Gallery, Projects and General > Project Logs
Centec 2A Rebuild
PekkaNF:
The preload should not be too great. Do you have acess at the both ends of the bearings and make sure that bearing rings are seated right? A small chip somewhere on the spindle shaft or bearing seat would set the bearing askew, cause too big preload and cause runout + play.
When you turn the spindle (only spindle, no belts or gears in mesh) manually, does it rotates smoothly whole round? Or does it has one (or two) spots of harder resistance or can you feel "cogging".
How this bearing arrangement does the preload? Two taper roller bearing and nut on the shaft pushing inner ring of the bearing?
*Fatty fingers&dystolexia
Fortis64:
I also had a bit of faffing about to get the bearings set up . The rear bearing was my problem,the bore for the bearing shell was in my opinion too tight of a fit to allow proper adjustment of the the bearings .
This is what I fitted to my Centec 2
Timken
Outer 1930 2-0
Inner 1985 1-6
Outer 1729 1-6
Inner 1775 4-6
mm289:
Success :beer:
SO the short story is I now have runout on the horizontal spindle under control :clap:
For the sake of anyone trying to do the same thing, or if you enjoy a Friday night read, the long story is as follows.........grab a cuppa :coffee:
Had a long chat with Pete this morning and in amongst other ramblings the discussion got round to how it would be almost impossible for a set of taper roller bearings to be so far out unless they were literally falling apart - which mine weren't. I was all for just slapping a new set in and getting on with doing some scraping, but Pete got me thinking so I went back to basics. :scratch:
First off - eliminate the spindle as a source of error i.e. is it straight? So over to the lathe which I had helpfully just taken the 4 jaw off and replaced with the 3 jaw yesterday :doh: so swapped the 4 jaw back again (which is mahoosive, note to self must get smaller 4 jaw chuck :palm:). So en route to setting up the spindle it gave me a chance to strip and clean the 4 jaw which i hadn't done since aquiring the lathe 18 months ago!
45 mins later... got the chuck set up and popped the spindle into the chuck then centred it. At first look I had a bit of a panic as it looked like the spindle had .45mm runout from one end to the other - bugger I thought - bent spindle (pic 1) :bang: At this point I will claim some sympathy for being a bell end as I haven't got any formal machinist or engineering background :bugeye: - obviously it would help when taking these sort of measurements if you had the other end of the spindle supported as well :doh:
So ran a live centre into the nose of the spindle and ran the tests again..... much better - runout down to +.04mm. Checked at the chuck end and the runout was -.03 though which seemed a bit odd. (Pic 2,3 & 4).
Initially I had the spindle gripped in the chuck near its end, so I moved the spindle in to where the rear bearings would sit and re did the test (again). Bingo - runout reduced to between .01mm and .015mm (4 to 8 tenths) which is good enough for me. (pic 5)
My conclusion being, the spindle between bearings is running true but is slightly bent between the rear bearing and the end of the shaft (where the pulley sits). Possibly only about .04mm or 2 thou so I am going to ignore it for now and press on :clap:
More to follow...... just grabbing some grub!
mm289:
OK, back from dinner (homemade meat pie made from Christmas leftovers :))
SO having convinced myself the spindle was OK back to re-assembly. Old bearings were dropped in some Jizer for a while to get them as clean as possible, meanwhile man from Royal Mail arrived with my RS Components delivery of an enclosure, 3 way switch and potentiometer to tidy up the 3 phase motor and VFD upgrade I am doing (more of that later :thumbup:)
So the way the spindle bearings are mounted is they are situated in the casting and held on each side by a retaining plate/cover. This is bolted together with 4 bolts - but it became apparent that when these were machined they must have been done in situ or as matched pairs. Reason for saying this is on mine at least, they would only bolt up in one orientation, the bolts would bind as they went into the covers otherwise, so both sets were test assembled to find the "best" fit then centre punched to mark orientation (pic 2)
I then drove the bearing cups into the case so they butted up against where the retaining plate would sit and reassembled the shaft with dry bearings. I just did the front bearing first and was puzzled that as I tightened up the retaining plate on the front of the knee the bearing became tight to the point of stopping the shaft moving. :scratch: AT first I thought it was pulling in crooked and binding on the nose of the shaft so despite having aligned the plate/cover I tried all 4 possible orientations but still had the same problem.
Eventually I engaged brain and broke out the depth gauge. Now this is going to be hard to explain without a diagram - and your not getting one of those!! :bugeye: - but, it seems that if the bearing cup is driven in flush with the retaining plate (which sits inside the case) then when you tighten up the cover it effectively preloads the bearing to the point of clamping it. This is because the cover has a flange which will press on the bearing cage. To fix this I drove the cup in further so as to give me 1mm clearance between the cover and the bearing.
SO all tightened up nicely and shaft rotating freely. Repeated process on the rear and exactly the same problem! Again pushed the cup in a bot further and the bearing ran freely.
Moment of truth... wound in a small amount of preload (1/8th turn) and put the Dial Gauge on the nose and..... 0.015mm runout :) :)
Rebuilt the spindle again having lightly packed with the NGLI 3 bearing grease (Pic 3) equivalent of the original Centec recommended lubricant and then ran a series of tests at different pre loads as per Pic 4.
Result, finished spindle showing only 0.015mm runout at nose and when under load only deflecting 0.006mm (~3 tenths). Also end float is now zero. I then measured runout in the taper rather than on the nose (as suggested earlier) and it reduced to 0.01mm.
RESULT :beer: :ddb: :ddb: :ddb:
SO bearings were OK (thanks Pete :headbang:) - I can only think that somehow the cups had got moved around that that whilst I was winding in pre load on the shaft it wasn't actually loading the bearings hence why I was getting so much movement in my original test. Anyway, now I have done this I can move onto setting up for measuring and scraping ( a day(or 4 ) later than planned :doh:
Cheers,
Paul.
mm289:
Have now removed the knee from the base and setup on workbench to make scraping easier.
In doing so I have discovered a few things:
1. The horizontal arbour when mounted in the overarm is uber accurate, only .01mm of runout at the far end of the arbour which is 6" from the nose of the shaft :thumbup:
2. My clarkson collet chuck is pants :bang: the chuck body gives about .02mm runout but when I mount a straight bar in it it is giving me .07mm run out, got the same result mounting it in the 4 jaw in the lathe so figure the chuck is rubbish.
3. My "spare" collet holder doesn't take any of my Clarkson collets (not sure what collets it needs :doh:)
4. My Jacobs chuck on an MT2 taper is my most accurate work holder with a runout of only .03mm :( -
5. Trying to figure out how to check the alignment of the knee from the horizontal arbour when none of your workpiece holders are accurate is a PITA :hammer:
Oh well, tomorrow is another day :D
Cheers,
Paul.
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