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Gear Hobber : Design ideas please ...

Compact and Simple ...

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SillyOldDuffer09/08/2018 21:01:34
3303 forum posts
658 photos

Posted by John Alexander Stewart on 09/08/2018 19:55:04:.

...

Of those, I find Windows obtuse, confusing, and hard to do anything on, compared to the others!

I think we should focus more on the ease of the end results rather on the specifics of getting there - there can be many paths to a destination.

Regards - John.

Agree. But Joe can't possibly believe Windows is an easy platform to program on can he? He must be comparing Linux with something else entirely, perhaps an embedded environment that has no operating system at all. Programming hardware is much easier when you don't have to share the computer with other users and processes.

Dave

XD 35109/08/2018 22:16:32
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1006 forum posts
38 photos

Another thought i had ( yes it is true that you only need two brain cells to think! ) is to use a single pulse generator feeding two identical steppers so in theory they should be rotating at the same speed . One drives the hob the other drives the gear blank but the gear blank mandrel is not directly driven like the hob spindle is .

The gear blank mandrel is driven by a worm wheel that is the same diameter and pitch as the hob and this is driving a gear which is the same tooth count and diameter as the gear to be cut . This should give the correct drive ratio required to cut the gear and the dummy gear and worm wheel would be drawn up in cad then 3D printed , once the hob has cut deep enough to free hob the drive system on the gear mandrel could be disengaged if needed .

Once again just a thought !

Mark Rand09/08/2018 22:32:03
527 forum posts
Posted by Bazyle on 09/08/2018 20:59:10:

We have discussed encoders to provide pulses but, especially in a stand alone machine, at what point doe it become a better idea to drive the hob spindle from a stepper as well? This also enables the system to ramp up to speed in a controlled way which may help.
What direction are the forces in both mechanical and electronic versions. As we know you can 'free hob' with the hob running a freewheeling blank does this mean in a Jacobs or electronic machine the blank is being pulled round and the gears/stepper is acting as a brake to keep it in step, rather than forcing it round?
Any ideas about detecting missing steps - not the electronics failing to command the step but the motor stalling and not making the move due to power/friction problems?

If there were going to be missing steps, they would occur on a stepper driving the hob, that has an intermittent load. The gear drive itself should only have friction to deal with, since the gear PD is going to be moving at exactly the same speed that the hob pitch is moving at, so there should be no torque imposed on the blank by the hob. On that basis, it's probably simpler to use a conventional drive and a rotary encoder for the hob and divide that down for a stepper to drive the blank.

Another advantage of the electronic solution is that there are no universal couplings, bevel drives needed, just spur gears (or straight helical gears, once they've been made) to turn the 200 steps/rev of the stepper motor into enough to give smooth rotation of the blank . Also the gears can be whatever changewheels are to hand initially.

 

Again, the reason for the electronics is to avoid the need for any specific collection of gears in order to create the gears you want to make.

Edited By Mark Rand on 09/08/2018 22:34:32

Clive Foster10/08/2018 10:52:42
1461 forum posts
34 photos

Missed step detection is, as I recall it, relatively easy with the electronic PLL based control system. Encoder on the output feeding a second PLL and count both output and input steps using a simple grey code divider chains. Pulse outputs should be temporally aligned so any miss is obvious. Recovery isn't quite so simple but it can be done.

System I made way back had this capability, including automatic recovery, but for different purposes. I used both halves of a CMOS Dual PLL chip and monitored the delay between the grey code counts to tweak the output of the voltage controlled oscillator driving the stepper so it could track a moderately unstable "axis". Including ramp up to speed from stationary over a somewhat undefined period of a few seconds varying by up to 50%. The major issue being persuading the PLL to operate over a too wide frequency range.

I'm really annoyed that I've lost the circuit because it would have done for a fair few things. Ideal for Michaels job. Simple too as it was built as part of technology demonstrator for a "£100" sensor and control package for terminally guided submunitions.

On the microchip user side I wonder if there is anything useful in this book :- **LINK** . As I recall it Elektor projects had the reputation of being reliable and well thought out from the "doing what it says on the tin" viewpoint.

Clive.

John Pace30/08/2018 13:54:28
104 forum posts
103 photos

Hi Michael

Have sent you a PM

John

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