How Tolerances Add Up on Stacked and Assembled Copper Parts
September 26, 2026
A copper busbar stack, a laminated bus, a heat sink bolted to a module: each is an assembly where several machined parts add their tolerances together. When it does not fit, every part measures fine and everyone is confused. This is how to tolerance the parts so the assembly works.
The arithmetic
If four bars are each specified at plus or minus 0.1 mm on thickness, the stack can be plus or minus 0.4 mm. On a worst case basis that is what you design for, and on copper it is not unusual for parts to sit near one end of the band because the material moves the same way each time.
Statistical stacking gives a smaller expected variation, and it is a legitimate method, but it is a decision to make deliberately, not an accident.
Three ways to make it fit
Tighten the parts. Works, and costs on every part forever. On copper, which moves with heat and clamping, tight thickness tolerances mean finishing passes and controlled inspection. See copper machining tolerances.
Absorb the stack in the design. A Belleville washer stack, a slotted hole, a flexible link, a shim. This is usually the cheapest answer and it is how switchgear busbar has been built for decades.
Control one datum. Tolerance the surfaces that touch and let the rest float. On a busbar stack that means the contact faces and their flatness, not the outline.
Where copper adds its own variation
Plating. Every plated face gains thickness. In a stack of bars plated all over, that is two plated faces per bar. Specify plating thickness by area, mask where you can, and state whether drawing dimensions apply before or after plating. See plating thickness specification.
Temperature. Copper expands around 17 micrometres per metre per degree. A stack measured warm from machining is not the stack you assemble cold.
Bends. A bent bar springs back and the flatness of the joint faces changes. Joint faces on bent bars are machined after bending.
What to put on the drawings
On each part:
- The dimensions that participate in the stack, toleranced for the stack
- A general tolerance block for everything else
- Flatness on the mating faces only
- Plating thickness by area, and before or after plating
On the assembly:
- The stack requirement itself, which tells the shop why the part tolerances are what they are
- The compensating element, if there is one
A shop that sees the assembly drawing can often suggest which part tolerance to relax. What buyers get wrong on copper drawings covers the other side of that conversation.
Busbars specifically
The copper busbar design guide covers hole tolerances, which are the other place stacks bite: a pattern of holes across several bars has to line up, and clearance holes plus two locating holes is the pattern that makes that cheap.
Frequently asked questions
Why does my busbar stack not fit when every bar is in tolerance?
Because the tolerances add. Four bars each at the top of a plus or minus band stack to four times the band. The fix is either tighter individual bars, which costs on every one, or a design that absorbs the stack with a washer, a slot or a flexible link.
Should I tighten every dimension to be safe?
No. Tighten the dimensions that participate in the stack and leave the rest at a general tolerance. Blanket tight tolerances on copper add inspection and finishing time to features that never mattered.
Does plating enter the stack?
Yes. Plating adds thickness on every plated face, and a stack of plated bars picks up plating on both sides of every bar. State whether dimensions are before or after plating, and include the plating in the stack.
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