Hi, it’s Morio!

The hot season is back again!
Some of our coworkers say it feels like the sun is stabbing their skin!
We virtual staff can’t really feel that, but please take good care of yourselves out there!

Do you remember the term “residual stress” from the last blog post?
I’ve heard it a lot since joining the company, but honestly, I didn’t really understand what it meant.
So this time, I looked into it—and I’ll do my best to explain it in a way that makes sense!

💪 What is “stress”?

First off, “stress” in this context refers to the internal force that occurs inside a material when it’s subjected to external forces.

For example, when the frame of a TF series machine is processing a spring,
all kinds of forces are being applied from the feeding unit, coiling unit, cutting unit, etc.

At that moment, inside the frame, it’s like saying “Whoa there!”
An internal counterforce kicks in to resist those external loads.

That internal force is what we call “stress”!

🧠 What is “residual stress”?

Now comes the trickier one.

Even after processing is finished and there are no more external forces,
sometimes a kind of “memory of force” remains inside the material.

That’s what we call “residual stress.”

🤕 Let’s compare it to muscle soreness!

Let’s take a human example.
After you do some muscle training, you feel sore the next day, right?
That soreness is the lingering impact of the force that was applied during exercise.

So basically:

Stress = the strain on your muscles while working out

Residual stress = the soreness you feel the next day

📌 And just like how it’s risky to move with sore muscles,
📌 running a machine with residual stress can lead to deformation or even breakage.

🛠 What happens when residual stress is present?

If precision parts like the frame or bed still contain residual stress, it can cause:

Minor distortions during machining

-Assembly precision issues

-Deformation or strange vibrations after long-term use

In other words, it seriously affects our all-important repeatability!

So let’s dive into the TF frame production process and dig deeper!

🧱 How is a cast-iron frame processed?

TF series frames are made from gray cast iron (FC250) and built with a two-piece (front and back) structure.
They’re then processed with drilling, chamfering, tapping, and precision finishing.

But the key point here is…

Cast iron has internal stress even before machining begins!

💥 Does cast iron have a “memory of force”?

Yup! Cast iron is made by pouring molten metal into molds and letting it solidify.
During this process, the outer parts cool and harden first, and the inner parts cool more slowly.

That time lag causes a mismatch in shrinkage forces between inside and outside,
leaving behind “internal force memory”—aka residual stress.

If we were to humanize it:

“It’s like waking up with a crick in your neck!”
(Hmm… does that analogy work? 😅)

🔧 So what happens during machining?

Here’s the core issue!

If you machine the frame while this internal force still exists…

-You’ll get warping or bending mid-process

-The shape might change over time even after finishing

-Once assembled, it could vibrate weirdly or make strange noises

In short:

You may think it’s perfectly machined, but things still shift later!

🛠 How do we prevent that?

At our company, we take great care to avoid problems caused by residual stress.
Here’s how we tackle it with thoughtful techniques and processes:

✅ Stress relief through aging and heat treatment

→ We leave the cast iron to “rest” for weeks or months (this is called “seasoning”),
or apply heat treatment in a furnace to gradually relieve internal stress.

In the past, people would simply let it sit for months (or even years!)
But nowadays, we use annealing to remove stress more quickly.
For gray cast iron like FC250, it’s common to hold it at about 530°C for 3–6 hours, then let it cool in the furnace.

✅ Careful machining sequence and fixturing

→ If we remove too much material from one side first, it throws off the stress balance.
So we machine gradually, flip sides, and work symmetrically to maintain stability.

✅ In-process distortion checks

→ Between steps, we measure perpendicularity and flatness using measuring tools.

Think of it like stretching after intense exercise—it prevents injuries later!

🎯 Why do we care so much?

Simple!

Because our core mission is to uphold:

Repeatability — the ability to manufacture high quality springs in exactly the same shape, every time!

If the frame is even slightly distorted:

-Bending positions will shift

-Cutting points will drift

-Coiling balance will be thrown off

So yes, we care a LOT!

🐾 Summary

Cast-iron frames aren’t just about cutting shapes.
They’re made with careful attention to invisible forces,
thanks to the craftsmanship and experience of our trusted partner technicians.

Hmm… now that I think about it—if we virtual guys don’t feel summer-heat, can we still get muscle soreness? 🤔

See you again next time on
“Dig Here, Bowwow!!” 🐾

This blog was created using generative AI.