The Stovebolt Site Forums The Shop Area The Tool Chest Checking Digital Caliper's Accuracy

I bought a "Pittsburgh" 6" digital caliper (SS jaws)from Harbor Freight for $10. It seems great but is there a way to check it's accuracy? Are there any common, cheap things to measure and check it?

When I get a measurement of 2.031 inches, how many "significant figures" do we really have? "Sig figs?" anyone remember this from Chem class?

Thanks!

...most places, that are ISO certified, have a set of weights and measurements that are "certified" to be correct from some company. Since we do not have these I usually just check mine to another set of dials or measure something with the dials and a mic I keep under lock and key.
(I have a mic I RARELY use and check my dials to that mic)...am I making any sense here!! You know what I mean!

I also have a HF dial caliper, probably the same model as yours. Mine is very accurate when checked against gage blocks. You could always take yours to the hardware store and check it against something like a ball bearing in the nut & bolt aisle. Ball bearings are manufactured to pretty tight tolerances. As far as the number of significant digits, I don't know.
Dave Evans
52_3100

If you want an exact figure you'll need to find what's called a 'standard'. Unless you're into tight tolerance machining that's probably not necessary. Dale's idea of checking it to a bearing will work for what most of us do.

I doubt you'll be able to calibrate or standardize your calipers. You need to know how far off they are at what size. Is it linear? Does it get worse as the jaws open? How close are the inside and outside jaws?

Ideally, you'd use gage blocks (also nicknamed "joe blocks") to verify accuracy. These are machined and polished blocks of various sizes. A set costs hundreds to thousands of dollars. If you have a friend who works at a factory with a metrology dept, you might be able to work out a 15 minute appointment to verify your calibration.

If it were me, and I didn't have access to gage blocks, I'd go to Tractor Supply or a big box hardware store. Look for "drill rod", or "polished and ground" steel stock. You can measure this and compare the advertised size with your results. Likewise, measure drill bit shanks and compare. If you have three pieces of stock, sandwich one between the other two and measure using the inside jaws. It is typical to get different readings between inside and outside jaws.

Also use feeler gages and, when all else fails, coins .

You say your $10 electronic calipers measure to THREE decimal places? Mine only measure to 2. This really limits their usefulness.

Significant digits: it depends on your calipers' output. If the last digit indexes by 0.001, then it should be good to 1/1000". If it rolls from 2.031 to 2.062, then you are only reading to the nearest 1/32" (0.03125" or more likely 0.031".) It wouldn't surprise me if it read 2.030 > 2.035 > 2.040 etc., the nearest 0.005". You get what you pay for, oftentimes less.

Bill

I have a six inch model and also one for measuring brake drums. They have a set zero that is used with the jaws closed. I think you will find it quite accurate.

If you're serious about checking your dial or digital calipers here's the tool made just for that purpose.
Picture of the tool
Now for the price...

Grigg

That's like saying "the hook on my tape measure is always on the end."

I'm still a fan of coins as a cheap standard. You can use their thickness and diameter to check your accuracy at the 1/1000" and 1.xxx" level. You can "buy" them for less than a couple bucks!

Bill

I am a firm believer in "you get what you pay for". I wouldn't trust a $10 instrument for anything precision. It takes me several hundred bucks of instruments just to measure valve stem to guide clearance. But it is cheap compared to having a valve seize which is quite common these days.

All precision measuring tools are only as good as the skill of the user and in most cases the average person does not hold the tool correctly resulting in an incorrect reading. If the jaws show no daylight between them when closed and the readout is zeroed it will be reasonably accurate throughout the entire scale range. Avoid getting cheap digital calipers wet as the reader head can fail and all micrometers over 1” will have a standard in the box that can be used to confirm caliper accuracy.

I have both cheap and expensive measuring tools. Leaving my cheap ones on the bench (where another wrench may land on it) is for a general use where getting a quick measurement comes in handy. My "good" ones are in the toolbox and in their own custom box for when I need to know I can trust the accuracy.
I do find those cheap ones basically do what they say they can though. I check them with my standards and find them quite accurate, and I'm not afraid to use them.