Machining Between Centers Unit 52

Objectives Set up the cutting tool for turning operations Turn parallel diameters to within ±.002-in. (0.05-mm) accuracy for size Produce a good surface finish by filing and polishing Machine square, filleted, and beveled shoulders to within .015-in. (0.3-mm) accuracy

Machining Between Centers Training programs (schools) Remove and replace work in lathe many times before completed Need assurance that machined diameter will run true with other diameters Machining between centers saves time in setting up Common operations Facing, rough and finish-turning, shoulder turning, filing and polishing

Setting Up a Cutting Tool Move toolpost to the left-hand side of the T-slot in the compound rest Mount tool- holder in toolpost so setscrew in toolholder 1 in. beyond toolpost Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Heavy Cuts: Set toolholder at right angles to work Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Setting Up a Cutting Tool: cont. Insert proper cutting tool into toolholder, having tool extend .500 in. beyond toolholder and never more than twice its thickness Set cutting-tool point to center height Check it against lathe center point Tighten toolpost securely to prevent it from moving during a cut

Mounting Work Between Lathe Centers Procedure same as for facing Unit 51 listed proper procedure for mounting work between centers

Parallel Turning Used when work must be cut to size and have same diameter along entire length Many factors determine amount of material that can be removed on lathe at one time Cut to size in two cuts: roughing and finishing Lathe centers must be in line Trial cut taken off diameter before setting depth of cut on crossfeed micrometer graduated collar

Purposes of a Trial Cut Produce accurate turned diameter Measured with micrometer Set cutting-tool point to the diameter Set crossfeed micrometer collar to the diameter

Procedure to Take a Trial Cut Set up workpiece and cutting tool as for turning Set proper speeds and feed to suit material Start lathe and position toolbit over work approximately .125 in. from end Turn compound rest handle clockwise ¼ of a turn to remove any backlash

feed toolbit into work by turning crossfeed handle clockwise until light ring appears around entire circumference of work Do NOT move crossfeed handle setting Turn carriage handwheel until toolbit clears end of workpiece by about .060 in. Turn crossfeed handle clockwise about .010 in. and take trial cut .250 in. along length of work Disengage automatic feed and clear toolbit past end of work with carriage handwheel

Stop the lathe Test accuracy of micrometer by cleaning and closing measuring faces and then measure trial-cut diameter Calculate how much material must still be removed from diameter of work Turn crossfeed handle clockwise ½ amount of material to be removed

Take another trial cut .250 in. long and stop the lathe Clear toolbit over end of work with carriage handwheel Measure diameter and readjust crossfeed handle until diameter is correct Machine diameter to length

Rough Turning Removes as much metal as possible in shortest length of time Accuracy and surface finish are not important in this operation .020- to .030-in. feed recommended Work rough-turned to Within .030 in. of finished size when removing up to .500 in. diameter Within .060 in. when removing > .500 in.

Procedure for Rough Turning Set lathe to correct speed for type and size of material being cut Adjust quick-change gearbox for a .010- to .030-in. feed Depends on depth of cut and condition of machine Move toolholder to left-hand side of compound rest and set toolbit height to center

Take light trial cut at right-hand end of work for a .250 in. length Tighten toolpost securely to prevent toolholder from moving during machining Take light trial cut at right-hand end of work for a .250 in. length Measure work and adjust toolbit for proper depth of cut Cut along for .250 in., stop lathe, and check diameter for size Diameter .030 in. over finish size Readjust depth of cut, if necessary

Finish Turning Follows rough turning Produces smooth surface finish and cuts work to an accurate size Factors affecting type of surface finish Condition of cutting tool Rigidity of machine and work Lathe speeds and feeds

Procedure For Finish Turning Make sure cutting edge of toolbit free from nicks, burrs, etc. Set toolbit on center; check it against lathe center point Set lathe to recommended speed and feed

Take light trial cut .250 in. long at right-hand end of work Produce true diameter Set cutting tool to diameter Set graduated collar to diameter Stop lathe and measure diameter Set depth of cut for half amount of material to be removed Cut along for .250 in., stop lathe, check Readjust depth of cut and finish-turn

