Flicker Effect ?

[DomValente]

While posting pictures in another section I noticed my own lighting and was reminded of something told to me by an old friend.
"Always put double fluorescent tubes over your table saw."
The reason for this, is that tubes flicker and if you use your table saw without a guard on, the lighting "flicker effect" can give the impression that the blade is not moving, much like the spokes on a bicycle wheel.
Having double fluorescent tube units will negate this because each one cancels out the other.
I'm no engineer and this may be an old wives tale but it sounds right.
All of this said, you should not use your saw without the correct guarding

Dom

 

[Keith Smith]

Dom the lights "flicker" in sync with the mains frequency, having two tubes won't necessarily make any difference. This is usually more of a problem with lathes because as you say they can appear stationary when they are spinning, not such a good idea.

High frequency fluorescents are available and they don't strobe, they are a bit more expensive but on the other hand they are more efficient so cost less to run.

Keith

 

[Scrit]

Yes, Dom, as Woodsmith says the better manufactories used to have high frequency flourescents with odd frequencies which were not multiples of the mains frequency. I used to have Thorn stuff like that in my last workshop, but my recollection was that it was about 1-1/2 to 2 times the price of the standard stuff a few years back. I thought that strobe effect was also part of the reason why low voltage machinery lamps used to be DC not AC.

All of this said, you should not use your saw without the correct guarding

:roll: :wink:

Scrit

 

[ndbrown]

I was concerned about the possibility of the strobe effect whilst completing the build of my small workshop recently, so I fitted the fully enclosed and dustproof Screwfix High Frequency ones (code 78671) at £37 per fitting plus the cost of a daylight tube at £2.50 from a local electrical distributor. Two of these fittings plus the daylight tubes have made a huge difference to working in my 9' x 9' shed, (tried one tube, but two are really required to reduce shadowing). Would highly recommend going down this route. No flicker, instant start and no annoying hum associated with "normal" fluorescents. Will definitely be equipping my garage workshop with 4 of these.

 

[Pecker]

Daylight bulbs? I just keep the workshop door open :wink:

woody

 

[Losos]

Hi Dom,
Methinks you've opened a can of worms here because there all sorts of theories both in favour and against the idea that fluoros can 'stop' machinery.

FWIW my recollections are that yes, if the blade is running at a multiple of the mains frequency (50Hz) and if there is no other lighting, and if the speed of the teeth (ie the outer circumference of the blade) is right then it might be possible for it (the blade) to appear stationary.

So the blade diam. / rpm / lack of any other lighting may combine to do this. All I can say is I've never seen it myself.

 

[DomValente]

No, can't say I've seen it either, but then I've got twin tubes

Dom

 

[dickm]

Agree with Losos that this is a theoretical possibility, but I've always felt that the probability of it occurring in a situation that could lead to an accident was pretty remote (exactly the right number of teeth, speed of rotation etc.).

Some other oldies may remember that posh record player turntables sometimes had patterns round the edge that would appear to be stationary if the table was illuminated by mains frequency lighting, and the table was rotating at the selected 33rpm or whatever. (I did once have a variable speed turntable which I was trying to set up outdoors, and wondered why the strobe did not work, till somebody pointed out that the sun doesn't operate at 50Hz.)

 

[Shultzy]

Am I missing something here? Even if the lights cause the saw to appear stationary, won't the noise give the game away?

 

[Bean]

Dom
As the tubes are on the same feed, at the same frequency they will flicker together so you would get a stationary saw or chuck, If they were running at exactly the correct speed without interference from other light sources, its hardly likely to happen.

Low voltage lamps on the front of machines are there to minimise the danger to the operative when they break, its the same reason that all control switchgear is low voltage

 

[OJ]

This used to be called "stood machine syndrome" and is a real effect.
Common fixes are to use an incandescent bulb in an angle poise type lamp (no flicker) or to put your fluorescent tube banks on different phases if you have a 2 or 3 phase supply.

OJ

 

[Anonymous]

Dom

Wth dual tubes and different inductors in the fitting - like a single winding transformer), the tubes will not flicker together

 

[DomValente]

Thanks for all the replies,

So I'm replacing the tubes with lamp bulbs

Dom

 

[Newbie_Neil]

Hi OJ

Welcome to the forum.

Cheers
Neil

 

[mudman]

Okay, just tried this using my M950 lathe.

Trouble is, to get a stationary effect I reckon that the lathe needs to rotate at 3000rpm (50hz *60 please correct me if I'm wrong). The M950 only goes up to 2,400 rpm so unlikely to get the effect but I gave it a go anyway.

Took the starters out of all but one of my four tubes and used the variable speed control to vary the speed whilst observing the chuck.
I got no stationary effect although I was getting some strobe effects with the light reflecting on the chuck. There was a sort of coloured bar that could be made to move up and down with variation in speed.
Another effect I noticed was that at certain speeds the chuck sort of appeared to be moving slower than it actually was. However, you wouldn't notice this unless you were watching for it.

Interested in what I might observe with all the tubes working, I put the starters in and did it again. I observed the same strobe effects but this time it was multiple coloured reflections. Trying to remember back to the strobe experiments from 0 and A level days, I expect it is something to do with the four tubes being at slightly differerent frequencies.

It would be interesting to try this with a variable speed machine that will go up to above 3000rpm. Anyone got one?

 

[9fingers]

The strobe effect occurs when the speed is Nx (2 x 50Hz x 60)/ no of similar features per rev.

Twice mains frequency as peak light output is obtained at the peak of each mains half cycle

Ie ie you have 4 jaw chuck, the jaws will appear stationary at 375,750, 1125,1500 rpm and so on.

The prominence of the effect depends on the light source. A proper strobe light turns on and off very quickly and gives an almost perfect effect. A fluorescent tube gives some effect because although the ionised gas in the tube goes on and off at 100Hz, the fluorescent phosphor in the tube has some degree of persistence and slows the on and off times of the light you see.
An incandescent bulb shows no discernable effect as the thermal inertia of the filament means that the light output does not vary very much during the mains cycle.

Bob

 

[mudman]

Bob,
I hadn't appreciated the fact that the tubes will be 'flashing' at 100Hz, not 50.
I also hadn't considered the 'similar features' bit. But, it all starts to come flooding back now.
However, I'm not sure I see how you get to your values for where it will appear stationary.
Shouldn't it be N x 6000/4 ?
I.e. 1500, 3000, 4500, 6000 etc?

 

[mudman]

Posting this all over again as the **@#*%$* software lost it all.

The old physics lessons are coming back to me now.
The reason it will appear stationary is due to the retardation of the position of each jaw at each flash and means that it will appear stationary at speeds defined by:
(N/4) x 6000/4 where N=1,2,3,4,5...
Or N x 375 where N=1,2,3,4,5...


Makes sense now.

Anyway I didn't observe any effect and so I think that there isn't a lot to worry about.