Sherline Indexing Attachment

P/N 3200

Many times a machinist is required to machine features into a part that are located radially instead of linearly. Examples of such features are the flats on a nut, the teeth on a gear or the splines on a shaft. The Sherline indexing attachment is a unique design that provides a very economical means of accurately rotating a part so that these kinds of features can be machined.

The attachment consists of an indexing head and a tailstock mounted on a 12-inch long dovetailed bed. Also included is a rack gear that is used to measure radial movement. The indexing head utilizes a spindle with a #1 Morse taper and a 3/4"-16 male thread identical to the spindles on the Sherline lathe and milling machines. This enables the use of all the Sherline accessories such as the faceplate, 3- and 4-jaw chucks, Jacobs drill chuck and centers with the indexer. A built-in mechanism detents the spindle every 5°, and the spindle is also graduated in 5° increments. The rack gear mentioned above provides a means of accurately positioning the spindle where positioning in other than 5° increments is required. The rack gear inserts into the indexing head and engages an index gear. This converts the rotary motion of the spindle into linear motion that can be measured with a Vernier caliper. Simple calculations then permit accurate indexing to an infinite number of positions.

The indexing attachment can be attached directly to the bed of the Sherline milling machine (See Figure 4), or the Indexing Head can be used by itself in a horizontal position similar to a rotary table (See Figure 2). The indexing attachment is not recommended for use on the Sherline lathe with the vertical milling column; however, with the use of shims to elevate the unit high enough to clear the crosslide handwheel, it can be done.

Operating Instructions

This indexing attachment has been designed to give the average hobbyist an all-purpose method of dividing circles into an equal number of segments to aid in cutting gears or any other repetitive, circular machining operation. It is of a price and size that makes it ideal for use with miniature machines. The dividing head can be used in both horizontal and vertical modes.

Although it has been designed to be used with the Sherline vertical mill, it can be adapted for use with other types of equipment or used for different purposes described in this booklet.

Before attempting any machining operation, be sure your setups use good machining principles and practices. Work in a careful, professional, craftsman-like manner, and ALWAYS wear SAFETY GLASSES.

NOTE DESIGN CHANGE: The illustrations in these instructions show the older style single piece tailstock with the center split and horizontal tightening screw. The design now incorporates a brass gib beneath the dovetail and a vertical tightening screw as is used on the lathe tailstock. The exploded view has been updated to show these parts.

Like any fine machine tool accessory, care should be taken to keep your indexing attachment clean and free from rust. Moving parts should be oiled periodically with sewing machine oil. The indexing head can be easily taken apart for cleaning when necessary.

Endplay can be removed from the head spindle by unlocking set screw #3214 (Ref #16 on exploded view) and turning clockwise to remove "play". Turning the set screw counter-clockwise reduces drag on the spindle.

1. INDEXED METHOD. This method is quite simple and uses the indexing lever and the graduated scale on the spindle. Internally the indexing lever engages with a 72-tooth gear, and each tooth equals 5° of movement. Obviously this method will only allow indexing of simple hole patterns since you can only work in multiples of 5°; however, this is usually sufficient for most jobs with the exception of cutting gears. Since very few gears will work out in even multiples of 5°, a second method of dividing called the "calculated method" can be used. It is described below.

It is important to remember to lock the spindle before attempting any machining. The indexing lever is NOT a lock and is not intended for any use other than to locate the index (See Figure 2).

2. CALCULATED METHOD. This method will yield an infinite number of divisions but takes considerably more time. To set up the head in this mode, the indexing lever must be raised to its uppermost position. The rack gear is then inserted from either side with the teeth towards the spindle under the lever. It is important that the spindle lock is loose so the spindle is free to move as the rack is inserted. The theory behind the calculated method should be apparent now. As the spindle is rotated, the rack moves in a linear motion that can be easily measured. If the total movement of the rack for one revolution is known, any number of divisions can be made by dividing this dimension by the number of divisions required.

The calculated linear dimension for one complete revolution is 4.712 inches (119.685mm), but this dimension may vary slightly from one indexing attachment to the next. For utmost precision, it is suggested that you accurately measure your particular indexing head and note the dimension for future use. Use a precise Vernier or dial caliper of at least 5 inches in length (6 inches is preferable) equipped with a depth rod (See Figure 3).

