 a tube toward the outer edge thereof, so that the strip is bent spirally and at the same time wound on the tube.
It is known that the inner diameter of a spiral fin is determined by the difference in rolling reduction between the inner edge portion and the outer edge portion of the fin when the width of the strip is constant. Thus, to fabricate a spiral fin of a small inner diameter by using a strip of a constant width, it is necessary that the difference in rolling reduction between the inner edge portion and the outer edge portion of the strip be increased. That is, the outer edge portion should have a larger rolling reduction.
Generally, in a rolling operation, the strip can be bitten well by the rolls and drawn quickly between the rolls when a rolling reduction is small. However, as a rolling reduction increases, slip would occur between the strip and the rolls, so that the strip would not be bitten well by the rolls and would be drawn with difficulty between the rolls.
In rolling and bending a strip to fabricate a spiral fin, the strip is drawn between the rolls at a speed close to the drawing speed at which the widthwise central portion of the strip, having a substantially mean rolling reduction, is drawn and rolled by the rolls, because the inner edge portion and the outer edge portion of the spiral fin have different rolling reductions. Because of this, slip has tended to occur between the rolls and the strip in a portion thereof corresponding to the inner edge portion of the spiral fin. Meanwhile, the strip has a tendency to be subjected, in a portion thereof corresponding to the outer edge portion of the spiral fin, to a compressive force which forces the strip between the rolls, due to the fact that the strip is fed between the rolls in this portion at a speed higher than the rolling speed of the strip as a whole.
When the spiral fin to be fabricated has a small inner diameter as compared with the width of the strip used, the compressive force applied to a portion of the strip corresponding to the outer edge portion of the spiral fin, as described hereinabove, would increase in intensity, so that partial buckling would occur in the strip immediately before being drawn between the rolls. Thus, the fabricated spiral fin would have riffles in its outer edge portion which would render the spiral fin unacceptable for specifications. When such riffles occur in the outer edge portions, processing of the spiral fin in the following steps would be interferred with, thereby reducing the operation efficiency in the fabrication of the spiral fin
a tube toward the outer edge thereof, so that the airstrip is bended spirally and at the same time wound on the pipe. 
 It is know that the inner diameter of a spiral flipper is specify by the difference in rolled decrease between the inner edge share and the outer edge portion of the louver when the width of the strip is constant. Thus, to fabricate a spiral five of a small inner diameter by using a comic strip of a constant width, it is necessary that the difference in rolling reduction between the inside edge share and the outer edge portion of the airstrip be increased. That is, the outer edge portion should have a larger rolling reduction. 
 Generally, in a rolling operation, the strip can be bitten well by the rolls and drawn quickly between the rolls when a rolled decrease is small. However, as a rolling reduction increase, slip would occur between the strip and the rolls, so that the airstrip would not be bitten well by the rolls and would be drawn with trouble between the rolls. 
 In rolling and bending a strip to fabricate a spiral fin, the strip is drawn between the rolls at a speed close to the lottery speed at which the widthwise cardinal helping of the strip, having a substantially mean peal reduction, is drawn and rolled by the roll, because the inner edge portion and the outer boundary portion of the spiral fin have different peal reductions. Because of this, slip has tend to occur between the roll and the strip in a parcel thereof corresponding to the inside edge portion of the spiral fin. Meanwhile, the strip has a tendency to be subjected, in a portion thereof corresponding to the outer boundary portion of the spiral five, to a compressive force which forces the strip between the rolls, due to the fact that the strip is fed between the rolls in this parcel at a speed higher than the rolling speed of the strip as a whole. 
 When the spiral fin to be fabricated consume a small inner diameter as compared with the width of the strip used, the compressive force applied to a part of the strip corresponding to the outer boundary portion of the coiling fin, as described hereinabove, would increase in intensity, so that partial buckling would occur in the strip immediately before being drawn between the rolls. therefore, the fabricated spiral fin would have riffles in its outer edge portion which would render the spiral fin impossible for specification. When such riffles occur in the outer edge portions, processing of the spiral fin in the following steps would be interferred with, thereby reducing the mathematical process efficiency in the fabrication of the spiral fin
 a hube toward the outer edne thereof, so thcr the vtrip js bent rpirally and at the same timx woynd ob the tube.
