Brown and Sharpe 252 Adjustment

[John CParticipant @johnc47954]

I have acquired a Brown and Sharpe 252 internal micrometer. This is a very nice item, very smooth and in good condition. However, I can see no way of adjusting or calibrating the device. There is no hole for a c spanner to rotate the lubber line, as on similar micrometers. There is what appears to be a stamped figure Z or 2 adjacent to the fixed anvil, and a screw with collar on the end of the barrel. The moving anvil is a good fit on its shaft, secured with a grub screw, but with no obvious sensitive adjustment.

Does anyone have experience of this model that can point me in the right direction to correctly zero the reading?

 

 

Many thanks,

John

Edited By John C on 10/10/2017 10:06:05

[John CParticipant @johnc47954]

[Clive FosterParticipant @clivefoster55965]

Is yours a fixed or friction thimble? On older Brown & Sharpe micrometers the sleeve is fixed so the thimble is moved for adjustment.

Asked many years ago in case I had to do my depth micrometer but have never needed to adjust it so no practical experience.

Rounded endcap is probably only finger tight. Once off a slotted screwdriver bit could be used to loosen the ring that actually holds the thimble.

Movement on mine is wonderfully silky smooth. Best of any micrometer I've used including new "proper" M&W, Starrett and Tesa.

Clive.

[John CParticipant @johnc47954]

It's a fixed thimble, Clive. I shall have a go at removing the screw. Many thanks for your input,

John

[John CParticipant @johnc47954]

Hmmmmm, curiouser and curiouser……

The thread inside the graduated thimble engages with the thread in the body with the fixed anvil. So their relationship is fixed. There appears no way to turn the lubber line relative to the thread in the body. The rod that moves within the body is driven by the shoulder against the inside of the graduated thimble – again this relationship is fixed. The hole at the end of the thimble is a sliding fit on the narrow portion of the graduated rod. The threaded washer and dome nut merely adjust the backlash. So unless I am missing the blindingly obvious, I can see no way of setting the mic except altering the position of the moving anvil on the rod – which seems a bit crude!

Any further suggestions gratefully received.

John

[Clive FosterParticipant @clivefoster55965]

John

That seems very odd especially as Brown & Sharpe had at least two patents concerning thimble adjustment viz :-

**LINK**

and

**LINK**

Some pictures close to the bottom of the page here :-    **LINK**    of one version.

So far as I can see that one has a slotted flange on the end of the thread driven by a collar carrying a mating blade. The collar is presumably a slip fit in the thimble. I assume the centre rod has a flange on which the collar sits and the thimble is held tight against the collar by an external "nut".

Yours appears to have a suitable slot in the flange at the end of the screw so perhaps the supposedly slip fitting and rotatable collar has taken up permanent residence in the end of the thimble. No idea how you'd coax it out without damage tho'.

Clive

Edited By Clive Foster on 10/10/2017 19:44:48

[John CParticipant @johnc47954]

Thanks again Clive. Having reassembled the thing, it now reads almost spot on at the thimble but 0.5mm high on the rod! The thread is 0.5mm pitch which may have a bearing on this, but I have yet to see how, apart from a huge coincidence.

More thinking required…….

[Clive FosterParticipant @clivefoster55965]

Hmmn. 0.25 mm change I could understand as that would correspond to a half turn change in the relative position of the slot on the end of the screw and the corresponding blade on the collar stuck inside the thimble.

I'd make a slot ended screwdriver to fit the blade on the collar by copying the one on the end of the screw onto a suitable piece of steel rod. Rod needs to be reasonably close running fit in the thimble so the slot and blade sty well lined up. Up to you whether you do a nice handle to help turn it, just knurl the end for finger grip, put a small hex on for a spanner or weld a broken hex drive screwdriver tip on so it fits a standard handle. I'd probably cut a 1/4" hex on the end to sit in a standard handle. But I'm set up to do hexes quite quickly.

See if it will turn, you might be lucky, and possibly work its way out. If not its down to the usual Plus Gas, heat, wiggle repeat treatment. Helps to bung the hole in the end up so the thimble can be stood up with the collar submerged. I've known this sort of thing take a week, or more, before something comes free. Daily changes of Plus Gas and tries of the wiggle or wiggle and heart process.

Clive

[John CParticipant @johnc47954]

OK, Clive, I'll give that a try. Thanks again!

John

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