External Turning. Outline Review of Turning. Cutters for Turning Centers

Transcription

1 Outline Review of Turning External Turning 3 External Turning Parameters Cutting Tools Inserts Toolholders Machining Operations Roughing Finishing General Recommendations Turning Calculations Machining Handbook Tool Selection Seco Tool Catalog Chapter 6 ME a p : Depth of cut (DOC) [mm], v c : Cutting speed [m/min], f n : Feed(rate) [mm/rev], n: Spindle speed [rpm], κ r : Setting angle [ o ], γ: Chip (side-rake) angle [ o ], λ: Inclination (back-rake) angle [ o ]. Chapter 6 ME Cutters for Turning Centers Conventional Lathe Tools Types of cutters: Carbide (and other hard materials) insert turning and boring tools Coated / uncoated High Speed Steel (HSS) drills and taps Information sources for turning calculations: Catalogs of Tool Manufacturers Machining Data Handbook Machinery s Handbook Chapter 6 ME Chapter 6 ME 440 5

2 Insert Types 1 V used for profiling, weakest insert, 2edges per side. D somewhat stronger, used for profiling when the angle allows it, 2 edges per side. T commonly used for turning because it has 3 edges per side. C popular insert because the same holder can be used for turning and facing. 2 edges per side. W newest shape. Can turn and face like the C, but 3 edges per side. S Very strong, but mostly used for chamfering because it won t cut a square shoulder. 4 edges per side. R strongest insert but least commonly used. Chapter 6 ME Inserts (Cont d) 3 Negative Inserts has a wedge angle of 90 o. The insert has to be inclined in the toolholder ld to provide a clearance angle tangential to the workpiece. Positive Inserts has a wedge angle smaller than 90 o. Since the clearance (relief) angle is built into the insert, there is no need for a special inclination in the toolholder. Chapter 6 ME Toolholders Tool Clamping Systems 2 Inserts must be attached to a robust shank that ideally has the following attributes: High rigidity, High damping, High stability (mechanical / thermal), Low cost. Clamping of inserts to the shank significantly affects the performance of turning. ISO recommends five basic toolholders + clamping systems: C (clamp) toolholder type D (clamp for inserts w/ hole) M (pin/clamp) P (pin/wedge) S (screw) Insert Clamping (External Turning) Insert Clamping (Internal Turning) Chapter 6 ME Chapter 6 ME 440 9

3 Toolholder Types (Versions) 1 Left Hand Right Hand Right Hand tool turns towards the chuck. Left Hand tool turns towards the tailstock. If the cutter is symmetrical with the shank, it is called neutral hand tool. For most CNC turning centers, the cutter is on the back side of the part. Tool is generally placed upside down in the tool turret. Tool-Turret Arrangements If the toolholder is NOT positioned upside down (just like illustrated), the right-hand tool becomes the left-hand one or vice-versa. If the tool is turned over, the spindle must turn clockwise. Otherwise, counter-clockwise! Pay attention to the direction of rotation: Spindle rotation from the turretside towards the operator is the clockwise (forward) spin. Chapter 6 ME Chapter 6 ME Tooling Considerations Tooling choices depend on the type of workpiece, the CNC machine, and dthe desired surface finish / tolerances. Hard workpieces require harder cutters. Modern cutters require the turning center to have high spindle speeds and powerful motors. Roughing Cut Just removing metal, surface finish does not matter. Aim is to obtain an approximate shape suitable for semi-finishing / finishing operations. Requires a strong cutter + rigid support. Generally have deep (depth of) cuts and high feed-rates. Cutting speed is generally lowered to keep heat down. Chapter 6 ME Chapter 6 ME

4 Finishing Cut Surface Finish Requirements 3 Must meet required surface finish i and size / form specifications. Requires a hard cutter to hold its shape well. Generally have small depth of cuts and low feed- rates. Low chip-load reduces cutting forces. Deflections of the cutter as well as the workpiece are decreased. Cutting speed is generally increased to give a better surface finish. R max = f 8 2 n r ε Surface finish depends on Feed (f n ) Cutter nose radius (r ε ). Generally, a large nose radius and a slow feed rate coupled with high cutting speed gives the best finish. However, too large of a nose radius induces chatter destroying the finish and the size. Chapter 6 ME Chapter 6 ME Wiper Technology 3 Wiper inserts have a completely different nose configuration. Also referred to as High Feed inserts Due to its flattened nose geometry, the insert is capable of generating the same finish at a much higher feed. General Recommendations For roughing: a p : 25t 2.5 to 5 mm f n : 0.3 to 0.45 mm/rev For finishing: a p : 0.75 to 1.25 mm f n : 0.15 to 0.25 mm/rev. Smaller feed can be selected if special inserts are utilized. As a general rule, make sure that a p > r ε f n < r ε Use this procedure for carbide, ceramic, and cermet inserts. We will fine tune the cutting speed based on the desired depth of cut and feedrate. Chapter 6 ME Chapter 6 ME

5 Cutting Speed Relative speed of the part wrt. cutter Tangential to the part Usually denoted as v c, V, CS, or S Tabulated in the handbooks as a function of Material Cutter type Type of cut (roughing or finishing) Needed d to calculate l spindle speed [rpm]. Spindle Speed Once the cutting speed (v c ) [m/min] is selected using the corresponding tables in machinist handbooks, the spindle speed (n) [rpm] can be calculated as 1000 v n = π D c [ rpm] where D is the (outer) diameter of the workpiece [mm] in external turning. Chapter 6 ME Chapter 6 ME Handbook Procedure for Calculating Spindle Speed 1. Select depth of cut -as deep as possible. 2. Select feed - appropriate p for roughing g or finishing. 3. Find the original cutting speed in Table Find the feed- and depth-of-cut factors in Table 5a. 5. Modify the original cutting speed based on step Calculate the spindle speed. Chapter 6 ME Example OD Turning Determine the spindle speed [rpm] for the following turning operation: Workpiece: Quenched and tempered 8620 steel with a Brinell hardness of 300. Diameter: 63.5 mm. Depth of cut: 6.35 mm. Feed: 0.3 mm/rev (roughing cut). Tool: Hard coated carbide cutter. Chapter 6 ME

