View Full Version : Hazard in latest M. W. article.

09-24-2002, 10:24 PM
I sent the following to Niel Knopf, email. I am also posting it here as I think it bears repeating.

Mr Knopf:

By now there has probably been a flood of correspondence concerning the "inexpensive EDM" article in the latest issue of MW. I will add my bit to it, as I just received my copy in today's mail.

The author correctly points out that the equipment is hazardous. However, the particular version is more than usually hazardous. In fact, I assert the liklihood of at least one electrocution or fire if even a small number of inexperienced persons try to construct the device according to the article.

1) There is no fuse included. This is a basic safety issue, as otherwise an accidental short depends solely on the branch circuit breaker or fuse for protection. Since these are rated at 15 or 20 amperes, a fire or other hazard is increased dramatically by the lack of a local fuse in the equipment.

2) Because all parts of the equipment are "conductively connected" to the power line, a shock hazard exists even from touching just ONE wire or part, if a foot or other part of the body touches grounded objects at the same time. The illustrated electrode lowering device lacks the most rudimentary shield against touching the metal directly below the knob. Use of an isolating transformer would protect against this hazard, although the inherent risk of shock from touching two parts of the apparatus at once would remain.

3) The author used electrolytic capacitors in the device, and failed to note the polarity on the schematic. This type of capacitor conducts well in one direction, but acts as a capacitor if polarity is reversed. Conduction is associated with heating and a very real risk of explosion of the part, particularly with no fuse present. Obviously the polarity is very important.

4) The suggestion of photoflash capacitors is made, which is reasonable as per ratings, but bad in the use of salvaged ones, which may have used up much of their life prior to the new use and be ready to fail. That type of capacitor has a hard life, and is expected to fail faster than normally used parts. This is despite the fact that they are built to withstand spark discharges such as experienced in this application.

5) A normal type of electrolytic capacitor is not made to withstand instant discharge as is normal with this type equipment. One would want to use a type intended for it, such as a photo type electrolytic, or certain types of non-electrolytic parts. The author fails to note this.

All in all, I think this article poses sufficient risks as-is that it may be advisable to go as far as a mailing advising at least subscribed readers not to construct the machine.

[This message has been edited by Oso (edited 09-24-2002).]

09-25-2002, 11:04 PM
I added more later, after reading even more carefully.

Additional information:

6) The author does not list the required voltage rating of the diode, although he does list a barely adequate rating for the bridge rectifier. I would suggest the use of a 400 volt rated device in either case, as the 200 volts is only a few percent higher than the peak line voltage.

7) Current rating is not discussed other than peripherally, but it should be at least 5 amperes, 10 amperes would be preferable. The 200 watt lamp will draw a little less than 2 amperes maximum, but the inrush current for a 200 watt bulb will be more like 15 or 20 amperes.

8) The insulation on the toolholder is minimal. It would be better to make the bulk of the toolholder from actual structural electrical insulation material, such as the "glastic" material available from MSC and others. This is a fiberglass material available in thicknesses of 1/4 inch and more, having adequate strength and insulation properties for this usage.

9) The article shows the clamps for the "tub", which are electrically "live", attached to the wooden base. Use of wood for insulation is not a good idea, and would not be approved by any safety agency. Wood can soak up water and become sufficiently conductive to present a shock hazard.

10) Covering the electrical wiring is mentioned in passing, but should have been emphasized.

11) Highlighting the author's lack of electrical knowlege is the reference to a half-wave rectifier giving 30 pulses per second, and a full wave rectifier giving 60. In fact, the numbers are 60 and 120, respectively.

Overall, as an electrical engineer designing consumer equipment, I am appalled at the hazards created by the lack of knowlege on the part of the author, as well as lack of elementary precautions. The fact that this made it into print unmodified and "un-reviewed" is equally appalling.
This is particularly perplexing after the more complex EDM article of several years ago in HSM. That prior article was apparently written by someone with actual engineering experience, and it shows.

