Measuring the Speed of Lap Rotation Using Strobe Discs

Over recent months I have seen a lot of articles and notes regarding polishing and cutting lap rotational speed. These articles give speeds in terms like fast, slow and medium speed. I’ve often felt that, what I regard as medium speed might be slow to other people. I then started to wonder what was the actual speed of my laps.

My attention was drawn to an article in Model Engineer 6th November ’98. The article ‘Strobe Dials’ by Bill Stevenson gives the information he used to calculate a number of single speed strobe dials. It became immediately obvious that this was the easiest method to accurately determine my lap speed. However, I did not like this set of dials as they were published, because they were single speed dials and to determine a particular speed setting they required changing on the machine until a match was found. In consequence I re-draughted the dials so that one disk covered the speed range 50 RPM to 300 RPM and the second disk covered 316 RPM to 1500 RPM. (The slightly odd choice of values is a natural consequence of the formula used to derive the disk.) The high speed disk is illustrated in Figure 1. The low speed disk is similar but each band has many more sectors.

Figure 1. The highspeed Strobe Disk

When used, one of these disks is placed on top of a lap and viewed under fluorescent lighting. As most people know, fluorescent tubes flicker because mains electricity is alternating current (AC) and the tube flashes twice during each cycle. You will know that the wheels of a wagon can appear stationary or revolve backwards when seen on the T.V. or in the cinema. A similar mechanism causes one of the bands on these disks to appear stationary when the speed of the lap matches that calculated for that particular band.

Stevenson, in his article, points out that fluorescent tubes flash twice per cycle so the tube will flash 6000 times per minute – 50 hertz x 2 x 60 seconds. Now, if the band has 4 sectors then it will appear stationary at 1500 RPM. Likewise a band with 10 sectors will appear stationary at 600 RPM and a band to indicate 50 RPM will have 120 sectors. As it is necessary to keep to whole numbers of sectors, a pattern drawn with 31/3 sectors, for instance, will not display properly.

As you know, the frequency of the mains supply in the UK is kept constant to within close tolerances – the accuracy which these disks give when viewed under fluorescent light is better than is required for our purposes. Because of different national standards these disks will only be accurate in countries where the mains frequency is 50 hertz (50 cycles). In America, for instance, mains frequency is 60 hertz so to obtain the speed of the lap you will need to multiply the calculated speed for the stationary band by 1.2 to give the correct speed; 60/50.

In most cases the pattern will not appear to be exactly stationary, but will rotate in one direction or the other. If the pattern appears to be rotating in the same direction as the lap is rotating, then it’s speed is faster than than calculated for the band, if it is rotating in the opposite direction then the lap is rotating slower than the calculated speed for that band. The precise speed can be worked out by timing the number of seconds it takes for the pattern to make one revolution. If you then divide 60 seconds by this time you get the number of revolutions per minute to add or subtract from the base speed for that band. Say it takes three seconds for the pattern to make one revolution, then the adjustment to the base speed is 60/3 or 20 RPM. This is added to the base speed for the band if the pattern is revolving in the direction of the lap rotation and subtracted if it is revolving in the other direction.

Sometimes you will notice that the stationary image appears to have twice the number of sectors than the band contains. The marks appear much fainter and closer together. This occurs when the disk is rotating at half the calibrated speed for that band – just be aware this can happen.

For my personal use, I’ve mounted these disks either side of a 200mm. disk that I’ve cut from some 8mm. thick foamed PVC board; card, ply, foamcore or any similar material will do just as well, I just happened to have some Foamalux spare.

A pair of disks suitable for use with machines which take 8” laps is included with this article; please feel free to photocopy them as you require. Those people who require disks for 6″ machines can reduce these disks in a photocopier to 75%.

A JPEG file to produce additional disks can be downloaded from this site. In the UK you need to download the files strobe50H.jpg and strobe50L.jpg for a mains frequency of 50 hertz. In the USA where the mains frequency is 60 hertz the correct version to download is strobe60H.jpg and strobe60L.jpg: the disks in this file have the correct RPM marked on the band – no conversion for the frequency is necessary.

Using these disks I’ve been able to determine the speed of my 1970′s Ultra Tec at each mark on the dial. I’ve found the speed of my machine doesn’t correspond to the rule of thumb given out for new machines. Howewer I now think I know the speed with sufficient accuracy for most purposes and am confident that when I think I’m running the machine at 300 RPM then it won’t be far off the correct speed.