Why is this guys mini lathe parting off so well?

Hello, first time posted here. I just bought my first lathe the other week, I went for a min-lathe due to space. Everything was going great, made my first delrin parts to fix the kids nerf gun, and then made my first 25mm steel part. And then everything went wrong when it came to parting off the steel.

I did some reading and found that mini lathe users tend to run the machine in reverse to part off. I also bought a HSS parting off tool which Arceurotrade said has been tested with the lathe. Yesterday I managed to finally part off my workpiece but it still looked like the tool was cutting small chips off the workpiece rather than a smooth slice.

Then this morning I saw this video, of a chap in the US testing a new tool on the mini lathe. His tool is taking lovely smooth slices off. I can't figure out what he's doing differently:

https://youtu.be/RWhIa-3gMfg?t=98

I've ordered some taper bearings and upgrade gib strips as I've read that these help with parting off.

Faulty link I think. For a start there should not be a "." in the word "tube"

Worked OK for me when copied and pasted. Though I would not say that was cutting particularly well.

Edited By JasonB on 22/10/2020 08:16:23

The spring tool holder effectively moves the pivot point from somewhere under the toolholder to the centre of the hole with a slot in it which is above the cut. Increased cutting forces thus tend to decrease the inthrust of the tool tip rather than increase it, so no dig in's. The positive feedback situation has been altered to negative feedback.

regards Martin

Parting is tricky and there is plenty to go wrong. The tool alignment needs to be right (especially being at 90 degrees to the spindle) so that it clears the swarf and the sides don't rub. Use a square to set the toolpost angle rather than doing it by eye. Everything needs to be as rigid as you can make it. Use lubrication.

Probably the trickiest aspect when you haven't done it before is to get a 'feel' for the required feed and speed, for example our natural reaction when things get noisy is to back off and do things more gently. With parting this can be the wrong answer and sometimes you need to grit your teeth and run the lathe faster and/or stuff the tool in harder to get through the chatter.

A rear toolpost for an inverted parting tool is a good thing to have if it will fit your lathe. It should be reliably square to the spindle with no faffing about, the swarf falls away under gravity and it helps with chatter because the forces on the work are downwards instead of upwards. Running backwards with the tool inverted in the front toolpost achieves most of this too... but only if your lathe does not use a screw thread to mount the chuck.

You will find plenty other threads on here and other places about parting off. Do read them but at the end of the day you will only get a 'feel' for what is right by doing it yourself.

I don't claim to have achieved perfection myself, but I have had both successes and failures.

Good luck!

.

I just copied and pasted that link on the iPad, and it works fine

... directs to here:

https://www.youtube.com/watch?v=RWhIa-3gMfg&feature=youtu.be&t=98

... as if by magic

**LINK**

MichaelG.

Edited By Michael Gilligan on 22/10/2020 08:42:37

His parting is, like JB says, pretty naff. We don’t actually know what material he is cutting (only watched the first 3 1/2minutes it took to make one small parting operation).

Clearly it is the tool that is making his work (presumably better than previous?).

A rear parting tool is better, for flexible set-ups, as the forces are downwards - generally a more rigid situation at the rear. I part of at the rear with the lathe turning in the normal direction(a screwed-on chuck)

Parting off outboard of the centre line of the cross slide and/or top slide is inviting more flex in the machine. Any inherent flex in the set-up will initially result in the cutting point being depressed - below the centre height if that is where it was set while static , and altering the top rake of the cutter - so QCTPs and flimsy extended cutters are bad news.

His arrangement is working (of sorts) with his spring tool, but apart from that it is mostly naff.

Andy's point should be emphasised - parting-off is famously difficult, and it's much harder to get right on small lathes than big ones. The operator has to balance cutting forces against his lathe's tendency to bend, and cutting forces vary wildly. They depend on the tool, it's exact height, feed-rate, surface speed, and the material. Problems multiply on small machines because they have light saddles that can lift, top-slides that might twist, and a bendy tool-post holding a long sticky out parting tool. Skilled work! Parting off on a big machine with power traverse is a doddle compared with a mini-lathe.

Brass was less trouble than Aluminium and Steel on my mini-lathe, but I avoided parting off preferring instead to saw and face-off.

What sort of steel are you working with Philip? Mild-steel formulated for enhanced machinability (EN1A, even better En1APb) is significantly friendlier than ordinary structural mild-steel (EN3). Many other steels don't machine well and some are vile. Beware of unknown scrap! Better for beginners to buy a machineable steel rather than get into a disappointing muddle with one of the awkward squad. Difficult metals can be tackled later.

Dave

Hello Philip

We have heard this song before and I plan to get rich on solving the problem.

More rigid tool.

Shorter overhang and stiffer spindle

Can we have a picture of Your setup showing lathe,tool and setup?

Such kind observations from Mick B1 !!

Yes, I too thought I saw the toolpost nodding, too much, I thought and I was not to impressed by the cutting.

It was no better than my own results, and that,s saying something !

