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Threading plastics

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Chris Suddell24/05/2019 06:35:02
5 forum posts


I need to make a thread to set a copper wire around a plastic or GRP tube. My preferance is GRP.

Just wondered if anyone had any tips fo doing this? I have read GRP is nasty and my other option white drainage pipe will heat up.

Any advice will be appreciated.

Speedy Builder524/05/2019 06:45:15
1801 forum posts
127 photos

Do you need to thread the tube ? Put the tube in the lathe, select TPI etc. Make a simple guide to wind the wire directly onto the tube. Before placing the wire, run a couple of lines of super glue along the tube. Wind the wire onto the tube and allow the super glue to dry, Then paint some resin over the wire.


Michael Gilligan24/05/2019 06:46:49
13827 forum posts
603 photos

I certainly wouldn't attempt to screwcut an existing GRP tube, Chris

Some indication of the size, and intended use, of what you want to make would help.

Would it be possible to approach the job the other way ? ... Apply a gel coat, and wrap the wire into it.



Bob beat me to it.

Edited By Michael Gilligan on 24/05/2019 06:47:46

David George 124/05/2019 06:51:52
880 forum posts
292 photos

Hi Chris what is the coil for will the coil get hot or warm in use? Have you thought of using tufnol you can get it in tube sections it is machinable and an insulator.


Chris Suddell24/05/2019 08:49:18
5 forum posts

Hi All,

The tube needs to be 2 foot long with a thread around 18 inches, 4 inches or 3 inches diameter. My lathe will do 3.75mm max thread. It's to build and inductor coil for an antenna. So there is RF going through the device, it would only get hot if the material reacts to RF.

Hope this helps?


Chris Suddell24/05/2019 08:54:25
5 forum posts

Hope this helps.Antenna

JohnF24/05/2019 09:01:20
853 forum posts
102 photos

Chris, GRP = Glass reinforced plastic ? if so I assume this is similar to the material used for fishing rods?

Assuming my supposition is correct the tubes for fishing rods whether glass or carbon reinforced are ground to produce the finish so using a suitable set up with say a dremel type tool you could grind the "thread" then apply the wire with an adhesive as above. Or consider applying a slow setting epoxy to the tube then wind on the wire in a similar manner for winding a coil spring and leave under tension until set.

Just my thoughts ---- John

Joseph Noci 124/05/2019 09:36:00
536 forum posts
832 photos


I have made a 'few' of those 'screwdriver jobs for mobile use - I used a 100mm schedule 4 PVC tube - had a wall thickness of 6mm. Is was light grey in colour, and To be sure of it's dielectric suitability I placed a 100mm section in the microwave, on high power, with a cup of water, for 10 minutes. It was mildly warm and worked ok in the application. Other tubes I tried - 100mm underground PVC drain pipe, a cream colour before the uWave test, dark brown and HOT after 4 minutes, a 75mm gutter down pipe, white, which also passed the test. I would have used that, but the wall thickness was onlt 2 or 3mm, difficult to cut the thread. I also made one using 40mm schedule 4 PVC tube that passed the uWave test - beware the typical house-hold 40mm PVC grain pipe - that stuff is very lossy.

I then made another using a solid 40mm x 300mm long Polyprop bar, and silver coated wire, with a roller contact - the bar rotated in bearings on a Polyprop through-shaft..It was a work of art, and the Hustler beat it..Was nice yo be able to change bands from the drives seat, but lost too many weak contacts - also, it was very difficult to eliminate the commutator brush noise on the bands - the motor is right neat the coil...

GPR is not good - the glass is fine, but the (general purpose ) resin is not. Is has high losses and used as a former in an HF amplifier tank coil, @150watts RMS, it got hot and softened. Polyester resins are worse..

Not sure how you intend making the sliding contact on the coil, but I used some of that berryllium finger strip, like on the peripheral inside of RF shielding cabinet doors, wound in a circle around the coil - that worked well.

I gave up on the antenna in the end - it was far more work that is gave back in performance - the good old hustler worked far better!

Have Fun..


not done it yet24/05/2019 09:41:09
3248 forum posts
11 photos

Inductors are normally made with insulated copper wire (enamelled, or other insul ation). Do these coils require a separation? Just wind it into a glue using the screw cutting setting for the carriage travel, like suggested above. You might need plug in each end of the tube to support it adequately.

Joseph Noci 124/05/2019 10:10:01
536 forum posts
832 photos

The screwdriver antenna inductor is normally uninsulated copper wire - the principle is that the antenna rod or whip is generally to short for the operating frequency ( <30MHz, typically)and so presents capacitively. To counter this in an attempt to get the antenna to resonate, we add a series inductor, the value of which will then vary with operating frequency. The Screwdriver antenna ( its name from the use of a battery powered screwdriver to drive the coil or shorting strip) has a coil of many turns, and for the higher frequencies, the coil is progressively shorted out, leave a resonating inductance value. This is normal done by have the RF entry to the coil via a movable contact on the coil periphery, able to move linearly from the start of the coil to the end, driven by a leadscrew or similar.


Andrew Johnston24/05/2019 10:58:59
4783 forum posts
538 photos

Ah, thought it might be an RF application.

