Re: adjustable square for SHS / box sections

"Richard Smith"  wrote in message news:m1y0yhas2t.fsf@void.com...
Hi all
Working with
Rectangular Hollow Sections
Square Hollow Sections
box sections
...
The rounded corners make using an adjustable square / sliding square
problematic.
Given number of machine parts I have to make quite accurately based on
SHS's, thinking of a tool specially optimised for marking distances
from the surfaces.
ie. whatever presents as "top", slide along marking down the "sides".
Typically the middle of the "sides".
Accuracy - well using a chalk / soapstone is plenty good enough.
Have to bring magnetic-base drill to the job most of the time, and
with using a shell-cutter / trepanning cutter with a central "pin" to
locate on the centre-pop - accuracy there is less than chalking gives.
So - would hold thing in place and chalk-mark.
I could trim offcuts of angle-iron to make single-dimension tools -
eg. marks 25mm from "top" surface.
Advantage with this or "settable" tool is spin around 180deg and mark
the other "side" and the two heights/levels should match-up.
Hope described this well enough.
------------------------
Compact versions of a carpenter's bulky framing square are called "L Squares".
I use a double or combination (90+45) square with the scale protruding the required distance and scribe across the end. It can be held vertical to bridge the rounded corner. This type and size is pocketable.
https://www.globalindustrial.com/p/igg-high-precision-double-square-w-4-inch-steel-blade-black?
For higher accuracy they can be set with dial calipers. 4" ones are fairly safe in a shirt pocket.
https://www.penntoolco.com/precise-white-face-dial-caliper-0-4-inch-range-spt-004/?
Marking the presumed center from both sides gives two closely spaced lines straddling the true center. At that point I guess and punch it.
A "prick" punch can locate the crossing of fine scribed lines to 0.1mm or so with care and a magnifier, or by feel along the line. Then "center" punch the dimple to better locate the drill bit. These make a good starting hole for larger bits.
https://www.amazon.com/Pieces-Center-60-Degree-Countersink-Metalworking/dp/B08ZYXXX1Y/ref=asc_df_B08ZYXXX1Y?
Long center drills transfer accurate hole locations to the other side of hollow sections.
https://drillsandcutters.com/5x6-extra-long-combined-drill-bit-and-countersink-qualtech/?
My scriber is a broken tap ground to a fine point, pressed into a hex standoff handle and covered with heatshrink for grip. The HSS is less brittle and easier to resharpen than carbide. In aluminium or plastic it doubles as the prick punch.
A light weight portable drilling guide like this lets a drill bit (or pointed tap) center itself on a punch mark. I don't own a mag base drill and clamp the guide to the beam to drill the full sized hole square, well enough for 3/8" bolts in 0.375" holes through both flanges of 4" channel. The bolt shanks measured 0.372". That was the gantry trolley beam center splice assembly.
https://www.amazon.com/Milescraft-1318-DrillMate-Drill-Guide/dp/B014A1Z92I/ref=asc_df_B014A1Z92I?
Machinists working with clean metal use bluing to make scribed lines more visible. Sticky paper labels work pretty well for pencil or scribed lines on wire-brushed rusty steel and surfaces that shouldn't be scratched. I save my dropped or otherwise less dependable calipers to use as scribers.
The cast or molded enclosures I mount snap-in meters in are too flexible in the milling vise to mill accurately with a DRO, scribing locates features on the uncompressed surfaces.
When working on large stock outdoors I keep my tools safe and handy on a small wheeled cart or folding camp table. This is my most heat-resistant and convenient to carry with one spare finger.
https://www.walmart.com/ip/Ozark-Trail-Aluminum-Camping-Table-Silver/531930310?
jsw