Rear Toolpost Parting Advice

[Hugh GilhespieParticipant @hughgilhespie56163]

I have acquired a rear tool post for my Colchester Student lathe and I want to set up a rear mounted parting tool. I use tipped tools for parting, Q-Cut, Glanze and one from Cutwel and I don’t have any major problems parting off from the normal tool post but as I have the rear tool post it will be handy to have the parting off facility permanently available.

 

As far as I can see, I have two options for using the rear set-up. Either I set up with the parting tool in it’s normal orientation – tip at the top – and reverse the lathe or have the tool inverted and then part off with the lathe spinning in it’s normal direction.

 

I actually prefer the second option as I won’t have to remember to reverse the machine before parting off but I am concerned that the tip won’t be properly supported. All my parting tool tips are held in the spring type holders and I worry that if used upside down the tip may come loose.

 

Any thoughts?

[Hugh GilhespieParticipant @hughgilhespie56163]

[JasonBModerator @jasonb]

Can’t see it being a problem as all the cutting forces will still be applied in the same direction pushing the tip into the holder.

 

Jason

[Chris TriceParticipant @christrice43267]

You’ll find another thread discussing the forces at work and why it’s advantageous to mount a parting off blade upside down to the rear (not digging in etc) but at the most basic level, if your chuck screws onto the lathe, there’s a chance it will unscrew if you run the lathe backwards particularly given the torque required. The other advantage to putting the parting off blade at the back but upside down is that the swarf wants to drop out of the slot rather than collecting and potentially jamming. I can’t see any point in mounting the tool at the back the right way up and running the lathe in reverse. In fact you’d only be increasing the chances of something going wrong for no advantage whatsoever. It’s also highly unlikely tip will come loose. It’s constantly being forced back into its slot.

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hugh (and Chris)

 

There is one other reason for not running in reverse and that is that the saddle is setup with the rear gibs fairly tight and the front ones a little slack as the cutting forces tend to lift the reand and press on the front.

Cutting with the tool face upwards and running in reverse will tend to cause the front of the saddle to bang like a dunny door in a gale and cause you lots of problems with finnish

 

Lawrie

[Hugh GilhespieParticipant @hughgilhespie56163]

Hi, Thanks for the advice. I have now installed the parting tool upside down in the rear post so that the lathe spin direction is normal. Lots of fun finding shims to get the height spot on but I managed it eventually.

 

Seems to work fine, I chopped up some 12 mm steel bar without problems – except remembering to reverse the power feed direction the first time. Can’t say if it is any improvement or not compared to ‘normal’ parting but it is very convenient to have the parting tool always mounted.

[Ian Bartlett 1Participant @ianbartlett1]

Hugh,

The "rear parting tool mounted upside down" idea is something taht works really well on very light lathes – I had an ML10, and always parted off in this way – it worked for me. Running the lathe backwards wasn't an option, as the chuck was screwed on.

Why does it work? Not sure – maybe it's simply because it eliminates the topslide from the equation. I had a lump of steel bolted to the back of the cross slide as a toolpost.

I now have a *much* heaftier lathe – a Harrison M400. I use the parting blades with the spring in-type inserts the 'right' way up, in a normal quick change toolholder on the topslide – never a worry, just stick it on an infeed of 4 or 5 thou per revolution and let it run. I've parted off 12" diameter, inch wall thickness pipe this way. Flood coolant helps.

I'd think that your Student would be equally suited to working like this, if it's not too worn or the slides aren't too loose.

Ian

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