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.........

differnet measurements from different tools 

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.