a) why? why not just use a M10 bolt?
b) if you machine it, pay very close attention to the fillet radius at the shoulder transition, and to the new thread runout before the shoulder. You could easily end use with a nasty stress concentration.

Hi Christo

It’s not something I have thought about before but providing the heat generated doesn’t exceed the temperatures of the original heat treatment I would of thought it’s mechanical properties would be unchanged, however if the screw thread was originally cold forged and you machine it, then that might be a different story. Can you please expand on why you need to turn a larger screw down and what is the application?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

In my experience (of several decades) it is generally a bad practice with less than desirable results to try to machine a standard commercial screw. Start from scratch and design the component you need.

Thanks for the feedback all. I have used this approcach before where it isn't possible to use "off the shelf screws" to achieve the connection required. Sometimes the design constraints can't be solved with standard fixings and a bespoke solution is required. I feel that it's a lot cheaper to buy a couple of hardened bolts than a bar of hardened machining steel.

Does anyone know if the heat generated during machining is high enough to alter mechanical properties of bolts? (Consider for example stainless steel which generates high surface temperatures during friction movement) Perhaps @EdStainless might know?

A sketch of what you need would be helpful.

I'm thinking :
- a spacer on COTS screw/bolt
- a COTS shoulder screw ( although a quick search for an M10 thread did not come up with shoulders over 14 mm or so)
- cut the thread off a long M20 screw with a shoulder/shank accurate enough for your needs. Leave the shoulder/shank the length you need. Then drill and tap the end of the stubby screw M10 20 mm deep, and install a 40 mm long M10 with red loctite.

Thanks for the ideas Tmoose. Unfortunately I don't have a sketch to share. This is a general question. The example in the original post is just an example, not a specific problem I'm trying to solve.

Since this is a "general question", this is a general answer - in general you will waste much more in time and money trying to machine a COTS fastener to achieve your final desired geometry than you might have saved by starting with a COTS bolt. If you insist on doing it anyway, carry on. In a few years you will agree with me.

Fasteners are generally case-hardened with soft cores so machining compromises most of the strength and hardness. I enjoy antique restoration as a hobby and consequently modify many COTS fasteners but would never do so for a safety-critical joint.

It isn't the heat from machining that you need to care about.
It is the fact that most fasteners are not the same strength throughout.
The cores will tend to be softer, and sometimes a lot.
If you are using an alloy such as 4130 that has been Q&T, then you are likely to only loose a few points HRC by removing 5mm of material.
If the threads were rolled then you could be losing closer to 10 points HRC.
None of these would work?
https://www.mcmaster.com/products/stripper-bolts/t...
https://www.mscdirect.com/browse/tn/Fasteners/Bolt...
https://us.misumi-ec.com/vona2/press/P0300000000/P...

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P.E. Metallurgy, consulting work welcomed

Thanks all, this is very helpful.
Yes Ed, those shoulder bolts definitely would do the job. I was just curious.