THERE is a wide range of experience and expertise in our readership, ranging from highly qualified engineering craftsmen with a lifetime of experience behind them to people with no engineering background and very little workshop experience. This article for the less experienced is about some workshop basics.

No doubt many of my readers will think that I am trying to teach my granny to suck eggs but even grannies are not immune from learning something new occasionally. After nearly 70 years on the workshop learning curve I am still climbing steadily! Bear with me if you already know it all and if you disagree with me, write to the editor and say so!!

Let me put my cards on the table straight away. I am not a subscriber to the “if it’s near enough it’s good enough” philosophy. I take the view that if the job calls for a particular dimension or finish for a component then every effort should be made to achieve that requirement. Inevitably the effort will not always be met with success – perfection will always be out of reach.  That is no reason not to strive for it however. 

The effort made to achieve accurate results is amply repaid by the ease with which parts fit together and by the appearance of the finished job. Appearance is important. The human eye is amazingly accurate and will detect extremely small errors in symmetry or parallelism and although you may feel that the accuracy of a non-critical feature is functionally adequate (the radius on the end of a link for example) errors of only a few thou may render the feature visually offensive. 

Personally, I make just as much effort to achieve accuracy in areas where it is of secondary importance as I do for important features such as cylinder bores. This is partly because of the satisfaction obtained from a job well done and partly because the discipline of getting the less important things right serves well in getting the important things correct.

What this preamble is leading up to is the subject of measuring and measuring equipment. I may be wrong, but I get the impression that the less experienced model engineers among us and, in particular newcomers to the hobby with little or no engineering background, place a low priority on equipping themselves with good quality and reliable measuring equipment. They will spend significant sums on machine tools, cutters and screwing tackle but consider an electronic digital slide gauge adequate for all of their measuring needs.  This, in my view, is a big mistake. I have nothing against slide gauges in principle (although I have had worrying experience of the electronic variety and always use traditional vernier type gauges) and there are some situations where they are the only practical solution to a measuring requirement, but in my opinion the humble screw micrometer provides the most accurate and reliable means of measurement for everyday workshop needs. 

Slide gauges do not provide the same ‘feel’ as is obtained with a micrometer and this inevitably places a limit on the accuracy that can be achieved.  Micrometers are not expensive tools and good quality second hand instruments (or ‘previously owned’ to use the ‘in’ jargon) are usually excellent value for money.  Most people will start with a 0 – 1in. (or 0 -25mm if you are a metric convert) micrometer but the measuring range available should be increased to at least 4in. (100mm).  My own measuring equipment includes micrometers up to 12in. capacity, but this is rather more than found in most model engineers’ workshops. 

By their nature, the measuring heads of micrometers are normally limited to a range of 1in. (25mm) and to cover a greater range than this it is necessary to use either separate instruments for each 1in. (25mm) increment of the range required or an instrument with interchangeable anvils in a single frame, enabling the range required to be selected by fitting an appropriate anvil. Presumably one of the attractions of the slide gauge to beginners is the ability of a single tool to cover a large range of measurement.

The zero setting of the basic 0-1in. (0-25mm) micrometer is easily checked by closing the spindle to the anvil and checking that the reading is zero. For an instrument with any other range a setting gauge or length standard is required. Appropriate length standards are supplied with new instruments over 1in. (25mm) range.  If you are buying a second hand instruments make sure that suitable length standards are included and try to have them checked by an independent authority.

Figure 1 shows a group of micrometers covering a range of 0 to 4in. together with a single instrument with interchangeable anvils covering the same range. Every

time an anvil is changed the setting should be checked – a scrap of dirt under the anvil seating will destroy the accuracy of the instrument. The length standards for the single instrument are included in its case while a set of standards up to 5in. is shown at the bottom right hand corner of the picture.

The principle of operation of a screw micrometer employs the very useful characteristic of a screw thread  - that one revolution of the screw advances the spindle by an amount equal to the pitch of the thread and part of a revolution advances the spindle by an exactly proportional part of the pitch. This may seem like stating the obvious, but the

principle extends to any device employing a screw thread  - the slides of the lathe or milling machine, for example.

The fact that these machines use screw threads to operate their slides makes them into accurate measuring devices in their own right, a characteristic frequently not exploited to the full by inexperienced model engineers. Someone told me recently that when he is turning something to a particular size he will sometimes find that he has put on a little too much cut and the job will finish under size. I asked how this comes about if he is using the micrometer dials of his machine to measure the applied cut. “I do that” was his reply, “but the dials are rather crude affairs”.

I know the type of dial referred to – a die casting with the graduations in the form of raised lines around the circumference. These dials may look crude compared with those on some machines, but the fact is that any device which divides the rotation of the feed screw into uniform increments will enable accurate control of the slide movement. If the slides are correctly adjusted and the tool sharp there is absolutely no reason why the micrometer function of the feed screws should not yield highly accurate results. 

A strip of paper marked in appropriate equal increments and wrapped round a wooden drum attached to the feed screw handle, crude though it may look, should enable accuracy within 0.001in. to be achieved.  I suspect that my friends’ problem lies not in the crudeness of his machines micrometer dials but in his use of them in conjunction with his measuring equipment, which is, I believe, a digital slide gauge!

“Ah” you may say, “but my machine is old and the feed screws are worn – there is half a turn of backlash in the feed”. While there are circumstances where this is significant, it is almost irrelevant when considering measurement of the movement of the lathe tool or milling machine table. The backlash in a feed mechanism is due to the clearance between the mating flanks of the male and female components, i.e. the screw and nut, and possibly any end float in the screw as a whole. 

Even brand new machines will have some clearance in these areas and therefore some backlash. As the components start to wear this clearance, and therefore the backlash, will increase. The only exceptions are machines equipped with re-circulating ball screws.  These are the norm for numerically controlled machine tools but are unlikely to be found in the average model engineer’s workshop. 

The wear on our conventional feed screw and its nut will take place on both components, although the nut will inevitably wear more than the screw. The nature of the screw thread is such that although the wear increases the clearance between the mating faces as shown in Figure 2 it does not alter the pitch of the thread. It therefore follows that, providing the same faces of the screw and nut are kept in contact (i.e. the feed is always in the same direction) the micrometer function of the feed mechanism remains accurate. 

The only qualification to be made to this statement is if a very long movement of the feed mechanism is to be made, outside the usual operating range. The longitudinal (X) movement of the milling machine table is the most likely example. If the machine vice is fitted to the centre of the table and most operations carried out within say, the centre third of the tables range of movement the feed screw will wear more in that area than at the extreme ends of the traverse. 

If a job arises which requires measured movement from the centre to one or other end of the available movement an error can occur due to the difference in wear on the flanks of the feed screw thread at the centre and ends of the screw.  Wear in the nut has no effect. I have detected an error of two or three thou in my own milling machine due to this cause. This is not the result of a pitch error however and does not affect the normal micrometer function of the feed mechanism. It is, however, a potential source of error to be aware of.