The sawzall and similar power tools have teeth that cut when pulled toward the saw. That action pulls chips, chunks, and in the case of wood, splinters, out from the exposed visible surface. For the most delicate, I use one of the small "compass" saws with a metal cutting blade. Fine tooth with the teeth pointing toward the surface being cut and the saw cutting when pushed.
Test the Dremel in a closet or other easily patched location. Plaster dulls cutting edges quickly.

Here is a link that might be useful: Saws

"The sawzall and similar power tools have teeth that cut when pulled toward the saw. "

With the correct blade (and you will need a lot of blades) and technique a Sawzall will do fine, and is arguably better sine it pulls on the wood lathe instead of pushing on it.

Use a relatively fine blade, and learn how to start it blind without any type of hole.
A short blade (around 5-6 inches) and low speed work best.
Cut the lone sides of the opening first, then using just the tip of the blade turn the corners to the short sides, then go back to clean out the curve cutting in the in the opposite direction as the initial curved cut.

Watch out for screws that fasten device boxes together.

You may need to nick the corners for clearance for them.

You can use new work boxes in many cases by putting the cable clamp on the cable first, then using the un-jacketed wires to pull the clamp into the box after the box is actually inserted into the hole in the wall.

The edges of the hole must be within 1/8 inch of the box walls or you need to add some filler.
The cover plate hides the actual cut edges nicely.

I would never recommend using a sawzall on a plaster lath wall,if the blade jams you risk destroying a good part of the wall (have seen it happen), get a good wood cutting bit or two for your Dremel, go slowly(but high speed on the Dremel), the tape might be a good idea if you want to be meticulous but i have never used any, if a piece of the wall chips off with the Dremel the cover or fixture/plus/switch is usually enough to cover it.

Good luck.

And if you use a high sped rotary tool you risk overheated wood lathe and starting a fires.

There is no reason a correctly used Sawzal will jamb and cause excessive damage.

Not forcing the tool and using sharp blades is a large part of cutting thorough things like plaster with minimal extraneous damage.

RotoSaws work very well on 2 coat plaster with gypsum lathe.
You can set the cutter to barely shy of the plaster thickness and cut out neat holes for junction boxes, ad even access holes for pulling when needed.
The piece remove can then be used to make the repair.

I agree with brick on this one. Actually, my tool of choice these days is a different Milwaukee product. They have cordless thing called a Hackzall which has sufficent strenth to cut through plaster/lath but is a lot lighter and easy to control.

Real plaster over lath is going to be hell on a rotary tool bit. I've seen the bit get red hot trying to plunge into just the plaster. God help you if you've got metal lath underneath.

It is the light touch with the cutting tool that does the job.

You might as well purchase a ten pack of blades before starting.

Plaster grinds the edge off very quickly.

A blade per device box hole (about 2in x 3 inches) would not be out of line.
If you use short blade you can keep the cutting shoe against the surface of the plaster and prevent excess flexing.

I have used blades broken off to just past the depth of the wall to be cut.
A longer blade is used to start the cut blind, then switch to the shortened blade.

Thanks all for your helpful comments. Doing the job on May 27, will let you know how it turns out. Brother-in-law is an industrial electrician, he's doing the wiring but leery of cutting the plaster - not his area of expertise.
I did use a Sawzall on a previous cut and lost a big chunk outside the cutout. But I might have been too forceful in cutting. Someone else suggested a fine hacksaw blade in an open-end holder. Probably will try that first, then try the sawzall if that doesn't work well. Probably use the Dremel as a last resort due to the risk of hot pieces falling in the wall. Fortunately, the plaster is in good condition.

I haven't been here for a long while but I would like to offer some help on how to start the opening and install the box.

With a thin screwdriver puncture the plaster and find a gap between two pieces of lath.

Plan on making the center of your box approximately five-eighths of an inch above or below the puncture hole.

Make sure that the nearest stud is more than one-half inch from the box you will install, and that there is nothing inside the wall that prevent the box from going in all the way (plumbing or heating stuff or an electrical box on the opposite side of the wall).

You will have to cut some or all of three pieces of lath to install the box... all of the middle lath, the bottom half of the top lath and the top half of the bottom lath.

