Field desk - part 1 - cabinet

Construction of the field desk

I have need for an electronics tool and experimentation kit which is self contained. I looked around on the internet for some ideas, but failed to find anything which suited my needs, or seemed practical for my planned budget.
The military used to use field desks (you occasionally see one in the old episodes of M*A*S*H if you want to see an example of a real one) and I thought that would make a good starting point for a design.

I googled (is that a real word yet?) "field desk plans" and found some dimensions from early American history re-enactors and museums. Most centered on dimensions of 24"W x 18"H x 10"D (nominally 600mmW x 450mmH x 250mmD) and that seemed like a reasonable size for my intended purposes.
Whilst planning the cutting for the timber I had, I decided to use an external depth of 12" (nominally 300mm) instead of the original 10" mentioned previously due to the depth of the plastic containers I am planning to use.
My timber was the remains of a UPS packing crate which I salvaged from a job about 2-3 years ago. The timber from the sides of the crate was 6mm (1/4") plywood, and the side which doubled as a ramp was 10mm (3/8") plywood. I drew up some plans and cutting lists using the 10mm ply for the top, bottom, riser, back and sides. I planned on using 6mm ply for the shelves, and some 12mm (1/2") plywood for the front. Due to a cutting error I ended up having to use some of the 12mm ply for the sides as well.

Figure 1 - Raw plywood material

Figure 2 - Cut into basic panels

After marking the timber to the largest finished dimension for each panel, I cut the panels out using a circular saw.
The rough panels were then marked up with the slots, dovetails, etc which were cut in using a jigsaw, hacksaw, and portable drill (the drill was used to start the slots for the finger joints. All external joints were made as dovetail joints to help the box stay together, whilst the internal joints (supporting the shelves) were made as finger joints (tab and slot).

Figure 3 - partially assembled carcass

During the construction I would clip the unit together to mark up the next panel - for some reason I got confuzzled at the end of the first day and assembled the desk with one piece reversed. This then meant the finished carcass went together upside down, and mirrored. Thankfully the photos I'd taken earlier in the day were able to be used to show me the correct orientation of the pieces, and allowed me to assemble it correctly the following day. Being right-handed, I really wanted the section for power to be on the RHS.

Figure 4 - Carcass assembled, but with pieces mirrored by accident - rear view

The back was marked up for dovetails and slots, then cut and fitted.

Figure 5 - Back panel marked up and cut for dovetails and finger joints.

Once happy with the finished carcass, I dismantled it and sanded all panels before assembling with construction adhesive (aka "Liquid nails") and PVA glue.

Figure 6 - Back panel in place on carcass of field desk, shelves not in place.

The front door was then cut and tested for fit. I found a slight bow in the LHS side panel - I'm still trying to determine if I can remove it,or if I will need to alter the door slightly to compensate. UPDATE - I modified the door slightly by allowing a slight amount of slack in the hinges, and put a heavy chamfer on the inner edges of the door, this allows the door to close and adjust it's centre based on the 1-2mm bow in the LHS panel.

Figure 7 - Field desk with shelves filled with sample material

Whilst the glue dried, I commenced work on building the hardware... hinges, catches, etc.

Next articles will include:
Hardware - hinges, catches, etc
Electrical - Lighting, PSU, etc
Finishing - Painting, trim, etc

Since I didn't get much done last week on the desk, I'm hoping to complete it over the next two weeks or so.

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.

Dividing Head for Taig (and others) - Part3 - division plate generation

The division plates used on this dividing head are made from old Hard Disk platters - the part of the harddrive which actually stores your data.
Each 3.5" hard disk will contain one or more of these disks which is 5.25" in diameter, with a 1" hole in the middle. The nominal thickness of the disks is around 1mm (less than 1/16") - I say nominal because I've found the more modern disks typically are thinner (less rotational inertia), whereas the older drives are thicker (up to 1/16")
The only thing you can rely on is that all disks in the same platter (collection of disks) will be the same thickness.


