Part types / Machine bases, beds and frames
Machining for Machine Bases, Beds and Frames
The part that everything else on the machine gets bolted to.
A base, bed or frame rarely has a tolerance on its own bulk. What it carries is the relationship between surfaces that other parts mount on: a rail strip parallel to a bore, a foot pad flat and coplanar with the three others, a mounting face square to a datum edge. Get that relationship wrong and every part bolted on top inherits the error.
These parts are large relative to how flat or square they have to be, which is exactly the combination that residual stress ruins. A cast or heavily welded frame that is rough machined, released from its clamps and left to move before it is finish machined will hold its dimensions. One that is roughed and finished in the same setup often will not.
What decides the tolerance
The features that carry most of the risk on this part type, before you get to the machine that cuts it.
- Flatness and coplanarityAcross mounting pads or rail strips, usually to a stated flatness over a stated length rather than across the whole face.
- Squareness between facesA vertical face square to a horizontal one, which sets the squareness of the machine or structure built on top.
- Grouting and levelling provisionPockets, jack points or a defined stock allowance left for grouting after installation.
- Bolt patterns and dowel boresPosition tolerance between a hole pattern and a datum face, often the tightest callout on the part.
- Stress relief before finishingA thermal or vibratory cycle between rough and finish machining so the part has already moved before the last cuts.
Where this work usually goes
Typical envelopes and table loads for each machine class are on the machine class reference.
Gantry or bridge mill
The wide flat faces and rail strips, clamped once so the whole reference frame stays fixed for the cut.
Horizontal boring mill
Bores through webs and uprights that a vertical spindle cannot reach without a right angle head.
A transfer between the two
Where flatness on one face and bore position on another both matter against the same datum.
Materials
Grey cast iron where damping matters, ductile iron where the load is more tensile or impact prone, fabricated steel plate for prototypes, low volumes or where lead time rules a casting out. Iron wins on vibration, fabrication wins on schedule and design changes.
Before you send the RFQ
- State the datum scheme, not just the tolerances. A flatness number means little without knowing which face it is measured against.
- Say whether grouting is planned. It changes whether stock should be left on the underside.
- Ask whether stress relief happens between rough and finish passes, and at what stage of the route.
- If the base has been in service and is being repaired, note prior wear or shimming so the shop is not chasing a moving target.
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