The plaster must be cut higher and lower than the lath to accommodate the box ears.

Know that the ears on old work boxes are adjustible and reversible. Set the proper depth with the ears and secure with small (I think #6) flathead Phillips screws (one in the top ear and one in the bottom ear not the same side as the top).

The screws must be threaded all the way to the head, if they have a smooth area just below the head they won't pull tight to the lath.

DO NOT USE DRY WALL SCREWS

#4 flathead tapping screws 5/8 long fit the boxes almost as if the box holes were countersunk.

Thanks, bus driver. I should have known that dw screws were #6 and that #4 is the correct size.

"Make sure that the nearest stud is more than one-half inch from the box you will install..."

This depends on what manufacturer's box you are using.

There are some old work boxes designed to be mounted to the stud using screws through the side of the box.

If you are using bendable strap clamps you need some room (these are bent over the front edge of the box while catching behind the surface material).

Other boxes use screw expanded 'wings' to grab the wall material from the back, while the top and bottom wings catch on the front of the wall material (the hole needs to be pretty tight for them to work well).

Screws through the wings are not nearly as solid as other methods, and rarely hold long term in plaster or drywall.

Simply opening a hole large enough to fasten the box to the side of a stud (use a new work box and screws) is often faster and more solid.

It is not hard to cut drywall to patch the area.
It may take a couple layers in plaster walls to get drywall flush to the face of thick plaster from a stud face, or a thick bed of setting compound to build up to the surface (not as hard as it sounds with EasySand).

Aligning boxes to wood lathe may result in them not being even throughout the room, and is generally not worth the trouble.

Making all the boxes the same height looks a lot better, and it is not hard to cut wood lathe as required, or start a sawzall style tool (or en a regular gig saw) 'blind' without a starting hole.
You just pivot the blade into the surface.

Thanks all for your advise. The job was an overwhelming success, but very time consuming! We found that a drill was our best friend. Drilled 5/8" holes around the inside perimeter of the box outlines, then worked the plaster mainly with a hacksaw blade. Then drilled holes in the lath and took it out with a fine tooth blade on the sawzall. Worked it very slow with the saw at high speed. Went through several blades. Thankfully, no metal mesh underneath or any other surprises!
Thanks again for all your advice!

We also put masking tape around the outside of the cutting area. Don't know if it helped or not, but it did no harm.

get an oscillating type saw..fien, sonic, bosch, dremel..
after using one on regular sheet rock..much easier than other saws and better results.

best tip..make sure you span 3 lathes..yes..you completely cut one and partially cut the upper and lower lathes..then the box will fit..otherwise it won't

if you use an oscillating type saw forget the tape..

-dkenny

I used to do repair work for my father in his house that had very dry brittle plaster on the walls. I tried many of the techniques mentioned in the posts, but with little success.

With the development of spiral saws installing a cut in electrical box became a lot easier. However, they are a bit difficult to accurately guide when cutting plaster and lath and can damage the surface of the plaster. It may take a couple of bits to make certain they are sharp and don't overheat the lath, but they still work better than any other tool I have tried.

There are simple tools available that can make life a lot easier and guard against costly errors.

They consist of a template to guide the spiral saw or Dremel and a base that attaches to the tools.

You can see the templates at the link I posted or go to Cutzout.com.

Here is a link that might be useful: Cutzout.com

This post was edited by David1943 on Mon, Mar 4, 13 at 14:18

Spiral saws and old dry wood lathe are NOT a good combination.

They work very well in 2-coat plaster (uses gyp panels for lathe, and i drywall.

you need to be very sure there is no metal mesh (expanded metal) in the cut area.

It stops the spiral saws cold (or hot if you keep pushing).

Expaned metal was used to secure corner bead, make inside corners less prone to cracking.

It was also very common in arch 'forms'.
A metal arch was formed using expanded metal and ten covered with plaster. The forms looked like the section from

The shape of the arch form started on each side all the way to the ceiling, and ten flared out on the ceiling few inches.

They also went past the sides of the opening to try and ensure there was no cracking at the upper corners of the opening.

Think of the whole section of wall above the arch to the ceiling.