Using a DRO on a rotary table

Using a 300 series readout on a rotary table

DRO on a rotary table
The DRO sensors from BW Electronics can be used to measure angle on a rotary table.

Because sensors in the 300 series of DROs from BW Electronics use a flexible stainless steel wire to measure with, this can be wrapped around a rotary table to measure angular distance. And because the sensors can be re-calibrated, this angular distance can be set to read degrees and hundredths of a degree.

Note that the exact diameter of the rotary table is irrelevant; the only restriction is that the measuring range of the sensor should be greater than the circumference of the rotary table. The maximum diameter of rotary table for a 361 sensor is 6 inches, while a 12 inch maximum is possible with the 362 sensor.

Fitting the sensor on the rotary table

The measuring wire needs to run in a groove around the outside of the rotary table. The ideal groove shape is a Vee with a flat bottom. Because the wire will need to overlap for a short distance, the flat needs to be at least twice the diameter of the wire, which is 0.016in or 0.4mm. Note that this flat must be exactly vertical, parallel to the axis of rotation of the rotary table. The easiest method of making this groove is to use an HSS Vee tool and gently stone a flat at the tip.

It is essential that the groove is exactly concentric to minimise errors. One practical way of achieving this is to machine the groove in place on the rotary table. First clamp the Vee tool to the table (use toolmakers’ clamps). Then gently tap the free end of the tool so that it digs into the table. Now rotate the table and the tool will cut the groove, which must end up exactly concentric. Keep feeding in the Vee tool and cutting the groove until the required depth is achieved ­ this needs only to be 0.020in or 0.5mm.

Now a fixing for the wire ring is needed. The easiest method is a tapped hole which will clamp the ring, allowing it to be positioned so that the wire enters the groove without masking it. (The groove must not be masked otherwise after a full turn of the table the wire will catch on the ring before going into the groove, thereby causing errors.)

The body of the sensor can be fitted to the rotary table. The wire needs to come straight out of the sensor to the groove, so some type of bracket on the side of the rotary table is needed. The sensor can be clipped, or fixed more permanently, where any built-in display (as on the MPS3) can be read.

Calibrating the sensor to read in degrees

With the sensor fixed and groove machined, now you can calibrate the sensor to read in degrees.

  • Set the display so that it powers on in millimetres.
  • Set the rotary table to a convenient zero point.
  • Power on the readout in re-calibration mode by pressing and holding the [set] key.
  • Move the rotary table a known angle, say 360, 90 or 60 degrees.
  • Use the numeric or inc/dec keys to adjust the display to show this angle.
  • Press and release [set] to end this procedure.

The readout is now calibrated to read in degrees and hundredths of a degree.

If an angle of less than 360° was used in the re-calibration, it is wise to check it. Therefore unwind or move the wire out of the groove and carefully clean the groove. Now replace the wire in the groove. Rotate the rotary table more than 360°, so there is a reasonable amount of overlap (say 1­2in, 25­5mm) at the 360° point. Very carefully, identify a zero datum point on the table, using a magnifying glass if necessary, and zero the readout.

Now rotate the table to unwind the wire until the previously identified zero point is reached. The reading on the readout should be 360.00. On the offchance that it isn’t, the discrepancy needs explaining. Possibilities are:

  • Eccentric table: when the groove was cut it may have been obvious that the groove varied in depth. In this case, re-calibrate the sensor again, but this time use the whole 360° rotation.
  • The difference is only 0.1° or 0.2°. This is potentially within the error of the sensor so it could be ignored.
  • The main rotary table bearing may have play that allows sideways movement, or the worm wheel is eccentric and binds on the worm causing strain and possible distortion. In either case there is little that can be done other than repairing or renovating the rotary table.

Using the readout and rotary table

With the rotary table and readout now calibrated, it can be used. In all cases, follow the procedure given below to eliminate any problems of swarf getting trapped between the wire and groove.

  • Carefully clean the groove, the side of the rotary table and the area between the rotary table and sensor.
  • Wrap the wire around the rotary table at least as far as the angle being machined.
  • Check the wire is in the groove and it is still clean.
  • Machine the component on the rotary table by unwinding the wire from the table into the sensor.
  • If the part requires two arcs at different radii, clean the wire and groove, rewind until past the start point, check for any trapped swarf, then machine by again unwinding the wire.

Use as a dividing head

Because the readout sees angles on the rotary table simply as numbers, it makes no difference if the 360° is calibrated as 121, 53, 487 or any other number. This allows the rotary table and sensor to be used as a universal dividing head, being able to cut any number of teeth to a practical limitation of several hundred teeth.

