Having trouble turning grooves

You can take that blade out of the holder and put it straight in the toolpost, thin strip of metal between blade and screw will help

.

Steve,

Going by your photo ... Yes that is a metric screw: The proportions of the heads differ noticeably between Metric and Imperial.

If it's M3 then the socket should be 2.5mm a/f ... So I guess you were using a 3/32" a/f wrench.

Buy some better quality screws

MichaelG.

Edited By Michael Gilligan on 05/08/2018 16:33:49

Edited By Frances IoM on 05/08/2018 16:38:45

0.5 pitch in my book for M3

Could also be UNC or even Whit as you often find odd sizes and poor screws on these type of holders

Edited By Steve Sherlock on 05/08/2018 16:48:28

> It left me with a donut shaped cap with the threaded part all in one peice

Sounds like the socket was too deep.

The problem with going upto an M4 fixing is that the shank is only 6mm or 1/4" square so there is not a lot of metal left, having snapped two of these holders across the screw whole I should know

.

Now I'm confused ... The head-to-thread proportion of the screws in your photo definitely looks right for metric.

If they are M3, then the head diameter should be nominally 5.5mm and the socket should take a 2.5mm key

Here is a proper one: **LINK**

https://www.westfieldfasteners.co.uk/BO_ScrewBolt_Unbrako_SHCap_M3.html

If they are Imperial [probably #4] then a 3/32" key is correct, but the head should be smaller diameter

**LINK**

https://www.westfieldfasteners.co.uk/A2_ScrewBolt_SHCap_UNCoarse_0.1120.html

For the sake of my sanity, could you please check the diameter of the head ?

Thanks

MichaelG.

Just checked Michael and the head is showing 5.4mm on my callipers here are some better photos.

On my to-do list is to make up a small version of Tubal Cain's (the English one) Gibraltar toolpost for my Perris/Cowells. Why? The top slide is always in the way unless you angle it a bit when it doesn't support the tool all that well. If I want to turn tapers I can very quickly put the topslide back on

They Look Great Duncan, Never heard of them before. Thanks for that. I shall look into that a little more

I'm confused myself to be honest. I always have a hard time deciphering fasteners. The thread actual threaded part is measuring 3mm with my calipers (I should add they are not fancy mitutoyo jobbies) which i suspect is a bit big for a m3 at (2.9mm ish)?

The only thread gauge I have is a whitworth and the closest if not exact match on that is 40 / 1/8"

do they in reality come out a tad smaller like the metric versions, or does 1/8" at a nominal 3.175mm sound too big?

They would usually come out a bit smaller so 1/8 x 40 whit or #5-40UNC are likely, not easy to measure thread angle at that size.

.

Thanks, Steve ... That all supports my original impression

It's a rather nasty approximation of a Metric cap-head screw [*]

... relieved to see that my 'eye is still in'

MichaelG.

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Of course, I have no idea why your 2.5mm key doesn't fit the socket.

.

[*] Edit, for Jason : Or a 5-40 with an oversized head. 

Edited By Michael Gilligan on 05/08/2018 20:35:14

Because it's more likely to be imperial at 40tpi which is a long way from the approx 51tpi of M3 and a 3/32" key fits

Edited By Frances IoM on 05/08/2018 20:34:30

Edited By Frances IoM on 05/08/2018 20:54:03

Rough calibration, based on Steve's 5.4mm measurement:

.

MichaelG.

With the eye of faith [and a large Vodka & Tonic] ...

The thread angle looks more like BA than anything else.

'Mongrel' is the word that springs to mind

MichaelG.

Steve,

On further contemplation; I think that broken screw is the proverbial 'red herring'

May I suggest that you concentrate on identifying the tapped hole instead ?

It should be reasonably easy to screw a plastic rod of suitable diameter [maybe a knitting needle] into the hole and take an impression of the thread pitch.

MichaelG.

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P.S. ... This is a useful reference document:

https://www.socket-allied.com/files/6414/7799/4114/SAS_Technical_Guide.pdf

Edited By Michael Gilligan on 06/08/2018 09:16:45

Thanks Michael - I found that inked document useful too. It can live alongside my Zeus Tables!

It amazing how often I encounter bolts with an unfamiliar thread.

John