09-25-2002, 11:59 PM
Very insightful Oso. Two thumbs up.


09-25-2002, 11:59 PM
I have not seen the article yet, but from what you say I am horrified. In Canada Open conductors must have a minimum 1" clearance between each bare conductor and any ground surface for all equipment run on 600V or less. Creepage between conductors should be checked as per CSA requirements with a Dielectric Strength Tester (we tested all 600V equipment to 2500V for one minute in the interest of safety). Proper non-conductive guarding is a must for a device such as this!

Lake of proper 100,000A interrupting capacity fusing or (thermal, magnetic, AFI, GFI) breakers as required by code is manditory and conducive to safe operation.

09-26-2002, 12:37 AM
Don't I just know it!
I have done agency compliance work per UL, CSA, and CE mark (europe), and I work with our compliance engineer from time to time still.

Actually, though, in equipment, through air or over surface clearances and creepage distance is only 3 to 6 mm with approved insulation materials used within ratings.
That is for voltages up to about 500V.

You may be referring to facility wiring, which is altogether different.

The fussiest guys are "TUKES" which is a non-government agency in Finland. They seem to be a sort of "greenpeace" for safety. They will test a product they pull from a store shelf, and then demand payment for their trouble if they claim it does not pass. Kind of sounds like a holdup to me.

Anyway, most of the special requirements are being "harmonized" which means we all cave in to whatever the europeans say.
The old CSA "flaming hot oil" test has gone by the wayside. I always thought that was a nice picturesque test description, although we did not have to do it.......

Compliance folks joke that Americans (and Canadians) live in wood houses and test for fire hazard, while europeans live in stone (or concrete) houses and test for shock hazard.

Since we use 12AX7, 6L6, EL34, and 6550 vacuum tubes in some equipment, we have in those cases to maintain clearances throughout our equipment, not just in the mains area. Makes things more interesting.

[This message has been edited by Oso (edited 09-25-2002).]

09-26-2002, 09:43 AM
thanks from me too.

don't know much about electronics, but this looked 'easy'. i was about to start looking for the parts. by this time i should have learned that anything that looks easy probably isn't.

all this said, does anyone have any information on a [safe] EDM machine that can be built in a well-equipped shop? i would like to have the capacity to do square holes, and to get all the taps i break out. always the small ones. no place to grab them to get out.


09-26-2002, 09:47 AM
That is difficult, as all inherently have high voltages.

The difference here was that there was no care at all taken to ensure that an unskilled person could build it correctly, and very little to reduce the inevitable hazards.

With proper components and materials, rated right, decent insulation, enclosure, etc, and an isolating transformer, it will be as safe as it can be made and still work. I will probably make one of that type myself.

09-26-2002, 10:44 AM
Hey, Oso:

When you make yours, why not submit an article with pictures to HSM so those of us challenged in the ways of the electron can make a SAFE home shop EDM? I realize it's a lot of work, but think of the fame and glory!

09-26-2002, 11:18 AM
<font face="Verdana, Arial" size="2">Originally posted by billr:
thanks from me too.

don't know much about electronics, but this looked 'easy'. i was about to start looking for the parts. by this time i should have learned that anything that looks easy probably isn't.

all this said, does anyone have any information on a [safe] EDM machine that can be built in a well-equipped shop? i would like to have the capacity to do square holes, and to get all the taps i break out. always the small ones. no place to grab them to get out.


A couple of years ago HSM had a series of articles on a pretty sofisticated one. I think it got good reviews from the Elec. Eng. Check the online index. I also think the author was selling circuit boards for this too.

Oso. Thanks for the heads up. From now on your the "go to" guy for electrical knowledge.LOL

metal mite
09-26-2002, 12:47 PM
oso is right,

A little tuning and it should be ok.
But many of our interests are a little risky.

