Surface grinding finish

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Surface grinding finish

This topic has 34 replies, 15 voices, and was last updated 16 May 2012 at 13:00 by Bubble.

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10 April 2012 at 12:02 #88851
Tractor man
Participant
@tractorman
Hello all,

I have renovated a Herbert manual surface grinder and it performs well BUT.

The surface finish I get is very flat, but looks a bit like fish scales or very very fine serrations.

Does anyone have any suggestions as to the cause for this surface finish? I thought that the long drive belt may be causing some kind of speed variation due to its tension. Is it something I am doing wrong on the speed of feed or depth of "cut"? Obviously I have no way to check the speed I am traversing the work at but fast or slow does not seem to make much difference.

Any thoughts or ideas gratefully accepted.

Regards T
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10 April 2012 at 12:02 #11914
Tractor man
Participant
@tractorman
10 April 2012 at 13:13 #88855
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Could be the drive system but since it presumably worked ok in the past I'd look elsewhere to begin with .

The most likely cause of problems is that the wheel is not dressed properly .
10 April 2012 at 15:02 #88866
David Colwill
Participant
@davidcolwill19261
I have also struggled with surface grinding. I have managed to get reasonable results by trial and error. This surely would make a good article for MEW. I would be interested to know how often should the wheel be dressed and how do you compensate for wheel wear?

David
10 April 2012 at 15:14 #88867
Bubble
Participant
@bubble
Hi Tractor Man

Had similar trouble when I renovated my eagle grinder (seen on http://www.lathes.co.uk/eagle_surface_grinder ) I fitted a powertwist belt with a belt tensioner which improved things but did not eliminate it, then fitted new spindle bearings which did likewise. Finally stripped it all down and discovered that the hardened spindle was slightly bent, about two thou in the middle. This caused shaft whirl at 2800 rpm which caused wheel wobble, even though the wheel was diamond dressed on the machine. Made a new spindle from EN8, ground all over, which provided a complete cure. Surface finish is now impeccable.

The bent shaft could have been caused by heat treatment during manufacture, but I suspect some clown lifted the machine with a sling around the shaft (not me I hasten to add).

Hope this helps

Jim

p.s. copy & paste the url, doesn't seem to work from within the post

Edited By Jim Cahill on 10/04/2012 15:16:31
10 April 2012 at 15:24 #88868
Les Jones 1
Participant
@lesjones1
Hi Jim,

I think the "_surface_grinder" needs to be left of the URL

I think this is it.

Les
10 April 2012 at 16:58 #88873
mick
Participant
@mick65121
Have you balanced the grinding wheel and are you dressing the wheel with a diamond? Other causes could be the wrong grade wheel for the material being ground, or play in the head bearings
10 April 2012 at 17:07 #88874
Stewart Hart
Participant
@stewarthart90345
Check balance of wheel, and dress wheel with a dimond, when you dress it travers the dimond across the wheel slowly at the final pass take it across a couple of times at the same setting.

Stew
10 April 2012 at 17:07 #88875
Stewart Hart
Participant
@stewarthart90345
Check balance of wheel, and dress wheel with a dimond, when you dress it travers the dimond across the wheel slowly at the final pass take it across a couple of times at the same setting.

Stew
10 April 2012 at 17:52 #88879
colin hawes
Participant
@colinhawes85982
It's a long time since I used a surface grinder but I would make sure the wheel is of a suitable grade. I can't exactly recall the wheel spec. but 46 I springs to mind. Wheels that are too hard tend to bounce. Colin
10 April 2012 at 20:46 #88899
Tractor man
Participant
@tractorman
Many thanks for the posts so far, The spindle is mounted internally and I don't think it could have been subjected to bending forces as described by Jim, So I am hoping there will be no need to replace it.

I recall the bearings were fine so they were not replaced, I am hopeful that I have refitted and adjusted them correctly but that is a check I will complete after I look into the wheel grade and balance/dressing of the wheel. Thanks for the advice on dressing, I am using a properly mounted diamond dresser and following instructions but may be rushing the process.

I fitted a single slack adjuster on one side of the belt but this made only a slight change to the finish, I am going to fit one to the other side to control slap on that long run too as it was a simple mod to do.

I will report back with any progress but is there any comment on speed of traversing the table? I have seen a you tube clip where the table is motorised and literally flies back and forth, what is an advisable speed to work the lever back and forth?

