Buying a Dial Test Indicator...?

You need some help in choosing a dial indicator or test indicator.? There are so many types and prices. I will use it to center objects in a 4 jaw chuck and to test run out of accessories and egocentricity of work pieces.
Does it really matter if You get metric or English? Either way You would be reading decimal mm or inches. Do you want an indicator or test indicator?
What range do you want? Should it be 0-100 or 0-50-0 type?
You can mount it on a magnetic base standing on a heavy piece of iron (old railroad tie plate) on the table behind my lathe.
For centering you want a DTI or Dial Test Indicator. That is the type with an arm that can be easily and quickly set to any angle. This
is a great help when mounting it for centering and for lining things up on a mill. For centering and set-ups in general, the actual scale, English or metric, is of little or no consequence. You are looking at how much it deviates as the work is rotated or moved along an axis. 0.001" = 0.025mm, approximately. You can easily keep that in mind and mentally translate from one to the other for the desired accuracy.
One thing to keep in mind with DTIs is the cosine error or the fact that the calibration is only accurate when the arm is perpendicular to the motion of the work. At other angles, as is often the case, it is less sensitive so you have to dial it in to a finer degree than the scale indicates. At 45 degrees the motion is actually 1.4 times the indicated amount.
A DI or dial indicator has a plunger type probe so it is easier to take accurate readings in real units. They can be used for centering but may be harder to use in some situations. For instance, it would be hard or impossible to indicate along the side of a groove. They are frequently used to make actual measurements either on test stands or mounted on a lathe bed to show carriage motion. I have never used one for centering or lining things up. Most modern ones will read in both directions without any need to switch anything. The scale 0-100 or 50-0-50 is of little consequence and you can get what you want. I rarely even look at the numbers, just how many divisions the needle wiggles or moves. More important is the total movement for one revolution of the dial. I would suggest about 0.100" or 2.5mm. This will make it easy to line things up +/- 0.001" or the metric equivalent. That will be OK for most work - all but the most exact and if you are starting out, this will be both a good starting point and should carry you far before you feel you need better.
As for expense vs quality or brand name, I have both name brand and import DTIs. So far, I see little difference between them in actual use. Perhaps if I drop them the name brand ones may survive better, but I do not plan to carry out that test. And you can buy half a dozen or more imports for the price of a name brand. The suppliers like Enco sometimes have sales and the last time I wanted to buy one, I purchased two just to make the shipping worth it. They worked just fine. Save your money until you find a definite need for a better one.


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