How to maximise material removal rate on a mini lathe?

Been making a H shaped piece of steel, 60mm large diameter, 34mm smaller diameter. Lots of turning hand wheels and getting burnt by bits of flying swarf.

Been using a CCMT 060402 carbide tool with the insert that came with the chinese pack of tools. Normally used side is somewhat blunt so been cutting left to right to use the fresh, usually unused edge.

Before I get a new pack of inserts I wanted to confer with the hivemind for some tips on efficiency.

I think the consensus for general turning on minilathes with CC** is to use CCGT finishing inserts since they are sharper. Would this carry over to trying to remove material as fast as possible or are CCMT still good for roughing?

Assuming I don't have to tilt it for clearance should I keep the toolpost as close to 90 degrees as possible?

Should I go for a different tool (holder/geometry) other than SCLC with CC**? My bench grinder is currently slightly dead so HSS for now is out.

Also have to say somewhat impressed by how deep the lathe can cut. I noticed the depth of cut was being limited by the cutting length of the insert rather than by chatter or motor stalls.

It seems as though you are managing to do quite ok as is... just listen to what the lathe is telling you then alter doc / feedrate / speed accordingly, if your not stalling your machine then it seems as though it can handle what you're giving it. In my opinion CC** inserts are fine for roughing out, some may say that a larger tip radius would be better but if it's doing the job why worry, finish off with smaller tip radius & higher speed, carbide tips are designed for high speed performance. There are formulae for working out doc-feedrate- rpm- metal removal rate - time et. so unless you're on a production run...

​George

1. Get a decent piece of HSS

2. Grind a RH roughing tool with a ridiculously steep top rake and generous front relief.

3. Watch it slice off material like butter on your low powered lathe.

+1 but ... 'My bench grinder is currently slightly dead so HSS for now is out'.

​... hence Rainbows is stuck with his CC** inserts...

George.

Much studied in industry because time is money!

It comes down to how much power your machine can put into the cut and for how long you and it can keep the pace up. Vibration, overheating and the risk of breakages limit what can be done.

Some typical figures for metal removal in Watt-Seconds per cubic mm are:

• Aluminium Alloys - 0.3 to 1.1
• Cast Irons - 1.0 to 6
• Steel - 1.9 to 9

As expected, it takes less power to cut Aluminium than a hard steel, and the power needed varies across a wide range.

Assuming mild steel comes off the job at 2 Watt Seconds per cubic mm, then a 500W output mini-lathe should be able to remove about 15000 cubic mm per minute. To do that you would have to keep the motor at maximum power output by balancing depth of cut and feed rate, ie not too fast and not too slow, say a couple of mm deep fast fed at about 90% of max rpm. The machine would be badly stressed and it's motor & electronics would quickly get hot . Unkind and foolhardy.

In theory the cutter doesn't matter much, in practice the properties of the metal being cut favour more-or-less sharpness and relief. Most important the cutter has to stay in shape despite considerable heat. Carbon steel is very hard, but it loses its edge at low temperatures. Good work can be done with it but only slowly. The various grades of HSS are much better, but they are all at the low end when it comes to removing metal quickly. For real speed you need (in order of rising wonderfulness and cost), carbide, coated carbide, cermets, and boron nitride.

Trouble is, getting the best out of carbide and friends needs a big fast powerful machine, the more monstrous the better. Like a machine centre costing £1.5M with four 12kW motors running at 15,000rpm each.

Although a mini-lathe at top speed has some chance of making carbide work efficiently, 600W isn't enough grunt to do a proper job. With at least a kilowatt at 2200rpm plus, a roughing cutter is best for removing metal quickly. They don't cut like HSS at all; rather than cutting with an edge the metal is sheared off just in front of a surprisingly blunt chamfer. More like a wedge than a knife. Efficiency is high partly because the high temperatures involved soften most metals.

