MAKING AN INDEXING PLATE

 

I have been singing the praises of indexing plates when used in conjunction with milling or drilling devises but the truth of the matter is that they are very expensive to purchase and impossible for the home machinist to make. HA! I'm lying. Nothing could be further from the truth. I have made a great multiple row set of index plates that directly mount between the spindle pulley and the head stock of my little TAIG lathe and provides me with a multitude of divisions for just about anything I may need to mill. The thing that causes the most anxiety is thinking about how in the heck are we going to space all those little holes around the plate. Lets look at the way it is traditionally done. Let us say that you want to create one hundred equal divisions or holes around a round object. You would take the object and chuck or clamp it to a rotary dividing table which by the way, will set you back at least $250 to $300 even for an import. The table has a full 360o protractor around its periphery and a side crank that is further subdivided into either 50 or 100 divisions. The gear ratio of most tables gear boxes is 40:1 so simply put, for each full turn of the crank, the table will rotate 1/40 of a revolution. Each turn or 1/40 of 360 equals 9o per turn of the crank. To divide a round disk into 100 divisions you index every 3.6o ( 360 divided by 100 = 3.6 ). By dividing 9o ( for each 1/40 turn) into the number of divisions on the crank and multiply the result by 3.6 you will get what fraction of a turn of crank you need to achieve a rotation equaling 3.6o. Take for instance a crank with 100 divisions and with calculator, divide by 9 resulting in 11.111111.... infinite number. Multiply 11.111111.... by 3.6 and you will get 40, That will be the number of divisions of the crank you will have to turn in order to rotate the table the required 3.6o. If the crank has 50 divisions the result would then be every 20 divisions. At this point you would have the work piece attached, centered and shifted to what ever lateral distance you require and holes or other milling work performed with a drill press or vertical miller. The rotary tables can be used horizontally or vertically with or without a supplementary set of indexing plates, but this really gets deep into the subject and a little bit beyond our present discussion.

