parting off probelm. Last tiny bit.
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  1. #1
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    Default parting off probelm. Last tiny bit.

    I was using an aloris cutoff tool holder with a quality tapered cutoff blade. I have ground a slight concavity on the top to curl the chip. It works well except for the last 1/4 inch or so. It stoped cutting and forces the entire WCTP to pivot.
    This was hand feeding and it felt like I had jammed the tool block into the work. It pivoted the setup so the blade was pointed to the headstock enough to be noticeable. Then I guess it was just dragging andburnishing the left side of the groove. I even widened the groove to two blade widths for clearance and got the same thing just a bigger angle before it jammed.
    I did not want to tighten the toolblock nut anymore then I had. it was a lot tighter then i normally do it up.
    I know parting off can be difficult just wondering what folks recommend. I finished with a hacksaw in the lathe. This was toolsteel cutting with HSS. The chips rolled up nicely and were not jamming. It was cutting nice for the first 3/4 of the cut
    Bil lD

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    It sounds like your tool is not on center.

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    That sound as if the tool is slightly too far above centre height.

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    And not exactly perpendicular to the spindle axis. This matters a lot with a solid steel cutoff blade like the original Aloris BXA7. The newer units may come with a carbide insert into a narrower steel blade; this is less sensitive to lack of perpendicularity.

    Universal Parting Blade Holder BXA-7

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    When this happens, at this diameter, the RPM is way too low, stop and increase RPM.

    Jeff

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    Parting-off has always been a right bitch on "solids", easy-peasy on thinwall features or where a solid has been drilled or bored so there isn't much depth to plunge until "done".

    Joe's mention of bespoke carbides helps a LOT vs any HSS-fu. Prolly an order of magnitude better than the "old skewl" funny-grinds on HSS plus sacrificing goats under a full moon and such.

    "thick enough, but only just barely" re-purposed power HS blade salvage my best-ever run of general-purpose good luck.

    Better solutions come with one of two approaches Hardinge popularized for fast, repeatable, reliable high-volume working:

    - a bespoke swing plate affixed to the HS for nought BUT parting-off.

    - a "back" tooling block, stiffer than the average QCTP, too - with tool-tip downward for "normal" direction of rotation, ELSE tip upward for such lathes as are easily reversed.

    Not too proud to admit that onesies and fewsies, mostly I cannot be bothered, just plunge as deep as is safe, (stiffer 4-Way, not fancy QCTP) finish with a hand-held hacksaw, spindle turning. Clean-up the stock with a facing cut, do a second-op on the parted-off item, but only if either/both are justified.

    Ugly? Time budget and Bank account disagree, more off the back of f**king-up less work, tooling or crashes avoided than any gain on throughput.

    "Your Mileage May Vary". It is SUPPOSED TO DO, after all. Not everyone is doing the same item and alloy on the same lathe.

    So DO take the time to find out what works best for YOU, your machine, any of many specific taskings or alloys you work on it. More wrong solutions than right ones, but there IS NO "one true religion" in parting-off - not even live-tooled sawing.

    Serious-volume, enough money involved? That second-op happens on some high-end CNC critters with a second powered spindle grasping the part, cleaning up the teat or fin, even adding further features, the whole deal hard to even keep up with, human reaction times as they are.

    That one IS "close" to perfect, though. IF, repeat IF, the economics are aligned to fund the stiff investment.

    2CW

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    Quote Originally Posted by thermite View Post
    - a "back" tooling block, stiffer than the average QCTP, too - with tool-tip downward for "normal" direction of rotation, ELSE tip upward for such lathes as are easily reversed.
    I do this a lot, using a Dorian holder that allows the insert blade to be installed upside down, and the lathe running in reverse. It works very well, but does require flood cooling - brush-on just doesn't get the coolant where needed.

    The Dorian toolholder is boldly marked "R" with a yellow paint pen - running the lathe forward invariably destroys the insert.

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    Quote Originally Posted by Joe Gwinn View Post
    I do this a lot, using a Dorian holder that allows the insert blade to be installed upside down, and the lathe running in reverse. It works very well, but does require flood cooling - brush-on just doesn't get the coolant where needed.

