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Mcgyver
04-08-2007, 01:19 PM
have been working all weekend on a spot welder I'm making. mechanical is my own creation, electrics will be more or less to Swede's (5 Bear) design. seeing as I've never even used a spot welder, the jury's out on whether my design is crap or not :D.

one design aspect i trying to figure out.... Its bench mounted, with a foot pedal to turn the juice on (actually trips the relay, but is timed and by relay). I've been thinking of have the arms spring loaded in the closed position, with something you step on to open them, the idea being to have both hands free to hold the two pieces of sheet metal. Do you see any issue with having the electrode's default being in the closed position?

ckelloug
04-08-2007, 01:33 PM
Step on the pedal without work in it and you get the electrodes welded together.

scott96088
04-08-2007, 01:37 PM
Step on the pedal without work in it and you get the electrodes welded together.


The spot welder I have used when you step on the pedal the it closes on your parts. Opens when you take your foot off. You still get to hold on with both hands. And a safety factor of being open and off at the same time.


Scott

C - ROSS
04-08-2007, 01:39 PM
Probably OK although I do everything backwards. I would have them default to the open position. Use only one foot control to both close electrodes and turn on the relay to activate, then release and reposition for the next.

Ross

(Oh boy, everyone types faster than I do)

scott96088
04-08-2007, 01:48 PM
Probably OK although I do everything backwards. I would have them default to the open position. Use only one foot control to both close electrodes and turn on the relay to activate, then release and reposition for the next.

Ross

(Oh boy, everyone types faster than I do)

Make one pedal do both, push down to close then push farther to activate the relay.

In other words use a spring to pull the jaws shut. The when evrything is where you want it push a bit more to hit the switch


Scott

A.K. Boomer
04-08-2007, 01:58 PM
I think Scotty has a good Idea there, in fact it reminds me of the hand crimp actiavated one I used in metal shop as a kid...

Weston Bye
04-08-2007, 05:39 PM
What scotty said.

My most recent efforts were with a Miachi welder. A pneumatic cylinder compressed a spring until a trigger switch tripped to start the weld controller. The spring compression was adjustable with a "micrometer" knob. Once set, this provided uniform force for all welds. As this was a production welder and all the welds were in the same place on all the parts, uniformity is necessary. If you are fabbing one-off parts of varying thickness materials you will be adjusting a lot, but experience will teach you where to set it for each weld situation.

tiptop
04-08-2007, 06:19 PM
Hey Mcgyver, are you going to post pics? Your projects are fun to veiw. :)

agrip
04-08-2007, 06:19 PM
Mc

Spotwelders benefit by having a great deal of load avaiable for the jaw closure.

Big cylinders or tuned toggle mechanisms for the load, you will not regret.

If your welders arms flex much, expect the parts to slip each other a bit. You will have difficulty sliding them when fully clamped.

Your cycle time for doing thin stainless sheets may be in milliseconds. This gets a good nugget without a buncha flash to cleanup. Three seconds is a Looooong time.

I rigged mine with a calibrated limit switch so as NOT to
fire unless the jaws were fully shut.

Then by controlling closing force, I could tinker a bit with half closed jaws IF I needed to.

Soon found out how to set the stuff in there and avoid that.

An alternative firing plan, is a head pad.
pardon the pun, but when you nod your head, hit it.

almost forgot,
1. If you are using a relay, rig the opening spring to make the closing coil work a bit. this is the only way to get 'em to bounce fast enough for the short shots.
2. INCLUDE, some serious contact opening arc supression.

Ag

Forrest Addy
04-08-2007, 06:54 PM
Sorry if it's already been suggested, I didn't take time to read all the posts.

I suggest a two stage pedal. Depress it part way and the electrodes close to hold the work. Depress it all that way and a contact closes to start the welding cycle.

For those reading but not in the know, good spot welders produce a jolt of current in excess of 3000 Amps sufficient to bond a nugget of metal in the work between the electrodes. In the better spot welders the welding is a two stage affair where there's a definite force between the electrodes in the heating phase that's increased to force the metal together for a second or so after the current shuts off. This ensures a metal to metal bond and is what causes the distinctive little dents you see in spot welds on cars. It takes from 1/10 to 1/2 second per weld depending on about a dozen factors.

The secondary of a spot welder is usually a single turn (or maybe two) of solid copper buss bar or a laminated copper conductor. Ordinary welding cable or any extra length in the current path will cause voltage drop. This limits curret so the weld may be poor;u bonded or maybe impossible. Figure 2000 Amps per square inch of solid secondary copper conductor.

