[acannell]

1997 Dyna Myte DM2900 mini-mill (about 1500lbs, 2hp)

X and Y axis smooth as silk at 150ipm rapids

Z axis shudders on downward movement at 60ipm. Starts out smooth for about a second then vibbrrATTEEEEEEEE. Its not an earthquake or anything, but its definitely not good.

Per tech support I tightened the Z axis gib (s?). This style has one screw on top and one screw on the bottom of the spindle casting, and you loosen one, then tighten the other. The gib was definitely loose because the head would slowly fall during E stop. So I tightened it a bit. It fixed the head falling issue, but not the shuddering issue.

If you are jogging the head down at 60ipm, it starts smooth for about a second, then starts shuddering. If you let go of jog, then start it up again, the shudder is gone, then comes back, in the same way as when you started. Seems to be pretty much the same anywhere on the Z axis.

If you slow down to 40ipm, it doesnt seem to shudder anymore.

I guessed that maybe i hadnt tightned the gibs enough, or I had tightened them too much, so I played around with much tighter and much looser. Much tighter (a few turns) seems to cause consistent, heavier vibration. Much looser doesnt seem to change anything.

I had been using jack hydraulic oil for lube, but I finally got my hands on some vactra #2. I wasnt sure how long if would take to clear out the jack fluid, so i manually dribbled vactra 2 all over the z axis ways and ballscrew. It DID improve slightly! But its definitely still there.

If you push hard on the head so you are pushing it perpendicular to the Z axis, you can make the vibration go away. But only if you push very hard (as hard as possible).

The steel cart the mill is mounted in is on casters. Perhaps I should install the levelling feet? i will do that tomorrow. Any other ideas? Please help!

[2ferrous]

Although I'm more familiar with the 4400, the design should be the same.

Any shuddering or odd noises I had on mine were traced back to the thrust bearings. I'm not sure if the 2900 has a gearbox in it like the 4400 but it would still have thrust bearings and support bearings. I would disassemble the machine and clean these at the very least, replace if possible. The bearings are all standard sizes so you should be able to get them from a local supplier or if you have a bit of time from an online supplier.

Timothy

[acannell]

Although I'm more familiar with the 4400, the design should be the same.

Any shuddering or odd noises I had on mine were traced back to the thrust bearings. I'm not sure if the 2900 has a gearbox in it like the 4400 but it would still have thrust bearings and support bearings. I would disassemble the machine and clean these at the very least, replace if possible. The bearings are all standard sizes so you should be able to get them from a local supplier or if you have a bit of time from an online supplier.

Timothy

thanks! i have a rough idea what you are talking about. it appears the DM2900 has a gearbox. i would assume the thrust bearings are somewhere between the interface of the gearbox and the ballscrew and/or gearbox and motor. ill ask tech support for an exploded view, otherwise ill just start taking the gearbox and motor apart and see what i can see!

[keebler303]

I don't know anything about this particular machine but it sounds like stick/slip to me. You have some backlash (possibly caused by thrust bearings) which allows the Z axis to fall a little bit until the backlash is taken up.

Basically the Z axis sticks (not necessarily a problem) and then the Z screw pushes it down. When it moves a bit, it unsticks, and then drops until the screw catches it again. This process repeats over and over again.

A properly lubed Z axis should minimize the sticking, a backlash free screw would eliminate it. You only experience it on the way down because gravity is making the Z axis drop by the amount of your backlash.

[acannell]

I don't know anything about this particular machine but it sounds like stick/slip to me. You have some backlash (possibly caused by thrust bearings) which allows the Z axis to fall a little bit until the backlash is taken up.

Basically the Z axis sticks (not necessarily a problem) and then the Z screw pushes it down. When it moves a bit, it unsticks, and then drops until the screw catches it again. This process repeats over and over again.

A properly lubed Z axis should minimize the sticking, a backlash free screw would eliminate it. You only experience it on the way down because gravity is making the Z axis drop by the amount of your backlash.

this sounds right, thanks. i emailed tech support to see if they have anything to say about getting to the thrust washers, before i just start taking stuff apart. i would assume the thrust bearings will be somewhere between the gearbox/ballscrew interface?

[2ferrous]

Yes, the thrust bearings are located in the main housing of the gearbox. I've attached some pics of the 4400 gearbox being taken apart.









