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Old 26th Nov 2020, 8:45 am   #1
Keith956
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Default Cutting irregular hole in panel

Mods, please move if this is not in the right place.

I've finally found a steel box to put an isolation transformer in, the front panel is what appears to be 1mm steel. I want to put a standard MK mains single socket on the panel, which entails cutting an irregular hole - almost square, but not quite.

What is the best way to cut such a hole (there's also a rectangular one needed for the on/off rocker switch)? I've got no fancy metalworking tools, just a power drill, vice, hacksaw and files.
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Old 26th Nov 2020, 8:50 am   #2
GrimJosef
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Default Re: Cutting irregular hole in panel

The standard way is called 'stitch drilling'. Mark the hole you want to cut, perhaps on masking tape applied to the panel. Drill a sequence of small holes around, and just inside, the edge you've marked. Join a few of them together with the tip of a needle file, then continue this if necessary with a junior hacksaw blade. Wiggle the waste piece of metal out. Tidy up by hand-filing.

Cheers,

GJ
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Old 26th Nov 2020, 8:53 am   #3
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Default Re: Cutting irregular hole in panel

Have you considered fitting a cavity wall style plastic Pattress then you could cut a simple square hole.

https://www.screwfix.com/p/appleby-1...B&gclsrc=aw.ds

Cheers

Mike T
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Old 26th Nov 2020, 8:59 am   #4
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Default Re: Cutting irregular hole in panel

I would use a steel cutting blade in my jigsaw, taking it slowly . Mick.
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Old 26th Nov 2020, 9:00 am   #5
Keith956
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Default Re: Cutting irregular hole in panel

Thanks Mike, I had never thought of a pattress, it would simplify matters a lot. Would need to check if there's enough clearance behind the panel to the transformer.

GJ, that sounds like a straightforward approach. Is it best to use lots of small holes or fewer large ones?
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Old 26th Nov 2020, 9:30 am   #6
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Default Re: Cutting irregular hole in panel

Small holes can better follow the shape and leave you less filing to do. But you can't drill overlapping holes with ordinary drill bits, so you leave small spaces between them, then you need to be able to enlarge a few to make space to get a hacksaw blade through to be able to saw the other holes together.

If you go the jigsaw route, Put masking tape on the surface so it doesn't get marked by the sole plate of the jigsaw.

Other methods would be:
Abrafile fitted in a hackaw frame.
Nibbler (Advel, Monodex, or a powered one)

It depends what tackle you have handy.
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Old 26th Nov 2020, 9:35 am   #7
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Default Re: Cutting irregular hole in panel

A surface mounted plastic pattress would make life even easier.

Alan
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Old 26th Nov 2020, 9:46 am   #8
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Default Re: Cutting irregular hole in panel

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Originally Posted by ajgriff View Post
A surface mounted plastic pattress would make life even easier.

Alan
You may need to fit some packing behind the lugs to enable you to tighten up the screws , Mick.
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Old 26th Nov 2020, 10:16 am   #9
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Default Re: Cutting irregular hole in panel

Not sure I understand the potential need for packing Mick. Just screw or bolt the pattress to the panel and fit the socket in the normal way. Hole needed for the wiring of course.

Alan

Last edited by ajgriff; 26th Nov 2020 at 10:21 am.
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Old 26th Nov 2020, 10:27 am   #10
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Default Re: Cutting irregular hole in panel

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Originally Posted by ajgriff View Post
Not sure I understand the potential need for packing Mick. Just screw or bolt the pattress to the panel and fit the socket in the normal way. Hole needed for the wiring of course.

Alan
I think Mick may have mistaken my post #3 for a cavity wall Pattress which will require packing for your suggestion which was for a surface mount Pattress which won't.

Cheers

Mike T
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Old 26th Nov 2020, 10:28 am   #11
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Default Re: Cutting irregular hole in panel

Corner holes then jigsaw and finish of with a file.

