MEW-169 Tool-post Grinder Article

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Steve, I was reading through and compiling my post during the time you posted your revised diagram. sorry for any confusion.

 

Regards Nick.

[Steve GarnettParticipant @stevegarnett62550]

Posted by tractionengine42 on 28/10/2010 01:41:32: 

 

Steve, thanks for the diagram, comfirms what you’ve been saying.

 

If the work is held between centres then you will have to ‘not climb grind’? Am I right?

 

Well I’m glad that somebody’s seen the point… but I don’t think that the way the work is mounted makes any difference. In the diagram, the centres would be aligned through the page, and  you could easily arrange to turn the work either way.

 

What on the face of it appears to be happening is that with climb grinding, the wheel leading edge attacks a fractionally larger amount of material, and in fact it’s easy to demonstrate that the faster the work rotates, the greater the attack amount is. With non-climb grinding, the opposite happens; the grind wheel attack amount is at a minimum, and this increases progressively through the contact. So the speed the work rotates in either scenario determines the rate at which a very small amount of material is removed.

 

What I don’t know though is how much difference this makes – we are talking about a very small contact area, certainly with a typically small wheel. But if there is going to be any difference in the results, this is where it will have come from, because it’s the only other mechanical effect involved when you’ve removed rigidity issues, etc. I’m quite prepared to believe that it’s a red herring – that’s fine, as long as I’ve at least attempted to understand why!

[GoCreateParticipant @gocreate]

Steve

Understood, thanks.

Nigel

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

As has been mentioned, when grinding you are taking off only tenths of thous, not eighths of inches as when milling. It is for this reason I maintain that conventional versus climb grinding if an irrelevance and a red herring. 

As has also been mentioned surface speed, of the grinding medium, is important for good grinding. Work going in the wrong direction at too high a speed would reduce the relative grinding speed, possibly to a dangerous level. 

If this thread has spurred some of you to try tool-post grinding, do try to work safely. The toolpost grinding wheels might be small but they can throw off sparks just like big ones, please remember that eyes and sparks do not mix well!

As the Sargeant in “Hill Street Blues” used to say “lets be careful, out there”

chriStephens 

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

As has been mentioned, when grinding you are taking off only tenths of thous, not eighths of inches as when milling. It is for this reason I maintain that conventional versus climb grinding if an irrelevance and a red herring. 

As has also been mentioned surface speed, of the grinding medium, is important for good grinding. Work going in the wrong direction at too high a speed would reduce the relative grinding speed, possibly to a dangerous level. 

If this thread has spurred some of you to try tool-post grinding, do try to work safely. The toolpost grinding wheels might be small but they can throw off sparks just like big ones, please remember that eyes and sparks do not mix well!

As the Sargeant in “Hill Street Blues” used to say “lets be careful, out there”

chriStephens 

[blowlampParticipant @blowlamp]

Greetings all.

 

If I can relate my experience with surface grinding, using a horizontal spindle machine, it might be of some help to those that are new to the process of precision grinding as the principles can be applied for cylindrical grinding too.

 

After doing all the preparatory work such as selecting and dressing the wheel and mounting the job to the table or chuck, comes the actual process of passing the workpiece under the wheel to remove the waste material.

 

Firstly, use a common sense approach to find the point at which the wheel just touches the top of the job, similar to those used for milling etc.

 

Position the workpiece in front of the wheel, with a small gap to allow the workpiece to traverse without passing under the wheel.

When you’ve done that, apply a roughing downfeed in the order of 0.01mm to 0.02mm and flood with coolant.

 

Start to traverse the workpiece smoothly, left to right at an appropriate speed – and if we’re talking about a manual traverse machine here, this usually means about as fast as you can move it!

 

Apply a crossfeed of about 2.0mm at the extremity of table travel, but note that with a wheel rotating in a clockwise direction, any crossfeed applied whilst the work table is moving right to left will be a climb cut and so more likely to grab, particulary if an anti-friction work table is fitted. If the wheel does grab, it can move on its flange mounting or become crushed locally, thus causing defects in the finish.

 

It’s quite possible to crossfeed at either or both extremes of travel and depending on how all the above factors are interacting with each other, this may or may not have an impact on surface finish.

 

If conditions are right, you’ll get a lovely smooth finish. If things aren’t going your way, you could be seeing judder, crossfeed lines, or burning. The first thing you should do here is dress the wheel to open it up and allow it to cut freely again.

 

Check out some of the models and tools at any exhibition and you’ll almost certainly see some of these defects. In my opinion it’s purely down to unfamiliarity with the grinding process and is not intended as a criticism of the builder.

