Shear Cutting Titanium Wire -force Req? Blade Material?

Here is a possible start.

OK, thanks Tornado. I found from the chart that:

1. By adding to the published 900MPa max tensile strength (for Ti 6Al/4V) a 20% overkill factor and then multiplying by the 80% of tensile strength for shear strength noted on page 4, I am looking at 864MPA for my shear cut, which converts to 125,000 psi. That doesn't help me convert to pounds of force, because I don't know how to calculate the square inches of the cut on a round pin shear. (by the way, I have examined some wire cuts on Titanium that I want to duplicate, and have decided that the cutting blades are not straight, but curved around the radius of the wire.) So how do I begin to calculate the force in pounds needed to cut this part? From the marks on the parts I examined, it looks as though the blade penetrates from 20-50% into the cross section before the part yields. If I were to guess by looking at the marks on the samples, the most malleable samples show perhaps at most 0.08 square inches of marked contact area. (Calculates to 10,000psi.) I would hate to use this examination for a guesstimate to calculate the force to make my cutting machine. Can anyone help me with this question?

2. I should expect to use hardened High Speed Steel for my cutting blade material. (Does this only apply to tool bits used on a lathe; is there a differnt material/spec for shear-cut blades like "tool steel" or ...?)

3. I know some heat treating experts that can answer the question about heat treatment for me.

4. From my examination of prior art cuts, it looks as though the blades may not be a perpendicular cut. I am guessing that an angle of incidence would allow for concentrating the area of yield close to the edge of the cut, so the blade would have more penetration along the cut line before the part is forced to yield completely. Also, it seems that I should probably make a cut with an angled-incident blade. How do I select an optimum angle? I would guess a 60 degree angle of incidence...?

5. Since the High Speed Steels are used for continuous duty lathes (with coolant), can I safely assume that the heat build-up will not be a problem, since my cut period translates to only about a 5% duty cycle? (I hesitate here, because I believe when I whack my wire to cut it, surely my impulse force far exceeds the constant forces on the tools on the lathe!) I just don't know enough to answer these questions, and don't want to build multiple prototypes to get it right, or to build too much overkill (expensive).

I'm just stabbing in the dark, guys... I'd greatly appreciate your interest in helping; the the brain trust here at CR4 can be amazing!

(Thanks for reading such a long post.)

hi WhiteHorse,

I'm just joining in here to see what I can learn. Since I have only worked with hand tools, I am interested in your design process for a cutting machine.

I do have one humble suggestion, with respect to the cuts you wanted to duplicate. Take samples of your materials to a hardware store that carries a full line of shears, and make a series of test cuts with the different models to see which best approximates your intended cut.

Hi artsmith,

Thanks for joining in.

When I have watched people use hand tools to cut these Titanium wires, I have seen them struggle with all their weight and with jerking pulls on cutters that are mounted on a firm stand or bench with 36-48" long handles -just to get the cut made. The cutters have a relatively blunt wedge-shaped blade set, much like common bolt cutters with 24-36" handles, only more beefy.

If the cutters had knife edge blades and were doing a shear cut, I would simply estimate or measure the real-world force required with the existing 'hand tool', and calculate the blade force from the mechanics of the lever arm. I have not seen any 'hand tool' that uses shear cut blades that will cut this size of titanium wire.

When I searched for hand-held hydraulic shear-cut wire cutters, I could not get any specifications as to what strength they had, nor what capacity of Titanium wire they would reliably cut. I have not seen a hardware store that had even one shear-cut wire cutter that I thought would cut 0.2"OD Titanium wire without damage.

I want to cut a minimum of 100,000 pieces per production run, and then I'm OK with changing blades for the next run. If the blades last reliably for the whole 8-30 hours it will take to make the cuts (cycle time is adjustable from 1.0-0.3 cuts/second), much time will be saved babysitting the quality of the production run.

If I could be sure of the cutting edge angle of incidence and profile (sharpness) that would give me the best blade life and a decent shear-cut, I will have the blades made, then take them to a lab for testing the force required using a compression tester.

Any tool & Die makers on CR4? Maybe one of you can help here.

Thanks for your help...

Tough stuff, eh.

There's relevant and specific information about cutting titanium, tool wear, heat issues, etc. starting page 9 of this pdf.

These folks advise that tungsten carbide C1-C4 or cobalt type tools are best used for cutting titanium wire. They supply cold shearing among other services, so may be a good source of advice about your angle of incidence/profile questions. Or these, or these.

Thanks again, artsmith, I'll look at those links momentarily.

Meanwhile, I have a concern about making the blades. When machining HSS before it is heat-treated, (I belive) it is relatively easy to machine/surface grind. I am a bit worried that drilling & grinding on the tungsten-carbide C1-C4 or cobalt tool material (doesn't need heat-treating) might be a problem. Am I off on this?

Thanks

Carbide is what I would use. No heat treating required. Long life assured if of suitable size and not abused.

