Amount Of Material Removal On Spring Cut

I was "trying" to do a press fit bore in some 5mm thick aluminium the other day and I missed by 6 thou, that was infuriating.

I was also using a dial on the cross feed.

I think the temprature wasn't helping me at the time :-0

Either that or it was the peson who talked to me whilst i was doing math's ?

:)

Stuart
 
A habit I'm bad at using is. When you take that last cut, stop cutting about an 1/8" in and traverse the carriage to get the cutting tool out of the cut. Don't touch the cross slide, leave it as is. Mikc the cut and see if it is where you want it. If it is where you want it, re-start the cut at the end, not where you left off. It has saved me a part or two over the years and it will leave a place at the start of part where the cutting tool cut over again. Sometimes at the end of the cut, I'll reverse the feed and let the cutting tool make a spring pass over the part.
You're old 21" LeBlond is going to take some time to get used to how it cuts. Have to go back to using some Rex 95 HSS ground to get good finishes with and hold tolerances with. Use negative rake carbide tooling for the heavy cuts.
 
Thanks, I haven't a clue either.
However the above quoted post is confusing, whilst understanding increasing negative rake and increasing clearance (if ones does not touch the tool to the part the clearance is 100%) the increase in tool force claim is fascinating. and requires explanation.

It is a bit scary to answer this to a person with your experience and knowledge (it far outweighs mine in all respects). If you call BS on this, I will humbly accept it. It would not be the first time I have stood up in public and made a fool out of myself :)

Hopefully the image below will help clarify. The top diagram is a 0 rake tool on center, the bottom diagram is an exaggerated 0 rake tool below center.
I am not a engineer, so please forgive any mistakes I make in both content and language. This is just my understanding of what is going on from reading technical descriptions combined with an education heavy on math and physics. The explanation is a bit idealized, in practice there are other things also happening.

It ultimately comes down to chip formation.
In the region about the point of contact with the tool and work, the metal becomes plastic and malleable. The metal is in something like a transition state between solid and liquid. The amount of force required to make the metal plastic is idealized in the physics behind SFM. I do not know exactly what all the factors are that determine the ideal plastic state, but I assume that it is a balance between malleability, heat byproduct, and flowability.
The metal that is in a plastic state expands in the area around the contact point, partly from heat expansion, and partly from the change density. This expansion is independent from the metal being fed into the cutter. This by the way is the physics behind forge welding, the hammer blow causes the metals to enter a plastic state and "join" even though they are below melting temperature.

During a cut with the tool on center.
The work applies all of it's force to the tool in the y-axis as pure down force (I am ignoring z-force altogether in this discussion).
The chip applies force in both the x and the y axes. Much if not most of the x-axis force from the chip is in the form of friction as the chip slides over the tool.
The transition area of the chip (the part of the chip that is plastic) expands and applies down pressure, and leftward force to the tool, and rightward force to the work.
Ideally, almost none of the heat penetrates the work itself. The tool should be drawing the hot chip away at the same rate the heat is generated.
So as the tool enters the work, the metal becomes plastic and flows over the tool. As the chip cools it becomes a solid, curls, and drags over the tool. All of the forces on the tool are to the left and down.

During a cut with the tool below center.
The work applies down force and some rightward force as the work tries to drag the tool under the work
The chip applies force in both the x and y axes similar to the tool being on center.
The transition area becomes larger, and contains a "pocket" from which the metal has more trouble flowing out of. The transition area becomes hotter, larger, and has greater dwell time. This extra heat penetrates farther up toward the top of the chip, farther into the tool, and into the work itself.
This results in a chip that both coils more easily, and coils more tightly. A more tightly wound chip applies less friction to the tool because it has lower contact area. This results in more of the tool pressure being applied to the work rather than to the chip.
The work being heated more, has an easier time entering the transition state.
Some of the downward and outward pressure from the expansion of the metal in it's plastic state is converted to heat energy and wasted.
The result is lower leftward force on the tool from the metal flowing over it.

In short, lowering the tool results in greater tool pressure because less of that pressure is wasted on parasitic losses.

