Two newbie turning questions

I hope some old hands can offer me some advice here:

I have a new Chinese 14x40 gearhead lathe, and it seems to be a reasonably solid unit although I'm still just finding out what I can do with it.

Question 1.

I have chucked a 500mm length of 25mm 4140 with about 80mm extended from the jaws. I can turn 50mm of that down to 17mm in a single pass at 2000 rpm and get a good finish (see second question) with a carbide insert. It's like a knife through butter, the machine is totally smooth and feels and sounds like it could do it all day. After letting it cool down a little I use my Mitutoyo 0.001mm vernier micrometer to measure the work - ok, the absolute value is a little out, no surprise, but the diameter is constant to within a small fraction of 1 micron i.e. approx

0.0002/0.0003mm along the length of the 50mm cut. It's a bit hard to visually interpolate between the vernier divisions on the micrometer, but it's *way* less than 0.001mm variation. Then, just for the hell of it, I mounted a DTI (0.01mm per division) on the carriage and rotated the chuck by hand. Shock and horror - I see just under 1 division (~ 0.008/0.009mm) fluctuation per rev. It's totally consistent, and if I move the carriage along the length of the 50mm the DTI behaviour remains identical. I ran the lathe at the lowest speed (70rpm), and the DTI response is exactly the same, right along the 50mm length. I removed the DTI and set about measuring diameter again, rotating the chuck 30 degrees, measure, rotate another 30 degrees, measure, etc. Dead circular. Did that at several places along the 50mm length and every measurement is within that fraction of a micron.

So here's the question; How is it that I can produce a perfectly circular result, yet which appears to be off-centre from the spindle axis? For the life of me I can't visualize the conditions that would lead to this.

For the record, I took another pass again at 2000rpm but with a 0.5mm DOC. The result is exactly the same.

Question 2.

As I said above, the cut was taken at 2000rpm. I couldn't get an acceptable finish at any speed below that, and it didn't matter whether the DOC was 0.5mm or 3 - 4 mm. Even more strange (to me, at least) is that the finish that I did get with a nominally 'finishing' insert (larger radius tip) was worse than with a smaller radius tip that is listed for 'medium' turning.

The inserts are TaeguTec CCMT 09T304, MT chipbreaker, TT5100 carbide, and CCMT 09T308, FG chipbreaker, TT5100 carbide, both in an SCLCR type holder.

The chip breakers don't work, possibly because of the speed(?). I get a single continuous streamer of swarf flowing up and back towards the tailstock before gravity takes over after a couple of inches and it curls down onto the cross-slide. Inevitably it falls forward towards the chuck and if I don't stop the carriage feed (thereby breaking the chip) the tangle of swarf gets caught up around the workpiece and mars the finish.

I almost don't even know what question to ask here. I would have expected that I could get a good finish at a lower speed, and I would have expected the chipbreaker to work. I would have expected the larger radius insert to give a better finish, not a worse one.

I will be getting some HSS to gain experience with grinding my own tools, but that won't happen for a month or more because my 'real' work is consuming me night and day at the moment.

All observations/suggestions will be gratefully received.

Roger

I can't help answer your questions but want to mention that just because your micrometer says it measures the same at many points around the diameter doesn't mean the part is round. See the following link

I hope some old hands can offer me some advice here:

No help with your question, but what brand of Lathe is this? I have been thinking about a 14 X 40 geared head lathe for some time. But I have heard that, even though many of them come from the same place, the quality "control", finish and details of some brands is a lot better. Any comments? Pete Stanaitis, western Wisconsin, USA

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rl> I hope some old hands can offer me some advice here:

...

... I bet your feed is too low for chipbreaking. My rule of thumb for carbide; increase your speed, increase your feed. When I got trouble seems its always one or the other. Find the vendors table for each individual insert, they will show a box of feeds, DOC, and speeds for good results. For your machine, these numbers are at the extreme top end of what the lathe can do.

On my 10EE, I use only CNMG inserts with the LF (light-fine) chipbreaker for finish cuts. I often have trouble with the long stringys though. For ruffing, I use a standard CNMG insert (any auction special), minimum 0.100" DOC, .007" or more IPR feed, and then run the machine fast enough for the chips to turn blue. 20 years ago, my instructor taught me: If your chips aren't blue, you're going too slow. Your machine may not have the power and rigidity to do this.

Personally, I'd strongly recommend that you use HSS almost exclusively on this lathe. HSS excels at great finish and accurate cuts at the speeds and feeds this lathe is made for. Brazed carbide bits might work well for you also.

Karl

Not really what you were getting at, but I'm a bit suprised you can read a vernier caliper to within one micron. Are you postive you are interpreting the readings correctly?

Hmm. First off don't run the dial indicator on the workpiece under power. That's bad for it.