Mcgyver
04-08-2007, 09:11 PM
wow some great ideas, lots there i hadn't thought of, its appreciated.

maybe a two stage foot switch with an air operated clamp trigger via solenoid? hmmm, also trying to keep it simple. the head pad was a good idea, foot to close the arms and head pad for relay. I'll have to sleep on it.

here;s a pic so far, kind of making it up as I go based on materials at hand. I had the arms sitting around for 10 years (they're .625 copper). The part holding the upper arm is keyed to the axle which protrudes on either side so I've lots of options on hooking up levers etc. I'll key the axle to whatever arms i come up with. The angle iron feet stick out the back so i can cantilever it off the bench to work with foot device (clamp etc). don't hold back on the brilliant ideas on how to complete this :)

i hope it works, it won't get Forrest's amps though. Electrics are based on http://www.5bears.com/welder.htm Nothing is bought yet, so if you think there is better way, I'd love to hear it. I was going to go transformer scrounging and wrap with the heaviest welding wire i could find. i see Forrest's point on this, if there was enough space on the transformer could I wrap 2 in parallel?? or is that a bad idea (maybe the voltages aren't perfectcly balanced)?

http://i20.photobucket.com/albums/b201/michael0100/DSCN9326.jpg

CCWKen
04-08-2007, 10:28 PM
Part of your welding process will be based on clamp pressure. I'd do what Scott suggested with a two-stage pedal. You should also allow for a means to adjust the pressure. Spot welding is based on many factors but the one's you'll be able to control will be time and pressure. Maybe amps depending on how you build it.

If you wind your own transformer, the secondary will only be about .9 of a winding turn. The secondaries on my welders are about 2"x1/8" copper ribbon wire. :eek: As Forest says, you'll need up around 2000+ amps to do any significant welding. The less amps you have the longer you'll need to heat. The longer you heat, the hotter your welder will get. (Tips and transformer) Most of my welding is in the 18-20ga. and timed for less than a second. Way less!

I've seen the 5bears welder some time ago and as I recall there were some questionable claims.

pntrbl
04-08-2007, 11:10 PM
Years ago I worked at a company that made all sorts of weld heads. In all cases there was an adjustable spring to actually apply the pressure. That's your clamping pressure control.

A lever would typically push on a sliding mechanism of some type that housed the spring. They made them with flat slides on linear balls. Round tubes in tubes. Etc. The spring would in turn clamp the electrodes together. Adjusting the tension of the spring and then pushing the actuation until the spring takes over gives you a way of controlling the clamping pressure.

A switch at the end of the actuator movement would fire the power so there was no way to weld without the parts being clamped together properly.

It was mostly geared at the dental industry but I expect the same principles apply.

SP

Swarf&Sparks
04-09-2007, 09:45 AM
I may be throwing a (small) spanner in the works, but... here goes.
For some time, I've been thinking about a small (model/jewellery) spot welder. Thinking along the lines of a bank of electro caps, switchable in parallel, according to required current.
When the caps are discharged, no further heating can take place, so no damage to job.

Any comments from this erudite group?

ckelloug
04-09-2007, 10:39 AM
Swarf,

I've seen spot welders for nickel battery tabs online that use the capacitor discharge principle. Google for battery tab spot welder.

--Cameron

Swarf&Sparks
04-09-2007, 10:52 AM
Thanks Cameron, food for thought.
I'd like to avoid high $$$ semiconductors if possible, hence the idea of using manually switched cap bank.
Rgds, Lin

A.K. Boomer
04-09-2007, 11:17 AM
I personally dont like the use of plastics for the main housing, i know you have to have an insulator but your dealing with many amps --- not volts, dont think you need anything more than a stout plastic bushing, The nature of two pointed electrods being clamped together and then for a split second almost liquifieing the material in between them will make any deviations apparent --- with the flex of the delrin? trying to hold this together at such an increased leverage ratio against it might make for a sloppy machine ---- i remember the tips of our machine in metal shop and they had to be sharpened very "square" to each other or this effect would imediatly show up and it was for the most part a metal based holder bracket, it will also drastically extend the tip life because if they slide during the arc proceedure they will arc intermittently themselves and give up a little material each time.

oh yeah --- paint your copper arms a cheap aluminum color so nobody steals them!;)

Timewarp
04-09-2007, 11:37 AM
Arthur Gansen ( kinetic sculpture) has a really nice looking handheld spot welder - homebuilt. I saw it on a video he produced.
Paul

agrip
04-09-2007, 12:35 PM
S&S
Heavier than battery tabs cap discharge spotwelders do exist, used one 30 years ago. It was pricey back then, maybe the IGFET revolution can help with that.

