Surface grinding finish

[Anonymous]

Martin,

If I understand correctly, for a thread pitch of say 0.2mm across a 13mm wheel that means one traverse of the wheel should be the equivalent of 65 rotations of the wheel. The wheel should be doing about 3200rpm, so it takes about 1.2 seconds to do 65 rotations. I'm turning the cross feed wheel as fast as I can, I estimate between 1 and 2 seconds for a complete traverse,so that seems about right? Coolant would be nice. The machine is not, and never had been, set up for coolant, although it was originally available as a cost extra. It wouldn't be difficult to add.

Michael,

Simple things first; no the wheel doesn't have a nylon centre bush, the spindle is 1-1/4" and that's what hole in the wheel is. The grinder has fully automatic table feed and variable auto cross feed capability, although all the above tests were done manually. The table drive is rack and pinion and all the feed mechanisms are purely mechanical.

Now, onto the question of frequency domain. I assume you're not suggesting I do Fourier Transform analysis, but see how things vary with table speed. I didn't do this initially as I had in my mind that I'd tried this a while back and it didn't make any difference. So I was wrong, not for the first time, nor the last I suspect. I've just done a quick experiment across one face of the block. The depth of cut was about 1 thou, one traverse across the face, and a return sparkout traverse. The stepover per pass was 1/4". The first half of the traverses were done as fast as I could turn the table handle, the second half much slower, a few seconds per pass. The results are rather interesting. The first half of the block is as before, a very obvious regular wavy pattern spaced at about 1/16". For the second half there is still a regular wavy pattern, but the spacing is closer, about 1/32", and the amplitude is much smaller. In fact you have to get the light on the block at the correct angle to see the waviness clearly. So it appears that both the amplitude and frequency of the wavy pattern is related to table speed. Now as to what that means is another story. I'll have to cogitate on it

Regards,

Andrew

[blowlampParticipant @blowlamp]

Andrew.

Can you slow the wheel down and/or increase the table traverse speed?

The results you're getting are typical of a blunt wheel burning the job and burnishing the workpiece surface.

The ripples are caused by judder, as the wheel needs opening up to allow it to cut freely and the lack of coolant won't help either.

Martin.

[DustyParticipant @dusty]

Gentlemen

Before I venture into the fray I would say that it is many years since I did any surface grinding. The passage of time may have dimmed my memory, but here is my two pennyworth. From your photo Andrew it is apparent that you are getting wheel bounce. This can be for a number of reasons. The most common is the wheel is out of balance (this is usually found on the simpler types of surface grinder where the wheel is mounted straight onto the spindle) The second is as Blowlamp has alluded to, that the wheel is not fully dressed (you need to be a bit brutal and cut the wheel back to virgin stone) What happens, is where particles of metal are still emmbeded in the wheel, as it traverses the workpiece the wheel skids on the emmbeded material. This has the effect of minutely lifting the wheel as the wheel continues to rotate it drops back with a slight dig in, this causes the effect in the photograph, it will also cause the burning, too fine a grit will exacerbate this as it will tend to clog much quicker. Coolant will help with the clogging and the surface finish. If you do fit it up remember to start the wheel before the coolant, if not things can get a bit exciting.

[Tony Pratt 1Participant @tonypratt1]

Hi Andrew, more advice I'm afraid. Mild steel in my experience is difficult to get a decent ground finish on at the best of times, you may want to consider grinding a hardened parallel or something similar [not HSS] You don't want to slow the wheel down that will makes things worse! Aluminium oxide of 60 grit is a good starting point , it is also suitable for cleaning up the mag chuck.

You wheel may be out of balance, has the arbor got balancing weights attached? As for dressing with a diamond, true the wheel with .001" cuts then .0005" to finish, keep passing the diamond back and forth across the wheel until it doesn't produce any more dust, feed for this operation should be moderate, it's a bit difficult to be more specific.

As for speed of table traverse and cross feed there may be examples of surface grinding on Youtube which may help you, also coolant is really important if you can manage it!

Best of luck,

Tony

[blowlampParticipant @blowlamp]

Andrew.

As a test. Once you've dressed the wheel at various speeds, try freehand grinding a bit of HSS as you would on a bench machine and let us know how freely the wheel cuts

Martin.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Andrew ,

Just for interest :

A mechanical system behaves just like an electronic circuit and many of the design , calculation , test and problem solving methods of electronics can be used to analyse the behaviour of mechanical systems .

Anyway you will be used to thinking in terms of frequencies and responses from your own electronics work – try applying same principles to your surface grinder .

A few things that you may like to think about :

There is an instability shown by the ripple pattern .Observe from the pictures that this is a quasi stable , periodic and relatively low frequency event therefore need to evaluate the inputs to the system and look for potential sources of low frequency excitation :

Unlikely to be the drive system or a faulty wheel since these would generate high frequency events .

Unlikely to be bearings since badly adjusted or damaged bearings tend to generate chaotic events rather than periodic .

