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How do I measure approx 60" ACCURATELY ?

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awemawson:
My Jones and Shipman 1300 EIUR cylindrical grinder spec says that it's  swivelling table is 60" long.

Now I need more information than that as I am measuring linear displacement of the table end to set a very accurate angle. The pivot point of the upper table is totally obscured but I think that a fair assumption is that it is dead in the centre of the table. The ends of the table are radiused, but again obscured by the table clamps but I think that I can project a tangent from these radiused ends forwards past all the obstructions using steel rulers and set squares to measure between them to get the length at the point that I am making my linear displacement.

So now I (theoretically) have two steel rulers projecting into free space and I need to know the distance between them - what options do I have:

A/ Tape measure - I'm sure that this isn't accurate enough - my displacement is to  tenths of a thou !

B/ Using my Leica Disto laser measurer, but it's only accurate to 1 mm

C/ Length Bars - I don't have any and certainly don't have enough Jo Blocks to stack up to 60 inches !

Now the accuracy of the 60" leg of my triangle can be less than the displacement (about 3.5") by swivelling the table by the ratio of their lengths and keep the same overall accuracy (I think) but any geniuses out there do please comment !

. . .all suggestions welcomed  :thumbup:


(best guess so far using a carpenters steel tape gives about 58.75" )

BillTodd:
Are trying to set the angle of the table relative to the bed , rather than the angle of the work piece to the bed?

Sounds like a job for a good sized sine-bar jo blocks and an indicator.

Not sure why you're talking about steel rules?

Ok re-reading a couple of times...for  some reason,  you are trying to measure the angle of the table.

I would rig up a straight edge along the T slot  (to use the slot as reference)  setup a sinebar with something like adjustable parallels as the spacer ( you need something adjustable and measurable with a mic)  set an indicator on the head an sweep the bar , adjusting the parallel until sweet, then measure the parallels and whatever blocks to get the the value to plug into the appropriate bit of trig for your length of sinebar.

Pete.:
Why not reverse-engineer the solution? Set a sine table to a known shallow angle using Jo blocks and put the base of the sine table against the side of the grinder table. Rotate the table until a tenths-reading (or better) dial reads 0-0-0 on the sine table as you traverse the grinder table and measure the amount of displacement at the point of measurement. From that you should be able to extrapolate the leg length from the pivot to the measurement point with at least as much accuracy as that system of measurement would provide from a known length.

The only difficult bit is that you won't be measuring from the actual pivot but off-set from it by half the table width. Just thinking about the trig for that is giving me a headache.

awemawson:
I’ll put pictures up tomorrow but basically I am using the machine as a sine bar with an approximately 30 inch long leg (half table length) with the jo blocks replaced by a nifty long travel displacement gauge that indicates to tenths of a tenth.

My issue is measuring accurately the “long leg” ie the equivalent of the spacing of the sine bar rollers. Clamping a far shorter sine bar to the ways will give me far less accuracy.

(The angle that I need to set is 7 degrees 7 minutes and 30 seconds)

For years I’ve used the approximate  value of 30 inches but for this it is not accurate enough.

Pete.:
I think that the problem is going to lie with the fact that your corner of the triangle that you're intending to use for trig calculations is moving with the table, given that it's not on the pivot point but on the front edge of the table adjacent to the pivot.

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