Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PowerPoint to accompany Krar Gill Smid Technology of Machine Tools 6 th Edition The Jig Grinder Unit 74

74-2 Objectives Select wheels and grinding methods required for jig-grinding holes Set up work and jig-grind straight hole to within tolerance of in. (0.005 mm)

74-3 Jig Grinder Uses Need for accurate hole locations in hardened material led to development in 1940 Other uses –Grinding of contour forms Radii Tangents Angles Flats

74-4 Advantages of Jig Grinding 1.Holes distorted during hardening process can be accurately brought to correct size and position 2.Holes and contours requiring taper or draft may be ground 3.Because more accurate fits and better surface finishes are possible. service life of part greatly prolonged 4.Many parts requiring contours can be made in solid form

74-5 Jig Grinder Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Similar to jig borer –Capable of positioning table within.0001-in. accuracy –Both vertical spindle machines –Main difference is in spindles Equipped with high-speed pneumatic turbine grinding spindle –Permits outfeed grinding and grinding of tapered holes

74-6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Jig Grinder Grinding Spindle May be offset from main spindle Planetary path of rotation

74-7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Main Spindle Assembly

74-8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Grinding Head Outfeed Horizontal dovetail slide connects grinding head to main spindle of jig grinder Grinding head offset from center of main spindle to grind various-size holes Amount of offset can be accurately controlled by internally threaded outfeed dial, mounted on nonrotating yoke

74-9 Three Depth Measuring Devices 1.Adjustable positive stop Located on left-hand end of pinion shaft Microadjustments made by limiting screw 2.Graduated dial Located on downfeed handwheel Indicates travel of quill Set to zero at any position and reads.001 in. steps 3.Micrometer stop Fastened to column of grinder

74-10 Diamond Dressing Arm Jig grinders must rapidly dress grinding wheels without disturbing setup May be quickly swung into approximate grinding wheel location and locked into position Final approach done by fine-adjusting knurled screw –Advances diamond through dressing arm

74-11 Grinding Two methods each with its own advantages –Outfeed grinding –Plunge grinding Small holes (less than ¼ in. in diameter –Ground using diamond-charged mandrels Holes larger in diameter than normal range –Use extension plate Up to 9 in. in diameter

74-12 Outfeed Grinding Similar to internal grinding where wheel fed radially into work –Passes as fine as.0001 in. at a time Cutting action takes place with periphery of grinding wheel Generally used to remove small amounts of stock when high finish and accurate hole size required

74-13 Plunge Grinding Compares to cutting action of boring tool Wheel fed radially to desired diameter and then into work Cutting done with bottom corner of wheel –Keeps work cooler than outfeed grinding Rapid method of removing excess stock Wheel properly dressed produces satisfactory finishes for some jobs

74-14 Diamond-Charged Mandrels Used instead of conventional grinding wheels for grinding holes <.250 in. diameter Made of cold-rolled steel turned to correct size and shape Grinding end placed in diamond dust and tapped sharply to embed dust in surface Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

74-15 Advantages Over Conventional Grinding Wheel 1.Mandrels have maximum strength and rigidity 2.Mandrels made for ideal diameter and length for each hole 3.Velocity required for efficient grinding ¼ of that for wheel 4.Cost per hole is less due to greater efficiency

74-16 General Principles in Selecting Grinding Wheels 1.Shank or mandrel of mounted wheels be short as possible 2.Grinding wheel diameter should be approximately ¾ diameter of hole to grind 3.Widely spaced abrasive grains in bond increase penetrating power of wheel 4.Hard abrasive grain with strong or hard bond used for soft, low-tensil-strength materials 5.Hard abrasive grain in soft or weak bond recommended for high-alloy hardened steels

74-17 Wheel Speed Majority operated most efficiently at 6000 surface feet per minute (sf/min) Diamond-charged mandrels at 1500 sf/min Spindle speed varied for types and diameters of wheels used Three grinding heads for Moore jig grinder –Speed varied by adjustment of pressure regulator

74-18 Conditions That Indicate Improperly Dressed Wheel 1.Poor surface finish on hole 2.Surface burns 3.Out-of-round holes 4.Taper or bell-mouth holes 5.Locational error

74-19 Techniques for Dressing a Grinding Wheel 1.While wheel running at reduced rate, dress top and bottom face with abrasive stick held in hand 2.Dress diameter of wheel with sharp diamond 3.Repeat steps 1 and 2 with wheel at proper operating speed

Relieve upper portion of diameter so approximately.250 in. of cutting face remains Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5.Bottom face of wheel should be concaved slightly with abrasive stick for grinding to shoulder on bottom of hole In outfeed grinding, only diameter of wheel should be dressed when required. When plunge grinding, dress bottom face of wheel with abrasive stick.