Lawrence.
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Old 26th Nov 2020, 11:13 am   #12
emeritus
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Default Re: Cutting irregular hole in panel

Mk do a "metal frame for panel mounting" that might do you: I have used them myself for this sort of thing. Screwfix don't do them now but a decent electrical shop should carry them.
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Old 26th Nov 2020, 12:26 pm   #13
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Default Re: Cutting irregular hole in panel

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Originally Posted by emeritus View Post
Mk do a "metal frame for panel mounting" that might do you: I have used them myself for this sort of thing. Screwfix don't do them now but a decent electrical shop should carry them.
Very interesting, thanks! That looks like it would make the cutout simpler. At 2.18 a pop from RS I think it's worth ordering one.
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Old 26th Nov 2020, 1:45 pm   #14
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Default Re: Cutting irregular hole in panel

Once you have drilled a series of holes they can be joined by tilting the drill/metal doing a self supported milling action. This does tend to drift to one side a bit, the correct angle of tilt will compensate. The little bits left joining the two can be snipped with cutters.
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Old 26th Nov 2020, 2:58 pm   #15
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Default Re: Cutting irregular hole in panel

Or one of these if you have a 50mm holesaw: https://uk.farnell.com/mk/735whi/mai...020-Desktop-Hi
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Old 26th Nov 2020, 4:18 pm   #16
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Default Re: Cutting irregular hole in panel

Quote:
Originally Posted by Cobaltblue View Post
Quote:
Originally Posted by ajgriff View Post
Not sure I understand the potential need for packing Mick. Just screw or bolt the pattress to the panel and fit the socket in the normal way. Hole needed for the wiring of course.

Alan
I think Mick may have mistaken my post #3 for a cavity wall Pattress which will require packing for your suggestion which was for a surface mount Pattress which won't.

Cheers

Mike T
Quite right I was thinking of cavity wall box's surfaces pattresses should be ok Mick.
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Old 26th Nov 2020, 4:27 pm   #17
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Default Re: Cutting irregular hole in panel

if you mount the socket conventionally you might want to think about if you want the earth on the socket to be connected electrically to the case of the transformer or not, as it will be via the mounting screws.

My transformer uses an all-insulated box and the screws and socket earth are isolated completely from the incoming mains earth.

Filtering on the appliance under test may otherwise cause parts in it to acquire a high floating voltage by referencing it to mains earth.
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Old 26th Nov 2020, 5:40 pm   #18
Keith956
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Default Re: Cutting irregular hole in panel

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Filtering on the appliance under test may otherwise cause parts in it to acquire a high floating voltage by referencing it to mains earth.
I was just planning on connecting the mains incoming earth to the chassis/transformer and to the socket earth. Not sure I understand how that is any different to earthing via plugging the appliance into the mains directly?

The intended usage is mainly for live chassis work which would not use the earth pin, but also for small switch mode power supply testing. In the latter case there is access to the transformer output (and socket L/N) to allow a current probe and high voltage differential probe, so the input power can be monitored via a scope.
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Old 26th Nov 2020, 6:49 pm   #19
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Default Re: Cutting irregular hole in panel

If you use a jigsaw cover the front panel and the saw bed plate with masking tape
this allows you to mark the panel and more importantly will stop the panel having its finish damaged
Did this 100,s of time while I was producing control panels etc
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Old 26th Nov 2020, 10:45 pm   #20
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Default Re: Cutting irregular hole in panel

because I assume you're using an isolating transformer so you can work on something live a bit more safely. If the appliance has components (eg capacitors, especially vintage ones, or a leaky mains transformer) connected between its earth conductor and its live or neutral, and you connect its earth to mains earth, then you are connecting components between the 'isolated' side and the mains earth, so the item under test is no longer completely isolated from the mains earth. Touching the supposedly isolated live or neutral may use you and a faulty component to complete the circuit. You may therefore be under a false sense of security.

If its a 2-wire only appliance eg live chassis radio then the output socket earth wont be connected. But if its an appliance that needs an earth under normal conditions, then leakage to earth may manifest as an earth-referenced live or neutral.

When I worked for another company our isolation transformer had a sense relay that cut the power if it saw any voltage coming back on the appliance earth conductor, like a voltage-operated ELCB.

I think
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