Until yesterday I'd never heard of the Wish online shopping site. I happened to Google an engineering item (Dial Test Indicator), and up came this price that looked too good to be true. I looked at it every which way, and decided that it was not much money to loose if it goes pear-shaped, so pulled the trigger. Then I discovered that list of items that interested me was endless, and became hooked! All told I've 'risked' NZ$100 on items that would otherwise of cost me at least five times as much.
I could have gone on...and on...and on.., but I don't even buy lottery tickets yet alone gamble on slot machines, and there is no way in this lifetime I'd walk into a bookmakers. So I Googled Wish, and found a whole lot of one star reviews. The average was two stars to be fair, which was balanced out by 29% five star ratings. I recently read about people who sell phony good reviews on Amazon, so it probably pays to be skeptical.
So I'd be interested to hear if anyone has shopped with Wish.com (I believe there is also an App) and actually received their purchase _and_ it was what they ordered? Or even what experience people have had with AliExpress or Alibaba? I hear that Jack Ma — the man behind Alibaba — is the richest person in China, so his company must deliver.
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This is a DTI. As the arm pivots, the dial needle moves. In this case, each division = 0.01mm (0.0004"). some of the best examples come from Switzerland, which is be the workings are not unlike a clock movement, but Japan makes excellent versions nowadays. China does knock-offs, but they do the job. They are mostly used to measure 'comparative' movement. Checking how concentric a work piece runs in a lathe chuck for instance. You'd also use them to square a vice to a milling machine slide. The important thing is that you are just trying to make the needle move as little as possible. I'll explain why in a moment.
This is a Dial Indicator. It does a similar thing, but in this case you can use it to 'measure' linear movement. The important thing is that the movement is linear. The DTI above moves the lever through an arc, which makes linear measurements have what is called 'cosine error'.
It is not a problem when measuring very small distances, and the arm is flat (they are generally designed to be accurate when the arm is at 12° to horizontal). But if the arm moves through more than a very few degrees, then the vertical movement is no longer directly proportional to the indicated movement on the dial. I found this wee diagram that demonstrates it. Hey, you are an ace with math, so you'll be totally on top of the trig.
Those prices are NZD. I think I paid NZ$10 for a Chinese DTI. The items all now show as shipped, and have tracking numbers, so it might even work out. 
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