MMD-45 Mill Tight Spindle

[cba_melbourne]

It seems to be essentially a Taiwanese Rong-Fu clone. There are many factories in mainland China making these. One of the better ones is sold in the US as the Grizzly G0755. You can download the parts list from here and compare it to your mill: http://www.grizzly.com/products/Heav...wer-Feed/G0755.
There is an exploded drawing of the head, and according to the parts list the main bearings are two taper rollers 30206P5 and 30207P5 (I doubt low cost versions of this mill use P5 class bearings, but the size is most likely the same).

These are usually the problems you will universally face with low cost Chinese machine tool spindles:
- bearing preload cannot be adjusted properly, because the inner cones are too tight a fit onto the spindle. The spindle journal needs to be reduced (ground, turned, or in a pinch at least polished with emery cloth) until the cup can slide on it (slide firmly but with hand force alone, no sledge hammer should be necessary).
- lots of casting sand left in craps and crevices that over time comes loose and contaminates lubricant and degrades bearings. It is advisable to completely strip such machines and clean out. Some of the better machines made in the more expensive factories may not suffer from this problem, but if unsure better take apart than be sorry.
- the most low cost factories may use inferior trailer bearings with excessive runout (making audible noise). Such bearings may initially get better after some hours running-in, but won't last for long. Replacement will be needed.

Notice that too much preload will cause the spindle to show excessive resistance when turning, this will cause the spindle to heat up excessively, and it will quickly ruin the bearings. Insufficient preload otoh will cause poor surface finish, and in more extreme cases chatter and inability to hold dimension.

[Oldneweng]

Thanks for the model number cba. As I had previously mentioned I had downloaded a parts list from Grizzly, but it was for the G0463 as I had no idea which one to choose.

I am pretty sure I have found the problem. When removing the locking nut at the top of the spindle I had to use a lot of force. I used the ER collet chuck spanner. I was just about to get a pipe extension when the bearing retaining nut cracked free. Yes I had released the locking tab.

I pressed the spindle out of the quill on my press. It did not take much force.

Lower pindle Bearing Removed.jpg

It is obvious that the lower bearing is less than ideal. The upper bearing does not show any signs of damage tho. Now I need to get some prices.

I did try the drill chuck in the taper after I had loosened the bearing retaining nut and then retightened it. I gave it more tension than it should normally be adjusted, but no where near as much as it had originally. I tightenened the draw bar a fair bit tighter than I would normally. There appeared to be no difference when turning the spindle.

The lower cover ring I was asking about was a problem. It was cross threaded. Its only plastic so I managed to get it off without totally destroying it.

Dean

[Oldneweng]

Some initial results from bearing research. The bearing sizes required are 30206 and 30208. The Grizzly list has a 30207 istead of the 30208.

The following bearing is available on EBay. It is a spindle bearing from a Myford 10 Lathe. It is rated at P6. The Grizzly parts list shows a requirement of P5.

http://www.ebay.com.au/itm/NEW-HIGH-...item19ed5b70cb

The local CBC store says there is only 1 grade for these bearings as listed in their books. The grade is unknown, but at $30 for the larger bearing it is unlikely to be very high. He did say he could try a search for better bearings, but who knows how much they would cost. Cost is a definite issue for me and I need to balance this with the benefits obtained considering that it is just a Chinese built machine.

Is the CL7C classification of any use in this situation? It was mentioned in a thread on Prac. Mach. in relation to a manual lathe. A quote from the thread which came from SKF originally.

The tolerances for CL7C specification bearings correspond to Normal tolerances except for the running accuracy which has been tightened ...

There appears to be a shortage of higher grade bearings available.

Dean

[cba_melbourne]

Some initial results from bearing research. The bearing sizes required are 30206 and 30208. The Grizzly list has a 30207 istead of the 30208.

The following bearing is available on EBay. It is a spindle bearing from a Myford 10 Lathe. It is rated at P6. The Grizzly parts list shows a requirement of P5.

http://www.ebay.com.au/itm/NEW-HIGH-...item19ed5b70cb

The local CBC store says there is only 1 grade for these bearings as listed in their books. The grade is unknown, but at $30 for the larger bearing it is unlikely to be very high. He did say he could try a search for better bearings, but who knows how much they would cost. Cost is a definite issue for me and I need to balance this with the benefits obtained considering that it is just a Chinese built machine.

