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Workholding – Rotary tables – introduction

The rotary table is simply a round flat surface that can be rotated. What makes it interesting is that the table is driven round using a worm and wormwheel arrangement. This means that if a workpiece is mounted on the table it can be machined as it rotates.

Often the workpiece is mounted directly on the table but it is also possible to mount other sorts of workholding device on the rotary tables surface.

fig typical rotary table set up for turning by degrees

fig rotary table set up for dividing

The worm and wormwheel

       On a rotary table the ratio between the worm and wheel is often about 40:1 on a small table but increases with the size of the table. For example a 360mm (12-inch) table might have a ratio of 120:1.

      Rotary tables are calibrated round the edge in degrees and have a handle which turns the worm and which, in turn, will rotate the table by 360º divided by the ratio of the worm and wheel. For example, a 360mm (12-inch) table with a 120:1 worm and wheel will rotate by 3º per turn. The handle mechanism has a rotating dial and a Vernier so the angle, on a larger rotary table, can be measured to a few minutes.

 Adjusting the worm

      Naturally any worm and wheel arrangement on a rotary table is likely to have some backlash. Sometimes this can be compensated for by adjusting the distance between the worm and wheel. Usually any backlash can be ignored if the movement, when machining, is always one way.

      The same mechanism can sometimes be used to disengage the worm from the wormwheel. This is useful on large rotary tables because it enables the user to turn the wheel quickly to get from one position to another. (It is not possible to machine the workpiece whilst doing this.)

 The hole in the middle

      All rotary tables have a hole in the middle of the table. This is usually a parallel-sided hole but some, especially on smaller rotary tables it is tapered.

       This hole can be used to take spigots that can be used to align the rotary table or align the workpiece on the table. Sometimes it is possible to fit a bolt through this hole using various spacers, washers, etc to hold the workpiece on the rotary table.

It is possible the get a device that has a taper on one end that is designed to fit the taper as found on some small rotary tables. The other end has a thread that is designed to fit the backplate as used on the chucks used on some lathes.

 Horizontal/vertical fixing

      All rotary tables can be mounted in the horizontal position on the milling table. Some are designed so they can also be mounted vertically without any other hardware.

      Most have slots in the base so they can be bolted to the milling table. Some do not but have a flange so that they can be clamped to the milling table.

      The combination of a horizontal rotary table mounted on an angle plate will give the equivalent of the vertically mounted type.

Tilting rotary tables

      It is possible to buy rotary tables that have the facility to tilt the table built in to them. Some even can be tilted at any angle in either or both of two planes at right angles. But all of this adds significantly to the height and weight of the rotary table.

      The usefulness of being able to tilt in two planes is very limited and would probably not justify the space it would take up. But a rotary table that tilts in one plane can be useful. This setup can easily be emulated by fitting a rotary table to a tilting table.

fig tilting rotary table emulated by using a tilting vice

Locking the rotary table

      Most rotary tables have some means of locking the table at any particular position. Very often an operation is done whilst the table is being rotated in which case the force of the cutter cancels any backlash. However when an operation such as drilling is being done at a particular point then the table should be locked.

 

Stops

      Very often a cut needs to be made between two points at the ends of a particular arc. Usually it is not possible to make the cut in one go but several passes are needed. In this case it is useful to have two stops so each cut will start and stop at exactly the same points. This is very useful for preventing mistakes.

      The fig. shows a stop. The movable part clamps to the top of the rotary table’s table. Two of these are needed. The fixed part has been fitted to the hole normally used for the locking mechanism as shown in the previous fig.

It will be noticed that the same hole on the rotary table is used for both locking and for a stop. But, of course, in practice, it  will, at any one time, only be needed for one function or the other.

      Of course, the ends of any cut like this will be round. If square ends are needed this has to be done using a file.

Size/weight of the rotary table.

For milling any particular workpiece on a rotary table one has to allow for the space around the workpiece for the clamps used to hold it. For example a 200mm rotary table might hold a workpiece that had to have 120mm hole cut into it. It will be shown later how to effectively extend the diameter of a rotary table. It is often desirable to get the largest rotary table that will fit the milling machine table. However larger rotary tables can be very heavy.

With a large milling machine the practical limit is probably the largest you can lift safely. It is possible to have some sort of lifting gear but this all takes time. It is worth looking carefully before buying because for a given diameter, different makes or different methods of construction can cause a rotary table to vary dramatically in weight.

 Height of the rotary table

      There will usually be enough space between to milling table and the cutting tool to fit a rotary table to do any required job. But there is always the height of the workpiece to consider. If other devices are to be mounted on the rotary table then the space rapidly disappears.

      Rotary tables with a low profile are rare but can be very useful.

 Rotary tables – operation

Most rotary table are set up as shown above to be rotated by a certain number of degrees. This is done using the calibrations on the table marked in degrees.

      If it is necessary, when using a rotary table to divide a circle into a number of equal sectors then it is necessary to divide 360º by the number of sectors required. On a small rotary table, the table might only be calibrated to 5°. On larger ones they might be calibrated to individual degrees round the edge but will have a vernier arrangement on the handle so they can be set to a certain number of minutes. This gives us the angle between the sectors. Each time we move from one sector to the next we have to add the angle per sector onto the last angle. For any but the simplest numbers, the chances of getting this right are not great.

      It is possible to have dividing plates fitted to a rotating table, as shown above, but this is unusual. But since dividing plates are always fitted to dividing heads these will be covered under dividing heads.

It is quite common to need to be able to divide a circle into so many parts. With dividing plates this is easy and is covered elsewhere. for a rotary table using just degrees and minutes a circle can be divided by one of the following methods.

 A     calculate the angle in your head or using a calculator for the origin for each sector and write them down. Most simple calculators will give decimal angles whereas the rotary table is marked in degrees and minutes.

B     use a spreadsheet to produce a list of angles. These will probably be decimal angles. But is then quite easy to turn decimal degrees into degrees and minutes.

 C     Use tables showing the angles for each position for a circle divided up to 200 sectors can be found in Appendix C. These are in degrees and minutes.

D     use the table for the first 200 sectors that can be found in “Tables for [the] Cooke Optical Dividing Head” published by Cooke, Troughton and Simms.