Newbie- callipers dial or digital do you use? to check a finished part ?

[bobrob]

Do you use them to check a finished part ?? when you are trying to hold a plus or minus one . tollerance?

Or would you use a micrometer? AND ALSO

PLEASE SAY WHY which of the two you would use.

[tobyaxis]

As rule of thumb Calipers are only to be used for Tolerances of +-.003. Of coarse many of us have still used calipers for slightly lower tolerances it is frowned upon. Calipers are great reference instruments. I own both Electronic and Dial Rack Gear type Calipers and Micrometers. Electronic are a bit pricey so you may not want to start with those. Be sure to keep the Rack Gear free of chips and oiled very now and then. The Caliper should open and close Smooth without skipping the needle.

Micrometers should always be used for +-.001 to check Finished Parts. Just be sure to check them often and keep them in spec. Do Not Drop, Bang, or Use them for Abrasive Work. Let the company supply instruments for crappy materials. Chances are you will not be able to afford new Instruments every year or two.

If your checking an OD use a Deltronic Plug Gage Style Pin to Confirm Accuracy close to the Tolerance your holding.

Checking Parallel Surfaces use Joe Blocks to Confirm Accuracy before using at least three times a day. You never know when during your lunch or break if some operator dropped it on the floor and didn't say anything.

This is pretty common practice for machinists. Other members methods may be different, but these are good standards to go by.

I haven't had any problems yet.

BTW: Don't Buy Cheapo, Buy Brand Name like Mitutoyo, B&S, Starret, These last and as long as you take care of them they will take care of you.

Do Not Buy SPI, Fowler, Economy, Crap.

[The Blight]

+/-0.003? I assume your talking inches? Thats a lot! My mitutoyo is guaranteed to hold +/-0.02mm (0.0008") and I have never seen it outside +/-0mm during calibration. The micrometers (digital) that we use also hold a tolerance of +/-0.002mm which is close to 0.00008"), but they rarely deviate more then 0.001mm. We calibrate our tools on a regular basis, so that we can count on the measurement we get.

I use mitutoyo for everything and I love it!

[tobyaxis]

Originally Posted by The Blight +/-0.003? I assume your talking inches? Thats a lot! My mitutoyo is guaranteed to hold +/-0.02mm (0.0008") and I have never seen it outside +/-0mm during calibration. The micrometers (digital) that we use also hold a tolerance of +/-0.002mm which is close to 0.00008"), but they rarely deviate more then 0.001mm. We calibrate our tools on a regular basis, so that we can count on the measurement we get. I use mitutoyo for everything and I love it!

With Calipers it has a lot to do with your feel. Remember that you have a feel for your measuring instruments. Micrometers have a Ratchet or Thimble to control the Torque applied.

I would never expect a Caliper to hold better than +-.003 inches. Your taking a risk measuring +-.0008 inches with calipers.

When you were in trade school did they teach you to use a butter knife as a screw driver??

Right Tool for the Right Job!!!

Do this in front of a Tradesman and you might wind up getting fired, I would surely fire you for using a caliper for checking finished parts with a tolerance of +-.001.

[The Blight]

Oh you should come see what we do. You would be surprised. I work with tolerances down in the 0.0002" range on a daily basis (+/-0.005mm). +/-0.003" is way too much. I can and I will show resaults with both calipers, digital micrometers and whatever you want to prove my point. Sure it has a lot to do with feel, but I'm educated in this, and I know what I'm talking about. Sure I won't work with tolerances below +/-0.0008 using a caliper, but everything above that, and it's fine.

And again. The digital calipers we use are guaranteed to hold a max tol. of +/-0.02mm, but I have never seen it deviate this much. +/-0.02mm is my limit when working with tolerances and using a caliper. But then again.. You americans have all your tools in inches. 0.001 is the lowest your calipers can go. Thats 0.0254mm. Our calipers go down to 0.01mm. Same applies to the micrometers.

And I bet that all my co-workers will say the same. But then again, a lot of them are educated fine mechanics and are well capeable of machining fine tolerances. Our most used tool is the digital caliper which goes through quality control on a regular basis under controlled enviornments. Our no.1 policy is "quality first. Always".

