GH1440 Lathe Bed Conversion

[m_c]

I was aware of the brake system design for a 4th axis. But my thoughts were to use the 4th axis simultaneously. The brake could be added as an option though, when only one face would need machining. But it adds to the complexity, and I'm trying to keep it as simple as possible. Wouldn't a geared 12Nm Nema34 do the job without a break? I don't need high speed & feeds. I will consider a brake if it doesn't.

You would need to run some figures using the torque your milling spindle could produce, and the diameter of your work piece, to calculate the torque required at the 4th axis.
I.e. using some random figures, say your milling spindle can produce 1Nm, and you have a 5mm radius (10mm diameter) cutter.
At the very edge of that cutter, you potentially have 200N of force (1Nm/(0.005m/2)).
Now apply that 200N to the edge of a 25mm radius work piece, you get 5Nm of torque produced (200N*0.025).

That means under perfect transfer of torque conditions (very unlikely), you need 5Nm to hold the spindle. You're not likely to get perfect transfer of torque, however the torque applied isn't constant, and that is compounded by should the cutter snag, you have the inertia of the spindle/cutter which will produce peaks far higher than 5Nm.
However that is assuming you're using some form of side cutting cutter, cutting at a tangent to the workpiece, and at 90deg to the 4th axis axis. Things like drilling holes around the circumference, will result in minimal rotational torque being applied to the 4th axis.

If you want to run figures, most of this can be worked out to a reasonable accuracy.

Also, good point m_c about the low speed motor driven by the other one at extremely high RPM. Haven't thought about it. That means the lathe motor (the 2.2kW low RPM komatias has been suggested) can be permanently attached to the spindle, and a servo (hybrid) stepper would need somehow to be detached when in lathe mode.

I see this guy nailed it. He made a lathe and a 4th axis mill run simultaneously. I'm sure you are well aware about his project.
https://www.youtube.com/watch?v=b2-Kdud7eiA

That is a video of the original design from that thread I posted. It's worth having a read of that thread, as Simpson explained quite a lot about how he designed and built the inturn's.

And with a single motor attached to the spindle. What's the catch?

Getting the required torque.

Plenty of homework for the week-end.
Thank you for your time. You guys are great!

Spreadsheets are your friend for working out some ball park figures for the required torque.

[Valfar]

Thank you m_c! It does make sense, and I've also found some spreadsheets for calculating stepper torque values. I also believe they are a good start before deciding to buy.

Also, an excellent tutorial (from my point of view) can be found bellow:
http://www.mycncuk.com/threads/1524-...otor-do-I-need

But now Hanermo2 wrote:

My experiences, working at this for 10+ years.
Steppers are no-where near accurate enough.
Even the best belt drives are not accurate enough.

Now from my understanding that means stepper motors driven by belts are not accurate enough (what's the exact value of this? 10 thou? 100?), and the way to go would be either a worm gear or a gear box. But the problem with these is the backlash they introduce, and a good quality one is out of the question for a DIY budget. Is that correct?

That leads me to believe is the belt itself causing the problems, being flexible, and not the stepper. Is that correct?

And of course, a brake system applied on the C axis would work, but that means the same issues are encountered on the other axis (XYZ) as well?

Getting the required torque.

I understand the torque value, but this contradicts with the high RPM value needed for the lathe operations?

Cheers!

[Valfar]

This is the AC motor I have, similar to the one in the link bellow.

http://www.brookcrompton.com/upload/...E_W_CI_IE2.pdf

Brook Crompton
kW: 2.2
V: 380-415
A: 5.0-4.8
RPM: 1415
Phase: 3

I shall use it for the lathe operations.

Is there a good VFD you would recommend, or should I just go for a reasonable one over the e-bay?

I will have to also design a lathe spindle encoder similar to the one in the link bellow, is that right?

http://ve7it.cowlug.org/spindle-encoder.html

Cheers!

[Neale]

Now from my understanding that means stepper motors driven by belts are not accurate enough (what's the exact value of this? 10 thou? 100?), and the way to go would be either a worm gear or a gear box. But the problem with these is the backlash they introduce, and a good quality one is out of the question for a DIY budget. Is that correct?

One data point based on my own recent experience. My setup might or might be applicable to your situation but it might give a little bit of insight. I've been working on a Z height setting macro on my router. I'm winding a 2.2KW watercooled spindle up and down using a 1605 ballscrew driven by a 3Nm stepper via a 1-1 HTD5 belt and pulleys. Max speed 2500mm/min but can't remember current acceleration settings off-hand - think it's about 400mm/s/s. I can touch off the setting plate, wind the Z up and down 50 times at full acceleration/rapid speed, and retouch. I get repeatability within one or two microsteps, time after time. That corresponds to 3-6 microns (0.003-0.006mm) variation. Ok, the machine is very new so the belt has hardly had time to stretch, and I don't claim that the machine has long-term accuracy of anything like this. Similarly, I can set one tool to the work surface, change tools, retouch off a fixed setting plate, and the new tool will be just touching the surface of the work. That is testing various aspects of the drive system, and I'm pretty happy with it. You will be needing very high precision gearing to get anything close to that, and/or very careful attention to fitting and adjusting and maintaining, and/or a lot of money to buy the components. Although my own experience is with a router, I would be happy to get that performance out of a vertical mill.

Belt drive might not be perfect but it beats any gearing available at an affordable price, I suspect. And while a stepper might not have the resolution of a high-count servo encoder, how often will you need that level of resolution?

[m_c]

You need speed for turning. To give an example, I run my lathe at 2000rpm for 20mm aluminium, and that is nowhere near the recommended speed. The reason I limit to that, is to stop the stock whipping in the spindle, as it's not got any spindle liners to support material.

