Do you lock the carriage and top slide?

Assuming that the tool is indeed advancing at right angles, you could try swapping out the tip; the present one may be cutting unequally across its width. Difficult to say from the photo but it should be examined critically under a lens.

On a deep cut you may find it better to set the tool projection so it will go half way and then set the tool further out of the holder to complete the cut.

A 2mm blade should be OK, thats what I use and don't get any deflection. If one corner of the tip has become damaged or worn that will also tend to push it sideways

What sort of diameter are you cutting as that will affect speed.

I seem to manage OK without a rear post and a similar 2mm blade, 1/16" slice off 2.5" cast iron and less than a thou variation in thickness!

JasonB:

Did you use tailstock support and if so at what stage did you retract the centre?

I work on a Sieg SC3, use a HSS-Co8 parting blade clamped in a home made holder that clamps the blade in the side face. The blade has no top rake, only ground on the front. HSS-Co8 is much tougher than a straight HSS blade, the ones I use are I assume Chinesium but they work. When I first had the lathe I had a carbide insert holder, but gave up on it after having to use a HSS blade when the supplier was shut.

Saddle is clamped to the bed, after positioning. Compund slide is tightened but not fully clamped, the reason for this is so I can make a wider cut than the single blade width. I do this so that the swarf clears the side of the blade, I take a plunge cut, then move slightly to the side to take a second as I move the blade in. Cutting speed is around 400RPM, dependant on diameter, adjust to suit diameter as you feel confident. Don't overhang the compound slide from the cross slide as this will allow movement on a small machine. I have cut unto 75mm diameter in steel, it took time but it did cut. Use a cutting oil to keep the tip cool, I use a spray cutting fluid from Toolstation. Most of my work is with steel, softer metals like brass and phosphor bronze are less of an issue.

My two penn'orth:

Looking at you photo of the tool, I'm thinking I can see a tiny wedge of support on the lower edge of the insert location that suggests the insert is skewed rightwards. Can't tell if the insert is properly vertical or not, but if it isn't the right-hand side of the tool might be rubbing, unable to cut, and so forcing the left-hand side to dig in further and deflect the supporting blade that way.

The fact that there's also a significant bit of the support projecting forward underneath the insert suggests to me that there's summat up with its location.

I recently turned a set of buffers from 35mm dia ali and Jasons advice above is pretty much what I found worked for me using a 1.5mm HSS tool. Moving up to a 3mm tool I would avoid if my experience with one is anything to go by. If you are using a mini-lathe as I am I would argue that a 3mm parting tool is too much for them. I have a HSS 3.2mm tool and the lathe just doesnt like it, even with ali, I tried everything I could but it was not happy.

Edited By Ron Laden on 11/10/2018 14:03:25

Edited By Ron Laden on 11/10/2018 14:18:05

I hit this problem occasionally with blade type holders when the blade is well out. It seems more likely to happen when the slides are unlocked.

My pet theory is that if the tool is slightly off-square, and the blade is extended, and the slides are unlocked, then there's enough leverage to either move the carriage or to flex the blade. Unfortunately I'm not able to prove it!

I've not found it easy to align a parting tool exactly square to the work. More likely one side or the other of the tip will start cutting first putting a sideways force on the blade. How big the force is I've no idea, but it might be enough cause petro1head's problem.

What's the best way of making sure a parting tool is exactly square as opposed to 'looks OK to me guv'?

Dave