Using a DRO

Using a digital readout

A digital readout (DRO) replaces the existing calibrated feed-screw dials with an independent measuring and display device. This may not sound a particularly impressive improvement, but the important words are ‘independent’ and ‘display’.

You do not need to have used a machine tool such as a lathe or mill for very long before the annoying problem of backlash in the feed-screws becomes apparent (backlash is the movement of the feed-screw handle before the axis, for example cross slide, starts to move). Because the DRO measures independent of the feed-screw, then this backlash is no longer of importance when positioning. Of course, backlash is still there, and allowance may need to be taken in case the tool snatches and moves the axis over the unrestrained distance that the backlash gives. The good news, for what it may be worth, is that the DRO will tell you where you are, even though you may not want to be there.

The latter important word, ‘display’, means that you now have something far easier to read than tiny, possibly well-worn, numbers around a calibrated dial. And if you have a machine with an 8 tpi feed-screw then positioning will definitely be much easier.

These considerable benefits of eliminating backlash and providing easier reading are not the only improvements a DRO can provide. There are a number of other features, equally useful but not perhaps so obvious, that provide benefits to help you. The only possible reason for including any of these features is because it is useful and will assist you in your work, making it easier, quicker and less error prone.

To explain how a DRO is more than a box with numbers on it, in the following sections we shall consider normal everyday machining operations, and how the DRO can help. Because BW Electronics make digital readouts, the following text uses one of these DROs as an example. The readout chosen uses the 332 Measuring Readout display together with two of the 300 series position sensors.

332 remote two-axis readout
The 332, a two-axis remote display, is a component of the 300 series modular readouts designed and manufactured by BW Electronics

Using the ‘zero’ keys to set datum points

A standard DRO will show an XY position on two displays. Each axis display can be individually zeroed to set a datum point. For example, when threading in the lathe the datum can be set at the outside diameter of the unthreaded bar. The cross slide can now be advanced for each cut until the correct depth of thread is reached. With the datum set at the outside diameter then rapid repositioning is possible because by looking for the reading changing from negative to positive you know the tool is in the metal. The next increment can be added and the cut taken.

External threads are normally straightforward to cut, not so with internal threads. In this case a second datum would be taken at the maximum length of the thread. Now, when taking the cut, as soon as the display sign changes from negative to positive then the feed nuts are released. With concentration the reaction time to releasing the feed nuts will be very similar, so the overshoot will also be consistent, certainly much better than a pencil line on the bed.

Using the ‘units’ key: switching between mm and inches

The second common facility on a DRO is the ability to display the position in either inches or millimetres. This has uses beyond the obvious one where a drawing is in millimetres, but the machine in inches. One example is cutting an imperial width slot in an otherwise metric-dimensioned part. With the DRO showing inches, touch the end mill first on the X axis then the Y axis to set the two zero datum points. With the end mill above the work, the axes can be positioned to both read zero; now move half of the end mill diameter so that the centre line of the end mill is over the combined XY datum, re-zero both axes. Now switch the DRO to read in millimetres, and move to the first end point of the slot.

Using the ‘scale’ key: finding half

But what if the slot is required to be in the centre of a bar? The slot drill can be touched on one side and the axis zeroed to set the first datum. The axis can then be moved and the slot drill touched on the other side of the bar. The position displayed will be the width of the bar plus the diameter of the slot drill. This value is halved to give the distance of the centre line from the first datum point.

While simple enough, this has two disadvantages.

  1. An arithmetic error will inevitably occur at some time, usually when machining a complex part that needs considerable concentration.
  2. The axis needs to be positioned to a non-zero value that has just been calculated. Again, this is not usually a problem until an interruption causes you to momentarily forget the position. Because by this time the starting point position has been lost there is no means of recalculating the centre position other than going back to re-establish the second datum position.

However, there is an easier method …

The 332 Measuring Readout has a key labelled ‘scale’, one of the functions of which is as follows. The procedure to find the centre line is as just described, but when the second datum point has been established, instead of calculating half of the displayed position, the ‘scale’ key is pressed. What this does is take the displayed position, which is the offset from the first datum point on one side of the bar to the second datum point on the other side of the bar, and halve it. This gives the offset from the first datum point to the centre line. Then, to make positioning on the centre line easier, the new position is corrected such that it now displays the offset of the second datum point from the centre line. What has happened is that the zero reference datum point has been moved from the first side of the bar to the centre line. Now all that needs doing is to move the axis to display zero, and that is the centre line.

Using the ‘scale’ key: radius/diameter working

The Measuring Readout can be used on many different type of machine tool, and the ability to find the halfway point between two positions will be useful. But on a lathe this is not the case; finding half of a diameter or length is not very useful. Therefore the Measuring Readout allows the ‘scale’ key to be changed to something more useful for a lathe, that is displaying in either radius (rad) or diameter (dia) dimensions for the X axis. If ‘rad’ is selected then the display will show the correct distance moved: moving the X axis 2.0mm will result in the position changing by 2.0mm. But if the ‘dia’ reading is selected then all movements on the X axis are doubled, so moving the X axis 2.0mm will result in the position display changing by 4.0mm. This is therefore displaying the change in diameter of the work, not the depth of cut. On a lathe the choice between displaying the depth of cut as the ‘rad’ or the change in diameter as the ‘dia’ can be changed by pressing the ‘scale’ key. Which of these is selected is shown by the illuminated indicators; if both are illuminated then the milling machine ‘half’ function is selected.

