bantam 1600 electronic problem

Just buy a modern single-speed motor and a VFD. Rotary converters are obsolete.

Everyone is saying the same John go for a 3ph 240v motor with a VFD i should be able to control the forward reverse with that setup.Would i be able to use the hi-lo or does the VFD make it variable speed ?

If anyone has motor/electrics problems on an old machine and especially those with a two speed motor I would recommend buying a modern 240V 3 phase induction motor and a VFD. However, having said that, I believe that there are some 3kW brushless DC motors available that have superior driving capabilities than an induction motor when operating at slow speed. Even if these motors are optimistically rated ie described as their peak rating rather than their continuous rating they are seriously powerful. Also, having permanent magnet rotors, rotor heating when operating at low speed and high torque isn't as big a problem as it is with an induction motor. Most induction motors are fully enclosed to prevent the spread of fire in the event of a motor overload and rely entirely on a shaft mounted cooling fan blowing air over external cooling fins. However at low speeds this would provide far less cooling so for continuous use at low speed a fixed speed electric fan should be used to cool the motor or use a much larger motor and use the VFD to limit the power it can provide.

Since the electronics of a VFD and a BLDC motor controller are virtually identical only the extra cost of a BLDC motor may be an issue. However their higher efficiency and their capability to produce high torque at low speed , I believe, gives the BLDC motor the edge. From an historical perspective DC motors have always been the favourite for traction applications and brushless motors will hasten their return.

Andrew

Yes, speed control is one of the features of a VFD (Variable frequency drive), increased frequency will increase motor speed and conversely reducing the frequency will reduce spindle speed.

The choice of 2 pole or 4 pole is debateable with a geared head lathe such as the Bantam, you have many choices of spindle speed, 35-800, via the gearbox so the motor can always be running fast enough to provide cooling so if your use will be for speeds between 800-1600 or greater then I would prefer the 2 pole motor so at 50Hz you will be in the range 72-1600 from the gearbox, increasing frequency will take you up to 2000 without issues, going much above this you have to consider chuck max speed and other rotating mass, don't forget the Bantam was designed to work happily up to 1600 or 2000 for later versions.

Emgee

Thank you Clive and Emgee you are all very clever when it comes to this kind of stuff,me on the other hand know some but not gears and speeds and so on.Emgee you said what range will i be using well i tried 800 rpm today on some 40mm bar and i found that to be to quick for my liking so i wonder will i ever use 1600 rpm it would be nice to have all the speeds through the gearbox using all the lathes leavers but it would appear that's not going to happen.

A handy rule of thumb for guesstimating RPM for cutting mild-steel with HSS is:

rpm = 10000 / diameter in mm

40mm would suggest a speed of 10000/40 = 250rpm, so as you say, 800rpm is too fast. It would be about right for 12mm or 1/2" diameter rod.

4mm diameter rod would be happy at 2500rpm.

Those speeds are for HSS, but carbide cuts better at much higher rpm - 2 to 10 times faster than HSS. So your spinning 40mm at 800rpm is reasonable when the lathe is fitted with a carbide insert.

Don't get too hung up on cutting speeds. I use the formula to get into the right ball-park, but speed up or slow-down as necessary to get a decent finish. The material matters as well a diameter, but the mild-steel formula still helps:

Aluminium, Brass and many plastics cut faster than mild-steel

Free cutting mild-steel (recommended!!!), bit faster than mild-steel

Copper, Bronze, Gunmetal and some cast-iron cut at about the same speed as mild steel

Tool, Cast, Silver and Stainless Steels cut slower than mild-steel

Most cast-iron cuts much slower than mild-steel

Three factors to experiment with:

1. RPM (as formula)
2. Depth of cut (not normally critical, but light cuts need a sharp edge)
3. Feed rate. (worth experimenting with: counter-intuitively fast feed rates sometimes produce better finish than slow.)

Beware scrap metal! Quite a lot of it doesn't machine well. I recommend starting with known metal where the description includes words like 'free-cutting' or 'good machinability'. Move on to unknown scrap when you know what to expect: lots of trouble is caused by beginners trying to learn on metal that would test an experienced machinist.

Have fun,

Dave

You don't say what material or what cutting tool so hard to comment.

Was it the rotational speed that was not to your liking or the speed the tool moved on autofeed, if the later you may simply have had too high a feed rate selected.

Thank you for answers i should of stated what material and cutter.It was aluminium with a carbide cutter and it was the speed of travel that put me off.I have just used a dial indicator for the first time on the test piece and the dial hardly moves so i am happy with the results on a slower speed.

With that combination you were running quite slowly, even your to speed of 1600 is about half the minimum book cutting speed.

You may want to select a finer feed rate say half of what it's set at at the moment but depth of cut can be increased so you end up taking a similar amount of metal off in the same time.

Small amount of paraffin or WD40 will give a better finish and also stop metal sticking to the tip of the insert.

Thanks Jason i use wd-40 as a lubricant i know it doesn't cool but helps cut.So you are saying 800rpm is to slow,why does it move so quick then is that normal for it to take about 4 sec to do 100mm

The finest feed that I use works out bat 0.0025" /rev, with either replaceable carbide tips or HSS.

