Member postings for Tony Jeffree

Posted by Clive Hartland on 03/05/2011 13:32:12:

Yes...it would seem that the real value to the allied war effort was in boosting morale (and demoralizing the Germans) rather than in the actual damage caused.

 

Regards,

Tony

Posted by Steve Garnett on 02/05/2011 17:34:34:
Oh come on, Tony. You've done enough electronics not to trust one of those, haven't you?

I'm afraid that is true....

 

Regards,

Tony

Posted by Steve Garnett on 02/05/2011 16:59:41:

Edited By Steve Garnett on 02/05/2011 17:02:28

A lot easier just to look at the spec sheet

Regards,

Tony

Posted by MICHAEL WILLIAMS on 02/05/2011 12:09:58:

Hi Michael -

 

That is of course true (and I did point that out in what I posted, although maybe not as explicitly as you state here).

 

However, I take issue with your statement that you cannot contrive greater accuracy in a system that is not accurate in the first place. If that was universally true, then it would be impossible to construct measurement systems that are more accurate than the ones we had before, and we would not have the instruments we have today.

 

Regards,

Tony

 

Posted by MICHAEL WILLIAMS on 02/05/2011 11:33:12:

Actually, you are wrong.

 

The situation is greatly improved if you use a worm and wheel; the positional error of a stepper motor isn't accumulative, so the accuracy of positioning improves by a factor equal to the worm/wheel drive ratio.

 

Direct indexing using full steps of a stepper motor gives you a positional accuracy of ~ +- 5% depending on the quality of the motor (and also gives you a very limited number of divisions, as there are only 200 discrete positions that the motor can index to). Stick that motor on the end of a 90:1 worm drive and the +- 5% reduces to +- 0.055%, modulo the accuracy of the worm and wheel.

 

This fact leads to a very usable means of improving the accuracy of dividing plates on a dividing head or rotary table; you cut one set of dividing plates as accurately as you can, then use that set with the reduction drive on your dividing head to generate a second, more accurate, set.

 

Accurate, manually operated division systems that work on that principle have been around for far longer than rotary encoders. The only fundamental difference between a manual dividing system using a high reduction ratio and the stepper-driven equivalent is that you push a button rather than cranking a handle.

 

Regards,

Tony

 

Regards,

Tony

One thing to bear in mind is that the Divisionmaster controller can only drive up to 2A/phase; if the motor that the OP is using is rated at significantly more than 2A then the controller won't be driving the motor as hard as it can go. Also, be sure to set the current level on the controller appropriately...I'm pretty sure that the default setting is less than the full 2A/phase capability.

 

Regards,

Tony

Posted by Spurry on 28/04/2011 16:01:38:

 

Pete -

 

There's no substitute for trial and error.

 

Regards,

Tony

I read with interest, and a little concern, the article in MEW #175/176 on the construction of a so-called "Accurate CNC 4th axis".

 

The main cause for concern was the choice of the athor to use a simple toothed belt reduction of 3.6:1, between the stepper and the spindle, and his calculation that with 8 microsteps, this would result in 5760 steps per rev of the spindle. All very well on paper but...

 

Firstly, stepper motors are not precision instruments in themselves. The positioning of whole steps on a stepper motor is typically quoted at 5-10%, but if you start going down the microstepping route, all bets are off as the microstep accuracy is highly dependent on the ability of the microstepping drive to accurately position the microsteps (which it achieves by applying varying currents to the two motor coils), and this ability is generally pretty poor. So if you are looking for the best accuracy that you can get out of a stepper based system, then you are better off going for full stepping and uping the drive ratio accordingly, because at least then the accuracy should be as good as the motor is inherently capable of.

 

Secondly, the accuracy quoted for a stepper motor is under no-load conditions. Apply any rotational force to the motor shaft and it will move to a point where the restoring torque generated by the coils on the magnets of the rotor is exactly equal and opposite to the force applied. So, assuming the force isn't great enough to cause the rotor to turn to the next step position, then an applied force could cause a positional error of up to 1/2 a step of the motor (i.e., a step position error of 50%). Obviously, there are two ways that this can be ameliorated; you can use a socking great big stepper that generates a gozillion oz-inches or newton-meters of torque, or you can increase the drive ratio to suit the real-life working conditions. In a CNC mill, those conditions can be fairly "interesting"; slap even a modest stepper motor on the end of a 1mm pitch screw and the resultant force moving the axis can be quite large - for example, with my Taig CNC mill (see MEW 120, 121) I figured out that the 120 oz-in motors I used were capable of generating around 100 kilos of force at the tool tip, with a mechanical advantage via the leadscrew of around 125:1. Now, stick a 4th axis with a mechanical advantage of only 3.6:1 on the table of my Taig, and once you start taking a decent sized cut, something is going to give; that something is the posinal accuracy of the 4th axis. So what is the solution? Obviously, use a drive ratio for the 4th axis that is considerably greater than 3.6:1; perhaps slightly less obviously, use a worm drive rather than simple gearing. Why? Because if you use a worm drive with a reasonable ratio (say, 30:1 or greater) it is impossible to drive it backwards against even a small force holding the worm in position. This is one of the major reasons why worm drives, often as high as 90:1, as mentioned by the author, are used for 4th axis drives; they provide, in a single compact package, a large reduction ratio and the important characteristic that they can't be back-driven by cutting forces.