Example, that little blow up in Ohio last year.
Anyone here ever blown a case in their rifle?
or had an unexpected discharge?

Fell off your bike, motor bike, vehical, boat, or horse?

Heck, I may make one myself.


Heck, we could all take up bowling like the former pres. suggested!

[This message has been edited by metal mite (edited 09-26-2002).]

09-26-2002, 09:58 PM
here here! I still have eight fingers and 30% of my brain cells intact, I'm ready to rock. Really now, a discharge machine would be sweet. But that one in HSM took 2 years worth of issues and a ton of cash to complete. What's the chances there were zero typos? Now it's in book form, so maybe all the bugs are worked out. (Wouldn't bet on it) Maybe Mr. Gregrichs' machine lacks some safety features, but I'd be more willing to tackle this one than the one in the book. So, Oso, we are all ears. Something simple, cheap, and safe, if you get one cooking we would be most appreciative if you share the know-how!

09-26-2002, 11:22 PM
Well, I would start by rectifying the items I mentioned.

The thing cannot be made non-hazardous, because it uses lethal voltages. So respect is required, just as with your other machines. The difference is the hazards of electrical equipment are somewhat invisible. Also, the other machines will probably just damage you, not kill. Electric can kill.

Most important first items to me are the fuse, insulation, and the 1:1 isolating transformer.

Oh, and one thing I didn't even mention.....can't believe I missed it.....AN ON/OFF SWITCH AND PILOT LIGHT! I am not by the magazine now, but I don't remember seeing that. I would have the switch break both sides of the line.

Correct capacitor polarity is clearly needed, and I think I would consider a non-electrolytic cap for the output cap.

400 volt rating on the diode or rectifier, and 10 amp rating minimum.

Cover up the electrical parts, and use real insulating material, not wood, for the enclosure. (or, grounded metal, but I would advise insulating material, as the fewer grounded items nearby the tub, the better)

Get the fiberglas insulating material and use a chunk for the toolholder support, between the adjuster and the tool. And insulate the top end of the tool with some sort of shroud, like PVC pipe.

Cover the stuff that is near the adjusting knob, so your wet hands don't slip off into the hot stuff.

That's a good start. basically go through my list and fix it.

The transformer is to prevent a shock if you just touch one wire. With everthing direct to the line, a single wire plus a wet shoe can get you.
The transformer means you have to touch two wires to get across hazardous voltage.

[This message has been edited by Oso (edited 09-26-2002).]

[This message has been edited by Oso (edited 09-27-2002).]

09-26-2002, 11:23 PM
Whats a "flaming hot oil" test?

09-26-2002, 11:31 PM
It was a test for certain types of equipment requiring an enclosed chassis for fire hazard reduction.

A certain amount of burning oil is poured through any vents in the chassis from the inside towards the outside. It must NOT be burning when it comes out the other side.

If the holes are sufficiently small, the oil will be extinguished.

That has to be one of my favorite test descriptions in all of the compliance biz.....

09-27-2002, 03:50 PM
Yes, I was going by industrial/HD wiring standards that are far stricter than houshold standards and for good reason - safety.

The test I thought was neat was the explosion proof enclosure test. Any electrical enclosure rated for hazardous locations had a spark plug installed and were filled with an explosive mixure of gas and pure oxygen and then place in the bottom of a 35' tank and detonated. If the enclosure leaked or distended too much it fails. It is good someone is watching out for our butts! And you wonder why that explosion proof stuff is so expensive...

[This message has been edited by Thrud (edited 09-27-2002).]

09-27-2002, 11:52 PM
yeah, haven't done that stuff, but it figures that would be the test. After all, even though its sealed, they figure the explosive gas will get in.

BTW, have heard nothing from MR Knopf, kinda expected to hear something back after sending him the message. I figure he is inundated with similar messages, judging by the letters that get sent when HSM has a pic with no guard on a grinding wheel.

