Finishing methods
Roughing vs. finishing Roughing removing material as fast as possible, without special demands on surface and low demand on precision high Q, high IT, high Ra Finishing making final surface with wanted properties. Remowing residual material, improving geometrical and dimensional tollerance low Q, low IT, better Ra
GRINDING the finishing method with very wide use area; grinding wheel is a multiple cutting edges tool consisting from abrasive grains and a bonding material; grinding can be likened to the milling process 3
Grinding process t chip thickness d depth of cut 4
Grinding wheels Abrasive grains: Al 2 O 3 SiC cbn diamond tough most common material for: steel, Fe alloys hard for ductile metals (Al, brass, stainless steel) and brittle metals (cast iron) for fine grinding dtto GRAIN SIZE (grit): 0,003 3 mm 5
Grinding wheels Bonding material: vitrified bond clay + ceramic materials ( baked ) silicate bond Na 2 SO 3 metallic bond bronze rubber bond (for cbn and diamond) (for cbn and diamond) 6
Grinding methods A) surface grinding plain flat surfaces - horizontal grinding horizontal spindle - vertical grinding vertical spindle B) cylindrical grinding rotational surfaces 7
Surface grinding 8
Surface grinder 9
Grinding methods A) surface grinding plain flat surfaces - horizontal grinding horizontal spindle - vertical grinding vertical spindle B) cylindrical grinding rotational surfaces 1. external grinding - centre grinding a workpiece is rotated between centres - centreless grinding a part is supported by a grinding wheel and a regulating wheel 2. internal grinding 10
Cylindrical grinding a) external b) internal 11
External cylindrical grinding Feed motion types in centre grinding: a) traverse feed b) plunge cut 12
Center grinding with an inclined in-feed an operation example 13
External centreless grinding 14
Internal grinding traverse feed 15
Internal grinding plunge cut 16
Grinding tolerances and surface finish Type of cut IT Ra roughing 9 11 0,8 6,3 finishing 5 7 0,2 1,6 fine finishing 3 4 0,025 0,4 17
Finishing methods With material removal generative methods (fine machining, honing, superfinishing, lapping) Without material removal forming methods (glazing, blasting, tumbling, polishing)
1. Fine machining Machining process Turning or milling with depth of cut close to minimum depth of cut Fine cylindrical of flat surfaces
Fine machining Tools Turning high precision inserts (PCD - alluminium, coated SC) Milling special shaped inserts, large diameter facing milling heads
Fine machining - examples Video 001
Fine machining properites Low depth of cut Ap = 0,01 0,05 mm High cutting speeds up to 1000 m.min-1 High cutting forces, high spindle load IT = 4 6 Ra = 0,1 0,4
2. Honing Abrassive process Small amout of material is removed by honing stones Mainly internal holes (bored and ground holes)
Honing Tool: Honing head with honing stones Honing stones: Made of abrasive material Adjustable Seated on pushing springs
Honing Material of stones Any abrasive material can Be used for a honing stone - Same materials like grinding stones (corundum, SiC, CBN, PCD) - more precise shape, smaller grain (10-50 μm) - Cutting fluids are used for lubrication
Honing Available as tool for CNC machines Various diameters (4-40 mm) Spring mounted (to adapt to hole) Movement with CNC cycle Small tools (1-20mm)
Honing process mechanics Honing head is 1) Rotating reciprocitive 2) Oscilating up and down 3) Honing stones are pushed to the surface Video 002
Honing process mechanics Two speeds combined together make unique movement and special form of tool path.
