Mini lathe 3 phase AC conversion

Firstly, thank you for the membership (I have just joined).

So, without further ado, I cooked the motor on my little 7x12 Chinese mini lathe (the one without the hi/lo gear) on Monday. It's only a couple of months old and I've probably machined 3 parts (been too busy on eBay buying bits, as I'm completely new to this lark), but I can't be bothered going through the stress of trying to get a new motor from the Belgian vendor. So I jumped back on eBay, won an auction for an "unused, but not new" 3 phase, 3hp AC motor (half the price of what appears to be retail) and bought a HY 2.2kW VFD on eBay as well. As you do.

The motor, as well as being ridiculously overpowered for the little lathe, is a 2850rpm motor at 220V, which is obviously also a bit fast.

My question, therefore, is shall I need to reduce the speed with a reducing pulley ratio, or can I run it similar to Neil's setup on the website at pretty much 1:1 (motor to spindle) and reduce the max speed on the VFD only?

I'd ideally like to run the pulley ratio at 1:1, since there is not a lot of space to be messing about with larger pulley diameters at the back of the spindle.

I've toyed with putting a large pulley on a kind of stepped out arrangement at the end of the spindle (so it clears to the outside of the leadscrew fwd/rev tumbler and change gears... and maintains the spindle bore), but I like Neil's arrangement as it requires very little modification (only hacking out a small belt entry/exit aperture).

Crucially, though, I need to know if I need to mechanically reduce the speed in spite of the VFD's ability to do so.

Hopefully this not too daft a question, but as I said, I'm a beginner!

Thanks!

I'd really be looking at why you cooked the motor in the first place, you may be better off learning how to use the machine within its capabilities rather then risk damaging other parts once you have a massively overpowered motor on it.

That boy Neil has a lot to answer for Think he started with a motor that ran at half the speed of yours at 50hz

Edited By JasonB on 30/05/2018 13:37:34

Hello Mark, your HY inverter will take care of the speed for you. In the instruction book there are circuit diagrams and component values which enable you to make up a simple controller, for/rev and speed. If you are new to HY inverters I can send you a simplified version of the program which I have used. (I'm no expert by the way). Your motor seem rather big for the size of your lathe, wattage and physically wise, but you are forced to use its full power. perhaps you can locate the motor away from the lathe via a long belt. If you want a copy of my simplified program send me a PM with your details. John

Hi and welcome from me too....

As for 3ph conversion, not a bad idea given the costs involved for replacing oem parts ( motor and speed controller..)

What you need to bear in mind, is the speed of the original motor....if the spindle speed was originally around 2500 rpm, then the new motor needs to run at the same speed as the original...

So for arguments sake, and keep the maths simple, the orig motor ran at 5000rpm, in high speed, then the new motor needs to run at the same...Now its a 2 pole motor, so at 50Hz it will run at 2800 rpm...But now we need it to run at 5000, so the frequency needs to be upped to 50/28 x 50 = 89 Hz....Round up to 90...

So the upper frequency on the inverter needs setting to this new value and your lathe should run at the same speeds as before....assuming you keep the pulley sizes as was.

I'd be looking at mounting the motor underneath the lathe for ease and use a jackshaft mounted in place of the original motor, driven by the new one...but your choice...

Ideally, look at limiting the motor current via the inverter, so you dont overload the little lathe....cant remember if you can with the HY inverters....( ideally a smaller motor would be better...)

Hope it goes well.....

Just to add, I went this route my clone WM290 lathe fitted with a 1.5hp dc motor.....that was running at around similar figures. I just upped the frequency on the inverter to get me back to the right spindle speeds..

Edited By John Rudd on 30/05/2018 13:52:38

Edited By John Rudd on 30/05/2018 13:55:41

Thanks all.

Jason, I was turning at a slow speed to face off a piece of 5" mild steel. Probably ran it too slow as it turns out and stalled the motor a few times. After a while smoke started wafting up from the motor and I immediately terminated proceedings! It may well still be ok, but there was quite a bit of smoke.

Thanks John F. I think on the HY I can limit max output. As John R says, I think I can even maintain the original pulley sizes (only, I'd probably prefer a V pulley over the "timing belt" type.