I've found using sharp carbide inserts at high speed with fast deep cuts does a good job on my lathe. The sharp edge cuts like HSS but can get much hotter - red-hot chips fly like shrapnel. Despite the brutality, the finish can be very good. It can also be very poor; usually I think because my maximum 2200rpm is on the slow side and/or I have the feed-rate too slow.

I hope you're all wondering if this is a sensible way to run a hobby lathe? Not for long it isn't! I keep a careful ear out for signs of distress, and only do hard work in short bursts within what I perceive to be the machine's envelope. One day I'll over do it. Driving a lathe hard isn't fun either - I'm a hobbyist, not slave labour. Generally I don't rush things at all; I'm more interested getting the job done right than removing metal quickly.

I did loads of work on a mini-lathe but always treated it with respect. It's not a heavy duty tool.

Dave

Rainbows,

This from a beginners experience with a half horse power lathe that maxes out at 1000 rpm......

A properly formed and sharpened HSS knife tool as recommend by Pete is very forgiving and you can take some hefty cuts and still have a reasonable finish. I followed the shape and angles as suggested by Sparey in "The Amateurs Lathe", and was impressed with the performance.

The difficulty is, you have to make them yourself and it's not an easy thing to do, particularly so as your workpiece needs two of them; both a left hand and right hand version.

If the grinder's an ex parrot go the CCGT route, they are excellent, and don't need high speed or lots of power to work very nicely. Be warned however that you'll break a few when learning how to use them; chatter and interrupted cuts will crack them in an instant.

Again the cuts can be pretty meaty and you'll get a good finish. When I was in your boat I spoke to Jenny at JB Cutting Tools and it was she who recommended a CCGT insert, specifically as being suitable for a low powered lathe. Mine are CCGT 09T304 NF25, so a bit bigger than yours. She supplies the holders too at reasonable prices. Usual disclaimer.

http://www.jbcuttingtools.com/epages/es461493.sf/en_GB/?ObjectPath=/Shops/es461493/Categories/%22Turning%20inserts%22/Ground__Polished_Inserts/CCGT

In short the quick fix is the CCGT insert; you'll be delighted.

Eug

Edited By Eugene on 01/06/2018 19:56:32

I think the question was rhetorical? Rainbows seems to have demonstrated just how much you can hack off with a mini lathe.

(Not often I see a lathe in more of a mess than mine)

Only comments, CCMT best for that sort of abuse, the geometry is stronger and will blunt less rapidly - it also heats the steel which makes cutting easier - if the feedrate is OK. Don't forget to speed up as the diameter drops.

CCGT better for finishing.

With CCMT in particular you need to make sure that as much as possible, if not all, of the corner radius is in the cut.

Took a 3-4mm deep cut in medium carbon steel on the SC4 for the last lathework series episode with a CCMT insert and it left a satin finish on steel that tends to look like a ploughed field.

CCGT are ground to sharp edges so you can more easily get away with the light cuts and lower speeds usually used with HSS.

The mechanics of a mini-lathe can easily cope with 3mm cuts in mild steel, but whether or not the electrics can depends on the types of motor, quality of control board tool, quality of tool and experience of user...

Toolpost angle should be chosen to present the tool edge as well as possible while avoiding any risk of hitting those nasty whirly jaw-bits.

Neil

Holy Ancient Psychic Tandem War Elephants! Looks like Stub hijacked my computer again.

Neil (dopellganger)

Get a new grinder, it won't cost more than a few TC tips, get some HSS blanks, 1/4" / 6 mm is as big as you need. If you have an angle grinder, use that to get the rough shape.

With TC tips and a low powered lathe, if you get a dig in, ie not enough power to keep cutting you have a fair chance of a chip off the tip, I know I did it a few times myself when I started using TC tips. Mostly the cause of the breaks was too slow rpm, too deep cut, you are better to take a shallower cut with a faster feed rate, and keep the revs up, that reads to me that what is needed is something like a Chipmaster.

Ian S C

Makes one wonder if the part couldn't have been redesigned as a bar with two washers?

