Avoidingi CHATTER/

. ! in turning

CHATTER, which leaves thesurface of the work withripples, can have various

single causes; or it may arise froma combination of factors, each ofwhich would be ineffective in othercircumstances. On some l ightlathes or on worn lathes it mayappear as soon as they are pushedbeyond a modest limit of rotationalspeed and depth of cut; and toproduce good work from thesecalls for the best conditions ofmachining.

In practice, working conditionsare so varied that a brief considerationof what happens in normal smoothturning may perhaps be the best wayof showing how irregularities leadingto chatter occur. The material isrigid in itself and its mounting,and moves with uniform speed pastthe tool, which is likewise rigid andforced into the material by feed. All-round rigidity, material speed, toolpressures and feed are the mainfactors.

Not too fastHence, lathe spindle and slides must

be in good adjustment, and bothmaterial and tool firmly mounted withminimum overhang. Speed must not/

\be too fast; and the tool must besharp and capable of forcing its wayinto the material.

/ Irregularities occur when there islack of rigidity in the lathe spindleor slides, or in the mounting of

t material or tool; and when the speedis too fast, or tool feed into the mater-ial is difficult. Reducing speed maythen eliminate chatter, as also ‘mayreducing the depth of the cut--or theperimeter on which the tool is cutting.

In this connection, in rough turning,alternative cuts are as at A; but theround-nosed tool is more likely thanthe straight-sided one to cause chatter,

1 OCTOBER 1959

W O R K S H O P H I N T S A N D T I P S

SMALLHONED

FLAT

ALTERNATIVE_ R O U G H I N Grc_ CUTS

because of its longer cutting edge fora given depth of cut. On the otherhand, a sharp round-nosed tool maynot cause chatter where a blunt onewould; and in a borderline case, theuse of oil or cutting suds may preventchatter-as against dry cutting.

For roughing cuts taken at slowspeed, in back gear when required,the straight-sided tool can be groundto a point, as at B (above), then the tipturned into a tiny radius with a handhone. For finishing at a fast speed, asmall feed-overlapping flat can behoned as shown (right), and a lightcut employed. A tool for facing cutson the same principle. may be ofopposite hand, as at C (above).

When tools are used for cutting onlong edges or perimeters, as in form-ing operations, chatter is likely tooccur. For chamfers (which can beformed with angled tools), the solutionis to set the topslide round and feedthe tool, as at C. Form tools, or deepgrooving tools, which must go straighton to the material, however, areusually improved, as at D, by grindinga small groove close to the cuttingedge, and providing packing to thecross-slide. Side overhang on thetopslide should be obviated if possibleby bringing the slide to mid-position;and for a smooth finish, it is helpfulto end the cut with the materialcreeping round to a halt.

209

BY GEOMETER

For boring operations, tools shouldbe used at minimum overhang, andwhere bore diameters admit, toolshanks or holders of maximum sizeare advisable. Open-ended bores canbe machined with small round toolbits held by clamping screws in stiffmild steel holders; and in grindingthese bits, the tool principles, as at B,should be applied.

For machining large diameters,particularly in roughing operations,rigid mounting is essential. Forflywheels and pulleys, this can be inan independent chuck with jawsreversed, or on the faceplate, as at E,if there are spokes between whichbolts can be fitted. Turning theoutside diameter, machining the frontface of the rim-using back gear,facing the boss and boring it, can allbe done at a setting. Then reversingthe wheel, the other side can befinished. Finally, should there beedge-wobble on it, light truing cutswith it mounted on a mandrel willCorrect.

In part-off operations, joggling thetool for cuts. as at F, is helpful, bothto reduce chatter and clear swarf-cut down 1, displace tool and cutdown 2, return and cut off 3.

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