MEW-169 Tool-post Grinder Article

[blowlampParticipant @blowlamp]

I think the reason for the grinding wheel and surface of the workpiece to be moving in opposite relative directions to one another is similar to the reason why climb milling isn’t recommended in all circumstances.

 

If the points of contact of both the workpiece and the grinding wheel are moving in the same direction, albeit at different speeds, then you are in effect ‘climb grinding’, which has the dual action of driving and pulling the job, deeper into the cut. If there is any lack of rigidity within the system then a poor finish is more likely.

 

Martin.

 

Posted by JDEng on 26/10/2010 08:39:15:

Thanks for the welcome Steve!

 

After I had made my post I had a look through various textbooks which I’ve gathered over the years and they all state that the work and the wheel should be moving in opposite directions at the point of contact but none of them actually give the reason why. Thus they both need to revolve in the same direction if external grinding and in opposite directions if internal grinding.

 

What they do all agree on is that when toolpost grinding you need to achieve the maximum amount of rigidity in the set up if you are to achieve anything like a decent finish.

 

I’m with you Bogs on using a machine; I think you need to take common sense precautions such as covering slideways, cleaning down well after wheel-dressing and at the end of the job etc but at the end of the day they are machine tools for using. I’ve seen some awful abuse of machinery in industry and it still keeps working – not that I’m condoning that but I do think you’ve got to keep a balance between practicality and keeping something in cotton wool.

 

By the way has anyone seen the new toolpost grinder by Hemingway Kits?

 

John.

 

[WALLACEParticipant @wallace]

Our American cousins seem to like tool post grinders – do a search on ebay and there’s loads for sale over the pond.

 

I do have one that I’m in the process of revamping – biggest problem with it is that the long stud that holds the motor in place and keeps the drive belt in tension tends to loosen mid – cut and knacker the finish.

 

The plan is to put a strap on the side  – something like a car alternator one to hold it all in place. 

 

w.

 

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

Time for an educated guess as to why you grind the way you do.

When most of the books that everybody learnt from were written, ( and reprinted, and cribbed from, ad nauseum, without much revision) the only stones available were natural ones, not the complex bits of kit that they are now. I think most will agree that the greater the surface speed of the stone, within reason, the better the cut and finish. By having the two surfaces, stone and work, rotating  against each other you have in effect an increase in stone speed, but with less risk of it shattering. I also think that the climb milling/grinding is a bit of a red herring.

With modern manufactured “stones” and high speed motorized spindles the direction of the work is purely incidental, as the stone’s surface speed is doing all the work., and the work is rotating purely to produce the round shape.

I am quite prepared to be shewn to be wrong on this matter, but that is how it seems to me and I offer my thoughts as a  start for a discussion.

christephens 

[John OlsenParticipant @johnolsen79199]

I think you are probably right Chris. I used the air die grinder in the toolpost last night to cut off  and groove some 1/4 inch diameter hardened steel die pins, and was thinking at the time that with the lathe doing about 100 rpm backwards and the die grinder doing about 30,000 forwards, the lathe speed could hardly be making much difference to the relative speed at the contact point. However, since the lathe has a reverse and a speed control, I may as well keep doing it!

 

regards

John

 

 

[blowlampParticipant @blowlamp]

I can tell you what I know to be important when it comes to precision grinding and getting a good surface finish on materials such as cast iron and mild steel, as can be found on machine tools – so we’re talking components like cross slides and spindles etc.

Obviously use the right type of wheel for the material being worked, which usually means silicon carbide for cast iron and aluminium oxide for steel. Use a wheel of correctly sized grit and openness, as well as the right degree of hardness for the job in hand.

In my experience, most people don’t do much of the above unless they have to and just use what’s already fitted on the machine, but still get acceptable results provided they aren’t doing long job runs.

