Simple Precision Transverse Slide

Suitable for Caustic Testing and other applications.

This uses a cheap, readily available component so people without precision

machining equipment may produce an accurate transverse slide.

Copyright – P. J. Smith

But permission is given to distribute this material in unaltered form as long as it is not sold for profit.



Some tests require precision measurements of transverse movements and spacings.  Usually the required travel is small. 

The Caustic Test is an example.  The requirements of the transverse slide are that it be of excellent quality in motion, measurement, and setting control.  In fact, the transverse slide requirements are far far more stringent than the longitudinal slide.

This has discouraged ATM’s from experimenting with the Caustic test – only a few with the facilities to make precision equipment seem to have attempted it.  This is a sad state of affairs.

The solution presented here is easy to attach to an existing Foucault tester of sturdy construction.

Over the years I have used various other solutions, but this has a more delicate feel than all of the others and finds application in many other situations.



Dial Indicators are readily available and some represent incredible value for money.  I have found even very cheap ones – often down to US$10 each – are surprisingly accurate over nearly 1 inch of movement.

Calibration tests were done with gauge blocks at a variety of spacings and reading accuracy seemed to be reliable to ¼  division over most of the travel.  Thus readings may be interpolated to 0.25 thousandths of an inch which is good enough to be useful in Caustic testing.

If a dial gauge is manufactured to comply with standards, they are supposed to be guaranteed to better than one division over the middle 90% of travel with a repeatability of 1/5 division.  Some I tested were much better.

Of course, there are better Dial Indicators available, with different travels and sensitivities, even up to digital ones of high precision and cost.  Some may even be interfaced directly with a computer.

Rather than encourage spending big on exotic gear,  this article is primarily to convince people that they have a cheap and simple solution at their fingertips.  You can at some later stage, if you wish, replace or calibrate a cheap dial indicator.  Even the simple expedient of changing the settings slightly so series of readings are made over different portions of its travel will provide a real measure of control over possible errors.

The other attribute built into this instrument is a precision non rotating slide (the spindle) with enough capacity to carry a fine wire or LED plus pinhole or slit.  Most alternatives result in a much heavier solution and micrometers usually have a rotating spindle.

There are two parts which must be made to adapt a dial indicator.  Since all of the precision translation and measurement takes place within the dial indicator, these additions may be quite crude and the instrument will still function perfectly. 

Examine the following drawings.

You will see that the dial indicator has an attachment inserted over the plunger end (at right above) and a small yoke fitted to the other end (at left).

The function of the right hand fitting is simply to provide a controlled, smooth movement to the measuring plunger.  There is no need for precision here – only smoothness of operation.  A Dial Indicator reading in thousandths of an inch may use a 2 inch long 3/16 inch diameter Whitworth bolt (24 threads per inch) for adjustment but a more sensitive indicator will require a finer thread.  By the way, I prefer to work in Metric but since I picked up some super cheap Imperial (inch) Dial Indicators very cheaply, I am not very concerned – unit conversion is easy.

Again, the yoke on the left could be made from a small piece of bent Aluminium.  It must, however,  be adjusted so the wire is truly vertical and perpendicular with the plunger axis.

Please note that this drawing is slightly different from the photographs below.  The drawing was made a few years ago to explain the principle to someone and since then a different one has been constructed. If the range of transverse movement required is smaller, a more compact instrument may use a different indicator.  There are also available ones with smaller or larger clock faces. 

The assembled instrument is shown above, while the parts are detailed below.  In this case, the right hand tube is made of PVC (an offcut from other work) which simply slips on and is held by friction since it is a reamed fit.  But the fit can be quite sloppy and be held on by a pinch bolt if desired.

A dimple is machined in the end of the bolt to fit over the Dial Indicator Spindle as shown below.

If this dimple cannot be made very accurately, it is better to have the threaded portion in the tube short and sloppy.  This will have no effect on accuracy, only on the smoothness of operation.  Teflon grease is also useful.

Finally, the yoke carrying the wire or thread is shown below. Sorry about the picture quality.

This was made by turning a piece in brass, drilling, and milling away unwanted material.

Small V grooves (just visible) were turned on the outside.  These now serve as guide slots for the attachment of wire or fibre strands.  One ‘face’ has been milled away almost to the centreline (the ‘face’ away from viewer) because keeping the wire in line with the plunger axis is good geometric design for better accuracy.  On the other hand, it may be an unnecessary refinement.

Wire may be twisted around an arm, then stretched across to the other arm, finally being twisted around this. Unfortunately wire has no elasticity and is difficult to tension so it is always straight and I recommend a fibre unravelled from Dental Floss (clean wax off in solvent first).  It can be stretched across into each V groove then glued in place.  Turning the grooves like this is a simple way to ensure the fibre is perpendicular to the axis of the indicator plunger.

It is also possible to mount a LED and pinhole or slit on this or another yoke.  In this case it may be necessary to increase the spring force returning the plunger because of trailing electric wires.  An elastic band between yoke and frame will suffice.

There is no reason the instrument cannot be made in many other ways.




There are two totally different ways to do the Caustic transverse measurements.

One is to use an eyepiece which shows the wire in the tester superimposed over an image of a slit.  It is possible to simply mount an eyepiece onto a separate stand which is placed behind the yoke.  In this case the eyepiece need not actually move during setting.  While rather simplistic, this has the advantage that the eyepiece can be removed without changing any settings.  This eyepiece stand is in no way part of the Foucault tester – it simply stands behind and to one side of it.

The other is to simply use the wire in the occulting mode without any eyepiece.  Diffraction effects around the edges help to centralise it.  Many prefer this method but I suggest trying an eyepiece first as it helps one understand what is happening during the test.

I personally feel very comfortable removing the variable of shadow balancing inherent in quantitative Foucault zonal testing and replacing it with a system depending on coincidence settings instead.  In my mind, this is the main advantage of the caustic test, although there are others.

I will not spend any more time on exactly how this device can be used.  When the method of Caustic testing is understood it is obvious.  Rather I will leave it up to individuals to assess whether this device has anything to offer.