To determine this dimension for your particular indexing head, drill a small hole with a center drill on the very top edge of the faceplate with the indexing head mounted on its bed. Before drilling, be sure the rack is positioned in such a way that one complete revolution can be turned and still have the rack properly engaged. Make sure to lock the spindle and accurately measure from the end of the rack to the indexing head. After drilling, unlock the spindle and rotate it one revolution using the hole and center drill to index the spindle. Measure the rack again and subtract the smaller number from the larger. The difference should come out quite close to 4.712" or 119.685 mm. Since the accuracy of all your machining is dependent on the precision of this measurement, it is suggested that you DOUBLE CHECK your work! Once you have this dimension, dividing a part into any number of divisions is easy. Just divide 4.712" (or the dimension for your attachment if different) by the number of divisions you wish to make. This will give you the distance the rack must move for each division. The math is simple, but a small pocket calculator saves a lot of time.

EXAMPLE:
Say you want to cut an 83-tooth gear, 56 pitch (The size of the blank can be arrived at from Machinery’s Handbook and a similar type gear can be used to arrive at the cutter shape). The blank can be mounted on an arbor and held between centers. The dog is clamped on the arbor in such a way that it engages with the faceplate. Care must be taken to eliminate all "play" where the dog engages with the faceplate. Grind a tool bit that will give the desired tooth shape and use this tool like a fly cutter. Cutters can be purchased that will generate an accurate shape, but the are very expensive and hard to find. With a little practice excellent results can be obtained with the "fly cutter method" (See figure 4).

Once the cutter is properly held in a holder and has been located on center using the tailstock center for a reference point, the indexing attachment is properly clamped to the machine table and aligned with an indicator, and the rack gear is located in such a way a complete revolution can be turned, you are ready to begin.

Before you begin "making chips", look again at your setup; is it really SAFE? Also make sure you're wearing safety glasses. Remember, cuts of this type require very rigid setups because of the intermittent cutting action. If the gear blank is thin it may require additional support by sandwiching the gear blank between support pieces shaped like a large washer and of a material which is easily machined. Turn on the machine spindle and move the "Y" axis toward the cutter while moving the X-axis back and forth until the cutter just starts to touch the blank. Write down the dial setting and calculate the total depth of the cut. (The information to calculate this can be found also in Machinery’s Handbook.) The first cut should be less than .010" (.20mm) deep. Observe the cutting action carefully. Is the cutter cutting properly? Is there excessive vibration in the setup? Is the cutting speed proper? There is no book written that can give you the answer to these questions; this is where experience and craftsmanship come into play. The best way to make good parts is to work VERY CAREFULLY! To cut an 83-tooth gear means you have to do 83 successive machining operations correctly to make a good part...82 out of 83 is a waste of time!

Once your cutting speed and feed are to a point you're sure you can repeat the same operation over and over again with excellent results, finish out your first cut to its final depth. Now it is time to index for the next cut. Measure the distance from the end of the rack to the index head carefully before unlocking the spindle. Write down this dimension (it will be referred to as "A"). From previous instructions you have already figured the total throw of your indexing head—say it is 4.712". You now divide this number by the number of divisions, in this case 83 to get: 4.712 x 1/83 = .056771", or rounding off, .057". This is now added to or subtracted from dimension "A". At first glance it would appear that all you need to do is add .057" for each cut because an error of only .000229" is so small it can be discounted. But if you multiply this error by the total number of teeth (in this case 83) you would end up with an error of .019" which would make the last tooth you cut a very "interesting" shape. This is what is known as "tolerance buildup" and is the reason you must use your basic formula at each step to calculate the next dimension rather than simply adding rounded off dimensions. The second cut and each succeeding cut are calculated as follows: 4.712 x 2/83 = .113542 or .114". Add this to or subtract from dimension "A", index and cut. With each cut you make your understanding of the techniques involved will increase. By working and thinking in a careful manner you should be successful on the first attempt.

Holding WW collets in the Indexing Attachment

An adapter is available that inserts into the #1 Morse spindle to accept WW collets. They are pulled into place with a special short drawbar that is P/N 11682. See the page on WW collets for a description of the collets and links to available sets and sizes.

Although the instructions given here have been related to cutting a gear, the same approach must be used for any type of indexed machining.

1. Work ACCURATELY.
2. Determine the best possible way of holding the part to be machined. This type of machining requires a very secure setup.
3. Carefully align the setup with the machine.
4. Take cuts that the setup you are using can easily withstand.
5. Don’t try to rush the job! Successive machining operations make some people lax; therefore, it is wise to consider the amount of time and effort you will lose if you destroy your part rather than how much is left to do.

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