If I did this in front of a tradesman and the tol. of the caliper was outside the range of the tol. of the part, then sure he would call me stupid because that is stupid. If I want to work with tighter tolerances, then I would choose the apropriate tool for the job. Everything else is also stupid. If I was at the edge of a tolerance, I would also use a finer tool to be on the safe side. When the caliper says that i'm at 8.00, and the tol is +/-0.025, then I would say it's perfectly fine.

[Superman]

I seem to remember from years gone by

the smallest thet can be measured is 1/2 of a division, any more then you are only guessing

on a 0.001" mic you can only report to +-0.0005" of the closest line ( all vernier type scales, even in a dial indicator, 1/2 a div. )

Digital scales are good for 1st inspection, but nothing beats the mic with the ratchet thumbset. The human factor [feel] is minimised )

The Dial Rack Gear type Calipers are thrown in the bin ( too many errors could happen- a bit of dust/swarf and you stuff a $5000 part- it's not worth it )

Aerospace stuff is full reports on calibrated instuments.
2 people must be able to measure and get very similar results

[HuFlungDung]

I use a digital slide caliper for OD inspection when the caliper is at least twice as accurate as the needed accuracy of the part. If the caliper resolution is +/-.0005", then it's really only accurate to double that because you never know if the thing is about to move up or down by one pulse. Forcing it doesn't contribute to its accuracy.

I prefer a screw type micrometer for the critical stuff and I'll often double check what I'm seeing with the digital caliper against what the micrometer tells me, just for the purpose of removing systematic error (such as a gib screw coming loose on the back of the caliper).

I regularly interpolate .0002" with a plain micrometer, just by estimating the position of the index. Having a .0001 vernier scale doesn't make it more accurate to read.

I thoroughly detest using a digital caliper for measuring inside dimensions, it just does not read linearly until the caliper gets nearly an inch open. This is due to parallax error in the ID measuring jaws.

For small ID's I have an assortment of more expensive inside mikes, with preference for the 3 legged type. I like them because I can work the mic around a bit and feel the wobble decrease as the 3 legs align with the bore. When its rock solid its ready for a truly accurate reading.

[beege]

I've always understood that you use the instrument that has at least 1/10 the resolution of your tolerance. For instance, if your cals can display in.0005", then you should only be measuring tolerances of +/-.005" or looser. I know you can trust them for better, but its a good rule of thumb. And if your micrometers read to .0001", then use them for tolerances of +/-.001 or looser. Have a tolerance of +/-.0001"? Better have an instrument that has a resolution of 10 millionths.

[The Blight]

The digital calipers have a internal resolution of close to 0.001mm, but are rounded to 0.01mm to get rid of jitter. So when measuring something with a resolution of +/-0.02mm you are using a tool that can register dimensions smaller then 1/10 of the tol.

0.01" tol for a caliper? Thats 0.254mm. Thats more then the standard tol. of unspecified dimensions on all the parts we work with. If I were to use a micrometer for every single part that I make, I would end up getting fired for being too slow! 1/10 of resolution in inches is 0.01" for a digital caliper. 1/10 for a metric caliper is 0.1mm (0.004").

For ID measurements, the caliper is crap. I only use it for indication, and tol. bigger then +/-0.05mm.

The calibration and inspeciton department where I work guarantees that my caliper holds tol. I take good care of it, and they adjust it if it needs to be adjusted.

Edit: I'm also going to add (to avoid confusion) that if I had to choose between a micrometer, and a caliper, I would choose the micrometer. It's more accurate, but if I had no choice, I could reproduce satisfactory resaults with a good quality caliper. And I guess I agree that when working with tolerances below 0.05mm (about 0.002") a micrometer would be the best choise, but I still think a caliper can work in the same situiation. Between +/-0.02 to +/-0.05mm a caliper can work with confirmation from a micrometer if the measurement is at the edge of the tolerance.

But not to get too far off topic, I would choose the digital over the dial.

And just as a side note: I'm refering to digital micrometers. I will only use the ones with a scale down to 0.005mm.