Regards Ian.

My experience with lathes is quite limited, I used an old Myford many moons ago but it was all very basic stuff.

In about a month when the shed is ready I will be getting myself a mini lathe and from what I,ve read and all the videos I,ve watched I thought of them as light duty hobby lathes that shouldnt be pushed or overloaded.

So I was quite surprised to see your approach to this job, I,m not criticising after all what do I know I,m a beginner but just wondered why you want to remove as much material as possible and in the shortest time.

Edited By Ron Laden on 02/06/2018 13:49:12

Guessing, it is because he is impatient or has a rapidly approaching deadline.

For reference this is the final part, 5 points to whoever can see why it is actually somewhat impossible to build in its current state. I did debate fabricating it but had the round bar already at hand. Also get to have the fun time of grinding a big flat on the top of the middle cylinder.

Thinking back 500RPM at 60mm might have been slow for carbide? Will have to experiment with kicking it into high gear.

The HSS recomendations reminded me of a thread somewhere where someone ground a tool from a south bend manual or the like. Took slow RPM and feeds (to suit a plain bearing lathe and carbon tools) but took a really deep cut. Made huge swarf coils too if I remember right.

Also while the lathe can evidently get the job done as mentioned Im inpatient and want to be able to do it faster next time

What a peculiar question! Why wouldn't you want to remove material as efficiently as possible?

As I mentioned I,m a beginner and can appreciate someone wanting to remove material as quick as possible but I guess in this case I was thinking more of the mini lathe, but if it can cope with it then its fine.

Edited By Ron Laden on 03/06/2018 14:27:02

I think it's a very sensible question Ron! All machines have a limit to the amount of work they can do without damage, and yes - it is possible to break a mini-lathe!

Cars are generally most efficient when cruised at about 56mph. On a motorway, most of us would choose to do 70mph, or ahem, a little faster. Not as efficient, but you get there sooner without blowing up the engine. Enter the same car in a rally. Now you're racing on back roads, lanes, and tracks. To stand any chance keeping up with real rally cars, you will have to drive aggressively. Loads of heavy breaking and hard acceleration in low gear. No cruising at all. Poor fuel economy is the least of your problems; the brakes will overheat and fail. Even worse your engine's cooling system ain't up to it either. It won't be long before the car is completely thrashed.

Mini-lathes are capable machines but they're designed for hobby work, not flat-out metal ripping. The breed vary. Some come with motors as low as 400W, others 600W and/or brushless. A 400W motor will overheat faster than a 600W motor and need longer to cool down between sessions. Some mini-lathes are fitted with metal gears in the HI-LO gearbox, others with plastic. Plastic gears are quiet, cheap to replace and break before something more expensive does. Metal geared head-stocks can be pushed harder, but it's liable to be far more painful when they break. And so on with the electronics etc. Best to treat the machine with respect, not to give it a good kicking just because you're heavy handed and impatient. (Buy a bigger lathe instead!)

As demonstrated at the beginning of the thread, Mini-lathes can cut a lot of metal quickly if you don't mind taking calculated risks. That doesn't mean it's sensible to drive like that all the time because it shortens the working life of the machine.

Difficult to describe in words how hard to push a mini-lathe 'normally'; I learned by listening (you want to hear the motor work, but not straining), and periodically checking how hot the motor was getting. This varies - for example taking heavy cuts at slow speed will overheat it disproportionally compared with normal operation.

You have to experiment to find what speeds, depth of cut, and feed-rate work best with different metals, diameters & types of tool. It's not critical and you soon learn what feels best. You don't want to scratch or rub the job, nor hack into it like a demented gorilla. I suppose most roughing cuts I took in mild-steel were 0.5mm deep. The lathe could certainly take more than that but I was more nervous back then! No need to be cowardly, they're reasonably sturdy. Start gently and work up. Don't force it. Enjoy!

