Parting off on a mini lathe

[DellParticipant @dell]

I have a Myford 10 & had problems parting but now I have rear parting tool HSS parting tool upside down swarf falls away ( couldn’t run lathe in reverse as screw on chuck ) I don’t have problems but everything needs to be as ridged as possible .

[Richard SimpsonParticipant @richardsimpson88330]

Just a question ladies and gents.

I see a lot of references in this thread to rear mounted tool posts.  Reading the reasoning behind this I can see the advantage but what justifies using a rear mounted tool post over simply turning the tool upside down in the normal tool post and rotating the job clockwise instead of anti clockwise?

[JasonBModerator @jasonb]

Most using the read toolpost will have Myfords or similar where the chuck screws onto the spindle nose. If you run them backwards the chuck can screw itself off!!

[BazyleParticipant @bazyle]

The main reason for the rear toolpost is the way most amateur lathe saddles are/were made with nothing to stop the rear of the saddle lifting off the bed, or if a plate was fitted eg Boxford it probably wasn’t set tight. So if the tool catches it lifts and when you examine the geometry that relieves the cut. The other reason is some people are obsessed with the fear of the chuck unscrewing when run in reverse.

[Richard Simpson]

On Richard Simpson Said:

… what justifies using a rear mounted tool post over simply turning the tool upside down in the normal tool post and rotating the job clockwise instead of anti clockwise?

Whilst running upside down in reverse is an advantage, rigidity is the main reason for using a rear tool-post.

Comparison!

My front tool-post is a towering multi-jointed edifice.  I’d describe it as spindly rather than solid, and it’s height causes it to increase forces by acting as a lever.  All front tool-posts are bendy, especially those on small lathes.

When cutting, Newton’s third law applies: ‘when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction.‘  A front-tool post’s ability to resist ‘opposite in direction’ forces is limited by how much metal is in it, the height,  the joints, and the route taken by reaction forces to ground.

In contrast rear tool-posts are a hefty solid block of metal firmly bolted to the saddle.  They are much stiffer than a conventional front tool-post.  Stubby with only two joints: the slot for the cutter, and where it joint the saddle.  Leverage is minimised too: cutting forces are taken almost directly straight down into the saddle.  It’s instructive to trace route taken by forces grounding through a front-tool post; it’s long and twisty – not ideal.

As front tool-posts go, though inferior to a rear type, the 4-way is fairly solid.  As QCTP tend to be more spindly with extra leverage through the tool-holders, they are often less rigid.  Therefore I suggest beginners think twice before rushing to fit them.  Ask what’s more valuable in your workshop – is it rigidity or the ability to swap HSS tools quickly?

Dave

[John HaineParticipant @johnhaine32865]

What revolutionised parting for me was to start doing it at higher speed using power cross feed.  Higher speed gives lower chip load per rev and better cutting action by the tool, PXF is smoother and isn’t prone to “stutter” like humans.  I use a insert type parting tool in a QC toolholder (Dickson type).  My S7 has a custom toolpost without topslide and the feed is stepper powered by the CNC controller.

[JasonBModerator @jasonb]

Front QCTP works for me even on a “spindly” cross slide

70mm cast iron parting a 1.5mm thick disc, part held by approx 15mm dia chucking spigot

200mm dia x 25mm thick wall steel tube with 1.5mm HSS. I did saw the last 0.75mm rather than have the ring drop on the bed.

[Martin KyteParticipant @martinkyte99762]

When cutting, Newton’s third law applies: ‘when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction.‘  A front-tool post’s ability to resist ‘opposite in direction’forces is limited by how much metal is in it, the height,  the joints, and the route taken by reaction forces to ground.

Not quite as simple as that with regard to tool posts. Opposite usually involves  a couple which tranfers the force opposing the cut to the front ‘foot’ of the tool post and the front shear. Relaxation of the cutting force can cause the tool to ‘nod’ forward which tends to increase the depth of cut generally causing chatter and sometimes a dig in. When the operator is tentative in approach to parting this can often happen whilst with a more robust approach everything is ‘hard back on the stops’ and well out of the danger zone. That’s my interpretation.

[old martParticipant @oldmart]

When I made the rear toolpost for the Smart & Brown model A, I added a second lock at the rear of the saddle. Having a very long cross slide helps greatly, and what makes the biggest benifit for parting in particular is not using the top slide, giving added stiffness. Having power cross feed is a benifit especially as then you can keep a steady trickle of coolant running. The power cross feed has gone wrong again on the S&B, it is such an antiquated design that we are constantly putting off mending it.

[Andy StopfordParticipant @andystopford50521]

I use either a GTN2 insert or, for small stuff, a HSS tool ground to ~0.9 mm wide, no top rake, in my WM180, standard top slide and QCTP.

With the insert, I try to run the lathe as fast as it seems happy at, and keep the feed heavy enough to ensure it keeps cutting. For diameters to great for the insert toolholder, I have a conventional HSS blade in the dedicated QCTP toolholder.

The main limiting factor on large diameters is the lack of torque at minimum speed (50 rpm). Maybe I’ll make a 3 step pulley sometime.

What does make a big difference is using a collet instead of the 3 jaw – unfortunately only possible up to 20mm diameter of course (ER32 collets), but it’s interesting how much smoother and faster parting off is, probably due to the reduced overhang, and maybe also to more positive location of the workpiece.

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