    The Dorian toolholder is boldly marked "R" with a yellow paint pen - running the lathe forward invariably destroys the insert.
    Prolly a mis-print, but an upside-down back-tool is used in FWD, may need to cope with potential for carriage or cross LIFT on light lathes with significant wear.

    Only a tip UPWARD "back" tool orientation wants spindle in reverse, but at least avoids the lifting force on the topslide of the cross.

    I'm - perhaps oddly - one for going "dry" on parting. That off the back of tool-tip and top grinds that "fold" the chip clear of rubbing the walls and BREAK chip to reduce wanting to wedge.

    Get that element working well, short curls of dry chip are flung out, clear, no build-up. Challenge as one approaches center is the usual one of geometry. SFM drops dramatically, pressure goes up, and "too fast".

    See also those bespoke Carbides. Good stuff indeed. VERY. And even in an otherwise all-HSS shop.


    4CW

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    He's using it on the front side, not the back; which requires "R." This orientation is very useful in chatter prone cuts.

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    It is not that far off center to leave a 1/4 Diameter uncutable portion. If it was like 1/8 inch I could understand it. I know that center height has to be adjusted as blade is extended out for deeper cuts etc.
    I agree it is the tool post is not perpendicular but it is walking away from perpendicular for some reason.

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    Quote Originally Posted by eKretz View Post
    He's using it on the front side, not the back; which requires "R." This orientation is very useful in chatter prone cuts.
    That's it exactly.

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    Quote Originally Posted by Bill D View Post
    It is not that far off center to leave a 1/4 Diameter uncutable portion. If it was like 1/8 inch I could understand it. I know that center height has to be adjusted as blade is extended out for deeper cuts etc.
    I agree it is the tool post is not perpendicular but it is walking away from perpendicular for some reason.
    S-B's HTRAL pamphlet even showed a diagram with a slight up-angle. But that was 1959, using a "lantern" toolpost, and on spaghetti-bed hobby lathes.

    What works better is to provide support on a line dropped VERTICALLY from right under the tool-tip clear down to "solid ground" - EG: the topslide.

    Tool has to extend far enough out to reach the center before the holder hits the OD, surely.

    But have a look. There is a gap UNDER the tool. Clear. Straight down. Can't AVOID having that clear space. You've just cut it.

    See to it there is metal in that slot, under the tool, deep. All the way deep if need be. Much as you can get instead of ignorant AIR?

    "Chatter" is moved off the lathe and back onto the iPad, earbuds, and internet forums.


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    If you ground a "hook" on top of the cutoff blade to get the chips to curl, that is a no-no. Any grinding on top of the blade will narrow the width of the blade to where it will bind and could stop cutting. The only grinding you can get by with on top of that blade is a slight flat to remove the "crown" off of the blade. But you have to be careful not to take too much or you will wind up with a narrow tip than the rest of the blade.
    I would recommend you get a "T" style blade to use with your tool holder, assuming it is a No. 7 Aloris holder. It's made to have the proper back rake to make "curly" chips. Ken

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    Quote Originally Posted by Bill D View Post
    I was using an aloris cutoff tool holder with a quality tapered cutoff blade. I have ground a slight concavity on the top to curl the chip. It works well except for the last 1/4 inch or so. It stoped cutting and forces the entire WCTP to pivot.
    This was hand feeding and it felt like I had jammed the tool block into the work. It pivoted the setup so the blade was pointed to the headstock enough to be noticeable. Then I guess it was just dragging andburnishing the left side of the groove. I even widened the groove to two blade widths for clearance and got the same thing just a bigger angle before it jammed.
    I did not want to tighten the toolblock nut anymore then I had. it was a lot tighter then i normally do it up.
    I know parting off can be difficult just wondering what folks recommend. I finished with a hacksaw in the lathe. This was toolsteel cutting with HSS. The chips rolled up nicely and were not jamming. It was cutting nice for the first 3/4 of the cut
    Bil lD
    What make/model/size lathe ?
    What material ?
    What dia. ?
    What RPM ?
    What Loob ?

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    Quote Originally Posted by 4GSR View Post
    I would recommend you get a "T" style blade..
    "Tee" or heavy top-rib blank is better than taper for-sure, and as tall as will fit. One has to modify one side of the 4-Way to hold the oversize.