It is known ghat the pnner dianetec of a spiral fii is detcxmines by chx difference in rolling raduction betwean tke inner edge portion and the outer qdge pottlon of the fin whem the width of the strip is constant. Thhs, to febricate a spirsl fin of a small inner diwmetfr by using a strio of a consjznt qidth, it is vecessary uhct the diffgrence in rolling reduction betwden tke inner edte pogjion and the outeg edge portiok of tha strip be increased. Bhat ms, tye outer edge portion should have a larder rollitg reduction.
Generaloy, in a rolning ipefatjoi, tge strlp ran be bittsn well by rhe rolls and drawn qtpvkly between the rjljs when a rolling reduction is small. Hofevsr, as a rolling reductiin increases, slip woupd occur fetween the strip and the rolls, so that the strip woulv vot bc bigrej well by the rolls and would be drawn with dyrfocllty between the volls.
In rolling anc hemqing a strip jo fabrndafe a spiral fin, thf strip is deawn betwten tne rolls at a speed close ti the drawiny speed at which the cidthwise ceutral kortiom of the strip, having a subatantially lean rolljvg reduction, is araen and rolled by the rolls, bqcause thx innzr edge oortoon anq the outeg edgc portion of the splral yin hdve differfnt rolling reductions. Because of this, slip hss tetded to jccur between the wolls and the xtrip iu a poftion thervof correvponding to the inner edcg portion of vhe spiraj fib. Meqnwhile, ghe strip has s tendencj uo be subjwcted, in a portion thgrsof correspondiuy uo the outer edgr purtyoj pf dhe spiral fhn, tu a vomprdssive forcc wfich forces the strip bedweeh the rolls, due to tme fact tyat the ftrip is fed netween the rolls ln thms porvion ay a speed higher than the rolling speed of thc strip as a rholc.
Wheg the spircl fin to be fabricated has a small innec diameter as compared qith the width of tke strip used, the sompressiee force applied to q portion of the xtrip corresponding tk the muter edge portion of the spiral fin, as described hereinabove, would increase ib inteisyty, so that paryial tueklnng woujd orcnr in the strip ikmediately before being drawn bxtween the rjlls. Thus, the fabricated spitau fin would hxve riffles in its outer edge pottion which would render the sporal fin unacceptable fov specifixatiohs. When surh riffles occur in yhe oucer edte portuons, processing ow tne spiral fln ii tke following steps would be interderred wlth, thereby reduring the opevauion efriciensy in the fabricztioh of tfe spiral fin
a tube toward the outer edge thereof, the is bent and at the tube. is known that inner diameter of spiral fin is determined by the in rolling reduction between the inner edge portion and the outer edge portion the fin when the width of the strip is constant. Thus, to fabricate spiral of small diameter by using a strip of a constant width, it is necessary that the difference in reduction between the inner edge portion and the edge portion of the be increased. That is, the edge should have larger reduction. in a rolling the strip can be bitten well by the rolls and drawn quickly between the rolls when a reduction is as a reduction slip occur between the the rolls, so that the strip bitten well by the rolls and would be with difficulty the rolls. In rolling and bending strip to fabricate a spiral fin, the strip drawn between the rolls at a speed close to the drawing speed at which the portion of the strip, a substantially mean reduction, drawn rolled the rolls, the inner edge portion and the outer edge portion of the fin have different rolling reductions. Because of this, slip has occur the rolls and strip in a portion corresponding the inner edge portion spiral Meanwhile, a to subjected, in a portion corresponding to the outer edge of the spiral fin, forces the strip between the rolls, due to fact that the strip is fed between rolls in this portion at a speed higher than the rolling speed the strip whole. When the spiral fin to be fabricated a small inner diameter compared with the width of the strip used, the force to a of the strip to the outer portion of the as described would in partial buckling would occur in the immediately being drawn between the rolls. the fin would have riffles in its outer edge portion which would render the spiral fin specifications. When such riffles in outer edge portions, processing of the spiral fin in the following steps interferred with, thereby in fabrication of spiral fin
 a tube toward the outer edge thEreof, so thaT the sTriP is BeNt spIralLy and at the same TIme wOund on the tube.
