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Thread: Surface grinder weirdness
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24th Nov 2021, 07:56 PM #1Diamond Member
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Surface grinder weirdness
A little surface grinder had to get moved out of a neighbour's shop, and to get out, it had to be disassembled; motor+wheel off, worktable off, bearing cages wrapped carefully, cross slide off, two men to lug base+column into my place.
Of course, while it was apart, it got a cleanup. Kero to wipe the old grease out, find one of the two missing ball bearings, bend the cage so the loose ones don't fall out again, et c.
Then, re-assemble, and roughly level it. And, because everything has moved, re-grind the magnetic chuck/plate/thingy.
Didn't have a diamond to dress the wheel, so the surface finish from the first pass isn't great:
IMG_2208.jpg
but this showed me the back of the plate was over 20 microns higher than the front (which was barely ground). The next day, I borrowed a diamond from a different machine, and with Dad's old vice, dressed the wheel:
IMG_2210.jpg
and made some passes. This is after 4:
IMG_2212.jpg
Note the front edge is still showing the rough cuts from the day before.
Anyway, after about 6 passes, it seemed flat (although surface finish wasn't brilliant. I should have dressed the wheel a few more times), but I decided to test it with a dial indicator:
IMG_2215.jpg
That is showing the back about 40 microns higher than the front. What the?
Toolmaker said "You can't measure it there: you have to be under the wheel !" and moved indicator over:
IMG_2213.jpg
Sure enough, it's as close to zero as that indicator and base can measure.
So, after the rest of the day thinking about this, I am troubled.
If the cross slide shows deviation anywhere, doesn't that mean the work table, or the magnetic chuck, is twisted?
(e.g. the longitudal feed is rising up at the front on that side, which means it is being ground away more)
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24th Nov 2021, 08:14 PM #2Most Valued Member
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Hi there,
Running a DI over your mag chuck (with the DI attached to the wheel or column) does not guarantee flatness, even if it shows no run out.
If your surface grinder has wear on the ways such that the wheel describes an arc or a convex when sliding across the ways then putting a DI on the column or near the wheel, it too will describe and move across the same arc, showing no deviation. Yet the surface is not flat.
I've also found that grinding accurate and flat is not a trial exercise. If you use the wrong wheel type for the material you are grinding or you don't freshly dress the wheel to keep it grinding clean or you introduce too much heat into the workpiece, you will get unreliable results.
Then there's feed speed and amount of overlap per pass to play with too.
Cheers,
Simon
Sent from a galaxy far far awayGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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24th Nov 2021, 09:01 PM #3Diamond Member
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Indeed, but I doubt the converse is true?
The "arc" idea is interesting. I'm sure the longitudinal tracks are worn in the middle (and thus the saddle/work-table rises up at the ends of travel), but rising up more on the front than the back seems strange.
I guess, if I was really serious about testing it, I would attach multiple DIs across the table/chuck, and compare the trends as the cross slide or longitudinal roll was traversed.
...but that sounds like too much work. It is a small grinder, and most of its life has/will be grinding small things in the middle of the chuck/work-table.
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25th Nov 2021, 01:05 AM #4
Hi Nigel, Guys,
Ignoring wear on the knee, are the knee gibs adjusted properly ?Best Regards:
Baron J.
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25th Nov 2021, 08:53 AM #5Most Valued Member
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Grease?... i thought these things used way oil?
my Power SG has a badge on it saying "NO Grease"...unsure if its original or was placed there by someone who may have known better.
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25th Nov 2021, 09:20 AM #6Most Valued Member
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Never tried to grind a chuck dry. How warm is it getting?
Did it measure the same this morning as it did last night?
How much of a cut are you taking?
For the minute I would stop grinding your chuck and make 5 test pieces and grind them until you are happy.
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25th Nov 2021, 12:53 PM #7
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25th Nov 2021, 07:08 PM #8Diamond Member
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Thanks for the suggestion.
I've actually never seen a surface grinder with a knee !
I assume it would be like a BridgePort style mill, where the table elevates?
Machine is small. A 1980's version of this Hafco:
Main.jpg
• There is probably one gib on the column, but I didn't touch it.
Can't imagine why it would affect flatness of table?
• The gib for the saddle cross feed was removed, cleaned, and greased.
I didn't check it in any way. It is your typical poor quality rubbish
(angle grinding marks still visible from the factory).
Did, however, notice significant wear on bottom of one dovetail.
