Eagle 25 Milling Machine
Yesterday I decided to have a look at the spindle on the mill and see if I could find out what was causing the vibration problems. Now I've got the VFD I tend to use much higher spindle speeds than before, especially as I tend to use carbide cutters most of the time.
I still suspect that it is the belt causing the problem as the amount of vibration varies with the spindle speed. The belt is quite long now compared to the two original ones and it does tend to flap about at certain speeds. However, I thought that I would check the spindle bearings anyway to be on the safe side. The spindle pulley has a small amount of sideways play which you could feel by grabbing the pulley and pushing and pulling on it. I suspected that the top bearing was probably loose or worn.
It's a bit of a pain getting the spindle out as you have to remove all the quill feed mechanism but it didn't take too long. The most fiddly bit is unhooking the end of the quill return spring!
It was at this point that I realised/remembered that the pulley shaft is completely separate from the spindle and has it's own set of bearings. The spindle has it's own bearings mounted in the quill and is driven by the splines on the outside of the spindle which engage with splines on the inside of the pulley shaft. The play in the pulley shaft was therefore nothing to do with the spindle bearings.
Whilst everything was in bits I decided to go ahead and order a complete new set of bearings. I went for some high quality SKF brand which I'm sure will be far better than the cheap chinese ones that are no doubt fitted at the moment. All these mills are built to a price and they are not going to fit expensive bearings to them at the factory. As it was the set of four bearings were still less than £60.
I tried to find out what type the bearings were before dismantling the spindle and came across the parts list for a Grizzly 0705 mill which is prety much the same. These mills are all clones of the Rong Fu RF 25 mill with just a few details different. This parts list did list the bearing numbers but it's a good job I didn't just blindly order the bearings listed there as mine are nearly all different! The only one the same is the top bearing for the pulley shaft but I still made a mistake. I assumed that both pulley shaft bearings would be the same and ordered two but they are not. The bottom bearing is smaller so I haven't ordered a new one of those yet. I can't see the number on that without actually removing the bearing as it's tucked up right inside the headstock.
The major problem now was going to be how easy would it be to remove the old bearings.
Firstly I had a go at removing the pulley shaft by bodging up an improvised puller using a bit of threaded rod and some bits from the mill clamping set. It looked as though the spindle had to come out from the top and by using the 'puller' with a nut on the end to engage the bottom of the pulley shaft that should have worked. The shaft should have come out leaving the bottom bearing in the headstock casting.
The shaft came up easily along with the top bearing for about 1/8" and then it got very tight and didn't want to come up any more. Obviously the top bearing was quite loose in the headstock casting but the spindle didn't want to come out of the bottom bearing. The bottom bearing can't have been fully home and had pulled upwards slightly and then hit the top of the bearing recess in the headstock and that was it. I didn't want to force anything so decided to give up for now for fear of damaging something! I removed the puller and tapped the top bearing back down into position.
Strangely, the shaft seemed a bit smoother than before and all the play had disappeared! The shaft was now really solid with no sideways play at all. Maybe moving the bearings had sorted something out?
When I first took the large pulley off I noticed a 10mm grub screw with a pointed end in the side of the casting just below the top bearing. It was loose and I couldn't see what it was supposed to do so I just left it as it was. It must be there for a reason but I don't know what it is at the moment!
Out of interest I refitted the pulley and the belt and ran the motor. It was pretty much the same as before with bad vibration at certain motor speeds. At some speeds it was quite smooth running and at others it shook the mill quite badly. The pulley seems to run very truly so that shouldn't be causing the vibration. This makes me suspect the belt more and more. The one that is on is a good quality belt but I've decided to order an AX section cogged belt and try that as these are supposed to be more flexible than the solid A section belt and might be smoother running. The other alternative to try is a link belt but they tend to be very expensive. I'll try the cogged belt first as they are much cheaper!