Figure 1 - Collection of platters destined to become division plates


I've been collecting hard disks for salvage for quite a while. The magnets are useful (see shed tip #1 ), and I also salvage bearings from them. The disks get shuffled into the pile for making division plates, and the casings go into the scrap aluminium bin (for foundry supplies) - only the boards, screws and little plastic doo-hickies get tossed. I learn a fair bit about mechanical design from looking inside the harddrive as well - there's some really clever braking mechanisms used to return the head, lock it, and so forth simply driven by ground effects from the spinning disk platter.

To make up a division plate:
I insert this mandrel into the back of the spindle of the dividing head. (see figure 2)

 Figure 2 - Direct indexing mandrel

The mandrel is made to expand and grip the inside of the spindle once the 1/4" nut is tightened up. A gear is placed between the 2 large washers which is an exact match (or multiple of) the desired index count. When I generated the 40 hole plate, I used a 40 tooth gear, but I could have used a 80 tooth gear if I had one.

 Figure 3 - Mandrel in place

The mandrel is sized to match the collection of C218 changewheels I purchased a few years ago. Those changewheels are the basis for the leadscrew of the Taig lathe and are the same metric mod 1, 20 degree PA changewheels used in the myriad of 7x12 lathes available in the US and other locations.

I place a chuck on the dividing head, and use a holder to grip the blank division plate in the chuck.
A centre drill is gripped in the lathe chuck using an arbor supporting a normal drill chuck.

Figure 4 - Dividing head spindle nose

A detent plunger mechanism is attached to the dividing head body which engages the gap between the teeth of the gear wheel. Turning and locking the spindle turns the blank plate, and all I need to do is feed the head into the drill to make the holes in the plate. The bracket supporting the detent plunger system is made from an offcut of an aluminium angle extrusion. The bracket is bolted to the body with two socket-head screws, and has holes to pass over the heads of the body bolts. Figure 5 (below) illustrates how it is fitted, with one securing bolt removed for demonstration purposes.

Figure 5 - Detent plunger system fitted

The detent plunger has a wedge shape when viewed from the side, but an inverted V shape when viewed from the front - this is to allow the indexing of the tooth tip instead of the gap between the teeth. This means I can index in the gaps of a 20 tooth gear, then rotate the plunger 90 degrees, and then index off the tooth tip and obtain another 20 positions - allowing me to generate a 40 hole plate from a 20 tooth gear.
The mount block for the detent plunger is bolted to the vertical arm of the bracket in one of 3 positions, allowing for a wide range of gear diameters. The large hole drilled through the body (from it's previous scrap origin) is used to secure the plunger with an elastic band if the internal spring is needing a little help.
The last article will cover (albeit briefly) the sector arms, plunger arm, and retaining mechanism.


Next project for documentation will be the field desk if I get it completed on schedule.

Dividing Head for Taig (and others) - Part2 - plate carrier and worm

The plate carrier was made from 1/2" plate. A piece similar to the brake was made and the plate carrier was bolted to it. One bolt holds the piece on, and another engages a curved slot which is used to adjust the amount of backlash in the worm/spur gear engagement.



Figure 1 - plate carrier/ worm assembly

Another piece of metal is made which has a 1" diameter spigot which is about 0.5mm thick. This spigot is made to match the hole in the middle of a hard-disk platter. A piece of 1/4" brass has been drilled in to intersect the edge of the hole and a corresponding notch is cut into the platter to stop it rotating. A cover plate is made with a corresponding hole and screw to lock the plate (Hard-disk platter) in place. Yes that's chatter marks from turning... that work was done on a stub arbor.


Figure 2 - plate carrier with spigot



Figure 3 - Plate mounted on spigot

The plate retainer is basically a brass plate with a hollow spigot on it. The worm shaft passes through the spigot, whereas the sector arms rotate about the spigot.

Figure 4 - plate retainer holding plate on spigot

The other main feature of the plate carrier is that the part which carries the plate can be rotated through approx 90 degrees. This was cut from a piece of plate, mounted on a stub arbor, and turned to match the curvature of the mating piece. A slot was cut in the side rim whic mates to a threaded hole in the main part. The purpose of this is to allow the plate carrier (and attached plate) to be rotated independently from the worm itself over a 90 degree range. Why? Imagine you mount something requiring 13 divisions in the chuck on the dividing head, and then need to index another section of the same piece with 19 divisions and one section needs to be perfectly in line. I can mount up, cut one set of indexed points, apply the spindle brake, then change the plate, and move the plate to align exactly with one plate hole, then re-commence indexing for 19 positions. Maybe something which will only happen once in a blue moon, but it cost me little but time to add it into the design.

