Spindle Reverse on a Bridgeport Interact?

[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]

Originally Posted by UKRobotics 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 ?