On to the spindle and the bearings in that did seem quite rough. The bearings are two different sized taper roller bearings with the larger one at the bottom. I haven't taken them out yet as I haven't worked out how to pull the spindle out of the quill yet. I think that I might get away with making another improvised puller so have ordered a long length of threaded rod. All the bits I have at the moment would be too short to go all the way through the spindle.
I have removed the cap that retains the bottom bearing. That simply threads into the bottom of the quill. I was expecting to find a rubber seal in it to keep chips etc. out of the bearing but it is just a plain metal cap. It is quite a close fit on the spindle so I suppose it probably works ok. There wasn't much evidence of chips or muck under the cap and on the bearing.
The top bearing is retained by a threaded nut and I presume that this has to be tightened to adjust the preload on the bearings. This nut has a lock washer which stops it rotating once the preload is set.
The locking tab washer looks as though it has never been used for it's intended purpose. It should have a central tab to engage one of the flutes on the spindle but that is virtually missing. There is just a little stub left which wouldn't engage the flutes at all. The outer tabs haven't been bent either. One should have been bent up to engage one of the slots in the nut.
The bearings had plenty of grease in them and it seemed clean enough although a bit thick and sticky. I've ordered a tube of high performance PTFE based grease to use on the new bearings when I fit them.
I might have a look and see if I can get a rubber seal to fit inside the bottom bearing retaining cap to seal it properly against foreign matter.
Even with the top nut undone the bearings still feel pretty rough. I'm wondering if the nut has been overtightened and too much preload put on the bearings which has damaged them. The nut was very tight and took a hammer on the end of a C spanner to get it undone. I'm sure it shouldn't have been that tight.
I might have to refit the spindle temporarily as I need the mill to make a couple of bits for a job so the bearing change may have to wait a few days even if the new bearings arrive tomorrow.
The bearings did come this morning so I decided to put off the milling job that I had as it wasn't that urgent and go ahead and have a go at fitting the new spindle bearings. It seemed a bit daft to put it back together only to have to dismantle it again.
The old bearings came off a lot easier than I was expecting. I manged to tap the spindle out using a soft bolt in the top to protect the end of the spindle and gentle taps with a hammer. The spindle was quite a tight fit in the top bearing but it came out slowly. The bottom bearing was tapped off the spindle using a piece of steel tube of a suitable size and gentle use of the hammer again. The outer races were a very easy fit in the quill and just tapped out using a bit of rod on the edges. Fortunately, the bores behind the bearings were larger in diameter than the bearing race so there was a lip that I could get to. Life would have been much easier with a press but there are times when you have to make do with what you have on hand!
I cleaned the old bearings in the ultrasonic cleaner just to see what condition they were in. The larger bottom bearing seems good and runs very smoothly. The smaller top bearing is a bit rough and the races and rollers have marks on them. I did find casting sand on the inside of the quill so possibly some had got into the bearing in the early days.
The outer race of the smaller bearing has some discolouration on the top half which could be due to it overheating. It also has vertical marks all the way round and it looks as though the rollers have made slight indentations in the surface.
After cleaning everything up I rebuilt the spindle with the new bearings and lubricated them with the new grease. The outer race of the bottom bearing just pushed in and the top one only needed a slight tap. The bottom bearing was fitted to the shaft using a length of thick wall copper tube which happened to be the right size and more taps from the hammer. The top race was a very tight fit on the spindle, maybe a bit too tight really, but was pulled into place using the nut that sets the preload. I did polish the bearing seats prior to refitting the spindle with some 1200 grade carborundum paper to remove a few scratches that had been made when the old bearings had been fitted.
I couldn't really find anything useful online about setting the preload so I just tightened the nut until there was a bit of drag when the spindle was rotated. It will probably need adjusting again when the new bearings have settled in and done a few hours running.
The spindle is now as smooth as silk and much better than it was before. I quickly refited it to the mill so that I could run it and check that the bearings didn't get too hot. I ran it at 2000rpm for about 15 minutes and the quill did get warm to the touch but certainly not hot. I did measure the temperature with an infrared thermometer that I've got and that said about 30°C. I'll leave things as they are and check it again after the mill has run for a few hours. It may need the preload increasing a bit.