I hope I don't sound like a wet blanket because that idea seems like a worthy project, if you have the need to justify the time and money.

The problems immediately to mind you need to face are.
1. The Caps are NOT ordinary.
The "vertical" dV/dT & dI/dT curves there, means a bit of engineering to do.

Useful spot welding currents are usually over 1000 amps.
To get that kind of flow in a few millisecond window you may need 50 volts in the caps just to get stuff to move.

They must be built for repeated rapid (heavy) discharge. Probably very low inductance too.

2. Not sure but the whole circuit may need to be built to a careful balance to handle the reactance/ resistance problems of getting the caps to share the load.

3. The BIG switch also needs some thought.
1000 - 2000 amp inrush hmmm
Contact first closure and bounce and alla that can bring on welding shut,
OR if solid state, frying the the solid state channel as it opens.

Maybe the EE's can chime in.

Hth Ag

ckelloug
04-09-2007, 01:05 PM
I would think a silicon controlled rectifier would make an excellent discharge mechanism controlled by a SPST switch connected to a DC power supply. A 5 minute web search in the thomas register led me to these guys who do have some SCR's big enough: http://www.ampsabundant.com/scr.html

The SCR will trigger when It's gate voltage is raised by connecting it to power. The SCR will open when the current falls below the holding current--when the capacitors are drained. Relatively little current is needed to power the gate but the device can handle unbelievable power. You probably would need two of these however, one for the positive half of the AC wave and one for the negative half. It might also be possible and preferable to use a Triac since it will directly switch AC but I'm not that familiar with them.

ptjw7uk
04-09-2007, 02:48 PM
I have been collecting the bits to make a capacitor discharge welder. The secret is to use a SCR with as high a fusing current as possible then as long as the resistance at the weld site is low enough all the current should go into the weld and not fry the SCR. In the original that I saw over 20 years ago the designer actually measured the proposed weld resistance and when the resistance was in the correct range zap the SCR to create the weld.
The problem component at the moment is the caps, ones that can stand repetative high current discharge are not cheap.
Peter

Mcgyver
04-09-2007, 03:03 PM
I have been collecting the bits to make a capacitor discharge welder.



what would the advantage of this be - why this root rather than mains/transformer? aren't you still striving for low volts/high current? with it powered by mains, at least they are easily timed and turned on and off

is it acceptable to hook the transformer up to 220 (the two 120's of split single phase are 180 degrees apart right?) what voltage are we striving for 1 or 2?

Forrest, you mentioned 2000 amps per square inch.....those arms @ .625 dia are less than 1/3 of square inch. kind of scuppers things there doesn't it, but they are off a commercial machine?? unless its a matter of duty cycle in which case i'm prepared for the diy sacrifice.

Seems a problem is going to be suitably large cross section on the secondary winding, can there be two windings or even two transformers parallel? or will that blow me up?

Boomer, I hear you on the plastic, it was what I had, if it doesn't work I'll have to figure something else out. short of asking you guys to design it (hehe like I'm doing now), i know little about them other than web reading so I'm making it up as i go

ptjw7uk
04-09-2007, 03:20 PM
The reason is that a capacitor spot welder can be made quite small and can even spot weld aluminium foil and my spot welder is to big for model making.
Anyway I thought it would be easier than waiting for the glue (what I use now) to dry!
Its basically another of those long term jobs that when I get a roundtoit I will finish!!
Peter

Evan
04-09-2007, 05:27 PM
Mcgyver,

You will need to rewind the welder secondary if that's what you use. You can use more than one wire in parallel if you have room for the bundle. Or, you can use copper sheet like this transformer.

http://vts.bc.ca/pics/xformer1.jpg

A.K. Boomer
04-10-2007, 12:15 AM
Might as well try it, I think our one in metalshop had a tough time just because of the length of the arms themselfs added allot of side flex and such so you never know till you give it a try, Your bushings sure look like a class act and thats probably even more important then what kind of material..,

Cant wait to see some spots --- what do you plan on spot welding?

