1.1 kw brushed motor Torque

Good day all,

I am thinking of buying a new lathe, as I live in Switzerland the choice has been narrowed to either the Sieg SC4 or the Optimum 2406v, Whilst the SC4 has a lot going for it the bed swing (105mm) is a bit limiting, for say a 9 inch flywheel, therefore I am leaning towards the Optimum (125mm swing), however the 2406v comes with 1.1 kw (P2) brushed motor with two speed ranges by belt change. lower range 150-1200 rpm

My questions is the following:- will there be enough torque available of low rpm (300) to turn a 9 inch cast iron flywheel? I am not worried about taking heavy cuts as time is not an issue, Any comments would be most appreciated!

Best wishes, Mark

Mark,

The low speed range is for that size.

Usually when we talk about swing, it is the diameter of the largest item, so for instance a centre height ot 3.5 inches like my Myford is a swing of 7 inches. It used to be that Americans went by swing and Brits by centre height, presumably so that they could fail to communicate more effectively!

But anyway, while I don't have any experience of either of these machines, my feeling is that these variable speed drives are fine for slowing the machine down for light work, but not so good for serious machining of big items. This is because the maximum torque at low speed is no more than it was at high speed, when for turning large diameter items you really want more torque at the low speed. This is especially true for turning a cast iron flywheel, where you may need a decent cut to get under the hard skin that castings often have. So although my own Myford has a VFD, the pulley steps and the back gear still get a fair bit of use when larger items come up, since you can step the speed down and then take a decent cut. It takes long enough at slow speeds, without having to reduce the cut to a mere skim. Also, if you are having to take light skims, you will probably want to use HSS tooling rather than carbide, the carbides tend to work better if you are removing a decent amount.

It does all depend though...if you are only doing a very few large jobs, you can probably put up with them taking a long time. You can also do things like taking off the hard layer with a grinder before putting it in the lathe, so long as you don't overdo it.

John

You will probably stall it if you take too deep a cut but if you use carbide tooling and run it 250-300rpm, 0.75mm DOC with a fine feed it will do the job and a lot faster than an old lathe in backgear would.

I have the next size up 140mm ctr height and that is happy to do iron flywheels upto 10 1/2" and has a 1.1kw brushed motor.

Edited By JasonB on 29/03/2021 10:53:50

Dear all,

Many thanks for the quick replies! for the threading speed I appreciate the downside of the lowest speed but I tend not to single point threads. As for large flywheels it is not something that I plan to do everday day or even every year? But on the Polly models site there seems to be quite a few stationary engines with 9" flywheels so I would like to be able to join in the fun. So I guess that it should not be a problem so long as I use carbide tooling with a bit more revs, comforting to know!

For the swing centre height I guess that it is a further example of two countries separated by a common language? I will use the UK version in future.

For information I have recently sold my Sieg SC3 (500w P2) and was quite happy turning flywheels within it's capacity, however as most of you know once you have a lathe you will soon want a bigger one

Thanks again,

Cheers, Mark

i suspect that's nonsense. A lathe with a proper backgear should have full power available at the spindle, whereas with a DC motor running at slower speed will not provide full power. In this case the cut is limited by the weedy boring bar:

Flywheel diameter is 16.5" and is running at 40rpm with DOC of 0.06" and feed of 0.01" per rev. That's about 1.25 cubic inches per minute, well below what the lathe can do, but limited by the boring bar. Not allowing for transmission losses the available torque is over 500Nm. Metal removal rate is almost all about available power at the spindle.

Andrew

Well I was going by the previous post that mentioned a Myford which I doubt will be able to remove as much metal as yours. Maybe a Myford owner could comment on what feeds, speeds and DOC they would use on a 9" flywheel casting particularly with HSS but even if I'm using 1/4 of the DOC that they would be using the fact I'm running at 300rpm rather than 40 means I'm able to take 7.5 cuts in the same amount of time as it won't take them to do one deeper cut.

Actually quite pleased with what Mine is removing in those videos 10.5" dia, 300rpm, 0.025" DOC 0.003"/rev is approx 0.75cu in/min considering it's at 1/3rd power and we know Warco have a habit of quoting input power not what the motor puts out.

As for turning capacity, having has a lathe that could only turn upto 8" diameter for a number of years it was very limiting in the choice of stationary engine models that were available this is mostly due to designs being produced that could be built on a Myford which had a narrow gap bed which was able to accomodate the common 9" flywheels seen on many models. So Mark, if your main interest is in stationaly steam then I would look to something that can swing 9.5 - 10" which puts you into the "240-250" size lathes if looking for new.

The OP should not assume the motor output power is 1.1kW unless specifically stated. That 1.1kW might be the input power - and some of that would be turned into heat within the motor. When the motor is turning more slowly, that heat will not be dissipated as easily as at full speed.

The swing of a lathe is often quoted as twice the centre height. That is not quite true - it may depend on the gap between the ways. Small difference, but when things get tight....

Hi Jason,

many thanks for the reply, it looks as though I will buy the Optimum because of the centre height, it gives me the capacity to turn 9" flywheels if I choose to. The way I look at it is if I buy a smaller capacity lathe I am stuck with it, if I buy the bigger capacity lathe and, at some time in the future, find that the motor is under powered then I can consider changing the motor, not likely to be honest but at least it gives me the option!

Thanks!

Good afternoon,

thanks for the advice, the figure quoted are from the sellers site and translated from German, the motor output power is listed as 1.1kw and the centre height 125mm with a maximum diameter workpiece of 250mm.

this will be confirmed before I put and hard earned cash down,

cheers, Mark

I only saw motor power, which did not state output or input. But makes no odds if you need a 125mm centre height lathe.

