[UKRobotics]

I'm doing an electronics retrofit on a bridgeport interact and I'm wondering how important it is to have the option to reverse the spindle direction?

Am I likley to need it much or even ever ? The only two cnc machines I have used previously had no such option.

Basically if theres a chance I might need it later I'd rather do it now so I can design it properly rather than bodging an add on at a later date. That said, if it's unlikley i'd need it i'd rather save mysef some time/space/money and remove that part from my design entirely.

Another question on the same subject .. the motor in my machine is a 400v DC series/shunt wound motor. Is it possible to reverse the motor direction by swapping the polarity of the field voltage or will this only work with the armature?

If I can get away with the former that is much lower voltage and current so I could get away with cheap relays rather than vastly expensive contactors.

[gus]

On back gear it is required to go forward in back gear, if you have them

Reversing is nowhere near that simple, not even close

[Al_The_Man]

Another question on the same subject .. the motor in my machine is a 400v DC series/shunt wound motor. Is it possible to reverse the motor direction by swapping the polarity of the field voltage or will this only work with the armature?
If I can get away with the former that is much lower voltage and current so I could get away with cheap relays rather than vastly expensive contactors.

The direction can be changed by swapping the field if need be, in series mode or shunt, I assume it is use in shunt mode?
Al.

[UKRobotics]

Forgive my ignorance but what is the difference between series and shunt?

Gus: there are no gears or anything like that. Reversing has always been done electronically through the speed control.

[gus]

The control has safeties and such to stop it reversing at 3000 rpm. I can't imagine any other dc controller would not have reversing built in.

If you ever wanted to tap, which is handy, you would need to reverse, otherwise I have never used it on my knee mills

[gridley51]

Don`t know what you mean by safties Gus but any decent spindle drive can reverse a motor at 3000 rpm.It is not done by changing direction at 3000 rpm but by taking the speed down to zero and back up again all under control and extremely quickly.
I have 80 hp dc lathe spindle motors than can reverse from 3000 revs in under 4 seconds.
I`m not sure if the OP understands how dc drives work or if he is trying to save money but if he plans on reversing the polarity of dc motors under load with relays or contactors he`s gonna get a surprise.
With a dc drive on analogue control you would just reverse the polarity of the 0-10v ref.voltage.

On edit.That would only apply to four quadrant drives.

[gus]

>>I`m not sure if the OP understands how dc drives work or if he is trying to save money but if he plans on reversing the polarity of dc motors under load with relays or contactors he`s gonna get a surprise.<<

Exactly my point. MY 1942 EE had it figured out, but people keep needing to reinvent the wheel....

[gridley51]

Gus,what`s a 1942 EE?
Mark.

[UKRobotics]

The drive I have purchased is a 2 Quadrant controller so it does not have the facility to reverse. I've been told the only way to have a reverse spindle feature is by using relays or contactors.

Ovbiously I'm not going to attempt to swap the polarity when the motor is going at full tilt (or even turning at all). Should be easy enough to tell mach3 not to allow such a condition.

Can I cut threads without reverse? i.e. using a thread cutter rather than a tap.

Also will it harm my motor to leave the field energised without the armature for long preiods ?

[gus]

why bother with such a low tech device?

is the original doa?

[gridley51]

Robotics,sorry about doubting your knowledge od dc motors but when you asked about reversing it I thought that you were going to treat it like an ac motor.
You can reverse a single quadrant drive by reversing the motor armature leads with a pair of interlocked contactors.Use the zero speed signal from the drive as an interlock so it can`t happen at speed.
As Gus says it`s not really a step forward doing it this way.I personally wouldn`t bother going to the bother of fitting reversing contactors to this drive as you`re not going to gain anything.Fit the drive as it is and if you pick up a reversing drive in the future it`s an easy changeover as the wiring is already in place.
Is this a Mentor drive that was on Ebay recently?
I have bought a few Mentor and Eurotherm drives of Ebay but only buy four quadrant ones.With the Mentor you have to check the serial number for an R in it as all the Mentors have a reverse led on the front whether they reverse or not so some sellers advertise them as reversing.
It will not harm the motor to leave the field energised for long periods,I have machines wired that way from the factory,it keeps the windings dry.Some drives like to cut the field power back when the drive is at standstill.Your field will of course be controlled and monitored through the drive.
Mark.

