Advice on V shaped inserts please?

T

tmenyc

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I'm exploring the use of rhombic shaped inserts for use in tight spaces with softer materials, particularly delrin, acrylic resins, ebonite but possibly also aluminum. The real issue here is tight spaces... I already have the Shars extended live center and an Aloris extended holder, but my work is small and getting smaller as I get more skilled, and I don't always have the luxury of cutting a relief groove to get started. Further, my experience with carbide inserts to date has all been with CCMT and CCGT, so I'm pretty comfortable with 7 deg relief, have not used any negative or zero relief inserts. Thus, I'm thinking of trying VCMT, VBGT, or VBMT.
Further, what would be the correct holder for one of these? The Black Book (3rd edition p. 199) has the SV series, and from the directional arrows it appears that SVVC might give me the most flexibility. Have I thought this through correctly?
Finally, assuming for the moment that this logic is OK, I'd love to hear your experience in purchasing this holder and inserts, which I realize will probably not be cheap since it's not seen all the time.

Alternately, and to anticipate Mike's inevitable response, since I am an adherent to his HSS method and have successful made my own square, threading, and zero rake tools, I'd love to hear if the long tight angle is workable in a grinder. I'm certainly willing to go that route. Would a longer threading tool shape work?

For context, although I'm teaching myself the basic skills and practicing mostly on 12L14, moving progressively smaller, my real use will be fabricating replacement parts for vintage fountain pens, which generally run in the size range of OD .200"-.250" and are often as thin as I can get them. Tolerances are pretty big, though, in the .0010-.020 range since the material expands and shrinks a bit and the old parts they fit on or into is often a bit wonky. So I'm looking at a lot of close-to-chuck work with short lengths.
My lathe is a Logan 820 with AXA tooling.

Many thanks!
Tim

PS: I just noticed that LMS has an AR Warner kit, Warner's kit #16, with VCMW and VCMT 7 deg relief inserts and appropriate L and R bars that aren't specified by its name. The set is $139 and individual inserts are $10, which would certainly be affordable. Would that work?
 
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I've used the Shars SVVBN toolholder with VBMT and VCMT inserts. It is not that expensive, 28 bucks for the insert and 5 to 7 bucks an insert. The SVVBN is pretty good for things like turning to a shoulder, so it is useful for tight spaces. I have not tried to get a fine finish with it; I suspect it is not rigid enough, but perhaps with the right insert and a light cut it would do well.

Regarding HSS, you can certainly grind a toolbit down to a narrow cutting face. Might save some time to just use a narrower bit (e.g. 1/4"). If rigidity seems to be an issue (and when isn't it?), you can use a rectangular toolbit (e.g. HHIP 2000-0203 1/4"x3/8"x3"), such that it is taller than it is wide. Grind one end |\ for right-handed turning and the other end /| for left-handed; you should be able to get it narrow enough to do shoulder turning and facing.

In terms of the materials you are working on, you might want to stick with HSS over carbide. With plastics, woods, etc, sharpness is going to matter more than toughness, and sharpness is the strong suit of HSS while toughness is the strong suit of carbide. You could even use carbon steel for these materials, provided you could find the (hardened) tool blanks. Another note for these materials is the grind angle: wood tools are often ground with a long shallow bevel for the clearance (15-30 deg), then a short sharp bevel at the edge (30-45 deg). These might cut better than the comparatively blunt angles needed for metal.

Before the fellas yank my membership, I hasten to point out that while I did discourage you from buying a tool, I suggested some tool blanks to buy, and that's gotta count for something.
 
I have to agree with Woodsman. HSS ground for soft materials seems like a better solution. Wood likes a shear cut. I doubt that stick out will be a problem even for 1/4" tooling.
 
Always happy to not buy... this sounds like a good solution! I'll give it thought.
Thanks!
Tim
 
Shape a HSS blank figuring what you know from the tutorial, and try to shape what you think will work. Give it a try, and see, then post up for fine tuning advise. Diamond card sharpen the bit too. Good luck
 
Based on what you are looking at cutting, I would opt for HSS, Tim. Delrin, acrylic and aluminum all cut well with HSS. I haven't cut Ebonite and don't know how well rubber cuts with HSS.

As you recall, the side and edge cutting edge angles define the shape of the tool and you can make it any shape you like, from any size blank you like. What really matters is the relief and rake angles and those will make a difference.

As you know, plastics tend to melt as heat builds so you want to keep cutting temps as low as possible, while also balancing the need for enough speed to give you a decent finish. The best way to do this is to keep the relief angles on the higher side, in the 12-18 degree range, while also increasing side rake into the 15 degree range. This will give you a very sharp edge that will reduce cutting forces and cutting temperatures. If you also use back rake in the 18-20 degree range, this will increase chip flow while also thinning the chip to lower cutting temperatures even more. What's more, the effect of back rake will be additive to the effect of side rake and this tool will cut plastics efficiently. The added back rake will also work well with aluminum. In fact, if you increase back rake into the 35-40 degree range it will cut aluminum extremely well while also working well with plastics.

Again, the goal is to keep temps low. To do this, keep your speeds in the low to moderate range and increase feed. With the tip geometry done well, you will find working with plastics simple. For aluminum, keep your speeds high and feed for the desired finish. If it strings, increase depth of cut and boost feed.
 
Mike, Aukai, Thin,
Thanks; this is my direction, for sure. I've also been talking to a pen maker in France who uses a deep rhomboid carbide tool, and he said that heat is his biggest enemy and if he knew how to do it with HSS he would. A pen maker in Arkansas, a super craftsman, told me he never could figure out how to grind a tool himself and would love one if I made it for him. So, onward!...
Tim
 
Tim, I hate to say this on this forum but, all the stuff about having to have such and such an angle for this and that is fine if you are a perfectionist. But as often as not sort-a-close is close enough. To get started just grind one that has (10+- degrees will work) relief on any side touching the work. A shape that works and a slight (1/32"+-) nose radius. If you need a square shoulder with no inside corner radius, do that on the left side of the tool (I'm assuming that is the side that needs the square shoulder) I'd actually do a bit of a back cut so you square the edge when you withdraw the tool. Grind the nose radius only on the other half of the nose. That will give you a tool that can produce a smooth surface going one direction and a square shoulder the other.

For a trail run I wouldn't hone it. After you like the way it works then hone but be careful to not ease the edge and loose your relief. I would just leave the top of the tool flat. Later if you feel you need some positive rake you can do it. Hand held grinding on a bench grinder is close enough for a start. just do it! It's not rocket science. Run the tool, see how it works, modify it if needed. There is a leaning curve but practice fixes that.
 
Sorry about the double post.
 
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Larry,
Thanks, appreciated. I've learned a tremendous amount from Mikey and the others here over the past couple of years, and particularly when some precision matters. Yes, getting the range is important, and for particular reasons, which I am continuing to learn. My grinding results are never exactly on those specific marks anyway, but they're close, as intended.
Tim
 
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