6 Calculation Steps 1 to 3 Step 1: Depth of cut = 6.35 mm (given) Step 2: Feedrate = 0.3 mm/rev (given) Step 3: Determine V from Table 1. V opt = 585 ft/min = m/min V avg = 790 ft/min = m/min f opt = inch/rev = mm/rev Calculation Step 4 Calculate the following ratios: f/f f opt = 0.3 / = V avg / V opt = / = 1.35 From Table 5a, read off the corresponding factors: Feed factor, F f = 1.22 Depth-of-cut tfactor, F d = Chapter 6 ME 440 Next Step 22 Chapter 6 ME Calculation Steps 5 and 6 Table 1 Step 5: Calculate the cutting speed V: V = V opt F f F d = V = [m/min] Step 6: Calculate the spindle speed n: n = 1000V / (πd) = / (π 63.5) n = [rpm] Next Chapter 6 ME Slide 40 Chapter 6 ME 440 Calculation Steps 25

7 Table 5a Lead Angle Lead angle is often times referred to as Side Cutting Edge Angle. It affects not only the chip thickness but also the width of the uncut chip layer. Chapter 6 ME 440 Calculation Step 4 26 Lead Angle Chapter 6 ME 440 Table 5a 27 Selecting Turning Tools using Manufacturers Catalogs 1. Determine the ISO material class (P, M, K, N, S) and its (sub)group. Group is usually determined based on the hardness of workpiece material. 2. Define machining operation: External / Internal / Parting / Threading Longitudinal / Profiling / Facing Roughing / Semi-finishing / Finishing Select the depth of cut (and initial feedrate) 3. Select insert: Grade / Geometry / Shape / Size 4. Choose tooling/clamping system 5. Use recommended cutting data to determine Cutting speed / feed 6. Calculate spindle speed and power If beyond the capabilities of the machine tool, revisit Step 2 Reduce the depth of cut 7. Determine chipbreaker geometry. Chapter 6 ME Example Revisited Seco Tool Selection 1. Define workpiece material properties: ISO P-class (AISI/SAE 8620 Steel) Group: Seco material group #4 (Slide 30) Specific cutting pressure : k c1.1 = 1700 [N/mm 2 ] Given for 1 mm of undeformed chip thickness Chip-load exponent: m c = Define machining operation: External machining (i.e. outer diameter turning) Longitudinal rough turning (with a p : 6.35 mm) Chapter 6 ME 440 Next Step 29

8 Specifie ed materia al is in (Seco) materia l Group # Seco Material Group 2 Seco Tool Selection (Cont d) 3. Choose insert: Type: C Negative insert with hole Appropriate for OD turning and facing (Slide 32) Grade: TP3000 (Coated WC) Very tough Suitable for interrupted rough cutting (Slides 33-34) Size: 16 mm (Slides 35-36) 4. Select toolholder for the insert (Slide 37): D-type clamp L-type toolholder Right-hand hand version 5. For our case, the recommended values (Slide 39) are f n = 0.4 mm/rev v c = 195 m/min Note this choice leads to a tool life of (approx.) 15 minutes. Chapter 6 ME 440 Back 30 Chapter 6 ME 440 Next Step 31 3 Insert Types Insert Grades 2 Chapter 6 ME 440 Back 32 Chapter 6 ME

9 3 - Insert Grades (Cont d) Insert Size 2 Chapter 6 ME 440 Back 34 Chapter 6 ME Insert Size (Cont d) 2 4 Toolholder and Clamping 2 Clamping System: Toolholder Type: Note that the user can select r ε from the set: {0.8, 1.2, 1.6} (mm). Chapter 6 ME 440 Back 36 Chapter 6 ME

10 4 Selected Toolholder Recommended Cutting Speed 2 Chapter 6 ME 440 Back 38 Slide 40 Back Chapter 6 ME Spindle Speed and Power Calculate the spindle speed n: n = 1000v c /(πd) = /(π 63.5) n = [rpm] Comparable to its counterpart calculated in Slide 24! Compute the spindle power using v c = 195 [m/min]; f n = 0.4 [mm/rev] a p = 6.35 mm; κ r = 95 o γ = -6 o (Side rake angle See Slide 38) k c1.1 = 1700 [N/mm 2 ]; m c = 0.24 Chapter 6 ME Power Requirement e e (Cont d) If the efficiency of the transmission system is assumed η = 0.85, the power requirement of the machine tool becomes where P = k k c c c v c a p f n η [ kw ] 0.01γ = kc 1.1 [ N / mm m ( f sinκ ) c 1 2 n r o ( 6 ) 2 kc = 1700 = 2247 [ N / mm ] o 0.24 (0.4sin 95 ) Hence, P c = 2247 = 21.83[ kw ] Chapter 6 ME ]

11 7 Chipbreaker 2 When machining ductile materials, the chip accumulation can be avoided with the utilization of chipbreakers: They are now integral parts of insert geometries. References Some of the materials used in these notes are adapted from the following sources: 1. MFET 275: CNC Applications, Purdue Calumet. 2. Seco Tool Catalog, Sandvik Coromant Tool Catalog, Chapter 6 ME Chapter 6 ME