[UKRobotics]

Gridley: Don't worry about it, no offence taken.

Ok, a bit more info for you:

When i bought the machine it came with an all singing all dancing contraves drive worth about £3k. Unforuntatly someone had let out the magical blue smoke before I got hold of it and the cheapest quote I got for repair was not under £1,000. (Someone had also let the blue smoke out the heidenhein but that is another story for another time) But lets just say there was enough wrong with the machine when I bought it to make retrofitting with my own mach/rutex control system the best option.

Unsure of the best way to proceed, I removed the spindle motor from the machine and took it to a local company which specalised in DC motor repairs, re-winds and controls. They took a look at the motor, realised it was in quite a state and did quite a lot of work on it including skimming/undercutting the comm, replacing the brushes, spindle bearings, taco brushes and cleaning out the carbon dust as well as converting the blower motor to single phase (I have no 3phase in my shop)

Once this was done they set about finding me a way of running the motor with cnc control from a single phase supply without breaking the bank.

Their solution was a 5kw autotransformer (240v - 415v) which supplies power to a Eurotherm 512C drive.





This is quite a simple DC drive and only provides control for the armature. The field windings must be powered and controlled separately. My advice from the company supplying me with this kit was to use a small variac (which they also supplied) which I could then use to 'tune' the voltage going to the field to find the best compromise between speed and torque.

The controller has all the normal things such as accel/decel ramps as well as tacho input for closed loop operation, and a 0-10v analouge input which will work with the CNC4pc speed control board. Having considered your suggestion of a zero speed hardware interlock, looking at the manual here reveals that it has a 24v digital output to indicate zerospeed condition so I guess I could use that without too much trouble.

To be honest I'm a little concerned about the number of comments which seem to be suggesting what I am doing is a bad idea. Should I carry on as I have started (considering I have already bought all the parts) or would I be better off looking for a different spindle control?

Bear in mind I am a uni student with no money and this is all having to be done on a rather tight budget. I think this control cost me in the region of £150 so the very most I could afford would be another £150 on top of that to get something better. Are their any controls in the <300 range which will control both field and armature and offer reverse? or even just reverse and armature control ?

[gridley51]

Dom,the reason people are saying that you are going the wrong way is because you are not gaining anything using a single ended drive.To make reverse of use to you I would think you need the four quadrant drive as you can then forward/reverse the motor very smoothly and quickly as for in tapping with a compensated holder,even rigid tapping if the control is up to it.
Now while the dc motor people have been very helpful (assuming they didn`t charge the earth) their solution is not the best.Two reasons,firstly because you need field monitoring.If you lose the field while you have power on the armature the motor speed will take of very quickly.If you do not have maximum speed monitiring on the armature control the result will be disastrous.
Secondly you do not say what the base speed of your motor is but,if you want to run above base speed you need field weakening.This is best controlled by the drive.
I also didn`t know that you can control dc voltage with a variac.In fact I`m sure you can`t.
I have a few Mentor 11 and Eurotherm 590 drives that I have bought of Ebay and I`ve never paid over £200,usually a lot less.
I have had problems with Contraves spindle drives and having no documentation on them have usually binned them.Then last week I found out that a guy I have known for years is ex Contraves and has all the info and test rigs to repair them.
I also know of a working Heid 145 control that will be for sale cheaply soon.
Mark.

[UKRobotics]

If the field supply failed for any reason whilst the motor was turning, wouldnt this cause the motor to stop instantly rather than speed up ?

The controller I have already puchased does have a maximum speed limit as well as current limit, stall trip and overtemp cutout.

Are you able to reccomed me a specific drive that works with a single phase 415v input, does everything else my current drive does as well as 4 quadrant reverse which wont break the bank ?