Can't remember if it is the side which has the gib.
I guess if one side was grabbing unevenly,
it could cause the saddle to rise up as the cross feed is wound in?
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25th Nov 2021, 07:22 PM #9
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25th Nov 2021, 07:26 PM #10Diamond Member
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It has two "grease" nipples (column, and cross slide),
and had old green/black grease in it that I cleaned out of the dovetails,
and the cross feed screw's nut.
The cross slide one has a large groove on each side,
and they seem to be connected by a cavity in the cast base
(i.e. grease gun causes fewsh grease to ooze out both sides)
I guess it might have been oil fed, but that does nothing for work table traversing rollers,
or its rack+pinion (which did not seem to be "fed" by the nipples)
Manual for current version:
https://images.machineryhouse.com.au...n%20Manual.pdf
mentions an oil lubrication system, but that would be
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25th Nov 2021, 07:51 PM #11Diamond Member
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All valid questions.
• After the deepest cut, it was about 5—10°C above ambient.
Between third and fourth pass, I sprayed the table with water to minimise growth,
but didn't like the surface finish it gave me.
Mind you, on last pass, I cross-fed backwards to make sure it was sparked out.
It made a weird checker board effect near the middle.
So much for my careful consistent dry grinding
• Haven't tried re-checking.
Might pretend it's a mill, and mount a "tramming" indicator bar?
• 1st pass 40–50 microns.
2nd thru 6th, 15–20 microns.
5th+6th, 4 or 5 microns (i.e. about half way between divisions, and then to next division).
Which is about as small as I can move the column due to the handle/gear play, and stiction.
Machine's column is interesting.
A lever DI showed repeatability to each 10micron division,
but in use, if you hit the traverse stop too hard,
column can drop down a bit, and dig in on your next pass!
Sadly, this can happen easily when grinding something long, like a magnetic chuck
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25th Nov 2021, 08:06 PM #12Diamond Member
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Thanks for those links!
"Grind the bottom of the magnetic chuck. Grind so there is approximately .0002” of dish per 6” of chuck length"
Blimey.
a) That is a challenge to setup, and
b) would cause "air" under the middle of the chuck, that might not be taken up by gravity's deformation of the chuck?
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26th Nov 2021, 08:20 PM #13Golden Member
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There's a guy who has some youtube videos that are very informative about this, Steve something, Solid Rock Machine Shop; probably the best video from him on this subject is actually on Keith Rucker's channel where Steve thingamajig helps him set up an old SG. He talks about the correct wheel for the job and the whole process to get it flat. Essentially the process is: 1: remove the mag chuck and grind the bed where the chuck mounts; 2: mount the chuck upside down with the magnet switched off, block it in place rather than clamp it, and grind the bottom of the chuck flat; 3: mount the chuck properly and then grind the top with the magnet switched on. The wheel has to be the correct grade (so you can grind the whole surface without losing effective wheel diameter) and dressed properly and you have to finish with zero passes to get each surface flat. The proof of whether it is flat is NOT in how an indicator reads when you sweep the surface on the machine- for the reasons mentioned above- but in whether you can then produce flat work.
I went through this process on my SG, took about 8 hours, then the crossfeed screw thread stripped on the next job!
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27th Nov 2021, 09:29 AM #14Most Valued Member
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18th Feb 2022, 09:35 PM #15Diamond Member
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After several months using this, I have noticed some interesting challenges;
1) There is a slight hump in the middle of the table (a line running left to right).
Just a slight noise if tapping a good long parallel on the chuck
(parallel in the middle, in a line from me to the back)
2) Grinding anything 6" or longer, longitudinally, gives a little over half a thou inaccuracy.
3) Rotating those at 45° drops it to about 3 tenths of a thou. Possible due to 1)
4) I can usually get about 4 hours of careful grinding done before the wheel needs dressing,
but grinding on the back edge of the wheel (i.e. with the job slowly stepping toward me),
gives a smoother finish than the front edge.
I'm wondering if one of the spindle bearings is worse then the other!
5) Today, after dressing the wheel a few times, the surface finish was worse!!!
(more of a matt finish, no matter how slow/careful I was)
Anyway, I decided to re-finish the top of the chuck. A "barely touching" pass:
IMG_2560.jpgIMG_2561.jpg
shows high spots on the left and right edges, most of the back half (with a slight dip in the middle), and an interesting line on the left edge of one of the magnetic strips!
I guess the toolmaker is right – these things really do settle and move over time!
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