Long, tangling chips

A long chip is a chip the breaker did not break. Sandvik Coromant describes the fault as “Long unbroken snarls winding around the tool or workpieces”; Mitsubishi’s chip chart says of its type A chips: “Tangle about tool and workpiece.” Sumitomo names what they cost: type A “Twines around the tool or workpiece, damaging the machined surface and affecting safety”, and type B “Causes problems in the automatic chip conveyor and chipping occurs easily”. On an automated machine a long chip also means a stop: Citizen writes that defined chip breaking “significantly reduces the frequency of unnecessary machine stops caused by long chips.”

Soft, low-carbon steel is the classic case. Korloy: “Continuous and long chip flowing on the tools is formed while machining mild steel”; Mitsubishi calls its FP breaker the first choice for finish cutting of “low-carbon and alloy steels”. Cast iron is the opposite, in Sandvik Coromant’s words “a short-chipping type of material”. The chip forms, and how a chip breaker bends a chip until it breaks, are on the chip types page; this page is the list to work through at the machine.

Below the breaker’s range

Every chip breaker works within a range of feed and depth of cut. “Cutting depth (ap) and feed (fn) must be adapted to the chipbreaking area of the geometry,” the Sandvik Coromant training handbook says, and its list for long snarls begins with the two ways out of that area at the bottom: “Feed too low for the chosen geometry” and “Depth of cut too shallow for the chosen geometry.” Tungaloy puts it the same way: there are optimum values, an effective range, in feed and depth of cut. The range moves with the conditions. Mitsubishi: “In general, when cutting speed increases, the chip control range tends to become narrower”, and with coolant “the effective range of chip breaking becomes wider”. Tungaloy says both too.

Makers print the range for each geometry. Three, as published:

Two cautions go with such figures. Sumitomo prints its own as “representative values only” that “may change according to the actual catalog number”, and Kennametal advises using “the center 60% of the range” of feed “for best results”. For work that has to stay below the usual range, the makers offer geometries made for light cuts: Mitsubishi’s FP “provides efficient chip breaking at small depths of cut”, Tungaloy’s TS is “proven for superior chip control at light cutting depths”, Kyocera’s VC promises “Stable Chip Control Even at Small Depths and Low Feed Rates”, and ISCAR’s SF is a super-finishing chipformer for very low feeds and depths of cut.

Depth of cut against the nose radius

At a small depth of cut the nose radius does most of the cutting, and the chip it makes is thin. Sandvik Coromant gives the rule in two forms: its training handbook says “The depth of cut should be no less than 2/3 of the nose radius (re)”, and an article it supplied to MSC Industrial Supply says “Run at a cutting depth larger than the nose radius”, which it says “will minimize the radial deflection of the insert, which is important in internal machining”. Mitsubishi: “a larger corner radius tends to worsen chip control”, and “This tendency can be especially seen at low depths of cut”. Sumitomo: “Even if the feed rate is the same, a smaller nose radius makes chips thicker and chip control improves.”

• chip section
• edge in contact
• the edge one revolution earlier
• workpiece

Depth of cut ap

1,00

mmFeed f

0,25

mm/rev

Nose radius rε

0.40.81.21.6

Entering angle κr

95°75°45°

Largest chip thickness

—

Edge length in contact

—

Width / thickness

—

—

A smaller radius has a price. Sumitomo: “A smaller nose radius produces a rougher surface finish at the same feed rate”, although a larger one “increases back force which induces chattering” (see chatter). And the rule depends on the geometry: Mitsubishi’s LS finishing breaker is sold for “depths of cut smaller than the corner R”.

The entering angle works the same way. Sandvik Coromant gives the chip thickness as about the feed at κr 90° and “hex ≈ fn x 0.71” at 45°; “The chip thickness is reduced and the width increased with a larger lead angle (smaller entering angle).” The other makers write it with the side cutting edge angle (lead angle) that Mitsubishi, Sumitomo and Tungaloy print, which is 90° − κr: Sumitomo, “a smaller side cutting edge angle makes chips thicker and chip control improves”; Tungaloy, “Side cutting edge angle is preferably small”; Mitsubishi, “As the lead angle increases, chip control deteriorates.” Against long chips, then, κr towards 90°. Mitsubishi makes one exception, for back-feed facing: “When back feed machining the end face, increase the lead angle”.