John R, thank you, too. I like the idea of a jack/counter shaft in place of the original motor (that was my first thought as it happens), but I really like Neil's solution. Although, now I'm thinking about it, the jackshaft is coming back in vogue in my thought process! I could even mount the new motor on the bench behind the lathe. Hmmm. As for the VFD, I'll need to have a play when it arrives. I'll deffo be looking at "de-rating" the 3 horses as much as possible!

I think my question has been answered, thanks!

M

Welcome to the forum Mark,

I am curious. What is the exact model of mini-lathe which you have if it is the one without hi/lo gear ?

Some pictures of the lathe/link to the lathe would be great.

Ketan at ARC.

Sorry, forgot to say, John R the original (OEM) top rpm was allegedly 2500ish, so I'm not a million miles off that. The only trouble is the high torque (of the new motor) which worries me a bit. In any event, I doubt I'll be using the upper reaches of the rpm range very often (if at all).

John F, actually I'd really be grateful if you could PM your programming of the HY (it's the 220V 2.2kW/3hp one with the removable control panel which I've ordered) to me. Thanks!

Thanks again everyone, great site and forum!

M

Hi Ketan, I'm not at home right now, but it is a generic blue and grey colour-schemed one. It just has a straight-through spindle in the headstock (no gearing at all of the spindle and no intermediate drive shaft) and the drive is direct to the spindle (where the other hi/lo ones have a plastic spacer).

Sorry I can't help with model numbers, but believe me, it really is ungeared! I've had the headstock off to attach a tach and it's pretty empty in there. Tbf though, it's not like I'm going to need gearing with a 3 horse motor!

Perhaps I need a bigger bank balance, Carl!

I'll just build one over time... I'm already looking at beds. Everything else is doable on the minilathe, it's only the bed which is beyond a halfway competent homegamer with a minilathe!

I would use a 1:1 direct drive with that motor, but limit its power output in the VFD setting - Muzzer posted some advice on this in a recent thread

2.2 kW is overkill, but it will give you nice smooth running at lower speeds with plenty of torque.

A mini lathe can tackle surprisingly hefty cuts, the problems are the temptation to keep cutting with blunt tooling, shock loads or if the cut does stall there can be some nasty straining going on.

Neil

I understand what you are saying Mark, but I am trying to picture such a lathe - other than brushless motor version.

I don't believe that you have a brushless motor version, so what exactly have you got?

So far, the results you have talked about raise plenty of alarm bells.

Our Hells angel Neil understands the mechanical principals and capacity limitations of his quasimodo modifications to his mini-lathe, which actually is a mini-lathe. The term mini-lathe is a loose term.

As far as I am concerned, everyone giving you advice here is in the dark about exactly what they are helping you with. When you get home, it would be a good idea for you to post pictures of what you actually have and/or link to the suppliers product page, so that people who are giving your their comments actually know what you are talking about.

Alarm bells are still on - you state to Jason: 'Jason, I was turning at a slow speed to face off a piece of 5" mild steel. Probably ran it too slow as it turns out and stalled the motor a few times. After a while smoke started wafting up from the motor and I immediately terminated proceedings! It may well still be ok, but there was quite a bit of smoke.' ... So, what exactly were you expecting?... and I say this respectfully without sarcasm.

As far as I am concerned, it is wrong to give a beginner like you advice in the 'electronic direction' in which this thread is developing, without knowing what you have exactly - current electrical with mechanical combination, what you were doing, held how - in what chuck, with what tooling, what ability, 5" what grade of steel!...

We just had a regular customer come in who has an old largish lathe. He did the same as you, against advise from the late JS. The chap even debated this with JS on the HMEM forum when JS was alive. Today, he told me how his actions resulted in serious mechanical failure.

Ketan at ARC.

Thanks for the support, Neil! My thoughts exactly. I really liked your piece on the website btw. I am, however, now conflicted between a) your setup (mounting-wise) which I really liked and b) using a countershaft in place of the OEM motor and running the drive to that horizontally from the new motor mounted directly onto the bench behind the lathe.

