Thread: easy inside corner radius?

This is mostly just for my general knowledge as a designer. We are making these widgets out of 316L stainless they need to have key ways cut into them at relatively precise (more precise and larger than a standard key way) positions. We have been getting them wire cut but we have been specifying a max of R 0.010 in the corners. Our supplier has not said anything about it but I was hoping to get some more insight into what it would take to actually do this. the keys are typically 1" wide about .625 deep and the length of the part can vary from as little as 2" up to 5" their are typically 3 of these per housing. So is 0.010 a reasonable rad in this material? what size wire would you use? is this a single cut operation or a roughing and then final? what kind of feed rates would you be looking at? sorry for all the questions I have never run one of these machines before like I said we just design them but if I have a better understanding of what needs done and how maybe I can help make a better design that is easier for everybody.

A .010" inside corner radius would be a very reasonable process on a wire machine, given the 2"-5" length of the parts you specified. The overall height of the part to be cut dictates the wire diameter (size of electrode to carry a given current), which in turn dictates the minimum inside radius that can be cut on that part. I usually cut with .008" and .012" wire, both of which are capable of cutting the taller of the parts that you listed. These would, in turn, proved a minimum of .004" and .006" inside radii, respectively. The overall tolerance and surface finish that you're requiring on the parts would indicate how many skim passes would be required. A single roughing pass through the 5" part should provide you with a tolerance in the +/-.0015" or so ballpark, while adding one skim pass should pull that back to +/-.0008" or so. These are pretty safe estimates given that the tolerance can be affected by many factors, least of which is the overall condition of the machine in question. Feedrates are probably going to be all over the map, depending on the age of the machine, cutting conditions and cutting technology. On the Mitsubishi's that I run, a safe number on the roughing pass at 5" with .012" wire would be .05"-.07"/minute roughing, followed by .10"-.12"/minute on the one skim. These are only estimates off the top of my head, though.

Thanks for the info. I always like to know what the guy making it actually has to do to make the parts I design. Another quick question am I correct in assuming that the physical hardness is not a primary factor when wire cutting. I assume that the real factors would be conductivity both thermal and electrical and melting (or vaproizing) point. Is that a correct assumption?

I have heard from more than one source that softer materials do cut slower than harder materials, but in my experience with steels, I've never noticed a difference. Better to say that I've never seen enough of a difference to catch my attention. Conductivity of material does play an important role, and more importantly, density of material. A good example would be aluminum vs. steel vs. carbide. I would say that aluminum will cut at roughly 2x+ the rate of steel, and steel at roughly 2-3x the rate of carbide. 99% of my wire time is with steel and some carbides, so I can't give an accurate answer in regards to materials such as copper, graphite, titanium, ect.

I agree with toolmaker 35. Another big factor for speed is if the part is solid with no inturruptions and flat on the top and bottom to get your flush cups to seal off on the part. Your tolerance and surface finish will determine how many skims you need. We cut at 14sq in / hour with good conditions for roughing.
Progressive Tool @ MFG. INC. Greensboro, NC

Thanks a lot guys I am learning a lot. Some more questions. On another product line we need to make small slots (0.005" to 0.015") in carbide that is 0.100 thick flat on both sides but with interruptions, would these sizes present any kind of issue typically? The other notable feature of these parts (or extrusion dies rather) is the tolerance we specify 0.0005" any issue with that kind of tolerance other than machine movement capability?

Ahh, now the rules are changing a little . The .005" to .015" slots are the width of the slots, correct? The .015" slot would be easy with .008" wire, but the .005" slot would pose a problem, at least for me. Currently, I can cut with .006" wire, and I have used .004" once (with marginal results). That one slot would require a wire size in the .002" range, given the .0005" total tolerance. When getting down to wire diameters such as this, the machine really needs to be specifically set up for fine wire, and some older machines simply aren't geared for this type of cutting. This means the machine has to be capable of handling that size wire (very good wire tension control), associated hardware needs to be changed on that machine (guides, contacts, rollers, ect.), and the appropriate cutting technology needs to be added or created. There are shops who specialize specifically in fine-wire work because of this.