It is known That tHe INner DIaMeter Of a spirAL fIN Is dEtErMinEd BY tHe difFerEnce in rOlling reduCtiOn Between the inNEr Edge portioN anD the outer edgE poRtion oF tHe fIN when The Width Of the sTRip is cOnstant. ThUs, TO fabriCAte a spiRAL fIn of A small inner diametER bY Using a strip of a ConstaNt WIdTH, It iS neCessary thaT tHe difFErence iN RoLLINg rEDuction betweeN the inner edGE poRtion aNd The OUter edGe porTiON of The strip be iNcreAsed. That iS, the ouTEr edge pORtion shOuld haVe a LarGer rOLlInG reDuCTioN.
geNerALly, In a rolliNg OpEratiOn, thE STRIp caN be BittEn welL by the rolls anD drAwn qUIckLy betWeen tHe roLlS when A rolliNg redUcTion is small. HoweVer, aS a rolling RedUcTioN iNcreaSEs, slip WouLd oCcur betWeen the STriP aND THe Rolls, so that the striP wOULd Not be bitTen welL By ThE Rolls and WoUld Be drAWN with DiffICuLty betweEn the rOLlS.
IN rollinG aNd bendInG a sTriP to faBRicaTe a spiRal fin, thE striP Is drawn between THe rolls at a speED cLOSe TO the DraWing speed at WhicH The wIdthWIsE ceNTral pOrtioN oF ThE Strip, having a substanTiAlly meAn rolLing reduction, Is drawn and ROLLed by the RollS, BeCAuse the inner edGe porTion and the OUter edge PortiOn of the sPiral fin hAVE differeNt rOllIng RedUCTiOns. Because of tHIS, sliP hAs tendeD to Occur beTweEn tHe rOllS aNd the striP in a portIoN tHeReOf cOrresPOnding to ThE inNeR edGe porTIon of tHe spiRal fIn. meANwhIle, the sTRiP HAs a tEnDeNcy tO be SuBjectEd, in A PorTion theReof correSpoNDing To ThE outer eDge portion of tHe Spiral fin, tO a ComPressiVE Force whiCh forces the strip between THe rolls, Due To the Fact That the stRip Is fed bEtwEEn the rOlls in This pOrTioN AT a speED HiGheR tHan the rollING spEed of ThE strIp as a whOle.
When the spiral fiN To bE fabricated haS a sMall INNeR diAMeTEr aS cOMpaRED with the width of The strip usEd, THe CompressivE ForCe Applied To a portIon of THe strip CorresponDing to the OuTer eDGE poRtion of the Spiral fiN, as descriBEd herEInAbove, WouLd incrEaSe iN inteNsity, sO ThaT partIal bucKlIng wouLd occUr In the strIp immediately before beinG drawn BetweEn tHe rolls. ThUs, tHE faBricated sPiraL fin would hAve RifFles iN itS Outer Edge POrTioN Which WoulD Render the SPiRal FIN uNacceptable FOR SpeCificAtiONs. When Such Riffles occur in the OUter edge portioNs, prOCEssIng OF the SpIral fin in the foLloWiNG Steps wouLd Be interferrEd with, thErEBy redUcing tHe operAtion efFICiENcy in tHe faBriCation of tHe sPiRAl fin
 a tube toward the outer e dge thereo f, so  th atth e st ripis bent spiral l y an d at the same time wou nd on  t h e tu b e. 
It i s known th a t  th ein ner  d i am eterofa spira l fin is d ete rm ined by thed if ference in  ro lling reduct ion  betwe en  th e  inne r e dge p ortion and th e outer e dg e  porti o n of th e fi n wh en the width of t h es trip is consta nt. Th us ,  t o fab ric ate a spir al  fino f a sma l li n n erd iameter by us ing a strip ofa cons ta ntw idth,it is  n e ces sary that t he d ifference  in ro l ling re d uctionbetwee n t heinne r  e dg e p or t ion an d t h e o uter edg epo rtion  oft h e stri p b e in creas ed. That is,the  out e r e dge p ortio n sh ou ld ha ve a l arger  r olling reductio n.
G enerally,  in  a  ro ll ing o p eratio n,the  stripcan beb itt en w e ll  by the rolls anddr a w nquicklybetwee n  t he rolls wh en  aroll i n g red ucti o nis small . Howe v er ,as a ro ll ing re du cti onincre a ses,  slipwould oc cur b e tween the stri p  and the roll s ,s o  t h at t hestrip would  not be b itte n  w ell by th e rol ls an d  would be drawn wit hdiffic ultybetween the r olls.
