Stress Relief and Flatness: Why a Machined Base Moves
Why welded machine bases distort after machining, how thermal and vibratory stress relief differ, and how to write a flatness requirement a shop can hold.
A machine base is not finished when it comes off the boring mill. It is finished when it is still flat six months later, bolted down in a customer’s plant. Everything in this guide is about the difference between those two moments.
Where the movement comes from
Welding is local, violent heating followed by fast cooling. The metal around each weld contracts against material that is not free to move, and what cannot move becomes locked-in residual stress. A finished weldment can be dimensionally perfect and still be full of stored energy.
Machining then releases it. Every cut removes material that was holding part of that balance, and the structure relaxes into a new shape. A base that measured flat on the table walks out of tolerance overnight. Castings have a milder version of the same problem, from uneven cooling in the mould.
So the question is never whether the part will move. It is whether it moves before you machine it or after.
The two methods
Thermal stress relief. The structure goes into a furnace, is brought up to a soak temperature well below any transformation point, held there, and cooled slowly under control. At temperature the steel creeps slightly and the peak residual stresses fall. It is the method specifications reach for, it is thoroughly documented, and it produces a chart you can file.
The costs are real: oven capacity large enough for the part, the crane moves, the queue behind other jobs, and scale on the surface that has to be blasted off afterwards. For a large base, furnace availability is often the single longest item in the lead time.
Vibratory stress relief. The structure is clamped, instrumented and excited at its own resonances for a fixed period. Peak stresses redistribute rather than disappear. It takes hours rather than days, costs a fraction of a furnace cycle, needs no oven, and leaves no scale.
It is used widely and successfully on machine frames. It is also not accepted everywhere, and some quality systems will not recognise it. This is a specification decision, not a technical preference you can leave to the shop.
The sequence that works
On any base where the machined faces matter:
- Weld the structure.
- Stress relieve.
- Rough machine, leaving stock on every functional face.
- Stress relieve again.
- Finish machine.
The second cycle is the one people delete to save money, and it is the one that stops the part moving after delivery. On non-critical work a single post-weld cycle is normal practice, but say which you are buying.
Leaving the part to sit between rough and finish machining also helps, and costs nothing but floor space and patience.
Writing a flatness requirement that means something
“Flat within 0.05” on a drawing is not a specification. It is missing everything that makes the number measurable.
A usable requirement states:
- Which surface. Each machined pad separately, or the whole top face as one.
- Over what length. 0.05 mm over 300 mm and 0.05 mm over 2000 mm are very different jobs.
- Against which datum. Parallelism and squareness are meaningless without one, and the grammar for writing them down is geometric dimensioning and tolerancing, as codified in ISO 1101 and ASME Y14.5. Pick the datum that matches how the machine is assembled and how it sits on its feet.
- Measured how. Straightedge and feeler, precision level, laser, CMM. Each has a different resolution and a different price.
- Measured when and where. On the machine after cutting, or on a plate in a metrology room at a stated temperature. A large steel structure changes size with ambient temperature, and on a two-metre base that is not a rounding error.
- Supported how. A base measured on three points reads differently from the same base on six. State it, or you are measuring how it was set down.
Get those six in, and quotes from different shops become comparable for the first time.
What this costs you
Tightening flatness is not linear. Going from 0.10 mm to 0.05 mm over a metre is usually another cut. Going from 0.05 mm to 0.02 mm can mean a different machine, an extra setup, a temperature-controlled bay and a second stress relief cycle, and the price moves accordingly.
So specify what the machine actually needs. Rails sitting on a grouted plate do not need what a metrology frame needs. The most expensive drawings we see are the ones where a default tolerance was applied to every surface because nobody asked which ones mattered.
Before you send it out
Mark the functional faces, name the datums, state the stress relief method and whether you want the chart, and say what inspection documents you expect in the box. That single page of notes is worth more than another week of email.
Send the drawing and we will tell you which shops can hold it. If you are still choosing a construction, start with cast iron against welded steel.
Common questions
Is vibratory stress relief as good as a furnace cycle?
They do different things. A furnace cycle raises the whole structure to a soak temperature and lets the metal creep its residual stresses away; it also changes the surface and costs oven time. Vibratory treatment excites the structure at resonance to redistribute peak stresses, is far faster and cheaper, and is widely used for machine frames. Some specifications and some customers accept only thermal treatment. Decide which you need before you compare prices, because the two are not the same product.
How flat can a machined machine base actually be held?
On a well-machined steel or cast iron base, flatness in the 0.02 to 0.10 mm band over a metre on a machined pad is the normal commercial range, and the tighter end needs a stable machine, a settled part and often a second cut. Below that you are into grinding and hand scraping, which are separate operations with their own cost. The number only means something with a stated measurement method and datum.
Should I stress relieve before or after rough machining?
Both, on work that matters. Relieve after welding, rough machine, relieve again, then finish machine. Rough machining removes material and releases stress that was locked in; a second cycle before the finish cut is what stops the part moving after it leaves the machine. On less critical work a single post-weld cycle is normal.
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