Greetings all,

I'm new to the hobby-cnc world, and am reading everything I can get my hands on to try and get up-to-speed, or atleast not appear ignorant.     One thing I have decided on is to use Mach 3 as I've read hundreds of good reviews from people who have home-built and/or retro fitted CNC machines.

My current/first project is the design and construction of a CNC mini lathe (4" x 12") and I'm hoping for some reccomendations on Spindle Encoders that are known to be 100% compatible with Mach 3.

I've been looking through the archives here and found posts of people asking for help with some encoders, but I didn't see many posts that talked about encoders working on the first go.

One thing I did learn in reading all the posts is that I really need to have a USB (Smooth Stepper) interface to handle higher RPMs.


And while I'm at it I'm going throw out a really ignorant question:    For a lathe where I'm going to be doing single point threading is a VFD or a Servo better for the spindle motor?


Thanks for reading, and any enlightenment will be greatly appreciated,

Curtis F.

Aaaaaaaaaand I forgot to mention that I'm looking for an encoder "kit" that'll fit on a 50mm OD shaft. 

I'm desiging the spindle around the C5 collet standards and will be pushing a max operating RPM of about 15k (wanted 30k, but scaled back the design due to bearing costs), which immediately kills most encoders with integreal bearings, thus the "kit" option.

Though I'm still interrested in any recomendations as there's a Mill project after the Lathe and it'll need encoders as well.


Thank you for your time again,

Curtis F.

You dont actually need encoders as Mach just uses a single input pulse per rev at this time and it works well.
As to what is a better motor for the spindle I would say Servo everytime over Induction/VFD. Obviously if you use a servo for the spindle then it will have an encoder so if it has the index pulse you can use that for input to Mach via the SS.
Hood

You should check out the thread "Success, mini machining center . . . " in this forum. I just finished building what amounts to a lathe head using a 5C spindle and it is servo driven. You might get some ideas there.

A servo motor is basically a precision purpose build DC motor (read expensive). There is no point to using one on a spindle unless you are going to use gears or timing belt drive. The typical flat or V-belt arrangement (which you will probably use at the RPM you are talking about) erases any advantage the servo has.

You will have to rig the encoder and also an index, I would think. Unless you can find an encoder with an index rated to 15k. Here again, really no purpose to the encoder if you don't use a servo, which there's no purpose in if you use a v-belt. Catch-22

15k RPM is a dangerous neighborhood to be playing in. When things go wrong at that speed, a lot of major damage gets done in an instant. You might consider a smaller collet size for that kind of speed. 

There are occasionally precision lathe heads for 3C  (I think) on ebay that sell for about $350, so you might consider that option.

Curtis,
Curious, what are you turning at 30,000 RPM??
RICH

Big benefit of an AC sero (not sure about DC) for a spindle is you have full continuous rated torque from Zero RPM right up to full continuous  rated RPM, a VFD will drop off rapidly on torque when you get low on the speed. Another good thing about AC Servo is you use Step/Dir so you have very accurate speed control and if using Swap Axis you can also position the spindle, could be handy for an auto tolchanger.
 Bad thing about AC Servo is the cost but deals can be had on eBay and the Chinese ones are forcing the price down.

Hood

Hood,

My bad. I hadn't even considered an AC servo. The OP is talking 4" x 12" lathe. You could probably fit that whole thing on your cross slide 

Do they make tiny AC servos?  If so . .  I may feel the need to tinker . . . .   

I could probably fit it in my toolpost
Yes AC servos can be had in all sizes, saw one the lat day that was 200Watt and I am sure you get even smaller.
Hood

Simpson,

Yes, AC servos are available in a wide variety of sizes. Got one on my Emco Compact lathe spindle. Yet another big advantage of an AC servo is its small frame size for the same power rating as DC motors. Mine is 1/3 the size and weight of the original 0.44KW DC motor and is 1.3kW!

Hood,

DC motors are the contrary to VFD - they have max torque at zero speed and it drops liniary as you increase the speed. At max speed the torque is zero.

Daniel

 

DC motors are the contrary to VFD - they have max torque at zero speed and it drops liniary as you increase the speed. At max speed the torque is zero.

Daniel
 

True only for DC steper motors.


I have read that AC servos have superior holding power  . .  an important consideration for my 4th axis application.

A huge disadvantage of an AC servo is that my wife will kill me if I spend any more money on this CNC project . . . LOL!!