Rack and pinion drive might cause right sort of frequency of events in two ways :

(1) Rack simply bumping or more likely tooth clearances varying as each rack tooth engages and disengages causing periodic small changes in feed rate and in particular periodic dwells . Need not be the fundamental cogging frequency – unlikely but could be a low level harmonic also .

(2) Combination of human operator , rack and pinion and feedback from wheel load all interact to give a spongy system and periodic variations in feed rate . This feed subsystem will have a natural frequency for any specific set of conditions .Frequency will tend to be low and not vary very much .

Numerical values can be put to the ripple frequency and the simple rack effect frequency . Tests would be needed to find instability in feed system and any resulting natural frequency .

The presence or absence of damping in the feed system may be relevant .

Regards ,

Michael Wiliams

[Anonymous]

Thanks one and all for the suggestions. There's plenty of ideas to mull over. I've done a couple of extra tests this evening.

FIrst I tried grinding a piece of HSS tool steel. All my parallels are soft, and I don't have any hardened carbon steel parts that I'd be prepared to experiment on. The results are similar, a fine wavy pattern. After the first trial I gave the wheel a good dressing down, at least 20 thou off the diameter, but the subsequent results were similar. Surface roughness measurements were:

0.01": 0.17um

0.03": 0.20um

0.1": 0.54um

For the second experiment I reverted to the original soft steel block and tried a pass across using the table power feed and auto cross feed. Superficially the finish looked good, without the obvious patterning of before. However, with the light at the correct angle it was clear that the surface was uneven, with an irregular waviness, but with the crests much further apart, about 0.3" rather than 0.05". I think this is because the table moves much faster in power mode than I can move it by hand. The hand wheel is almost a blur, if you got in the way it'd take your hand off.

I like the idea of wheel bounce, even after dressing the wheel gets loaded quite quickly. The question is why is the wheel bouncing? Quite possibly because I've got the combination of wheel type, speed and feed incorrect. However, I wouldn't rule out badgered spindle bearings. The spindle bearings are of the plain opposed taper type, with springs in the middle holding the taper bearing apart. The machine has a plate on it specifying a strange concoction of 'water white kerosene' and a light mineral oil for the spindle. In discussion with Hallett Oils we decided this was equivalent to an ISO3 oil. It's just like water. If you pour it into the sight glass it just pisses straight out of the front of the spindle! The same is true for the ISO15 and ISO32 oils that I use on my cylindrical grinder. That seems to imply that the bearing clearances are rather larger than they should be?

Tomorrow I'll try freehand grinding. By the way the wheel spindle has no balance weights.

Regards,

Andrew

[Anonymous]

I haven't tried freehand grinding, but I have now tried a couple of new grinding wheels. Both wheels are 150x13x31.75mm. These are smaller than the previous wheel, so probably running nearer 5000 sfpm rather than the 6000 sfpm before. When I get round to it I'll borrow a tacho and get an accurate wheel speed.

Both wheels are white aluminium oxide, one is 46KV and the other 60KV. After mounting and dressing each wheel I tried the same experiment on a block of hot rolled steel. The parameters were 0.5 thou depth of cut, stepover of 1/8" and 3 three sparkout passes, all manual feed. I took Ra (in µm) measurements with the direction of feed and across it. For the 46 grit wheel:

with across

0.01" 0.16 0.65

0.03" 0.44 0.67

0.1" 0.54 1.00

and for the 60 grit wheel:

with across

0.01" 0.16 1.11

0.03" 0.36 1.24

0.1" 0.54 1.15

Oddly enough the 60 grit wheel gave the worst finish in terms of Ra. This may be because the wheel loaded up quite quickly, whereas the 46 grit wheel hardly loaded at all. In both cases the finish looked pretty good and felt smooth. A faint pattern was just about discernable, but not spaced or regular as before. I'd say it was more a natural consequence of the grit size rather than a problem. With the light at the correct angle some faint iridescence could be seen, indicating that the surface roughness is less than the wavelength of visible light.

I also did a similar experiment with the 46 grit wheel under power feed, except that the stepover was 20 thou:

with across

0.01" 0.11 0.96

0.03" 0.47 1.01

0.1" 0.71 0.85

The surface feels smooth but has clearly visible striations perpendicular to the wheel travel. These striations are irregularly spaced, but are consistent across the width of the block. I am sure that these are a consequence of the power feed to the table. One can see and feel the whole machine vibrate as the table moves. Given the movement of the machine I'm amazed that the effect is so small.

Once again thanks one and all for the help and suggestions.

Regards,

Andrew

[James BParticipant @jamesb]

Hi Jim (Cahill),

You mentioned in an earlier post that you renovated the Eagle Surface grinder on lathes.co.uk – have looked at those pictures many times..!

I have recently acquired an Eagle surface grinder, and am at the stage of removing the spindle – could you let me know how you went about this on your machine? Considering the above information, I don't want to force anything, and I don't have any manuals or information to work from.

Great information on this thread – can't wait to see the results on my machine…!

Thanks,

James

[BubbleParticipant @bubble]

Hi James

I hav sent you a PM

Jim

[Author]

Posts