 

When I started, I found it best have quite a fast table speed, as it helps keep the wheel sharp. Having it too fast will open the wheel up so much that you’ll get a very coarse finish on the job. You’ll then need to re-dress the wheel and slow the table until you reach a good compromise of finish vs wheel wear.

 

Bear in mind that varying the wheel speed whilst keeping the table speed the same can also be done, as the important factor is the ratio between wheel speed and work traverse speed, because this ratio determines chip size and tooth (wheel grit) load.

 

As an example:- With the wheel locked in a stationary position with only the workpiece moving past the wheel, would result in a chip of size equal to the length of the job multiplied by the depth of cut. This is an impossibly large tooth load and would break down the wheel quickly. Reversing this scenario should highlight the other extreme, that being, tiny chip equals tiny tooth load which in turn equals a soon to be blunt wheel (frequent dressing).

 

Finding the right balance of the above is the key to good grinding.

 

 

Martin.
 

[WALLACEParticipant @wallace]

Just an observation – the commercial toolpost grinder I have doesn’t have a forward / reverse switch on it  – even though originally it came with an internal and external wheel head (internal one’s missing – something to make up one day . . !).

May I should fit one .  . 

 

w.

 

[Steve GarnettParticipant @stevegarnett62550]

Duplicate post, sorry – see below:

Edited By Steve Garnett on 29/10/2010 10:29:40

[Steve GarnettParticipant @stevegarnett62550]

Whether you could do this safely rather depends on how the wheels are fitted and retained on the heads. If they are held in place by any sort of screw threaded arrangement it could potentially undo itself if reversed, unless steps are taken to prevent this. And of course it has to have a motor that’s capable of being reversed…

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

The direction of rotation of the workpiece relative to the wheel does not of itself affect the grinding process at all . The wheel and work are arranged to travel in opposite directions at the point of contact so that all the backlash in the workpiece headstock (and drive ) is taken up and the grinding process is then stable . If the work is arranged to travel in the same direction at the point of contact then the backlash is always present and the grinding process is unstable . Grinding in the unstable way can give rise to things like poor finish , faceting and out of roundness . In less rigid set ups (like internal grinding of a small holes) grinding in the unstable way greatly increases the probability of wheel snatch occuring .

 

The problem of grinding dust damaging lathes is overstated – the abrasive rubbish that comes off a wheel casting being machined is much worse . Some protection with safely arranged coverings and a good clean and oil afterwards is sufficient . The only place that seems to trap grinding dust harmfully is the internal workings of lathe chucks and these can be protected quite easily .

 

Grinding wheels are considered as hazzards in industry and special rules apply to their use . Some of you may like to read the Grinding Wheel Regulations which give good information about how to stay safe when using grinding wheels .

[WALLACEParticipant @wallace]

 

Hi Steve.

 

You have a very good point !

 

Seem to remember the bolt holding the spindle drive pulley is reversed thread, the one on the grinding wheel is  normal.

 

I shall leave as is . .

  

Incidently, I only used it once to skim off a flywheel  -see below  – looking at the switch on the lathe, I reckon it was going clockwise . .

 

 

W.

 

 

 

 

 

 

Edited By WALLACE on 29/10/2010 14:20:01

Edited By Katy Purvis on 01/06/2015 12:47:40

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

This makes first line of my posting above a little clearer :

 

 

The direction of rotation of the workpiece relative to the wheel does not affect the grinding process at all if there is no backlash present . Since there is always some backlash this has to be taken into account in practical grinding . ——-

[John OlsenParticipant @johnolsen79199]

On the non reversable grinder…You only need to be able to reverse one of the spindles, and most larger lathes would have a reverse available I would think.

 

Speaking of safety…if anyone does use the idea I mentioned with the small Dremel type cutoff wheels, be aware that these wheels are very brittle. I make a point of not being in the direct line of the wheel at all, as when they decide to go they fling bits off in all directions. Having said that, I have broken more of them putting them on or taking them off than in service. Luckily they are very cheap.

 

regards

John

[Steve GarnettParticipant @stevegarnett62550]

Posted by MICHAEL WILLIAMS on 29/10/2010 14:31:37: 

 

The direction of rotation of the workpiece relative to the wheel does not affect the grinding process at all if there is no backlash present . Since there is always some backlash this has to be taken into account in practical grinding.

So that puts you firmly in what I called the ‘non-climb’ grinding school. It’s the one that makes intuitive sense to me, but just so that I’m sure of what you are saying, can you confirm that you are suggesting that the grinding wheel energy (even with minimal contact) is sufficient to take up the backlash in the drive to the work? If that is indeed the case, then it would be a very convincing reason for having the surface travel directions opposing each other.