Evan
04-10-2007, 03:44 AM
The angle iron feet stick out the back so i can cantilever it off the bench to work with foot device (clamp etc). don't hold back on the brilliant ideas on how to complete this

Hydraulic clutch master and slave, flexible brake hose between them and a pressure switch to turn it on. Go find a Ford Ranger at the junk yard. They have a hydraulic clutch.

dicks42000
04-10-2007, 08:19 AM
Now there's an idea Evan. Brake pressure switches are a standard auto & bike part too. (Tail light...)
I just started dissecting a cast iron 4 cyl. from a Ranger for use as the column for my G. Ewen mill & am using the rods for a power hacksaw. (Half finished...)
Proof that Ford Rangers are useful....Yes, I still drive mine, too.

Forrest Addy
04-10-2007, 09:52 AM
McGyver: Those spot welding electrodes are built for a small portable spot welder and I happen to own one. It works well enough on sheet metal but at its full 16 gaga capacity you have to heat the work until you get a red spot the size of a dime. This causes a local anneal and a point of fatigue/bending failure on things like repaired sheet metal stools etc.

My little portable spotwelder is handy so long as I don't try to work beyond its practical limit of about 22 gage. On that and thnner it will make a good spotweld but quality goes down and difficulty increases with greater thickness.

I wish I had a spotwelder (they are simple to make given a dry type, 5 KVA transformer with a suitable 230 volt winding) that was good for welding 1/8 to 1/8 but that's a pretty stout machine at 12,000 amps or more.

You guys talking SCR contrrol, that's on the primary right? A good many modern spotwelders have SCR controls. You can get the puck stype solid state relays up to 50 Amps. Back in the day they were mercury tube (ignatron as I recall) and they used a phase shift, pulse counting circuit very similar to that used today.

If you want to keep it simple and you're thinking small, stick with the foot pedal operation. The linkage is simple and the available force is large. A toggle action adapted from a vise grip design will increase your clamping force to thousands of pounds if you need it. If your actuating force is small (a push button) then a cylinder makes sense. If you need to juggle and position the work then clamp and weld a foot pedal's flexibility and sensitivity is the better way to go.

Spot welders benefit from the grizzly bear approach; if you're gonna be a bear, be a grizzly bear. It's fine to build small if your requirements are small but life is always handing us projects each a bit larger and more advanced than the previous. Thus in the absense of common sense I tend to advocate bigger is better.

scott96088
04-10-2007, 11:25 AM
Now there's an idea Evan. Brake pressure switches are a standard auto & bike part too. (Tail light...)
I just started dissecting a cast iron 4 cyl. from a Ranger for use as the column for my G. Ewen mill & am using the rods for a power hacksaw. (Half finished...)
Proof that Ford Rangers are useful....Yes, I still drive mine, too.


Nice trick to drive with no block or rods. You must have a big hamster wheel on the tranny. :D :D


Scott

Evan
04-10-2007, 11:56 AM
http://vts.bc.ca/pics/hw1.gif

Note the phase lag between the spokes and the squirrel cage...

Continuous torque.

scott96088
04-10-2007, 12:14 PM
http://vts.bc.ca/pics/hw1.gif

Note the phase lag between the spokes and the squirrel cage...

Continuous torque.

Evan


No AC ripple in that motor,

with 4 legs at least one is always driving.

You had me on the floor in stitches :)

Scott

jim davies
04-11-2007, 12:02 AM
It seems to me that an el-Cheapo chinese bolt cutter would make a
good place to start as far as a spot welder clamping mechanism...

Wirecutter
04-12-2007, 01:13 PM
I'm a big fan of capacitive discharge spot welding, like the larger brothers of the battery tab welders. I used one quite a bit at work about 15 years ago. The marvelous thing about them is that they deliver a lot of energy in a very short pulse, and produce pretty good welds on really small stuff. Also, I've never had a weld that was too hot to touch. You just couldn't get it out of the electrodes before the very small concentrated area of heat had dissipated into the parts.

The welders I used were made by Hughes and Unitek, and the type still commands a lot of money, even for 20 year old machines. They also do a nice job of bonding dissimilar metals together. The less conductive materials bond best - stainless for example. Brass can be done, but it's difficult. If you're bonding less conductive metals like stainless, you want highly conductive electrodes, like copper, and vice versa. There's a whole world of alloys used for welding electrodes, as many of you know. But I just think it's a cool process. It also makes a nice alternative to soldering on circuits that will get exposed to very high temperatures, like the instrumentation sometimes used at the business end of oil drilling.

I've also assembled a lot of the parts for a welder, but other pursuits are keeping me from it right now.


-Mark