Filing in a Lathe Only to remove small amount of stock, remove burrs, or round off sharp corners Work should be turned to within .002 to .003 in. of size For safety, file with left hand so arms and hands kept clear of revolving chuck Remove toolbit from toolholder before filing Cover lathe bed with paper before filing

Procedure to File in a Lathe Set spindle speed to twice that for turning Mount work between centers, lubricate, and carefully adjust dead center in work Move carriage as far to right as possible and remove toolpost Disengage lead screw and feed rod Select 10- or 12-in. mill file or long-angle lathe file

Start lathe Grasp file handle in left hand and support file point with fingers of right hand Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Apply light pressure and push file forward to its full length; release pressure on return stroke Move file about half width of file for each stroke and continue filing until finished Use 30-40 strokes per minute Safety precautions Roll sleeves above elbow Remove watches and rings Never use file without properly fitted handle Never apply too much pressure Clean file frequently with file brush

Procedure for Polishing in a Lathe Select correct type and grade of abrasive cloth for finish desired Piece about 6 – 8 in. long and 1 in. wide Use aluminum oxide abrasive cloth for ferrous metals Use silicon carbide abrasive cloth should be used for nonferrous metals Set lathe to run at high speed Disengage feed rod and lead screw

Remove toolpost and toolholder Lubricate and adjust dead center Roll sleeves up above elbows and tuck in any loose clothing Start lathe Hold abrasive cloth on work With right hand, press cloth firmly on work while tightly holding other end of abrasive cloth with left hand Move cloth slowly back and forth

Shoulder Shoulder: the change in diameters, or step, when turning more than one diameter on a piece of work Three common types of shoulders Square Filleted Angular or Tapered Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Three Types of Shoulders Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Procedure to Turn a Square Shoulder With work mounted in lathe, lay out shoulder position from finished end of work for filleted shoulders, allow sufficient length to permit proper radius to be formed Place point of toolbit at this mark and cut a small groove around circumference to mark off length

With turning tool, rough- and finish-turn work to within. 060 in With turning tool, rough- and finish-turn work to within .060 in. of required length Set up end-facing tool, chalk small diameter of work, and bring cutting tool up until it just removes chalk mark Note reading on graduated crossfeed handle Face (square) shoulder, cutting to line using hand feed Return crossfeed handle to same graduated collar setting for more cuts

Fillets Used at a shoulder To overcome sharpness of corner To strengthen part at this point Toolbit having same radius used to finish shoulder if filleted corner required

Procedure to Machine a Filleted Shoulder Lay out length of shoulder with center-punch mark or by cutting light groove Rough- and finish-turn small diameter to correct length minus the radius to be cut Mount correct radius toolbit and set it to center

Set lathe for one-half turning speed Coat small diameter near shoulder with chalk or layout dye Start lathe and feed cutting tool in until it lightly marks small diameter near shoulder Slowly feed cutting tool sideways with carriage handwheel until shoulder cut to correct length

Beveled (Angular) Shoulder Used to Eliminate sharp corners and edges Make parts easier to handle Improve the appearance of the part Sometimes used to strengthen part by eliminating sharp corner of square shoulder Beveled at angles ranging from 30º to 60º Most common is 45º

Procedure For Creating Beveled Shoulders Turn large diameter to size Lay out position of shoulder along length of workpiece Rough- and finish-turn small diameter to size Mount side cutting tool in toolholder and set it to center Use protractor and set side cutting edge of toolbit to desired angle

Apply chalk or layout dye to small diameter close to shoulder location Set lathe spindle to ½ turning speed Bring point of toolbit in until it just removes chalk or layout dye Turn carriage handwheel by hand to feed cutting tool into shoulder Apply cutting fluid to assist cutting action Machine beveled shoulder until it is required size

Procedure to Cut Beveled Shoulder Using the Compound Rest If size of shoulder large and chatter occurs during cutting with the side of the toolbit, may be necessary to use the compound rest. Set compound rest to desired angle Adjust toolbit so only point does the cutting Machine the bevel by feeding the compound rest by hand