As part of a team build I am involved in, I need to make around 15 cams that will eventually be sent to other model engineers, and be included in their model engine build. Being such a member, you want to make sure your part doesn't let others down, so keeping to the dimensions on the plan is critical.
When the subject regarding levels of accuracy that home machinists or model engineers, should seek to achieve, the number 1 often appears. That is, 1 thousandths of 1 inch which is commonly suggested as the goal. However a more fundamental question perhaps should be asked. What level of accuracy is achievable with the tools of measurement available to the model engineer?
First, a common tool (I have two) is the digital caliper. It is easy to read and use, and until today, I thought extremely accurate. Secondly, there are many good, or even high quality micrometers, available via on line auctions such as eBay. So how do they compare? Well today I happened to check work I had measured up using my trusty old second hand micrometer with the digital caliper I have been using for some time. Surprisingly there was a difference of 3 times the accuracy level of 0.001" or 3 thou. Now that was quite disconcerting as I had by that time made three of the fifteen blanks, and all were correct according to my micrometer, but undersized using the caliper!
The first thought was, is there dirt, swarf on either or both instruments? So both were thouroughly cleaned however this is something I am almost paranoid about, and always make sure the part and the mic or caliper is clean. Once the thorough clean was done, the part was measured again and the discrepency was still there. So I got out a virtually new digital micrometer I have, and sought out my second digital caliper. I then remembered that I had another brand new digital caliper which belongs to my son (free gift from a firm he had bought some tool from for my birthday) and I unwrapped that one as well and decided I would see what the readings were against one component. The results were very surprising to me! The differences between all of the items was larger than I would have believed. It was interesting though that one digital caliper and the two micrometers were very close, but the other two digital calipers were out more than the one thousandth of one inch I try to work to.
Now if I am making a part that fits another then as long as the same instrument is used it all works. Its when two differing instruments, or in the team build maybe as many as 30 plus instruments, that possible problems could occur. If I sent parts out exactly to size according to my kit but it is 2 thou under and it goes to another whose instrument is 2 thou over, then you can see the problem.
Have a look at the photo below to see the exact readings in this excercise.........
Unfortunately I do not have another 0-1" micrometer, so I cannot make that comparison but I believe that quality instruments of the type my micrometer is, would not show such variations. if you know better please let me know.
So, the answer to the question posed? I do not have sufficient instruments to make a scientific study and this article does not seek to promote itself as anything other than an interesting outcome to my own measuring instruments. But I know that in future I shall rely on the micrometer for accurate work. I only hope my fellow team builders do not have the same problems with there measurement instruments!
Please comment if you feel this is wrong. Or if I am doing something wrong. I am after all reasonably new to this model engineering game.
In my opinion, you are right. You can also learn how accurate the digital calipers are here.
As a toolmaker with 30yrs experience ,Id never expect to work much closer than say .002 with a digital vernier calipers..maybe less depending on the geometry being measured,and the quality of the calipers…dont expect an Aldi or Lidl cheapy to be as consistent as a good quality mitutoyo vernier.I conistently work within ten thousandths with a Mitutoyo digital micrometer in a temperature controlled environment.Any pro engineer should be able to work consistently within .0005 with a good quality standard micrometer.Id suggest checking your equipment regularly with gauges(i use slip gauges) to maintain accuracy
Hello Richard, your comments are welcome. I understand what you say since you are a professional . However this entry was made as a relatively new model engineer and hobbyist. It wasn’t made for a proffesional audience and I doubt many home model engineers could or need to work to the accuracy you do. Also I doubt that they would be prepared to pay for the quality of digital caliper you mention. Again some model engineers have slip gauges but they are by no means widely used.
I have promised a set of secondhand slip gauges in the future and they do come up on ebay from time to time.
Finally from my understanding (and reading some tool suppliers catalogues) the need to have slip gauges calibrated is very expensive so whilst the professionals would insist on the calibration, the majority of model engineers would not.
Again thank you for the views of a proffesional. Mike
> Surprisingly there was a difference of 3 times the accuracy level of 0.001″ or 3 thou.
Precision/resolution is not the same as accuracy.