Is the CL7C classification of any use in this situation? It was mentioned in a thread on Prac. Mach. in relation to a manual lathe. A quote from the thread which came from SKF originally.

There appears to be a shortage of higher grade bearings available.

Dean

Hi Dean,

in this case common automotive grade bearings (precision class P0 or PX, or no suffix at all) are probably the best choice. But choose them to come from a reputable "western" maker like SKF. Stay away from fantasy maker names that you have never heard mentioned before, you do not want to install boat trailer grade bearings. P5 and better bearings would be overkill, because the spindle of this mill is itself probably only manufactured to 0.02mm runout tolerance.

When installing the new bearings, make sure that the top inner cone is a sliding fit onto the spindle. Meaning that if lightly oiled, you can just slide it up and down with hand force only. If its too tight, you will not be able to finely adjust the bearing preload, because the cone will only move in "jerks" when tightening the adjuster nut, and it wont release preload when loosening the adjuster nut (you will need to use a hammer on the spindle to loosen preload).

Next, do not overfill the bearings with grease. If you do, they will run hot and the oil will separate out from the grease, leaving just gunk behind that is not a good lube. Remember, a machine tool spindle requires a much higher preload than a car wheel and thus will run hotter. Use a good quality bearing grease, you do not want to take that spindle out regularly to re-grease.


Disclaimer: I am no bearing expert and do not claim to be one. I am just a hobby user happening to have some experience with bearings inside hobby machine tools.

[Oldneweng]

Hi Dean,

in this case common automotive grade bearings (precision class P0 or PX, or no suffix at all) are probably the best choice. But choose them to come from a reputable "western" maker like SKF. Stay away from fantasy maker names that you have never heard mentioned before, you do not want to install boat trailer grade bearings. P5 and better bearings would be overkill, because the spindle of this mill is itself probably only manufactured to 0.02mm runout tolerance.

When installing the new bearings, make sure that the top inner cone is a sliding fit onto the spindle. Meaning that if lightly oiled, you can just slide it up and down with hand force only. If its too tight, you will not be able to finely adjust the bearing preload, because the cone will only move in "jerks" when tightening the adjuster nut, and it wont release preload when loosening the adjuster nut (you will need to use a hammer on the spindle to loosen preload).

Next, do not overfill the bearings with grease. If you do, they will run hot and the oil will separate out from the grease, leaving just gunk behind that is not a good lube. Remember, a machine tool spindle requires a much higher preload than a car wheel and thus will run hotter. Use a good quality bearing grease, you do not want to take that spindle out regularly to re-grease.


Disclaimer: I am no bearing expert and do not claim to be one. I am just a hobby user happening to have some experience with bearings inside hobby machine tools.

Thank cba. In that case I will probably just get the local bearings as I could afford them. He did say there were also used for industrial purposes? I just need to get there. I will try a much more local/closer industrial "everything" type store. At least I go past regularly. They may be expensive tho.

I will check the fit of the top cone as suggested.

Half full or so of grease is what is mentioned in the repair threads have mentioned before.

I may have trouble removing the bottom bearing cone. There is very little room. It appears to be only a shallow shoulder and then a taper. There is not much to grip on because of the taper. I was thinking of a disk with a taper to roughly match. Another disk which fits inside the cone and bolts to the larger disk then cut the larger disk into pieces so I can fit it in position behind the cone and lock in place with the smaller disk.

Dean

[Oldneweng]

The LOML talked me into taking her into town. I decided to try the Industrial Shop there for the bearings. (Not the town I was thinking of) This shop caters primarily for agricultural equipment supplies. No matter, "Blue" (he has red hair) went straight to the boxes and whipped them out.

CBC price for both $49.70.
ATI price for both $36.60. (The ones I bought)

I am happy except for working out how to get that lower cone out.