Above_below.jpg

So before we all run out and start lowering our tools...
Lowering the tool can result in dramatic reduction in tool life, especially during heavy cuts. Part of the reason that lowering the tool during a finish cut works(sometimes) is that we are no longer cutting at the ideal chip load and SFM. typically a finish cut is at increased RPM (artificially increasing the SFM), and lower feed (artificially reducing chip load). Lowering the tool can help improve the chip load in this situation.
Lowering the tool also results in greater upward flexing of the work, which can result in unwanted harmonic vibration because the tool itself is not effective at dampening up and down motion.
Lowering the tool can result in more heat penetrating the work, meaning we have to stop and let things cool down longer.
Lowering the tool will increase the tendency of the chip to weld itself to the tool, aluminum suffers from this problem enough already.

Lastly, tool grind and rake will obviously have a huge impact on all of this.
Myself, I am a hobbyist not a pro. I grind all my tools for finishing, and rough with them. I make a semi-finish pass to help remove spring, and then a final pass, all with the tool on center height in most cases. I only lower the tool when making very fine skim cuts on harder steels, though I prefer a vertical shearing tool for this and use one whenever possible. If I am having to skim off 0.001" of material, I consider it a mistake on my part (I make plenty of mistakes...)
 
It's amazing how much spring is in steel. I have a 1.5 inch test bar that extends 12 inches out of the MT5 spindle. I can move the end of that bar .001 just with one finger. It's all spring because an indicator near the spindle doesn't move at all.
 
One thing that I have not seen mentioned about tools at above or below centre is that your tool is no longer transversing along the radius of the workpiece but along a cord (as seen on the cross section). This means that you will be removing less metal than what your dials or DRO tell you you are removing. As you approach the centre of your workpiece (spindle axis) this error will get greater.

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It's amazing how much spring is in steel. I have a 1.5 inch test bar that extends 12 inches out of the MT5 spindle. I can move the end of that bar .001 just with one finger. It's all spring because an indicator near the spindle doesn't move at all.[/QUOTE


You may be getting some of that from your spindle, when you are measuring 12" out you are amplifying everything the spindle does wrong. I noticed on the 1144 stress proof I cut at about 12" out there was the ability to move it about .003 and it was 2-1/16"diameter, don't get me wrong I was pushing and pulling hard to move it, however I think the results would be different if it was sticking out of concrete 1'
 
Ken, I used to do the same thing when I was running parts with a close tolerance. Saved me a few times from either a skim cut, or a half hour with abrasive cloth. Got into that habit running Incoloy hydraulic sleeves for SII DataDrill's pulser tool. Couple of feet of used tubing was $300, so really hated to lose any.
 
One thing that I have not seen mentioned about tools at above or below centre is that your tool is no longer transversing along the radius of the workpiece but along a cord (as seen on the cross section). This means that you will be removing less metal than what your dials or DRO tell you you are removing.

When the set-down is just right the reduced spring will compensate for that.

As you approach the centre of your workpiece (spindle axis) this error will get greater.

Which is why the set-down has to decrease with decreasing diameter so that the tool moves along a radius (the optimum path is probably not a straight line).
 
I reckon in the future I will make a spring cut without any feed before


I have a leblonde model 21 (24" diameter X 8') prolly older than both of us together if your under 80.

Hello, the dangers of skim reading,

How similar is the sadle to the little brothers to your? I have a 13" leblond and if i rewind it by hand i get an increased depth of cut (about 5-15th) I think the small wear plates on mine have some slop that alows the sadle to "cock" a little when going backwards.

Their on the list of things to be fettled on my lathe :).

I think shim sheet can be installed under them to get them a bit tighter.

Stuart
 
When the set-down is just right the reduced spring will compensate for that.



Which is why the set-down has to decrease with decreasing diameter so that the tool moves along a radius (the optimum path is probably not a straight line).
How do you calculate the exact amount of set down to apply that will compensate for the fact that you are now not transversing along the radius of the workpiece.

This would be a very complicated equation that would be a function of the materials elasticity, the depth of cut and the carriage position relative to the axis of revolution. It would very well be a exponential curve

Further you would then need to have the tool height increase while the carriage is being fed in,further complicating the calculations.

Basically having your tool below centre will ensure that you always cut less than what your dials/DRO are indicating regardless of flexing of the workpiece (purely on geometry). This will mean that you are simply guessing how much you are taking off the workpiece with every pass.

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