    Much, repeat MUCH depends on the alloy, but:

    The top-grind that works for me with the least effort, fastest prep is simply a mild central groove. "In theory" that puts the centerpoint of the tip plunging into the work incorrectly below centre.

    In practice it acts almost is if it were TWO tools, but close enough together the chip does not split but curls, rather and clears both sides.

    Another version of grind, more tedious to prep, the face is stepped - two or more steps, all square, not angled - so one side cuts "first". The chips generated also clear as they go rather than rub or jam.

    "Fishtail" grind, the inverted vee's "tail points" drive into the cut, also works to shape the chip, but needs regrind sooner than squared steps need.

    Thereafter, it is "which alloy today?" that makes the choice.

    Helps to have a 4-Way or dedicated block monolith - compound is often off the lathe, anyway - with all mounting options stiff as a piss-hard-on out of Hell.

    If all you have is fettuccine-noodle holders off a QCTP swivel-chair atop a weak and shakey compound?

    Just use the Carbide insert system the most grown ups are bragging about, any given year, and yer as well-served as it is likely to get 'til competition delivers an even better one a year or three in future.

    Mind - I still detest parting-off solids by enough I quit modifying power-hacksaw blades and bought a decent power-hacksaw to use them "directly" so as to less often NEED to do. My spindle-bores wont pass but rod and rather small OD bar in any case.


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    I must have a horseshoe up my butt because I never have a problem with cutoffs. I get the tool reasonably square with the work, make sure the height is right so I wont have to deal with a tit on the end and I always use power crossfeed. Using hand feed for me is just not smooth enough to keep a good curl going and jams more.

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    Harrison m300 lathe about 200 rpm( too slow) It is a axial groove ground down the center so the blade width is the same. I guess it would lower the cutting height in the center section but I did not extend the groove clear to the edge. Stopping the grove a fraction behind the cutting edge. I used a chainsaw grinding stone on a dremel to grind. This is a tapered blade similar to a tee blade.

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    If you haven't yet, verify that your parting tool is on center, even if just bringing it to a live center in your tail stock. What you are describing sounds like your tool is above center and it sounds like you have never checked to see if the tip of the blade is on the centerline of the lathe.

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    Quote Originally Posted by kenton View Post
    If you haven't yet, verify that your parting tool is on center, even if just bringing it to a live center in your tail stock. What you are describing sounds like your tool is above center and it sounds like you have never checked to see if the tip of the blade is on the centerline of the lathe.
    I'd bet he assuredly HAS done. Some things really are PFB.

    The far greater problem, and near-as-dammit "standard issue" with any parting-off tool is KEEPING it on-center. Few things will stress a lathe to its max flex or twist on toolpost or compound axis as fast or with as great a certainty. Any and all "surprises" in-between as to when and how badly are only the Devil's idea of a joke.

    Call that mission-damn-near-impossible for lanterns, HARD for QCTP's, and "not assured" even for a 4-Way if it still has a compound rest under it, 'stead of direct-mount to the cross's top-slide.

    Reliable, safe, fast ergo cheap, and clean parting-off of solids is akin to a whole separate science and skillset compared to any other turning work. Either master it. Or avoid the need. There is no "in-between" as works long or well without ruining work, the lathe, yerself, or some combination thereof.

    Same as "landings" are to judging skill of pilots of aircraft. Do it well ... or only the one time!



    Tubing and hollow - even if only a small central hole - by contrast are "cake", the goal being mostly how cleanly it can be done to reduce or eliminate need of any significant second-op.

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    Quote Originally Posted by Bill D View Post
    Harrison m300 lathe about 200 rpm( too slow) It is a axial groove ground down the center so the blade width is the same. I guess it would lower the cutting height in the center section but I did not extend the groove clear to the edge. Stopping the grove a fraction behind the cutting edge. I used a chainsaw grinding stone on a dremel to grind. This is a tapered blade similar to a tee blade.
    At 200 RPM, once you get to a 1/4" diameter, your surface speed is down around what, 10 or 12 SFM? No wonder it grinds to a halt, you might as well be trying to ram a chisel through a stationary bar.

    Wind the tool out, put the spindle speed up to at least 1000 RPM for the last 1/4" (or 500 at 1/2")or so and the tool might actually cut.

    Dave H. (the other one) (who hooked up a slider potentiometer to the cross-slide to control the VFD and spindle speed...)


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