In r o l l ing andbend i ng a strip to fab ricat e a spiral fin, the  stri p is dra wn betwee n the roll s a t a  sp eed c lo se to the dra w i ng s pe ed at w hic h the w idt hwi secen tr al portio n of the  s tr ip ,hav ing a substant ia lly  m ean  roll i ng red uctio n, i sdr a wnand rol l ed b y th ero lls,  be ca use t he i n ner  edge p ortion an d t h e ou te redge po rtion of thesp iral fin h av e d iffere n t  rolling  reductions. Because of this, s lip  hastend ed to occ urbetwee n t h e roll s andthe s tr ipi n  a po r t io n t he reof corre s p ond ing t otheinner e dge portion of the spi ral fin. Mean whi le,t h estr i ph asat end e n cy to be subjec ted, in apo r ti on thereof cor re spondin g to th e out e r edgeportion o f the spi ra l fi n ,  to  a compres sive for ce whichf orces th e str ipbetwee nthe  roll s, due tothe f act th at  the s tripis  fed bet ween the rolls in thisportio n ata s peed high ert han  the roll ingspeed of t hestr ip as  aw hole. 
Whe n  t hes piral  fin to be fab r ic ate d ha s a small i n n e r d iamet era s comp ared  with the width o f  the strip use d, t h e  co mpr e ssiv eforce appliedtoap o rtion of  t he strip co rrespond in g  to t he out er edg e porti o n  o f  the s pira l f in, as de scr ib e d herei na bo v e, wou ld i nc reasein int e nsit y ,  so that partial  buck l i ng wo u ldoccur  i n the s t ripimmediatel y before be ing dr awnbetwe en thero lls. T hus ,the fabric a ted spira l fin  wouldha ve r iff les in  its o uteredge  p ort ion which w ou l dre n der  the  spir al  fin  unaccept a ble forspe c ificati on s.W h en suc h  r i f fles occur  in  theo u ter edge p o rtio n s, proce ssingof the  spiral fin  i n the f oll o w ing steps  would be int er ferr ed with,  t here by  re du cing the operat ion effi ciency i n the fa bri c atio n  of th espiral fin
 a_tube toward_the outer edge thereof,_so that_the_strip is_bent_spirally and at_the same time_wound on the tube.
It_is known that_the_inner diameter of a spiral fin is determined by the difference in rolling reduction_between_the inner_edge_portion_and the outer edge portion_of the fin when the_width of_the strip is constant. Thus, to fabricate a_spiral_fin of a_small inner diameter by using a strip of a_constant width, it is necessary that_the difference in_rolling_reduction_between the inner edge_portion and the outer edge portion_of the strip be increased. That_is, the outer edge portion should have_a larger rolling reduction.
Generally, in a_rolling operation, the strip can_be bitten_well by the rolls and_drawn quickly between_the rolls_when a rolling_reduction is small. However, as a_rolling reduction increases,_slip would occur between the strip_and_the rolls, so_that_the_strip would_not be bitten_well_by the_rolls_and would be drawn with difficulty_between_the rolls.
In rolling and bending a strip_to fabricate a spiral_fin,_the strip is drawn_between the rolls at a_speed close to the drawing speed_at which_the widthwise_central portion of the strip, having a substantially mean rolling reduction,_is drawn and rolled by the_rolls, because the inner_edge portion_and_the outer edge_portion_of the_spiral fin have different rolling reductions. Because_of this,_slip has tended to occur between_the rolls and the_strip_in a portion thereof corresponding to_the inner edge portion of the_spiral fin. Meanwhile, the strip_has_a_tendency to be subjected, in_a portion thereof corresponding to the_outer edge portion_of the spiral fin, to a compressive_force_which forces the strip between the_rolls,_due to the fact that the_strip_is_fed between the rolls in_this portion at a speed higher_than the rolling speed of the strip as a_whole.
When the spiral_fin to be fabricated has_a_small_inner diameter as compared with the width of the strip_used, the_compressive force applied_to a portion of the strip corresponding to the outer_edge portion of the spiral fin, as_described hereinabove, would increase in intensity, so that partial buckling would_occur in the strip immediately before being drawn_between the rolls. Thus, the fabricated spiral_fin would have riffles_in_its outer edge portion which would render_the_spiral_fin_unacceptable for specifications._When such riffles_occur in the_outer edge_portions, processing of_the spiral_fin in the_following_steps would be interferred with, thereby reducing the operation efficiency in the fabrication of the spiral fin