My buddy bought one pair of HF calipers and was wondering why his stuff didn't fit on a project with small machined parts. HF calipers and other cheap ones are generally very weak and will give you measurements that don't repeat depending where on the jaw you measure. I recommended to him to return them and buy a better quality caliper and 1" micrometer.
I've seen a lot of youtube videos showing how to rebuild engines, general mechanical stuff and I see a lot of people using the cheapest calipers they can find, namely the no-name harbor-freight type digital calipers and measuring crankshaft journals thinking they are measuring within thousandths. I cringe when I see people using cheap stuff to make critical measurements.
I realized most people don't know a thing about this stuff.. and so here I am showing you guys what I showed my buddy.
In this guide I will show you the range of quality and what to look for when buying measuring tools. It is always better to buy new tools from a reputable brand name, but if you find used tools in good condition, there is no reason why they shouldn't be just as good as new ones, provided you know what to look for.
Let's start with calipers. Calipers are the most widely used measurement tool: Calipers come in standard sizes of 6", 8", 12", and larger. You can measure OD's, ID's, depths, steps, and use the side of it to measure flatness across a surface, by sliding different feeler gages between your surface and the side of the caliper length. I suggest you stay away from digital calipers on the low-end. There are many quality issues with them, and you don't really know if they are skipping when you pick them up a few months later.
For the 6" size of calipers (or 8"), BUY NEW.
This is the most common size, and these tools will have the greatest wear. Everything else, indicators, mics, larger calipers, whatever, don't get used so much and can be bought used safely.
Here are my own calipers from when I used to work as a machinist. The top left was my main caliper, a top-quality Brown & Sharpe 6" that could measure down to .0005" with a good feel. Bottom left is the first caliper I ever bought: a 6" Fowler dial caliper on sale for $20 from Enco. This one was good out of the box, however it had some burrs that I cleaned up with an ultra-fine file. Since I acquired more calipers, I relieved the static jaw to be able to measure small parts' overall length close to the collet on a Hardinge manual lathe. The upper right is a Mitutoyo made in Brazil - Mitutoyo top notch tools are made in Japan, but this one from Brazil is good too. The lower right is the cheap HF type brand, what you want to stay away from.
Down below, you can see a Mitutoyo Japan 12" dial caliper I picked up for $30 used. The guy told me it was over 15 years old.. more now. It came without the donkey dick (the depth measuring stick) which made it very useful when reaching inside a CNC lathe to measure diameters and bores larger than 7", because the caliper was still a foot long instead of being that much larger due to the donkey dick. For my motorized bicycle needs, I have not found a use for a depth measurement greater than 6".
With precision measuring tools, always remember to have a super-light feel. Use the same super-light feel to measure everything. Use your feel to make sure the part is square and never push hard on it. This will cause your tools to wear and eventually give you different readings.
Round parts should be measured with micrometers, because it is hard to get a good caliper feel on a small round diameter (i.e. 1/16" diameter pin). If you don't have or don't want to invest in a micrometer, just remember to use an ultra-light feel when measuring round parts with a caliper, and measure as close to the slide as possible to reduce all flexing forces.
The first thing you will find with cheap calipers is that they are built cheap and have multiple quality issues on all sides. The easiest thing to check is the quality of the OD jaws. Clean the jaws with your finger to make sure there is no dirt, then close them gently and hold them up to the light. Quality calipers will have no gap, HF calipers you will see light shining through. This gap could be anywhere from a few tenths up to one thousandth (.0003" - .001").
It was difficult to photograph. My HF caliper I got from a buddy NIB came stock like this. Used calipers might show this too - the used 12" mitutoyo I got had no gap at all.. that's quality.
The next thing to look for is the ID tips. OD tips as well. The cheap caliper came with a flat on the tips. I measured a .1997" ring gage which measured .005" off with the hf caliper. BAD BAD BAD!! This caliper came stock with a flat on one tip:
Compare to a Brown & Sharpe:
The cheap HF caliper also had an issue with the donkey dick. At the end of the caliper, everything should line up flat an level. The depth measurement was approx. .003" shallow, so from zero leveling it, it would read .003".. not good for precision.. if you are milling your head down or whatnot.. you want something that is good to .001 that you don't need to zero out every time you use a different side of it.
Rather than going and buying an HF cheapo caliper, I recommend you get a new name-brand dial caliper. You don't have to buy the top quality stuff, but now you know what to look for. I like Fowler for cheap stuff. They make everything from calipers, to mics, indicators, all sorts of stuff. The dial caliper I got was $20 on sale and five years later there is still no gap between the jaws, despite being my "relief caliper".
If you find a Starrett, Brown & Sharpe, or Mitutoyo Japan caliper used, make sure the above parts are good and there are no broken teeth. Most times, used calipers will have broken teeth on the gears from being dropped. A broken tooth will make it skip .025 or .05". If it does skip, but not often, that is OK too, just look on the .1" scale and see that is is approximately in the middle if you're reading .050" on the dial, or right on the line if you're near 0.000/.100" on the dial. My 12" will skip occasionally if I ramp it up too fast.. no biggie. If it skips a lot and you can feel the hiccup, don't buy it.