SQUARING THE CIRCLE

Part two by Graham Meek

It goes without saying that attention must be paid to the concentricity of all the components during manufacture. Carrying out as much machining at one setting before parting off and finishing the component off on a known true mandrel will take care of this requirement. Taking time to do a good job of a mandrel pays dividends on jobs like these. Not only can the mandrel be used for turning purposes but it can also be used in the Dividing Head to cut gears, or machine facets on workpieces.

Getting back to the design you may recall I mentioned the Symonds nut on the end of the handwheel pinion shaft was used to adjust endfloat.
Looking at the GA, the reader will see this threaded portion is now buried inside the handwheel dial assembly. To overcome this problem the pinion shaft is now free of any endwise adjustment. A small step on the end of the Input gear makes contact with the end of the pinion shaft and makes sure this end of the pinion shaft is always below the Apron wall surface. Thus ensuring that when the nut securing the handwheel is locked up, the whole assembly is still able to rotate. Any endfloat in the assembly is taken care of within the ball race, and by careful attention to the respective lengths on the drawing. The unit is further restricted by the Dial Backplate, as this is attached to the Apron wall by two M4 Cheese Head screws, Photo 4 (below).
Care needs to be exercised when drilling the Apron as the wall thickness is not too great and it is not desirable to introduce swarf into the Apron gear box. These retaining holes are spotted through one at a time when the dial assembly is about to be fitted to the machine. Temporarily introducing a spacer, (a couple of thick washers), between the backplate and the Apron wall will permit the backplate to be locked in position for the purpose of spotting the first hole. Just remember that there is a ball race, (6803 RS), in this gear/backplate assembly when tightening the nut, it does only require ‘nipping up’. The second hole is located once the first hole is drilled and tapped. Using this tapped hole to position the backplate for the second hole, ie no spacers required. A depth stop made to suit the tapping drill to limit the depth to no more than 6 mm is well recommended.



The reader will see from the drawings that he Input gear has two keyways.The keyway in the bore of the Input gear was cut in the lathe using a lever operated attachment I made many years ago, Photo 5 (above).

This photograph was taken during the manufacture of the Super 7 handwheel dial designed many years ago. It will also be noted that the existing keyway in the Emco handwheel plays no part in transmitting the drive to the pinion shaft. This is taken care of by a second Woodruff key in the handwheel boss. This boss incidentally needs to be reduced in diameter to give a good bearing area in the input gear. As well as to get rid of the existing undercut which used to house the friction spring. The boss was further reduced to make the wall thickness in the Input gear a little stronger.

The outer face of the Input gear is what takes the locking forces of the Securing Nut/Bolt. If this wall section were too thin then the Input gear could go barrel shaped under the clamping forces, thereby locking the unit solid.

To enable the machining of the handwheel a spare part was obtained, but with careful planning the handwheel from the tailstock could be borrowed, or used for the machining process. Cutting the Woodruff key seat also needs careful planning.

Having set this job up once to cut this key seat I found that the body of the collet chuck fouled the Rib of the handwheel. Luckily I spotted my error before starting to cut metal and no damage was done. This resulted in a reset, finally cutting the key seat at 45 degrees to the handwheel ribs. The keyseat is purposely offset on the 10 mm spigot, ie, not central. This is because a 10 x 3 mm Woodruff key cutter is usually 10.5 mm in diameter. Thus if the keyseat was put in at 5 mm from the inner face the cutter would promptly cut a gash in this face of the handwheel, or worse. This dimension also means the Woodruff key needs to be shortened on one side, so that it does not foul the inside face of the input gear on assembly.


To utilise my existing stamping jig, Photo 6, used on the Super 7 design, the inside of the dial is bored out to 60.25 mm diameter by 5 mm deep. This makes room for the teeth of the Sleeve gear and clears the teeth of the idler gear. The outside face of the dial is also bored out to mimic the existing Emco handwheel dial. I decided at the outset to take advantage of the larger material to hand and make the dial larger in diameter. This does mean the dial overhangs the edge of the Apron wall slightly, whereas before it didn’t.

As luck would have it, the increase in diameter, and therefore the circumference, means the additional divisions for the extra millimeter of travel work out to be the same width as the original Emco design. There being 100 divisions in total, giving 0.2 mm per division. It is very easy to subdivide these divisions to get 0.1 or even 0.05 mm increments if desired, although I do tend to do these with the top slide where I can.

The reader may be wondering what the two centre lines shown at 42 degrees on the outside diameter of the backplate are for. These are the locations of two small holes that will take the original modified Emco index plate.

This plate was gently eased off the Apron wall using a wide Craft tool blade in my X-acto handle. The index plate was then machined to remove the two ears of the curve which originally followed the original dial circumference. Once this was completed the existing holes in the plate were drilled and countersunk to take the attachment screws. These are small Pozi self tapping screws salvaged from a VHS recorder long ago.


The plate was then passed through my small set of George Thomas bending rolls, Photo 7, to form the radius to match the dial backplate. There is no position given for the index plate holes on the backplate with reference to the dial face, as each index plate will be machined slightly differently.

Having set the backplate up on a mandrel in the dividing head and indexing the first hole position. The index plate is offered up to the backplate, while a small turned pin in the drill chuck is lowered via the quill to locate in the respective countersunk hole. Moving the milling table on the X-axis will position the index plate to where it is needed. Securing the index plate in this hole the second hole can be indexed round in the dividing head and drilled. Just to be on the safe side offer up the turned pin to make sure the hole lines up, before drilling.

The outside diameter of the backplate was purposely made smaller by two thicknesses of the Index plate. Thereby having the dial divisions and the index line on the same level. Should the reader decide to do away with this index plate and engrave a line for the index mark, then the backplate can be turned to the same diameter as the dial.

When it came to ‘blacking’ of the dial to enhance the clarity of the divisions and numbers, as well as form some corrosion protection for the backplate two options were open. One was to have the parts blacked commercially, but upon enquiring the minimum order price this was a non-starter. The other was to purchase one of the many blacking kits available and do the job in-house.

Having loads of chemicals about the workshop which I would only use once in a ‘Blue Moon’ was another thing I was not keen on. Eventually after much searching I found an antiquing fluid for blackening brass, Curator Antiquing Fluid, (the usual disclaimer). Reading through the product info and the instructions for use, it is stated that the solution can be used on Steel. The solution can be used as is, or diluted and the item immersed. At a 10 to 1 dilution rate 20 ml of the product would give me enough solution to immerse the dial in a small plastic tub. The procedure takes 2 minutes to do and the results are as shown in the photographs.

I have to admit I was sceptical of the 2 minutes and the fact that I could use this on brass as well means I am one confirmed user now. Plus I have only one 150 ml container in storage with 130 ml left for another day. Which will definitely get used on some brass parts I have in mind for my Clayton Timber Tractor.

In use the only thing that needs to be remembered is to adjust the dial in the direction of carriage travel at the time. While making sure the handwheel is static and if anything being gently urged in the direction of travel to eliminate the backlash in the Apron gearbox. Plus all dimensional cuts are finished with a manual input, ie, not allowed to be established using the feed mechanism as this will give an error due to the backlash not being taken out of the system. Which incidentally on my lathe is all in the Apron gearbox as no backlash exists in the dial assembly.

In use I have been unable to detect any indicated error over several turns of the dial, plus the mistakes due to mental arithmetic have dropped to zero, with a subsequent dramatic reduction in scrappage. The addition of the radial ball bearing has made the whole carriage movement feel silky smooth. My only regret is that I wish I had carried out this modification about 30 years ago.

Set of drawings will be included next week.

Part one here.

 
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