Dial indicators are precision measuring tools with a myriad of applications in the machine shop.
Once you move beyond the basic machining operations you will definitely want to have one or more of these in your workshop. On this page I will describe the instruments and holders, but the how-to-use-them information will be added to my other pages as I have time.
A plunger moves in and out from the body of the indicator and rotates the measuring needle on a dial face.
Dial indicators usually have either a 1″ or 2″ range and are calibrated in increments of .001″. A smaller dial reads each revolution of the larger dial in increments of 0.100″.
The outer bezel rotates and turns the numeric scale with it so that you can set the indicator to zero at any plunger position.
Most D/Is have two little movable markers on the outside of the dial face that can be used as reference points. I have removed them from mine, since I rarely use them.
Here are some things you can use a dial indicator for:
- Centering cylindrical stock in a 4-jaw chuck
- Determining accuracy of lathe or mill alignment
- Determining runout of lathe spindle and chucks
- Aligning stock for milling
- Monitoring depth of drilled holes
- Monitoring vertical movement of milling head
- Determining if edges of a rectangular workpiece are parallel
This is far from a complete list. Once you gain familiarity with this tool you will think of many ways to use it.
Here’s a photo of a dial indicator (DI) set up to center a bolt in a 4-jaw chuck. If the bolt is off-center, the DI plunger will move in and out as the chuck rotates.
When the bolt is properly centered there will be little or no movement of the plunger and the needle of the DI will move only .001 or less.
You can buy decent Chinese made dial indicators for under $15.00 from tool suppliers such as J&L, MSC, Enco, Harbor Freight and Grizzly.
Recently (02/02) I have seen them for around $7.00 – amazing if you look inside and see how many parts there are. While not as good as a Starrett or Browne & Sharpe instrument, these low-cost Chinese made tools are fine for the home shop and are consistent with the low cost and accuracy limits of the mini-lathe and mini-mill.
Magnetic Base Indicator Holders
Perhaps even more important than the indicator itself, is the holding device for it. You can purchase magnetic base holders for under $20, but I have found these to be awkward to adjust. Here’s a typical low-cost holder:
On the right side of the base is a lever which has the effect of turning the magnetic field on and off.
Since the magnetic force is quite powerful, on the order of 50 lbs., turning the field off makes it much easier to remove and adjust the base. Naturally, the base will only clamp onto steel, cast iron or other ferrous metals.
Usually I clamp it to the headstock, the ways or the top of the compound. On the back side of the DI is a mounting lug through which a locking bolt is passed to attach the DI to the holder.
The problem with the cheap holders like that shown above is that it that you must independently adjust the two axes to postion the indicator.
Especially within the tight confines of the mini-lathe and mill, you will find that getting the indicator in the exact position you want it is often difficult. Maximum flexibility of the holder is thus a great time saver.
Fortunately, holders are available which can be locked in place using a single knob after the indicator is in the desired position. The one I use, shown below, was purchased from J&L on sale for $59 including a dial test indicator which sells separately for around $27.
Later I bought a similar but much nicer one made by Noga. It is really smooth in operation and easier to align to where you want it.
Even though it lacks the ability to slide the DI along the arm, I have found this holder to be very versatile and much quicker to set up than the cheaper one.
Dial Test Indicators
Dial test indicators are similar to dial indicators, but are typically more precise and have a smaller range of movement. Rather than a plunger that moves in and out, they have a small lever arm with a ball-shaped tip that moves up and down.
This enables the tip to be inserted into a small hole so that the hole can be precisely centered on the lathe axis – an operation that could not be done with a dial indicator.
Check the typical import tool catatlogs (especially the sale flyers) and you should be able to find these from under $30.
The indicator shown here has a measuring range of 0.030 – much less than a dial indicator – and reads plus or minus from the zero point.
When the tip is at rest at its neutral point, it can be moved 0.015 in either direction. The tip can be set at different angles for convenience in setup.
As on the dial indicator, the bezel and numeric scale can be rotated to zero the reading. Each division is 0.0005 (5 ten-thousandths or half a thousandth per division).
On three sides of the DTI are small dovetails used to mount it. Mine came with two adaptors that attach to these dovetails and have short mounting posts of different diameters, as shown in the next photograph.
The magnetic holder that my DTI came with also has a dovetail clamp which clamps directly onto the DTI body.
Here’s a photo showing the DTI in use centering a bolt with a hole drilled in in in the 4-jaw chuck. You could not use a DI in this situation since there is no exposed surface of the bolt other than the hex head.
Instead, we take the reading off the edge of the hole, taking advantage of the narrow ball tip of the DTI. Since the DTI has only a very limited range, you must have the bolt pretty nearly centered already before you can use the DTI.
You can get pretty close by observing how far each jaw extends from the edge of the chuck and making these as nearly equal as possible.