Timothy

[2ferrous]

If you feel really enthusiastic you can remove the entire head and clean all the ways and turcite. All three of my machines were filthy. And two of them had clogged oil feed holes.


(this is the carriage off of my number 1 machine. no pics of the spindle head unfortunately)

Timothy

[acannell]

If you feel really enthusiastic you can remove the entire head and clean all the ways and turcite. All three of my machines were filthy. And two of them had clogged oil feed holes.




Timothy

thanks for the pics! i peeping-tom'd the other ones in your album, that machine was pretty dirty. i didnt see any "after" pics..did you get it all cleaned up and reassembled? do you still use it?

if you needed to replace a ballscrew, is there a way to do it thats cheaper than the $1600 each from dyna mech?

im thinking about getting into the head to check the thrust bearings today, is it something i can do with the mill in place, i.e. just unbolt stuff in the gearbox region, or am i going to need to do other disassembly beforehand?

[2ferrous]

I haven't had a chance to upload all the new pics yet. I'll get to it soon.

All the ball screws out of my machines have been in good shape once I cleaned them. And by cleaning, I mean total disassembly right down to the 136 precision balls in the screw. Wash, dry and inspect. Then buff, wash, dry and inspect again. Then sit down for an hour and reload all the balls. They run super smooth after that.

Other than putting a block under the head to stop it from winding down while you're working, the whole assembly can come off the machine from the top. You can leave the counterbalance weight in place too.

Timothy

[acannell]

I haven't had a chance to upload all the new pics yet. I'll get to it soon.

All the ball screws out of my machines have been in good shape once I cleaned them. And by cleaning, I mean total disassembly right down to the 136 precision balls in the screw. Wash, dry and inspect. Then buff, wash, dry and inspect again. Then sit down for an hour and reload all the balls. They run super smooth after that.

Other than putting a block under the head to stop it from winding down while you're working, the whole assembly can come off the machine from the top. You can leave the counterbalance weight in place too.

Timothy

i just got word from dyna tech support, the bearings are $42 each from them, 18 in stock. not too bad..they even sent me a simple diagram showing where they are.

id really like to completely disassemble this machine, but im not used to working with such heavy components and its hard for me to plan it out without it becoming an insane nightmare of filthy super heavy mill parts all over my tiny workshop...if i can get the shudder to go away, then pretty much everything on it works great, i need to start making some money with it and then give it an overhaul once its paid for itself a couple times over hopefully hehe

[2ferrous]

I found bearings on eBay that were pretty inexpensive.

2 Angular Contact 7302B 15x42x13 15mm 42mm 13mm Ball Screw Spindle Ball Bearings | eBay




And yes, you're going to need some room if you want to take it apart. My shop floor was completely covered in skids with parts on them.

Timothy

[acannell]

I found bearings on eBay that were pretty inexpensive.

2 Angular Contact 7302B 15x42x13 15mm 42mm 13mm Ball Screw Spindle Ball Bearings | eBay




And yes, you're going to need some room if you want to take it apart. My shop floor was completely covered in skids with parts on them.

Timothy

wow thanks! i wasnt sure what to expect..i didnt know these would be ball bearings. is there any way to determine if they are good or bad? no reason to just replace them if its not the issue right?

hopefully its obvious and they slop around. or i find something loose or filthy

when i attempted to measure z axis backlash it was a few thou..the x and y were about 1 thou

[2ferrous]

I can't guarantee that you have the same size bearings in your machine. You'll find out when you get it apart. Then clean, inspect, measure and THEN order.

There will be two of them. They are installed back to back with a ground washer in between. Don't mix up the washers as they are sets too.

Post some pics when you get everything apart.

Timothy

[keebler303]

If it is done right, your "washers" are actually precision ground spacers and are matched to THOSE bearings. There is no guarantee the same spacers will give you the same preload with a different set of bearings. Perhaps one reason why the factory bearings are more expensive?

[2ferrous]

And both bearings are supposed to be a matched set too.
But I didn't want to scare the poor guy too much.

I've installed both on my machines. I can measure the difference but it's really hard to tell at the cutter.
A new bearing slightly out of tolerance is better that a worn out or damaged bearing.