My lathe (NOT a Colchester ) was built with 12 speeds (including Back Gear ) by a 2 sheave motor pulley, and three sheave pulleys inside the Headstock .

The original single phase motor was a 2 hp, but for the VFD with a 240V three phase motor, I chose 1.5 hp

Only one sheave of the pulley on the 3 phase motor can be used..

So far, cannot recall taking a cut deeper than 0.100" ( 2.5 mm ) deep.

Being idle, I usually leave the belt on the middle sheave, and vary speed by means of the VFD, although, often it runs at full speed.

In top "gear" it becomes clear that the drive train is is not in perfect balance (Probably the spring loaded driving pin in the bull gear throws things out of balance ).

FWIW my advice is worth, put things back to original, as much as possible but fit a VFD and 240V three phase motor, of the same power as the original. The VFD will give you an almost infinite variety of speeds. Since the VFD can run upto 75 Hz you might need to change the motor and driven pulleys, to limit top speed to 1600 rpm..

HTH

Howard

Thank you Howard i have another question for you all.I live in a council house so if i need a 16amp fuse it will cost £500 by the council to fit.I enquired with numerous electricians and they wouldn't touch me because of regulations with the council.Will a VFD run this lathe on 13 amp ?

Sounds like you have a very coarse screwcutting feed selected and are engaging the leadscrew half nuts rather than the feedshaft. The middle of your 3 shafts should be turning when cutting not the leadscrew at the top

May also be you have a screwcutting gear train rather than a feed one which will mean the input speed to the gearbox is far too fast. You need to take off the end cover and see what gear train you have. You want one of the gear trains bottom left of the headstock charts

Edited By JasonB on 22/12/2021 19:14:58

My VFD runs off a suppressed 13 Amp socket on single phase ring main, fed from a RCD in the house.

It has run like this since installation in September 2003 without any problems

The socket is suppressed to prevent any nasty spikes that might go back into the mains and upset our house, or any other on the same phase.. Probably an overkill..

1.5 hp plus losses in the VFD will probably amount to less than 2 Kw (Just over 8 Amps ) total, so a 13 Amp single phase socket will be below its maximum loading of 3.2 Kw.

When measured, the current draw has been well below 8 amps, nearer half of that, so I have not been using the full power of the motor.

So you don't need a 16 Amp feed for VFD, just plug it into a 13 Amp socket..

Howard.

Unless screwcutting, you should be using the gears to set the feed rate.

0.004" /rev will do for finishing cuts, possibly 0.016" / rev for roughing.

Most of the time you will not be making 4 tpi Leadscrews!.

The coarsest thread that I cut was 4 mm pitch, x 1.5 mm deep, and for that, i used Back Gear to keep speeds low.

Your Colchester may not have back gear, but at minimum speed, especially with VFD will probably be slow enough to cut threads without a need for lightning fast reactions, WHEN you get to that stage, which is unlikely to be for a while.

become familiar with the machine before doing anything complicated with it.

Read a few books on lathe work, for guidance, even if they deal with hobby lathes rather than an ex industrial machine like yours. The principles are the same, just that the details will be different.

Buy a set of Zeus Charts - an invaluable reference

.Ian Bradley, "The Amateurs Workshop"

L H Sparey "The Amateurs Lathe".

Harold hall - "La\thework"

Neil Wyatt "Lathework"

Later buy Tubal Cain "Model Engineers Handbook"

What you spend on books may save you wasting material, or damaging the machine, and will teach you the way to go about things.

HTH

Howard

Howard, the input speed to the gearbox needs to be set with change gears for feed rates, there are three change gear options N, P & Q. You can then use the gearbox to get six speeds for each of those change gear combinations.

If the change gears are one of the combinations for screwcutting then the input to the gearbox will be far too fast and no matter what position the "joystick" is in the carriage will move far to fast.

I hope the lathe came with it's change gears

There's no need to faff with pulleys to limit max speed if you are running with a VFD, adjust max Hz in the parameters.

Plenty of choice of feedrates provided the correct gear train is fitted.

Emgee

Edited By Emgee on 22/12/2021 19:49:33

It's a Mk2 so different charts.

Here is why it is important to engage the feed screw for turning and not the lead screw.

On the left are metric screw pitches, on the right metric feeds. I've highlighted gear train "P" , 3 position knob in "S" and joystick in "2"

If you engage the feed shaft the carriage will move at 0.15mm per rev, if you mistakenly engage the leadscrew when you actually want to cut it will move at 1.75mm per rev. Now if you apply that to the 800rpm being used in 4 secons your tool will move 93mm in 4 seconds which is close to what Andrew says he got when it should have been a sedate 0.15mm/per rev which would have covered 8mm in the same time.

For feeds you should be using the lever middle bottom of the apron and for screw cutting the lever on the left of the apron

Edited By JasonB on 22/12/2021 20:23:26

Thank you well at least i can use a 13 amp socket thats one thing sorted.As for cutting i always use the bottom middle and the same for the cross feed.I will check the change gears to see what gears he has been using.