Game on!

[Kitwn]

Thankfully I have a humungous (1.5KVA) isolation transformer I use when developing switch mode supplies, putting that in the line has at least enabled me to get going.

You know this, but I'm going to say it anyway in case anyone else wants to have a go: You are now completely unprotected from electric shocks. What would previously (thanks to the annoying 30mA RCD) have resulted in nothing worse than a bout of swearing could now lead to a funeral.

[brman]

You know this, but I'm going to say it anyway in case anyone else wants to have a go: You are now completely unprotected from electric shocks. What would previously (thanks to the annoying 30mA RCD) have resulted in nothing worse than a bout of swearing could now lead to a funeral.

Care to explain that? Because, like voicecoil I use an isolation transformer when developing 240V switching devices (in my case 3 phase pmsm drives) specifically to prevent myself getting an electric shock. That and stopping me melting 'scope probe leads anyway.... ;)

[Kitwn]

Care to explain that? Because, like voicecoil I use an isolation transformer when developing 240V switching devices (in my case 3 phase pmsm drives) specifically to prevent myself getting an electric shock. That and stopping me melting 'scope probe leads anyway.... ;)

Yes it does need qualifying.

If there is no earth reference connection anywhere following the transformer and you touch a live connection then that side of the supply will float to your potential and no current will flow. Same with your scope probes, the scope floats to the same potential as that side of the supply (or vice versa) and there is no problem.

The problem arises when one side of the supply is connected to exposed metalwork either deliberately or by accident such as touching a metal bench or via another piece of equipment plugged into a dis board fed from the transformer. This creates the risk of you contacting both sides of the supply at once.

You are quite right that an isolating transformer can protect you in certain circumstances but most electronic workshops have strict guidelines about the use of isolating transformers including only ever plugging in one piece of equipment at a time, covering metal benches with rubber mats etc. My concern was if the whole CNC machine and other gear was going to be isolated by the transformer something somewhere could be providing an earth reference and that creates a danger.

The primary danger I was concerned with is the fact that a transformer negates the protection provided by an RCD. If you connect yourself between a standard mains supply and earth it's the leakage to earth (which creates a small difference in the live and neutral currents at the RCD) that is picked up by the RCD and that trips the supply. Put a transformer downstream of the RCD and connect yourself across it's output and there is no earth leakage back to the mains supply so the RCD won't protect you.

[Voicecoil]

Y

The problem arises when one side of the supply is connected to exposed metalwork either deliberately or by accident such as touching a metal bench or via another piece of equipment plugged into a dis board fed from the transformer. This creates the risk of you contacting both sides of the supply at once.

......That's why SMPS engineers tend to work with one hand behind their back I guess!

Seriously though, the danger isn't that great as to get a shock off an isolated supply you'd basically need to be touching something live with one hand and something neutral with the other, which is likely a smaller risk than touching something live and leakage to earth via feet etc.. And of course as part of the procedure I've always gone through when working in such a way, you do a check that there's mains earth continuity through the iso. transformer, so that in this case the machine chassis is properly earthed.

[Muzzer]

You may be overthinking or misunderstanding this. Having spent most of my career designing and developing mains powered SMPS products, I would endorse the use of an isolation transformer. It's true that the RCD no longer protects you against accidental contact with an HV circuit - but when it's floating, by definition one such point of contact isn't in itself an issue. On the other hand, if it's non-isolated mains, you can get a lethal (deadly) belt just by touching a live node unless you are fully enclosed in a heavy duty rubber gimp suit. Unfortunately these are expensive, uncomfortable and can get you arrested.

Most professional labs use an mains isolation transformer and a variable source, either a variac (variable autotransformer) or a modern switchmode AC source. There is generally an e-stop to completely kill the power remotely in case you observe anything untoward. And you don't let anyone near the stuff without full and formal training.

There's no protecting against complete idiocy and at some point you need to learn the importance of NOT touching 2 different nodes, particularly with different hands. I once did that across 415Vac and somehow live to tell the tale. It hurt like f*ck and I never did it again but might not have been so lucky. For that reason, only using one hand to attach probes etc, combined with an isolation transformer is an <almost> foolproof way to avoid unnecessary bouts of death. Nowadays, we are trained in CPR and have access to defibrillators but those are the ambulance at the bottom of the cliff rather than the fence at the top.

[Voicecoil]

Got the machine up and running, then realised I needed to get some coolant sorted before cutting anything much in the metal department. Rather than reinvent the wheel from scratch I thought I'd start with one of these off the Bay of fleas:

Trying it out it seemed to deliver coolant without too much fog, but was rather feeble in the chip clearing department running off my rather low pressure air supply..Dis-assembling the thing it is made of 4 parts like so:

the inner coolant nozzle has the same pitch thread as the outer collar, so threads into the collar when it's put on. The lack of puff looked like it was being caused by the very small apertures:

Then I noticed the internal diameter of the yellow plastic end bit looked like it would fit a bit of 6mm AF hex bar, and that the collar and final guide had the same M10 x 1 threads. So I turned and slotted a bit of 6mm AF hex brass to make an alternative inner piece:

This push fits firmly inside the end of the yellow bit thus:

Screw the conical end guide back on and hey presto there was a good blow & coolant, quite a decently narrow stream too, about 10mm diameter at 100mm distance. And the bits cost less than a fiver

[AndyUK]

Nice one! Looks great!

Don't suppose you'd knock off another insert for those of us without a lathe handy?

(Technically the lathe is just a few hundred miles away covered in a mountain of crap, but I've never used it, and plan to eventually start gently...)

[Voicecoil]

Nice one! Looks great!

Don't suppose you'd knock off another insert for those of us without a lathe handy?

(Technically the lathe is just a few hundred miles away covered in a mountain of crap, but I've never used it, and plan to eventually start gently...)

I'll have a go - means I'll have to measure that one though! Might be a week or two as v.busy at the mo.

[Voicecoil]

This was borne out of a cockup, but might be of interest to someone else. I'd originally intended to have some bellows covering the rails and ballscrew on the gantry (my X axis), but when I came to fit them I found I'd made an error in the drawing and there wasn't quite enough clearance at the bottom so they fouled the carriage plates. It won't be a problem in the long run as I was intending to remake the carriage plates anyway now that I have proper milling capacity, but that's been put back a bit due to work pressure and supply problems with aluminium stock due to the virus. Observing that the X axis ballscrew was frequently getting sprinkled with chips, I thought some other form of protection was in order. I rang a few of the machine guarding people but everything they had on offer was too big and/or too expensive. After a bit of thought and fiddling, it looked like some kind of blind arrangement would be the best option given the limited space. I fiddled around with springs out of an old window blind, but the force was far from constant and it was a pain to put together without getting custom springs made. Then the thought struck me that a DC motor supplied with a constant current gave constant torque..... So I came up with these:


Not a 100% perfect solution, but they seem to be keeping the ballscrew clean, and apart from a couple of £10 motors off the Bay of fleas, were made from scrap bits lying around. I've set them up so they're switched on by the X axis enable signal, though the motors don't even get warm if on continuously.

[JAZZCNC]

that picture really got me confussed. I looked at it and thought why as he got green caps on the ends of those rails... then i realised the green was the bearing caps...