My 6" X 26" mill broke this weekend

[Powder Keg]

I was trying to get the last tools made for my shaper. I was about 10 minutes into getting started. I had cleaned off the mill, set the parts in the vice where I needed them, then started to raise the table. I was about a half inch away and the knee and table just dropped clear to the base.

I didn't get any pictures. But I had to remove the table and cross slide to get to the up and down screw. I used it as a gauge to make a new 1" - 8 left handed Acme thread for my mill. I used a piece of oil lite bronze that i had been saving. It took me pretty much all day to get it all back together and working.

About two years ago, I bought one of those universal power feeds for the table. There was know way I could see it fitting. But since I have seen a couple of pictures of similar builds and It doesn't look too bad. So, I am going to do that now. I'll try and remember pictures this time.

Wes

[ken572]

Hi Wes,

Was the original 1" acme screw also made of oilite bronze??
I Thought they were made of steel.

Ken.

[Powder Keg]

The nut is what stripped. I used the steel Acme shaft as a thread gauge. The original nut was cast iron. I don't know why It has wore out so fast? I think I bought the mill in 2003 and I'm pretty good about oiling things up before I start. It must have just been its time.

[Powder Keg]

I've been so busy the last week or so. Between work and everything else. I have managed to get out in the shop and I did get my power feed all mounted up and running. I did neglect the pictures again though. I basically made a 6" shaft extension for the table screw that is held on with a couple of setscrews. I made a 1.75" spacer to take up the space between the table and the feed unit where the extension fastens to the screw. Everything works pretty good. I had to machine the hand wheel thinner and I have to figure out how to mount the dial. It mounts completely different than the original. Its a little smaller so what would be good is to make a new, bigger dial. But I really don't want to mess with that now.

Wes

[Powder Keg]

When I first got my mill I added a 6" extension to to the head. That way I wouldn't run out of daylight using drills and such. It turns out that that is the cause for my mill troubles. When you raise the table up to get closer to the head there isn't as much thread engaged. Sometimes I even went too high and the thread would go up over the top of the nut and drop back down. This would chip the nut a little and jam things together. To fix it I need a new thread and a new nut. I had originally made the replacement nut. But I found some left handed acme that will work from McMaster Carr. I'll get that ordered this week. I tore my mill apart today.

~Wes~

[doubleboost]

Hi
Wes
I have heard of this happening before
The cast iron they use is very soft
My millill has a riser as well , i have screwed out of the nut before (i have a paint mark now so i can keep a full nut engaged)
A bronze nut with a grease nipple is on the to do list
Can you imagine that table whacking you on the way down

[Rob.Wilson]

  you could have the  screw and nut from my 626  Wes ,,,,,,,, as i am not using it , its just sitting there .

Rob

[WillieL]

When I first got my mill I added a 6" extension to to the head. That way I wouldn't run out of daylight using drills and such. It turns out that that is the cause for my mill troubles. When you raise the table up to get closer to the head there isn't as much thread engaged. Sometimes I even went too high and the thread would go up over the top of the nut and drop back down.

~Wes~

I'm glad I saw this thread, as I just ordered a riser for my mill last night. I didn't really think about it causing issues like this. 
Something that comes to mind now is, if you add a longer screw to the knee - when you raise it the additional 6 inches, is there enough dovetail on the column to fully support the knee?

Also, is there any chance of the longer screw "bottoming out" when trying to get the knee to it's lowest point?

[doubleboost]

Hi
There is plenty of dove tail left
All you need is a mark on the main casting to tell you when to stop
Or better still a physical method of stopping the screw coming out of the nut
The riser block is the best mod i have done on my mill
If you are running out of table lift just wind the quill down a bit
John

[Powder Keg]

Thanks for the offer Rob But It would just happen again. It needs to be fixed right. I'll have the parts ordered this week.

~Wes~

[Jonny]

Like me Wes do it right or not at all, should see my hosehold diy.