It's definitely the belt causing the vibration problems. At certain speeds you can see the belt flapping about like crazy and the whole mill shakes! The new cogged belt should be here on Monday and I'm hoping that that will improve matters.
Out of interest I checked the run out of the spindla again now that the new bearings have been fitted. I was pleasantly surprised to see that it was half what it had been with the old bearings and was about 2 to 3 tenths of a thou. I think I can live with that!
After I had finished in the workshop last night I sat in front of the computer and thought about the problem of not being able to get the pulley shaft out of the headstock. I thought about that mysterious grub screw and thought 'I wonder if that holds a sleeve or something between the bearings and needs to be loosened to allow the shaft assembly to come out'.
I was at the club most of the day for the public running but went out in the workshop late this evening. I took the quill assembly out again and had another go at withdrawing the pulley shaft assembly with the makeshift puller. This time I took the grub screw out and lo and behold the shaft and bearings came out really easily! I was right about there being a sleeve between the two bearings and this sleeve had a vee groove in it that the end of the grub screw located in.
The other good news was that both bearings are the same after all so I could now replace both of them.
The bottom bearing is retained with a circlip and after removing that, the shaft tapped out fairly easily from the two bearings.
Both bearings seemed very smooth with no play in them so I don't know what was causing the play in the pulley shaft in the beginning. Very strange.
Anyway, I still changed the bearings seeing as I had the new ones and refitted the shaft assembly to the headstock. I refitted the pulley and ran the motor. It did seem a bit quieter and smoother than before but maybe that was my imagination!
I carried on and reinstalled the quill and the feed mechanism etc. so the mill is now back in service. Hopefully the new belt will arrive tomorrow and we'll see if that makes any difference to the vibration problem.
The new cogged belt didn't arrive until this morning but I rushed out to the workshop and quickly fitted it to the mill. What a difference! The vibration has virtually disappeared apart from a slight amount at about 2,000 rpm. I took it all the way up to 2,500 rpm with no problems and noticed that that was the highest speed that I could get. I'll have to alter the maximum frequency parameter on the inverter to get more rpm at the spindle but I'm sure that the majority of the time 2,500 rpm will be more than adequate.
I still need to sort out the low end of the rpm range as the motor won't run below about 150rpm and it has very little torque at these low frequences. It's quite possible that I may not be able to improve this without fitting a smaller pulley to the motor which I don't really want to do unless absolutely necessary. It depends on what the maximum speed is that the motor can safely run at. If I halved the diameter of the motor pulley then for a spindle speed of 3,000 rpm the motor would have to run at 6,000rpm which seems rather high.
One possibility would be to fit a two step pulley to the motor and just change the belt for very low speeds.
I've used the mill a couple of times now for some drilling and milling and I can't believe how quiet and smooth it is compared to how it was before. It's like a different mill!
I had a look for some specs on the TEC motor that I've fitted to the mill to try and find out what the maximum rpm is that the motor can be run at. From what I've found, the maximum rpm is 5,000 so there would be no problem running the motor faster than at the moment (about 2,900rpm). The spec sheets show a maximum frequency of 80Hz on all the charts but at top speed I'm running the motor at 100Hz. The sheets do suggest that the motors can be run at higher frequencies but torque and efficiency will suffer but that probably wouldn't be a problem. Anyway, for now I'll just leave the top speed as it is at 2,500 rpm.
I've had a bit of a play with the torque settings on the VFD. There are two settings that affect the torque at low rpm but one setting didn't seem to do anything at all. The other did give a noticeable increase in the torque at very low rpm but it seemed to make the motor a lot noisier when running, especially at higher rpm. Maybe I need to look into the settings more but the instructions are too basic to really do this. I think that I'll just leave things as they are. I can always just alter the VFD setting if I do need to run the spindle very slowly and then put it back for normal running.
I am so impressed with the cogged belt that I've ordered two for the Myford ML7. That can be a bit noisy at times so we'll se what happens when I change the belts on that.