A Guide to Buying Machine Tool Castings
This page is for engineers and sourcing staff who are buying machine tool castings — beds, bases, columns, spindle housings, saddles. It covers what to specify on the drawing, what to ask before the PO, and how to tell a good quote from a thin one.
In this article
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Key takeaways
Buying machine tool castings: route vs. part
Match the route to the part before you compare prices.
| Casting route | Typical part | Wall thickness | Best when |
|---|---|---|---|
| Grey iron, resin sand | Bed, base, column | 8–25 mm | High damping, low cost per kg |
| Grey iron, green sand | Large base, cover | 12–40 mm | Big volumes, simple geometry |
| Ductile iron (QT450) | Saddle, ram | 10–30 mm | Shock loads, bending stiffness |
| Steel casting | Spindle housing | 15–40 mm | High tensile, welded repair |
| Billet / plate stock | Small brackets | n/a | One-off, no pattern budget |
| Aluminum casting | Light gantry, cover | 5–15 mm | Moving mass matters |
| Epoxy granite | Bench, gantry base | n/a | Vibration damping is critical |
| Welded steel frame | Prototype bed | n/a | Speed over damping |
The short version
Specify grade, hardness, stress relief, allowance and flatness as separate line items. A quote that answers all five is worth more than one that is 10% cheaper and silent on three of them.
Iron grade and hardness: what the drawing should say
Most machine tool beds and bases are grey iron. GG25 (HT250) is the default for beds, columns and bases because it damps vibration well and machines cleanly. Tensile strength lands around 250 MPa. When the part sees shock or repeated bending — a ram, a saddle under interrupted cuts — ductile iron QT450-10 or QT500-7 is the safer pick.
Hardness should be specified on the as-cast surface, not after machining. A common window is 170–240 HB for grey iron beds. Below 170 HB the surface smears during milling and the finish suffers. Above 240 HB tool wear climbs and the insert cost lands in the quote.
Two suppliers quoting the same grade can ship different graphite structure. Ask for the microstructure photo with the first article. Flake graphite size 4–6 with a pearlite matrix is normal for a bed. If the supplier cannot show a photo from the actual pour, treat the grade claim as unverified.
Section thickness drives cooling speed, and cooling speed drives hardness. A 40 mm boss next to a 10 mm rib will not cool at the same rate. The foundry should either equalize sections on the pattern or tell you which zones will sit at the hard end of the range.
Stress relief: the step that decides long-term accuracy
Iron castings arrive with locked-in stress from cooling. If you machine the bed before that stress is released, the part moves over weeks. A bed that was flat within 0.02 mm on the bench can drift past 0.05 mm after roughing and a few days of sitting.
The usual route is natural aging plus a thermal cycle. Natural aging means leaving the raw casting outdoors or in an unheated bay for 3–6 months. Thermal stress relief holds the casting at 550–600 °C for 4–8 hours, then cools it slowly inside the furnace. Many shops do both: thermal cycle first, then a short natural aging period.
Ask two questions. What is the holding temperature and time? And how is the casting cooled — in the furnace or pulled out hot? Furnace cooling to below 200 °C is the answer you want. Pulling the casting out at 400 °C releases stress unevenly and undoes most of the benefit.
If the supplier skips stress relief to hit a short lead time, the accuracy problem shows up at your assembly station, not theirs. Put the stress-relief requirement on the PO with the temperature and holding time, and ask for the furnace chart with the inspection report.
Machining allowance and flatness: two numbers, not one
Machining allowance is the stock left on faces that will be cut. For a bed 1,000–2,000 mm long, 3–5 mm per side is normal for the guideway faces. Cosmetic faces can run 2–3 mm. Below 2 mm on a large casting, the risk is that a surface defect or a hard spot pulls the cut below the finished dimension.
Flatness of the as-cast guideway face is the second number. A common spec is 1.0–2.0 mm total over the full length before machining. Without that window, a twisted casting may not clean up at the specified allowance, and the shop either scraps the part or asks for a drawing change.
Weight tolerance matters for moving parts. A saddle or ram that comes in 6% heavy changes the axis dynamics. Put a weight window on the drawing, typically ±5% for machined parts, and have the first article weighed on a calibrated scale.