Bob Quale
09-28-2002, 09:41 AM

Would it be better to use something other then light bulbs? Something that would not allow the high "rush in current". Then you could fuse the whole circuit for perhaps five amps instead of ten?

Also, what type of caps and what values would you recomend?


09-28-2002, 11:14 AM
Well, the inrush is really a problem only once when you turn it on (or plug it in, in the case of theirs *(Thrud , see below). Otherwise it is a benefit.

The lights are there for a reason, they are a variable resistor, and they are a current limiter if you run the tool into the work.

When the spark has discharged the first capacitor (the one by the tool) it gets re-charged from the other. The lamp limits the recharging current surges, but is out of the circuit again (low resistance) after current flow drops, so it charges faster than a resistor would.

Cap type needs to be one with the capability for repeated fast discharges, meaning photo duty ones are not bad, but "film and foil" "pulse duty" non-electrolytic types would be better. Catch is it takes more of these parts to reach the desired capacity than electrolytics.

As far as capacitor value, that depends on the energy per pulse you want. The stored energy is &lt;1/2 x capacitor value in farads x volts squared&gt;, expressed in watt-seconds, or "joules".
Larger energy eats more metal, probably gives a larger clearance around the tool, and recharges slower.
Smaller energy is the reverse, and should allow intricate shapes to be more closely reproduced.

Your choice.

Thrud...while we are discussing regulations, the lack of a power switch means the plug is connected and a charge-up surge flows. The plug is accepted as a disconnecting means, but NOT as a switch.

Therefore at each connection the arc will probably erode the plug or socket, which is one reason that usage of the plug as a switch (connection/disconnection under load) is not accepted by any agency.

Equipment is generally required to have a switch (not the same thing as guaranteeing a dopey user will actually USE the switch though).

09-28-2002, 06:30 PM
Explosion tests, there's a forgotten batch of brain cells jogged into gear! Once upon a time I did explosion tests at Delco Remy on their h.d. truck alternators. The creepy part was the work area was attached to the side of the building, with blow-away joints. So if you screwed up and got too much combustible mat'l in the atmosphere before you arc'd a spark, damage would be minimized TO THE BUILDING. Didn't inspire confidence, I must say. Froze my butt off out there too, trying to blow something up besides myself.

Weston Bye
09-28-2002, 06:48 PM
I read the article and, like all of you, was aghast at the electrical practices in the article. However, I certainly wouldn't discourage Neil from running articles with electrical or other potentially hazardous content in the magazines. Indeed, any and all phases of metal working, from hand filing to welding, to conventional machining, to EDM can be fraught with perils in the hands of the stupid and careless.

This article has presented a considerable learning opportunity for all involved: author, editor & reader. I agree with most of your criticisms, but even if the article were perfect, I might have included this disclaimer:


Well, probably something a little milder, but you get the idea.

If perfect safety in article content is required, our magazines (HSM, MW & LS) will limited to running articles on the safe use of crayons and play-doh. (Oops, don't eat that, it will make you sick)

All that being said, remember: there are lawyers out there...


09-28-2002, 06:58 PM
All stationary equipment must have a FUSED disconnect/service panel in Canada that can be locked out. I don't disagree with the regs at all, they make perfect sense and are there to save lives.

Power lockout relays/contactors should be manditory on the equipment we use (machine must be manually restarted after power outage) - a must as far as I am concerned. A mere plug-in on equipment such as an ESD is absurd - even if it is transformer isolated! It should have had at least a panel mounted 2-pole circuit breaker (both neutral and hot, or both hots on 240v, - ground must always be connected).

Nothing scares the crap out of me like a knuclehead near a power switch when I am working on a machine. I feel a hell of a lot better if I can slap my personal padlock on the power disconnect lever to prevent stupid accidents.

