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holes in sheet metal

Posted by ionized (My Page) on
Fri, Mar 16, 12 at 18:02

What is the best way to drill half-inch diameter holes through sheet metal that I can't easily move around. I am talking with a hand drill and I want to avoid twisting up the metal in the fluorescent tube fixtures when I install switches to control them individually.

Should I clamp a piece of wood on the back side?
Will a small pilot hole help, or hinder?
Will a special type of bit make it easier?
Is cutting oil helpful with thin metal?


Follow-Up Postings:

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RE: holes in sheet metal

The best way to do this is not do this at all. Metal holes with wires running through them is a big safety hazard. And you aren't allowed to direct wire fixtures that aren't designed for it from the beginning.


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RE: holes in sheet metal

So, why would you decide to alter a UL rated fixture, which may already have up to seven factory knockouts, which could be fitted with a switch?


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RE: holes in sheet metal

Thanks you are right. A drill is the wrong way to go. I think I will get a punch that is the correct size.


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RE: holes in sheet metal

I think it did this years ago. I put string pull type switches in ceiling mounted fluorescent fixtures that were controlled by a wall switch to turn them on/off individually to save some electricity. Actually I never used them as much as I thought I would.

As I recall, the standard knockout is too large for the switch and I could not (at least easily) find the appropriate bushing. Clamping a small block of wood to the steel to act as a backer is a good idea. The metal is so thin that it doesn't much matter what kind of drill you use. A hole saw or a ordinary twist bit with a pilot hole works. Clean up any burrs with a file.


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RE: holes in sheet metal

Thanks, Mike. That is pretty much what I am doing. I have, IIRC, I have a total of 25, 8' tubes in the garage plus 6, 4' tubes. They are on three switches. That really is not enough because the 8' fixtures are, of course, instant-start and turning them on and off a lot will really tear up the tubes electrodes.


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RE: holes in sheet metal

"mike_kaiser (My Page) on Sat, Mar 17, 12 at 7:28

As I recall, the standard knockout is too large for the switch and I could not (at least easily) find the appropriate bushing".

Fender washers have been around since the 40's, that I know of.


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RE: holes in sheet metal

There are dozens of ways to drill the holes but even when you get the holes done, your idea won't work.

Florescent lamps require a "Ballast" to step the line voltage up to high voltage to activate the lamps.

Generally even though the fixture has two lamps it only has one "Ballast" and both lamps work off that one ballast. If you were to put a switch on the secondary side of the ballast and only run one lamp off of it, the ballast would overheat and quickly burn out.


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RE: holes in sheet metal

Some of you guys are making this way to complicated. There are no additional wires going in or out of the fixtures. There is no switching of the lamp side of the ballast. I just install a switch in the case, intercept the hot conductor and install with wire nuts to the switch pigtail. The only one that is slightly complicated is the fixture with 4 tubes and two ballasts.


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RE: holes in sheet metal

"I just install a switch in the case, intercept the hot conductor and install with wire nuts to the switch pigtail."

Most if the issue appears to be how to make a hole sized for the switch in the fixture, not how to wire the switch.

A pair of fender washers drilled out to accept the switch and larger than a knockout would be fine, or use a punch to create a switch sized hole.


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RE: holes in sheet metal

Yes, that is so correct brickeye. I probably could have saved myself a lot of trouble by leaving the lights, wires, switches etc. out of my post and focusing on the sheet metal. My assumption is always that people will be interested in the larger problem.

I ended up clamping scrap lumber to the back side, drilling a small pilot and then using a 7/16 bit. The lumber made all the difference in the world. I cleaned it up a little with a round file and got a good 1/2 inch hole. I tried a stepped hole bit, but it was going to take longer than I have the patience for and I quit before I got one hole done. Using one of those things means cutting the "same" hole over and over until you get to the size you want.

I made good progress for starting late Sunday afternoon. In the shop, I had already done one fixture with 4 tubes and two ballasts. (It now has two stitches. The holes are sloppy). Yesterday I did one more fixture in there. It was a tough one with the reflector being a full 8' long instead of being split. I am down to turning on three tubes, with the wall switch, instead of 9.

On one side of the garage I am down to two tubes on one side instead of 8. I'll work on the other side another day.

There are a number of reasons to drill a new hole rather than use an existing knockout. I am using canopy switches that I got for $1.29 each. I have to keep this inexpensive to make it worthwhile. Knock-outs are not in good places to make these switches work well since they don't operate well at too high an angle. Sometimes using a knock-out means installing a new length of wire inside the fixture. I know that a little wire and a few washers and wire nuts are cheap, but so am I. I might even have the right washers around, but I think that whole package is likely too thick for the switches.