The procedure is exactly as described previously, except:

  • Calibrate over a whole turn to the number of teeth required.
  • Move the rotary table to whole numbers, 1, 2, 3 etc. to position for cutting the teeth.
  • If large teeth are being cut, then do every tenth or twentieth tooth to minimise errors due to thermal expansion of the block.

When calibrating it might be easier to follow this procedure:

  • Clean the groove, sides and area between the rotary table and the sensor.
  • Pull out the wire while cleaning it using a paper kitchen towel.
  • Attach the wire to the rotary table with approximately 10° to 20° contact distance.
  • Identify a zero datum.
  • Calibrate the sensor to the teeth number by winding the wire onto the rotary table until the previous zero datum is arrived at, i.e. 360° exactly.
  • Enter the teeth number into the readout and end the re-calibration.

With the wire wound on you can start cutting the teeth.

Accuracy and resolution

The sensor accuracy is ±0.05mm or ±0.08mm depending on sensor type. If the wire is wrapped around a 4″ or 100mm rotary table, then because the circumference is very nearly 360mm the accuracy will be ±0.05° or 0.08°. In this case the sensor resolution, approximately 0.006mm, is less than the display resolution. This means that every value can be obtained.

If a 12″ rotary table is used, then the circumference is 940mm. This means that every 10mm of wire movement has been rescaled to display as (360 ÷ 940)mm = 3.8mm, or 3.8°. Because the value displayed is less than the distance moved, the sensor resolution of 0.006mm is the same, but it is now 0.006 × (360÷940) = 0.0023°. This also means that the accuracy, ±0.08mm, has now become ±0.08 × (360÷940) = ±0.03°.

What if inches rather than millimetres were used? In this case the circumference will be 360″, and every 1″ of wire has been rescaled to display as (360÷38), about 10″ or 10°. The value displayed is now greater than the distance moved, by the factor (360÷38) or about 10. This means that the sensor resolution has also been multiplied by this factor when it is displayed, so the display will change in steps of 0.000 25″×10, or 2.5 thou. Similarly the accuracy will also have changed by 10, to ±0.030″. This may seem to be far worse than previous, when the rotary table was re-calibrated in millimetres, but it isn’t in reality.

The important point is that the display is in degrees, not inches or millimetres. Therefore the previously calculated resolution of 0.0023° and accuracy of 0.03° are exactly the same (given that the maths has been rounded to one decimal place) as the ‘inch’ values of 0.0025″ resolution and 0.030″ accuracy. All that has changed is that by using inches an extra digit of resolution has been obtained. Is this important? No, not really. Since dividing is a process of moving between sequential positions, repeatability does not come into it. Because the error is larger than the least significant digit (±0.03 error to display 0.01) then nothing can be gained.

All this may seem of little importance, and even less interest. After all, one hundredth of a degree is is equivalent to one inch around a circle of diameter nearly 1000 feet. However, where it is of importance is when the rotary table is used as a dividing head. Because the number of divisions, teeth, per 360° is changing, then there are benefits to using either inches or millimetres as the basis for re-calibration. As the number of a teeth reduces there comes a point where the extra digit of resolution on an inches display becomes useful.

For example, suppose you need a 45-tooth gear and you are using a 12″ table:

  • Metric scaling: 1 tooth is 21mm
  • Inch scaling: 1 tooth is 0.8″.

With metric, the scaling factor is 21, so the sensor resolution is (0.006÷21) = 0.0003, and accuracy (0.08÷21) = 0.04. The accuracy is less than the display resolution so it is better to use inches to gain better accuracy because of the additional digit in the display.

As a rule of thumb, if the number of teeth divided by the diameter of the rotary table in inches is greater than 10, use millimetres; if less than 10, use inches.

There is another restriction on the minimum number of teeth that can be cut because of increasing rounding errors in the readout as the number of teeth decreases. This puts an effective minimum limit of the number of teeth of twice the diameter of the rotary table in inches; e.g. 24 teeth for a 12″ rotary table. There are two possible workarounds:

  • Use a smaller rotary table.
  • Re-calibrate for twice or ten times the number of teeth.

Conclusion

This is a different approach to dividing. A dividing head, because it is circular, will always come back to zero errors after one revolution. The rotary table and sensor method uses linear measurement and is therefore not guaranteed to come back to zero after one revolution.

So if you are cutting an important gear it is wise to do the following:

  • Re-calibrate and set up as described.
  • Cut the first tooth space, but only a few thousandths of an inch or tenths of a millimetre deep.
  • Brush all swarf away from the rotary table.
  • Turn the rotary table through 360.00°.
  • Re-cut the first tooth space, deeper than before, but not across the whole gear face.
  • You can now visually check that the two cuts are in the same place.
  • If this is OK, turn back 360.00° and cut the teeth.

Digital Position Readouts for smaller machine tools