Microwave brazing machine.

[Retired]

Hi,

This is a follow on from;

https://www.diy-forums.com/threads/brazing-tc-tips.290598/

I'm adding this for interest only and not to encourage anyone to play around with mains electricty which can and will kill given the slightest chance so if you copy this you do so entirely at your own risk of serious injury or death.

I've watched many YouTube videos showing how to convert an old micro wave oven transformer so here's my version which seems to have taken forever due to no end of problems and Covid messing everyone's life up. It's a brazing machine where the transformer secondary winding is two turns of 500A welding cable but I'm aware this drops amps to 975A hence the need to control the beast otherwise the cable will quickly overload and burn out.

My micro wave transformer is a commercial high power 1950W bought through eBay. Many of the videos are simply crude just applying full mains power but I wanted to experimant and try to control output amps; controlling amps is difficult but controlling volts is much easier at least it is for me. Years ago a friend told me about a very cheap Chinese motor speed controller he had bought so could I incorporate one of these working on the principle amps x volts = watts; if I drop the volts will it control the amps?

Motor speed control circuit board; connections are very easy indeed but what isn't stated is that these only work with a brush type motor which I found out the hard way but to cut a long story short this speed controller actually works with my transformer; I do lots of experimenting enjoying these projects. "Fan" connects to the motor live.

Here's another refinement it being a timer with timing right up to 30 minutes depending where the jumpers are positioned. I removed the potentiometer and discarded it instaling a new much heavier duty potentiometer of correct 500K ohms allowing remote control. I've since bought two more of these circuit boards but they look to have remote potentiometer supplied which is much better.

Here's a 3/4hp motor being used to test circuits rather than connect the transformer and where I found out the motor speed controller circuit board doesn't work because it's not a brush type motor?

The motor speed controler mounted on a backing plate using "stand offs" making it easier to securely mount safely.

Please don't do this unless you've a lot of experience; I worked restoring vintage valve radios as an hobby for ten years and these vintage radios are truly lethal with live chassis so I'm at home with this knowing the risks involved.

In order to run the controls from a safe 12VDC I added relays; these having the 12VDC coil but capable of switching up to 15A 240VAC; a great deal of extra work and planning but I was experimenting; I used a 1A 240VAC to 12VDC printer power supply. I don't throw old switches or power supplies away they are handy to play with.

I'm not electrically trained so please don't throw stones at me after all I'm only sharing what I like playing with. It took four redesigns before finally getting the relays in their correct places; I used "Emachineshop" totally free CAD which I've used for years to draw the circuit; this CAD is the only one of many I tried and can actually use; the others both free and bought I struggled just to draw a straight line. I first came across this CAD whilst wanting to make 45 cast iron gears and was delighted to find it contains a "Spur gear wizard".

The front of the machine showing the electrodes; it's virtually a spot welder by design.

The toggle switch is the manual control at the top; the momentary push button at the bottom is pressed once to engage the timer; the large rocker switch is mains power the smaller rocker switch supplies either the motor speed controller or timer each isolated from each other; LED's are included these on 12VDC. The top control knob regulates the motor speed controller the bottom knob regulates the timing period; it's fully working and I've heated the end of 10mm square BMS bar stock to cherry red; I need to experiment more with the electrodes the 6mm dia solid brass are prone to over heating but at the moment I'm busy elsewhere so can't play further with this project.

I've been on the go since getting up at 6 o'clock this morning and am tired out so time to call it a day; I hope this story makes sense but I'm short of time to expand further; it's the downside of retirement; not enough hours in a day and now at 73 less energy than I used to have; I think I'm slowing down.

Kind regards, Colin.

 

[Retired]

Hi,

Another long hard day but before knocking off time to add a couple of pictures.

Before getting the speed controller to work I tried using the machine on full 975A and it sure was fierce; it melted the solid 6mm dia brass electrodes and also I tried 6mm dia steel bolts whese went to white hot in seconds forcing a quick shut down.

