attaching a small high torque shelf to a plaster wall

I need to attach what would look like a small shelf (2"a x 3"b x 4"hyp): to a plaster wall. It is to support a magnifying lamp with a clamp-on pivot arm. The lamps weight is a bit of a beast. The shelf itself is not a problem, I can make it out of 3/4" plywood, but I do not have experience with this type of wall. It is not drywall, its the kind of very hard to knock on stuff, from 70 yrs ago. The small shelf is a <||>, turned 90 degrees, for a beast of a clamp-on pivot arm to the || shelf. A small horizontal shelf dadoed into vertical pieces bolted into an inside corner wall will withstand the torque stresses. I have a 1/2" hammer drill and I will need to drill into the normal wall (inside corner) both above and below the shelf at right angles so the clamp-on pivot arm torque forces on the shelf will be pushing on the shear of the four fasteners. The distance from the adjacent wall will have to be the width of the 1/2 profile of the hammer drill (maybe 2" from the inside corners, about 27" up a 8' high wall). I have a feeling I will need to build a jig for the holes, probably with plywood with washers hammered in spaced spade drill holes, so the masonry bit doesn't jump all over the place. So I will be using masonry bits, but since I have never attempted such a hold, not drilling in this type of wall, I don't know what type of fasteners to use, or if I need to worry about drilling out with HSS-type regular-drill for lath or whatever might be in there from before I was born.
So the question is: What's in there & how do I drill it out (several drills, drill holes in wood shelf after?)? What kind of fasteners do I use through 3/4" plywood & into wall (lag bolts, expanding bolts)?
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I don't understand much of what you wrote but plaster walls will have wood lathing behind it. (strips of wood that the plaster was applied to) You won't need a hammer drill. If you're going to use a mansonry bit it will also go through the lathing. You won't need a jig. Lag bolts will not work. I like to use the type that spring open (can't think of what they're called right now) after you push them through the wall. If the plaster wall is in good condition it will withstand a lot of stress/torque. Also, you might want to hunt up a stud to screw directly into.
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bent says...

Think moly bolts, they are designed for plastered walls.
Lew
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You never said on what the "plaster wall" was supported, though you seem to assume masonry.
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MichaelB
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I am assuming that he thinks he needs a hammer drill with masonry bits to get through the "very hard" plaster and then possibly switch to an HSS bit to get through the lathing strips or "whatever might be in there."
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If it's a frame wall, regular bits will do it, but he has to find the studs. Look for electrical boxes (attached to studs) or for nails in baseboards, which will also indicate stud locations.
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Drilling into a plaster wall isn't difficult. Just use a normal drill and a regular steel drill bit. Plaster is harder than drywall, but it's still quite soft compared to steel or stone.

Mark your holes, and start with a small drill bit. Then switch to the full size bit, or work your way up to the full size with intermediate sized bits.

If this is a normal plaster over wood lath wall, I would attempt to find a stud and use screws long enough to go through the shelf, the plaster, the lath, and into the stud (a 2-1/2" to 3" screw should be fine).
An electronic stud finder works OK with plaster walls too, just double and triple check in multiple places because the varying thickness of the plaster can sometimes give false positives.
If there isn't a stud where you need to insert a fastener, use the spring loaded moly type bolts. Drill a small hole through the shelf and wall, then take down the shelf and widen the hole in the wall so it's big enough to pass the moly through. Assemble the bolts on the shelf, then holding the shelf to the wall, carefully push the moly's through the holes. They'll spring out on the back side, and you can pull them tight against the wall while you tighten the screw. If it's a heavy shelf, you might need to get some help with that.
If the plaster is over a masonry wall (concrete or brick), you'll probably need to drill the holes with a masonry bit (start small, work your way to the needed size), then use expanding masonry bolts.
Hope this helps,
Anthony
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