RegardsT
11 April 2012 at 12:12 #88937
colin hawes
Participant
@colinhawes85982
You should not be taking more than a two thou depth of cut ; for a good finish 2 or 3 tenths of a thou is about right. Table speed about 3 ins per second should be satisfactory. Colin
11 April 2012 at 16:45 #88947
Bubble
Participant
@bubble
Hi Tractor Man

I found this anonymous article from Practical Machinist from 2002 which I think is very useful.

http://www.practicalmachinist.com/vb/greatest-hits-links/surface-grinding-deeper-cuts-better-finishes-110659/

regards

Jim
11 April 2012 at 20:55 #88962
Tractor man
Participant
@tractorman
Hi all,

some success with the problem I posted about. I was using a 60 grit wheel and I dressed it according to advice and the finish was markedly better.

Seems the feed and table speed are a dark art so plenty of experimenting to get the right feel.

But I changed to an 80 grit wheel and dressed it and things are a great deal better. Very much smoother and visibly superior to the best efforts with the 60 grit.

I took off the drive belt and the shaft feels smooth and free running to me but I could be wrong so I might have a go at re-adjusting the bearings.

I will also make up another belt tensioner this weekend and report back when that is done.

The original 3 phase motor is long gone and a clarke single phase has been substituted, could that be causing problems? Have read other threads that suggest phase converters on three phase can cause issues with surface finish as the motor does not run dead smooth.

Not sure about deep cuts and shallow feeds as the machine has no coolant provision so heat might become a problem.

Thanks so far.

Regards T
12 April 2012 at 09:22 #88979
David Clark 13
Participant
@davidclark13
Hi There

A60K5V or A46K5V for the wheel.

If grinding a magnetic chuck, steel and brass, you will get good results with a silicon Carbide wheel.

Dressing the wheel

The diamond should be at an angle to the wheel so the grit is torn out leaving it sharp. If you juist wind the wheel across the top of the diamond it will be blunt.

Dress the wheel as above and then relieve the wheel across 2/3 to 3/4 of its width by about 5 thou deep like this ————___ This takes the load off the face of the wheel.

Maximum depth of cut would be about a thou although we used to take 60 thou cuts on cast iron.

regards David

Edited By David Clark 1 on 12/04/2012 09:23:16
12 April 2012 at 15:42 #88991
Anonymous
Hi David,

If I read correctly you're recommending a silicon carbide wheel for grinding steel? I'm trying to surface grind some steel (low carbon, hot rolled) at the moment, and while the opposing faces feel flat and I cannot measure any difference in thickness there is definitely a fine, but regular pattern on the surface. I'd assumed that it was due to the wheel bearings being knackered, but might it be due to the wheel skipping? The wheel I'm using is white aluminium oxide, 46 grit, I can't read the rest of the description but I think it was medium hardness.

Tractor man: A single phase induction motor has a pulsating magnetic field, which may, or may not, affect the surface finish. A 3-phase motor does not have this pulsating effect. There are those who state that a single phase motor can affect the finish when using a lathe: I would have thought that a surface grinder would be more sensitive. I'm using a 3-phase motor, as I have a 3-phase supply. My grinder would have originally been supplied with a single motor driving both wheel and table feed (all mechanical) but had been converted at some time in the past to two single phase motors, one for each. The wheel drive motor smoked soon after I got the grinder, so I threw the whole lot away and reverted to a single 3-phase motor driving everything.

Regards,

Andrew
12 April 2012 at 16:18 #88992
David Clark 13
Participant
@davidclark13
Hi Andrew

No, I did not recomend a Silicon Carbide wheel for steel.

I recomended it for a brass and steel magnetic chuck. the one where the brass and steel interleave.

regards David
12 April 2012 at 21:49 #89007
Anonymous
David: Now I reread your post I see that I mis-understood. I'll probably buy a silicon carbide wheel anyway, as I need to clean up the top of my magnetic chuck.

Regards,

Andrew
13 April 2012 at 02:35 #89021
Jon
Participant
@jon
Coolant will help a lot too.

Takes me back a few decades mainly Snows, excellents responses above.
13 April 2012 at 11:53 #89030
Tractor man
Participant
@tractorman
Again some really helpful posts and a lot of experience shared so many thanks to all who have contributed.

My machine does have cross feed dials and I fitted a DRO to the Z axis so I can gauge depth of cut, but it only reads down to five tenthous so its hard to get really small finishing cuts, How do you do that with such a course lead screw on the column?

I also found quite a bit of movement in the table gibb so I will adjust that tonight to ensure smooth operation with no possible vibration etc.

I'll also try to reset the spindle bearing as I did not have any information on doing that when I rebuilt her.

