[coffeetek]

Hence, it appears that you have a bent Y-axis leadscrew.

Last thing I wanted to hear, but its very likely you are correct. Fact that even under load it would move predictably would strongly suggest that its lead-screw. However, I did observe that I could do a very heavy cut and get lines to almost disappear... We will see what Tormach says. What would cause Y-axis lead screw to bend though?

[LeeWay]

It is either a bent lead screw or the ball nut may be misaligned. Could be loose too, but then I would expect to see slop in the form of backlash from the nut rocking.
Do you know anyone that has one of those little inspection cameras?
I bought one a while back from Harbor freight and it is very cool.
Perfect tool for inspecting things under tables and other tight places. I have used it quite a bit more than I initially thought I would.
https://www.harborfreight.com/digita...era-62359.html

[mountaindew]

If you cut another piece of stock the same size and clamp it on the other side of the vise it balances the load and the movable jaw stays straight.
With an aggressive cut and the setup you had the part is likely to pull out of the vise because the front jaw is really only contacting the left corner of the workpiece.

I often use a threaded hole on a 123 block with a bolt that I adjust to be a little short of the material size being held on other side of vise. Then tighten vise and check when the material and the 123 block are both tight. this helps to limit any skew I get working on one side of vise.

As for the problem shown in this thread. Looks way to consistent to be anything but code or a mechanical defect like something is bent, bound or loose .

[coffeetek]

Thinking about this some more I think very likely this is lead-screw problem, its wobbling. If you watch the video it seems as table gets closer to start of lead-screw where it attaches to motor the wobble is less than on the far end...

Anyone knows how involved is it to replace this? A whole day job or more?

[philbur]

What am I missing. Surely rotation of even a bent y axis ballscrew would have no influence on the tables' x axis location if the y axis gib is adjusted correctly and there is no slack in the x axis screw or mounting bearing. My money's on a sloppy y axis gib.

Phil

[Zetopan]

Yes, the Y-axis gib must also be somewhat loose as well. Furthermore the "bend" could be a non-alignment at the motor end of the ball screw since the wobble seems to be largest there and smallest when the saddle is towards the distal end of the ball screw (the latter end is not rigidly supported by a frame bearing). If the coupling is trashed such that the motor shaft and ball screw are not concentric, or the motor shaft or ball screw end are bent (or both) that could cause the symptoms. The owner needs to pull the back cover on the column and examine the motor to ball screw coupling when it is rotating. Likewise that end of the ball screw will be visible.

[Bdrangdo]

Just an idea, maybe there is a damaged ball in the ball nut that jams, or stiffens up at it passes through the return causing a momentary twisting action in the nut every 360 degrees and then
corresponding movement in the slide which must either be flexing or has loose or badly fit jibs/dovetails. This could be easily checked by turning the screw by hand and feeling
for a stiff point every revolution. Dave

[coffeetek]

What am I missing. Surely rotation of even a bent y axis ballscrew would have no influence on the tables' x axis location if the y axis gib is adjusted correctly and there is no slack in the x axis screw or mounting bearing. My money's on a sloppy y axis gib.

Phil

Y and X gibs are adjusted per manual. Manual also specifies that lost motion on X or Y axis should be less than .0018”, mine is at about .001" I think. Increasing the gib tightness will only lead to increased lost motion. It also does not explain why movement on X axis does not wobble.

One thing might be worth mentioning, and I don't know whether that is expected, but I cannot use gib adjustment screws to completely stop X or Y axis movement due to gibs being too tight. From some reading it appears that this should be possible, but not on my 440.

[Need TECH Help!]

Have you tried rotating screw by hand , with machine off ,as Dave suggested? I would also place an indicator like you have on side of vise and push table back and forth to see if there is the same amount of movement as when you are feeding axis. Do this with Y axis in different positions. Also, check X axis like this.

HTH,
Troy

[LeeWay]

You should theoretically be able to lock them down with the adjustment screw. This means that they are not getting as snug as possible and may be contributing to a non concentric screw alignment translating to the part.

This will be a warranty issue I think. If it is something that can be adjusted out to correct, then that is good and the machine may not need to be disassembled. Fingers crossed for you.

[Need TECH Help!]

Do these machines have a stop screw at other end of gib?

Troy

[coffeetek]

You should theoretically be able to lock them down with the adjustment screw. This means that they are not getting as snug as possible and may be contributing to a non concentric screw alignment translating to the part.

This will be a warranty issue I think. If it is something that can be adjusted out to correct, then that is good and the machine may not need to be disassembled. Fingers crossed for you.

I have tried, its impossible to lock either X or Y down to stop movement. This however might be how they made 440 since both axis behave the same... I don't know... I don't think this will be an easy fix, but I really need to have this machine working correctly to use for seconds ops... It works quite well when it works :-)

[Bdrangdo]

I am not familiar with these machines so I could be wide of the mark. A badly worn Bridgeport or a badly made Chines clone can often have jibs that do not run parallel with the
slideways. They can be adjusted to the point of binding and yet still there will be a degree of lift on the saddle or bed because the gib strip (which are often bent) is no longer
rectangular in section and can twist. An easy way to check is to put a clock on the pillar to the bed and try and lift the bed. I am guessing that what appears to be a side way
oscillation during a cut is actually is actually a rocking action caused by the screw and nut binding for some reason. Dave

[LeeWay]

Both the saddle and table have a screw on each end of the gib.
Looks like a typical gib setup from the exploded parts drawing in the manual.

[coffeetek]

When I think about it I should be able to really lock the table with gibs adjustment screws, but I can tighten them as far as they will go while backing the one on opposite side all the way out and see really minuscule effect on lost motion measurement. While adjusting them I thought that they are not doing anything at all... And that's on both X and Y axis. No clue if this is how it should be but I doubt it.

[LeeWay]

Start watching John Saunders video here.
https://www.youtube.com/watch?v=dDuPcSKb6uk

Scroll to about 6:30 or so.

[coffeetek]

Yes, watched that before, that's the reason I am thinking that I should be able to seize the axis with gib adjustment. He can. On my machine, impossible.

[coffeetek]

Here is another video on gib adjustment, its for different machine but its different approach:


I have tried this and adjusting gibs this way results in out of spec lost motion i.e. 0.00311" when manual recommended way to adjust gibs yield less than 0.0018"

I have also experimented with getting Y gib tight so it stops this wobble. Its possible (still shades of wobble visible), but results in lost motion greater than 0.0031"

I have new Y ball screw on the way from Tormach. I will replace that, adjust gibs per manual and see what happens.

[Bdrangdo]

I don't believe your loose gibs are your sole problem, in themselves they are not causing your problem but merely exposing the fact there is an underlying problem with your ball screw/nut which is causing the bed to jump around. You may well end up having to shim the gibs. I repair machine tools for a living and often find all manner of things used for shim, even strips of paper will work. Dave

[philbur]

Here is another video on gib adjustment, its for different machine but its different approach:

I have tried this and adjusting gibs this way results in out of spec lost motion i.e. 0.00311" when manual recommended way to adjust gibs yield less than 0.0018"

I have also experimented with getting Y gib tight so it stops this wobble. Its possible (still shades of wobble visible), but results in lost motion greater than 0.0031"

I have new Y ball screw on the way from Tormach. I will replace that, adjust gibs per manual and see what happens.

I thought you solved the problem back in post 32. Quote:

Wohooo! Solved! So Tormach support suggested adjusting Y gib as you guys did here. So I went and adjusted it as described in manual and problem went away. Here is picture of finish while moving on Y axis. Adjusted Y gib on top, original problem on bottom. HUGE difference:



Phil