Many of the common plastics have relatively high RF losses. You might be better off making a frame (like a squirrel cage) from small sections of a low loss material such as polystyrene. Each rod can be notched with a file to act as a guide for the wire. The empirical Wheeler formula gives good results for the inductance of simple coils; I doubt the value would change much if the cross-section was polygonal rather than round.


Nicholas Farr24/05/2019 11:03:58
1945 forum posts
925 photos

Hi, basically, it is the same type of construction as the variable resistor/potentiometer/rheostat shown below, but only the slider and one end of the coil would be used. Therefore enamelled or insulated wire would be of no use.


I guess the idea of a threaded tube would give a consistent spacing while keeping each turn of the coil separated and only have the slider able to make contact with the outermost side of the winding.

Regards Nick.

Edited By Nicholas Farr on 24/05/2019 11:12:45

Howard Lewis24/05/2019 11:32:55
2217 forum posts
2 photos

I would not be in favour of screwcutting GRP. The dust / swarf produced would not be good for you, on the skin or inhaled, (Glass fibre or the resin ). And, never having tried this, it would require a sharp tool, which may need resharpening quite frequently, to minimise tearing.

The advice to wind the wire on, as if winding a spring would be my preferred method. Feed through a chamfered hole in a bar set in the toolpost. Just choose your pitch, and set up the lathe to produce it, as if screwcutting. If the Inductor is a fixed value, the wire could be enamelled or bare. If the Inductor has to, be variable, presumably to tune the antenna, it needs to be uncoated, and the turns cannot be allowed to touch each other..

The speed should be low, possibly hand rather than motor powered. This could mean making up some device to grip the wire lightly, before the bar in the toolpost, to maintain a steady tension.

Let us know how you get on!


Joseph Noci 124/05/2019 12:07:15
536 forum posts
832 photos

This should give an idea of the intent...Although this one progressed a little beyond the screwdriver concept..


Coil and rolling contact

Coil drive assy1

Coild drive assy 2


SillyOldDuffer24/05/2019 12:51:22
4601 forum posts
988 photos
Posted by Chris Suddell on 24/05/2019 06:35:02:


I need to make a thread to set a copper wire around a plastic or GRP tube. My preferance is GRP.

Just wondered if anyone had any tips fo doing this? I have read GRP is nasty and my other option white drainage pipe will heat up.

Any advice will be appreciated.

Bit more detail would help Chris - presumably this is a transmitting HF whip antenna? If so the frequency and power matters, white drainage pipe probably fine with 50W at 3.5MHz, and dodgy with 150W at 28MHz. Well worth applying Joe's microwave test - I've found some PVC drainpipe to be much better than others, probably because they use different fillers.

Does the coil have to be part of the structure? The other possibility is to air-wind a large diameter coil and run the smaller whip through the axis, the bigger the gap the better. This example kept in shape by gluing wire to four thin black plastic strips.



Chris Suddell24/05/2019 15:04:06
5 forum posts

Hi All,

Ah ha, seems like there may be a few hams here!

Anyway, it is a screwdriver antenna, not just a loading coil. So it needs to go on the ourside of a tube. Unfortunately the Q will only be 300-400 max, but it will do. I've 3d printed 3" and 4" inductors and wound them they work fine. This one needs some strength. I have done the microwave test on a few materials, not all went well.

Joe, I like your variable inductor, very nice.



Tim Stevens25/05/2019 12:07:43
1056 forum posts

Find a length of twine or string the diameter of the gap you need between your turns. Then using the lathe (etc) wind on the twine and the wire side by side. There is no reason to remove the twine before giving a coat of varnish to set things firmly in place - but if you are fussy, be sure the winding ends are held firm before trying to remove the twine.

I have used the same trick for winding springs from stainless steel, using a length of pvc insulated vehicle wire - it makes a proper looking job more quickly than setting up a lead screw etc.

Cheers, Tim

Joseph Noci 125/05/2019 12:23:41
536 forum posts
832 photos

As has been said a few times.The wire needs to be un-insulated, so no varnish, etc. For the screwdriver antenna the windings NEED to be in a cut thread as there is a wiper that is in very firm contact with the coil periphery and slides from one end to the other. Any movement of the wire may cause a short to an adjacent winding with useless consequences...shorted turns = poor Q factor, moving windings = inductance variation, etc. The simple delta in temp coefficient between the copper wire and the plastic tube in a variable environment makes the turns sometimes slack, sometimes taut, so no amount of pulling and tugging on the wire whilst winding will keep the windings from sliding around post the process. It needs a thread to locate them.

One could try to dribble epoxy over the wound assembly while it still rotates in the lathe, giving an even coating to all, and then when cured use sandpaper held against the coil outer while spinning in the lathe, to expose the wires again - but that is just a heck of a lot more work than cutting a neat thread. And you would need to be sure that the varnish/epoxy or whatever does not perform poorly under high RF exposure..


Edited By Joseph Noci 1 on 25/05/2019 12:27:04

Michael Gilligan25/05/2019 12:29:11
13827 forum posts
603 photos

Excellent idea, Tim yes


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