A centering Indicator is a useful tool. It really improves speed and accuracy when trying to locate the axis of a cylindrical part or feature, or locating a future hole precisely by just using a punch mark and the indicator.
It is equally useful in mills as in lathe setups.
They centering indicators are available at different machining outlets, online and sometimes used. Depending on the precision required and brand, they range from under $100 to several hundred $$$. This model is on the low end and the precision is 0.0005" which is good enough for the equipment brand used and parts machined by the hobbyist under normal circumstances.
All the steps are performed with the machine turned OFF.
Make sure you observe all rules and procedures in the outlined in the SBU Mill class.
I made it at TechShop Chandler, techshop.ws.
Step 1: Get Your Tools and Part Ready
Get the indicator out of the box and install the appropriate feeler arm. There is usually an assortment inside the box, some are straight some curved and usually three different sizes are supplied. There is also an articulated feeler used for centering around punch marks.
In this particular case I used the shortest curved arm because the part was rather tall and the mill knee only dropped so much. Using the wrench tool, supplied in the case, set and tighten the filler in place. See first Picture
Select a known good R8 chuck and install it in the spindle as you would use a regular drill. Unlock the spindle so it rotates free. Select a small drill or a tap with a pointy tip. Chuck it in.
Use a V block and chuck the part in the vise securely ready to be machined (see second picture). You can also use a rotating table to chuck the part in, or a collet holder. Once indicated the part cannot be further tighten as it will move and get off absolute Zero that was set in X and Y. So do it good at this point.
Bring the part under the pointing tip as close as possible to the desired location you wish to indicate the center on.
The range on the indicator is rather narrow so you need to get close. See second picture.Once close, take the drill or whatever you use out, and chuck the indicator in place (see picture three). Adjust the feeler arm and make sure it is in firm contact with the surface of the part. Pick a part that is rather smooth so the ball at the end of the feeler has an easy time gliding over. Lift the contact point slightly several times and make sure it comes down firmly and the indicator dial needle is consistently in the same spot. See forth picture
Screw the stop arm on to the body of the dial. You can use left or right front or back. I prefer right side, as in the picture. Set up a magnetic base with the arm straight up (see third picture) and rest the arm against it, thus preventing rotation of the indicator. It helps to secure arm with a rubber band.
Step 2: Indicator Use
You are now ready for the main event, part centering.
Rotate the chuck by hand until the feeler is in the 3 o'clock position. Set the dial to Zero. Rotate the chuck again to the opposite side 9 o'clock. There should be a difference of several graduation. Move the handle of the X table in the direction that will result in narrowing the gap between the actual needle and zero set. Mow about half the way. Now go back to the original position. It should be somewhat away from zero. Set zero again. Rotate to 9 o'clock and repeat the same as above. In about three or four back and forth sets of movements, you are now right on Zero when you swing around to the either side.
As you rotate through the 12 o'clock and 6 o'clock you will notice some deflection on the needle, that is because the Y axis is not set yet.
Go to the 6 o'clock position and set Zero on the dial, swing around and go to the 12 o'clock, note the difference. Turn the handle for the Y axis so the difference gets smaller just like in the above X axis steps. Swing around again, set zero and repeat the steps. The steps are identical to setting the X axis. After several tries you should have the needle sitting pretty close to Zero as you rotate the indicator through a complete 360 degree circle. There is no rule what side you start with. X or Y, lef or right as long as the points are opposite pairs...
When that is achieved you are now set true to the center. Lock the X and Y ways so there is no way of accidentally hitting something and moving your setup. Set Zero in X and Y on the DRO Display so it can be later used as a reference also. You are within 0.0005 to the actual center.
Step 3: Disconnect and Store Tool
Now that you are all centered up, go ahead and remove the indicator and the chuck.
If you are ready to mill something. Install the appropriate holder and mill or leave the chuck on, and install your tool (drill, tap etc).
Put the indicator back in its box after all the accessories have been dismantled from it. Being a precision instrument treat it with care and keep it safe from shocks, dirt and moisture
You are now ready to Mill or Drill. In the picture above the drill is positioned to drill exactly in the center of the round part. It helps to use a center drill first, so the actual drill will not walk off the center and you end up with a hole that is off center, after all that work.
The same basic steps and procedure can be used in the CNC mill or the pedestal drill.
There are other methods for centering work using different tools. This centering indicator is purposely build for centering work, and it is used by professionals for quick accurate positioning, it beats other methods by a long shot :)
Hope this helps improve precision for all aspiring machinists out there