Running some basic figures for 20mm bar through HSMadvisor, for 6061 series aluminium recommended speed is 10222rpm, low carbon steel is around 1900rpm, and 316SS is 724rpm.

With stepper motors, the rated torque is holding torque. As soon as the motor starts spinning, the actual produced torque at the shaft quickly drops off.
Also, you can only guarantee a stepper motors position to one full step (1.8 degrees for a standard 200 step motor). The main purpose of microstepping is to improve smoothness at low RPM, not to improve positioning. Microstepping may provide fractional steps in movement, however due to friction/stiction within the system, the exact distance cannot be assured.

[Valfar]

What you guys are saying makes perfect sense to me, but here's one other thing I cannot understand without actually testing a stepper.

Say a (standard 1.8 degrees/200 steps) stepper motor is rated at 10Nm holding torque (never mind the micro stepping), and we apply a tangent force. With no rotation, just holding the spindle in place, what does this actually mean in practice?

Will the 10Nm stepper keep the spindle dead locked until the 10Nm value is reached, and then it starts skipping in the direction of the force?

Ok, say not until 10Nm value is reached, but will it keep the spindle dead locked until at least 5Nm is reached and than it start's vibrating within the 1.7 degrees interval, and when it reaches 1.8 degrees (10Nm torque) value, it skips in the direction of the force?

Or will it vibrate anyway, no matter the applied force? So for 1Nm it'll vibrate 0.2 degrees, for 2Nm -> 0.4 degrees and so on (is it a linear or logarithmic graph?). If yes, I can understand why a brake is needed in this case.

I hope it makes sense.

Cheers!

[hanermo2]

Depends.
At the full-step point, it will hold at 10 Nm, until the torque is over 10 Nm, and then it will move one step.
Because the holding torque is very much stronger at full-steps only.

So at less than 10 Nm stress, at the full-step position, it will not rotate at all.

At 1/10 microsteps the torque is == 10% iirc.

So, if the stop point is at 9 microsteps off a full step, it will "bend" or comply until it gets to the full step point.
It does not lose steps, but acts as a spring.
When torque is removed, it goes back to the position it was meant to be in.

Servos are different.
A servo "knows" where it is supposed to be, ie what the encoder count must be to stop.
If I rotate the chuck the servo drive led shows how many counts off it is.
It tries to use peak torque, 3x of rated torque, to get back to the position it needs to be in.

So, the 2.5 kW servo, at 10Nm torque and 30 Nm peak torque, tries to use 30 Nm torque x 1:3 belt drive = 90 Nm to get back to the position it "wants" to be in.
The peak torque applies for upto 3 secs, and then goes to sustained rated torque, 10 Nm in my case (2.5 kW).

The effect is very obvious and intuitive.
You can see on the servo drive led, the error by encoder count, in real time.
If you use so much force that you overpower the servo max-error setting, D iirc, it faults.

This takes less than 1 ms, or 0.001 secs.
Typical servo loops are 12 kHz, or 0.12 ms.

Mostly, the spindle can be off by == 0.1 mm at outer edge of 12" chuck before fault.
With very very loose (poor) servo tuning (factory default).

Servos do not have "microsteps" and cannot loose steps.
Servos position perfectly at rest, and then lock (modern ac servos).
When in use, they lag a bit, and this depends on tuning.
The lag is shown on the led at the drive.

The faster the servo runs (or more acceleration), the more lag it has.

[Valfar]

It makes sense, what I understand is when a 200 steps stepper motor is used for the 4th axis, it stays dead locked until the rated torque is reached. So basically I could machine a stock on 200 faces with the ultimate precision (which is a 3 to 6 microns variations as stated by Neale - more than enough for me, as I'm not planing to machine air bearing for instance) dictated by the belt (which is the weakest link in the chain). The stepper will never act like a spring unless micro stepping comes into the equation. To me it sounds like a good option to drive the C axis.

Regarding your particular project and the servo motors, I understand your not happy with how it performs when the spindle takes the C axis role. Is that because the servos tend to act like a spring all the time, no mater the position, due to the lack of steps?

Also, there are these hybrid servo (servo stepper) motors. Do they also come with 200 steps + microsteps + all the other benefits of a servo motor? Or are they just as described by you, and 'hybrid' is just marketing? It's a bit confusing for me. If just marketing, I understand a 200 steps stepper would still be the best option for the C axis, provided there is no brake involved, and provided one must never ever go beyond the rated torque? Also, I'm sure there should be some electrics which could shut down everything in case a step is skipped (but this is not important right now).

I want to connect 2 motors to the headstock spindle. One for turning and one for the C axis. For turning I've got a 2.2kW AC. I'll invest in some 3kW VFD and make an indexer (not sure yet how). But at least I know the direction I should investigate. Now I'm looking for a good compromise for the C axis. The machinery will act either as a lathe, or as a 4 axis milling machine, but NOT at the same time. The AC motor will be permanently engaged (but powered off while in milling mode). I need to find a good way to disconnect the C axis motor while in lathe mode (even if I have to manually disengage the belt), so there would't be any backlash involved. The lathe and the mill would share the X Y axis motors. A BT30 mill would be fitted on the Z axis, to take care of the milling operations. I hope it makes sense.

Please don't hesitate to write down anything you think it may help (or not :) ).

Cheers!

[Valfar]

Servos position perfectly at rest, and then lock (modern ac servos).

Do these modern servos you've mentioned come with an integrated brake of some sort? Similar to what m_c has been suggesting, but integrated?