distortion in gear after hobbing

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Posted by harsh gupta on October 09, 2000 at 05:59:21:

dear sir,
i am hobbing a 5 teeth, 19mm o.d., 41deg helix, 1.98 module, 20deg press. angle gear on a shaft about 170mm long, gear length being 35mm starting from 18mm of one end of the shaft on gleason's 125 gh cnc hobber, while holding the shaft between centres and further locating and driving with the help of a collet at the centre of the shaft using a two cut method, finish cut being a conventional cut 0.5mm deep and at feed 0.7mm per work revolution. the runout on colleting dia.(15mm) having been maintained as low as 0.002~0.004 on 15mm dia. we're facing a problem of shaft getting distorted (bent to 0.050~0.100 of runout between centres)after hobbing operation, i.e. runout increases from 0.002 of before hobbing to 0.050 of after hobbing.tis reflects very badly in lead inspection. i agree there are a number of factors responsible for this viz. raw material,feed rate, hob condition, cutting oil, runout on hob collar etc.
i am interested to know the various responsible factors as seen by an expert like you and the priority in which these must be targetted (optimum conditions to cut this gear).

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