# Bent Stent



## MRA (Sep 26, 2021)

Hi folks
I just bought most of a 'Stent' type T&C grinder from ebay, very cheap.  It seems to have been worked on by different people - some of it is quite nice, some is very rough indeed!  A couple of questions, to anyone who has made one of these or uses one:

This slides on pairs of 3/4" rod in x and y.  It's not _too_ bad - the carriage slides all the way in both planes (although the hole for the 'y' feed screw is misaligned so it gets very tight at one end - but I can sort this out).  But - the holes in the block (with 2 pairs of 3/4" holes in it, at 90 deg) which slides in x and y, have been drilled, not reamed, and there is a little 'rattle' in it in both planes.  I _could_ have a go at making this again (can I do better?  ), but before I do - what about adding a couple of 2BA-ish brass screws bearing on each rod, in an attempt at gib screws (but without a gib strip)?  For the small forces and intermittent use involved, I wonder if I would get away with it - what do you think?

There is quite a nice collet holder and one (home-made) collet - but the size is a little small.  I hope to modify a Chinese ER25 collet on a 20mm shank up to 1" (the size used here) so I can use standard ER25 collets, which is what I use on my milling machine - so no need to go any bigger.  

 Well, I am interested in your thoughts
cheers
MRA


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## SmithDoor (Sep 26, 2021)

MRA said:


> Hi folks
> I just bought most of a 'Stent' type T&C grinder from ebay, very cheap.  It seems to have been worked on by different people - some of it is quite nice, some is very rough indeed!  A couple of questions, to anyone who has made one of these or uses one:
> 
> This slides on pairs of 3/4" rod in x and y.  It's not _too_ bad - the carriage slides all the way in both planes (although the hole for the 'y' feed screw is misaligned so it gets very tight at one end - but I can sort this out).  But - the holes in the block (with 2 pairs of 3/4" holes in it, at 90 deg) which slides in x and y, have been drilled, not reamed, and there is a little 'rattle' in it in both planes.  I _could_ have a go at making this again (can I do better?  ), but before I do - what about adding a couple of 2BA-ish brass screws bearing on each rod, in an attempt at gib screws (but without a gib strip)?  For the small forces and intermittent use involved, I wonder if I would get away with it - what do you think?
> ...


Do you photos you can post

Dave


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## goldstar31 (Sep 27, 2021)

I doubt that it is a Stent. It is more likely to be FROM a design by Derek Brookes

The drawings appeared in Model Engineers Workshop issues 16 and 17

I own a fabricated( yes) short Stent.
My severe macular degeneration at 91 prerukes further comment,

I suspect that BaronJ has something similar to what laughingly desired as the Brooks Stent

Norman


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## goldstar31 (Sep 27, 2021)

As an 'addendum', the ER25 collet holder etc is interesting. However, I gave up on another web site as there seems to be a stout refusal to spend money on the benefits of the 'new'-ish Quorn Mark3 and the ability not only to use the ER25 collet holder and to do the modifications to 'round off'  the cutting edges which the fabricated rotary table affords.  The plans for the Mark3 and the instructions are  copyright and cost- well mine- were £65 +postage etc.
Despite having several t&c's, I thought it  well worth it and bought the kit for the Mark3 as an afterthought to add to what is an ancient Quorn Mark1
My opinion, of course


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## BaronJ (Sep 27, 2021)

goldstar31 said:


> I doubt that it is a Stent. It is more likely to be FROM a design by Derek Brookes
> 
> The drawings appeared in Model Engineers Workshop issues 16 and 17
> 
> ...



Hi Norman,

For those that like pictures, some of my modified Brooks TCG !






Looking from the front. Each division on the dial is 1 thou.





Looking at the the motor and table drive end.  Again each division on the dial is 1 thou.  The height adjustment dial on top is currently un-calibrated, and drives an M10 by 1 threaded rod





A view from the left side showing the table stop and the ER25 collet holder. The large brass knob locks the collet in position.  I've since modified that assembly to allow rotary indexing.





That arm with the finger is used to index the cutter lip.  I've since modified that to allow positive two and four point indexing.  It also dispenses with the fine feed screw on the back of the collet shaft. The bronze bush bearing you can see supports the free end of the table leadscrew.  The M6 CSK screw has an oil hole drilled in it to lubricate the bearing.  A similar screw is used at the drive end.  The "T" slot is the same as the Myford crosslide.  The table leadscrew is a left hand M6 thread supported by a small steel ball at the far end and drives the table by a backlash free 1" inch long plastic molded nut press fit secured in the table body.  The whole thing runs on 13 mm diameter ball races.


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## BaronJ (Sep 27, 2021)

Some more pictures of the modifications to the collet and indexing mechanism.





This is the modified collet tool holder assembly.  That bit with hex headed pin in started life as a Yale lock, which explains the six holes drilled in it. Its held in place by two M4 cap screws.





The alloy ring was originally at the back with an M3 by 0.5 screw through it as a fine adjustment screw, now defunct and not needed as I found that the table feed was adequate adjustment.





