i'm just striping down my old 9x20 lathe and getting prepared to convert to cnc.
i'm after some (any) suggestions and pointers with these types of lathe.
i have a couple of main questions.
1, lead screw pitch? Whats good? i have been looking at 4 or 5mm lead I want a reasonable amount of precision, probably more important than ultimate speed as it's not the most rigid lathe in the world.
2, i'm intending to attach the motors with timing belts, any advise on type/matirial? I'm thinking of the steel belted T5 belts at the moment.
3, i see a lot of 9x20 lathes have the ball nut mounted to one side of the cross slide, are their any issues with this? E.g. Racking the slide/angular misalignment or flex.
4, how much torque do i need on a lathe this size? I'm thinking much torque would be helpful for power slotting etc. should i try using a bendy torque meter on the screws to see or will this be pointless as I have brass nuts currently (well its cold in the uk , arrg bad joke).
5, i'm thinking of getting the low to medium price ball screws and nuts to start with to help with getting it together now and to work out any problems then replace with better ones if it's all working out. Are the nut sizes similar between brands etc. or totally different?
5b, ball screw backlash, is it an issue? How much is common on cheeper ball screws? Should i be using double nuts on each axis and pre load them against each other?
6, tool change, whats the best way to go? Can you use multiple tool posts as a cheep option? Or a fount tool and a back tool (either upside down or reversing the spindle)
7, rs232/485 3 phase vfd, brushed DC or BLDC motor?
(This is probably later down the line as i will most likely first set it up with the single phase motor it has currently)
8, reflective spindle sensor , encoder wheel or ? For spindle sync.
any other pointers welcomed.
I'm intending to use emc2 as it seems to support threading and be very reconfigureable.
Many thanks in advance, sorry if it's too many silly questions all at once. :)
1) 4 or 5 will be good enough.
2) Use whatever you can get that suits your budget. You won't be pushing for every last bit of performance, and the neglible amount of play in a belt isn't going to matter in the grand scheme of things.
3) It will cause racking, especially if you're gib isn't adjusted correctly, however you just compensate for it in either via tool offsets, or the part g-code.
4) Measuring is pointless unless you already have ballscrews on. My last lathe used a 1.47nm stepper directly coupled to a 12x4mm ballscrew on the Z-axis, and it could sink a 8mm drill into aluminium, but would only mark steel. My current lathe has a 750w servo connected at some stupid ratio (about 1.3:1 IIRC) to a 16x5mm ballscrew, and it'll knock the turret out of alignment before it'll stall the servo!
Personally, I'd say go for some of the good quality high torque NEMA 34 steppers combined with good drives and see how you get on with a 2:1 reduction. You can always change ratio later.
5)On a lathe, backlash isn't really an issue, as you're nearly always cutting from a single direction with each tool, so you can compensate pretty well. It's not like a mill, where backlash can cause havoc when trying to cut with the cutter digging in and moving in the backlash. Do some searching on ballscrews. Generally you get them already assembled and from the same supplier.
6) QCTP, Gangtooling, or a combination of both are the cheaper options.
7) Whatever. They're all a much of a muchness in terms of performance on a lathe. It all comes down to how much you want to spend.
8) That will depend on what EMC will accept. I use Mach3 and my old lathe only had a single pulse opto slot sensor connected to a smoothstepper, and it threaded perfectly well even though it only had a 750w DC motor.Avoiding the rubbish customer service from AluminiumWarehouse since July '13.
I have converted mine but not done the encoder bit yet. Happy New year. Clive
i've done a bit of reading and it seems a 1 pulse per rev for positional info and a slot encoder with either one or two sensors (depends if you want to know what way your turning) is whats good wih emc.
i have a bit of experiance with emc for controling some cnc camera rigs etc. so i'm fairly comfertable with it.
i'm hopeing to be able to do off round shapes. Like latheing hexes etc.
I will mostly be machining steel.
Quite a few of the external motion controllers can manage that (apparently Mach4 and Darwin/PP can manage it, but Mach4 turn isn't commonly available yet), but for the vast majority of users, they won't be able to tell the difference between a thread cut with a single slot sensor and one cut with a quadrature encoder. I only used my old lathe as an example as it worked perfectly well.
My current lathe has a quadrature encoder connected to a KFlop/Kanalog, so it's capable of the Z-axis tracking the spindle perfectly and even reversing back, but it's got no major benefit during normal threading.Avoiding the rubbish customer service from AluminiumWarehouse since July '13.
Last edited by Stupoty; 31-12-2014 at 06:16 PM. Reason: A typo , opps
If you're needing to interpolate moves on different axis, then you'll need to use a proper quadrature encoder.
However, do you realise you won't be able to turn a sharp cornered square using this method?
I'd even question the ability to produce a reasonable quality square using this method, given the cutting angles that would be involved.Avoiding the rubbish customer service from AluminiumWarehouse since July '13.
yeah i would imagine their would be some inacuracy in tool paths, i plan on going for many slots on the encoder just not sure how many i nead, i can always experiment wih this.
i'm not building it with anything perticular in mind it's just i cant have another lathe sitting around :) (i have my first lathe on a shelf in the bedroom it's a piece of art and i'm a bit attached to it)
at the moment I'm aiming highh and seing how far it can go :)
even if i end up with not as much capability as I would like it will be an interesting journey.
A mostly flat surface could be handy for easy finishing on a mill for example. Less nead to index part for hex heads for example.
thanks for the input I am taking it all in and i prommis i'm not going to try and run before i walk and all that.
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