Why is this guys mini lathe parting off so well?

[Philip AParticipant @philipa30666]

Just realised how stupid my point was that the blade is on the wrong side of the Arc holder. I just flipped the blade upside down and could now make the cut right by the chuck. I just successfully parted off, much better this time, but still space for improvement. What's changed since last time:

• Cutting much closed to the chuck
• Fitted brass gibs which allow them to be much tighter
• Checked the blade was straight
• Kept steady feedrate to reduce work hardening.
• Lubrication

Face of the cut is still rough and chip doesn't look great but I'll work on that.

[Mick B1Participant @mickb1]

Looking at the ground HSS tool in the top pic of post 10:20:16, I'd say:-

i) There doesn't look to be any back taper, so the sides will potentially rub, and

ii) the top face is rough, and won't promote smooth sliding of chips off the top of the tool

Tool finish and sharpness count for quite a bit when parting. I usually stone mine with a medium India oilstone.

Grinding a dip for top rake has advantages for the particular component, but if you ever have to extend the tool outward to part off something of larger diameter, you have to make sure you don't go beyond the runout of the dip, or you get binding and potential chip jams there. And your tool height now demands adjustment. I use a cheap RDG partoff that holds a blade like the bottom pic, but I only grind the front face. Losing the top rake seems a lower price to pay than having to faff about with tool height or rake runout.

[John BaronParticipant @johnbaron31275]

Hi Guys,

This is a picture of my rear parting tool and holder.

This parting blade is 2 mm by 12 mm by 200 mm long. The blade is parallel, no tapers anywhere.  I only grind the front face to sharpen it.

Whilst the post is not as rigid as it could be, its only 30 mm in diameter and really could use a block placed over it to increase the mounting area and provide a platform for height adjustment. At the moment I've just used a block under the bottom for the screw to bear against.

Notice that the blade is dead square to the chuck and is inside the width of the cross slide reducing any tendency for the cutting forces to pull the tool block over and twist the slide. The biggest diameter that I've parted off so far with this setup has been 52 mm in steel. Though I must admit that since getting a 6×4 bandsaw I tend to use that in preference.

 

Edited By John Baron on 25/10/2020 15:42:20

[Philip AParticipant @philipa30666]

I just tried some of that EN1A steel you recommended, and the parting blade went through it like a hot knife through butter !!

[Spyro ManzuffaParticipant @spyromanzuffa86869]

Hello i stumbled upon this thread while looking on the internet.

As a new member and owner of a mini lathe (the vario ones those with the double V groove one for the carriage and one for the tailstock)

Aside from the myriad of serious problems it has (its been almost 3 months since i bought it and still haven't been able to run it properly) i got into parting and been having problems serious troubles with it since.

The only blade that has been working for me are those indexable SPB blades with the 2mm inserts. They require water or oil but they have been the only parting tool to work. HSS sharpened/unsharpened won't work for me.

To the bit about rigidity i have been trying to figure out what the ''problem'' was: after countless measurements with the dial indicator, scraping in the saddle, stiffening the compound (another serious issue there, too shallow of a dovetail meant the compound rocked ) etc i settled on the fact that the spindle was flexing.
I have the super cool 38mm through hole, that means 3mm spindle wall thickness.
With the dial and a rod chucked up you could flex the spindle flange between 15 to 25 houndredths of a mm.
Not much force is needed, add that with the 10 houndredths of the compound and tool hoolder + the who knows saddle / cross slide play and flex….
So **** out of luck, i may have to turn a new spindle…….

Fast forward and i was still measuring the flex with the dial indicator, but this time i started measuring between the bed and the headstock and there i saw it: it isnt the spindle but the headstock itself.

The lathe bed beneath the headstock is an open C section to house the motor and it flexed like a ********************

By chance i ended up turning a steel rod, 1 mm pass: stop cutting, the rod and cutter start to ring as per normal behavior; but i need to just press with my thumb (firmly but gently) on the headstock to remove the ring.

With just the thumb you can flex the headstock up to 3/4 houndredths, if you press with the palm of the hand you end up with 15/20.

So there you have it.

Edited By JasonB on 17/12/2020 17:34:34

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