09-28-2002, 10:27 PM
Here you are supposed to be able to lock out the disconnect (OSHA).
I have heard of a company where someone (I think it was a manager) did turn on the power and shocked (surprised but did not injure) another worker.
The worker came over and decked the manager big time. Despite the no fighting/automatic firing rules, the worker still works there.........dunno about the manager.

As far as MW, I DO NOT think there should be any removal of articles on hazardous things.
This particular article was SO BAD, that I would remotely replace it with blank pages if I could.

The problem is that information given is insufficient to make the machine work at all, certainly too little to make it work sufficiently free of egregious hazards.
In fact, there is significant risk of things literally blowing up in the face of the builder, due to this lack.
THAT cannot be allowed.

After that we can talk about the disclaimers.

BTW, per the cap values, I would suggest thinking smaller than the values used in the article. This is simply on the basis of the very rounded square holes in his pictures. I think there is too much spark energy for good reproduction of the shape. Some of that can be pre-distorted in the tool, but eventually there is a corner, and if you want it sharper, you need a lower energy spark as I understand it.

I am no EDM expert, I know what I know from designing plastic parts that had the molds EDM sunk. Learned a bit about it then as to what I could expect them to re-produce effectively without a lot of extra work.

[This message has been edited by Oso (edited 09-28-2002).]

Bob Quale
09-29-2002, 11:42 AM
How do you size your isolation transformer? Do you figure two amps or 240VA or ten amps 1.2KVA? Big price difference.


Bob Quale
09-29-2002, 12:39 PM
I'm a full time Fire Fighter in CT. We responded to an industrial accident. A maintenace worker was up in a shear doing a repair. A worker came in and turned on the machine to start the day. He was decapitated. Left a wife, and two kids under the age of five. OSHA was all over them, but the man is still dead-ACCIDENTS HAPPEN- you can't be too safe.

Crazy Ed
09-30-2002, 02:07 AM
Lockouts are great! I quit a job in '74 as the boss told me NOT to lock out something.

10-01-2002, 06:39 PM
[QUOTE]Originally posted by Oso:
[B]Here you are supposed to be able to lock out the disconnect (OSHA).
I have heard of a company where someone (I think it was a manager) did turn on the power and shocked (surprised but did not injure) another worker.

The person who flipped the switch was not a manager... he/she was a wannabe. Speaking as a manager myself, you cannot supervise anything that you are doing. Supervision is a full time job (that is often done inappropriately: hence the connections between managers, lawyers, and other scum of the earth professions).

The person who flipped the switch without checking for others in harm's way does not belong around hazardous energies in any capacity, and I hope the person was moved along (usually promoted) to somewhere where he/she could do less damage.

10-02-2002, 09:48 AM
As far as I can see, well over 90% of managers are wannabe's per your definition.

Maybe they really are managers, and you are the wannabe.
Apparently the qualification to be a manager is the instant loss of all sense and reason. Since you appear to still have yours, evidently you don't qualify.......... http://bbs.homeshopmachinist.net//biggrin.gif

Uncle Dunc
10-08-2002, 12:29 PM
Let's propose a new magazine for Village Press. We'll call it Home Shock Machinist.

Uncle Dunc
10-08-2002, 12:55 PM
&gt;&gt; ... you can't be too safe.

Sure you can. When a safety measure costs more than the risk it is designed to abate, that's being too safe. When the car makers put sensors in the front seat to make sure the seatbelts were buckled, the American public decided that was too safe. And when airport security measures made sure that no one on a plane would be equipped to take down four mujahidin with box cutters, that was too safe. The risks of a gunfight on an airliner don't even compare to the risks of flying it into a building.

People weigh risk against reward all the time, and choose to bear some risk. There's a reason why coal miners get paid more than sales clerks at Wal-mart.

10-08-2002, 01:48 PM
To all:
Thank you for your comments regarding the EDM article in the latest MW. Certainly, it is not our intent to place our readers in harm's way. Your applicable comments will be printed in the next MW.


paul j smeltzer
10-09-2002, 01:03 AM
I thought everything in the shop could bite you sting you or kill you if you didnt use just a little bit of common sense. Paul

[This message has been edited by paul j smeltzer (edited 10-09-2002).]