74-21 Factors Influencing Grinding Allowances 1.Type of surface finish in bored hole 2.Size of hole 3.Material of workpiece 4.Distortions that occur during hardening process

74-22 General Rules For Amount of Material Left For Grinding 1.Holes of up to.500 in. diameter should be.005 to.008 in. undersize for grinding 2.Holes of over.500 in. diameter should be.010 to.015 in. undersize for grinding

74-23 Setting Up Work 1.When bolts or strap clamps used, keep bolts close to work 2.Strap clamps placed exactly over parallels supporting work 3.Bolts should not be tightened any more than required to hold workpiece 4.Do not clamp work too tightly in precision vise clamp 5.Set up work on parallels high enough to allow bottom of hole being ground to be measured

74-24 To Locate the Workpiece Workpiece may be set up parallel to table travel by three methods: 1.Indicate edge of workpiece 2.Set work against table straightedge; then check alignment with indicator 3.On heat-treated piece, indicate two or more holes and set up work to suit average location of group of holes

74-25 Grinding Sequence 1.Rough-grind all holes first 2.Finish-grind all holes that can be ground with same grinding head 3.Holes whose relationship to others is most important should be ground in one continuous period of time 4.Grind holes with shoulders or steps only once

74-26 To Grind a Tapered Hole 1.Convert angle into thousandths taper per inch by mathematical calculations 2.Mount indicator in machine spindle 3.Set angle plate or master square on machine table 4.Move indicator through 1 in. of vertical movement as read on downfeed dial 5.Set adjusting screws until desired taper attained Steps when an extremely accurate angular setting required:

74-27 Tapered Holes Most also have a straight section –Taper ground to certain distance from top Difficult to see where taper begins Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

74-28 Two Methods To Show Where Tapered Section Begins 1.Apply layout dye to top portion of hole with pipe cleaner Dye removed from taper portion during grinding operation allowing length of straight hole to be measured 2.On holes too small or difficult to see and measure, taper ground first to dimension X. Use formula to calculate size hole would be at dimension X Once tapered hole ground, straight hole ground to proper diameter

Select proper-size grinding wheel for hole size 2.Make bottom of wheel slightly concave with an abrasive stick 3.Set depth stop so that wheel just touches bottom or shoulder of hole 4.Rough-grind sides and shoulder of hole at same time 5.Dress wheel and proceed to finish-grind hole Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Suggestions for Grinding Shouldered Holes

74-30 CBN Wheels Perform best when jig grinding tool and die, carbon, and alloy steels hardened to Rc >50; hard abrasive cast iron; superalloys Rc >35 Advantages over aluminum oxide wheels –Long wheel life –Less wheel maintenance –Positive size control –Consistent surface finishes

74-31 Wheel Selection Guidelines 1.Abrasive type 2.Bond types Resin- and vitrified-bond wheels Metal-bond wheels Electroplated wheels 3.Grit sizes Choose largest abrasive size Choose finer abrasive size

74-32 Mounting Have shank extending as little as possible to avoid overhang –Results in chatter, vibration and spindle deflection if large overhang Use indicator on wheel shank and rotate spindle slowly by hand to check runout of spindle –Should not be more than.001 in.

74-33 Truing Resin-, vitrified-, and metal-bond jig- grinding wheels must be trued –Never true or condition electroplated wheels Always use 150-grit-size diamond- impregnated nibs for truing Never use single-point diamond

74-34 Truing Mount diamond-impregnated truing nib in sturdy holder Position diamond nib close to CBN wheel Take light infeed truing increments of.0005 in. or less Vertically feed full wheel length past diamond truing nib at feed rate of in./min Continue truing until wheel is true

74-35 Dressing Operation removes some of wheel bond and exposes sharp edges of CBN crystal –Truing glazes CBN wheels and leaves abrasive crystals and bond on same plane 1.Use 220-grit, medium hardness aluminum oxide dressing stick 2.Soak dressing stick in water-soluble oil so slurry created while dressing

Start wheel and force dressing stick in horizontally for depth of SBN abrasive section Never use an up-and-down motion for dressing 4.Withdraw dressing stick and move it up a distance equal to wheel length 5.Again force dressing stick in to depth of wheel's abrasive section 6.Continue this procedure until wheel seems to draw dressing stick in with ease

74-37 Jig-Grinding Guidelines 1.For best results, grind wet 2.Use proper wheel speeds, outfeeds, and reciprocal and planetary speeds –Never use wheel at higher speeds that those recommended by manufacturers –Outfeeds depend on spindle r/min, wheel diameter, wheel bond, and workpiece material –Reciprocal and planetary speeds should be fairly fast at continuous light outfeed rate

Use proper grinding mode to suit operation required. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Hole or outfeed grinding Chop grinding Wipe grinding Shoulder grinding

74-39 Summary CBN jig-grinding wheels and pins twice as hard and more abrasion-resistant than aluminum oxide wheels Expensive, but cost-effective due to longer wheel life, shorter grinding time, cool cutting action, and less wheel maintenance Remove 30% to 50% faster resulting in higher productivity

74-40 Jig-Grinding Hints Calculate all coordinate hole locations first Clamp work just enough to hold it in place Select grinding wheel three-quarters diameter of hole to be ground Wheel with widely spaced grains should be selected for rough grinding

74-41 More Hints Relieve wheel diameters so that only.250 in. (6 mm) of cutting face remains Never use glazed wheel for grinding Rough-grind all holes by plunge grinding Allow work to cool before finish grinding Finish-grind holes with freshly dressed wheel by outfeed grinding