Dean

[cba_melbourne]

............
I am happy except for working out how to get that lower cone out.
Dean

On these spindles the cone journal often grossly oversize. To make matters worse, the journal is often only turned or coarsely ground instead of being precisely ground to a fine finish. Then at the factory they use brutal force to get the cone on. Which btw distorts the cone and ruins the precision..... Anyway, they can be very hard indeed to get off. One method is to carefully grind a slit into the cone with an abrasive disc, then split it with a center punch. The cones are hardened and usually very brittle. Or you can grind a flat onto the cone, and as you come very close to the spindle give it a punch and the cone will break and spring open. If you have an acetylene torch all you need is a fine pointed hot flame to heat the cone only (not the spindle) and it will pop loose. Before putting the new cones on, measure the spindle and compare to the required installation tolerance from the bearing website or catalog. It the spindle journal needs to get smaller, put the spindle in your lathe and use fine emery cloth over a file, 400 grit then 600. This method is a big "nonono" for precision spindles. But given these spindles are not made very accurately to begin with (about 0.02mm tir is normal for Chinese hobbylathe and hobbymill spindles) and most of those low cost spindles are not hardened, the emery cloth method is an appropriate shortcut. You typically only have to take off a few 0.001mm from the journal diameter to bring it into the bearing maker tolerance for a loose interference fit. Do it by trial fit if you do not own a micrometer capable to resolve 0.001mm - a common 0.01mm micrometer is not good enough to measure/make a bearing fit.

Removing the outer races are easier. Just weld a bead to the inside and they shrink and pop out.

[Oldneweng]

On these spindles the cone journal often grossly oversize. To make matters worse, the journal is often only turned or coarsely ground instead of being precisely ground to a fine finish. Then at the factory they use brutal force to get the cone on. Which btw distorts the cone and ruins the precision..... Anyway, they can be very hard indeed to get off. One method is to carefully grind a slit into the cone with an abrasive disc, then split it with a center punch. The cones are hardened and usually very brittle. Or you can grind a flat onto the cone, and as you come very close to the spindle give it a punch and the cone will break and spring open. If you have an acetylene torch all you need is a fine pointed hot flame to heat the cone only (not the spindle) and it will pop loose. Before putting the new cones on, measure the spindle and compare to the required installation tolerance from the bearing website or catalog. It the spindle journal needs to get smaller, put the spindle in your lathe and use fine emery cloth over a file, 400 grit then 600. This method is a big "nonono" for precision spindles. But given these spindles are not made very accurately to begin with (about 0.02mm tir is normal for Chinese hobbylathe and hobbymill spindles) and most of those low cost spindles are not hardened, the emery cloth method is an appropriate shortcut. You typically only have to take off a few 0.001mm from the journal diameter to bring it into the bearing maker tolerance for a loose interference fit. Do it by trial fit if you do not own a micrometer capable to resolve 0.001mm - a common 0.01mm micrometer is not good enough to measure/make a bearing fit.

Removing the outer races are easier. Just weld a bead to the inside and they shrink and pop out.

Thanks for this excellent information and particularly for the bead welding trick. I had totally forgotten that one.

The inner cones were quite easy to press off. I hope to only need a little touch up of the spindle to acheive a neat slide fit as as you have mentioned.

I forgot to mention in the previous post that both bearings were NSK.

Dean

[cba_melbourne]

........
I forgot to mention in the previous post that both bearings were NSK.....
Dean

I am surprised. NSK is a very reputable maker. I would have expected a much cheaper no-name bearing in there. This means they must have ruined that bearing when installing it. Maybe they used a too heavy mallet to adjust the preload. Or maybe there was more than usual casting sand contamination in the grease they used. Maybe condensation water collected inside the quill whilst in transit...

[Oldneweng]

I am surprised. NSK is a very reputable maker. I would have expected a much cheaper no-name bearing in there. This means they must have ruined that bearing when installing it. Maybe they used a too heavy mallet to adjust the preload. Or maybe there was more than usual casting sand contamination in the grease they used. Maybe condensation water collected inside the quill whilst in transit...

Oops, my mistake there.

I should have been clearer. The bearings I bought were both NSK. Sorry about that.

Dean

[pipeclay]

Was very clear to me what you meant,but then I am pretty simple and don't over think things to much.

