All posts by smithn

Mandrel to machine large copper tube

A friend asked if I could machine the ends of a soft copper tube around 70mm diameter to be flat and square, and to an exact length. Not easy to hold in the lathe, so I made up some expanding mandrels from engineering plastic.

I made a silver steel shaft with an M12 threads on one end and drilled a centre. The other end mounts in a 12mm ER40 collet and there is a bracing tube so tightening the nut forces the the tapered plugs into the split mandrels and they open and press very tightly against the inside of the copper tube. The mandrels when uncompressed are a tight fit in the tube.

Using this technique allows full access to both ends and the whole outside of the tube for machining and polishing.

The raw tube ready to machine
Split mandrel with a tapered bore and a tapered plug on a threaded shaft
Mandrel in place on the shaft
Mandrels in place in the tube, there is a spacer tube inside to keep the mandrels apart
Finished product

Waveguide Adaptor for Octagon LNB to 22mm

The simple approach to attach an Octagon LNB to a circular waveguide is to saw off the ridged scalar horn and push the cut end into a split 22mm pipe, but I decided to do it the long way. I made up a conical plug from brass to fit against the cone of the horn, then bored a tapered hole inside the plug to match the 17.5mm ID of the Octagon to the 20.2mm ID of 22mm water pipe waveguide. Probably totally unnecessary, but I had fun doing it.

Parts of the adaptor and mount with the 22mm waveguide and bare LNB

First step was a CAD design

CAD Model of the Octagon LNB horn and tube section
The brass adaptor plug in place
Adaptor plug and the locking ring with the O ring compression seal
Front collar in place, without locking ring
Complete assembly with 22mm copper tube inserted into the plug with collar and locking ring
Tapered end of the plug
22mm bored socket for the copper tube
Collar slid over the plug
Collar showing the thread
Collar with locking ring in place
Collar, adaptor and locking ring in place
View into the tapered bore of the adaptor
Complete assembly ready to attach the dual-band POTY feedhorn

500 watt Dummy Load

A friend asked me to make up a heatspreader for a 500W load resistor and fit it and a 7-16 DIN socket to a giant heatsink weighing about 15 lbs

Close up of the 90 x 70 x 12mm spreader
Mounting block for the 7-16 DIN connector and RG401 1/4 inch semi-rigid coax

The load and coax will be soldered to the spreader, and high performance conductive paste will be used between the spreader and the heatsink.

Laser-cut QO-100 POTY patch antenna

Two friends have asked me to assemble dual-band 2.4GHz/10.4GHz patch/horn antenna feeds, known aa a POTY (for Patch of the Year).

The first one I made was from 1.5mm brass sheet, and I made it on the lathe and milling machine, but for these, a group buy was arranged on the UKMicrowaves group to get them cut on a CNC laser. The cuts are very slightly oversize, by about 0.2mm, but the edges are very clean and only needed minimal fettling to remove the inevitable pip at the start/finish of each cut. I had to open up the centre holes using a deburring tool to make the fit perfect. I also had to mark up and drill/ream the hole for the socket. I made a custom collet for the previous one, it supports the reflector at exactly 90 degrees. It is relieved near the solder joint.

Reflector and central waveguide/support mounted on the custom collet and clamped
Drilled the pilot hole in the reflector ready to open up to 8.8mm then ream to 9.0mm
Reaming the mounting hole in the reflector for the N socket

It was a lot of effort and actually it would have taken about the same amount of time to machine them from scratch, but for folks without a machine shop, they are excellent.

The Radiall N socket my friend supplied is not the usual small-format N with 4.1mm PTFE sleeve over a 1.27mm gold-plated brass pin. It has a solder spill. I had to cut it back a little and solder in a suitable extension pin. Again, a lot of work for no benefit.

In addition the N socket and the TNC supplier by the other friend for hit POTY both have a 1.5mm raised boss behind the socket.

TNC with solder spill just like the Radiall N

The N connectors I use are like this:

4.1mm PTFE shrouded N socket like the one I use for POTYs, but much longer
Six-slot N socket that I prefer for the POTYs

On the 0.9mm laser-cut material, that means the boss would intrude into the space between the element and reflector. I made up a precision washer to ensure the boss was flush with the reflector surface.

