I Need help with carbide insert application design

Take a look:
http://www.metalworking.com/DropBox/wire_cutter.jpg
This rough drawing shows a cutter to cut 3" pieces of flat steel wire, 3 at
a time, every 0.5 seconds. The wire is advanced through the block between the two shown 3/4" x 3/4" x 1/4" carbide inserts. Notice the notch in the lower insert. The arm pivots on a 3/4" shaft in 2 Timken bearings in the bored hole in the block nearest the step. The other bored hole has bearings for the 1" shaft that has a cam follower with a 5/16" offset to act as an actuator cam -not shown. The cutter arm also has a 3/4" x 3/4" x 1/4" carbide insert. As the cam makes the arm pivot the wire is cut and a spring returns it to the open position.
The part I need thoughts on are how to pre-load the carbides in such a way that it is easy to unload to change the carbides...(notice I get 8 cutting edges on each insert!) The lower carbide should last almost forever as it is not used in cutting and has the guide notch ground into it. I wonder how much pre-load is needed on the carbides, they will be sliding on each other's faces. One thought is to have the carbides backed up with a block of steel that slides in a cut-out in the main block but can be clamped tightly and have a differential screw apply the loading pressure then clamp the sliding block. The goal is to be able to change the carbides in a few minutes and have a positive, easily adjustable and reproducible pre-load. Oh, by the way, the assembly is not extremely accessible. I figure if the carbides are real easy to change, they will last a long time and if it takes an hour, they will need to be changed often...Right?
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Tom,
Neat gadget. Have you already decided on the inserts, or are you open to suggestions? You can buy inserts with holes for a securing screw. The screws aren't cheap, but they're not crazy and that would be a quick way to index them.
It would probably be a good idea to have the inserts nested in the block. When we build our dies with steel punches and trim steels, we go to great lengths to have solid steel/cast iron support (shoulders or keys) in every steel in order to keep them from moving. Carbide would be significantly more susceptable to failure due to a sloppy fit.
I would not have the carbide inserts sliding against each other, if you can avoid it. This rubbing will cause premature failure, as well as excessive stress on the arm and base block. While rubbing will give you the advantage of nearly zero burr, your insert life will be shorter.
I'm not sure what an ideal clearance between your inserts would be. Typically, 10% of material thickness is used in many punching operations, but you may find that this much clearance will create a massive burr, deform the end of your wire, and smear metal on your inserts (causing premature failure and nasty cuts). Ideally, you could make some shims and try out different clearances.
As your clearance increases, the required force should decrease, which should increase the lifetime of the wear components within your tool (bearings, inserts, wear surface on arm, cam, motor, etc.)
Finally, I would take care to make sure there is no slop in your tool at all. Again, if things don't fit correctly this will cause premature failure (which can be fun when it's burried inside some monster machine).
Just my $0.02 worth.
Regards,
Robin
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Screws would mean complete dissassembly to change.

I have the inserts shown floating out of their pockets for clairity, imagine they are seated and clamped.

I have a similar cutter with the wire passing through a hole in in 1/2" round carbides. They have to be but-tight to each other or the hard steel wire forces them apart and destroys the carbides and leaves a bad burr.

All the wear should be on the carbides. My target is anything better than 125,000 cuts or a week.

Just the flex in the arm is enough to demand a pre-load.

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I can see the upper insert being clamped with some type of over-arm, but it looks like there's no room for any type of clamping system which sits proud of the lower insert.
I'll assume there are other components which prevent the arm from swiveling to a vertical position (which could clear both inserts)?

I understand what you drew, but I thought I should emphasize the importance of a good fit.

I stand corrected. In sheet metal work, trim steels never touch because of the increased tonnage and reduced life of the cutting edge.

In my experience (heavy stamping dies), anything which moves will wear. While the inserts will no doubt show excessive wear, having a sloppy fit or soft bearing surfaces (cam and lever arm) will probably cause premature failure and rework. Of course, I've never worked with wire...
Regards,
Robin
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I haven't done any virtual cuts yet but imaging an end cap, like on the arm, clamping the inserts into place. I'm thinking of a sliding piece that backs-up the carbides then clamp the piss out of them.