10-09-2002, 01:47 AM
Hell Guys,
If it ain't dangerous then it ain't fun..!
I remember you all talking about blowing an anvil into the air with black powder. Every tool bit will cut you to the bone. Compressed air will cause serious damage to ears and other body parts. You don't hesitate a moment to melt bronze, aluminum, iron and cast intricate parts.
Why the hooplah about an electrical hazard. Just apply the same level of caution and understanding as you would to handling a crusable full of bronze. My partner and I teach electrical safety to electricians and they bring to our classes a load of stories about the installations that they work on. They do like this chat room does -- they tell each other about the hazards in an effort to help and educate. This is a good thing.
Please don't limit the articles that are published. This communication will illuminate and enlighten the readers.
Just my 2 cents

10-09-2002, 09:56 AM
Jack, you are missing the point. Badly.

Presumably you drive with no seatbelt, no brakes and the door open, for fun, all of your extension cords have missing insulation, and you store gasoline in the kitchen under the stove?

In this case, directions were published in a magazine, on how to construct a piece of equipment.

The instructions given are intended for people who do not know what they are doing, or they would not NEED directions.

The directions are incomplete, leaving out information required to avoid having things literally explode.

AND, aside from that, common safety items present even in the cheapest chinese import POS item of equipment are totally missing and unmentioned.

So according to your reasoning, its OK if I explain to your son how to make a neat firecracker by mixing a few chemicals to make nitroglycerine, and leave out a few steps so it will probably go off while its cooking?
I think not.

10-10-2002, 01:55 AM
Have YOU read the Anarchists Cookbook..? They do have a recipe for Nitro and several of the steps are incorrect or incomplete. This was explained to me by a chemical engineer who knew how to make the stuff.

And my son has read the book and he and I have talked about the process and what was wrong and missing. They say to stir the mix constantly ... what they don’t say is that if you stir the mix with a metal spoon the mix will self ignite the moment it becomes Nitro. They use compressed air to bubble through the mix to stir it in the commercial world. My chemical engineer laughed hard about the wrong technique.

My point was this..
This person submitted an article to a magazine to show others how to make a EDM. This was not done with malice.

Immediately when it came out you saw the errors and omissions in the construction and operation of this primitive EDM. You acted to enlighten the readers about the dangers. All of this is good.

The output of this discussion will be a better, safer EDM and a more educated audience. Already several have asked you for your expert information. A small safe accurate EDM is on the way already.

Nothing springs full grown from the womb.

A person I work with cut himself with an Exacto knife. He did a great job of it..bone deep...lots of blood.

The management of the company acted to ban Exacto knifes from everybody. I’m just worried that you are trying to ban Exacto knifes in your concern about this article. I disagree, we need more articles, and more good people like yourself to raise the bar.


10-10-2002, 02:36 AM
No, they don't allow educational material like that in Canada - we might get ideas.

The point was that the equipment is a hazard not just to the user, but because proper regulations are not followed other people can get killed from mere inattention to primary electrical safety - like children in the shop showing their friends dad's homebrew EDM.

Could you live with that on your shoulders?


Someone mentioned anvil shooting - anvil shooting is supposed to be carried out in a safe controlled manner just like rifle & pistol rangee are. There is little danger when approached in an intelligent, responsible manner.

10-10-2002, 09:35 AM
Naw, I may be from USA, but I'm not a gun-grabber, nor an advocate of safety at all costs (you know the nutty routine, "if it saves the life of one.....its worth it".)

Point is that if the article had been reviewed before publication, it could have been re-written re-designed to be as safe as reasonably possible.

If you had read the posts above you would have noticed that I mention this equipment cannot be made "non-hazardous". It can however be made reasonably safe to use.