What is this bolt?!

LancsRick · 20 May 2016

Just finishing setting up the bowl turning attachment on my Coronet, and I've discovered I'm a bolt short. Thankfully I have one of them so I know what I'm after for the second one, but I'm not doing well at establishing what it is!!

It's a square head bolt.
The head is 25.5mm square (i.e. 1"), and 5mm deep.
The thread itself is 39mm long, with a major diameter of 12.7mm, and a thread pitch somewhere in the region of 1.6mm(ish).

I think that means this is a 1/2" coarse UNC, but I'll be damned if I can find anything with a suitable square head to run in the t-slot!!! Any suggestions what I should be searching for?

Thanks.

bugbear · 20 May 2016

LancsRick":4rrw2x5o said:
Just finishing setting up the bowl turning attachment on my Coronet, and I've discovered I'm a bolt short. Thankfully I have one of them so I know what I'm after for the second one, but I'm not doing well at establishing what it is!!

It's a square head bolt.
The head is 25.5mm square (i.e. 1"), and 5mm deep.
The thread itself is 39mm long, with a major diameter of 12.7mm, and a thread pitch somewhere in the region of 1.6mm(ish).

I think that means this is a 1/2" coarse UNC, but I'll be damned if I can find anything with a suitable square head to run in the t-slot!!! Any suggestions what I should be searching for?

Thanks.

Be very careful. 1/2" is famously (depending on the circles you move in) the one point where Whitworth and UNC are different.

1/2" UNC is 13 TPI, 1/2" Whitworth is 12 TPI.

My prejudice, on a machine as English as your Coronet, would lead me to suspect Whitworth, but I would check carefully.

Bugbear

LancsRick · 20 May 2016

Thanks for that counsel. I've dropped Derek Pyatt an email to see if he can advise/supply.

Cheshirechappie · 20 May 2016

The 1.6mm pitch is (near as dammit) 1/16", suggesting 16tpi. A thread of 1/2" OD by 16tpi is 1/2" BSF.

1/2" BSW is 12tpi, and 1/2" UNC is 13tpi (as BB mentioned above). 1/2" UNF is 20tpi. Thus, there's a considerable disparity between the pitches of 1/2" bolts to different standards, so it would be as well to test the pitch carefully. With no access to a thread pitch gauge, use a good steel rule, lay it against the thread, and count the number of complete threadforms against one inch length - note - NOT the number of thread peaks!

LancsRick · 20 May 2016

Joyous. I might just replace the whole lot with equivalent metric ones if I can get something to run in the t-slot, or if not then I'll machine up some t-blocks to be drilled and tapped, and bolt it into the runner that way around.

Bloody imperial!!!

bugbear · 20 May 2016

LancsRick":32xd8v2t said:
Joyous. I might just replace the whole lot with equivalent metric ones if I can get something to run in the t-slot, or if not then I'll machine up some t-blocks to be drilled and tapped, and bolt it into the runner that way around.

Bloody imperial!!!

A quick glance at my chart shows that for 12mm metric, pitches of 1.75, 1, 1.25, 1.5 are all "standard".

I don't see metric as a solution to this problem...

(and to CC - good spot on the pitch, I was lazy and just ref'd the OD

To complete the list:
UNC 1/2 x 13
UNF 1/2 x 20
UNEF 1/2 x 28
BSW 1/2 x 12
BSF 1/2 x 16
)

Oh - another way to measure small things like threads is a clear hires photograph with a ruler in shot, so that you can pixel count, or simply zoom in for magnification.

BugBear