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Spark erosion


Peaklander
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I recently bought a secondhand Hannibal roof rack. Where the gutter clamps pass back up through the side arms, to be pulled up on their threaded rods, there are M6 grub screws which act against the square section of those rods, just below the threaded part. The previous owner had never used the grub screws because they were tight, stainless into the aluminium. I released five of the six easily with just a little heat on the aluminium but one couldn't be operated as there was a rusty, snapped-off hex in there, flush with the surface of the grub screw.

I realised that I couldn't do anything as it's a hard hex metal stuck in a less-hard stainless grub-screw, in a soft aluminium base. So I looked for a spark erosion place. Amazingly, I found one in a hamlet just a few miles away. I've driven past dozens of times over the years and had no idea that it was there. It turns out that in a small building, in pristine cleanliness, there's a one man business that has a whole host of high precision CNC tooling including wire erosion to very close tolerances and he is a very busy man.

The guy said he'd have a go and I waited a few weeks for him to have time. The bulkiness of the part meant that the machine's coolant bath had to be opened and he could only use the nozzles to direct coolant onto the piece and this limited the power. However with a lot of effort to clamp an awkward piece, he set it up and sparked through and preserved the thread too.

I thought I'd post the two pictures that I have.

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IMG_1045.jpeg.1cf519cb0e6174cb665cdf0f0140ed1c.jpeg

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Ha!

Unfortunately I wasn’t there. I had only rung him twice in three weeks to see if it was done and the second time was when it was on the machine. By the time I got there it was done. 

He took out the grub screw too. Not sure what happened but the plan was to wind it through and out the back once there was a hole through it.

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All injection and aluminium mould makers will have this kit and they're in little units all over the place. If anyone needs to find one search for a toolmaker. 

The cooling fluid is a dielectric and used to control the distance that the spark jumps and as you say, is usually submerged, however they still have to use a jet of fluid to try to blow the 'burn't away' metal out of the way otherwise it just conducts and uses all your current to go nowhere. If you watch them work they will go up and down like peck drilling to try to clear this. They have to make an electrode out of copper or graphite the shape you want less spark gap tolerance and may make more than one, a rougher and a finisher. It sparks away in little craters so you can get a rougher or smoother surface depending on the current. If I want a rough surface on a mould tool I spec a rough spark (there are gauges you can look it up on) which is a cheap way to get a rough surface finish on the plastic part. CNC sparkers can also jiggle around in a circle so you can go down a hole then jiggle about to increase the diameter or put a thread in a hardened material. As its very slow they also usually mill as much away as possible before they go to the sparker. A none CNC machine can be had for as little as a grand if you have the work for one. 

 

You can buy a little machine for sparking bolts out, never seen one though so don't know what they are like. 

https://www.ebay.co.uk/itm/Portable-EDM-8C-Broken-Tap-Remover-Small-Mini-Spark-Erosion-Bolt-Extractor-800W-/112641091923

 

If you like gizmos look up laser welders and laser hardeners. They are mounted on tracks so if you are in a fix they bring them out in the back of a van and will do the welding in your machine to save you taking everything apart. 

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46 minutes ago, Badger110 said:

Without sounding too thick, and i'm sure i'm not the only one, but how does it work?

Does it melt the metal grub screw?

 

Imagine the contacts of a switch... if you keep getting sparks across it you wear it out. It’s the same concept as that. The dielectric fluid it’s bathed in controls how it works. From memory it’s there to stop it arcing and then just sticking together. It means you can make the electrodes all sorts of funky shapes and it will make a hole that shape. Be it a bar with a hex on the end up to massive 3D shapes.  

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7 minutes ago, landroversforever said:

Imagine the contacts of a switch... if you keep getting sparks across it you wear it out. It’s the same concept as that. The dielectric fluid it’s bathed in controls how it works. From memory it’s there to stop it arcing and then just sticking together. It means you can make the electrodes all sorts of funky shapes and it will make a hole that shape. Be it a bar with a hex on the end up to massive 3D shapes.  

I think i might point you over there

👈

to my avatar for my initial response to your reply 😂

 

However it's not becasue you're not explaining it correctly, more to the point, my brain isn't understanding it.  

 

i didn't know electrical sparks cause erosion of sorts, but is that what you're saying?

 

 As in the title of the thread dumbass! <--my brain again ;)

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The guy who did this has a wire erosion machine running pretty much all the time. I presume the idea is the same except in this case the wire passes through a leader hole and then erodes sideways in order to cut. It is a continuous wire feed with a takeup reel underneath somewhere.

I think he said 1/2 micron accuracy. I may be wrong on that but he is proud that he does about a close as you can get. 

Edited by Peaklander
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39 minutes ago, Badger110 said:

I think i might point you over there

👈

to my avatar for my initial response to your reply 😂

 

However it's not becasue you're not explaining it correctly, more to the point, my brain isn't understanding it.  

 

i didn't know electrical sparks cause erosion of sorts, but is that what you're saying?

 

 As in the title of the thread dumbass! <--my brain again ;)

Yeah that’s it, all the little sparks between the electrode and the workpiece remove material, which the fluid also flushed away.  As it goes the electrode also wears so with the wire erosion type of machine, the wire is actually constantly being fed through the machine.

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Any gap on a mould more than 0.0125mm wide will flash, ie the plastic will escape down the gap. For plastics like nylon its half that thickness. The biggest mould I have is 500kg and that makes a plastic part weighing a couple of hundred grams. Imagine how big the mould has to be to make wheelie bins, car bumpers and so on and you can see why such level of accuracy is required. 

The mould I have will typically churn out 4 parts every 25 seconds and run continuously for decades so you can also see why good quality hardened tool steel is required and why things like sparkling and wiring have their place. 

Bottle tops might be made 128off per tool and might run on a 5 second cycle, or less. Now that's going for it! 

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18 hours ago, Cynic-al said:

Any gap on a mould more than 0.0125mm wide will flash, ie the plastic will escape down the gap. For plastics like nylon its half that thickness. The biggest mould I have is 500kg and that makes a plastic part weighing a couple of hundred grams. Imagine how big the mould has to be to make wheelie bins, car bumpers and so on and you can see why such level of accuracy is required. 

The mould I have will typically churn out 4 parts every 25 seconds and run continuously for decades so you can also see why good quality hardened tool steel is required and why things like sparkling and wiring have their place. 

Bottle tops might be made 128off per tool and might run on a 5 second cycle, or less. Now that's going for it! 

I was in China recently visiting tool making companies with a colleague. This mould produces plastic pallets. The gantry that lifts it was marked 40 tonne.

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23 hours ago, mickeyw said:

I was in China recently visiting tool making companies with a colleague. This mould produces plastic pallets. The gantry that lifts it was marked 40 tonne.

That's a big old girl, wouldn't want to drop that on your finger!

Might even be a stack mould, unless there are two next to each other :unsure:

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