Best type of bolt head

[Kicked Back]

I needed to align my table saw mitre slot to the blade, which required undoing the bolts that hold the cast iron top to the base. These were M6 x 25mm bolts with round heads and a hex slot.

One of the hex slots got chewed really badly and I had to grind flat edges onto the head to get mole grips to stick - and even that required a massive amount of force to loosen.

I want to replace the bolts with higher quality ones and realised I don't really know what makes a bolt high quality...

What kind of material should I be looking for and what shape head is most reliable? Also any advice on brands would be appreciated.

Thanks

 

[TFrench]

Bolts are graded by strength - 5.8 being weakest, up to 12.9.
8.8 is fine for most purposes. If your saw is relatively modern, the bolts will probably be cheap chinesium which is a grade of its own of a similar strength to cheddar cheese.

 

[Sporky McGuffin]

I had a batch of speakers for work;not only were the bolts of the cheddar-grade previously mentioned, but the brackets seemed to be composed of Wensleydale.

 

[Sandyn]

Socket screws are really good as long as the screws are HT steel. Use a quality allen key of the correct size to undo them. Make sure they are not full of dirt before trying to undo them.
Check out this site (just an example) they say-:""Allen bolts / Socket Screws used in many engineering & automotive applications come in four main different styles. Socket Cap, Socket Countersunk, Socket Button and Socket Set or Grub. In steel, these bolts / screws are supplied by us as High Tensile. Cap Screws are 12.9 Grade, Countersunk and Button are 10.9 Grade. Socket Sets / Grubs are 14.9. Most of our range (apart from socket sets / grubs) is plated with ROHS compliant zinc – self colour or ‘black’ can be supplied on request.""

 

[Sideways]

Find a local engineers merchant and take along one of the damaged bolts. Ask him for the same thing, but a proper quality high tensile part. The manufacturer "Unbrako" is a top quality brand in these fixings if you need to wave around a name to indicate top quality.
The type of fastener you describe with a round head and a hexagon recess for an Allen key are called "Cap Screws". Some are made in cheap tat steel, often bright zinc plated, but the real ones are black (the metal itself looks black, not painted), and made of high tensile steel. These are some of the toughest fasteners you'll easily obtain and more than up to the application. They are not expensive for the handful that you will need. Probably less than 30p each for M6 25's. Get 20 or 50 plain steel M6 washers to go with them, those will only be pennies each.
I don't see any reason to over tighten the bolts between table and frame when you reassemble the saw. Use a long M6 Allen key to fasten them (4 or 5 inches, if you don't have one, maybe buy a single key to fit your screws at the engineers merchants) and tighten until the key just begins to bend. That should be plenty and will be nowhere near enough force to damage the socket.

 

[pe2dave]

Recent fun and games with 'cap screws', I purchased (my first) T bar allen key. I found it so much easier to use (and torque up) then an allen key. Bought singly, I'll be favouring these in future.

 

[TheTiddles]

For your application a cheap fastener will be fine. Socket caps are often damaged by people using worn hexagonal keys, they’re disposable tools. Greasing a thread often enables a substantially higher force to be generated, often too much for the fastener

If it was hard to get the last one out, check the tapped thread for damage, that might have bee the cause.

 

[Kicked Back]

Socket caps are often damaged by people using worn hexagonal keys, they’re disposable tools.

In this case it was a Wera allen key, which doesn't have a scratch on it.

Good call about checking the threads.