Why is this guys mini lathe parting off so well?

My Drummond Admiralty B type is not the most rigid machine so I avoid insert tooling and anything else that might upset it. However it's party trick is a Glanze parting insert tool and powered crossfeed . Consistent feed and a free hand to keep the cutting oil flowing does the trick.

One question though Ron, I would have thought that most of the play in the slides is due to the gib moving/rotating and not due to the steel flexing, so I'm surprised that increasing the size of the slide is a major upgrade. Am I missing something?

NDIY hinted at the Z axis overhang early on (Parting off outboard of the centre line of the cross slide and/or top slide is inviting more flex in the machine.). This is how far towards the cutting tip is towards the chuck beyond the carriage support point for the cross slide. In the video by the OP this is clearly a major error in the setup. One of the things rear tool post parting systems do is effectively reduce this overhang to zero. I suspect that a dedicated mounting for a parting blade that mounts directly to the cross slide at the front will have no z axis overhang and so remove one of the potential errors in set up. Parting requires everything to be set up correctly, any error will be punished. My starting point for setting up for parting is to move the carriage under the point of parting off (or as near as I can get it) before anything else. Neil's photo with chatter marks looks like it may have some z axis overhang.

Martin C

Be slightly careful with the shimming - it cures a symptom rather than the cause. The gib shouldn't bear on the bottom of the slide (or the top). If it does, it is either not being held properly by the adjustment screws, or is rotating slightly, which forces the lower corner into the bottom of the slide.

In an ideal world, the pressure on the gib should be at right angles to the sliding face - it isn't on the mini lathe (and many other designs, too) so there is a tendency for the gib to be pushed downwards as the adjustments are snugged up. There needs to be enough engagement of the adjustment screw tips to stop this. Also, as the gibs are so narrow, the point of action for the adjusting screws needs to be close to the sliding face to reduce the tendency of the gib to rotate, not on the back of the gib as is the default.

After trying several internet solutions without making a huge difference, the thing that sorted mine was to depen the recesses in the gib to take the tips of the dog-point adjustment screws. These need to be angled to match the adjustment screws, not flat to the gib strip itself. I managed to do this by clamping the gib to the slide itself (with the hole location overhanging) and drilling them deeper. I also had to add clearance to prevent the threaded part of the adjusting screws fouling the gib. I managed this with a 4mm end mill in a drill press. (A bit hairy, but worked OK).

It sounds as though mini lathe gibs would benefit from being dowelled, a procedure that has been covered here and is very well described by GHT in his Workshop Manual.

Edited By ega on 23/10/2020 14:15:13

The gib on mine was fine, but when I made a t-slotted cross slide and a gib for it, I had to do exactly this.

Philip when I said the heavier duty cross slide is a fairly serious upgrade what I meant is it is not a five minute job its quite a bit of machining to turn a large lump of cast iron into a cross slide with the T slots, dovetails and the stepped hole mount for the compound. The main reason I went down this path was wanting a rear tool post and T slots, there is not the space or more importantly the meat to achieve this on the smaller standard cross slide the lathe comes with.

I dont agree that the play in the slides is the only cause of flex in the mini lathe, others may disagree but the mini lathe lacks rigidity, thats just a fact. I,m not knocking the mini lathe I wouldnt do that as mine got me started and I managed to do some nice work on it. I would argue that you can fit good quality gibs correctly adjusted to eliminate any play but that wont cure the lack of rigidity, it will help but it wont cure it and yes the metal will flex as I know from experience.

I stalled my mini lathe whilst parting off (twice) and the second time broke the plastic high/low gears in the head, when it happened the amount of movement through the tool post/top slide was scary even though the top slide gib was very well adjusted and set up.

Ron

Hi Andy, I was actually talking about the gibs that hold the carriage down (pic below), which were riding on the edge of the underside of the ways. I used shim washers to get those horizontal, and was wondering why others are using more awkward shim strips:

But you are right about the slide gibs, the grub screws are forcing them to rotate, it doesn't look right at all, but seems to be working as there's no play now. I'll probably do something better once I can afford to buy a milling machine:

I haven't done much to test all the parting tips you've given me yet. Mainly because the Arc parting tool has the blade on the wrong side so it's not possible to part less than 16mm away from the chuck which is way further than the 1/4 - 1/2" you're recommending.

I've ordered a sample of EN1A steel to see if my problem is that I'm working with the wrong material. I was wrong to say that EN1A can't be zinc plated, it's only the leaded EN1A-Pb which can't be plated.

Would you be able to explain whether the top side of the parting blade needs to be horizontal or at an angle? The parting tools shown on the mini-lathe.com website show blades which have a horizontal top side:

However the Arc tool and many other tools and websites recommend that there is an angle. The arc tool achieves this by having a half moon ground into the top side:

I think practically speaking a parting tool with a horizontal blade would be more convinient because if used with a quick change tool post it's height would only ever need to be set once.

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.

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.

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 6x4 bandsaw I tend to use that in preference.

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

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

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