special processes for finishing gears

8/9/2019 Special Processes for Finishing Gears

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Special processes for nishing gears

By Dr. Bianco www.gianfrancobianco.c

Skivinghobs

Rollingtools


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Skiving hobs


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Skiving hobs

A Skiving Hob is a hob that canfnish aheat-treated workpiece.

Working with skiving hobs is thereore afnishing operation which may be an

alternative to grinding or honing process


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Skiving hobs

Hard gear

dynamic


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Skiving hobs

In this process the gear is roghhobbed in the green sing a pre-skivehob which leaves an envelope ostock! slightly greater than or pre-

shave! or removal ater hardening othe gear

"he process is a hard gear fnishingdynamic in which the gear is fnishedin its hard state! correctingdistorsions indced by the heat treatprocess


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Skiving hobs

Skiving hobs are manactred incarbide and they have a very largenegative ctting angle rom -#$

to -%$&

'egativectting

angle


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Skiving hobs"he carbide skiving hob is designed to

remove material only rom the tooth (anks annot rom the root

)onse*ently! the typical pre-skive roghingprocess involve nderct tooth (anks orcorrected pressre angle

0,15 0,20mm


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Skiving hobs

Accracy ater skivingprocess

With skivingprocess itspossible to obtaingear class /0I'%123

!o"g#ness!a $ 0,25 0,50%m

&eli' (ro)le


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Skiving hobs

"he skiving hob have been sccessllyapplied on , to #$ modle gear having ahardness $ 6 2 HR) at ctting speed o $to #$$ m7min and eed rate o # to 2 mm7rev.

"he hob material mst be carbide grop 8 orgrop 9

"i' coating is sed only or carbide solid hob/ma+ diam. #$$ mm3

Working

conditions

Skiving operation not se coolant"he chips generated have the shape o small

needles"he chips are very hot. :o mst remove the chips

very *ickly


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Skiving hobsWorking

conditions

;a+imn tollerable wear is arond $!#< mm

"he level o wear shold be no higher!however! since the *ality o the gearteeth wold worsen and there wold bethe risk o the hob teeth chipping.


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Skiving hobsWorking

conditions

;echanical partor inde+ing the

pre-ctworkpiece

/manally3

=ear

)') atomaticmachine has a

electronic inde+ingdevice


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Skiving hobs

Resharpening

D

utg

=

2

;ovement abovecentre

>+ample?Hob diameter @ #2$ mm'egative racke angle @ %$&0eplacement above center @%!# mm


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Skiving hobsResharpeni

ng4eore resharpening yo mst check the hobdiameter and calclate the deplacement o thegrinding wheel in order to obtain the right rackeangle.

>very error prodces a profle modifcation

I the deplacement increase the pressreangle increaseI the deplacement decrease the pressre angledecreaseB corse resharpening mst se diamond

grinding wheels

'ormally the hob is spplied with a relativeresharpening diagram in order to easilycalclate the deplacement in relation o hobdiameter


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Skiving hobs

Advantage of skiving

process

-- 'ot need a special machine. "he hobbingmachine can be sed also or skiving

operation i its enogh rigid.

-- Its possible to fnish also a very largegears.

-- Ater skiving operation the gears have a

very good srace

-- Some time with skiving hob its possible tofnish gears not grindable /close to a sholder.


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Skiving hobs

Disadvantage of skivingprocess

-- "he cycle time is longer compared toshaving or grinding.

-- "he accracy is less than grindingoperation.

-- Resharpening re*ires more attention inorder to obtain a good accracy o the gear.

-- Its very di5clt to obtain proflemodifcation in the gear.

-- Heli+ modifcation can be obtained only with

)') hobbing machine.


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Finish byRolling


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Finish byRolling

Cinish the gears by rolling is a di5cltoperation

9rofle o rolling tool mst be modifed witha long stdy. "he modifcation depends on

many actors

;ain points to consider

"ype o gear )ylindrical and helical

0imension o

gear

Small and medim /ma+

modle@ 2!


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Finish byRolling

Workingscheme with

one head

"here are rolling machines with one head orwith two heads


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Finish byRolling

Workingscheme with

two head


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Finish byRolling


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Finish byRolling


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Finish byRolling

)hip closeto rootdiameter

Strctremodifcation de totoo high

hardness

9rofleirreglarityclose to pitchdiam.

=ood

profleand

strctre


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"he two sides o the teeth havediEerent profles

"he heat treatment cases greaterdeormation o the profle

Finish byRolling


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Finish byRolling

9rofle o thegear canchange i?

change thehardness o thesteel

change the

strctre o thesteelchange chemicalcomposition o thesteel

change the stockremoval


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Finish byRolling

Smmary

-- "he rolling tool is very e+pensive bt canmake a lot o pieces

-- Rolling gear mst made beore heat treatment.

-- In the gear prodction this method is not veryoten applied

-- 'owadays the modern grinding machines havea cycle time very short and the *ality is veryhigh and absoltely constant


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special processes for finishing gears

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