How about a welder for edm power source?

I broke a 4-40 tap in my filing machine casting. It was for the top cap, which is there to hold packing as a seal for the file rod, so not
that big of a deal since there are five good screws. But it feels like a berry seed in my tooth. Aggravating.
I got Langlois' book on building a hobbyist edm, but have a few other thoughts. I have a very nice tig welder, Miller Dynasty 200DX. In tig mode, it'll go down to 1A DC or 5A AC, plus a pulser on dc up to 250 Hz and lots of control over ac frequency and wave form. Seems I might be able to use it in lift start mode for edm.
I'd still need to build the carriage, stepper drive, controls, etc. I have a Beaglebone Black which could be applied to that job. I can get a reasonable stepper for $11 from the salvage place in town, plus some linear bearings and a bellows coupler for not much.
Any known reasons the welder is a bad idea?
Thanks.
Pete Keillor
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

No... but why not just 'urge' the thing out mechanically (punch, core drill, magnets, etc), then if the hole is messed up, weld the hole full, and then re-drill and tap?
It would sure take less time than building new machines for a 1-hole job!
Of course, if you really WANT an EDM around, go for it.
Lloyd
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 15 Aug 2015 17:00:52 -0500, Pete Keillor

It's an interesting idea. Basic EDMs deliver current from a big capacitor, however, so the energy in that spark is a lot greater than you think.
We had a tap buster (a proto-EDM) in my shop back in the '70s. I think we paid $25 for it, used. The power supply was a simple RC circuit and it was the size of a lunch box. It had a hand-cranked feedscrew rather than a servo. It worked very well. I spark-eroded a stud out of the head of my Honda motorcycle with it (a frequent need in those days -- Honda's bolts and studs were made of frozen shit), and eroded several taps out of pieces we were working on. The hand feed made it kind of slow, but the real electric servo systems in EDMs are kind of special. Steppers are FAR easier to implement for this job than servo motors, which is why Sodick used them into the 1980s. If you want to know why, I'll explain.
What I don't know is if a welder power supply will do it. You might want to take a look at this description of early EDMs from Poco Graphite, which is close to being accurate:
http://edmtechman.com/about.cfm?pg=2&chap=1
I think you could try it out by hand, to see if you get the results you want. Take a screw and try repeatedly lift-starting on the end of it. See if it actually eats the screw quickly enough. If so, it will be worth taking the next step.
BTW, tap busters did not use liquid dielectric, with few exceptions. They just work in air. Be careful; hot globs can spit out of the hole.
Good luck!
--
Ed Huntress (former marketing manager for Sodick)


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Saturday, August 15, 2015 at 6:00:47 PM UTC-4, Pete Keillor wrote:

There are some DIY edm plans on the internet. You might look at some of th em for ideas.
Many years ago, somewhere around 1969 a friend and I kludged up a edm. We used his drill press for the feed by setting the depth nuts and then leaned on the handle to feed the tool a very small amount. As I remember we used a good sized maybe 500 VA transformer and I think a voltage doubler. A l ight bulb in series with the supply had low resistanece when things were wo rking and a higher resistance when we had the tool shorting to the work. A kludge , but it worked and we could put square holes into tool steel.
We were charging the cap to about 300 volts. So a bunch higher voltage tha n your welder would supply, but the plans on the internet all use lower vol tage. So in answer to your qoestion , I do not know.
Dan
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 15 Aug 2015 17:59:24 -0700 (PDT), " snipped-for-privacy@krl.org"

Thanks all. Lloyd, yeah I'm retired and I think it'd be educational, plus I already spent an hour or two picking and pecking at it.
Good ideas, Ed. I might combine that with Dan's drill press idea for a test. I'll try the welder at much lower voltage. If that doesn't work, then I'll ge further into Lanlois' design using some big caps.
Pete Keillor
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 16 Aug 2015 07:36:19 -0500, Pete Keillor

Here's another bit of inside info that may be good to know about -- not that you're trying to build a real EDM, but it may be useful to know how they do it.
When they're using a hydrocarbon dielectric (kerosene, for all intents), the voltage required to initiate a spark ranges from 90V to 300V. Keep in mind that their parameters are for producing a highly accurate hole with minimum damage to the workpiece.
Around 1980, Agie had the world by the tail with a patented power supply that initiated a spark by using a *string* of lower-voltage pulses at high frequency. The voltage probably was around 90V. The Japanese avoided the patent on their export machines. They didn't worry about it on the machines they sold in Japan. <g>
Anyway, Sodick, for example, broke the process into three steps, without the patented pulse circuit. There is more than one way to skin that cat. First, a high-impedance 300V circuit polarized the channel between the electrode and the workpiece, producing "stringers" of ionized dielectric that reduced resistance for the second step.
The second step was a medium-impedance circuit at 150V. This one began to turn the liquid dielectric into a low-resistance channel that delivered enough amperage to turn the dielectric into ionized gas. As it heated up, it became a plasma.
The final step was the money step. This one operated more like a welder, at least in terms of delivering power. This was a very low-impedance circuit (nearly a short circuit, with a big capacitor supplying the energy) at around 15V - 30V. This step in the power delivery used the plasma channel to deliver an extremely concentrated spark, which was delivered to the workpiece at a rate of thousands of amperes per square inch. The actual average amperage delivered through the whole cycle might be 30A.
Anyway, that's how they did it then. I don't know how they do it now. But it's useful to know that real EDMs employ *sparks*, rather than an *arc*, because an arc is difficult to shut off and it can damage the workpiece.
For a tap buster, that's less of a problem. You'll get sparks, and also some arcs. You shut off the arcs the same way you stop it in arc welding -- by pulling the electrode away from the work enough to break the arc.
In real EDM, the retraction is only a few thousandths of an inch, at most. That's not enough to stop an arc, so there is a lot of power-supply and servo design directed at producing sparks but not arcs. Again, that will be less of an issue for busting taps, so you don't need all of that complication.
But it's good to know what's going on physically in those machines.
--
Ed Huntress

