How We Saved 2 Days on a Tight Delivery by Switching One Step from Casting to CNC
The Project Brief
A client in the petrochemical industry placed an order for 2,500 sets of Bubble Caps — precision components used in industrial distillation columns to control vapor-liquid contact. The material specified was 310 Stainless Steel, a high-temperature, high-chromium alloy known for its excellent oxidation resistance but notoriously difficult to machine due to its work-hardening tendency.
The part geometry was complex: a hollow cylindrical cap approximately 10 inches long, featuring angled gas holes around the circumference, a precision internal groove for sealing, and a threaded end for assembly. The client had a firm delivery deadline — any delay would impact their plant maintenance schedule.
The Initial Challenge: A Casting-Only Approach Was Too Slow
Our first engineering review identified the internal groove as the critical bottleneck. To produce this groove by investment casting alone, we would need to use a soluble wax core — a specialized technique where a pre-formed core is placed inside the wax pattern to create the internal cavity.
Why Casting the Groove Was a Problem
Soluble wax cores require extended drying time between ceramic shell layers — adding approximately 2 days to the shell-building phase. For a 2,500-piece order, this delay would cascade through the entire production schedule.
The Engineering Decision: Hybrid Process
Our engineering team proposed a hybrid manufacturing approach:
- Investment casting to form the complex hollow body, angled gas holes, and overall geometry of the cap — all features that would be prohibitively expensive to machine from solid bar stock.
- CNC turning to machine the internal groove and the threaded end into the cast blank — eliminating the need for the soluble wax core entirely.
The Key Insight
The internal groove was a simple, rotationally symmetric feature — exactly the type of feature a CNC lathe can produce in minutes. There was no engineering reason to cast it. By moving this one feature to CNC, we eliminated the 2-day core drying delay without compromising part quality.
Production Timeline Comparison
| Production Step | Casting-Only | Hybrid (Actual) |
|---|---|---|
| Wax pattern injection | 2 days | 2 days |
| Ceramic shell building (with core) | 6 days | 4 days |
| Casting, knockout & cleaning | 3 days | 3 days |
| CNC machining (groove + thread) | — | 2 days |
| Heat treatment & passivation | 2 days | 2 days |
| Final inspection & packaging | 2 days | 2 days |
| Total Lead Time | 17 days | 15 days |
The Result
By switching the internal groove from casting to CNC, we reduced the total production timeline from 17 days to 15 days — saving the client 2 full days and delivering the order ahead of their plant maintenance deadline.
The quality outcome was also superior. The CNC-machined groove achieved a tighter tolerance (±0.05mm) than casting would have provided, ensuring a better seal in the assembled distillation column.
Key Takeaways for Procurement Engineers
This project illustrates three principles that apply to almost any stainless steel casting project:
Not every feature needs to be cast
Investment casting is powerful, but some features — especially simple grooves, threads, and flat mating surfaces — are faster and more accurate when machined. The question to ask is: 'Is this feature worth the casting complexity it adds?'
Hybrid processes are often the fastest route
Combining casting and CNC machining in a single facility eliminates transit time, scheduling delays, and quality disputes between suppliers. The total lead time is almost always shorter than a two-supplier chain.
Involve your supplier in DFM review early
If we had not reviewed the engineering drawing before production started, we would have followed the original casting-only plan and missed the deadline. Early DFM review is free — the cost of missing a delivery deadline is not.
For more on choosing between casting and machining, see our guide: Investment Casting vs. CNC Machining for Stainless Steel Parts.
If you are managing a tight delivery deadline, read: How a one-stop supplier eliminates supply chain friction.
To avoid similar bottlenecks in your own designs, read: 5 DFM Tips to Cut Lead Time on Investment Cast Parts.