Gears machining by milling tools
by Fabio Sada 2002

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15 Comparision between hobbing and milling
  


:      Some notes are referred to one only machining mode:
:           ** : referred to milling by continous rolling mode
:            * : referred to milling by multi-plane cuts mode


Technology

H O B B I N G
M I L L I N G

gear design

Module value up to 30-35
No limit to module value
Maximum diameter related to machine dimensions
No limit to diameter in case of machining of partial sectors
Pressure angle must be standard Immediate machining only if hob is available
Any pressure angle may be machined Immediate machining of any kind of gear
Teeth must be simmetric
Teeth may have different pressure angle on working and backlash flanks
Central groove must be wider
Central groove may be thinner
Toothing must be possible on complete 360 degrees tour
Toothing may be interrumpted by filling volumes OUT OF root diameter

Machine tool

May be used for gear cutting only
May be used for milling and boring machining on other pieces or other surfaces on the gear itself
Required CNC axe along hob axe direction
Not required
Required CNC axe for hob rotation
Not required
Not required
Required tangential axe X
Vertical axe controlled while cutting
Vertical axe Y controlled only while positioning
Indexable hob-holder head is very large and heavy
Indexable tool-holder head is smaller and lighter
May be machined 1 piece at a time Gear change is done with stopped machine
Machine may have two tables in order to change workpiece while working
Only cylindric gears may be machined
Also spur conic gears may be machined, even with very large dimensions
Required high speed CNC axe for rotary table: table and hob rotate very quickly in case of high speed or few teeth Movement synchronization is very difficult to be assured
Enough low speed CNC axe for rotary table: table rotates very slowly (**) or doesn't move (*) Only a few (**) or no (*) moving axes affect gear precision
An high precision hobbing machine is necessary
Machining may be carried out also by a machining center with good static precision and not so good dynamic precision (* only)
May happen that cutting speed must be low due to table speed limit
Cutting speed and feed may be as required for best use
Great damaging in case of power failure Rough cut of whole spacewidth removes a lot of material
Small (**) or no (*) damages in case of power failure For large modules, rough cut may be done by two V thin side cuts

tool

An HardMetal inserts hob is very expensive and easy to be damaged
A standard facing mill is very cheap and easily changable
At least one hob for each module or pressure angle
Just 1 tool for large range of modules or pressure angle
Better to have right and left hobs
The same tool machines both R/L helix
Large diameter so to lower spindle speed for high cutting speed
May choose best tool diameter since no limits to cutting speed
Tools and insert have a very long supply time
Tools and inserts are usually ready at supplier store
Job chances may be lost due to not immediately available or too expensive hob
Immediate and cheap machining of any gear, even 1 piece only
Hob suppliers are just a few and often act as a trust
Milling tools suppliers are a lot and fight each other by good service
For large modules double-lead hob is not possible
May work by two tools properly spaced and reduce time=cost up to 50%
HM hob inserts are very expensive and have usually a few edges
Standard milling inserts are cheaper and have usually more many edges
There are few qualities of inserts
A lot of qualities are available
The tool is very heavy and must be handled by crane and hands
The tool is very light and may be stored and changed automatically
Insert index or change must be carried out on the machine
Insert index or change may be carried out separately, with 2 tools available
The hob must be changed if module or helix direction change
The tool may be used for next gear or further toothing on same gear
Skyving hob requires sharpening
No sharpening, sharperer and sharpening machine is required
Skyving brazed hobs live short
Milling insert tool lives long
Tool profile at root is not easily changable
Tool profile at root may be choosen and changed easily
Hob is an high precision tool and may be damaged with high probability
The tool just requires to be correctly measured. Its own precision is good for chip removal but no necessary
The cut is on side with a long and thin chip Cut angles are not simmetric Tip-relief needs proper profile
The cut is in front with a shorter and thicker chip Tip-relief is built by tool path

tool and machine setup

Gear must be perfectly centered on rotary table
Gear may be desplaced from table axe
Centering a reference surface may often be difficult
Centering a reference surface is very fast and easy
Main gear datas must be entered into CNC macro-programm
Same

machining and machining time

Stock removal is smaller at the edges of facewidth
Stock removal is constant along whole facewidth
Extra-stroke is necessary for large helix angle or when gear elix is not as hob helix
No further stroke is required Eventually add 1 entering level step in case of large stock removal
Skyving hob may remove only a little material
Milling tool removes more much material even if hardened
Feed and rotation cannot be stopped instantly
Feed and rotation may be stopped istantly and indipendently
Cutting area is central and is not easily in sight
Cutting area is at side, tool is smaller and easily in sight
Cutting speed directly affects rotation speed of gear & table
Cutting speed may be controlled indipendently
High forces for high module
Cut forces low and indipendent from module
Machining takes approximately the same time as for a plane surface whose area is equal to involute flanks one
By using 2nd tool properly spaced, cut time may reduce up to 50% more
Low forces on tool and bearings

finish and testing

Span base tangent measure cannot be carried out for high BETA or Z
Span measurement may be carried out by machine axes in transverse section
By managing axial step, tool diameter and indexable insert design and precis it is possible to have a very good final precision No problem if further grinding is done



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