Stock allowance is material that is intentionally left on parts and welded assemblies at various stages of production. A necessary insurance policy, stock allowance enables manufacturers to compensate for distortion or warping by machining fits at intermediate stages of fabrication and at final machining. Since the mechanics of machining and welding are universal, every manufacturer of metal fabrications must consider stock allowance both during planning and intermittently as the project progresses.
Quantifying Stock Allowance
The appropriate level of stock allowance is almost always determined on a case-by-case basis. Planners must analyze the entire project from start to finish before any metal is cut, but stock allowance also depends on a series of small decisions made over the course of the project. Accurately assessing this allowance requires an understanding of how each set of parts will go together, and how each affects the next assembly.
Considerations start at the raw material stage as planners work to answer the following questions:
- What will it cost to pay for the extra material up front?
- Do we need to buy thicker material in order to have this extra stock, or is it just a matter of cutting the rough part a little bigger?
- How much extra stock is enough (too little is as bad as none)?
- What will it cost to remove the extra stock?
- Is it a separate machining setup and operation, or just a few more cuts in a setup that's already taking place?
- Do you need the extra stock on thickness or just on the outline?
- Can you start with standard material sizes?
- How is the part or assembly going to react to cutting, machining and welding?
- Will it distort out of tolerance or is there enough tolerance to allow some movement?
- Do we need to machine the part for a mating assembly — and where does that material come from?
- Finally, how do we recover if the part or assembly goes out of tolerance and we don’t have stock to fix it?
The Dangers of Under-Budgeting Stock Allowance
Leaving too much extra material can be expensive, but failing to leave enough can be even more costly to both schedules and budgets.
In some cases where stock allowance is insufficient, manufacturers will have to add replacement material by welding, straightening the part or modifying the mating piece to accommodate the deviation. In the most severe cases, the part may not be usable at all, forcing the team to start all over again, which comes with obvious implications for both cost and schedule.
Stock allowance is a type of insurance that enables manufacturers to compensate for distortion and warping. Determining how much extra material to include is a delicate art form that comes with experience. To get it right, manufacturers have to plan up front, but they also must adjust and revisit their calculations as the project moves forward.