The four flute cutter in the collet is a broken one that I just cut the broken end off with an abrasive wheel !  As you can see it got a little warm.










A close up from the front showing the curve machined to match the collet.

So far I've ground a few drills, made a "D" bit cutter and ground a couple of milling cutters.  I've still to make a holder to grind lathe tools.  Which actually was the original reason for building this machine.

I've since found that its easier and quicker to use carbide threading inserts than trying to grind HSS threading tools.  Though I admit to using "Coventry Dies" as threading tools.


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## goldstar31 (Sep 27, 2021)

John

Congratulations. Nice to see my old motor is of use

With tongue in cheek,  two things are worth considering on your collet holder

As we all know, it might be worth controlling the advance or retard of the ER collet- which gives a pivot
The second thing is to move  things- sideways. Got it?

I guess that someone else was following the determination of the Megalithic Yard as propounded by Professor Them but sort of written about by Knight and Lomas  who lecture or lectured in Freemasonry at Bradford University.  All to do with the planet Venus.  No dirty ditties please!
Actually, it is getting near the Autumnal Equinox. 
Thoughts--- this  3, 4, 5 right angled triangle formula. Friend of the Opposite Sex quipped about Trachtenberg Maths and Pelmanism. . ' So you are sober at times'
But I digress

Nice work- John

Keep Well

Norm


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## BaronJ (Sep 27, 2021)

Hi Norman,

Yes your motor is doing a sterling job of spinning that grinding wheel. I've got the gearing such that I have just over 5K rpm with the current pair of timing gears.  I have a second one that can be used to replace the one on the spindle cartridge that will lower the RPM to about 3200 rpm.  That grinding wheel is rated at 5.5K and is the recommended maximum.

I've a sneaky feeling that I've anticipated your recommendation about moving the tool collet holder sideways since the whole table can move linearly from left to right using the calibrated knob at the end. Also the tool collet assembly can be rotated on the table as can the motor and wheel assembly.

I'm not doing much at the moment since I've somehow managed to damage my right bicep.  The doc thinks that its either a tear in the muscle or tendon damage !  I won't know until after I have an ultrasound scan. In the meantime I'm being treated as if its a damaged muscle with Ibuprofen gel.  I admit it does help ease the pain that stops me sleeping at night.

Anyway that's all for now.  You take care, I might sneak up there for a visit.  Stay safe.


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## MRA (Sep 27, 2021)

That's funny, my right bicep is giving me loads of stick at the mo.  Well, I was just in hospital, not too keen to spend a day up there again just yet!

Here's a couple of photos of the pile of bits I picked up.  Looks like 'Brooks Stent' it is.  I think something like 'gib screws' are the next thing to try, to take some wobble out of it as I mentioned.  I'm not sure yet, I might replace the wheel on the motor like that, with a spindle which I happen to have, and drive it on a belt like yours BaronJ.


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## BaronJ (Sep 27, 2021)

MRA said:


> That's funny, my right bicep is giving me loads of stick at the mo.  Well, I was just in hospital, not too keen to spend a day up there again just yet!
> 
> Here's a couple of photos of the pile of bits I picked up.  Looks like 'Brooks Stent' it is.  I think something like 'gib screws' are the next thing to try, to take some wobble out of it as I mentioned.  I'm not sure yet, I might replace the wheel on the motor like that, with a spindle which I happen to have, and drive it on a belt like yours BaronJ.



Hi there MRA, thanks for the pictures.

Yes it does look like a "Brooks" !

I would be a bit wary about that motor though !  It looks like a universal motor, so be aware that it can easily run at 10 to 20 thousand RPM and that speed is way above the safe speed for that grinding wheel.  Looking at the second picture, there may be some damage to the wheel.

I've set the gearing up so that the grinding wheel is running at its maximum rated speed.  I do need to make a guard for it though.  If it burst it would do some damage, at the speed a universal motor runs at it could be catastrophic.

I rejected the idea of using precision ground bar simply because on a grinder it would be very difficult to keep them clean and free of grinding dust.  Even protecting the ways on mine the dust gets in everywhere.  I'm using ball races to support the table and saddle, I can feel the grit when winding the saddle from one extreme to the other even though I can't see it.  A wipe with a paper towel soon shows how dirty it really is.
I bought some 4 mm thick felting to make wipers, which should help.

Sorry to hear that you are having a similar muscle problem !  I've been prescribed some 10% Ibuprofen gell to apply three times a day.  It seems reasonably effective, particularly at night before bed !  At least I get some sleep.


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## goldstar31 (Sep 27, 2021)

BaronJ said:


> I've a sneaky feeling that I've anticipated your recommendation about moving the tool collet holder sideways since the whole table can move linearly from left to right using the calibrated knob at the end. Also the tool collet assembly can be rotated on the table as can the motor and wheel assembly.
> 
> I
> 
> Anyway that's all for now.  You take care, I might sneak up there for a visit.  Stay safe.