24mm OD washer bored 9mm and parted-off then superglued to a custom mandrel
Turning the washer to 0.46mm thick
Finished spacer washer under the Radiall N socket
Rear of the N socket now flush with the reflector face

I soldered the reflector to the tube at the right spacing from the machined top edge of the tube (6mm+0.9mm+3mm) using 183C tin-lead solder paste, which makes a neat joint,

I used a 3mm U-shaped spacer to fix the driven element in place and clamped it, then soldered it. to the tube. Then I stuffed paper towel into the tube and soaked it with water to prevent the existing joints from melting, then soldered the N connector and spacer to the rear of the reflector.

After a good cleaning with isopropyl alcohol then hot water and detergent, I fitted the 10GHz dielectric lens and connected the 2,3GHz port to an HP directional coupler, signal generator and spectrum analyser

POTY patch under test on 2.4GHz
Return loss versus frequency from 2320 to 2480MHz

So far, I have not been able to get the correct double-dip response. I can get a very deep single dip to -33dB (!) but even doing asymmetric bends or dielectric slips, I can’t split the dips. At max RL, the axial ratio is pretty much that of a linear feed, and with it much flatter, the axial ratio is still around 6dB. I am making another of the original patter with 1.5mm brass parts and machining them on the lather and mill, with a good small-format N socket with 4.1mm PTFE over 1.27mm brass pin so see if I can find what is going on. Then I need to make another with these TNCs…

SCAM mast Spigot for 48mm Scaffold Pole

For the Goole Radio and Electronics Society entry in the RSGB SSB Field Day contest, we needed to fit some 3m extension scaffold poles on top of a pair of SCAM12s to support our doublet. I made up some 40mm spigots which are a push fit into the poles, and fitted a locking bolt, and made the SCAM spigot as a sliding fit at -0.15mm as usual. Made from aluminium extruded bar stock.

I made up some M8 threaded lockbolts by turning down some M10 bolts to 8mm diameter and cutting an M8 thread on the lathe.

Spigot with a 50p coin for scale
Spigots with M10 bolts turned and rethreaded as M8
Spigots fitted into the ends of the scaffold poles

Tailstock DRO Part 3

The tailstock DRO is now completed. The last element was a block to bolt to the existing end clamp, to hold another rare-earth 12mm magnet which keeps the end of the scale firmly attached to the tab on the quill. See part 1 and part 2 for details

DRO attached to the tailstock casting by the four magnets under the wedge and the one in the end block
DRO in context in the tailstock
Collar and magnetic block
The magnet block attaches using the original end bracket cap
I left the magnetr slightly proud of the block in case of any misalignment
Milled finish will have to do, this isn’t jewellery!

Tailstock DRO Part 2

Having made the clamp collar and bracket to fit the quill (see part one), next step was to make the magnetic wedge to fit on the angled side of the tailstock on the Colchester 1800. It is inclined at 6.6 degrees. I wanted the digital display to be at 45 degrees to avoid glare and keep the LCD reading angle good for best contrast.

CAD sketch in Fusion360, showing the angled counterbores
The part in real lifer
Back view – the M3 mounting screws poke out of the holes by 2mm and screw into the backplate of the electronic scale
Four rare-earth magnets epoxied into the pockets so it will attach to the cast-iron of the tailstock
Bolted in place and ready for a test fit

Tailstock DRO Part 1

I needed a DRO (digital read-out) on the tailstock quill of my Colchester 1800 lathe. OK, the quill has an engraved mm scale and the wheel on the leadscrew is calibrated in mm, with 2.5mm per turn in multiples of 100 micrometres. Still a pain when you need to drill a hole 52.95mm deep. After watching a couple of Youtube vids, I saw a neat solution by ChrisB257. Make a collar to be a very close fit to the end of the quill, split it and put in some clamp screws, then fix a right-angled tab which can be used to attach the magnet on the end of a sliding electronic digital scale.

So, part 1 is just the quill clamp collar and magnetic tab. Collar is turned from 70mm EN3 steel round bar. Tab is from an offcut of EN8 I found in the scrap bin.

See part 2 for the magnetic wedge mount for the scale and part 3 for the magnetic end attachment.

CAD design idea in Fusion 360. Only one clamp screw modelled. Just getting some idea of scale
The first part, split collar and clamp with tab attached. I changed the orientation of the clamp to make access easier.
The main slit was made using a 1/16th inch slitting saw, the partial slit was done with the bandsaw
The collar pushed on to the tailstock quill and clamped in place.