The next thing is to true the wheel to make it round, followed by dressing, to create the right surface texture. It is vital to dress the wheel properly, but unfortunately many folks make the mistake of passing the diamond across the cutting face of the wheel far too slowly. This has the effect of blunting the wheel and leaving it useless for its intended purpose. Evidence of this can be seen by the presence of any or all of the following:- Burning, judder marks and a burnished finish on the job.

Running the wheel too fast and/or the worpiece too slow, has the effect of making the wheel act harder and prone to blunting, in the same way a milling cutter will blunt quickly if spun too fast. Similarly, if the wheel is run too slowly or the workpiece too fast, the grit will be torn from the wheel because the chip load per tooth (per grit) is too high and results in a wheel which won’t hold its size, but will cut like a demon as fresh new grit is constantly being exposed.

The art is in getting the right balance between all the above factors so that new grit is exposed as the old grit is torn away as it becomes blunt, ie. chip load increases to the point where the wheel breaks down and so self-sharpens.

If you accept any of this, then far from being a red herring, you can see that the direction of rotation of the workpiece, as well as its speed in relation to the wheel is important, as it is linked to the wheel-speed/work-speed ratio.

Martin.

Edited By blowlamp on 26/10/2010 22:22:55

[Nicholas FarrParticipant @nicholasfarr14254]

 

Hi,

 

Posted by Steve Garnett 25/10/2010

If the job and the wheel are moving in the same direction at the interface (ie. one is going clockwise and the other anti-clockwise) there is a chance that, at certain speed combinations, the job will be ground as a series of flats which is clearly undesirable and rather defeats the point!

 

I think the reason is this. If the stone is not truly round, and they don’t wear truly even, then at certain speeds when they are rotating in opposite directions, any low or high spots on the stones circumference can coincide in exactly the same places on the work piece, thus causing un-roundness. However your stones speed is much faster than your work piece, therefore, your stones entire circumference will contact every point of the circumference of the work piece when they are rotating in the same direction.

 

Think of a printed strobe on a record player, this relied on the 100 times a second of flashing of your room light to coincide with the strobe to make the marks to appear to stand still. If you span the turntable freely by hand, you would find the marks stand still a various different speeds, albeit with different gaps and at a different brightness.

 

Of course, I might be thinking incorrectly.

Regards Nick.

Edited By Nicholas Farr on 26/10/2010 22:43:20

[chris stephensParticipant @chrisstephens63393]

Hi Martin,

Of course I accept what you say, but I don’t see the relevance to “red herrings” . The wheel is only seeing the work in one direction, no matter which way the work is turning, because of the speed differential. 

chriStephens 

[blowlampParticipant @blowlamp]

Chris.

I thought it was you that mentioned red herrings? My reply was to that comment only.

 

I have to disagree with you about the wheel only seeing the work (move) in one direction though, because if you look at the wheel/work rotation relationship again, you’ll see it can’t be true.

 

With the wheel spinning at any constant speed and direction and then presented to the outer periphery of a workpiece spinning in the same direction as the wheel, the chip will begin life at zero thickness and by the time it severs from the job will be equal in thickness to the cutting depth of the wheel.

 

If you now reverse the direction of rotation of the workpiece only, then the chip thickness starts equal to wheel cutting depth and ends at zero thickness. These examples are fully equivalent to conventional milling and climb milling respectively.

 

Martin.

[Steve GarnettParticipant @stevegarnett62550]

Even though it wasn’t me that said it originally, I can sort-of see what Nick Farr is getting at – although I think that you’d have to be pretty unlucky to have a wheel going at an exact synchronous multiple of the work rotation speed and worn badly enough to do this – I think you’d probably notice!

 

And yes, a moment’s thought says that Martin is quite correct about the climb milling – if this wasn’t a round surface that the grinding wheel comes into contact with, but a flat one then it’s just the same as running a surface grinder in one direction over a job rather than the other one – but isn’t that pretty much what happens on a surface grinder anyway? You pass the wheel forwards and then back again? If mine was actually running, I’d attempt to find out whether there was any noticeable difference, although I rather suspect that if there was, I’d need a microscope to see it.