To many of us, owning a rotary table with dividing plates is a luxury that should only be considered after we have made a certain commitment to the hobby as it can be a costly undertaking. Before we get into the alternatives to the traditional methods, I would like to describe the basic process of making index plate blanks. I will show how I made them for my TAIG, but again, you could easily modify the specifications to adapt it to the SHERLINE or any other small similarly styled lathe. I begin with 3/16" or 1/4" thick aluminum plate about 4" x 4" and I scribe a circle a bit smaller than 4" with a pair of dividers or even a plain grade school compass. It does not matter as long as it is readily visible. Scroll cut or band saw the circle so the cut is just outside the line. Take the rough cut plate and glue it with CA glue to a small 2" faceplate so the center mark created by the compass is centered by eye as you look through the rear of the faceplate. Take a turning cut until the plate is perfectly round. Follow this with a facing cut. Remove the plate by gently apply a torch fame to loosen the glue. I machined a shallow step about 1/8 from the edge of the jaws on my four jaw scroll chuck so I can grip large diameter thin workpieces such as this. Grip the disk so the un-machined face is out and the clean surface is flush against the jaws. Set up the tail stock drilling guide and drill a 1/2" hole and then enlarge it to .625" and check the bore will a know .625" diameter rod. You are now done with the basic index plate blank. If you plan on making more than one of these you should make them all now and save them for future use. You will have to make a special arbor out of a 2" long piece of 1" diameter rod, facing and turning the end down to 5/8" diameter snug fit for a distance of 7/32". Drill and tap a hole 1/2" deep for a 10-32 screw. Flip the rod end for end and drill a #11 hole about 1/4" deep and set up a parting tool to part off a 1/4" thick washer from the end of the rod and remove all burrs. Reduce the 1" diameter portion of the rod to 1-1/4" in length by taking repeated facing cuts. In use, the plate is slipped over the small end and the washer is placed over the plate and is secured with a 10-32 cap screw. Put this aside for now so we can concentrate on the various dividing methods we can use. I will assume that you own or at least have access to a drill press. The simplest method is to use a circular saw blade and index off the teeth. As you know, circular saw blades come in many different teeth numbers to cut different types of materials. A 60 tooth blade can make a dividing plate with 60 or any other number of holes that can be equally divided into 60. There are 60, 80, 100, and even a 200 tooth blade. These are 7-1/4" in diameter and lo and behold they have a 5/8" diameter arbor. This was actually one of those happy coincidences you always hope for. I next cut a square of 3/4" thick plywood 7" x 7" and after drawing two diagonal corner to corner lines to find the center, I take a compass and set it to draw a 7" circle, band saw it the circle carefully to the line and finish by boring a 1" diameter through hole with a Forstner drill bit. Glue and press the arbor so the 1" to 5/8"shoulder is a hair above the surface of the board. The saw blade with the desired number of teeth is slipped over the 5/8" arbor and the plate blank is added on top . The top of the 5/8" diameter portion of the arbor should be sitting below the surface of the plate blank. Add the washer and cap screw to secure the sandwich. If you cut the round wooden plate to 6-3/4" diameter, the blade's teeth should protrude 1/4" all around the perimeter of the board. It is important that the tooth gullet extend beyond the edge of the plate. Prepare another 7" x 7" piece of 3/4" plywood and after drawing diagonal lines, bore another 1" diameter hole in the center to accept the bottom 1/2" long portion of the arbor. The top assembly should rotate easily but snugly. Draw a vertical line at 3-1/2" from either side of the square bottom board to locate the center point and attach a 1-1/2" long piece of 1/16" or so diameter piano wire or any other sturdy spring steel wire with several heavy duty staples and the cover the area with a thick layer of 5 minute epoxy. Make sure the wire does not shift or slide side ways in any way. This will make a flexible detent that will lock the top plate at each of the positions between each tooth. Pull back the wire detent to disconnect it from a tooth gullet and rotate the top plate to the next gullet and release the detent. The whole unit is clamped to the drill press table and centered by eye with chucked center point and aligning over the cap screw center. The table is now shifted laterally so the holes can be located and drilled around the edge of the plate blank. About 3/16" from the edge is good. Lets drill some holes! Begin by chucking a #1 center drill and carefully adjusting the depth travel so that a 3/32" diameter conical hole is created. Locate the top disk to any tooth at this time and drill the first hole. Pull back on the spring wire detent and rotate the top disk to the next tooth and let go of the detent to lock and the top plate at the next position and drill the second hole. Repeat this same procedure until all of the holes have been drilled and you are back where you first began from. If you want to create a multi row index wheel, you should use the traditional method of making the subsequent inner rows 3/16 apart center to center, with a lower number of holes. For example, you could make the outer row using a 100 tooth blade and follow with 60, a 40 tooth blade. Now that you have easily conquered what was once a huge obstacle, you can index and drill three mounting holes so you can attach it to the large end of the spindle pulley. Use the 60 tooth blade for this and shift the table laterally to place the drill point about 3/4" from the edge of the 5/8" center hole. Do not drill all the way through. Remember, the saw blade is still underneath the index plate. Once the locations for the holes are center drilled, you can remove the plate off the indexing apparatus and drill the holes completely through on the drill press with #11 bit. Counter sink the holes for flat head machine screws. Take the piece of 5/8" diameter rod you originally used to gauge the bore of the index plate, slide it through the pulley bore and stick a couple of small pieces of double stick tape to the large face of the faceplate and slip the index plate through the rod against tape and hold tight for a minute or so. The two should be in perfect alignment now. Place a #11 transfer punch through each of the three holes and tap it with a hammer to transfer the three hole locations. un-stick the plate blank and remove the tape. The punched holes are center drilled, drilled with a #21 bit 1/2" deep and tapped with a 10-32 tap. Screw the index plate and pulley together with three 1/2 long 10-32 machine screws and re-install it on the head spindle so the plate is facing the head stock. Prepare a block of aluminum by drilling four 1/8" holes in line and each 3/16" apart located so they match the distance of the different rows of the plate. Turn a piece of 1/8" drill rod to a conical 60o point to serve as a detent. The block is aligned and glued to the side of the TAIG head stock with CA glue so that each of the holes lines up with each of the rows of holes. The opposite end of the detent can be cross drilled and a smaller piece of drill rod inserted to act as a lever.

Other methods utilize a length of old bandsaw blade cut to length to give the desired number of teeth. Measure the length of the piece of bandsaw blade and divide by 3.146 to arrive at the proper diameter wooden disk. Wrap the blade around the disk and epoxy in place. Indent from the tooth notches as in the previous example. This method is a bit more difficult as the disks have to be very accurately cut so the ends of the blade meet flush as it is wrapped around. A third method employs making strips of paper with as many divisions as are needed penciled in every 1/4" or 3/8" apart or what ever other convenient increment. Figure out the diameter circle needed for the length of paper strip and indent by eye by lining up the wire pointer with each of the marks. This may nor seem accurate, but it actually is, because the disk you are using may be up to 8" diameter and you will be laying out a row of holes at about 3-1/2" diameter on the index plate and any subsequent work that is indexed during machining will be even smaller so any small inaccuracies on the large disk will be reduced so much as to become negligible. This does not allow for a sloppy approach!

