Stent Tool & Cutter grinder

I'm still working on my Stent restoration based on Charles Woodward’s article in MEW 2008. I have implemented the rotating captive z axis nut Fig 3 and the y axis lead screw Fig. 4 Pg. 34.

He also describes a cable operated X axis as an alternative to the rack & pinion. I can't quite see from photo 8 pg. 33 how the pulley is arranged. The questions I have are:

1. How might he cable be anchored to the drive pulley by the grub screw without having a nasty kink as it winds onto the pulley?

2. I believe the pulley is grooved. If it requires more than one turn of wire around it as seems likely due to the travel required, how is that groove arranged? I.e. would it be cut like a thread at a pitch that exceeds the wire diameter?

Has anyone made this modification? If anyone has a sketch or a picture of such an arrangement I would be most grateful.

Many thanks

Tony

Not familiar with the article you are refering to but possibly if you make say 2 turns of the wire on the pulley and apply a little tention it will have sufficient friction to drive the table back and forth.

I used a simmilar system on a fishing line measure for spooling line from bulk spools/

Good luck.

Hi Tony

I'm part way through a Stent ( and have been for some time!) and saw Charles Woodward's design which he adopted, it seems, partly on grounds of cost. As I have the rack & gear I didn't pay much attention though. When I look now it appears to me that the arrangement is for a wire to be secured at one end of the base casting, to be wound around a grooved pulley fixed to the table traverse handle, tensioned, and then fixed to the other end of the base. The stated advantage of this system is that backlash is eliminated. The photograph in the mag shows a pulley with multiple groves around its periphery in the manner of a barrel for a weight driven clock - and these are cut like a screw thread. I can't determine from the photo how many times the wire is wound around the drum but I suspect John is right and it is the friction that transmits the drive force and the wire is not fixed to the pulley. So maybe try 2 turns as he suggests?

I would be interested to hear of your progress.

Regards

Norman

Surely, as the table is traversed using the wire it will ride up and down the spiral grooves, simples!

So set the cable in the grooves to climb up the spiral in the direction of the table. Also include a stiff spring in the cable to tension it.

Clive

Tony,

I'm sure that Norman and John are correct

This arrangement was used on many of the early Scanners and Printers.

Typically; the wire is fixed to the travelling component, and runs round a single-groove idler pulley. ... The helical-groove drive pulley is attached to the motor, which is on a spring-loaded mount for tensioning the wire. The two ends of the wire are joined to form a continuous loop.

[other configurations are possible, but the principle is the same]

The helical-groove must have sufficient turns to let the 2 or 3 turns of wire "float" across he spool [all of which depends upon the relative dimensions of the components]

Hope that makes sense ...If not, then I may be able to post a photo next week.

MichaelG.

.

Clive beat me to it

Edited By Michael Gilligan on 20/01/2013 18:00:15

Would suggest grooves would be counter productive, a plain pully should be best and the wire will I think just self align? I think grooves , unless in spiral [thread??] the wire at some point would have to cross the groove crest.

Just my thoughts----John

unless in spiral [thread??]

... Yes, that's how they do it.  " The helical-groove drive pulley "

MichaelG.

Edited By Michael Gilligan on 21/01/2013 12:56:29

John

As I posted previously I think the grooves are a spiral thread - a system used in weight driven clocks from an era long before anyone thought of model engineering! (Or maybe before the Stent was a twinkle in it's designer's ancestors eye)

Here, as they say, is one I made earlier!

Cheers Norman

Edited By NJH on 21/01/2013 13:01:47

Gents,

Many thanks for the advice. I don't think the Woodward design included a spring tensioner but I like that idea. I will try the grooves as per the clock barrel and I was only thinking of a few turns.

Confesion time. I too have a rack & pinion and although its a bit ugly ( probably 16 DP) it worked. However in installing an 18mm tube carrying thrust bearings & brass bushes for the Y axis there is insufficient clearance to refit the rack - Doh! - although I could relieve the tube to obtain the required clearance. I too however like the cable idea so will give that a go first.

When I bought my Stent it looked like it was in need of a bit of TLC but this has turned into a complete rebuild. It seems to me that either it has had two owners, one with good machining skills and the other not so much or one owner who was good at turning and pretty lousy at milling. I do wonder sometimes if I would have been better starting from scratch, though I'm hoping that I will gain some time as I don't have to make the support fingers.

Anyway Norman I'll let you know how the cable drive works out.

Tony

Tony,

Just an afterthought: The other place that this sort of wire-drive was very common

Drawing Boards !

MichaelG.

Edited By Michael Gilligan on 22/01/2013 23:17:09

I'm reminded of the arrangement in older portable radio tuners, with a knob turning a small pulley, a nylon cord to a larger pulley to which the free ends of the cord are attached, the larger pulley being on the shaft of the tuning capacitor. A tensioning spring is incorporated in the cord. The pointer for the long tuning dial is attached to the nylon cord. It may be (I haven't been inside one for years) that the cord took a couple of turns round the small pulley for extra friction. The small/large pulleys "geared" the arrangement for finer tuning than a knob on the capacitor shaft could provide.

This might translate to a wire drive for a table, if the table was substituted for the pointer. But a scale of some sort to show the table movement to any degree of accuracy would be difficult, unless a dial indicator or DRO was employed.

Andy

Hi Tony,
Have you considered using a toothed belt as used on the Brooks Stent grinder ?

Les.

A good point, Les

Incidentally, there are some clever little springs for tensioning these.

MichaelG.

Les,

I had but thought that getting the correct belt could be tricky. I have used a toothed belt on the spindle drive.

Micahel - that tensioner idea is clever.

Hope to start on the cable drive inthe next few days.

Tony