Edited By Steve Garnett on 27/10/2010 00:52:41

[chris stephensParticipant @chrisstephens63393]

Hi Martin,

 Contemplate if you will on the relative speeds of the stone to a forward and backward work piece and, come to think of it, a stationary one as well. Provided the stone has a surface speed faster than the surface speed of the work, it will only see the work in one direction, as they said in my Maths class QED.

 

I stand by my statement that climb grinding/milling is, in this case, a “Red Herring” 

By the way, why is there an aversion to climb milling, there is nothing wrong with the practice provided there is no backlash in the system and cut depth is small. In fact surface finish can be improved by its use. I would not recommend its use to rank beginners though as its improper use can be hazardous to ones mental and financial health.

chriStephens 

[John OlsenParticipant @johnolsen79199]

Well, I don’t think any of my machines meet the “no backlash” condition.  Maybe if I had a little more money to spend?

 

I wonder though, if we consider a more substantial grinding wheel than my little air die grinder, then when they are opposing,  (eg both  rotating  the same way for an outside cut)  the forces on the two drive trains are both such as to try to slow them down. If they are not opposing, the grinding wheel is trying to make the job turn faster, and with a big enough wheel and a small job may succeed in doing so, eg it could grab a bit of extra cut, as can happen with the aforementioned climb milling. That might not be a good thing to have happen.

 

Incidently clockwise and anticlockwise are a  potentially  ambiguous, since it depends which end of the shaft you are looking at.

 

regards

John

[Ian S CParticipant @iansc]

About ten years ago I built a TP grinder with a sewing machine motor, dismally under powered, soon got the smoke coming out. At that time I got some books in, and all of them say the wheel should rotate in oppostion to the work. Interested in the idea of using a router as a power source. Even my Dremel tool has more power than a sewing machine motor. Ian S C

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Steve, sorry for using your post as reference, looking over this thread again, I see that is was JEDng’s post I should have referred to. However, it was just a theory of mine on this particular point. I have no real experience on tool post grinding, so this thread is educational to me. >>

 >>

Regards Nick.>>

> >

Edited By Nicholas Farr on 27/10/2010 10:12:35

[Steve GarnettParticipant @stevegarnett62550]

Posted by chris stephens on 27/10/2010 02:07:12:

 Contemplate if you will on the relative speeds of the stone to a forward and backward work piece and, come to think of it, a stationary one as well. Provided the stone has a surface speed faster than the surface speed of the work, it will only see the work in one direction, as they said in my Maths class QED.

 

I’m sorry Chris, but that’s not quite correct; the issue is not about speed. In one direction the wheel will be travelling into an uncut area with an upward motion, and in the other direction it will be travelling with a downward one. If what you are saying was true, then the concept of climb milling wouldn’t exist – but it does, and it’s real. QE really D!

[chris stephensParticipant @chrisstephens63393]

Hi John,

In truth neither do any of mine, but as long as the energy imparted by the tool  into the work, and mill table, is less than the energy needed to overcome the static mass of the bed, then all will be well. This means, in my case, taking very light finishing cuts.

 

Hi Steve,

The wheel will always be travelling into uncut area, otherwise it would not be taking a cut, would it. 

Try to imagine a car travelling along a road, if it is going faster than another car it will overtake it at a speed lesser than its actual road speed, but equal to its speed differential. If our car is now approaching another car travelling in the opposite direction, their closing speed is equal to the sum of their respective road speeds. Now change cars for a fast rotating grinding stone and a slow turning workpiece! Get the idea, it’s all to do with closing speed, one way it is greater than the other, but they are still closing whichever way the work is turning. 

 

I am still waiting to hear, from someone whose opinion I respect, to find the flaw in my argument.

chriStephens 

[blowlampParticipant @blowlamp]

Chris.