Timothy

[acannell]

And both bearings are supposed to be a matched set too.
But I didn't want to scare the poor guy too much.

I've installed both on my machines. I can measure the difference but it's really hard to tell at the cutter.
A new bearing slightly out of tolerance is better that a worn out or damaged bearing.

Timothy

thanks guys.

ill be losing my fast, heavy, precision virginity on this disassembly, so im ready and waiting for the juicy eye candy at just how a mill is engineered in its nether regions.

i spent all day getting my air compressor working so i didnt get to the thrust bearings. but the air is for the mill so its going to happen sometime this week im sure...ill make sure to post lotsa pics

[acannell]

okay!

so i took apart the z axis drive assembly...mostly

off comes the side covers and you can see the counterweight pulley system..

the cover for the z axis servomotor has some funny business going on..instead of the 4 screw holes clearly meant to mount the cover being in use, it looks like someone drilled two new holes and slotted the cover, and screwed in screws at a funny angle. the screws dont match each other. the new slots in the cover seem fairly well made. strange..why did someone do this? maybe if i try to use the 4 original holes ill find out

under the cover is the servomotor, looking rather simpler and smaller than i thought. the z axis home switch cable mounting ties have broken or been broken..hmm odd

and now to the "transmission"..i thought there was a gearbox, but to my surprise i took the motor off and there is just a teeny belt with a couple pulleys lol! how does the belt get tensioned?

i noticed i could turn the pulley back and forth about 5 degrees (about one tooth worth)..that seems like alot of play to me..hmm

can someone explain to me how a belt like this can be used in cnc machines? doesnt it have far more flexibility than can be allowed? (obviously not..but i dont get it)

so, i took off the ballscrew to pulley lock "nut" and then attempted to pull the pulley off. im glad i gave up early on because its screwed onto the ballscrew! duh!

once i unscrewd it i could see the top thrust bearing. i tried to figure out a way to get it out, but then just pulled it out with a magnet.

for curiousity i tried screwing the pulley back on a little tighter (by hand)..and the back and forth rotational play was much reduced..hmm...so perhaps i could improve backlash via that adjustment..i wonder if thats related to the shuddering...

the second thrust bearing looks like ill have to take the motor mount casting off in order to get it off..i believe, (if im thinking about this right) that the second bearing (the one I didnt take out) is only loaded when the z axis is going UP..so it shoudlnt be the culprit for my downward shudder..although i should probably replace it too

so out comes the bearing and..imo it feels like sh**..its a little "grainy" feeling, but it depends on how you push on the center of it while you turn it..hmm..its obviously not brand new...its got part numbers on it..ill see if i can find a cheap replacement for it..

this was a very easy disassembly so far! im glad i did it..i need to take this machine apart more and more so im more confident about how it works, but i also need to get it up and running right away so ill probably have to stop after this bearing issue

[acannell]

hmm...im trying to determine if the bearing is bad or not

while pinching the inner race with two fingers (so it cant move), i can push/pull the outer race about 10 to 20 thou axially..thats insanely terrible right?

should the bearing be absoutely solid in every direction? i.e. no wiggling of anything relative to anything else

im seeing crazy prices for this bearing..from $240 to $75...dynas price of $42 is a steal. im wondering if im picking out the wrong bearing elsewhere..this one is marked 7001A on the outer race but 7001 on the inner race (no A). its NSK, JAPAN, with a "V" and a dot on the outer race....seems like there should be more part number suffix or something to fully specify it..
hmm

[2ferrous]

If the bearing isn't assembled on the shaft and without it's mating pair then the bearing will feel loose as you've found out. That's normal for angular contact bearings.

Try a 7001B bearing instead.

7001B 12x28x8 Angular Contact 12mm 28mm 8mm Ball Screw Spindle Ball Bearings | eBay

Looks like you're going to have to remove the gearbox housing to get at the lower bearing. Unfortunately that also means removing the chain rollers too. Support the counter weight and pop them off.

Timothy

[keebler303]

The belts normally have fiberglass or similar belts inside them so they have very little stretch and are quite strong. With a good belt, you should be able to get it tight with just the small amount of slop in the mounting holes. My big old CNC bridgeport is done the same way.

While you have it apart, you should definitely replace both bearings and clean out any chips or crud which could fall into your new bearings and kill them.