Figure 5 - one extreme of plate carrier adjustment



Figure 6 - another extreme of plate carrier adjustment


The worm itself was cut down from the original shape when removed from the sewing machine. A triangular groove was turned in it which intersects with a pointed socket-head screw used to lock it in longitudinal position, and a pair of flats and 1/4-20 thread was cut into the end. These correspond with the sector arm, and retaining nut respectively.



Figure 7 - worm in carrier showing flats and threaded sections

All bushings in this part of the dividing head are simple yellow brass, not bronze. At the time I built this I could get surplus electrical test probes from the local salvage place for a couple of dollars. Each probe comprised about 12" of 1" diameter nylon with a 1/4" bore, and a corresponding length of 1/4" brass rod in the middle. A few other bits made up the probe, so I was able to get nylon (for rail buttons) and brass rod quite cheap. The interface disk from the probe was used to make the plate retainer since it was 1 1/2" in diameter, and about 1/4" thick. I miss that shop. Pretty much all the remaining brass I have is from various scrap merchants, or salvaged from all sorts of junk.

I'll cover generating plates and direct indexing next.

Dividing Head for Taig (and others) - Part 1 - Body and introduction

The dividing head was inspired by the work done by Tony Jeffree, and then heavily modified to suit my own circumstances and situation. (Note - Tony has a second design which uses a Taig spindle - see this link)
All photos in this series of articles were taken after construction had concluded since I did not have a camera during the build. This build occurred in Q2-3 of 2005 and was documented on Nick Carter's page back then.

Figure 1 - Dividing head in use on Taig Lathe

You can buy dividing heads - example shown here from Amazon, and there are books which touch on building your own (another example from Amazon) - in short, you need to decide what resources you have,and determine the best course... in my case I did not have a great amount of money, and I did have some time, and a willingness to learn - this meant I made my own. If I had the money... the start of many dreams. I don't have either of these products, but show them as examples of alternatives to scrap metal, wrecked sewing machines, etc.


















Building the dividing head body and base.
I started by making up a spindle. Tony's one used a drill chuck, but after reading many text books which talked about not disturbing work in chucks, I decided it would be better to make my dividing head use the lathe chuck if possible. This meant I had to make a spindle with a nose of 3/4"-16 tpi, with a 30 degree included angle for collets, and a through spindle bore of at least 3/8".
I had previously bored the spindle of my Taig lathe to 7/16" not long after purchasing it so I could pass 3/8" stock through the head - I used the same drill size to bore out the spindle of the dividing head.

Using a larger lathe (Thanks Dad!) I turned a 3/4-16 thread on a piece of 3/4 shafting, and drilled/bored the 7/16" through hole. I then made a collar which was shrunk on to form the register face for the lather spindle nose. This register face ensures the alignment of the lathe chuck. Whilst facing the register face, I also bored the tapered seat for the taig collets.
At the same time as all this lathework, I also threaded the other end of the spindle (3/4-16) to use for thrust nuts, and securing worms , etc.

By the time I'd finished the spindle, I was back home, and unable to access Dad's lathe - this meant all subsequent work was done on the Taig Lathe.

The body of the dividing head was made from a short length of 50x50 (2' x 2") aluminium I picked up at the scrap dealers.
The base plate of the dividing head was made from some 1/2" plate which was a reject from some CNC mob (I picked it up at the same scrap dealer as the 2"x2" piece)
I found if I stacked the 2" x2" on the plate, on the carriage, the centreline was in line with the spindle. Perfect for my immediate plans, and in line with my longer term goal.
The theory was I'd build the head on the plate base, and then should I every need to use the head on another lathe, it was simply a case of making a new base plate.. all other parts will transfer across.

The base plate was cut to square the end (The off-cut piece became the handle of my 3/16" allen key) and appropriately drilled and counterbored for the Taig slot pattern.


