Brideport to CNC ... do you copy

[zoeper]

Inspired by some of your projects i have finally decided it's time to
bring my milling machine to life.
It is a full size, bridgeport clone, table XYZ of 815,381,406, and is equiped with a rapid
lift motor on the knee. The powerfeed fitted to the x-axis specifies a torque of 650 in-lb, so i think that is a good indication of what the motor size should be. (I thought so until i saw some guys using 40 in-lb servo's )The leadscrew pitch is 5mm on the x and y axis, and if you use the 1/10 reduction gearbox fitted to the rapid lift motor, one revolution there gives 0.25mm of travel on the knee. This also provides for convenient place to mount Servo.
I machine aluminium alloys 95% of the time, and a little bit of brass and mild steel.
Does anyone have some ideas/advice on how to proceed?
Specifically:
Servo or stepper, (ac or dc for servo's)
Motor size
Drivers for the motors
other controll gear
As well as sources for parts/surplus motors/drivers
When using servo's, is it better to put the feedback encoder on the motor, or to use digital readout scales
directly on the slideways? Have anyone interfaced with these before?
Thanx
Pete

[BigDaddyG]

Pete,
Your about to have some fun! With a full size mill like yours, I would definately be looking at Servos! Steppers will work, but the size you need you can get better deals, not to mention better preformance out of the servos. Motor size, equal or greater to your power feed is a good starting point, there is no going over-board for a full size machine. As for the Drivers, there are plenty of good choices out there. Gecko is very highly though of, but I have yet to use their servo drivers. You would definately be better off using encoders on the motors. The will give a more acurate reading of what the motor is actually doing, and respond back to the controller better. Most servos will come with the encoder in place anyway.
Good Luck. Hope this helped.
Regards,
Glen

[Al_The_Man]

I think you will find that scales will give you better accuracy everytime, due to the fact that they by pass the backlash etc, they are after all right at the final motion, they are not used often due to the high cost,They also need protection from coolant and chips etc.
There is no difference with interfacing them as they look like encoders to the control, as long as you have ones with square wave quadrature out, some have a sine wave out and should be avoided for this application. There are usually plenty of 3rd party servo amps on ebay etc and some of the makes to look for are A-M-C and Copley Controls both make very good amps and are easy to work with. You will probabally have less trouble with mix and matching DC servos. If you want brushless, look at brushless DC, as the motor resembles an AC servo but is easier to match up to 3rd party amps than sinusoidal AC types.
Al

[arvidb]

To combine what Glen and Al said about encoders:

Use linear scales only if you have true zero backlash components throughout the system. Scales will give you better accuracy, as Al says.

However if there's even a small amount of backlash present, use motor mounted encoders. Otherwise the motors will hunt back and forth, trying to "compensate" for the backlash and quickly wearing out themselves and the screws. On the whole, I think you will find motor mounted encoders more forgiving.

Arvid

[Al_The_Man]

Originally Posted by arvidb However if there's even a small amount of backlash present, use motor mounted encoders. Otherwise the motors will hunt back and forth, trying to "compensate" for the backlash and quickly wearing out themselves and the screws. Arvid

I have not had that experience, if the scale indictes it is in position the servo will null, the servo will only move if it 'sees' the table move. In other words if there is thrust against the table which is generally in one direction, the servo will compensate as it would if the encoder was on the motor, if the table is at rest with no thrust then the servo will not be active if there is no position error.
Al

[zoeper]

Thanx guys (no grls i prsme)

I will start hunting for the stuff.
Just clarity on the servo size.. There is a big difference between 40in-lb and 650 in-lb
I like to over engineer rather than under, but there's a jungle of stuff out there.
Does servo's turn freely when not under power? (will i still be able to use it in manual mode). Interfacing with the pc, i hear of printer ports with breakout boards- that i understand, but do you then still need the (black box, mach 2 etc). Also i deduct that i should look for servo drivers that accepts step and direction inputs, what other inputs can you give a servo driver then, and is the size of the step user configurable?

Thanx for your replies, i feel something big coming.. (no not that)

[Al_The_Man]

I would consider 40in-lb servo's large enough especially if you reduce by 2:1 (timing belt etc).
Servo will turn freely when the drive is disabled, make sure you disable the drive, many drives have an enable input terminal, but you have to make sure if that if the control is still active, it also zero's the output to the drive when in the manual mode, because if not the control will see the manual move as a large error and try to compensate, so if you suddenly place the drive active again, there will be a big signal to the motor to try and bring it back to what it thinks is the correct position. There are sophisticated motion control cards that sit in the PC slot, these generally use +-10vdc analogue signal to the servo's. Some have I/O for machine control so the printer port is not used in this instance. The control ability is capable of CNC control comparable to the larger commercial types.
Al

[zoeper]

Hi,
How far can you get with the pc printer port, and is there much advantage to be had from the motion controller cards? Also i hear a lot about using the quill as the z axis as opposed to the knee. Is this simply to minimise wear and tear on the heavy knee (slideways) for drilling and related operations? I am not a big fan of using the quill for milling operations, will you get good acuracy on the quill if you retofit a ballscrew and servo? lastly, is it essential to get the whole machine on ballscrews, or can you make do with the 5mm pitch leadscrews that are currently in place?
Thanx for all the help sofar
Pete

[Al_The_Man]

nearly all the knee mills I run across make the quill the Z axis, the trouble with the knee it has a very large mass plus the load of the work peice, which makes a decent feed rate near impossible. Most just motorise the knee just for initial positioning, ball screws are the way to go for accuracy etc and when climb milling. There is a lot of difference with a motion control card, it is just quite a bit more expensive, I am spoilt I guess, as once you have used them, it is a job to go back.
Al

[zoeper]

Went hunting for stuff, and have identified some nice looking stuff. Rutex R990h2 motor drivers.. 200v @10 Amps looks like they should do the trick. They take step and direction inputs. As i understand i should be able to interface these directly with the printerport or motion controller. Where does the limit switches come into the picture, and can they also be connected to the printer port and thus controll the motors via the software, or should they be hard wired to the motor to cut it out when overstepping the line?
Does anyone know of good sources for ballscrews?
I have requested some quotes for motors, but if anyone knows of good deals to be had on dc servo's in the 40 in-lb range, please let me know
Best i go make some chips (by hand )