Welcome to our new PM Component Spotlight series!
These articles will identify ideal components for metal powder manufacturing. No theory here, only real-world problems and real-world solutions -- all achieved with powder metal parts.
For our Fall 2019 edition, see how a major rifle maker was able to upgrade the rifle’s cost-competitiveness and “feel” with a new rifle trigger mechanism:
Introducing the Bolt Action Rifle Trigger Mechanism
The Winchester Model 70 is a bolt action sporting rifle known around the world for its reliability. With quality, though, comes a price.
Engineers were working on a three-lever trigger design. Their goal was twofold:
- Create trigger with zero takeup (how far the trigger has to move before it engages)
- Create a trigger with less than 4 lbs. of pull pressure (the amount of pressure needed to pull the trigger)
And so the Winchester 70 MOA trigger project was born.
The answer to Winchester’s request wasn’t simple. It needed a materials, processing AND joining solution.
This was a newly redesigned gun. Notably, it was a part previously produced with a competing metal forming method. Because of the design, conventional powder metal molding was a possibility and provided an opportunity to reduce cost.
The rifle maker had major hardness requirements for the project. Key aspects of the design were:
- A stainless steel trigger assembly over an iron-based design, which improved the aesthetics and the corrosion resistance
- Hardness above 35-45 RC on the sear, sear actuator, and trigger, along with a housing for the assembly that had a network of channels on the inside
How does a powder metallurgy manufacturer make a high-hardness part? Not just with material selection, but with advanced sintering as well.
The Winchester Model 70 Trigger featured an advanced process called high-temperature sintering. A high-temperature sintering furnace typically runs at about 2300° F -- 100-250° hotter than a standard sintering furnace. This extra-crispy setting may seem like a minor thing, but it has excellent implications for component properties such as hardness.
In its original design, the housing was to be produced through a complex and expensive machining process. Instead, Horizon developed an idea to produce the part as two pieces, which allowed all of the complex features to be molded into one half. We then produced a simple plate as a cap and brazed the two halves together.
While the brazing provided a joint stronger than the material used to produce the part, engineers wanted to develop a redundant means of holding the two parts together. Horizon devised a rivet method in which features are molded onto the part and can be used to orient the cap. Those features are struck in a press to displace material and offer another means of holding the pieces together.
This assembly would have been very hard (no pun intended) to machine. The enhanced joining process opened design possibilities for the Winchester far beyond the previous version.
And just like that, the manufacturing and assembly process was simplified.
The owners of the Winchester name first bought the assembly conceptually. Excitement spread quickly, and the part moved into full production -- at the lower cost the business needed.
This is a story from 2009. All these years later, the Winchester continues to rely on the Horizon trigger assembly. There have been zero changes to the design since then.
Until Then ...
Hope you check out Winter 2020 edition of our PM Component Spotlight Series! There are many more stories to come about breakthroughs in:
- Cost reduction
- Magnetic performance
- Mechanical performance
- Shape-making capability
In the meantime, feel free to ask an engineer about the Winchester project or any other questions related to sintering and powder metallurgy design.