You did say that you don't want to contaminate with coolant so HSS is not really an option.

Carbide blades can run red hot, limited by the method of fixing to the tool holder only. Like, the brazing may/will melt...

If you are whacking the cut then you aren't shearing you are....doing something other??

As for calculating the required forces, I would exploit a student for that task.

Hi Wal,

Are you using these tools in high speed (lathe) applications, where the blade is constantly cutting? If so, I can see the importance of applying lubricant on the blade during cutting. The actual force of the cut in my application will be only at about a 5-10% duty cycle, I estimate. I do hope that whatever type of blade material I use, that will be plenty of time for the heat to dissapate from the cutting edge. I can't imagine my blades (2.5" x 2.5" x 3/8") will get very hot, especially since they will be bolted into some pretty hefty mandrels.

As for the 'whacking' that I describe, I say it that way to emphasize that my shear cut is done quickly. The overall cycle time is .25 to 1.0 sec. and the actual time to cut the part to yield will be about 5-10% of that time -thus: a "whack".

I used to review the first few designs that EE types made for my employer, when they were 'green', fresh out of school. I'd rather get something more practical, thanks.

Thanks for your help.

Hi again, Wal,

I should emphasize that my blades will do a shear cut. In fact, the plan is to drill a hole into the lower blade to allow the wire to pass through to a stop, setting the wire cut-length, and have the upper blade cut with a half-round cutting edge at an angle of incidence of 60? degrees. How difficult is this type of machining for the tungsten carbide, etc. materials that don't need heat-treating?

Thanks

Hello Whitehorse

I would imagine the cutting parts be made from two fairly hefty pieces of carbide where a cylindrical hole has been made to line up when used, pretty close to the OD of your wire. Their matching faces sliding on each other and your wire getting through both holes to the stop between each cycle. They should be clamped into a strong stell structure that allows for precission assembly. The moving part operated by a cam and the cooling, lubrication and removal of dust and debris from the mechanism should be done with high pressure compressed air.

Using cylindrical holes of close to the wire diameter will serve to keep the wire straight and minimise the lateral forces during the cut and reduce deformation at the cut ends.

Grinding is the only method I know of that can be used to machine the carbide

Hi Far,

Thanks for the input.

You have described fairly well the shearing blade formation and assembly that I am now imagining. Grinding 0.1360" holes in beefy carbide blade blanks seems unlikely, perhaps EDM...

I found from artsmith's suggested starting place a comment on page 3:

" In general, hard steel tools are used (for cutting/shearing large quantities of titanium by machines with sufficient rigidity and high power capacity) and high-speed carbide tool are used (for cutting/shearing small quantities of titanium by machine with low power capacity."

I will certainly be cutting large quantities of titanium (>1M/year), and will design beefy mandrels and power engine to drive the blades with plenty of force. Yet everyone seems to be pushing for a cutting blade material that is hard to machine. What am I missing?

As for cam drive... I would like to consider the cam drive idea carefully because I can control the loading rate on the part to take better care of the blades. I want the cam to have a long life, and for the number of cycles and forces required for this machine, I am wondering: would a roller bearing be a good idea, or a bad idea to ride on the cam surface? What do you think?

Thanks so much!

Why am I hung up on carbide cutting tools? Personal preference that's evolved from its positive attributes. Long cutting edge life and dry cutting capability mostly.

The non HSS cutting bits referred to in that reference were described as being coated not solid.

The high wear rate means that a coated tool would not be coated for long. I believe that this is an economic consideration. That's what I think, I could be wrong. Anyway you will be using solid carbide, yes?

Carbide is not hard to grind. Diamond drill bits are available in sizes close to what you need. You can always use abrasive water jet drilling. Does EDM work on ceramics?

I reckon a roller cam bearing is a good idea. Easy to maintain and very efficient.

Hi again Whitehorse

you will certainly be using the finished machine a lot, so you can get the rough carbide bits made close to your finished specs so they only need fine grinding at your shop, or possibly better still, get them manufactured to your specs.

the important part will be to make the bits large enough to ensure the mandrel parts do not wear out too fast. You could even have a second mandrel holding the mandrel that holds the carbide bit to facilitate faster maintenance when they become too sloppy.

As for the use of a roller bearing on the cam surface.....hmmm, I think it is probably not the right idea with the hard bumps and high speed and all the damage a broken roller can cause. How about having the camshaft below the cutter and the cam running in an oilbath sealed off such that the oil can not splash up into the cutter. The cam should be of large diameter and the camshaft run in large diameter solid bearings (whitemetal or bronze) with pressure lubrication. a cam follower should be between the cam and the operating rod.

I also had another idea when thinking about your problem: how about using a beefy solenoid ( maybe watercooled) to do the cutting and the trigger for each cycle being the endstop for the wire when in the right position. This setup will ensure no miscuts and would ensure the maximum safe rate of production - perhaps even faster than your target.

sorry I took so long to answer, I don't have much time.