As part of my Seal engine build I need to inline bore the cylinder block. Whilst making the boring bar is relatively simple, setting the tool in clear increments of adjustment after each cut seemed a little more complicated.

I needed a method of moving the tip of the cutting tool in steps that were measurable and raised this issue on my blog website. One response received included an article by Eddy Rebbe, who described his double clamp system incorporating a screw that moved the tool point. The article appeared in the May/June issue of MEW (Model Engineering Workshop) 1994.

I liked Eddies’ design but thought it could be simplified, and the level of accuracy of the tool increased by the fitting of a micrometer head. So out came the drawing paper and some scribbled designs were soon made. I also wanted to use the design process to help me get to grips with CAD. Using the Alibre free package I soon managed to get something drawn up and then it was into the workshop to make a start.

By a combination of building, back to design then building again, I arrived at the final design seen in the photos. It has fewer parts than Eddy Rebbes, so should be easier to construct. My version was made from materials in the ‘waiting to be resized’ box (scrap!) the only item bought was the micrometer head from MSC/J&L (UK based) (reference SPU-12375H) and cost £17.87p. I have no link to this company other than buying the micrometer head from them but their delivery was swift and it is a nice item. No doubt there are other sources out there for similar items.

I got on to MSC/J&L from an advert they placed in the last issue of Model Engineer and if you order their catalogue be prepared to reinforce your table. It’s massive, and from memory around 1,800 pages! I don’t think there is anything engineering wise that they don’t stock! I must take my hat off to you professionals out there. Some of the prices of top quality tooling is frightening…

Now back to the tool. It didn’t take too long to make and when finished I thought it would be nice to put it in a box to protect it. Perhaps it’s the fact that it has a micrometer head that led me to make its box. Bearing in mind I don’t like woodwork you can see how much I wanted to protect it!

Over the next few days I shall get to use it and will post the results when I have them. If you want a copy of the plans please email me BUT they are my first attempt at producing drawings so be warned!!!

After my post yesterday about variations in digital calipers I took the advice of an experienced model engineer and used material of known dimensionsl. In my case a set of ground parallel metal bars. One digital caliper, the digital micrometer and the screw driven micrometer all performed well giving the same results. However two other digital calipers varied each time (I open, closed, reset them each time) but as much as 4 thou. may not seem much but it is.

So what am I going to do about it? Well in part my faith has been restored as I have one digital caliper working fine so Its a case of keep checking the good one against either of the micrometers. As to the others, well they will be stored somewhere 'safe'. It has been a worthwhile exercise and perhaps with digital calipers cheap may not necessarily be best!

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.