For example, the digital calipers for sell at “Little Machine Shop” have a “0.0005″ (0.01 mm) resolution” but only a “0.001″ (0.02 mm) accuracy”. That means when you see .6015 it could really be anywhere from .6005 to .6025.
http://littlemachineshop.com/products/product_view.php?ProductID=2874&category=
Interestingly I have also recently come across Mike’s problem in that I have two micrometers (imperial & metric), two digital calipers (Rolson & Workzone/Aldi), a dial caliper and a slide vernier caliper. I have also borrowed an imperial micrometer reading to tenths. All give different readings.
As a result, I have bought the Mitutoyo 25mm, 50mm & 75mm micrometer standards and intend to do some tests. The standards are supposedly accurate to within certain limits at a specific temperature (20°C or 25°?) so what I intend to do is allow all the equipment to reach the same temperature by leaving them in the same room for a few hours, and then to tabulate results at 25mm, 50mm, 75mm, 100mm(25 + 75), 125mm(50 + 75), and possibly 150mm (by using all three devices). This should then give me some idea of
the various discrepancies, and, with luck, their linearities.
There are problems of course in that the three higher readings will not be aa accurate as the lower readings due to the tolerances of the standards being added (or subtracted in which case a combination may be more accurate).
Obviously, for one-off work, then using just one item should result in good work, hopefully, the above test will determine which device to use.
Regards,
Peter G. Shaw
I look forward to your results Peter with great interest
I’ve now done my testing – see my posting two above.
As I said, the standards, accurate at 20°C, and the measuring instruments were left overnight in a warmish room and the tests done in the same room with a temperature of 20.9°C at the start and 20.7°C at the end. All tests were done at least twice. The measuring instruments are:
Imperial Micrometer, resolution 0.001″ – Mic 1
Metric micrometer, resolution 0.01mm – Mic 2
Vernier caliper, marked INOX (but this may just be another name for stainless steel), resolution 0.05mm – INOX
Draper dial caliper, resolution 0.02mm – Draper
Rolson digital caliper, resolution 0.01mm, accuracy 0.02mm
(0-100mm), 0.03 (100-150mm), repeatability 0.01mm – Rolson
Workzone digital caliper, resolution 0.01mm, – Workzone
Note the limited specification of the Workzone caliper.
The results are shown: Device name – reading 1/reading 2
Mic 1 – 0.9843/0.9843″ (Equivalent to 25.00122mm)
Mic 2 – 25.004/25.005
INOX – 25.00/25.00, 50.00/50.00, 75.00/75.00, 100.00/100.00, 125.00/125.05, 150.00/150.00
Draper – 25.045/25.045, 50.085/50.085, 75.087/75.082, 100.140/100.100, 125.095/125.085, 150.185/150.100
Rolson – 24.98/24.98, 49.98/49.98, 74.98/74.99, 99.99/99.98, 125.02/124.99, 149.99/150.05
Workzone – 25.03/25.04, 50.05/50.03, 75.08/75.05, 100.10/100.04, 125.00/125.00, 150.00/150.00
Comments:
The two micrometers and the Draper dial vernier were estimated by splitting the graduations.
The INOX vernier is difficult to determine which graduation to use. I suspect that some of these readings are actually between 0.00 to 0.05mm higher.
I suspect that the slight differences on the purely mechanical devices are down to varying pressure when measuring. I suspect the same will apply to the digital units, however the Rolson repeatability seems very good, albeit consistently low. Maybe I am applying too much pressure.
The Workzone device does not seem particularly consistent to me.
Well, after all that, I don’t know what to do. It does seem to me, that as an amateur making stuff for myself, that at any one time,I need to use one device only to the exclusion of the others. This does mean that the idea, as I originally intended, of using the Workzone caliper to make a digital height guage is a non-starter. This does also make me wonder just how accurate are all those lathes and milling machines fitted with digital readouts.
Regards,
Peter G. Shaw
ps. I’ve tried to set the readings as in a table. At the time of writing, I do not know how it will appear. If it’s a mess, well, sorry, I don’t know what else to do.
Follow-up.
Oh dear, the formatting hasn’t worked!
Sorry, you’ll just have to transcribe them onto paper or a spreadsheet.
Regards,
Peter G. Shaw
Just a further thought.
I have a friend who is a retired tool room engineer and who has a micrometer with a vernier scale thus reading to “tenths”. On comparing readings from my micrometers (no vernier scales) and estimating in between graduations, I was very gratified to discover that I could estimate almost as accurately as my friends “tenths” scale micrometer was reading. My friend said that this was quite normal in that the eye/brain combination is capable of splitting small divisions quite accurately.