What to try, in order

The order follows Sandvik Coromant’s list for long snarls, with the other makers where they say the same.

Coolant pressure

ISCAR’s threading handbook names three pressure bands, written for thread turning:

LP

HP

UHP

Sandvik Coromant: up to 80 bar

1020305080120200400bar, log scale

LP · low pressure ≤ 30 bar

May improve tool life; usually no effect on chip control.

HP · high pressure 30–120 bar

The range ISCAR says is most used with its JHP tools: tool life, cutting speed and chip control improve.

UHP · ultra high pressure 120–400 bar

Needs a special tool design to make use of the extra pressure.

Up to 30 bar is low pressure, which “usually will not have an effect on chip control”; 30 to 120 bar is high pressure, which ISCAR calls “the most commonly used pressure range used with JHP tools”, with improved chip control; 120 to 400 bar is ultra-high pressure, which “requires special tool design to take advantage of the extra pressure”. ISCAR’s rule for setting it: “If the chips are not breaking, the pressure should be increased until chip control is achieved”; as the pressure is raised beyond that, “the chips become smaller and smaller.” What the jet does, in ISCAR’s words (in an FAQ answer on milling): “HPC chills chips quickly, making them hard and brittle.” Sandvik Coromant shows high-pressure coolant “at a pressure of up to 80 bars/1160 psi” in super alloys, delivered through “the high precision nozzles mounted on the tool”.

Drilling and grooving

In a hole the chip has nowhere to go. ISCAR: “Long chips that rotate around the drill are problematic.” Its remedies for indexable-insert drills: “Increase feed. If drilling a very soft material, reduce feed and increase speed.” Then “Choose a geometry with a tighter chipbreaker for lower feeds (GF)”, and as the last step: “If chip formation can not be improved by changing the machining conditions, use a pecking cycle.” Mitsubishi gives the end-milling version for long chips round the tool when feeding down: “Increase the feed rate.” and “Use step machining.”

In grooving and parting the chip must leave a slot no wider than the insert. Sandvik Coromant describes the typical chip as “clock-spring shaped, narrower than the groove” and adds that “The insert geometry shrinks the chip width”; Tungaloy calls managing chips “in such a narrow space” “a big challenge”.

Let the machine break the chip

Where no breaker will do it, the feed axis can. FANUC’s Servo Learning Oscillation “is a function that makes a feed axis oscillate to break chips during machining”; FANUC says it “eliminates the need for chip breakers” and that “It can be used for drilling on milling machines and not just for lathes”, on its Series 30i/31i/32i-B, -B Plus and 0i-F Plus controls. Citizen’s LFV does the same on its lathes: “Chips are broken up in a controlled way thus eliminating machine stops due to tangled long chips”. Its modes specify “the number of vibrations for one spindle rotation” or “the number of spindle revolutions per vibration”, and one does thread chasing with defined chip breaking. Takisawa names the mechanism: the oscillation creates an “air cut” time, “allowing for chip breaking during turning operation.”

• this revolution
• surface left by earlier revolutions
• chip (one colour per piece)

Amplitude / feed

0,80

Oscillations per revolution

0.511.52.5

Breaks per revolution

—

Time in air

—

—

The opposite fault: chips broken too hard

Breaking can go too far. Sandvik Coromant: “Very short chips, often sticking together, caused by too hard chip breaking”, which “often causes reduced tool life or even insert breakages due to too high chip load on the cutting edge.” Sumitomo’s type E chips cause “spraying of chips and poor surface finish due to chattering, along with chipping”; Mitsubishi says much the same of its type E. Sandvik Coromant’s remedies run the other way from the list above: “Select a larger nose radius”, “Reduce the feed”, and a holder with a lead angle of ψr 45–15°, κr 45–75°. Chips that break against the insert can also hammer it: Sandvik Coromant warns that this “can in some cases lead to chip hammering, where the chip damages the insert”, and ISCAR gives some of its chipformers a wavy surface “to prevent chip hammering”.