Draft angle and fillet radii belong on the drawing too. Missing draft makes pattern withdrawal hard and raises the tooling cost. Sharp internal corners create shrinkage porosity exactly where you plan to bolt the linear rail.
What to verify before you place the order
Ask for the pattern ownership terms in writing. If the supplier keeps the pattern and you move the part later, you may have to pay for it twice. Standard practice is that the buyer pays for the pattern and owns it. Get that in the contract, not in an email thread.
Check the in-house machining capability. A foundry that only casts will send your bed out for machining, and the flatness responsibility splits between two companies. A shop that casts, stress-relieves and machines under one roof can hold one tolerance stack and answer for the whole part.
Certification should match the industry you sell into. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies if the machine ships into automotive production. ISO 13485:2016 applies to medical device manufacturing equipment. Ask for the certificate scope, not just the logo.
Finally, agree on the inspection report format before the first shipment. Dimensional report, hardness readings, flatness map, furnace chart. If the report format is agreed up front, the first article review takes an afternoon instead of a week of back-and-forth.
Seven steps from RFQ to accepted casting
Use this order. Skipping step 4 is the most common and most expensive mistake.
- 11. Send the 3D model and 2D drawingInclude grade, hardness range, machining allowance, flatness window and weight tolerance. STEP plus PDF is enough for a first review.
- 22. Request the casting route in writingAsk which process (resin sand, green sand, investment), which stress-relief cycle, and who machines the guideways.
- 33. Confirm pattern cost and ownershipPattern cost is usually one-time. Get ownership terms in the contract so a future supplier change does not restart the cost.
- 44. Approve the first article before batchCheck dimensions, hardness (170–240 HB for grey iron), flatness map and weight. Do not release the batch on a drawing review alone.
- 55. Set the roughing and finishing sequenceRough within 2–3 mm, let the part rest, then finish. Cutting 5 mm in one pass on a 1,500 mm bed invites distortion.
- 66. Agree the inspection report formatDimensional report, furnace chart, hardness log. Confirm the format before shipment one, not after.
- 77. Log the as-received conditionPhotograph the casting and record flatness on arrival. This baseline settles any dispute about handling damage in transit.
Buying machine tool castings: common questions
What iron grade should I specify for a lathe bed?
For a bed up to about 2,000 mm, GG25 / HT250 grey iron at 170–240 HB covers most builds. It damps vibration and machines predictably.
If the bed carries heavy interruption or the machine sees shock loading, step up to ductile iron QT450-10 and expect a higher cost per kg.
How much machining allowance is enough?
For guideway faces on a bed 1,000–2,000 mm long, plan 3–5 mm per side. Cosmetic faces can run 2–3 mm.
Below 2 mm on a large casting, a hard spot or surface defect can pull the cut under the finished dimension, and the part is scrapped or reworked.
Can I skip stress relief to save lead time?
You can, and the part will still measure correctly on the day of shipment. The movement comes later, after roughing and a few weeks of sitting.
A bed that was flat within 0.02 mm can drift past 0.05 mm. Put the thermal cycle on the PO with temperature and holding time.
What certifications should a casting supplier hold?
ISO 9001:2015 is the baseline for general machine tool work. IATF 16949:2016 applies when the machine ships into automotive production lines.
ISO 13485:2016 applies to equipment used in medical device manufacturing. Ask for the certificate scope — a logo alone does not tell you which processes are covered.
Is there a minimum order quantity for castings?
It depends on the pattern. A new pattern carries a one-time cost, so a single casting is possible but the per-piece cost is high.
Once the pattern exists, small repeat batches are straightforward. Volume pricing starts to make sense when the pattern cost is spread over a real production run.
How do I check a casting without a CMM?
A granite surface plate, a dial indicator and a height gauge cover most bed and base checks. Record flatness on a grid and note the high and low points.
Hardness needs a portable tester, usually a Leeb or Brinell unit. Ask the supplier for their readings and compare them against your own on the same spots.
Send the drawing, get a casting route back
Upload the model and drawing and we will come back with the casting process, stress-relief cycle, machining allowance and inspection plan — quotation and free DFM analysis within 12 hours.
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