Should you cast threads directly to save money, or machine them for precision? We analyze the M6 threshold, the hidden costs of unscrewing molds, and why Sureton recommends auto-tapping for 90% of projects.

It’s a conversation we have almost every week at Sureton.
A client sends us a drawing for a new aluminum housing. It has twelve M4 mounting holes.
The Procurement Manager asks: “Can’t you just cast these threads directly? We want to eliminate the secondary machining cost.”

The short answer is: Yes, we can. But you probably don’t want us to.

While the idea of a “finished part right out of the mold” sounds like the holy grail of cost reduction, the reality of physics and tolerance stacks often makes Cast-in Threads a false economy.

Here is the honest engineering breakdown of when to Cast, when to Machine, and where the break-even point lies.

1. The Reality of “Cast-in” Threads

To cast a thread, we have to mold the metal into a spiral shape. This creates two distinct challenges depending on whether the thread is External (Male) or Internal (Female).

External Threads (The “Flash” Problem)

If you cast a bolt-like feature on a part, the mold must split open to release it.

• The Issue: The parting line will run right across the threads. Even with a perfect mold, you will get a microscopic line of Flash (excess metal) across the thread path.

• The Consequence: When your assembly line tries to screw a nut onto it, it jams. You end up having to “chase” the thread with a die to clean it anyway.

• Verdict: Only viable for very coarse threads (like a garden hose connector) where fit is loose.

Internal Threads (The “Unscrewing” Problem)

To cast an internal thread, we can’t just pull the core pin out (it would rip the threads). The core pin must unscrew itself before the mold opens.

• The Cost: This requires a complex “Unscrewing Mold” with gears and hydraulic motors inside the tool. This adds $3,000 – $5,000 to your tooling cost.

• The Cycle Time: The unscrewing action takes time (5-10 seconds per cycle). In die casting, time is money. You save on machining, but you pay more for the casting cycle.

2. The “M6 Threshold”: The Golden Rule

So, when does it make sense? At Sureton, we use the M6 Rule.

Below M6 (M2, M3, M4, M5): Machine It.

For small threads, the die casting grain structure is too coarse to hold a fine pitch.

• Risk: The threads are brittle. If an operator uses a power screwdriver, they will strip the cast threads instantly.

• Solution: We cast the Pilot Hole (Cored Hole) to the minor diameter, and then we tap it. This gives you strong, cut threads with Class 6H tolerance.

Above M6 (M8, M10, M20+): It Depends.

As the diameter gets larger, the thread becomes stronger.

• Scenario: If you have a large M20 drain plug on an oil pan, Cast it. The thread is big enough to be robust, and machining a hole that big removes a lot of material (slow).

3. The Third Option: Auto-Tapping (The Sweet Spot)

Many clients fear machining because they imagine a guy with a hand drill standing next to the machine. That is the old way.

At Sureton, we use Multi-Spindle Auto-Tapping Centers.

• Process: We load the part into a fixture. The machine taps 12 holes simultaneously in seconds.

• Cost: The cost is pennies per part.

• Quality: You get the precision of a machined thread with the speed of automation.

Pro Tip: For high-volume consumer electronics (like zinc housings), we often use Self-Tapping Screws. We cast a smooth pilot hole, and you use a hardened screw that cuts its own thread during assembly. This eliminates the tapping cost entirely.

4. Tolerance: The Deal Breaker

This is the final nail in the coffin for cast-in threads for precision parts.

• Die Casting Tolerance: Typically ISO 8062 CT6. A cast feature might move +/- 0.1mm due to shrinkage.

• Thread Tolerance: A standard machine screw requires tight engagement.

If you cast a thread, and the aluminum shrinks slightly more than predicted (due to a hot day or a slight alloy variance), your screw won’t fit. You cannot “adjust” a cast thread without modifying the steel mold. A machined thread, however, is always perfect because the tap doesn’t shrink.

Summary: The Decision Matrix

Save this chart for your next design review:

Let’s Optimize Your Assembly

Don’t let a stripped thread stop your production line.
The best approach is usually: Cast the hole, Machine the thread.

Not sure if your design is optimal?
Send your drawing to Sureton. Our DFM engineers will highlight every hole and tell you exactly which ones should be cored, which ones should be drilled, and which ones should be tapped to save you the most money without sacrificing quality.