In addition to that, the tubes/fixtures are 8' long so putting a switch at the "wrong" end makes a difference in convenience unless you have the wingspan of Larry Bird.

I was watching with a little interest last week an auction on ebay. Someone was unloading new 8'T8 fixtures (including ballasts) that each held 4, 48" tubes. They went for $13.54, for all 28 of them. Shipping would have been $575.


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RE: holes in sheet metal

So, now that you have "modified" a previously UL rated appliance, subverting that UL rating.
Being cheap, as you stated. If you have a loss and its attributed to one of your "modified" appliances, and the insurance "elects" not to honor your claim.

Exactly what portion of "budgeting for contingency" did you "elect" not to comprehend, and how "cheap" will the repairs be?


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RE: holes in sheet metal

My insurance company can inspect my property and elect to withdraw coverage citing some safety issues. They can not refuse to pay a claim after the fact unless fraud is involved.

Let's assume that you are correct, however, and go from there. How likely is it that some sort of failure due to the switches will occur and cause a fire or electrocution? The switches are rated well in excess of the load involved. The switches are actually designed for a similar application. The flammable parts of the switches are enclosed inside a device that is approved to contain junctions. There is plaster board above the nonflammable parts of the switches that protrude from the light fixtures.

I will consider getting my PE associate to write me a memo with his approval signature if it will help you sleep better at night. I've got a thousand other things higher up on my sleep-inhibiting list that are not keeping me up at night.


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RE: holes in sheet metal

You need a UL rating to manufacture and sell your appliance.

An individual can build their own light from scratch or modify one that they bought. As ionized said, unless you intentionally start a fire your insurance will cover you. In the same way you are protected if you accidentally leave a candle burning and it lights your drapes on fire and burns your house down, you are protected from your own ignorance or stupidity.


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RE: holes in sheet metal

"I tried a stepped hole bit"

Good thing you gave up.

They have a habit of making a mess of thing sheet metal.

The can work on styuff about 1/16 inch thick reasonable well, but punches are far neater and easier.
The big drawback of punches is the expense.

Putting thin metal between some 1x lumber and then drilling works well to limit damage.
It can be a real PITA to get the wood on both sides of a large item though.

If the holes wil be concealed, just running some screws through one piece into the other on each side of the larger hole location often works.

"So, now that you have "modified" a previously UL rated appliance, subverting that UL rating."

Adding a switch to a listed fixture is not going to void its listing like you are thinking, and your homeowners insurance will still cover you just fine.

You should see the holes that have to be cut in fluorescent fixtures that are going to cover a junction box.
The inside of the box cannot be covered by �discharge lighting.�

Out comes the drill and the sheet metal nibblers.

A round hole can be nibbled out and then filed clean if needed.
Since you are likely to have a switch body mounted in the hole, it does not have to be all that nice.

Some fender washers on each side and the switch mounting hardware to clamp the washers will also work.


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RE: holes in sheet metal

Thanks, brickeye. Just for interest, I don't understand what you mean here:

"The inside of the box cannot be covered by �discharge lighting.�"

In my partial account of my weekend activities, I forgot to mention that I could not find a punch in a small enough size. I found a set of knock-out punches that would have been worth investing in, but the smallest one was too big at about an inch diameter. At $25 they might not last that long, but it probably would have been long enough.


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RE: holes in sheet metal

The NEC does not allow discharge lighting enclosures to make a junction box inaccessible.

Since florescent lights are a type of discharge lighting, their (often large) reflector that forms the back of the light cannot cover a junction box.

A hole must be cut in the back to allow access.


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RE: holes in sheet metal

Thanks, that is good to know. I suppose that usually this is only an issue when someone is replacing a luminaire with one of a different type that has a larger footprint.


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RE: holes in sheet metal

"...different type that has a larger footprint."

Or failed to cut the required access the first time around (though I do not remember when the requirement appeared).

If you make a change it must comply with the code in affect when the change is made.

Grandfathering ends.


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RE: holes in sheet metal

Easily - as in walk across the shop and look in the drawer labeled 1/2" fender washers. Must have been empty that day.

Just as easy to drill a hole than drive to the store. ;-)


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RE: holes in sheet metal

Just for giggles, I will try to remember to stack up a couple of fender washers, add them to the thickness of an accessible fixture, and compare that to the way the switches are made. I don't think it is a practical way to do this anyway. It will be too thick for the switches to mount properly.

Add that to the fact that it does not really put the switches where I want them and you have a less suitable solution.


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RE: holes in sheet metal

"It will be too thick for the switches to mount properly. "

Pretty crappy switches.

Most have at lease a decent amount of threaded body for mounting.


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