Good morning,

further to our earlier exchange it seems that the 1.1 Kw figured quoted for the motor is not the motor power output (P2) but the motor power requirements! The actual power output is 750w. It seems that there are one or two suppliers who insist on using this fairly confusing method of specifying their products. It certainly makes you appreciate companies such as ARC and Paulimot (amongst others) who specify power output. Needless to sayI will not being pursuing this purchase. Oh well back to the internet for more research!

Have a good day!

If you really want torque you must use backgear on a hobby lathe

Quickie jobs are not a big deal but if you need it for any length of time then backgear is the only way

If looking for a variable speed lathe I would want to make sure that it has a low speed range by means of a belt position change to make sure of enough torque available at low speeds.

Russell

Whoa! Slow down. Wrote a longish post about motor power the other day, but never mind, it was boring and I won't repeat it. Instead, ask yourself the questions: 'How fast do I need to cut metal', and 'does the extra time taken by a 0.75kW rather than a 1.1kW motor matter?'

I wish machine tool adverts quoted both input and output power plus the graph needed to relate input power to torque over a speed range, but alas they rarely do. So, unless it says otherwise, best assume the power advertised is Input and that the motor isn't continuously rated. (Duty cycle is far more important than power rating, because it limits how long a motor will run before overheating. A motor rated at 750W input continuous, is likely 'better' than a 1.5kW output motor, 25% duty cycle.)

Murky waters, but in practice it may not make much difference in a hobby workshop, even though I'm more attracted to vendors quoting honest output power rather than sales speak. For what it's worth, a wattmeter on my 1.5kW hobby lathe, a WM280, shows it's difficult to take cuts deep enough to fully load the motor. Apart from the unpleasant spray of red-hot chips it produces, the lathe isn't quite rigid enough to be worked that hard. Putting a bigger motor on this model would probably be a pointless waste of money. The only valid reason I can think of is to maximise low speed torque, but some sort of back-gear would be a lot more effective if a lot of slow cutting is on the agenda.

Bear in mind a motor rated at 750W out will deliver more power than that in short bursts, which may be 'good enough': just be careful not to overheat the motor and electronics.

There is no perfect lathe, especially when held back by budget and location. Whatever you get will be a compromise, it's just that some will be closer to what you need than others. My advice, don't dismiss options too quickly!

Dave

Edited By SillyOldDuffer on 30/03/2021 10:01:48

The main factor is that (electronic) variable speed machines need a bigger rated motor than a similar sized belt or geared machine so that when the motor is running slow it does not run out of grunt too soon.

Simple example is a Warco WM240 which has electronic variable speed comes with a 1100W motor yet the WM240B which uses blets and pullies comes with a 550w motor

Good morning,

Thanks for the reply and very relevant comments, I would not be totally honest if I did not say that there was a bit of a knee jerk reaction to my decision to not purchase the Optimum, I too have clear preference for open and transparent manufacturers and it annoys me that when I was actually one click away from buying this lathe when this fairly important point arose, my (potential) supplier was open about it but said this was the way the manufacture works with regard to motor power specifications.

As mentioned earlier I had a Sieg SC2 (400mm point to point version) and this had a 500w power output motor, I was therefore looking for something more powerful for what would be a substantially larger capacity lathe. I looked at the SC4 and apart from the 105mm height above the bed it suited my requirements, as you say there is no perfect lathe and obviously I will have to compromise somewhere. I will reflect on the matter and see which way the cards fall, in any case time marches on and I "need" a lathe

many thanks for the reply, best wishes Mark

Mark, regarding the SC2. it has to have 500w motor as there is no secondary reduction due to it being direct drive from motor to spindle. The makers have to put in motor with a sufficiently high max power to compensate for when it is running at say 25% or less when using the lathe at low speed.

The Optimum and similar machine shave two speed ranges in the region of 50-1000 and 100-2000. This means that the motor can be run closer to it's max power when the speed is dropped

As an example I have 2 Seig manual mills the Brushless SX2.7 (750w) with one fixed ratio will stall if I run a 100mm slitting saw at calculated cutting speed. The Brushed X3 (600W) won't yet it has a less powerful motor but it will cut as it has a high and low ratio so I'm able to run the motor faster relative to spindle speed and get into the power band.

Also worth pointing out that the requirement for a shallow cut at low speed really only applies as the diameter gets towards the lathe's maximum, I said earlier that 0.025" depth of cut was about the comfortable max on a 9-10" flywheel.This is a piece of 1" steel being reduced to 1/2" dia so 0.250" depth pf cut in one pass, Not something I would do every day but useful to explore the machines limits

Edited By JasonB on 30/03/2021 13:15:11

Hi Jason,

Many thanks for the detailed reply, the forum is certainly a great source of knowledge! I must admit I am coming round to reconsidering the Optimum after having digested some of the wisdom available here. As you rightly say the problem may only occur at near to maximum diameter, not something that I am likely to be confronted with on a regular basis. A lathe for home use certainly seems to involve compromise, choosing the right compromise is the trick of course, I will post an update after I finally order a lathe, hopefully that will be soon!

Best wishes, Mark

Things were much simpler when motors were rated in HP, everyone knew what was meant.

Now we get power (any power mechanical or electrical) is only in KW and yet randomly checking ebay if you buy a motor on it's own they are listed both ways such as 0.75kw - 1HP so clearly output and you understand what you are getting.

One wonders why some machine tool suppliers or the manufacturers feel the need to inflate the figures.

Mark,

There are suppliers out there who still supply new machines with old school belt drives for spindle speed so the motor always runs at full revs