Shear strength of M6 8.8 mild steel bolts?

[ELL]

I want to make a bolt together work bench that will hold at least 250kg
Will M6 bolts be strong enough for this job?
The frame will be made from 40x40x4mm mild steel angle, with 2 bolts going through each piece of angle to hold it together.
Thanks.

 

[Craig-S]

In short: Yes, M6 bolts are strong enough to hold 250kg, provided you use a decent grade of bolt (like Grade 8.8) and assemble the frame correctly.
While M6 might look "small" compared to the beefy 40mm steel angle you’ve chosen, the physics of steel fasteners is on your side. Here is the breakdown of why this works and where you should be careful.

1. Bolt Strength vs. Your Load​

A standard Grade 8.8 M6 bolt has a shear strength (the force required to "chop" it in half) of roughly 600kg to 900kg depending on whether the threads are in the shear plane.
Since you are using two bolts per joint, each corner is held by a combined shear capacity of well over 1,200kg. Even with a significant safety factor, 250kg is well within the limits of M6 fasteners.

2. The "Weak Point" is rarely the Bolt​

In a 250kg-rated workbench, the bolts are rarely the first thing to fail. You should focus on these factors instead:

• Bearing Strength: Your 40x40x4mm angle is 4mm thick.1 If the bolts are loose, the hard steel of the bolt can "oval out" or tear through the 4mm mild steel hole under extreme vibration or shifting.
• Triangulation (Racking): A bolt-together frame is prone to "racking" (swaying side-to-side). Since bolts allow for a tiny amount of movement, you must use gussets or diagonal bracing to keep the bench from folding like a deck of cards when you push on it.
• Grade Matters: Do not use "mystery meat" bolts from a cheap bin. Ensure the head is stamped with 8.8. Avoid Grade 4.6 (common in decorative hardware), as it is much softer.

3. Recommendations for your Build​

Component	Recommendation
Bolt Grade	Use ISO Class 8.8 high-tensile steel bolts.
Hole Size	Drill 6.5mm holes. Too much "slop" in the hole will cause the bench to wobble.
Washers	Use large flat washers (fender washers) on both sides to spread the load across the 4mm steel surface.
Locking	Use Nyloc nuts or spring washers. Workbenches vibrate (hammering, sawing), which can vibrate standard nuts loose over time.

 

[BrokenBiker]

Yes...just design it so the load is shared well between all the bolts...I have a pressed steel bench that my grinders and pillar drill are mounted on and it uses M6 bolts and claims to hold 150kg and it's the flimsiest bench I've ever seen, or was til I replaced the top with 18mm ply instead the of 9mm MDF they supplied

 

[Agroshield]

In short:

Which AI did you use to write that for you?

 

[dieselsamurai]

Which AI did you use to write that for you?

Direct from google if you use AI mode

 

[bricol]

Which AI did you use to write that for you?

Could have been written by our quality department who also appear to have no concept of using the correct units

We'll ignore the idea that spring washers do anything to lock fasteners together . . .

 

[Craig-S]

Which AI did you use to write that for you?

Gemini, because whats the point in asking a question he'll get loads of replies with opinions when he could have just googled it.

 

[dieselsamurai]

To be fair it gave a good enough answer to the question , I sometimes think of asking a question on here but google answers a lot of them if you filter out the rubbish

 

[ELL]

Gemini, because whats the point in asking a question he'll get loads of replies with opinions when he could have just googled it.

Because AI often makes mistakes. So my dumb ass thought I'd get the opinion of professional metal fabricators.

 

[tom2207]

why use m6 when you can use m8 at the same price , the biggest load will probably be from dragging the bench about the floor , and the legs become a lever ,,

 

[Munkul]

why use m6 when you can use m8 at the same price , the biggest load will probably be from dragging the bench about the floor , and the legs become a lever ,,

This. The cost difference is absolutely nothing.

 

[bricol]

An even better time to learn some welding .. .

 

[Bornfree]

250kg static load is one thing but moving stuff on it. hammering and levering are something different especially if your fitting a vice on it.

 

[tom2207]

An even better time to learn some welding .. .

hard to get it in the boot of a mini if its welded ,,,,

 

[Ton-up]

Use the correct bolt (with plain shank) not set screws too.

 

[BrokenBiker]

hard to get it in the boot of a mini if its welded ,,,,

thats why they make convertibles

 

[Gareth J]

Total weight rating and fastener size aren't the most important factors in my opinion.

Dynamic loading, i.e whacking stuff on it with a big hammer. Or levering things in an attached vice to induce cyclical twisting forces in the frame will produce much more destructive forces than a 250kg engine plonked onto it.

I think the answer to whether m6 could be suitable is heavily dependent on the level of bracing.

 

[Farside]

hard to get it in the boot of a mini if its welded ,,,,

Getting it in is easy enough, getting it back out again...

 

[waddycall]

Gemini, because whats the point in asking a question he'll get loads of replies with opinions when he could have just googled it.

What’s the point in a forum then. What a low point this is

 

[Craig-S]

What’s the point in a forum then. What a low point this is

So your issue is basically that the first response was accurate for the yes/no scenario with a quick question to AI, instead of 18 replies of waffle? And as a result, this negates any positives of a forum?