Using a Co-Axial Dial Indicator to Find the Center of a Circle

Ever wondered how someone drilled those perfectly offset holes in that piece of round stock or how that person on the mill centered the rotary table so perfectly?  Wonder no more.  Using a Co-axial indicator to find the center of your stock is quick and painless.  It allows you to center the mill off an inside bore, outside surface, and center point on your stock.  Let's get started!

Use this Co-Axial Indicator at Techshop!

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Step 1: Setting Up the Co-Axial Indicator

The Co-Axial Indicator has several different attachments.  The curved attachments are for outside centering, the straight ones for inside, and the short pointy one is for centering on a center point or small hole.  Select which one best suits your purpose and insert it into the bottom of the indicator.  Use the small wrench to snugly tighten it, but not too tight.

Step 2: Getting Ready for the Mill

The Indicator mount is 3/8" and the mills at Techshop use R8 collets, so find the 3/8 R8 Collet and get ready to collet up the indicator.  Slide the collet into the mill, turning it until the slot lines up with the one on the spindle and turn the draw bar on the top a couple turns to secure the collet.  Now insert the mount end of the Co-axial indicator into the collet and hold it with your hand while tightening the draw bar until it is hand tight.  Hold down the spindle brake with one hand while tightening the draw bar with the draw bar wrench with the other.  Again snug up the draw bar but do not over tighten it.  Don't forget to put the draw bar wrench back in it's spot!

Step 3: Setting Up the Mill

Once the Co-axial indicator is colleted up in the mill, you are ready to set up the mill for low speed.  On the right side of the mill is the HIGH/LOW gear selector.  The handle faces you in HIGH speed and away from you in LOW.  If it is already in low you can leave it, but if it is in high, slightly press in the handle while rotating it counterclockwise until it turns 180 degrees.  Most of the time it won't seat right into low gear, you will have to slightly turn the spindle with your left hand while pressing in the gear selector lever.  Now you are ready to set your RPM.  On the front of the mill there is a RPM indicator in both blue and red.  Blue is for low gear.  You want to start out in the lowest RPM setting to make it easier to make adjustments the first time you use the indicator.  An RPM of 100 (lowest) is perfect.  After you have used it a couple times and have the hang of it you can speed it up if need be.  The indicator can be used up to 800 RPM maximum.  REMEMBER though, you can only change the RPM while the motor is running.  Turn on the mill in the FWD position and use the nob to slow the mill down to 100 RPM.  The last thing to do is add the holding handle to the indicator.  You can insert it either on the right or left hand side.  It is the long black threaded handle.  You are ready now for your stock or work piece. 

Step 4: Stock Setup

Before you start milling remember to turn on the DRO (digital read out) and clear it out, then set it to "ZERO" mode.  Now you can raise up your material to just below the indicator tip.  Eye up the center of your piece as best you can and make any adjustments on your X and Y axis before you begin.  The better you eye up the start the easier it will be.  Now it's time to set up the play adjustment on the indicator. The indicator range is 0.25' or a quarter inch.  However it does move quite a bit in order to accommodate your material.  You want to adjust it so that when it hits your material it is in the middle of its range, roughly .125".  Now hold it clear of your work and lower the spindle and lock it, or raise the knee up so that the ball on the tip of the indicator is sitting on your zero surface.  You are ready to begin.

Step 5: Centerin the Mill

The first couple turns make sure the ball on the tip of the indicator is making good contact all the way around your zero surface.  If that is good let's take a look at the dial.  The dial scale slides/turns in order to set the middle of the play at 0.  You can move this and adjust it until it's perfect or just use a different number for reference.  The dial on the indicator reads in increments of 0.001" and has a smaller dial that reads down to the 0.0005".  The easiest way to start is pick one axis, let's start with Y first.  You want to move the Y a little one way to see what the indicator does.  If the play is getting larger you are going the wrong way.  Now move it in the opposite direction.  You should see the indicator needle's play getting smaller.  It will continue getting smaller until at one point it will stop and then get larger again.  This is where you stop.  Once it starts getting larger again bring it back to the limit and move the X axis.  Continue with the X axis exactly as you did with the Y until your play is very nominal.  Now go back between the Y and X to make the last adjustments until the play is within your desired tolerance.  Zero your X and Y axis on the DRO and you are set!

The Co-Axial indicator is kept in the machine shop lock up at TechShop, ask a staff member and we will gladly grab it for you!