Dave

As was pointed out there,Sparey was not too far off with his tool angles for HSS. OK,you might have to grind them yourself,but the material is cheap enough and if you break or chip it,it,s no big deal.

If I was interested in removing material at a rate of knots,I would buy a bigger machine with a huge motor hanging out of it,and watch my electric meter doing laps ! Your lathe will let you know quick enough when it,s being pushed,just make sure you listen to it,it might not tell you twice !

Hobby work is not about speed or high output,it,s all about enjoyment [and a few sad stories!]

You know the old saying "Horses for courses"

About cars on motorways,my Hiace is happy at around 60mph,good return,but start doing 70 plus and the fuel disappears rapidly,same thing,I suppose.

So I always used to believe. I've always been obsessed with 'hyper miling' by efficient driving and the advent of accurate MPG meters has fed my obsession.

With the use of cruise control my 'new' car seems to show its best economy at 70. On a run back and forth to Durham it achieved 67mpg, on more normal runs back and forth, mostly on dual carriageway, it was giving a consistent 58 cruising at about 66, but upping cruise to 70 is pushing the average towards 60.

The Mondeo was similar, benefiting from a 6th gear, but topped out at about 60mpg on long motorway runs, and averaging 49 on mixed driving. The Stilo was most curious giving about 50-52 mpg regardless of driving conditions from motorway cruising to round town...

Neil

My hobby is not making piles of pretty swarf. The machines are tools for producing parts, and using them is just work - tedious work most of the time. Then there's the wearing out the machine argument; is taking a decent cut that doesn't slow the machine down or damage the tool/part any worse than continually racking the slides to and fro tickling off material?

The main reason I replaced my minilathe after 10 years use was that the WM250 drastically improved my productivity, capabilities and failure rate(as fewer cuts reduces the opportunity to cock up). Selling it and most of the accessories for the £350 I paid for the bare machine was a bonus

I really must get around to fitting all the CNC conversion parts I have to my mini-mill, having that work while I do something interesting or useful will be a big help

As for the most efficient at 56mph, that is entirely dependent on the car's engine and gearing. Going back 20 years, we converted several; 4 speed Fords to 5 speed gearboxes. I didn't see any improvement in the cruising economy, as the overdrive gear allowed the car to run at its sweet spot 12mph faster. My current 3.0l Omega costs about £3 more to do an 800mile trip at 90mph than it does at 70, something I have done many times.

Edited By Nicholas Wheeler 1 on 03/06/2018 18:50:42

I have found that cutting steel with aluminium inserts works really well on the home hobby lathe. Mainly because the hobby lathe does not have the power or rigidity of a commercial industrial lathe, although most do still have the rpm range. The Al inserts on steel will still outlast a HSS tool and run a higher surface speed. I have also found that the smaller nose radius has a lower power requirement also. Another approach is to rough out using a grooving tool can also be really effective on smaller low power lathes. Like a 2mm wide insert with a 0.2mm corner radius or even a 0.4mm corner radius. There are MDT(Multi Direction Turning) inserts for some of the grooving series tools.These tools allow you to plunge and turn left hand or right hand, with a 1mm radius insert. Also is very effective when turning an area that is relieved, like what you are turning in the photo. Turning with a left hand and a right hand turning tools also work. Like using DCMT style inserts or the DNMG style inserts, are 55 deg included angle inserts. I use DCMT11T3xx , mainly because of the huge range of geometries that are available, and industry standard inserts are often cheaper than the smaller hobby based sized inserts. I have a selection of inserts in my album. Chip control is the real key to any turning, and finding the depth of cut and feed rate to make little chips instead of long strings of swarf is equally important. With grooving, this is easily achieved with the 1/4 or 1/2 turn of the cross slide for example, or the pecking style of Z axis feed when turning along towards or away from the chuck. Seldom have I found a negative steel insert for industry to be suitable on smaller lathes. The ones that are effective for me have been the cermit sharp finishing inserts, but I now rarely use them.