Onwards and upwards, T
22 April 2012 at 20:31 #89564
Anonymous
I've finally found time to write up the results of some experiments with my surface grinder! The machine is a Brown & Sharpe No.2; it has a mechanical table and cross feed drive, but all the results to follow were obtained by hand feeding. The current wheel is 180x13x31.75mm, aluminium oxide, 46 grit. All surface roughness measurements are Ra, and are in micrometres. Values of 0.01", 0.03" and 0.1" refer to the distance over which the surface roughness gauge takes its average. The test block is hot rolled steel, rough machined on a horizontal mill. Here's the block before grinding:

I tried two strategies, on one side I used one turn of the cross feed (1/4&quot between table feeds and for the other side I used a quarter turn (1/16&quot per table feed. For each side I took cuts of about 1thou until the surfaces were clear of milling marks. I then took a final cut of half a thou plus a sparkout traverse. Here is the surface after grinding with a stepover of 0.25":

and here it is with a 1/16" stepover, it looks worse, with some evidence of wheel burn(?):

In both cases one can see a regular striation perpendicular to the direction of grinding. It doesn't show up on a 'tenths' dial gauge with a surface plate, but is clearly visible. Here are the surface roughness measurements for the 1/4" stepover:

0.01": 0.17um

0.03": 0.54um

0.1": 0.62um

and for the 1/16" stepover:

0.01": 0.10um

0.03": 0.36um

0.1": 0.46um

Oddly enough, the smaller stepover side seems to have a better finish, despite looking the worst. As far as I could tell the block is parallel to within a couple of tenths.

Both sides of the block were then rubbed on 800 wet 'n' dry until the striations disappeared. The surface roughness was then re-measured. Results for the 1/4" stepover are:

0.01": 0.06um

0.03": 0.07um

0.1": 0.14um

and for the 1/16" stepover:

0.01": 0.06um

0.03": 0.33um

0.1": 1.26um

Here's the finish on the 1/4" stepover side after wet 'n' dry:

Clearly there's something odd about the measurements on the 1/16" stepover side after wet 'n' dry; presumably finger trouble on my part.

The measurement parameter Ra is an average roughness, unlike the more modern parameter Rz, which is a peak to peak measurement, and so possibly more useful. There is no direct relationship between Ra and Rz, but a rule of thumb is that Rz is about an order of magnitude bigger than Ra. For the results on the 'as ground' 1/4" stepover (0.1&quot the peak to peak value is about 6.2 micrometres and after wet 'n' dry it is about 1.4 micrometres. The difference is 4.8 micrometres. A tenth of a thou is about 2.5 micrometres, so about two tenths. This rough 'n' ready estimate is borne out by the block now being thinner by around two tenths of a thou, according to the micrometer, in a comparative measurement.

A final word on the striations. I guess they could be wheel bounce, or the wheel bearings are badgered. I lean towards the latter, but in due course I will try different wheels and materials to see if it makes any difference. In the short term if I have to use a bit of wet 'n' dry to get rid of the marks, so what, I'm building traction engines, not a rocket!

Regards,

Andrew

Edited By Andrew Johnston on 22/04/2012 21:05:58
22 April 2012 at 21:53 #89571
blowlamp
Participant
@blowlamp
It looks more like a blunt wheel to me, Andrew.

Open up the grain by passing the diamond across the wheel much faster than you are doing. Also be sure to traverse the table faster too, as this will help to rip out the blunt grit from the wheel.

Martin.
22 April 2012 at 22:55 #89573
Anonymous
Martin: Hmmm, didn't think of that, could be, thanks. I am winding the cross feed as fast as I can when truing the wheel. Say less than a couple of seconds across a 13mm thick wheel. How fast should I be traversing it?

I could also try it under table power feed, but as far as I remember it has always done this, manual or fully automatic feeds. More experiments needed!

Regards,

Andrew
22 April 2012 at 23:15 #89574
blowlamp
Participant
@blowlamp
Andrew.

I don't know in mm's per second, but just try it faster than you would normally think is necessary – probably equivalent to a thread pitch across the wheel of say 0.1 to 0.2mm.

Slow the wheel if you can so each grain has something to bite in to when cutting the job – just think how easily any tool blunts if it's driven too fast, especially if combined with a shallow cut.

Use coolant to remove the swarf and keep the wheel clean etc.

Martin.
22 April 2012 at 23:29 #89576
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Hi Andrew ,

(1) Do your analysis of the problem in the frequency domain as well as the spatial domain .

(2) Has your wheel got Nylon centre bush ?

(3) Is manual and/or power feed by rack and pinion ?

Regards ,

Michael Williams .
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