Making a Dial Indicator Snug

Contents

• The Problem
• The Clamps
• The Screw
• Finishing

A dial incicator and accessories

I found a nice dial indicator at a local antique show. A number of accessories were included in the original plastic box, including alternate end caps for the indicator and a mouting bar designed to be held in a lathe toolpost. The bottom item in the attached photo was missing, so I had to make it.

The little assembly of two clamps and a screw is called a snug, apparently because you can snug up the two clamps to hold the indicator rigidly in almost any orientation.

The Clamps

Close-up of the snug parts

There are 3 parts to the snug, a large clamp to hold the body of the indicator, a small clamp to hold the post on the mounting bar, and a clamping screw that both holds the two clamps together and tightens them.

I made the two clamps from scraps of 5/16" thick steel plate that were left over from making my drill press depth stop. (The steel was salvaged from a house built in 1947-48, so it is probably A7 (as revised in 1936) structural steel; the modern A36 structural standard was introduced in 1960 and replace A7 in 1965.) I used photos of other snugs and measurements of the diameters of the indicator body and mounting post to get the rough dimensions of my snug.

I used my slitting saw to cut the two steel bricks that would become the clamps. When I slit my drill press depth stop, I did it the conventional way, with the saw pushing the workpiece away from itself as it rotated. This meant cutting with the workpiece behind the saw and advancing the cut by pulling the cross slide toward the front. To cut my steel bricks, I used climb cutting, where the saw tends to pull the workpiece towards itself while the workpiece sits in front of the saw and the cross-slide pushes it back.

The advantage of climb cutting is that it maintains a puddle of oil on the top of the workpiece, while with conventional cutting, the kerf scrapes oil off the blade below the workpiece, where the excess dribbles away. The disadvantage of climb cutting is that backlash is a problem, but on a small lathe like the Taig, finger pressure pushing the cross-slide into the work is sufficient to eliminate this problem.

I center punched the two holes in eacn clamp and scribed the outlines of the circular ends with a pair of dividers, and then used the slitting saw again to cut the corners at roughly 45°. The final step in making the outside shape of each clamp was to file to the scribed line.

I did the drilling with the blocks clamped in my drill press vise, shimmed flush with the top of the vise with strips of hardwood scrap. I began by drilling through each center punching with a small drill before drilling to size, and I drilled the large hole for the indicator body to an intermediate size before finishing it.

I drilled the screw holes in both clamps with the tap dril for 10-32 threads before drilling clearance holes all the way through the smaller clamp and halfway through the larger clamp.

After drilling, I went back to my lathe and used the slitting saw again to slit each clamp. I did this by eyeball, and if you inspect the photos, you can see that the slits are not quite centered. Each slit is about 1.5mm wide, and they were cut with a 0.5mm saw with two parallel cuts, again spaced by eye. The result is best described as adequate.

The final step was to tap the hole in the larger clamp. The clearance hole drilled halfway through that clamp made an excellent tap guide, so hand-held tapping went very quickly.

The Screw

I turned the screw on my Taig lathe, starting with a chunk of 1/2" bolt. After skimming off the zinc plating on the outside of the bolt, I turned the bolt down to about 3/8" diameter for the total length of the two screw holes in the clamps plus about an extra 1/4". Then I turned exactly the length of the clamping holes down to the outside diameter of a number 10 screw.

Before tapping the screw, I hand filed the end of the screw to be a bit rounded and then threaded it, still in the lathe. I don't have a die holder for my lathe, but pressing the drill press chuck against the back of the die squares things up enough to get a decent concentric thread.

After parting off the screw, I had a problem. I don't have knurls and I wanted a knurled screw. The solution I've used for this is not great, but it does give good enough knurls to grip. What I do is clamp the screw in a bench vise between two mill files and two strips of wood (so that the vise jaws don't wreck the moving file). Then, using a block of wood to cushion the end of the file, I gently hammer on the flat end of one file, sliding it horizontally through the vise and making the screw roll between the two files. If you're really careful, you can do this without knocking the whole assembly out of the vise and losing the screw under the workbench.

Finishing

The result in use

The toolpost mounting bar for the dial indicator is nicely blued and I decided to try bluing the snug to match. I did this by heating each of the parts in the flame of my kitchen stove until the color was right and then quenching in molten wax. I used red cheese wax, the kind Gouda cheese comes coated with. (Cheese wax has many uses around a workshop.)

This was not a matter of heat treating the steel, since A7 structural steel is a mild steel alloy and therefore does not harden or temper. Rather, all I wanted was a good layer of oxide impregnated with burnt hydrocarbons (that is, wax). The result is a close enough match to the blued mounting bar that I'm content and I will probably use the same technique to blue other small parts.

The final photo here shows the snug, hard at work holding the dial indicator so I could the measure TRO on my Sherline chuck.