Rear mounting parting-off tools

[KWILParticipant @kwil]

Nobby, As far as I can see, if the tool digs in it still has the backlash freedom to move into the work.  Working from the front you are pulling the leadscrew on the back of the nut and therefore backlash allows it forward towards the work, if working from the back, the leadscrew is pulling from the front of the nut and can still  move towards the work??

 

As I understood the theory, from the front you are pressing down on the slides which “loosens” the dovetails, thereby making their movement easier, whereas at the back you are pulling the dovetails tighter,  (but are also trying to lift the carriage).

Edited By KWIL on 11/01/2011 09:05:38

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Recent explanations and the 50 odd years of much the same from ‘great experts’

in the magazines are mindless twoddle . The problems of parting off in Myford style lathes don’t originate in the slideway system at all . All relatively lightly built lathes with belt drive have a large amount of elasticity in the drive system – primarily existing as elastic stretch in the tight side of each belt . This elasticity allows the spindle and hence work piece to oscillate about its own axis whilst turning .Under conditions of heavy load the system becomes unstable and during cutting the work can vibrate in a rotary sense relative to the tool and this is what is seen as chatter .

 

Generally speaking the tighter the belts are the less this effect is likely to occur but there is obviously a practical limit . For specifically Myford lathes best parting off resuts will be obtained when the belts are just slightly on the tight side of the manufacturers recommended setting .

 

A parting tool mounted upside down in a rear tool post mounted direct on the cross slide is much more rigid than one mounted indirectly on the topslide . Since any vibration of the tool and mounting is likely to initiate parting chatter a more rigid set up will always work better than a non rigid – hence the benefit of rear toolposts .

 

As a matter of interest if you mount a forward facing parting tool in a tool post directly mounted on the front of the cross slide and mount an inverted tool on a topslide temporarily mounted at the rear the situation reverses – the tool in toolpost at the front works better .

 

The often given advice of plunging in boldly to get stable parting off works because a

high but relatively constant load tends to tighten the drive sides of the belts making the set up stiffer – this is a very hit and miss solution however and over enthusiastic cutting is as likely to initiate problems as prevent them .

 

 

[blowlampParticipant @blowlamp]

Provided that the workpiece itself is mounted rigidly, there can only be two causes for a cram-up.

Either the tool digs in somehow, due to something like backlash or bending of the support system, or there is chip crowding of the groove being machined.

 

In any particular situation, it might be worth experimenting to find which is the cause of this bother, by cutting a groove just a few thou’ in from the end of a bar, such that the full width of the tool is utilised, but (apart from the thickness of this flimsy shim) one side remains open for swarf clearance.

 

If a cram-up still happens, then it must be that the tool is moving into the workpiece for some reason. If the problem disappears, it must be chip crowding.

 

Martin.

[NobbyParticipant @nobby]

Hi

Backlash ?

With m/c turned off .Try this?  Wind x slide in stop then push forward and see it move forward    Then wind in as if parting off from rear try pulling ?  be carefull of any sharp tool in toolpost .

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