As S-O-D said, parting off is never a simple operation, I get good results/bad results with my Craftsman lathe although I use a rear tool post all the time. Seem to remember Sparey made the same point in his book. and he did mention a spring type holder for threading, which he claimed gave better results.

I now use tips from China, held in a thin steel strip which I find work fairly well. I did not buy the complete holder, just the blade and tip in order to try it out The "holder" was made from what was to hand, but worked so well that I made two. They are crude, and I mean rough, but they work. It might be worth Philip,s while to try one of these units

Well I think that could be classed as video that demonstrates how NOT to part off. Apart from leaving the chuck key in the chuck, which even on a mini-lathe is a bad move, there are a number of other faults apparent.

I would say that the parting tool holder is too big for that lathe, it has way too much distance between the toolpost and the tool tip. There is clearly much too much slack in the system whether that is from the top-slide gibs, the toolpost clamp or further down I can't tell but you should definitely not be able to see that amount of movement when the cut is applied. The top-slide is wound way too far out, the idea should be to get the tool supported so that a line down from the tool goes through the toolpost, top-slide, cross-slide, saddle, bed. I appreciate that the chap has wound it out so that the tool holder will fit but that just supports the idea that the tool holder is too big for that lathe.

A general tightening of gibs, removing other slack, clamping the saddle to the bed, clamping the top-slide and reducing tool overhang as much as possible would probably get a better result than fancy tool holders and replacing bearings etc.

John

The best parting tools I’ve used on my mini lathe are the HSS chip breaker type.

**LINK**

The best parting tools are the ones that work. My lathe came with a rear tool post. It has a cutter holder with a simple blade installed. I ‘tickle‘ it up on the belt sander occasionally, set it to cut very close to the chuck, lock the saddle, engage power feed and concentrate on applying cutting oil as appropriate. It just parts off, with most materials I use, with no issues other than perhaps altering the feed rate.

Hello and welcome to the forum

Parting off can be the most daunting exercise for a novice on the lathe.

When machining steel once you start cutting you must keep cutting otherwise it work hardens. EN1A-Pb is a great steel to learn with as it is free-cutting though it is not suitable for all applications.

General guidance for parting off -

1. Parting off tool must be sharp, set to the centre height of the workpiece and absolutely perpendicular to it.
2. Reduce lathe speed by 75%
3. Lubrication helps.
4. The narrower the parting off tool the less stress to your lathe...
5. But don't allow swarf to build up in the groove you are making.

When parting thicker pieces some people like to take the tool part way in, back out, move the tool to one side, and go in again. This creates a wider groove so swarf and the workpiece are not rubbing on as much of the tool leading to friction and heat build up. Depending on the thickness of the part, the material involved, and the confidence of the operator (or lack thereof) this process may be repeated more than once to complete parting off.

Other things that people do to achieve successful parting off include inverting the parting off tool in the toolholder and running the lathe in reverse. Much depends on the toolholder you have since the cutting point must still be at the centre height of the workpiece. DO NOT try this method if the lathe chuck is threaded onto the spindle!!! Also, may subject an unmodified mini-lathe to unusual stresses since you are now also reversing the forces acting on the carriage - i.e. trying to lift it instead of pressing down. YMMV.

If you are still nervous about parting off (which is perfectly understandable) just use the parting off tool to go part of the way through the workpiece then back the tool out and switch off the lathe. Finish the job with a hacksaw. There is no shame in taking this approach, it just needs a little more elbow grease.

In my experience, the things that make the biggest difference when parting are:

• Well-adjusted lathe.
• Sharp tool set on centre height.
• Lubrication into the cut, by brush is fine.
• Confidence, so that you keep up the feedrate.

Neil

Finally, this isn't parting and you can see some chatter, but it shows the sort of DOC (3mm) you can achieve on a mini lathe at modest diameters.

Edited By Neil Wyatt on 22/10/2020 11:39:50

Re. video... How NOT to part off !!. he says later that he's 'happy with it' ... easily pleased I guess.

+1 for Journeyman's comment.

George.

Yes, I think I was too harsh there. I'd like it deleted if possible.

Yes that compound was rocking badly. I had to stop watching it. Thought it was going to go under the work.

I do not have a mini lathe so cannot comment on how to tighten things up. But it needs to be much better than the video shows.

I have a friend with a very nice Colchester student lathe. A beasty. He hates parting off & has reverted to putting a bed protector over the ways & using his angle grinder with a cuttoff disc. He says it is quicker & wastes less metal.

I personally have done quite a bit with various blades & always go back to my Eclipse blade in the home made holder. Although i do sometimes struggle with it. I think i am going too easy on the feed rate.

A learning curve i guess.

Steve.

I feel a bit uncomfortable about being critical, but the feedrate is far too low, it's scraping not cutting, and the gibs desperately need adjusting. The flexi toolholder is pointless unless the lathe itself is set up not to flex.

The only thing more worrying is how much trouble the OP must be having to feel that the cut in the video is so good!

Phillip exactly which of the parting blades did you get from ARC?