 

So, while Mr Gordon's device is nicely designed and well explained, I fear that building one in the hope of it being useful for real CNC work would be a waste of time.

 

A secondary cause for concern was the heavy weather the author made of his problems with the quality of toothed pulleys, and the astonishing solution that he mentioned in part 2 of re-machining one of the pulleys after layering on car body filler. Frankly if I had gone down that kind of route I would have kept very quiet about it when I wrote up the project! Treality is that toothed pulleys and belts of a suitable quality are readily available - RS Components for example - however, as mentioned above, this was in any case an example of the old Irish joke - "I wouldn't start from here!".

 

Turning a worm/wheel driven dividing head or a rotary table into a CNC 4th axis is a far more rewarding, and far easier, solution; it will give far greater precision than the one described in the articles, and will not suffer from its very obvious, and in my view fatal, flaws.

 

Regards,

Tony

Posted by Geoff Theasby on 05/04/2011 15:23:17:

Geoff -

 

In clockmaking, the objective is to get a good "running fit" between each wheel/pinion pair. In an ideal world, yes you could predict the appropriate centre-to-centre distance to achieve that for a given pair; however, in a world where you are making your own wheels and pinions and the PCD of the resultant object may not necessarily be precisely what it should, you need to use a depthing tool in order to figure out precisely what C-T-C distance you need for the components that you have in hand.

 

There's a saying in clockmaking - "if it rattles, it will run" - what you are aiming for isn't the perfect backlash-free fit, because in all probablility, the resultant friction will be way too high and the clock won't run. So a clockmaker will err on the side of a loose fit, which will help reduce the gearing friction. Bear in mind that the drive train of a clock involves a massive gearing up; the cord that suspends the driving weight in my long-case clock is wrapped around a barrel 16 times to give 8 days of running, so 1 turn of the barrel represents 12 hours of running, so the escape wheel (which carries the seconds hand) rotates 720 times for each rotation of the barrel. Introduce any unnecessary friction into that system and the clock won't run.

 

And of course, given that the drive train is always being driven in one direction, any backlash in the train that may result from your "rattling" fit is of no concern whatever.

 

Regards,

Tony

Posted by magpie on 05/04/2011 16:33:09:

I guess it would definitely have been larger if it was a lager pipe...

 

Regards,

Tony

Posted by Sam Stones on 04/04/2011 07:03:11:

Looks to me like a rotten old fence post (strainer) with the remains of the fencing wire still attached - was it about the same diam as a telegraph pole?

 

Regards,

Tony

Posted by John Stevenson on 27/03/2011 13:24:49:

 

John -

 

Sounds like he taught you everything you needed to know about how to handle customers <G>

 

Regards,

Tony

Posted by NJH on 26/03/2011 19:56:46:

 

I'll give it some thought!

 

Regards,

Tony

Posted by Peter G. Shaw on 26/03/2011 21:04:41:

Peter -

 

Thanks for the vote of confidence. Perhaps slightly overstating the case to compare my efforts with those of the great R Barker esq, but many thanks for the compliment!

 

Regards,

Tony

Posted by Graham Meek on 26/03/2011 22:55:53:

There you go

 

Regards,

Tony

Posted by John Stevenson on 26/03/2011 21:18:23:

Too late for the whippet - he's gone to the dogs...

 

Regards,

Tony

Posted by Alex gibson on 26/03/2011 19:33:18:

Thanks!

 

Regards,

Tony

Posted by Ted 047 on 26/03/2011 14:58:51:

 

Hi Ted -

 

Are you talking about the letter or the article? I would agree that the letter was about 50% humourous, but I don't think that applies to the Worden article itself, uness you have discovered a new branch of mathematics. By my calculation, the latter was about 1 column (1/3 of a page) of humour to 6 pages of article in Part 1, and pretty much no humour in the 6 pages of Part 2. Remains to be seen what Part 3 will bring

 

Regards,

Tony

 

Posted by Graham Meek on 26/03/2011 19:33:17:

Hi Gray -

 

I couldn't agree more. I will then take great delight in returning the favour by critiquing their efforts!

 

Regards,

Tony

Posted by Terryd on 24/03/2011 22:20:49:

 

You too Terry - after all, the worm may turn. Using an offset tailstock, of course...

 

Regards,

Tony