[Oldneweng]

It turned out to be a bit of an anticlimax to finish up with.

The top outer cone was accessible with a bar thru from the other end. There was enough edge showing. I used a square piece and tapped it round and round the cone. Movement was soon visible. I was not using much force. I soon had it right out.

What the heck. I made an impact puller from all thread with a thick domed washer welded on one end. In the lathe I tapered the edge of the washer to fit into the narrow gap. It was about 2mm at the edge. Initially I had to use it angled out slightly, but again I soon managed to get my fingernail behind the cone. Proof it had moved. I soon had it out as well. I only used a small weight and did not use much force. Just lots of round and round the ring. This cone had P6 marked on the outer diameter! What are the chances?

Now all bearings were off.

As per cba's suggestion, I put the spindle in the lathe with copper protection under the jaws, the live centre in the other end, and polished up the upper bearing locating surface with emery paper and.......... a parallel. A little bit at a time. I was finally satisfied with one of those not really wanting to go on, but then a slow slide and click experiences. Perfect. I also polished the other bearing locating surface as well because it looked a bit rough. It had lines running axially from bearing contact/pulling off. Only a short polish had it looking very clean tho. Maybe a result of heat?

The quill has had a very good wire brushing and clean inside on the cast surface and both new outer cones have been put in. Both of them actually tapped in most of the way with my small soft face hammer and gentle taps around the ring. I pressed in the last bit just to be sure.
I was very careful to use the new ones, and put them in the right way round!

I am now ready to reassemble the rest.

A couple of queries tho.

There are 2 holes drilled on an angle to exit internally just above the bottom bearing, entering from the external surface of the quill, just above the depth rod mount. These would appear to be oil holes. They are about 4mm diameter. Any comments about this? Oil or grease? This is the first time I have seen them. They are hidden under the rubber bump ring which protects the bottom of the quill if you let go and it flies up.

On a similar note, the spindle end cap which I have mentioned before (plastic and cross threaded) has about 7mm clearance between its centre diameter and the spindle end diameter. This appears somewhat pointless to me. I was wondering whether I could mount a metal end cap and proper seal here. Would it be worth it?

Dean

[BaronJ]

Hi Dean,

Is the conclusion that the lower bearing damage was the culprit for the spindle tightening up ? Or haven't we got there yet...

[Oldneweng]

Hi Dean,

Is the conclusion that the lower bearing damage was the culprit for the spindle tightening up ? Or haven't we got there yet...

I don't recall the spindle ever being very easy to turn although I did not try turning it in a neutral gear position specifically to see, that I recall. I have turned it in a neutral gear position for other reasons.

My best guess based on the force required to release the bearing locking nut is that over tightening of this nut caused the problem. A bit(?) of extra pressure to line up the next locking tab?

The damage to the lower bearing appears to be heat related. I cannot feel any variation in the surfaces. I would have thought that in order for this bearing damage to tighten the spindle to that degree, there would be other visible changes to the bearing. Other than the surface discolouration it looked and ran fairly normally in the hand.

The spindle has always run pretty hot as well. Guess I should have investigated this sooner. I have never owned a mill before. If I had realised how easy it was to remove the quill I may have been tempted.

Dean.

[Oldneweng]

My mill is fully back together and working just great. It is a bit quieter now. I made a few minor improvements. I only backed off the bearing adjustment nut slightly from contact. There is limited options with the locking tabs. The next tab was too far away for my liking. I will pull the quill out and recheck the adjustment after I have used the mill for a while.

It took about an hour to set up to press the bottom bearing onto the spindle and about 9 seconds to actually do it.

I lost one of the roll pins that holds the gear levers on. I had it partly in the lever and knocked it out so I could set alignment with the holes and haven't seen it since! Luckily I had already decided to use grub screws to secure them anyway. The holes for the pins were terrible. One shaft had 2 holes in it and the hole actually used was at a ridiculous angle. The levers were already drilled and tapped M6 with a steel central ring for strength. All I had to do was pop in a couple of grub screws. This meant I could adjust the lever with 3 gear ratios to align with the numbers better. Later I will file a bit of a flat at the contact point.

Dean