Both turret and front toolpost are hydraulic I think (not 100% sure) and I seem to remember both have mention of "rack in" and "rack out" in the schematics but again not 100% sure. Will go and look soon but they are huge so I will have to spread them out on the floor.
 
Also  I have seen some 2000ppr encoders at a semi reasonable price. I am not sure exactly if I have it correct in my head so I will run through my calculations and would be grateful if you could let me know how stupid I am
 Presuming the ballscrew is 5mm pitch, 2000ppr encoders, motor speed of 1500RPM and no reduction
 Steps per unit in Mach will be 2000/5 =400
 Max pulse rate needed would be 2000ppr x 1500rpm / 60seconds =10,000Hz = 10KHz
 G Rex pulse rate is 4MHz I think so that should be ok.
My resolution would then be 5/2000 = 0.0025mm

Hood

2000 lines?   Thats 8000 pulses per rev.

Usually encoders are rated in lines per rev, and you multiply by 4.   2000 pulses would be a 500 line encoder.



Where are the encoders to be mounted?  do you have reduction between the motor and ballscrew?

For 2000 lines

If you had a 5mm pitch ballscrew, 2:1 reduction and the encoder were mounted on the motors, then you steps per mm would be

(5/2) = 2.5mm per rev
8000/2.5 = 3200 steps per mm.

If the encoder were mounted on the ballscrews then it's 8000/5 =  1600 steps/mm.

Or for 500 lines divide the results by 4 which is what your example gave.

Wayne....

mmmmmmmmmmm now I am really confused
 The encoders are stated to be 2000 ppr, their is no mention of lines, just ppr. The encoders will be mounted on the end of the motor and the motors, although driving through a timing belt, are 1:1 with the ballscrew so no reduction.
 So if they are indeed 2000ppr (500lines) then are my calcs correct?
Heres a link to the encoders. http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=270023265848&fromMakeTrack=true
Do you think they would suit? Is the price reasonable?
Sorry for all the questions but as you will gather I dont have a clue about servos.
Hood

Just noticed you said your encoders were 2500 CPR so I am presuming that is counts per Rev, the ones I saw are PPR which I presume to be pulse per rev. So I preume that CPR and PPR are the same but just written in a different way.
Hood

Sorry to confuse you!    I use CPR, Lines per rev as the same thing.  PPR is the quadrature output IMO.

Looking at the pic of that ebay listing, it looks like it says 2000 LPR on the encoder, so thats 8000 PPR.  Manufacturers don't usually rate them at the quadrature rate.

They look very much like Heidenhain, but with a different colour case.

Whatever happens you are going to need some serious drives for those motors!   You can run them at below the max voltage at the expense of some speed.   The current is the killer.   Depends on how heavy a cut you want to take I guess.

Something about the Rutex drives always has worried me.   Hence the reason I'm looking at the Teknix drives, at least the people behind them give me confidence.

2000 lines and Gecko's are not a good marriage either.


Wayne...

Ha ha dont worry about confusing me, its very easy to do, i even manage to confuse myself by just trying to think.

I have had a look at the pic and it seems to be a different encoder than the description. There are several differences, the main one is the part number then there is the voltage and lastly the 2000 I see 200LMP or imp, what the heck that means I dont know.
 I am sorry to burden you so if you are getting exasperated at my ignorance please feel free to ignore me
 What amount of lines should I be looking for? I think a resolution of 0.0025mm should be ok but then again I dont really know
 The motors are run at 90v at the moment and I was planning to maybe even drop that to 80V as speed wont really be too important.
 On looking at the pics I have I think it is the power supply current thats  40Amps so I would presume that the motors are at present running maybe around the 20Amps.
 I was thinking of running Geckos or Rutex drives at 75-80V and 20Amps but now you have me worried, just wondering if I should think about getting some steppers instead, would make things slower but at least I have experience of them.

Hood

2.5 micron resolution would require on a 5mm ballscrew 500 line encoders.   These would be better with a Gecko than 1000 line +.   The gecko's fault at +-128 counts which is a little fine for heavy machines.

The Rutex drives seem to offer more, but it's just my cynical nature causing me concern.  It seems I've never got the answers I wanted whenever I enquired about them.  CNC Teknix on the otherhand have been very forthcoming, and so far can't fault their enthusiasm.

Don't if you can help it goto steppers on that machine.   The difference is like night and day.

I'll gladly help wherever I can, so no fears.

Wayne....

Thanks Wayne
 Will check out these teknix drives.
Also been looking at the pdf for the encoders and it does seem that they quote ppr,  in the model number the last two digits, 28 in this case, represent 2000 pulses, so I presume this means they are 500 line.

Hood

you'd need to check on that.   

The usdigital encoders are not that expensive.   Just make sure they dont send via air courier as they did with me years ago!

Wayne....

Wayne I think I will have to clarify what PPR actually means. I have just been look at specs for encoders on click automation and they say1000ppr then in the next sentance they say CPR so looks like they may be one and the same.

 Had a quick look at the teknix drives and the 80v one seems pretty good, especially the 25Amps it can handle.Though  I was a bit surprised to see the other one that can do 200V can only handle 15Amps.

Better go do some searching.

Hood