But of course! The block is kinda' buried and the block is bolted onto a flat apron from underneath.

Oh yea! On the other cutter with the round inserts I had to make hardened pockets for the carbides. The first iteration was cold-roll and deformed like wet clay in a few months.

I'll use a McGill follower for the activator and ride it on a Stelite surface. Nothing lasts forever so I want the wear parts to be off the shelf rather than make them.

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Tom Gardner wrote:

Any chance of brazing the cutter to a holder that has a recess that will support the carbide?
John
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Can you preload via pneumatics or a mechanical spring (say belville springs) with an adjustable solid stop on the backside to prevent over stressing the components?
--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
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Good thought! Aren't Belvill's cool?

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I use them in a lot of fixture and automation designs. Very versatile if you keep the design constraints of belvilles in mind.
--
Anthony

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On Thu, 14 Jul 2005 18:50:40 +0000, Tom Gardner wrote:

Tom, this might be a crazy idea, but I wonder if it'd be too complicated to incorporate a mechanism to rotate the cutter 90 degrees on each return stroke, thereby presenting a "new" cutting edge to the work on each cutting stroke. This would make the cutter last 4 times as long between changes, requiring the cutter only to be flipped over when dulled. I'm imagining some sort of a ratchet/indexing mechanism.
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snipped-for-privacy@sheayright.com says...

With the right details it's possible to do this passively. The rotating tools need to have a circular form and the force that drives the rotation is usually a result of placing the cut point somewhat off center. I've built cutters for tungsten wire (tungsten is horribly abrasive) that operate up to 1800 pieces per minute with auto- rotating tools.
Ned Simmons
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On Fri, 15 Jul 2005 12:52:32 -0400, Ned Simmons wrote:

Yes, that's much better for a number of reasons. When I was writing up my description I was thinking what a waste it was to not be able to use up all of the cutting edge on each face of the square cutter. With a passively rotating circular cutter, much more of the total cutting surface can be exploited. I would think that another advantage to an offset circular cutter would be in its cutting action; more of a slicing action than a chopping action (think of the angle on a guillotine blade), potentially reducing burrs and reducing the power needed to make the cut (and that saved power can be translated into increased speed).
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snipped-for-privacy@sheayright.com says...

Exactly, but you have to be careful how far off center you go or you can have problems with the wire slipping out of the desired nip point as the shearing angle between the blades gets too large.
Ned Simmons
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Good thinking but it would be difficult to get the rigidity. I'd be happy to just flip the carbides once a week in 5 minutes or so.
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Tom Gardner wrote:

You also wrote "The block is kinda' buried and the block is bolted onto a flat apron from underneath" but if perchance you have access to the back side adjacent to the white end (of the upper-right block in diagram), maybe the following would work: Instead of just a recess, cut all the way through; braze the square carbide inserts on to a length of square key stock. Use clamp bolts through the block top like on a lathe tool holder. To change cutting edge, loosen bolts, slide stock out the back, rotate 90, slide back in, tighten bolts.
Alternately, leave the recess as shown; cut the block between the insert end and the bushing bore; bolt that part to the apron from above rather than underneath; to change the inserts, unbolt the block part, pull it out, change the inserts, and bolt back in.
Re the moving arm, preload, and clearance -- if you tilt the axle forward 1 degree and mount the guide carbide at matching angle, you could have 0 clearance when the moving carbide hits the wire, and a few thousandths of clearance at end of cut. Or negative clearance at start of cut, and 0 clearance at end.
You wrote somewhere that cutters "have to be but-tight to each other or the hard steel wire forces them apart and destroys the carbides and leaves a bad burr." Would it decrease wear if the center of the carbide face were relieved? Also, how much difference in cutter life and burring does rake angle make?
-jiw
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A burr causes problems after the cut. The wire then gets bent 180 deg., like a Bobby pin, then gets a 1/2" long 16 ga. staple and jammed into a hole in a block of wood. 98 of these tufts make a grill cleaning brush. A burr will cause the cut wire to drag a little on one side or the other and the bend won't be in the center.
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