Re: adjustable square for SHS / box sections

"Richard Smith"  wrote in message news:m1zfivdmyu.fsf@void.com...
Hello everyone
Your various ideas caused a thought to "gel".
I will try this.
Get a smooth piece of plate a bit wider than the box-section.
Place your "ordinary" adjustable square on it, at the edge.
Make the blade/rule of the adjustable square protrude beneath the plate
lower surface by the amount you want to mark-off - eg. 25mm.
Bring "free" plate's edge to align with "side" surface of box you want
to mark, with the already-set adjustable square on it, and with hand
pressure keeping eveything touching where it should, slide along with
chalk under tip of adjustable-square blade.
Spin plate with adjustable square through 180deg and do same marking
down other side.
Those "heights" on the two "sides" will agree - probably the most
important thing.
Mark distances from the end of the box at which holes are and centre-pop
at the intersections.
Drill.
Done.
Hope this makes sense...
Will see whether this works - and report back, volente dio.
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It should, you've given yourself a reference surface.
Depending on desired accuracy and repeatability (Chalk??) you could fasten a 50mm strip of the plate to the vertical on the square with double stick tape.
A possibly less fumbly approach would be to use hermaphrodite calipers to scribe from the plate's inner surface, which permits direct length transfer and saves an element of alignment.
https://www.kbctools.com/itemdetail/1-804-457?gQT=1
Pointed dividers would work, hermaphrodites folded shut are safer in your pocket. If the tool is stainless you could use a magnet as the plate.
If you mostly need the center this should help.
https://www.grizzly.com/products/grizzly-center-finding-ruler-set-3-pc./t34082?
A good trick for dividing off-sized stock in half, thirds, quarters etc is to angle a ruler (scale) across the width of the stock with the ruler's ends (or other marks) at the edges. If for instance you place a 6" rule on 4" stock with its ends at the edges, the center of the stock will be at 3". 1/3 and 2/3 will be at 2" and 4". Remember to transfer the center parallel to the edges, not square to the ruler.
I center holes in mounting brackets etc by balancing a 6" ruler across stock in the milling vise and equalizing the edge readings from 3", like the center finding ruler but counting backwards on the 6" end. This gets me centered within a millimeter or better on rough edged or compressible stock. As with the previous trick the ruler doesn't have to be exactly squared.
A primitive but effective way is to roughly center a pencil point near the center with your little finger sliding on the edge of the stock, then repeat from the other side. You can refine the guess between the two lines and repeat. Chalk is good for circling and not losing a location with finer marks, like a fine point Sharpie.
I'll add here that placing a ruler marked to 0.1" on the stock lets me locate from the 0,0 reference corner with the mill's table dials without counting turns.
jsw