Here the 10mm square BMS bar end is at cherry red under full control of the speed controller using the 6mm dia brass electrodes; I've not had time to try brazing TC tips but I'm sure it will work.

Kind regards, ColinW.

 

[Ian]

This is a fascinating project @Retired! That is some diameter cable there.

I've got a few questions if you don't mind .

What happens if you accidentally shorted the brazing terminals - would it just blow an internal fuse after a few seconds?

How did you make the sheet metal chassis lid, is that something you had already, or did you just fold it to fit?

Do you get much of an inductive heating effect to the sides of the terminals, or is it very much restricted to between them?

 

[Jerny]

WOW!

 

[Retired]

Hi,

Thanks Ian. It's a pity the transformer "windows" wouldn't accept an even heavier gauge cable because the output from the transformer is about twice the rated current of the cable; by adding the motor speed controller though I can keep the current at a more acceptable level. A domestic microwave oven transformer would only supply about 450A with two turns on the secondary winding but I feel the smaller transformer would limit cable sizing even more; it's fascinating to play around with this though when I've got a bit of rare spare time.

Thanks for taking the interest to ask questions. If the brazing terminals (electrodes) are accidentally shorted it's possible the electodes would be the first to fail followed by the cable insulation. For a complete burnout I run the power through an MCB and it's further protected by an RCD but before anything really drastic happened just switch the machine off; I never ever leave machines under power without being present. Lots of YouTube video's show full mains power being applied but with high risk of burning the electrodes and cables out.

The sheet metal panels are aluminium which I carefully measured then used two lengths of heavy angle iron mounted in the vice allowing the edges to be neatly folded over at right angles; the angle iron is extended from the vice suffiently to accept the length of sheet being folded; both ends of the angle iron are nipped in the vice with the panel sandwiched and the extended ends are secured with either a strong clamp or in this case I drilled to accept a set screw and nut; a bit fiddly to set up but worth it.

Yes the electrode holders started to heat up but the main concern is in heating the electrode cable insulation; the transformer being well on top of the job remained cold. Anyone using a machine such as this I'm sure will have a feeling regarding safety and be ready to quickly switch it off.

If only I had spare time to play around but I've only just broken off for a mug of tea; I'm on #5 of six window jobs I'm doing and these are taking forever but need doing before winter kicks in. I'd better get off my backside and resume hostilties with the glazing.

Thanks Jerny .

Kind regards, ColinW.

 

[Ian]

Thanks for the answers Colin, it's a nifty bit of kit. Sounds like you'll need to keep a close eye on things when using it, but it's not like it's a machine you could leave unattended anyway.

I think I'll try replicating your sheet metal folding process. I normally just use painted wood panels for the most part, but using aluminium sheets should make things much easier.

 

[Retired]

Hi,

You're welcome Ian; yes it's not a toy to leave alone under power; it's safe enough even to touch the terminals whilst they are live because they are on less than 2VAC but not recommended.

I started to tidy the workshop after spending lots of time machining timber hence all the dust in the picture as I did a mock up of my sheet metal bending method. I have a big engineering vice allowing the heavy angle irons to be secured; the open end I can secure either with a set screw and nut or a strong clamp; I nip the sheet gently allowing it to be accurately positiond then tighten the angle irons. It's surprising how much force is required to bend over aluminium; if hammer marks are to be avoided then placing a strip of say MDF against the aluminium and hitting the MDF with the hammer; to finish off to a nice tight corner then replace the MDF with a length of steel and hit this. I've used this method for many years; it pays to drill or add openings in the sheet before bending.

I'm still wondering what retirement is about; I was up at 5am this morning it being Rufforth Auto Jumble day and was at Rufforth just before 7 o'clock. The picture shows the selection of paint brushes I bought; 15 on the right at £1 each Harris synthetic bristle and these are brilliant; 4 on the left disposable brushes for use with glue etc at 50p each; Harris have changed the handles since I bought 20 a few years ago the previous ones had wooden handles. I'd better resume tidying the workshop; retired 20 years and still working flat out.

Kind regards, ColinW.