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 16 Aug 2015 07:36:19 -0500
<snip>

I know you want an excuse/reason to build your own EDM but...
Jody has a nice youtube video on getting taps and such out with a TIG welder. Which now we know you have a nice one available to use ;-)
=="Remove a Broken Tap, Exhaust Stud, or Pressed Pin with TIG welder"

https://www.youtube.com/watch?v
R2pGVGHbg ==
--
Leon Fisk
Grand Rapids MI/Zone 5b
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 16 Aug 2015 12:02:51 -0400, Leon Fisk

Cool video, Leon. That'll be my first try.
Pete
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 16 Aug 2015 12:46:52 -0500

Yeah, Jody makes it look easy-peasy...
Some practice runs first, using similar materials of little value would probably be helpful...
When I was young and foolish I used to cut/burn out problem items similar to that with an acetylene torch. Nowadays I think I'm too cautious and worry too much about screwing up the good parts some how. Some practice first would be really helpful but it's against my nature to use up consumables in that manner...
--
Leon Fisk
Grand Rapids MI/Zone 5b
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 16 Aug 2015 14:16:29 -0400, Leon Fisk

Yeah, I'm nowhere near as steady on my best day as the guy in the video. I'll practice. A 4-40 tap is one tiny target.
I remember an old gunsmithing trick to remove a busted tap in a receiver; hit the tap with the oa torch, the tap will heat up much more quickly than the receiver. Then hit it with just oxy. One short burst and a shower of sparks... But I don't have the nerve to try that.
Pete
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 16 Aug 2015 13:51:31 -0500
<snip>

Don't let Jody fool you. He takes advantage of any sort of prop/bracing technique to steady himself that he can think of. He quite often will grab hold of the electrode (welding gloves on) while stick (SMAW) welding to keep it steady.
I've never had the opportunity to try TIG welding but would try resting the cup on something convenient for trying to build up your broken tap.

Yeah, that's the kind of stuff I remember doing as a dumb teenager. Bolt broke off in a casting, just blow it out of there with the torch. Never gave the consequences of screwing up a second thought... Of course I used to ride my motorcycle down the road back then at night too. Full throttle (~65mph), no headlight... if I stared straight ahead my peripheral vision would register the sides of the road. I tried to stay in the middle somewhere. I added a second headlight to my Honda Magna and would really rather not ride at night if at all possible :-)
--
Leon Fisk
Grand Rapids MI/Zone 5b
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 8/16/2015 1:51 PM, Pete Keillor wrote:

I used to have a very limited aluminum production item with 14 4-40 holes that I tapped. After I spent a few hours mechanically removing broken taps, I switched it over to 6-32. My life was easier. Also soon after that I bought a HSS tap locally and that was the best tap I ever used in aluminum. I have limited metal working experience, just got lucky when I needed a tap. Don't really know what it was, just labelled HSS. Mikek
--
This email has been checked for viruses by Avast antivirus software.
http://www.avast.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 16 Aug 2015 12:46:52 -0500, Pete Keillor

That guy is a great tutor, isn't he? I like his vids a lot. They're clear, bright, and consistent.
--
The beauty of the 2nd Amendment is that it will not be needed
until they try to take it. --Thomas Jefferson
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I guess I'm lucky (know I'm blessed!). I haven't broken off a tap in the work in about five years. I do make sure they're all sharp, and go straight-in into a properly-sized tap hole.
Lloyd
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 16 Aug 2015 16:41:52 -0500, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote: >

This was a new Greenfield, so I can't blame the tap, and it was well in the hole, just my wobble. Also couldn't use my bench block. Maybe a hand tapping machine would be a better use of my time than an edm. We'll see how the welding goes after I practice.
Pete
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I don't usually buy Chinese tools unless they're for one-off jobs, but I got a (pretty) nice hand-tapper from Enco that only required a couple of small mods to make it a worthwhile shop addition.
It's probably part of the reason my "tap breakage ratio" has fallen off from historical figures.
LLoyd
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Make sure that no spark energy goes through the ball bearings. Insulate the tool from the drill press, and put a short circuit between DP chuck and DP frame.
Joe Gwinn
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Monday, August 17, 2015 at 9:00:16 AM UTC-4, Joe Gwinn wrote:

I should have mentioned that. We used a wood dowel in the drill press chuck and whatever shaped tool attached to the other end of the dowel.
To be safe we should have had some sort of guard around the tool to keep anyone from touching the tool.
Dan
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

It's a shame they're going to the dump/scrap yard, but at $1k to ship plus a lot of space to set it up, it's no big wonder. Are smaller units built? I'll bet tiny EDMs find new homes a lot more easily, especially with hobbyists like us.
--
The beauty of the 2nd Amendment is that it will not be needed
until they try to take it. --Thomas Jefferson
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Mon, 17 Aug 2015 18:23:26 -0700, Larry Jaques

Old Hansvedt benchtop machines. They were pretty good. You did need a big benchtop, but they were about the smallest sinker EDMs around.
--
Ed Huntress

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.