This goes out to those who either don’t have the money, or space, to set up a nice big milling machine with DRO (digital readout). They’re not necessary if you learn to use the micrometer readings on the hand cranks.
First thing to learn is the principle of backlash.
Turn the crank on your machine to-and-fro, then observe the “loose” feeling between the directions you turn. This slack space is backlash. The threads on the table screws aren’t engaged in pulling/pushing the table in the direction you wish to travel. Consequently, the markings on the dials are meaningless at this point.
The indicator is only reading accurately when the slack is taken up, and there is resistance when turning the hand crank.
Any time you zero the dial, it must be done when everything has the slack removed.
Finding An Edge
Any machining will require finding the edge of the workpiece first. This is the basis for all other measurements.
The piece in the video is a 2″ wide piece of flat stock that needs a hole in the center.
I’m using a 13/16 annular cutter to make the hole.
FIRST…….. We need to find the edge of the piece to determine measurements on the Y Axis. The way the table will move away, and towards, you.
FIRST CALCULATION…… We need to know the center of the cutter we’re going to use (I’m not using an edge finder, instead…I’m using the cutter to determine the edge). Sometimes I don’t use the edge finder (something I’ll cover at another time) if I’m not after absolute precision where a few thou doesn’t matter. And, If I don’t mind a tiny nick on the edge of the metal.
13/16=.8125 this is the diameter of the cutter
.8125/2=.406 this is the center of the cutter, the measurement we need
MAKING CONTACT…. Make sure the cutter isn’t contacting the workpiece, then turn the machine on. Slowly advance the workpiece against the cutter until you hear it start to turn against the piece. Be gentle, you don’t actually want to cut the metal. (If your machine has a reverse feature, it’s better to run the cutter so that it isn’t rotating in the direction it’s made to cut, generally you’d want to run counterclockwise). Do this slowly. DO NOT TURN THE CRANK ANY DIRECTION BUT IN THE DIRECTION YOU WANT THE TABLE TO MOVE, THIS IS THE BACKLASH THING I COVERED.
FINDING THE EDGE…. After the cutter makes contact, stop the machine, and drop the workpiece below the cutter. ZERO YOUR DIAL AT THIS TIME.
Continue turning the crank to move the piece .406 towards the center of the cutter. Once you get to .406, you’ve found the edge of your piece. Because you’ve found the edge with the center of the cutter, any further movement of the table will place the hole exactly where you want it, already with the center of the hole determined.
After finding the edge, zero the dial again.
I need a hole smack dab in the center of the piece, so for this illustration, we’re gonna find the center. The hole could be anywhere you want it to be.
So, we have the cutter centered over the edge of the piece, now we have to determine how far to move the piece so that the cutter is centered over the location of the hole we want to make.
In this case, I want the hole in the center of the piece of 2″ stock, so I need to move the piece exactly 1″ on the Y Axis.
My machine has hand cranks that advance the table .125 per revolution. 1/8″ per full turn. Some machines are set up differently.
1″ (the center of our piece) is 8/8, or 1.000 This happens to be 8 full turns of my hand crank. After completing 8 turns, without allowing backlash, I’m at the exact center of the piece in the Y Axis.
Where you want the hole on the X Axis is calculated, and set up, in the same manner.
How It Works
Pictures are worth a thousand words 🙂