Trouble with cut off operations

[Richard Carlstedt]

Peter, the point I was making was keeping the cutting load centered on the support provided by the cross-slide AND tool-slide areas. Removing the tool-slide function as shown in your pics is also a great help as it removes another "Flex" joint in the assembly . No doubt a lathe without a tool-slide is much more rigid in operation.
I had a 1943 14" LeBlond that had a tall cross-slide ( and no Tool-slide) and it was extremely rigid. Tool-slides are not a necessity for Lathes but most lathes have them for thread work or micro taper boring , so you work within the means you have .

Rear tool posts also have disadvantages , but lack of rigidity is not one of them
Rich

 

[awake]

I'm not sure if it has been mentioned - I might have missed it - but there is another subtle difference in the upside-down parting (whether at the front with the spindle in reverse, or at the rear with the spindle in forward): The forces from the cutting action pull the cross slide up, wedging the cross slide dovetails against the carriage dovetails, and thus increasing rigidity.

Of course, for parting right-way up, the corresponding thought it is to be sure the cross slide gibs (and compound gibs, if using the compound) are snugged up properly. Also be sure the carriage is locked and can't shift left or right.

Another point worth mentioning: If you are using a chuck that is at all bell-mouthed, it can let the part flex just enough to catch. DAMHIKT!

 

[holmes_ca]

I have a Taig Mini Lathe and I scaled down a copy of an old fashioned spring parting off toolholder it really works great,

Edmund.........Alberta

 

[Tim1974]

Why not all just get a decent lathe and be dun with all this bickering

 

[Peter Twissell]

I choose to persevere with a Drummond round bed lathe which my grandfather bought in 1926.
I take pleasure in working out ways to perform operations which would normally be considered beyond the machines capacity.
For parting off, I find the traveling steady very useful. The loads from the tool are reacted through the steady directly to the saddle, eliminating chatter and producing a smooth, straight cut.

 

[Wizard69]

I see this advice a lot but when I search an authoritative source (such as Sandvik) they suggest virtually on center, or slightly above center on deeper cuts to allow for tool flex. Setting the tool low leads to rubbing and possible part climbing over the tool as diameter decreases. Based on the usual lack of real rigidity in most hobby lathes, I would advocate for slightly above centre (+0.1mm/4 thou) for best results.

Lately the only parting I've been doing at work has been either hardened Thompson rod or a bit of 4340. Hardened rod absolutely requires that you be either exactly on center or a tiny bit below. If not the tool rubs. Now different materials will deliver different results so I will not dismiss Sandviks advise completely. It should also be noted that I suggested no more than a few thousands lower, you can certainly run into issues going too far.

 

[Wizard69]

A tool height gauge is the most useful little tool I have ever made. Here is the idea: http://www.hemingwaykits.com/acatalog/Centre_Height_Gauge.html

Wizard69, you say you don't see why a rear toolpost should be an improvement, and then get pretty close to explaining it yourself! The argument goes like this: a tendency to dig in causes deflection (some, no matter how stiff the machine). Part of that defelection is in the form of a rotation about the mounting. At the front toolpost the deflection is down and forward into the workpiece, whereas with a rear toolpost the deflection is up and backwards, out of the work. If you have trouble visualizing this, imagine a tower crane. In the first case a dig-in is a self-reinforcing feedback system, while at the rear it tends to be self correcting. (I didn't notice the above post by tornitore45 before posting this)

I have no trouble imagining this sort of operation. What I see is the tool working against the mechanics of the machine. Instead of compressing the various parts of the machine together you are effectively pulling them apart.

 

[goldstar31]

Why not all just get a decent lathe and be dun with all this bickering

Tim

I choose to persevere with a Drummond round bed lathe which my grandfather bought in 1926.
cut.

Well Yes but I bought my first RB Drummond for £9, eventually gave it away and bought GHT's book for £30-- and then wore it out and bought a replacement

I don't think that I want to meet challenges in either another RB or have another damned nuisance of a bell mouthed chuck.

Perhaps the one thing that stuck in my mind in all my years is that
this is Big Show for one life only and it is not the dress rehearsal.

 

[tornitore45]

Why not all just get a decent lathe and be dun with all this bickering

Because a decent lathe use too much room and then I can not park my Roll Royce next to the Maserati next to the wife's Bentley.
Isn't obvious?

 

[tornitore45]

About tool height.
A tool that is high, even a few thousands, is not going to cut until it deflects enough to bring the cutting edge into contact is just plain geometry.
A tool that too low adds a bit of negative rake to the cutting angle.
This reduce the positive rake if the tool has any.
The rake angle reduction will increase as one moves to center.
As long as the tool is not going to leave too large a pip in the center is fine.
How large is a pip too large.
If the part is sturdy enough not to climb over the tool you can be 0.030" low and nothing happens.
If you are parting a 1/4" bar 1/2" from the jaws then better be no more than 0.010" low, the 0.020" pip is not strong enough to cause problems.
Being 0.005" high you may push the bar before the tool deflects enoug to start cutting. Beside, tool/tool holding deflection is what causes chatter.


No wonder parting is so troublesome and controversial given the amount of misunderstanding of the physic and geometry of the subject.

 

[awake]

At the risk of beating a dead horse ... I have a HSS parting tool, T-shaped but also with a U-shaped relief in the top to help curl the chips for clearance. It works very well (at least, when sharp - the other day I was thinking the material was awful, but the tool just needed to be sharpened). But here's the odd thing - the U-shaped geometry changes where it wants to be positioned. If positioned in the usual way "on center," the tips of the U are in the right place, but the bottom of the U is too low. I have to position it a bit higher for it to cut correctly.

 

[Peter Twissell]

Provided the tool is ground with any clearance on the leading face, it can cut when set above centre height. The limit case occurs when the clearance is tangent to the surface of the work. When using old machines, it may help to have a geometry which forces the work to lift slightly before the tool begins to cut. This will take up any vertical freeplay in both the workholding and the slides, reducing the opportunity for chatter.