Did you remove the workpiece from the chuck, and then replace it there before performing this test?

Jim

Hi Pete,

I'm in Australia, so my comments won't be directly useful to you. The lathe I have now is whatever Hare and Forbes (one of the bigger machinery importers here) bring in and sell under their own name. I say 'the lathe I have now', because the first one that was delivered made some very subtle noises in the headstock, so I went looking and found a couple of problems. They looked fixable, and H&F got a professional machine rebuilding company in to repair it. I stayed and got my hands dirty while they pulled it down, and the further we went into it the more horrible it became. The rebuilders told H&F that it would cost more to 'try' and put right than it was worth, and, all credit due, H&F replace it with the next model up. This machine runs well, but the finish is poor by comparison with any of the older 'name' brands. What you will find is fairly crudely finished castings, paintwork that is crudely applied and obviously has no base preparation (chips/flakes as soon as you look at it), and other ugly things that make you feel less than happy. But if you can ignore that for a while and look at the mechanical side of things I think you can see some differences. Make sure you run the actual machine that you are going to buy, and look and listen to the operation at different speeds. Listen and observe the power feeds and leadscrew operation. Gradually tighten up the gibs and see how the 'feel' changes. Extend the topslide and see how rigid (or otherwise) it is. You have to realize that you're not buying a Hardinge, but it is your hard-earned dollar that you're parting with, so check everything you can think of - don't just stand back and think of how good it will look in your garage! If you can find a machinist friend who will have a look at a couple of models with you, so much the better. I'm new to actually useing a lathe, but I have been around some well equiped workshops for a long time, so I had a bit of an idea of what I should see. Nevertheless, that first one blinded me in the showrooms.

Good luck, and don't be in a rush to get something. You're going to have to live with it for a long time.

Roger

Pete & sheri wrote:

[...]

A possibility - the drive gear is pressing against the spindle-mounted gear while under power at 2000 rpm, and moving the spindle's axis position. At low speed a different gear is engaged, and turning by hand there is no real force.

As I said, only a possibility.

btw I find your actual measurements slightly unrealistic. Are you sure you did them right?

Jim,

He actually wrote "vernier micrometer"...

Regards,

Robin

The most important part of leveling is that the carriage remains level as it travels down the ways. if the tool tips in or out a half thousandth you lose or gain a thou. in diameter.

John

Thanks Karl,

For medium work with the MT chipbreaker and this steel TaeguTec are showing 1 - 3.5mm DOC and about 200m/s at 0.2mm/rev feed. I guess I was only making about 75% on the speed, and I estimate about the same or a little less on the feed. I was feeding by hand so that I could feel what the machine thought about what I was doing. It was very smooth and seemed to be completely untroubled by the cut that I was putting on, but the chip was glowing red for about 1/4" or more from the cutting edge, and I was feeling sorry (and a little nervous) for the insert, so I didn't push it any harder. I'll get some coolant going and see just how hard I can push it, just so that I know. And then I'll get some HSS.

Roger

Karl Townsend wrote:

Hi Jim,

Yes, I know I shouldn't have done it, but the surface was very smooth... (hangs head in shame).

I agree, it seems a bit strange to be able to measure (and visually interpolate below) 1 micron with a workshop tool (after all, the wavelength of light is abour 0.6 microns), but there you go. The body divisions are 1 and 0.5 mm, the 50 thimble divisions are a decent size, and the physical movement to get from one vernier division to the next is enough that I can interpolate to 0.2/0.3 (under a mag-lamp). I don't know what Mitutoyo claim for absolute accuracy, but it's very repeatable and certainly makes for close comparisons.

No, I most certainly didn't even touch the chuck key.

Roger

jim rozen wrote:

Hi John,

Apart from the weight of the lathe itself, the base is pretty damn heavy cast iron, and I tied both ends of the base together at floor level, just so there couldn't be any tendency to spread or otherwise move.

I borrowed a *really* nice precision level from the university when I was setting it up, and lengthways, front/back, etc is all within 1 minor division on the scale.

Roger

john wrote:

Then it sounds as if the machine may be cutting slightly tri-lobed.

Jim

I have a new 12 x 36 chinese lathe that's still in the crate. I have the cabinets also, which are about 1 1/2 - 2" shorter than what I'd like them to be. I also didn't want the metal cabinets sitting directly on the concrete due to potential rust stains. I was thinking of laying down the correct level of plywood under the cabinets. This thing weighs about 1,200 lbs and I'm concerned about the level gradually changing and/or twisting the bed as the plywood compresses over time.

Is it a bad idea to lay some plywood down under the cabinets? Figure if I'm going to learn how to run this machine, I'd best start off with it set up correctly. Sorry for the newbie questions.