In regards to the tolerance and material, that actually sounds pretty straightforward. I'm assuming that the interruptions that you describe are existing holes or slots? Being that the material is carbide, that actually works out to be an advantage. Carbide may cut slowly, but it doesn't stress-relieve, or "move" nearly as much as steel does, so cutting through existing geometry doesn't pose as many problems as steel would in that regard. Being only .100" thick, it should cut fairly quick though, even considering the material. Also, the .0005" total tolerance shouldn't pose much of an issue. I would think that I could hold that number consistently with 1 roughing pass and 3 skim passes. I can generally stay within a .0003" window safely with 4 skims up to about an inch thick, once the machine's parameters are dialed in.

Originally Posted by toolmaker35

Ahh, now the rules are changing a little . The .005" to .015" slots are the width of the slots, correct? The .015" slot would be easy with .008" wire, but the .005" slot would pose a problem, at least for me. Currently, I can cut with .006" wire, and I have used .004" once (with marginal results). That one slot would require a wire size in the .002" range, given the .0005" total tolerance. When getting down to wire diameters such as this, the machine really needs to be specifically set up for fine wire, and some older machines simply aren't geared for this type of cutting. This means the machine has to be capable of handling that size wire (very good wire tension control), associated hardware needs to be changed on that machine (guides, contacts, rollers, ect.), and the appropriate cutting technology needs to be added or created. There are shops who specialize specifically in fine-wire work because of this.

In regards to the tolerance and material, that actually sounds pretty straightforward. I'm assuming that the interruptions that you describe are existing holes or slots? Being that the material is carbide, that actually works out to be an advantage. Carbide may cut slowly, but it doesn't stress-relieve, or "move" nearly as much as steel does, so cutting through existing geometry doesn't pose as many problems as steel would in that regard. Being only .100" thick, it should cut fairly quick though, even considering the material. Also, the .0005" total tolerance shouldn't pose much of an issue. I would think that I could hold that number consistently with 1 roughing pass and 3 skim passes. I can generally stay within a .0003" window safely with 4 skims up to about an inch thick, once the machine's parameters are dialed in.

Yes the rules change daily around here. today I am designing an e-stop system for some custom shop equipment. yesterday I worked on 7" dia transition rings in a 120 ton extrusion press. last week I had to make the die for extruding 3mm OD 1mm ID tubes for the semi semiconductor industry. the week before that a new bearing design for a galvanizing rig with over 10000 lbs line tension in Germany. It keeps me busy and always learning new things

What no one has commented on here is you're asking questions, rather than simply letting ego get in the way of a machinable cost effective part.

Increasingly rare to encounter that attitude any more.

Lee (the saw guy)

"The only thing I know is that I know nothing." -Socrates

Some of us young-ens did pay attention in school.

Hi tjd10684:
On your carbide parts, are the slots open at one end or closed at both ends.
The reason I ask, is that if they're blind at both ends, the start hole will be harder to do than the slot.
0.003" diameter holes in carbide 0.100" thick is a major challenge, and as others have commented, running 0.001" or 0.002" wire is not for any but those who have special machines with generators and threaders set up for these ultra fine wires.
T'will be fun; let us know if you can find anyone willing to take it on.
The Metal Doctor who moderates this forum has fine wire capability; maybe he can help you.
Cheers

Marcus
Implant Mechanix – Design & Innovation - home
Vancouver Wire EDM -- Wire EDM Machining

In regards to the slotted parts, if they do not require a start hole, is there any chance you need to have smaller slots? I need to check, but I think we can produce a slot width of .0012 in .1000 thick EDM grade carbide. Perhaps we may be able to go smaller. Toolmaker35 hit the mark when he said, "When getting down to wire diameters such as this, the machine really needs to be specifically set up for fine wire...".

their are some other larger holes that the slots pass thru so I don't think starting would be a problem. actually the "blank" already have a .050 hole in the middle to start from. I would really like you show you all exactly what I am talking about but it is proprietary information that I cant release on a public forum. Like I mentioned earlier we have a supplier that evidently is set up to do this work since he does it for us a couple times a month. I am just trying to wrap my head around the correlation between what I design and what the guys on the floor have to do to make it. Thanks for teaching me about this stuff I really appreciate it.