 

I’m not trying to provoke any arguments here, just trying to get a good grip on this so that next time I try it, I’ll have some sort of idea what might really be happening. There appear to be two somewhat different approaches to what might actually make a difference…

[blowlampParticipant @blowlamp]

Steve.

If you’re grinding in the conventional way, that is, by the non-climb method, then the backlash is automatically removed as soon as the work head is powered up and turning the job.

Think of how all the backlash becomes irrelevant from the screwcutting train on a lathe when threading.

 

Using a climb grinding technique requires the workhead drive motor to actually function as a brake, because the workpiece will tend to be driven by the wheel and isn’t ideal.

 

Martin. 

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Blowlamp : well said – that’s exactly how it is .

 

Steve :

 

The transmission force between the surface of the wheel and the surface of the work is usually small but it is by no means negligible – quite enough to drive the work around the wrong way if given freedom to do so . Your experience with an ordinary bench grinder will give you an idea of the magnitude of forces involved .

 

 

 

 

[chris stephensParticipant @chrisstephens63393]

Hi Michael,

As an example of how little force is involved, I have been known to true up rotating centre cones, which rotate with no more drive than the oil film in the bearings! If that is not negligible, it must be very close to it.

chriStephens 

[Steve GarnettParticipant @stevegarnett62550]

Yes I do understand what backlash is – that’s not the issue. I just wanted to make sure that I’d understood what Michael was saying correctly. I think that Martin’s point about the workhead motor then acting as a brake sums it all up rather nicely – the less the force applied by the grinding head, the greater the chance of ‘chatter’ in the work drive, which translates into changes in the rate of surface progress of grinding – obviously to be avoided.

 

It’s been a useful discussion though, because it now means that if I contrive a means of rotating work on the tool cutter (which I was thinking of), then I’ll do it with a stepper motor and a toothed drive, I think. More control over the rate of progress, and potentially zero backlash, whatever I do with it.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Steve ,

 

The drag load on the work (and wheel) in cylindrical grinding varies in a highly non linear way with depth of cut . The worst case is in internal grinding where the drag force varies as something like depth of cut to the power three so careful management and light cuts allow you to do delicate work with almost no drag whereas a quite modest increase of depth of cut generates a much higher drag and an ill judged or accidental over deep depth of cut leads causes a massive level of drag and wheel snatch inevitably follows with all its destructive and potentially dangerous consequences .

 

Its interesting to reflect on how long grinding technology has been around and studied . Certainly the Romans had powered (treadmill or water) grinding wheels turning at quite high speed for making and sharpening their weapons and they probably only copied and improved on what the ancient Greeks had done many years before .

[Steve GarnettParticipant @stevegarnett62550]

Posted by MICHAEL WILLIAMS on 31/10/2010 10:48:01:

Hi Steve ,

 

The drag load on the work (and wheel) in cylindrical grinding varies in a highly non linear way with depth of cut . The worst case is in internal grinding where the drag force varies as something like depth of cut to the power three so careful management and light cuts allow you to do delicate work with almost no drag whereas a quite modest increase of depth of cut generates a much higher drag and an ill judged or accidental over deep depth of cut leads causes a massive level of drag and wheel snatch inevitably follows with all its destructive and potentially dangerous consequences .

 

I had begun to wonder about the degree of drag and how it might vary, so I did a bit more research into that myself. The obvious additional factor seems to be that you have to add the rate of progress to this (work rotation speed), as well as the cut depth, but interestingly, I also found a reference to potential problems to do with grinding spindle flex, especially with small wheels. It went on to say in this particular context that there is also a corollary issue with letting the grinding wheel completely lose contact with the work at any point (if it is going to return to it, of course), and suggests that steps should be taken to prevent this from happening.  Anyway, it’s certainly not too difficult to see why the drag load vs. everything else relationship is complex!

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi again Steve ,

 

When a poorly supported grinding wheel is overloaded the condition of wheel snatch can occur – this is when the wheel instead of running true about its own axis tries to roll along or around the workpiece . Almost unknown in surface grinding but it can happen occassionally in external cylindrical grinding and is a common problem in internal grinding . 

 

When wheel snatch occurs in internal grinding the wheel tries to roll around the inside of the work in the manner of an epicyclic gear and since it’s spindle is now both bent and running at high speed the whole process goes totally out of control and there is danger to the operator and inevitable damage to the work .

 

Wheel snatch in internal grinding is a particular problem because the wheel support arbours tend to be small diameter and sometimes long giving poor stiffness and  the convex surface of the wheel is settled into the concave surface of the work with a very long arc of contact .

 

Wheel snatch is made more likely to occur by any loss of rigidity anywhere in the whole set up and by choosing a wheel size which is too big relative to the work diameter .

 

Wheel loading has as you say many input variables !

 

 

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