Figure 2 - original base plate shape - allen key handle made from off-cut.






The square body was then bolted to the plate, and line drilled and line bored on the Taig. Somehwer in all the line boring I used a shaft as an arbor, and turned the square body on the shafting to form a spigot at one end of the body.

The line boring provided a clearance fit for the 3/4" diameter spindle, and a light press fit for the bushes used at each end.

Figure 3 - Body on base plate

A brake was made and fitted to the front of the body which engages the collar shrunk onto the spindle.


Figure 4 - Front of body showing brake on collar

The spigot which was turned on the rear of the body is used to support the plate carrier. I saw somewhere that some dividing heads are used horizontally and vertically, and that some models allow the position of the plate to be changed to make things easier on the user... seemed like a good idea to incorporate into my build.

The worm and gear for my dividing head was salvaged from an old Singer sewing machine I found on the side of the road on curb dump day. It's a 24:1 ratio set which is OK for this design. I bored out the spur gear to suit the spindle, and cross drilled and threaded some grub screws in place. A normal 3/4-16 nut was cut down and faced to become a thrust adjustment nut. The other half of the thinned nut is often spun on to the spindle after the spur gear as a lock-nut. The grub screws on the spur gear engage in filed flats in the spindle thread.

Figure 5 - spigot, thrust-nut and worm on spindle.

The plate carrier will be discussed in another article, but here is a photo of it in it's place on the spigot.

Figure 6 - Plate carrier in place on spigot

The body really isn't much more than support for the spindle, and a means to hold everything else on the dividing head. I tried to design the body to make the head transferable from one machine to another - the base plate is the designed mechanism to permit that.

The plate is designed with holes in a grid to allow the plate to be mounted on Taig T-slots either parallel, or perpendicular to the slots.

Next articles: Plate carrier, plate generation, and sector arms and worm driving.


In the meanwhile I got the carcass of the field desk made yesterday, and hope to have the woodwork completed over the next week or so, then make up the catches, hinges, and handle over the next week, then inletting, finishing and it's complete. Everything takes time since I only have one day per week to do work - I try to keep the Sabbath holy, and work does a good job taking the other 5+ days... My wife is a darling since she encourages me to spend at least half of every Saturday in the shed - for that I'm most grateful.

Third hand - electronics tool

I have some work on the horizon where I may be away from my shed for periods of time. It's still up in the air, in the hands of Heavenly Father, managers, and others. In the meantime I'm getting things ready...

One of the things which I'm going to need is a portable electronics kit. My current setup is based on an old fishing box full of tools, and several tubs full of parts in various trays, etc. I've decided the best method to deal with the proposed situation will be a "field desk" with the minimum of what I require, and a contained work area.

The desk will be covered in another article series, but this article introduces the first tool made for the field desk... the third hand.


Figure 1 - third-hand in use - salvaged PCB from old fire-panel

For those who aren't familiar with the term, a "third hand" is simply a means of holding something whilst keeping your own two hand free. I used a third hand whilst at uni and found the design quite good and duplicated the essential features here.

There are other designs out there. most work on the principle of a heavy weighted base for stability, and then posable arms terminated with clamps. The one I used at uni (made by the lab techs there) used lightweight materials and a door hinge for the "pos-ability".

The only concession from their design to mine was that mine had to be collapsible so it took up less space in the field desk. The original unit did not come apart, and as such occupied a space of 200 x 125 x 150mm (8"x 5" x 6") - this design occupies the same space in use, but folds down to 180 x 125 x 35mm (7" x 5" x 1 1/2") for storage.


Figure 2 - flipped to other side for soldering work.

Since the unit can flip too far in one direction, a small piece of perspex can be inserted to limit the travel of the hinge as shown in Figure 3.


Figure 3 - inserted piece of perspex limits hinge travel.

I used some of the perspex I salvaged from some shop shelving, and cut it to utilise the existing lip which was on it.
A pair of pieces were cut to match and support the hinge, and this pair were then drilled to sit between the two halves of the base.
Some bolts were modified to make them "tool-less" by soldering their heads into a brass piece which had a square washer affixed - I'd have preferred wing nuts and wing-bolts but didn't have any.