At this point in design, I'd say the smaller the feature you require, the higher the cost. This will not be a linear relation, as far as feature size and cost of production are concerned. If feasible for your design, and part function, keep your slot sizes approximately 30% over nominal wire electrode sizes. This will allow the wire EDM operation on your parts more than sufficient stock to achieve accuracy and surface finish, as well as achieving the internal radial features that you desire.

I forgot to mention the obvious, so I'll do that now. As far as the design process goes, is it possible to specify the widest, most generous tolerance right on the print/CAD. If there is wiggle room in the design of the part, then wiggle away. Every tenth, thousandth and fraction contributes to cost and manufacturability options. It will save your company money. Then you can do the work in house, if able, or shop it around. Many manufacturers, machinist, skilled tradesmen, etc., have seen and produced many parts with lots of cost, but no additional value added, because that is what their customer asked for. If you want to control costs, control design intent by using the most open tolerance on features that will still allow a functional workpiece.

Many here might not know this, and I don't like being the bearer of this news -- but The Metal Doctor passed away many months ago... he was the victim in an extremely unfortunate accident involving a drunk driver.

The Metal Doctor is who got me started in the wire business, and who mentored and coached me through those difficult times we have all experienced at the beginning of our wire careers. He was a great friend.

Originally Posted by EDM JOE

I forgot to mention the obvious, so I'll do that now. As far as the design process goes, is it possible to specify the widest, most generous tolerance right on the print/CAD. If there is wiggle room in the design of the part, then wiggle away. Every tenth, thousandth and fraction contributes to cost and manufacturability options. It will save your company money. Then you can do the work in house, if able, or shop it around. Many manufacturers, machinist, skilled tradesmen, etc., have seen and produced many parts with lots of cost, but no additional value added, because that is what their customer asked for. If you want to control costs, control design intent by using the most open tolerance on features that will still allow a functional workpiece.

Dont worry I don't just design I cut metal too (just not with edm typically) I know what your saying. I have fought engineers quite a few times for an extra 0.005 here and there heck if I can get away with it I try to give the guys 1/32"+. I always try to design with function in mind and only tolerance to within the function of the part in the assembly. The engineers say I give them what the need not necessarily what they ask for .

tjd,

I wrote this up quite some time back, but might be something of use as well for you: Muller Machine

PM

Hi Precisionmetal:
I'm greatly saddened to hear that TMD has passed on under such awful circumstances; I had no idea.
Tough for you as well to lose a good friend; my sincere condolences.

On another note, I read with interest, your website content, and I'm struck by what we've lost with your retirement from the wire EDM world.
What are you up to now that the machine is out of your life?
Do you miss it? Do you have any plans to get back into it?
I can't imagine trying to run my shop without a wire in it; it's such a useful piece of kit.
In any event, I'm certainly glad you're still willing to participate here and pass on the benefit of your expertise.
Cheers

Marcus
Implant Mechanix – Design & Innovation - home
Vancouver Wire EDM -- Wire EDM Machining

Marcus,

Thank you -- appreciate your words.

I am now employed at Apple. I took a very interesting position in the manufacturing side of things.

I will still be involved with wire edm here, even though that's not specifically my position.

I still need to do a bit of updating to my web site -- it will no longer be for my "business", but I plan to add a lot more information simply because I know how difficult it was for me to get started in the business. Maybe I can help some new people have a bit easier time of it.

PM