cutting, lapping and shipping are slow and cost a lot for large pieces

Guide 1

Established niche

Epoxy granite (polymer concrete, mineral cast)

Graded mineral aggregate in epoxy resin, supplier formulation

Pros

• The highest damping of the options listed here
• Cast cold into a mould, so there is no stress relief and little distortion
• Inserts, ducts and cooling channels cast in place, cutting machining and assembly
• No corrosion, no paint
• Density of roughly a third of iron, so a lighter part at the cost of bulk

Cons

• Lower stiffness per unit volume, so walls are thick and the part bulky
• Weak in tension, loads go through cast-in inserts and need careful design
• The mould is the design: a late change is the most expensive of all constructions
• Cannot be repaired, and has no grading standard to specify against
• Pays off only above a certain volume, or where damping is the whole point

Guide 1 · Guide 2

Specialist, small machines

Aluminium plate or casting

EN AW-6061-T6, 5083, cast A356

Pros

• About a third the density of steel, very light for handling
• Machines fast and cleanly, with no welding or casting needed for plate bases
• Does not rust

Cons

• Modulus about a third of steel, so a base of the same stiffness needs thicker sections
• Expands about twice as much as steel with temperature
• Low damping, and a cutting-load structure often rings
• Price per kilogram is several times that of steel
• Heavy machines are not a fit: size and load are limited