Part types / Press frames, bolsters and die sets
Machining for Press Frames, Bolsters and Die Sets
Flat is only useful if it stays flat once the press is loaded.
A bolster plate or press frame is one of the few large parts where the number on the inspection report is not really the number that matters. What matters is how flat the part stays once it is installed, shimmed and put under load, which is a different question from how flat it measured on the machine table right after cutting.
That gap is why stock allowance and sequence get discussed as much as the final tolerance itself. A plate roughed, stress relieved and finish machined in a controlled sequence tends to hold its shape after installation. One machined to a final number in a single pass is more likely to move once it is bolted down and loaded.
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 under working loadThe real requirement, which the shop can only approximate on the table and the installer confirms after shimming.
- T-slot and clamping featuresPosition and depth relative to the working face, cut after the face itself is finished.
- Guide post and bushing boresParallelism between bores that carry the ram, often the tightest tolerance on the assembly.
- Die set register featuresLocating diameters or keys that position a die set repeatably against the bolster.
- Stock left for scrapingA deliberate allowance on the working face for hand fitting after machining, common on precision press tooling.
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 large flat working face, machined in one clamping so the whole face shares one reference frame.
Horizontal boring mill
Guide post and bushing bores that have to stay parallel over the depth of the frame.
Materials
Grey or ductile cast iron for damping under repeated impact, fabricated steel plate where lead time or a one-off design favours welding over a casting pattern.
Before you send the RFQ
- Describe the installation, including shimming and the number of support points, since that is what the flatness has to survive.
- State whether scraping is planned after machining, and if so leave the stock allowance explicit rather than implied.
- Give guide post bore parallelism as its own callout, separate from the general flatness of the working face.
- For a repair or a second bolster to match an existing die set, provide the existing part or its record rather than a clean-sheet drawing.
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