Before I continue with a further reply, could you just tell me whether you accept that with both wheel and workpiece rotating in the same direction, it equates to the conventional milling analogy and that by reversing the rotation of the workpiece, the analogy changes to that of climb milling?

 

I’m intrigued by what you mean when you say it’s a “Red Herring”. Perhaps you could expand on that also, please?

 

Martin.

[Steve GarnettParticipant @stevegarnett62550]

Chris, could you try to imagine what it is that Martin and I are on about? I can assure you that it’s true… and it’s not a speed issue.

[Steve GarnettParticipant @stevegarnett62550]

In an attempt to make it easier to see what it is we are on about….

 

 

[Gone AwayParticipant @goneaway]

If the analogy is to milling (conventional vs climb) aren’t the labels reversed on the diagram?

[Steve GarnettParticipant @stevegarnett62550]

Er, yes… whoops! I’ll redo that bit. Doesn’t alter the principle, though.

 

This has screwed the thread up slightly, but I think it’s still clear enough; only the correctly-labelled version is now visible.

Edited By Steve Garnett on 27/10/2010 23:56:23

[chris stephensParticipant @chrisstephens63393]

Hi Martin,

 There is no point in any further discussion, someone is clearly set in their ways and not open to reason. I, on the other hand, am quite willing to be proved wrong, when a well reasoned argument is put forward  to show me the fault in my logic.  

Let me buy you a pint at MEX in Dec and I will try to persuade you to my view, if I can’t then at least we can shake hands and agree to differ.

chriiStephens 

[Steve GarnettParticipant @stevegarnett62550]

I should point out that I do not know what difference it makes (apart from where all the dust flies), although the effect will surely be small. The contact angle will be very slight, and if the work and the grinder are both rigidly mounted, then presumably you wouldn’t get the problems you get with climb milling anyway. I had a look at several other sites where people talk about which way they do this, and there seem to be an equal number in both camps. And I have a completely open mind about it…

Edited By Steve Garnett on 28/10/2010 00:19:11

[Nicholas FarrParticipant @nicholasfarr14254]

 Hi Chris, Martin and Steve, as I’ve said, I have little to none in using tool post grinders. Picking up on Martins comment about the grinding wheel going to slow and tearing it apart ect., I can relate to. All my working life I have used hand held angle grinders in a lot of aspects of my work. I had to obtain an abrasive wheel certificate in conjunction with this activity, and while I was taught that wheel speed is important, it is the perimertary speed of the wheel that must not exceed the specs regardless of the RPM, to avoid wheels bursting. The analogy I’m relating to is the use of cutting discs moreover the 115mm variety. When new they cut quite a lot of metal before they start to lose any real diameter. Once they get to around 90mm in diameter they still cut, but the loss on the diameter is very much faster to the point of being to small to use. This would I think relate to Martins comments about going to slow. However, the difference in RPM terms relating to the perimertary speed is around 3000 rpm. With this in mind I don’t see how a differential speed of 100 rpm or so would have much effect.

 

I like Sid would think the lables should be reversed on the diagram.

 

Regards Nick.

[Steve GarnettParticipant @stevegarnett62550]

Nick, they have been reversed in the only version currently in the thread.

[GoCreateParticipant @gocreate]

Hi

 

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?

 

Maybe way back this is where the need for the grinding wheel and work surfaces moving in opposite directions originated, I would imagine in days gone that working between centres would be more common.

 

I have always shayed away from trying toolpost grinding but I have learnt so much from this thread I will be giving it ago. I have a Sherline headstock with variable speed upto 10,000 rpm, I am thinking that I should be able to set up a fairly ridged arrangement on my 6″ centre lathe and use the variable speed to get the right periferal speed for the grinding wheel diameter. Would not be suitable for using grinding points though, higher rpm would be needed.

 

Nigel

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