Dividing Head for Taig (and others) - Part 4 - sector arms, plunger, etc

Last article for the Dividing head.... What's left to discuss?
Sector arms
Plunger arm
Retaining Knob and spacers


Sector arms
The sector arms are patterned on the arm design from Tony Jeffree's website. The arms were first patterned out in cardboard, then cut out using a hacksaw, files, and drills.

Figure 1 - Sector arms with locking screw

The lower arm is then soft-soldered to a brass boss which protrudes to form a spigot for the upper arm. A screw has been drilled and threaded so it fully engages in the boss, but it's head overlaps the moving arm. A small brass cylinder was made to concentrate/ exaggerate the clamping force from the screw head.
In normal operation the screw is loosened by about 1 turn, and the upper arm can rotate freely on the spigot of the lower arm, once set at the appropriate arc angle (hole spacing), the screw is simply re-tightened to lock the arms in positions relative to each other. Both arms are still able to rotate as an assembly on the spigot of the plate carrier.

Figure 2 - Sector arms on plate retainer

A cover, which is actually a spacer, sits over the arm assembly in use, but serves no functional purpose other than to increase the distance between the arm, and the surface of the division plate.
Not long after starting to use this dividing head, I found the tips of the arms were difficult to operate if they crossed each other - kinda like trying to open scissors by using the tips... To alleviate that issue I turned the tip of the upper arm upwards to form a handle. I could have added a nice little knob, but I was worried about the extra weight on the slender arm.

Figure 3 - Spacer cover on sector arms

Plunger arm
The plunger arm - the arm which actually rotates the worm, is made of 1/4" thick brass strip. A slot (1/4" wide) was made through the middle by the use of chain drilling, and filing. This slot engages the flats filed on the worm shaft. A brass plunger mechanism was fabricated and the body soft-soldered to the arm.
The slot was made so the division plates could have multiple rows of holes, although typically I use only 3 rows of holes per plate at most.

Figure 4 - Plunger arm fitted to worm shaft

One problem I have with this plunger is that the threaded portion which holds the shaft and handle together will sometimes spin undone whilst using the knob for rotating the worm. I'll remember to dab a drop of superglue in there one day, but until then I remember to tighten the threaded joint before use.

Figure 5 - plunger arm secured by retaining knob

Retaining Knob and spacers
The retaining knob is simply a brass turned object, and my first attempt at knurling. The picture makes it look better than it actually is... the knob won't roll of the table because there is a flat spot on the bottom where the knurling "crunched up" - I can't explain why it happened, and I've since tried to rebuild the scissor knurler, but I keep having issues there.

Figure 6 - Retaining knob in profile

As mentioned with the sector arms, there is a cover which acts to space up the plunger arm. There is another spacer which sits above the arm to space the retaining knob. With both spacers in their correct locations, the arm is essentially clamped to the worm shaft not only by the slot, but also by the clamping of the spacers. It basically removes any clamping effects from the knob on the sector arms, since if I place the upper spacer in the wrong position (under the plunger arm) it will attempt to turn the sector arms when I operate the worm.

Figure 7 - Spacer ring above sector arm

Most of the brass was from the scrap merchant I mentioned in my previous articles, but the brass sheet for the sector arms was bought as scrap from the local radiator place (along with a clapped out 8" bench grinder which only needed $12 worth of bearings), and the brass for the plunger arm was purchased from the scrap bin of a local fabrication mob (NOT cheap)


 As previously mentioned, the spigot at the rear of the dividing head body permits the plate/worm assembly to rotated through about 180 degrees allowing the dividing head to be used vertically, or horizontally and still have the sector arms, division plate, plunger arm facing the operator.

Figure 8 - Plate assembly rotated 45 degrees to illustrate movement

That's about it for the Dividing head.. it's been used on and off over the years for a few jobs and will continue to be used for many more. The most recent job was making up a wrench called a "Torx-plus" so we could access the internals of a harddrive enclosure. The Torx-plus is a 5 lobed version of the more common 6 lobed "torx" bit. I made the bit  by drilling holes in the end of a piece of steel shaft to create 5 holes on the appropriate PCD, and then turned the holes away to only leave half the hole. The metal between each hole was used left in place to form the 5 lobes needed to turn the screws out of the enclosure.

I'll have to sit down one day and see if I can improve the design around the sector arms and plunger - it works, now, but it does need some improvement so the spacers aren't necessary - they are a pain if the top one gets put in out of sequence (below the plunger arm).

The field desk is progressing along, and will be the next article series at this stage. I've designed the locks and latch, and have commenced designing the hinges, support arms, lighting and handle. Somewhere in all that I'll need to decide what colour to paint it. This week I have jury duty, so it's possible I may get an hour each arfternoon/evening to work on the hardware.. here's hoping.