Pros for option a: less work (no need to fabricate the countershaft housing). Direct drive to spindle. Ability to upgrade to V belt.

Cons against a: Topheaviness perhaps? Cutting into the tumbler gear mount housing. belt in the way (ish) of the tumbler gear selector.

Pros for for b: Able to use existing belt and housings on the final drive leg (countershaft to spindle). Physical stability of the whole rig. No need to fabricate motor mount.

Cons against b: Need to fabricate countershaft mount (not beyond the pale, but a pain in the proverbial)... actually, probably easier than fabricating a motor mount, come to think of it! Existing drive belt (which would be the final drive from the countershaft to the spindle) is relatively flimsy and might not handle the increased loads (a stall would probably sheer its teeth rather than stall the motor).

Ugh, that didn't help! I must say, though, I am now leaning more towards b than a (perhaps with an upgraded aftermarket final drive toothed belt). At least just to get the thing turning again.

M

Think I would avoid a toothed belt so that next time you take too big a cut and stall the machine the belt will slip rather than mangle something else.

I also stalled my 280 machine the first time I went to cut a large diameter (225mm), soon adjusted my methods and have not had it stall since and that is with work right upto the max 280mm dia. Typical reason for stalling is you are taking too deep a cut and/or to high a feed rate. Also forget about the text book speeds as they can be too slow which puts the motor out of its power band, better to run faster and take light cuts.

Edited By JasonB on 30/05/2018 16:15:17

Ketan, I am in no position to argue with you (like your regular customer did with JS... sorry, I don't know the late JS, I'm new here). All I want to do is turn bits of metal round and round and change their shape. Currently, my lathe can't do that. It should be able to and I am going to make it able to.

Whilst I am not an engineer with 40 year's industrial experience, turning metal is not some form of sourcery shrouded in mystery, understandable only by an elite chosen few. I'm sorry if this offends you (it's not intended to, honestly), but I am not some halfwit going "wayhay, I have a lathe, let's build the Flying Scotsman!!" My goodness, I've seen enough of those cowboys on YouTube!

In short, I have a healthy respect for machine tools, understand my experience limitations (and the experience I DO have), am extremely eager to learn and gain understanding from those who do have the experience (which I am doing right now) and am surprisingly capable in terms of understanding the process of creating stuff, but what I refuse to do is back down from a challenge thrown at me by a 400 quid minilathe!

And, yes, I am a beginner, but so were you once.

The replies on this thread have been extremely helpful, for which I am grateful and they have greatly helped in both my further understanding of my intended setup and the design of that setup.

I promise I shall proceed with caution in my mods, which is what I was doing anyway (in fact, probably too cautious, which caused the thing to burn out in the first place... lesson learnt).

Absolutely agree, Jason. Only, it would be a very convenient setup. Of course, I could change out the existing toothed pulleys to V pulleys... there's a bit of wiggle room in there.

Don't quote me on this, but it is potentially a CJ0618-3B. It's only a guess, because I got that by zooming into the photo on the eBay listing of my purchased items (on my phone).

At any rate, it's the 550W brushed motor 7x12" one that is slathered all over eBay (at around the £400 to £500 mark) when you search for minilathe.

I don't see how this is relevant, though. How is this going to affect the advice you want to give me? It's a bog-standard 7x12 cheap Chinese minilathe... just like the one everyone is modding with AC motors (even Steve Jordan)!!

Well the first hit on that lathe suggests it has two speed ranges which are arrived at by gear or belt not a direct drive and not the 50-2500 shown on the front. Now you can see why more detail has been asked for.

Even then they have all copied the same spec and al got it wrong as the 2 speed ranges are 50-1250 and 100-2500

Take a closer look at Steve Jordan's machine and you will see a big lable that says "High-Low" which referes to the gears for the non direct drive from motor to spindle

Edited By JasonB on 30/05/2018 17:15:33

Could it be one of these that does not have the speed change lever on the back?

If it is then quite likely you cooked the motor as a direct drive brushed motor won't have a lot of power at low speeds but that's what you get for going to a cheap copy.

This one here claims to be brushless.