Thanks for the Post, I have the same punch but for the life of me it won't dissemble , it's not a LH thread is it. ThanksIt's now relatively easy to hold the piece and carry out the rest of the operations on it.
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Having taken some time to ensure that I had the drill in the correct position in the X axis, I then realized I didn't have enough height in the Z axis to get my centring attachment in. Rather than fanny around raising the head, and going through the setting again, I eyeballed the drill to achieve centre in the Y axis. It wasn't spot on but, as it happened, it didn't matter.
The finished trunnion
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Time to assemble it for a test
It didn't matter because it didn't b***** work!
After some head scratching, and not a little bit of cursing, plus some measuring too, I realised what was wrong.
The hole being slightly out of centre was not the issue (see above). I'd machined the sloping face and then faced off to what I thought was the correct step length (0.5mm). From that I measured the longitudinal position for the hole. In fact, the critical dimension is the distance from the lower edge of the slope to the hole centre. The small step only locates the end of the spring so could be longer, or even a gnats shorter, and still work.
A quick explanation here. The trunnion sits across the piston and the spring strip sits into the flat of the piston with its edge located on the step of the trunnion. Are you following this?
When fitted in the housing, there is sideways pressure from the spring which pushes the trunnion to one side so that the rounded other end protrudes a small amount. The internal bore of the housing is parallel for a short length and then tapers towards the top of the housing. The large compression spring pushes the piston away from the taper and into the parallel area of the bore.
The cross-drilled hole in the trunnion is now off-centre with the hole which is drilled into the piston end and into which the small spigot on the point locates. The point is also spring loaded in its housing.
When the punch is pressed onto a surface, the lighter spring in the point housing compresses, and allows the point to come into contact with the flat which is milled on the trunnion. This now moves the piston assembly back against the main spring. As this movement increases the spring obviously loads up but, the taper in the main housing now comes into play. The rounded trunnion and nears against the taper and is slid across until the point is reached where the point spigot lines up with the trunnion hole and the piston is released to strike the point housing and create the centre mark. The main housing has a screwed end so that the spring tension can be adjusted to suit the material being marked.
See me after class if you haven't understood my description
Remember I said I'd milled both ends of the rod in the event that something didn't work out?
Well, why use that? Why not go back to attempt No.1 and see if you can work out a way to hold it? It's much better to make a rod for your own back
After some thought I came up with this idea. I also raised the drill head so that I could get my centring tool in this time.
Much more professional, don't you think?
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This time I used the needle point to pick up on the root of the step. I still had to allow 0.25mm for the tip diameter.
Hmm, why isn't the dial gauge moving???
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That's better.
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Turn it over and mill the flat. I picked up some small carbide end mills at Ally Pally last year, a perfect opportunity to try one out. At this stage the limitations of the cheap Chinese vice became obvious, the moving jaw lifts as you tighten. It was virtually impossible to guarantee the position of the trunnion to get the hole axis square true to the spindle. I got it close enough to have a bash and hey, I've always got the other spare...
I suppose I should have got my large Elliott vice out form the pile and used that.
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The finished item, hardened and tempered. It looks okay and the hole is decidedly off-centre when fitted to the piston.
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Time to reassemble it and test, I suppose
RESULT!!!
It only took me 7 hours to do