Monday, 1 October 2012

Broken grub screw repair

A couple of weeks ago we had another one of those rainy summer days, here at Laughton Towers so all those outdoor jobs were on hold. I couldn’t paint the windows or build steps, couldn’t even mow the lawn (although Sue usually does that when the ground staff are busy) so with heavy heart I had to spend the afternoon messing about in the workshop.

Ready for action!
 I decided to have a quick steam up just to get the right atmosphere and filled the boiler of my new (ish) to me Mamod stationary engine - great fun! When it came to the cleaning up I realised the grub screw was broken and I could not get any purchase on it - Grrr! I suspect it had been there for a while and was well and truly stuck especially as one of the cheeks of the slot had given way. This is often the case when the grub screw it over tightened in an attempt to secure, in this instance, the output pulley to the shaft. I have often seen this with Meccano models. The following is equally valid for any grub screw that has been damaged in this way. 

A small blow torch was used to heat the grub screw and the boss and allowed to cool in air. This will do two things: the heating may help to loosen the screw as the boss will expand at a different rate to the grub screw, breaking any mechanical bond. The heating an slow cooling will also soften the grub screw slightly making it much easier to drill.

A nice clean hole in the damaged grub screw
A centre punch was used to ‘pop’ the top of the grub screw. I then bored a hole about a 1/16 inch deep as close to the centre of the grub screw as possible. small scrw extractors can be used here but they are very hard and can easily snap off at the tip if any sideways force is applied. I decided to go for a less conventional solution and use the taper of a nial punch set into the hole. 
A good tight fit and...
...and out is comes
After sorting through my selection of nail punches (wonderfully useful tools) I found one that was a tight fit in the hole. A single firm tap (not too hard!) with a hammer is all that is required to set the tip of the nail punch tightly into the hole I had made in the grub screw. I could now undo the damaged grub screw by rotating the punch. I replaced the old grub screw with a steal hex socket grub-screw (5/32WW - modern Meccano).

All done!
 Problem solved and ready for another steam up,another day. It never did stop raining so I was stuck in the workshop all day - and I still have not got the painting done but nobody will know...


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