All of these methods are good although some are less convenient than other and require various sizes wooden tops, bottom disks, strips of paper, saw blades and even expensive rotary tables. I have instead come up with a superior method which requires only two simple fixtures made of aluminum that can be made directly on your lathe in about one hour. The idea came to me as I was looking at the thread cutting gear set that came with my larger lathe. I thought about how great it would be if I could use these gears as templates to index from. The complete set has gears from 65 to 20 teeth. All I needed was to figure out a way to simultaneously hold the index plate blank and the chosen gear together in sandwich fashion. I began by taking one of my TAIG, 2" diameter faceplates and as in the chapter about faceplates, I turned an aluminum disk about 3-1/2" diameter that I attached to the faceplate and machined true and round. The center is drilled and the bore enlarged to accept a 3/4 diameter rod about 1" long. As you test the fit of the rod, make sure that you do not bore into the threaded portion of the faceplate. With a bit a CA glue, press fit the rod into the bored hole and once it is fully cured, I true up the sides and face it just so you can start with a true running rod. Reduce the overall diameter of the rod to .625" right up to the faceplate by adjusting the depth stop of the TAIG. Now that the rod is 5/8" diameter, bring the cutter to the stop and with a dial indicator bearing against the side of the carriage, advance the tool toward the right for .187" or 3/16" and reset the depth stop to that setting. Begin to reduce the diameter of the remainder of the rod to a snug press fit into the gear's center hole. Begin taking facing cuts to gradually reduce the length of the gear stub to .010" less than the width of the gear hub. Check the finished gear stub with an actual gear by pressing it in place. If it is too tight, take off the tiniest amount of material and check again. Drill the end with a #21 bit all the way through and tap for a 10-32 thread. Slightly counter sink the hole to remove any threading burrs. Unscrew the faceplate and chuck a short piece of scarp 3/4" aluminum rod, turn it true and face it. Drill a #11 hole about 1/2" deep and set up a parting tool and part off a 3/16" thick washer from the rod. Take one off the indexing plate blanks and slip it over the end of the faceplate arbor, over the 5/8" diameter portion of the stub, press the chosen gear onto the gear stub and install the washer with a 1/2" long 10-32 cap screw to lock the two securely against the aluminum face of the faceplate. To lock the spindle between each tooth you need to built a simple device to hold the indent. My TAIG 's head stock is " T " slotted on the top so I built a 2" square 1" thick slab of aluminum and after machining all six sides square, I drilled four #1 vertical holes 1" apart center to center for screws and " T " nuts. A 3/4"x 1/2" rectangular 4" long piece of bar stock is clamped flush with the left edge and square to the block and the lathe's center axis, between the locking bolts so that it is overhanging the faceplate. The 3/4" square bar can either be bolted to the cube or glued with CA glue. With the plate blank and gear in place, locate for a vertical hole centered on the end of the over hang so that it is centered on the bar and between the gear. Remove the bar and cube, clamp and drill vertical hole to 3/8". Drill and tap a hole through the end of the bar along the center line of the hole for a 10-32 knurled knob that will lock the indent rod in place during use. The indent is made from a piece of 3/8 drill rod about 3" long with one of the ends ground to match one of the gear teeth. Check as you are grinding against an actual gear to check your progress and stop when the point is a perfect mesh between a tooth space. Harden the whole rod and leave the oxide that forms on it and lubricate with some machine oil. Install the intent into the vertical hole and lightly tighten the knurled knob. Slide the 2" square along the "T" slots and adjust the alignment so the 3/4" bar is parallel to the lathe axis and the tip of the indent is above the middle line of the gear and about 1/4" above the gear teeth and tighten everything down. The 10-32 cap screws that secure the unit to the headstock pass through #1 holes which are large enough to allow sufficient room for adjustment to square the block to the spindle. The Spindle is indexed from teeth to teeth by lifting the indent, turning the spindle and sliding the indent down to the next tooth position and tightening the knob. You will need the flexible shaft tool post drilling unit and the depth stop set to create 3/32" diameter conical holes with a #1 center drill. Set the drill point about 3/16" from the edge of the disk and record the setting of the cross slide dial so the next few rows of holes can be accurately placed every .187" ( 3/16" ) in from the previous one. I normally would begin with the 60 tooth gear for the outer row, followed by the 50 and 40 tooth gears. This device works great and if you so desire, will let you mass produce these plates and maybe you can start to offer them for sale to others. So as you can see, with a simple rather small device and three of four different gears you can easily create as many indexing plates as you would ever need

Number of divisions possible with the following number of holes:

 

40 holes 50 holes 60holes

2 2 2

- - 3

4 - 4

5 5 5

- - 6

8 - -

10 10 10

- - 12

- - 15

20 - 20

- 25 -

- - 30

40 - -

50 - 60

 

You can see that the 60 hole row will provide to most equal divisions and the 40 and 50 hole rows only fill in the 8 - 25 - 40 - 50 gaps. The 50 hole row will let you make engraved dials will 50 divisions so that one by itself is important for me. I can't see needing 40 holes but I would indeed need eight to make eight fluted pillars and other miniature wood members. It will also let you mill a forty tooth gear if you ever need one.