Extend thread on a socket head bolt

[justask4]

Hello there -

My question is related to extending existing threads on socket head bolts - Allen Key bolts.

How do you clamp a socket head bolt into a 3 jaw chuck to cut the existing thread longer?
For example, I have a M8x50 bolt and the thread is only 20mm but I need 35mm thread.
I tried to hold it in a bench vice, but that’s not very good, I tried in a 3 jaw chuck on my lathe, but it does not run true and also does damage the head on the bolt. I tried to put some copper around the head of the bolt and also then tried the same with aluminium, but both attempts did not work very well. Any suggestions?
thanks heaps

[Chris Parks]

Hello there -

My question is related to extending existing threads on socket head bolts - Allen Key bolts.

How do you clamp a socket head bolt into a 3 jaw chuck to cut the existing thread longer?
For example, I have a M8x50 bolt and the thread is only 20mm but I need 35mm thread.
I tried to hold it in a bench vice, but that’s not very good, I tried in a 3 jaw chuck on my lathe, but it does not run true and also does damage the head on the bolt. I tried to put some copper around the head of the bolt and also then tried the same with aluminium, but both attempts did not work very well. Any suggestions?
thanks heaps

Put a key in the chuck, put the bolt head onto the key and lock it to the key with the tail stock. You could lock the key in a vice and using a hand held die apply the load when turning the die to keep the key in the head of the bolt.

[justask4]

Put a key in the chuck, put the bolt head onto the key and lock it to the key with the tail stock. You could lock the key in a vice and using a hand held die apply the load when turning the die to keep the key in the head of the bolt.

Thanks Chris, I tried that, but sometimes that actually makes the head round, i.e. the bolt spins on the key. I forgot to mention that I tried that. I tried so many things, I lost track.

[nearnexus]

Hello there -

My question is related to extending existing threads on socket head bolts - Allen Key bolts.

How do you clamp a socket head bolt into a 3 jaw chuck to cut the existing thread longer?

You don't. You use a collet to hold it, but you don't have collets I expect, like 90% of lathe owners out there.

So your next best method is to get/make some vice jaw plates with a "V" cut into each face vertically and use those to grip the round head section of the bolt - tap the thread by hand with a die. You can get by with just one "V" plate and the original flat vice jaw in most cases.

[justask4]

You don't. You use a collet to hold it, but you don't have collets I expect, like 90% of lathe owners out there.

So your next best method is to get/make some vice jaw plates with a "V" cut into each face vertically and use those to grip the round head section of the bolt - tap the thread by hand with a die. You can get by with just one "V" plate and the original flat vice jaw in most cases.

you are correct, I have no collets for the lathe. Unfortunately. may be /hopefully at a later stage.
I have put the bolts into a "V" block within a vice, but have not thought of making actual vertical grooved vice jaw plates. so thank you very much for the idea.

[BobL]

Make some small collets by putting a cut through a couple of nuts and screwing these onto the screw thread and hold the nuts in the 3 jaw, also repeated use of split does can ease the threading/cutting load

[kwijibo99]

If its a grade 12.9 screw it's going to be hard to machine however you hold it and if you're using a die be careful not to chip the teeth.
Rather than mess around I would just buy a new cap screw with a full length thread, shouldn't be a problem for 50mm long.
Cheers,
Greg.

[Chris Parks]

What size is the Key? Unless it is smaller than 4mm I can't see a cap screw rounding the head or key if they aren't Chinese crap. Button heads yes, they will strip but not cap screws using a good condition key. Just my experience over many years of using thousands of Allen heads. If you don't have a chuck and have a drill press with a Morse taper chuck, reove the chuck from the drill hold the flat of the morse taper shaft in a vise and use the chuck to hold the bolt.

[shedhappens]

justask have you tried griping one of the handles of your diestock in your vice, with soft jaws or 2 bits of wood, then
screw the cap screw into the die and cut the thread using an allen key and cutting oil or trefolex.
Whether that works or not depends on how hard the bolt is and how good your die is but it might be worth a shot.

cheers, shed

[China]

Some things are just not worth the bother just buy the correct bolts

[Chris Parks]

Some things are just not worth the bother just buy the correct bolts

I agree unless it is a totally unique application but that is very rare. Find a good fastener supplier and enjoy the low prices they generally sell at.

[BobL]

I agree unless it is a totally unique application but that is very rare. Find a good fastener supplier and enjoy the low prices they generally sell at.

I must be an odd one out as I tend to do this quite often.

One reason is, my stash of screws seems to be growing faster than I use them up and long ago ran out of storage space.
The screw stash comes from gear that came out of the skips at work, and where possible, for screws that are >50 mm long, in the past I've usually purchased boxes of screws with short threads as is much easier to add threads than to put the missing threads back on.

But the main reason is that as excellent and well priced as my specialist fastener supplier is, they are 25 minutes away (one way with no traffic - up to an hour or more with traffic) and open 7:30 am to 4 pm M-F.
So unless I need a multiple fasteners it is far quicker for me to add a couple of threads to an existing fastener or two.
I'm sure the same applies to an even greater extent for anyone in the country.

The times I usually don't do it is when screw strength is important

[swarfless]

Hello there -

My question is related to extending existing threads on socket head bolts - Allen Key bolts.

How do you clamp a socket head bolt into a 3 jaw chuck to cut the existing thread longer?
For example, I have a M8x50 bolt and the thread is only 20mm but I need 35mm thread.
I tried to hold it in a bench vice, but that’s not very good, I tried in a 3 jaw chuck on my lathe, but it does not run true and also does damage the head on the bolt. I tried to put some copper around the head of the bolt and also then tried the same with aluminium, but both attempts did not work very well. Any suggestions?
thanks heaps

What's the fuss about? Did exactly what you want 5 days ago with a 65 y.o P&N carbon steel die .. I know coz I was there when my father bought it.
Aluminium angle protectors on the vice jaws, don't be frightened to tighten the vice you can't hurt the cap head if it's reasonable quality BUT take care to 'swallow' the head i.e. position the head down in the mid section of the jaws ( 5" Dawn vice). Use a GOOD cutting compound, I use the Bordo stuff that smells like 1950s hair oil. You can't get cap heads with more thread coz THAT is the specification .. no one makes cap head set screws(British terminology). GO FOR IT!

[BobL]

OP

For example, I have a M8x50 bolt and the thread is only 20mm but I need 35mm thread.

You can't get cap heads with more thread coz THAT is the specification .. no one makes cap head set screws(British terminology). GO FOR IT!

I had a quick look in my stash.
As far as M6 caps go I have plenty of 25 mm that have 25mm of thread, Some that have 30 mm, and lots of CS socket heads with 32 mm of thread.
For M8s the longest threads are 30 mm.

[BaronJ]

Hi Guys,

I don't understand why there is all this fuss about fully threaded cap head screws ! If I want to buy fully threaded ones, I just ask for them ! Admittedly they cost a little more but they are readily available, particularly if you want stainless steel ones.

Otherwise as has been said its easy enough to just add a few threads as needed. I just put a suitable die in the holder and clamp one handle in the vise and thread the screw in using a good fitting Allen key, I've even used my expensive "T" handle ones for smaller screws.