The clamp which supports the PCB is simply one leaf of the hinge, and a piece of aluminium which is made from a drawer divider. The cranked over fold is used to form one part of a toe-clamp, and to provide clearance over the nut which holds the bolt in place.
A stiffening plate is captured under these bolts to provide more gripping surface. Again due to my lack of wingnuts I made up some nuts using brass. The brass used for making the wing nuts (and bolts) is from discarded tap spindles, the sheet from an old door strip.


Figure 4 - cross- view of PCB clamp with perspex gripped for illustrative purposes



Figure 5 - View of base assembled showing component parts.



Figure 6 - dis-assembled third-hand showing all parts

Soldering is simply done with a propane torch, using "Baker's fluid" as the flux, with normal 60/40 soft solder. Only the minimal amount of solder is applied, and excess is trimmed away to prevent absorption through normal use.

Figure 7 - Third-hand collapsed ready to be stowed

The field desk will be built with PICAXE projects in mind. I have a few which I need to get completed, and since all I can have at the camp is books, and minor electronics (no Lathe or other powertools) I figure this will make good use of what spare time I have.
Even if this proposed change falls in a heap, the investment in making this desk, and associated tools will still benefit my electronics hobby.

DTI Mag-Base repairs

a while ago I discussed a broken mag-base I salvaged from being tossed out. That article discussed the dis-assembly of the mag base.

In an effort to get the "To Do List" a little smaller I finished off the repairs to the indicator base.
I used the spindle from a discarded tap to make up an actuating system for rotating the mag-core. I simply filed the required square shape into the section which used to hold the jump valve.
The threads which actuate the original tap were turned away, and replaced with a parallel section.


Figure 1 - Tap spindle filed to square

A plastic bush was turned to locate the mag-core inside the void in the mag-base, and to provide support to the spindle. The plastic was from a sheet of 25mm (1") thick plastic (nylon I suspect) that I rescued from a bin. A suitable square was cut from one corner, and a 10mm hole drilled through it. A 10mm bolt and nut were inserted, tightened up, and used as an arbor for the turning. Whatever the plastic was, it certainly was "stringy" in the swarf.


Figure 2 - Commencing turning the plastic bushing

A brass indicator/ handle was made from an old brass fitting, and some sheet brass, and soldered together.


Figure 3 - Actuating knob and pointer prior to soldering


The spindle was designed to pass through the front plate which was made from some brass strip. The front plate is shown on the RHS of the exploded view below (Figure 4)


Figure 4 - exploded view of mag-base

In the above exploded view, everything to the right of the magnetic core, and everything above the magnetic base were made from salvaged materials.

The magnetic core was filled back to remove the old damaged paint, and engraving markings. A few minor dings were cleaned up, and then the base was primed with cold-gal paint (Zinc-it) then followed up with a couple of light coats of Silver Hammer finish paint.

Upon assembly it was deemed too difficult to re-drill the existing holes to their proper spots, so new holes were drilled for securing the face plate.



Figure 5 - the magnetic core in the base with the spindle and bushing in place.




Figure 6 - The mag-base assembled

One of the salvaged lengths of steel from a gas strut was used to make the mast on this base - although a brass socket and washer was turned from an old extinguisher part (CO2 nozzle) to stiffen the joint instead of simply using the M8 thread.



Figure 7 - The  mast on the mag-base made from gas strut

An offcut of the gas strut remains, being about 125mm (5") long which will most likely be used for making one of the connecting rods.




Figure 8 - The completed mag-base next to the Taig lathe.

I've possibly made the mast too tall, but standing next to the Taig lathe, it has sufficient height to ensure access to anything I put in the lathe, or the vertical slide. I've yet to make the other smaller poles, joiners, and DTI connectors, but that should be fairly easy to do over the next few weeks.

That said, guess who's added more projects to the list... You'd think I'd have learnt by now, or maybe it's my curse (or is it blessing) to always have more jobs than hours to complete them...


I did up a quick explanation of how a mag-base works for someone, and decided to include it here in case anyone else needs to know how they work. I've since cleaned it up and converted it to a JPG.


Figure 9 - Theory of operation - magnetic stand