Base speed of the motor is approx 3000rpm. There is a 1.5:1 ratio between the motor and spindle so it will be fine running at its rated speed.

The solution provided by Forco was not meant to be the best, but it was meant to be an affordable way to get the motor turning. You are right in thinking that you cant put dc voltage through a transformer. - I will be putting AC into my transformer and the output will go into a bridge-rectifier which will then go to my field.

I have no interest in trying to replace the original components like for like. I have long since removed all the original kit and spent several thousand pounds on new components to build a pc based control system of my own design.

I am very close to finishing this project rather than just starting so its too late to make drastic changes at this stage anyway. If I was made of money then i'd have bought a working VMC with ATC and be done with it but the world doesnt work like that. I'm starting with somthing I can afford and will use this one to pay for somthing bigger and better; at least that is the plan.

[gus]

>If the field supply failed for any reason whilst the motor was turning, wouldnt this cause the motor to stop instantly rather than speed up ?<

Amusingly enough, no, it will run away if you loose field. On the aforementioned 10EE, the motor voltage is raised to increase speed, then at a point the field is weakened to increase speed.

[Al_The_Man]

The advice given to you by that company is a bit dangerous, as stated, the motor can destroy it self in some cases, as losing the field while powered causes runaway.
A series connected motor (field in series with the armature, also tends to runaway) this is why a series motor (traction,starter etc) should always be run loaded.
Depending on the HP, I have picked up some 240v 4 quadrant SCR drives on ebay for cheap price.
You mentioned before you did not have 3ph, did you realise you can run a 3 phase motor on 240v 1ph using a VFD? this gives you speed and direction control.
Al.

[gridley51]

Dom,
I noticed your little SSD drive has field connections on it,is it the wrong range or what when you are not using them.

[UKRobotics]

Al, the motor is DC 415v. The Majoirty of controllers that work at those voltages require a 3phase input. It was uneconomical to swap the motor for a 240v 3phase.

gridley: I could be wrong but I belive that the field connections on my controller simply go to that bridge rectifier you see on the board above them. Just saves having to mount one somewhere else.

[gridley51]

Suggest you download the pdf manual for your drive from the SSD drives website.A quick glance confirms it does have the field output although on that model you have it might be a bit low on field amps.

[UKRobotics]

I think the issue of tapping/thread cutting is the single biggest thing that is holding back my design right now as my decisions here effect everything from spindle control, reversing and indexing.

I think the ability to be able to cut threads is important to me as this can be a very time consuming job that would be great to automate. I dont really mind how I have to do it as long as I can do it in some way, and considering I am on quite a tight budget that must factor in quite highly as well. If I can get away with not needing to include electronics to reverse the spindle at all that would be even better.

I suppose this leaves me with 4 options: Ridgid tapping, Floating tap holder, auto reversing tapping head, or thread milling.

Immediatly I can discount the first one due to excessive cost and complication. - I've been told its not realistic to expect to be able rigid tap with mach 3 and if it could be done at all it would require the spindle motor having its own encoder and driven by a servodrive.

With a floating tap holder I've been told I would need reverse, but would I need an index pulse on the spindle as well or would that not be necessary?

What about an auto reversing tapping head? Does anyone know much about these things ? As the name implies I would not need to reverse the spindle to do this but again would I need the index on the spindle ? Can I even use a reversing tapping head at all or is the way it works too similar to rigid tapping- (without a 'floating' action) ?

The final option is to use the thread milling wizzard that comes with mach3 which looks like quite a nice simple way of doing it but does anyone know the limitations? Do I need the ability to index or reverse my spindle to use this wizard ? Also what are the practical limitations on thread size? (internal threads especially!). Do the laws of physics and general practicality limit this wizard to only cutting really large diameter threads or would it work even down to M4 or smaller?

Would aprechiate some feedback from anyone with knowledge in this area or better yet anyone who has managed to cut threads somehow using a machine that they have built/retroed.

What option would you take if you were in my position?