The Shop > CNC
Electricuting the lead screw
efrench:
I'm using a Teensy 3.2 microprocessor (Arduino compatible) to drive the spindle on my rose engine. The stepper is a NEMA 17 94 oz-in. Stepper driver is a DRV8825. The Teensy can easily do 60,000 steps per second with this stepper and driver. The Teensy 3.5 and 3.6 can do even more. This stepper library claims 300,000 steps per second.
eskoilola:
--- Quote from: PK on April 21, 2018, 12:41:30 AM ---One pulse per rev. It's all you need.
--- End quote ---
I agree ... but ... I have the rubberband motor setup which means that the spindle speed is somewhat dependent on load. Driving the spindle with stepper or maybe a servo might make things lot easier. However, I am not willing to do that. As You stated in Your page - I will not believe You - I just have to bang my head into the same tree carcasses on the rocky way ....
--- Quote from: efrench on April 21, 2018, 02:37:14 AM ---I'm using a Teensy 3.2 microprocessor (Arduino compatible) to drive the spindle on my rose engine. <snip> The stepper is a NEMA 17 94 oz-in. Stepper driver is a DRV8825. The Teensy can easily do 60,000 steps per second <snip>
--- End quote ---
What kind of steps are these ? I suppose we are talking about microsteps and not the ordinary 200 steps/revolution steps. The final truth is actually the max. RPM of the stepper which in my case using the Slo SYn stepper would be about 600 rpm. I have a few steppers which can do at least 5.000 rpm but those have very special construction and have only 4 steps / revolution...
Noitoen:
You could look into this. http://bbs.homeshopmachinist.net/threads/34165-Yahoo-group-electronic-lead-screw
efrench:
I don't think it matters if it's full steps or microsteps. The driver sends one pulse for either.
eskoilola:
--- Quote from: efrench on April 21, 2018, 11:56:04 PM ---I don't think it matters if it's full steps or microsteps. The driver sends one pulse for either.
--- End quote ---
From the controller point of view this really is irrelevant. Modern electronics can easily do 10.000 steps per second with 100Vpp voltage swing. However, it is the motor itself that becomes a problem. Combining high step count with high RPM converts into high voltage and frequency.
It should also kept in mind that it is actually the current (not the voltage) that has to be stepped. Since the motor is an inductive device switching the current in any reasonable value with 10.000 Hz frequency will convert into very high voltage and also high losses in the motor due to the eddy currents.
Microsteps on the other hand divide the full step cycle into smaller ... ehhh .... microsteps. The minimum for a stepper to operate is 4 which can be divided further into 8 or even 16 microsteps.
The motor specifications (holding torque etc.) are always referencing the full steps (4 steps / full cycle). The intermediate steps have less holding torque and are also less accurate.
The Slo Syn stepper/synchronous motor is designed to be run either by stepping or by two sine waves 90 degrees apart. And again, it is the current that needs to be 90 degrees apart - not the voltage. Driving this motor with sine waves produces a very smooth movement without any resonances etc.
As I stated earlier - this can very easily run out of hands. So I should first think what I really need:
- power feed for lead screw in both directions regardless the spindle rotation direction
- accurate and adjustable stop of the feed on each of the directions
Anything else I can cope with. The power feed of my lathe is all too fast even with the slowest gear set selection.
So as long as I really cannot guarantee the spindle position with one pulse per revolution (with a 3-phase VFD one cannot) I have to forget about the electronic threading control. From this point of view PK's comments make perfect sense.
So let's assume I have an arrangement with some sort of a motor that is able to turn the lead screw and which has some sort of a speed control - what would I achieve with this:
- adjustable feed in both directions
- easy to do electronic feed stops
In this case I would be doing all threading with the existing gear swap system. This would not be that much of a problem as threading is not something one does every day. On the other hand feeding is something that is associated on almost every lathe operation.
The only thing I would really be missing are the left hand threads. How often do I come across those ... not very. Further more it would be an interesting project to install a middle gear somewhere there in between.
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