Gallery, Projects and General > How do I??
How do I measure approx 60" ACCURATELY ?
awemawson:
Phil, it’s a ‘chicken and the egg’ sort of thing.
Yes I could turn it on the Cnc lathe, but my only ‘standard’ is the recess in the back of the chuck that would be holding it. OK if it’s spot on first time but zero opportunity for adjusting later as the part has to be removed from the chuck to remove the chuck, and being a three jaw cannot be tweaked back into truth.
By grinding, not only do I get the far superior finish, but I can try the taper still on its arbor in the removed chuck with blue, and remount the arbor spot on if I need to adjust, this way I can ‘creep up’ on engagement depth.
I’m pretty certain it doesn’t need to be as exact as I’m aiming for, but that’s just me being obsessive!
Also of course I’m trying to solve the ‘general case’ for future use of the grinder.
awemawson:
So i was determined to push this a bit further today.
I realised that J&S have engraved a nice angle scale on the table for setting up, and presumably their engraving has some mathematical foundation. If I could measure the 'chord length' between angular graduations for a given angle I could calculate the radius !
OK due to vernier caliper limitations the largest angle I could reasonably measure was the '10 degree included' one, and plugging the figures into an online chord calculator gave me a radius length of 26.44 inches. To this needs to be added a further 1.31 inches to get to my place where I measure the displacement giving a radius of 27.75 inches
Is this a correct figure :scratch: I have no idea but it must be closer than the assumed previous thirty inches !
Meanwhile I got on grinding the parallel part of the taper gauge, covered over in the other thread.
awemawson:
The plot thickens!
Perhaps foolishly I decided to do a rough and ready table swing radius test using my Disto D3 laser measure. I clamped some flat bars to either end of the table, and measured the distance between them with the laser. Then measuring 'end corrections' of 49.80 mm and 51.54 mm from the flat bars to where I'm actually measuring displacement and applying them I got . . .
29.46" radius
So I now have values of 30" (assumed) 27.75" (calculated) and 29.46" (sort of measured)
That's quite a spread!
So what is the implication of these figures - well the displacement that I need to set to achieve 7 degrees 7 minutes and 30 seconds is :
27.75 gives 3.4486 displacement
29.46 gives 3.6611 displacement
30.00 gives 3.7282 displacement
I think' the 27.75 value can be discounted - it's method of calculation is rather dubious
But that still leaves a displacement spread of 67 thou in a measurement that I try to set to tenths of a thou !
. . .argh lets run round like lost sheep :bugeye:
(oh two more of the blighters born this morning)
vtsteam:
Andrew don't you have a manual lathe that you could cut the taper with?
vtsteam:
Also another thing that I don't understand is, if the tapered part is only a few inches long why is it necessary to essentially use a 60" long standard?
Can't you make up a simple gauge say 12" long at the angle you want accurately enough to use that to set the table against?
Well as usual, not being there I'm probably not picturing the machine(s) and true situation, so forgive the naivete.
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