Hi Radar987:
I do a fair amount of sinker EDM work, albeit with obsolescent equipment, so here's my take:
First, sinker EDM is a great tool to have in your pocket but gets invoked as a production process pretty rarely compared to chipmaking.
There are several reasons:
1) It's very slow because it proceeds one spark at a time...blasting a little divot out of the workpiece, then blasting another divot out, and another and another sequentially.
The sparks happen pretty often (milliseconds) but it's still one spark at a time, and the way you make it go faster is to make more powerful sparks that blast out bigger divots and leave a crappier result.
2) You have to make and fixture and align electrodes, so now your process needs at least two machines and two setups and two inspections and two cleanup operations etc etc etc.
3) Those electrodes you have to make don't last forever, especially if you have to preserve small details like sharp corners.
4) The electrode materials are expensive to buy and expensive to process, and will wreak havoc in your shop unless you implement a means to keep the abrasive and electrically conductive dust and dirt down from when you make the trodes.
5) The whole process is finicky and temperamental...much less so now that CNC sinkers with linear motors and adaptive controls are on the market, but you can spend an hour on a burn one day, and a day on the same burn the next day because of only trivial seeming changes in the conditions...usually changes in the thoroughness of the flushing.
This drawback is compounded if you try to improve your throughput by running several burns simultaneously on a single pallet of parts...one starved spot in the flushing will stall the whole burn.
6) A good finish and high accuracy takes a huge amount of time compared to a coarse finish and low accuracy.
The trode wears much faster during fine burns and the MRR is much much slower.
You can rough a burn to within 0.001" and 120 microinch in a tenth of the time it takes to finish the last 0.001" and bring the finish to 32 microinch; especially if you have details to burn and you need to create small features using multiple trodes in succession.
It gets far worse if you need exquisite precision and an even finer finish...orders of magnitude worse.
Next, the power consumption is high and the management of expensive consumables is high too.
For sinker (unlike wire) the consumables costs are trivial but still higher on average than a chipmaking process.
Typically this is the cost of dielectric oil and the cost of electrode material and the cutters to mill it.
The dielectric oil is not too bad...expensive to buy but pretty long lasting if it's not contaminated.
The graphite most commonly used for trodes is a big chunk of the budget, and most trode makers use diamond coated carbide endmills (pricey!).
Next, the machines move at what can most charitably be described as a "stately pace"
Watch a trode change or a rapid traverse on a sinker and you will fall into despair or boredom as it crawls along if you're accustomed to watching a CNC mill do a toolchange or a rapid traverse.
Rapids on my sinker are about 30 IPM...the more modern ones are a bit better but not a lot better.
Next, you can optimize the process best by pre-machining it as close as you can or dare to go...you can't just jack up the feedrate until the cutters break.
The sinker approaches the workpiece with the electrode and senses the changing voltage needed to jump the gap between workpiece and trode.
When it reaches a threshold value it triggers a discharge across the gap to blast out the divot.
You can't push it...you can only approach faster, or blow out a bigger chunk, or blow out more chunks per unit time.
The relationship between the variables you can control to achieve this is complex...modern controls will help you but it's still quite limited, and one of the key limitations is how fast you can flush the cut debris out of the gap.
So sinker EDM, while extraordinarily useful for many things you can't do any other way, is still much more limited than the marketing hype would have you believe.
My rule of thumb is "If you can machine it any other way...do so!"
Cheers
Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