I had to laugh!!!!!!!!!  You've saved 65 Quid  

Best Wishes


N


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## MRA (Sep 27, 2021)

That's a timely reminder about wheel speeds, thanks!  It is chipped, but not cracked I think (though I'll want to look closely at it) - I might have a go with the wheel dresser at work, and I can drive the motor with a little variac and watch it with a tacho.  I like the idea of slideway bearings - maybe sliding plastic things such as we use at work - but I have no room for them, so unless I make that bit again, then brass rubbing screws will have to be tried first.  The detent holes in the collet indexer are really, really all over the place - the rest of the part is quite nice, so I have to choose whether to plug the holes with some rod or other and start again, or weld it up (which is likely to end untidily if I'm doing it).  Even epoxy putty would serve, come to think of it - I might be able to make a tidy job of that.   

I rather hope my arm is a muscle thing - I've just gone back on thinners for a blood clot in my leg, and another one in the arm would be, errr, unwelcome!


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## MRA (Sep 29, 2021)

Well, rubbing brass screws are not going to do it - the surface area of the rubbing patch is so small, that it wears off as soon as you adjust it.  Doh!  So - I think the next bodge will be to use a slitting saw to cut each of the four 3/4" holes in the sliding block down its length, and then to make slim clamps which can be used to nip these slits up to take the shake out.  There's a little bit of work there, but it seems worthwhile - I think I can manage it, and it will allow me to take wear up, which seems likely with grinding dust floating around.


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## BaronJ (Oct 1, 2021)

MRA said:


> Well, rubbing brass screws are not going to do it - the surface area of the rubbing patch is so small, that it wears off as soon as you adjust it.  Doh!  So - I think the next bodge will be to use a slitting saw to cut each of the four 3/4" holes in the sliding block down its length, and then to make slim clamps which can be used to nip these slits up to take the shake out.  There's a little bit of work there, but it seems worthwhile - I think I can manage it, and it will allow me to take wear up, which seems likely with grinding dust floating around.



I'm not surprised that using brass screws to take up the play doesn't work !
My suggestion would be to take the bars out and machine a groove in each end of the block bore to take an "O" ring.  Polish the bars and lubricate the inside of the bores with silicon oil, re-fit and test for smooth movement.  The "O" rings will act as scrapers and keep the bars and bores clean.


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## BaronJ (Oct 3, 2021)

Hi MRA,

It would be nice to know if my suggestion was/is any good to you !
A similar technique is used in machinery that relies on sliding motion to provide lubrication and alignment.


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## MRA (Oct 4, 2021)

Hi Baronj

O rings sound like a good idea, thanks.  I had to put a circlip groove at the bottom of a deep, mostly blind (that is, I had to come at it from the far end) hole lately and it was a bit of a pain, but I guess at least O ring grooves would be at the top of a hole.  But that won't cure the basic wobble (I don't think - you weren't suggesting it would run on the O rings, were you?) - so I am going to go with slitting the holes along and adding clamps to give 'gib' adjustment.  If that scraps it, I guess I'll start again.


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## BaronJ (Oct 4, 2021)

Hi MRA,

Yes that is exactly what I was suggesting !

The bars currently run through the bores so if you just make sure that the bars are clean and lubricated an "O" ring fitted into a recess in the bore at either end of the bore will support and guide the bar.  

I think that the only issue would be if the bores are not parallel to each other.  But slitting and clamping would have the same problem.


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## MRA (Oct 4, 2021)

Oh!  Do you mean O ring as in rubber (or whatever they tend to be made of now) O ring?  I assumed there would be rather too much deflection as such rings might deform, even though the forces on this grinder are rather small - the tip of the cutter hitting the stone can put quite a long moment on the sliding block.  It might be possible to counterbore the holes a little at the ends (rather than recess true grooves) to half-seat an O ring, and then do a plate up against it to compress it - which would give a small amount of adjustment.  Can you tell I tend to design-in ways out of an expected lack of accuracy, before my habitual lack of accuracy even manifests itself?  

I think the bars are parallel, given the motion thus far is nice and free - just slightly shaky.  That's my motivation for trying to improve it rather than start again - I'm not confident my efforts to get up to the current point, would be any better. 

The U2 grinder I'm also working with, deals with backlash by spring-loading the only part of the motion not locked-off during use, so that the tool is already pulled off the stone (by that spring) as far as it will go against the backlash, before contact is even made.  That would be another way of doing this one, but the greater versatility of the x-y-z setup might make it harder to assume that the force of the stone was always going to act in the same direction.


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## BaronJ (Oct 4, 2021)

Hi MRA,

Actually, now that you mention it, a recess with a compression plate might be a better way to go.  I was thinking "Viton" "O" rings or Nitrile rubber ones.  They seem more forgiving of radial deflection.

As far as backlash is concerned, the moulded plastic nut that I've used on mine has essentially zero backlash !  Certainly non that I can measure.

I made a digital height gauge using one, the repeatability of it is far better than I can measure considering that it only measures to two decimal places.  ie 1/100 mm.


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