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School me on Ram EDM

Radar987

Cast Iron
Joined
Dec 30, 2011
Location
CA
As the title suggests, I'm looking into Ram EDM to expand our capabilities. I'd specifically like to know more about electrode wear, cycle time, and power consumption, which I'd imagine are all closely related.

Sample application: a 2" diameter bore in hardened D2 tool steel, 2" deep, blind, so wire EDM not possible. Sequence: machine, heat treate, EDM.

Questions:

  1. Best electrode material? Graphite? Carbide? I have neighbors who can help me with carbide (grinders and wire)
  2. What kind of tolerance can I hold on diameter if the electrode is hypothetically perfect? H6 or H7 possible?
  3. How much material allowance would I leave?
  4. How many bores can I make before the electrode needs replacement?
  5. Ballpark cycle time and power consumption in constant amps?

Now let's look at another sample application. Same as above, but now it's a tapered bore. So the electrode makes full contact on the walls. Same questions as above.

Thanks.
 

plastikdreams

Diamond
Joined
May 31, 2011
Location
upstate nj
The electrode makes no contact with the part in edm.

Most processes use multiple electrodes for roughing, semifinishing, and finishing.

Cycle time depends on how much material you are leaving and how many electrodes you run. Also, while you can get a good finish, you will have recast to remove and probably polishing or some other process to smooth it out.

Tolerances held can be in the tenths or better...figure you can hold a better number than you can reliably measure.

I've only run graphite electrodes, but I understand copper works very well too.

Can't help ya on the power consumption though.

It's an expensive game to get it, and a slow game to play. If you left around 50k all around you could probably burn it in a few hours to close tolerance. Maybe 2 electrodes if you have a good efficient machine. The taper...depends on the amount of taper. Could be 3-5 hours and 3 electrodes or more.
 

Luke.kerbey

Cast Iron
Joined
Mar 2, 2019
Will you be using a manual eroder and only plunge machining? Or with an orbiting head/CNC? This is going too effect your electrode design and machining strategy. Will you be rotating your electrode at the same time for better accuracy?

If you are cutting a large hole after heat treat, would an internal grinding or machining operation be cheaper or easier? Milling Cutters for hard D2 are readily available.
 

Radar987

Cast Iron
Joined
Dec 30, 2011
Location
CA
The taper...depends on the amount of taper. Could be 3-5 hours and 3 electrodes or more.

The taper would be steep, 15 degrees or less.

What if we hardmilled and left 0.005" of material allowance all around? Would it really take 3-5 hours to get through 0.005" of material?
 

Radar987

Cast Iron
Joined
Dec 30, 2011
Location
CA
If you are cutting a large hole after heat treat, would an internal grinding or machining operation be cheaper or easier? Milling Cutters for hard D2 are readily available.

We hardmill now but we're trying to reduce the need for operator dependence.

We roughly calculated that the EDM process could be 10X slower and we'd break even. If we can get the EDM to be 5X slower, we're hitting it out of the park.
 

implmex

Titanium
Joined
Jun 23, 2002
Location
Vancouver BC Canada
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
 

plastikdreams

Diamond
Joined
May 31, 2011
Location
upstate nj
Marcus is 100% right. Ram edm is used to make parts designed to be edm'd because there's no other way to do it. For a good new machine, key word good, not great or amazing you are looking at maybe 400k minimum. Start adding a tool changer and continuous c axis (rotates the electrode) you are up into 750k for a decent sized mits.

Nothing is cheap or fast about it. If hard milling is working for you I'd stick with it.
 

RC Mech

Stainless
Joined
Jul 21, 2014
Location
Ontario, Canada
Turning D2 at 64 Rc the other day, the above sentiment about machining any other way before considering sinking is sound advice.

Keep in mind RE: machine selection, there is a limit to the area that can be burned by a single electrode. The limiting factor is the available amperage. My sinker has an upper limit of 50A, correlates to something like 2 square inches. Your 2” bore would need to be done in annular sections. I’d do it that way in any case as the flushing is easier.

I’ve done mold surfaces where text was required on a vertical wall inside a cavity with about 0.4” opening around 0.3” deep on the wall. Only reason for EDM was the impossibility of milling. The core was done on a VMC.
 

007Rob

Aluminum
Joined
Sep 24, 2015
Agree with everything mentioned above. We make mold parts and do a good amount of both hardmilling and EDM and usually the rule of thumb for us is if we can avoid EDM in favor of hardmilling - do it. It's usually cheaper/more efficient for the reasons mentioned above.

We use copper electrodes, which don't burn quite as fast as graphite but they're cleaner to make and hold finer detail a bit better. For your example with the 2" bore I would rough the steel to within .005" or so. On our machine with the spark gaps we use 1 set of electrodes (1 rough and 1 finish) would be good for 2 holes before wear would be a problem.

Don't normally do stuff quite that big so hard to guess times but I would expect 2-3 hours per hole.
 








 
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