Lay the steel plate on the concrete where you want it. Draw a line around it. Cut a piece of paper to the same size as the steel plate. Place it on the concrete in the same position as the marked outline. Locate the concrete holes by gently pressing down where you think they are (tracing paper makes this even easier). Poke a hole through the paper at the center of each hole. Place the paper on top of the steel plate, and mark the centers of the holes.
The usual methods are:
Careful measurement. Really this doesn't need to be that precise, you are not looking for a press-fit between bolts and clearance holes.
If it goes wrong, just elongate a hole into a slot using whatever tools you have to hand (e.g. a round file, clapped-out old Bridgeport, ...) - Remember: "A grinder file/filler and paint make me the welder machinist I ain't".
Put pointy or painty things into the holes then press the steel plate down in place to mark it. In the dead-tree-carcass world there are things called "dowel pins" used for this kind of thing.
The hole spacing is more important than the location of the holes on the metal plate. Therefore, working off of option 1 from @RedGrittyBrick, this is the approach I would take using a wax pencil or marker and a carpenter's square.
(1) On one edge of the plate mark a (rough) centerline.
(2) Align this edge of the plate to the holes in the concrete, with the centerline mark (roughly) centered between the holes, and carefully mark the center of each hole on the edge of the plate. You should now have 3 marks on the edge of the plate.
(3) Carefully transfer the outer marks as two lines across the steel plate using a square.
(4) Mark a line perpendicular to these two lines and (roughly) on center on the plate. You should have three lines on the steel plate. The intersection of the three lines marks the center of the two holes to be drilled.
Repeat the entire procedure if you have multiple pre-drilled locations. Don't assume that other holes in other places in the concrete are equally spaced or centered.
If you have a laserpointer (or 2) "hang them" (they should not move around of course) above if possible so they point down vertically into the center of the hole(s). Put the steelplate in position and the laser marks the spot(s).
I like Mark's answer best (create a paper template and mark the holes) but another technique came to mind.
Take a straightedge and a construction pencil.
- Mark the centerline of the two holes longer than your steel plate.
- Mark a line perpendicular, on each hole, wider than your plate.
- Put the plate down, use your straightedge and the marks to locate the hole centers.
[hmm. similar but different to Stanwoods solution which I did not see at first]
Since nobody else has mentioned it, I'll add another option that I've used in the past for spacing holes.
Put some paint on the floor around the holes.
While the paint is still wet, place the metal plate on the floor.
When you remove the metal plate from the floor, the wet paint will have transferred to the plate but will leave two spots behind where the holes were. Mark those spots, clean up the wet paint, and drill.
This also works great for spacing screw holes for handles on cabinets & drawers although I would usually use lipstick for those instead of paint.
Do you really need to use the holes drilled in the concrete?
If not, pick another pair to be drilled far enough from the original ones. Make the hoes in the metal sheet first, put the sheet in position, secure it from movement and mark the spots or drill through the holes in the concrete.
Another point to be taken is how much the holes shall be coaxial. If the misfit can be in order of milimeters you can measure the position with a scale. If it must fit perfectly, two extra holes are a good tradeoff.
An alternative to Mark's idea would be to use plexiglass. Lay that down and mark either the center of the holes or the holes themselves. Drill through the plexiglass, make sure the holes are aligned properly, and then use that as a template to drill holes in the steel.
I would have added this as a comment under his answer but I don't have 50 reputation yet.
You need something with a slot in it, the same width as the bolts intended to go in the holes. Pass one bolt through, run a nut down and finger-tighten in place. Pass other bolt through, run a nut down it and tighten until you can slide the bolt easily along the slot. Now you have a gauge. Put your fixed bolt in one hole, slide the other along a slot until the bolt drops into the other hole. Tighten the nut with your fingers. Now the bolts are the correct distance apart.
Remove gauge from the concrete and use it indicate where to drill on the steel.
Assuming you need the steel to be located precisely, here's an idea: get two bits of threaded rod (or any steel) the same length as the holes + a smidgin, so it pokes up a few mm. Grind one end sharp, trying to get the sharp point in the centre. Put them in the holes and locate your steel plate where it needs to be. Give it a whack with a hammer. Now you have your centres to drill.
Tape one side of a piece of paper onto the concrete and trace the holes, then put the steel plate under the piece of paper and center punch the holes onto the steel plate.