Pre-setting a position

Being able to turn and directly display the diameter is of considerable help; but how do you know what the diameter should be? The tool can be advanced to the exact centre line of the work, and the X axis zeroed to give a datum. This will now give the correct diameter readings with that particular tool. The snag of course lies in the word ‘exact’; if the centre datum is out by 0.1mm, then the diameters will be out by 0.2mm.

To remove this problem the Measuring Readout allows a position to be pre-set for either axis. The work is set up in the lathe so that a light skim cut can be made; this cut needs to have as good a surface finish as possible. The diameter of this cut, the reference cut, is then measured. With the Measuring Readout in ‘dia’ mode, the diameter of the reference cut is then entered into the X axis display. Until either the tool or work is changed the X axis will show the diameter being turned.

This pre-setting of a position can be done at any time to either one or both axes. On a milling machine the work may be so large that the reference datum point is outside the movement available for one or both axes. Then a secondary datum can be selected that is within the movement of the axes, and the offset of this secondary datum relative to the primary datum can be entered into the X and Y axes. This allows all subsequent machining, location of holes etc. to be done relative to the datum as defined on the drawing, without the need for any mental arithmetic in positioning.

Another use for presenting a position is the common occurrence of repeated hole patterns that need drilling – for example, for a series of wheel bearings along the edge of a plate. The axle centre of each wheel has an offset from a datum, and each set of bearing fixing holes are identical, being referenced to the axle centre. The first axle centre is moved to, then the offset from the axle centre to the first bearing hole is entered. If the mill is now moved to display zero then the first hole can be drilled. Subsequent holes can then be drilled by moving one or both axes to the correct offset from the first hole. When all the holes are drilled the mill is moved back to the axle centre. This position is an offset from the first hole drilled, with the offset sign being changed. Once positioned over the axle centre the offset of this axle from the datum can be re-entered and the move to the second axle can be performed. The procedure to drill the holes is now repeated for this axle and any subsequent axles.

Using the ‘datum’ key: setting global/local datum points

The ability to pre-set a position allows for easier working, in that full use can be made of groups of localised dimensions that refer to features within the larger job. It would be slow and error prone to have to add these local dimensions onto offsets from the reference datum position. But even so, this method is still somewhat clumsy in that position offsets need to be re-entered. There is some assistance the Measuring Readout can give from its ability to keep track of two separate datum points.

In the above example, the reference datum point is for the whole job. This datum point is located and both the X and Y axes are zeroed. The position of the first axle centre can then be moved to. Now the alternative datum point is selected for the local hole positions around the axle. The offset from the axle centre to the first hole is entered and moved to; from this position all the other holes can be located. When the first axle is complete the reference datum point is re-selected and the position of the second axle centre moved to directly. Then the local datum point is selected and the holes drilled as before. Because the two datum points are used, one for the whole job and one for local detail within that job, they have been called the ‘global’ and ‘local’ base respectively. There is in fact no difference other than this naming convention.

Recalibrating the DRO

The position sensors have been designed to measure linear movement, but they can be simply adapted to other measuring needs – for example the positioning of holes around a circle. There are well-established tables converting the angular position of the holes to XY coordinates.

An alternative is the rotary table. If the rotary table has a smooth circumferential surface then the sensor measuring wire can rest on it and hence measure angular rotation. An improvement is to machine a small groove for the measuring wire to run in. The problem is that the readout would still be measuring in millimetres or inches, and unless the rotary table was exactly the right size so 1mm = 1 degree, this is not very convenient.

However, the Measuring Readout allows each sensor to be recalibrated. So with the rotary table the sensor could be recalibrated such that 1.0mm does equal 1.0 degree, irrespective of the size of the rotary table. With a digital readout of angular position then setting out pitch circle diameter holes becomes easier.

But it will also allow dividing to be done, without the restriction of unobtainable teeth counts of normal dividing heads. A calculator is needed to calculate the angle of each tooth and the rotary table moved to that angle and the tooth cut. This method gives some advantages over using a dividing head, other than previously unobtainable division ratios. It is possible to cut variable pitch teeth. Even so it is still not very convenient because the angle of each tooth still needs calculating; this is automatic using the sector arms on the dividing head.

A considerable improvement would be to recalibrate the sensor not to read angular degrees, but to read tooth numbers. For example if an 88 tooth gear was needed, then the sensor could be recalibrated to read 22.0 with a rotation of 90 degrees. Now positioning for each tooth is simple; just rotate until the display reads with all zeros after the decimal point.

Digital Position Readouts for smaller machine tools