Regards,
Peter G. Shaw.
Just another thought. In Mikes picture well above, he shows an Axminster digital caliper reading 0.597″ and which appears to be the lowest reading. Ironically, the design of the caliper, ie the display unit and the shaft or bar decoration is exactly the same as my Rolson which also reads low! Perhaps there is something here, possibly even made in the same factory etc.
Regards,
Peter G. Shaw
just my two cents
I have been a mackinits for 32 years and work onlarge parts now 500 to 2500 lbs holding tolerances between .0003 to .002 in daily we use mic’s for up to 15 -16 in but the cost of mics for up to 36 in in 1 inch increments was probhibitive (sp) I guess because we use old vernier calipers and some digitals the digitals will almost always read .001 to .002 smaller than the mics but you can set zero anywhere you want so if the are gauged to be small at all reading then just set them small at 0 but i question how oftenare you really holding .001 and at what dia’s if its under 1.0 or 2.0 inches i would suggest mics for the fin size and clipers for getting close before final sizing they are faster but not much can match a good set of mics.
Just a few things,
I have been working in in the metal trade for 32yrs and I generaly work within tolorances of 0.001mm and have to use both micrometers and digital verniers,here is my veiw,
1-ALLWAYS use a micrometer if posable but check both min & max WITH A MIC GUAGE! If your mic is old and has been used more at one end than the other then there will be a slight diferance and ajust.
2-Digital calipers are great and easy to use BUT!!!!! It depends on who uses them and funny thing, on there mood!!
even the same person will get a variation of readings and that is with a new set.
3-With an old set of digital calipers there will be worn parts that have to be taken in to account- The out side jaws will be accurate at the base but will generaly be out any where from 0.001mm to 0.2mm at the tip depending on who has used them and how worn they are not how old, With the inside jaws the same aplies.
4-To summerise Use a micrometer if posable.
Digital verniers are good but use a mic guage to test or
set and use as a comparitor ie: set or test your calipers at the part of the jaws that you are going to use.
5-Invest in some good quality outside & inside Micrometers
and only use your verniers where these can’t be used and you can’t go wrong.
Thanks Paul,
Good advice, from a hobbyist rather than a professionals view you can buy professional quality second hand equipment. I have bought micrometers of all types from Ebay and I would sooner use a second hand micrometer than a new (and often much dearer!) digital caliper. That said for many jobs (especially conversion between metric and imperial) then digital calipers have an important place. As the old saying goes its ….horses for courses…..
As a time served Toolmaker involved in high precision presstools and injection moulds I would advise the purchase of a quality long travel dial indicator with a few different point attachments. It may take a while to feel comfortable with the indicator but it is worthwhile. My reasons for saying this is that the dial indicator eliminates operator-touch-and-feel errors, so accuracy is repeatable as opposed to hit and miss. In other words the readings will be precise with no ifs and buts.
The article on the digital calipers was interesting to me, l tried similar experiments
with electronic calipers and found identical results, l made sure all the instruments
were perfectly clean, and measurements all done under the same tempreture, as it
can effect measurement, l noted that in the main the measurements were o.o1 to o.o4
out on the same item being measured, l did not use cheap calipers mine where quiet
high quality, my older manual steel caliper was the most accurate, the moral or the
story must be, electronic calipers and micrometers still have room for improvement,
l must say l no longer use the electronic ones for crucial measurements.
One other thing, the digital calipers you show in the picture all seem to be (no offense intended) the “bottom of the barrel” Chinese made calipers. There’s a reason people pay more for Mitituyo, Brown & Sharpe, etc. You would be much better off with a Mitituyo dial caliper (non-digital) than with the digital ones you are using. They can be picked up off Ebay pretty reasonably as they’re not as popular these days.
People often confuse “digital” with “accurate”, there’s no particular relationship between the two. Invest in some good quality equipment. Calipers are generally thought of as convenient but not particularly accurate. If you need accurate use micrometers. Once you have a good micrometer you can figure out whether your calipers are accurate or not, and if not what the error is and whether you can just correct for it.
Shop Ebay, don’t be in a hurry and you’ll be amazed at what you can buy. Last weekend I picked up some Brown & Sharpe #1 mikes, about as high quality as can be found, made by Etalon in Switzerland, for $25 each in like new condition. They’re probably $250 each new, well out of my price range. Invest in quality measuring tools, they’ll last the rest of your life and your kids’ if you take care of them.