Honing machine tools Special horizontal or vertical honing machines
Honing machine tools CNC horizontal or vertical honing machines
Honing - examples
Honing - properties Improving the geometrical accuracy (IT) mainly Setting certain surface pattern (lubrication) Expensive and time consuming IT = 3 to 6 Ra = (0,025)0,1 0,4
3. Superfinishing Abrassive process Small amount of material is removed by moving superfinishing stone Both cylindrical and planar surfaces
Superfinishing Tool: Superfinishing stone Has the negative shape of the machined surface (or it can be flat for large surfaces) Abrasive material again fine grained (to 20 μm)
Principle The superfinishing stone is sliding on a layer of lubricant on the surface and pushed by certain force 1) The surface is rough peaks looking out of the lubracant layer 2) The stone cuts the peaks the surface for lubricant is bigger, more sliding 3) The stone cuts all peaks, no more material removal
Superfinishing - surface
Superfinishing - methods Cylinders Cones Flat surface Shape surface
Superfinishing - properties Not improving the geometry!!! (IT remains) Lower speeds and force than gringing or honing Method Cut. Speed (m.min-1) Move Presure (MPa) Grinding 1800-3500 None (infeed) 14-140 Honing 100-250 superfinishing 6-14 50 cycles (length of hole 150 mm) 200-1000 cycles (1-5 mm) 3,5 7 0,02 0,07
Superfinishing - properties Ra = 0,025 0,1 Lubricant is used to carry away chips and minimize heat (deformation) kerosene is used
Superfinishing machine tool Superfinishing device mounted on lathe Superfinishing Machines special, Expensive type (multiple parts, long travel distance) Video 003 Video 004
4. Lapping Abrassive process Removing very small amount of material Removing surface undulations, roughness, tool marks and surface fuzz from grinding For all types of surfaces
Lapping Tool: A flat disc or plate made of cast iron, alalloy, copper, brass (low strength material) Abrasive particles are carried by an agent grease, oil, water (higher viscosity means higher abrasion)
Lapping - methods The lapping tool is rotated (oscilated) and moved irregularly across the machined surface - Equalising laping removing previous surface shape, flatening (spur, helical gears) - Forming laping lap is the negative shape of the surface
Lapping - methods For: Holes Flat surfaces cylinders Made: Manually mechanically
Lapping - methods Flat surfaces Lapping plate (different sizes, manual, machine operated - Moved irregularly along the surface
Lapping - methods Machine lapping Part are placed between a rotary and stationary lap. Part are carried by a workpiece holder. Part are placed excentrically so is the rotating lap. This makes the parts rotate and move
Lapping - examples Video 005 Video 006
Lapping - properties Lapping removes all tool traces improves durability, lowers friction Gives best surface quality (glass, mirror lapping) IT = 1 5 Ra = 0,01 0,16
6. Blasting Two forms - Forming (angle 80 ) - machining method (for angle of impact bellow 45 ) Particles are thrown at surface All kinds of shape
Blasting - tool Blasting elements are transported by high presure air to the surface Hardened steel balls cast iron balls Steel wire pieces (rounded) Glass balls aerospace industry
Blasting - examples Video 007 Video 008
Blasting - Aerolap
7. Tumbling Forming method, abrassive method Part to be machined and tumbling particles are placed in rotating cylinders Non defined surfaces
Tumbling - method Abbrasive particles and parts mixed together Special rotary or vibrational mills steel cylinders, inner rubber linning (to prevent abrasion) Sometimes active cutting fluid (water, NaCl, NaNO3, H2SO4 ) helping to remove the surface layer
Tumbling surface quality
Tumbling - tools Abbrasive particles non-uniform pieces of grinding stones, porcelain, ceramics (5-20 mm) Hardened steel balls (4-10) little surface hardnes improvement Soft particles for tumbling-polishing (dry corn peels, palm bark, wood chips)
Video 010 Video 009
Tumbling - machines
8. Polishing Abrasive method A friction between the polishing tool and part is caused by polishing media (paste) heat melts the peaks of surface All surface mainly flat surface with no strength demands
Polishing Abbrasive material is bond to a tool (disc, belt) or it is a media (paste) freely between tool (textile disc, belt) and part NO improve in shape or accuracy!! Just improves the surface quality
Polishing - tools Discs multi-plated : of fabric, leather, cotton - Solid : rubber, wood, cork, mild metal Brushes - Made of fibres : plastic, metal wire (0,1-1,2 mm) Belts - Endless belt pushed to the part. Better and more productive than discs.
Video 011 Video 012 Video 013
Polishing - properties Cutting speed 25-40 m.sec-1 No IT improvement Ra 0,2-0,4 (removes tool paths)!!! Implicates residual stress due to high temperature!!!
Schedule WS 2016/2017 lesson 11 5.1.2017 Lesson 12 10.1.2017 (Tuesday) Lesson 13 12.1.2017
Video links: Fine turning 001 - http://www.youtube.com/watch?v=do6bja7g3r0 https://www.youtube.com/watch?v=ctcegi1hvjo Honing 002 - http://www.youtube.com/watch?v=iuzbqlyfgxo Superfinishing 003 - http://www.youtube.com/watch?v=wnkngsnov7o 004 - http://www.youtube.com/watch?v=ephaijdjoue Lapping 005 - http://www.youtube.com/watch?v=rfc5h4da-ds 006 - http://www.youtube.com/watch?v=fnovv-rwiwy Blasting 007 - http://www.youtube.com/watch?v=zk258am2axo 008 - http://www.youtube.com/watch?v=q8qn9qylzvi
Video links: Tumbling 009_http://www.youtube.com/watch?v=QR1mBCFhlp M 010_http://www.youtube.com/watch?v=iIdc2YhkG5o Polishing 011_http://www.youtube.com/watch?v=TnzEMkkCxpw 012_http://www.youtube.com/watch?v=G4xC_GzYAK M INOX polishing 013_http://www.youtube.com/watch?v=l-Zf2AAgA5U