Receiving a quote for machined parts often feels like receiving a hospital bill: you see the final number, but the breakdown is a “black box.”
For procurement managers and engineers, understanding the math behind the quote is the first step to optimizing a Manufacturing Budget. It allows you to design for cost, rather than just designing for function.
While every machine shop has its own overhead rates, the core physics of a Part Cost Estimate are universal.
Here is the transparent, step-by-step guide to calculating it yourself.
1. The Material Cost: You Are “Buying Air”
The first variable is the raw stock.
The most common mistake engineers make is calculating the volume of the finished part.
Reality Check: You don’t pay for the finished part; you pay for the block it was carved from.
The Formula: (Stock Block Volume × Material Density × Price/kg) + 10-20% Markup
The “Air” Tax: If your part is an L-bracket, you are paying for a rectangular block. The material we turn into chips is waste, but you still pay for it.
Pro Tip: Stick to standard bar stock sizes.
If you design a part that is 52mm thick, we have to mill it down from a 60mm or 63.5mm (2.5″) block.
If you design it at 48mm, we can use 50mm stock. That 4mm change saves you 20% on material and significant facing time.
(Learn more about Material Selection strategies).
2. Machine Time: The “Taxi Meter” Effect
This is the biggest cost driver. CNC Machining is billed by the hour.
Think of it like a taxi: The meter is running from the moment the spindle starts turning.
The Hourly Rate:
3-Axis Mill: Typically $40 – $60 / hour.
5-Axis Mill: Typically $90 – $150 / hour.
The Variable: Material Removal Rate (MRR)
How fast can we cut?
Aluminum (6061): Soft and easy. We can run high feed rates. Low cost.
Stainless Steel (316) or Titanium: Hard and gummy. We must slow the machine down to prevent tool breakage. High cost.
The Geometry: Deep pockets require long tools. Long tools vibrate (chatter), forcing us to slow down further.
3. The “Setup” Tax (NRE)
This is the variable that kills low-volume budgets.
Before the first chip is cut, a skilled machinist must:
Program the tool paths (CAM).
Design and machine custom jaws or fixtures.
Load tools and “zero” the machine.
The Math of Amortization:
If setup takes 3 hours at $80/hour (= $240 NRE):
Order 1 Part: Setup adds $240 to the unit price.
Order 100 Parts: Setup adds $2.40 to the unit price.
Strategic Advice: If you are prototyping, ask if your part can be made using “Soft Jaws” or standard vises to minimize NRE.
4. Complexity: The Multiplier
Complexity isn’t just about “looking cool.” It’s about Workholding.
A CNC machine can only cut what it can reach.
3-Axis Limitation: If your part has features on all 6 sides, an operator must physically open the door, unclamp the part, flip it, and re-clamp it 6 times.
The Cost: Every flip adds manual labor time and introduces a new opportunity for tolerance stack-up errors.
Design Fix: Try to design parts that can be machined from one or two sides. If complex geometry is unavoidable, consider if Die Casting is a viable alternative for higher volumes.
5. Tolerances: The Exponential Curve
This is where engineers accidentally double the cost.
Standard Tolerance (ISO 2768-m): This is the “machine standard” (typically ±0.1mm). It is fast.
Tight Tolerance (±0.01mm): This requires:
Slower finishing passes.
Frequent tool wear checks.
Manual verification with a CMM (Coordinate Measuring Machine).
The Rule: Only apply tight tolerances to mating surfaces (bearing bores, interference fits). If a surface touches nothing but air, leave the tolerance open.
The Bottom Line
Cost reduction isn’t about squeezing your supplier’s margin; it’s about optimizing the physics of production.
To lower your quote:
Standardize your stock size.
Minimize the number of setups (flips).
Relax non-critical tolerances.
Need a sanity check on your design?
Upload your 3D file to Sureton today. Our engineering team will provide a DFM review to highlight exactly where your design is driving up costs—and how to fix it.