Its a common weak point of all chinese mills. The same i am waiting to happen on mine, cant see how 5 1/2 revs of backlash has any threads left. Must have two revs on both X axis mills.
Mad panic ball screw jobby when breaks.

[Powder Keg]

Parts are ordered\o/

Jonny, You might want to research and see if you can run a manual mill with Ballscrews installed? I've heard you cant. I might be wrong?

~Wes~

[75Plus]

There should not be a problem manually operating a ball screw. The Saginaw steering gear used by all GM cars for many decades had ball screws. Saginaw called it "recirculating ball and nut".

This shows how  the steering gear works and would apply to ball screws also.

http://www.stangerssite.com/HowItWorksSteeringBox.html

This site actually shows a Ford version of the Saginaw design.

Joe

[jgroom]

HI guys, my Dad has a small Jet (Chinese) benchtop knee-mill that at some point in it's past was converted to cnc.  When he bought it the motors and electronics were gone but the ballscrews are still in it.  As a manual mill the ballscrews work great with just a couple small problems, they are both right-handed so the y-axis feed is backwards from 'normal', and the graduated dials are now useless (a DRO fixed that).

Cheers

Jeff

[WillieL]

Parts are ordered\o/

Jonny, You might want to research and see if you can run a manual mill with Ballscrews installed? I've heard you cant. I might be wrong?

~Wes~

I might be wrong as well, but I have heard the same thing. Something about the ball screws being so free-running that if the cutter should dig into the work, (climb milling) it could shoot the table off on it's own and out of your control?

Could be fact or fiction - I don't know. But I do remember reading that somewhere, on the net.   

[Davo J]

Parts are ordered\o/

Jonny, You might want to research and see if you can run a manual mill with Ballscrews installed? I've heard you cant. I might be wrong?

~Wes~

Hi Wes,
No trouble operating a manual mill with ball screws. Ray over on the Wood work forum converted his X axis over to ball screws because he is CNCing it, he stopped half way though because of other jobs so he is using it manually. He was asked this question and he said other than the hand wheel direction noted above there fine to use manually.
I have heard this said by many other people as well, but there seems to be a myth going around you cant use ball screws on a manual mill.

Some of the manual mills sold over here from China come factory fitted with ball screws, so even the manufacturers think it's fine to use them on a manual machine. One thing I did notice on some machines I have seen, is they have a lock directly onto the screw near the end cap bearing to lock that axis, but I think the gib locks would still work fine. The only thing you might find is you need the gib locks just lightly done up to give a bit of drag.

I have ball screws for my mill to CNC it, I would fit them now but need to wait until I sort out the motors etc to machine them to suit. It would be the best of both worlds having them with no real backlash and no need to adjusting for backlash. It would also allow climb milling.

Dave

[Davo J]

There should not be a problem manually operating a ball screw. The Saginaw steering gear used by all GM cars for many decades had ball screws. Saginaw called it "recirculating ball and nut".

This shows how  the steering gear works and would apply to ball screws also.

http://www.stangerssite.com/HowItWorksSteeringBox.html

This site actually shows a Ford version of the Saginaw design.

Joe

I have a Nissan 720 and have taken a few of the steering boxes apart and they have ball screws as well.

Dave

[loply]

Parts are ordered\o/

Jonny, You might want to research and see if you can run a manual mill with Ballscrews installed? I've heard you cant. I might be wrong?

~Wes~

Hi Wes,
No trouble operating a manual mill with ball screws. Ray over on the Wood work forum converted his X axis over to ball screws because he is CNCing it, he stopped half way though because of other jobs so he is using it manually. He was asked this question and he said other than the hand wheel direction noted above there fine to use manually.
I have heard this said by many other people as well, but there seems to be a myth going around you cant use ball screws on a manual mill.

Some of the manual mills sold over here from China come factory fitted with ball screws, so even the manufacturers think it's fine to use them on a manual machine. One thing I did notice on some machines I have seen, is they have a lock directly onto the screw near the end cap bearing to lock that axis, but I think the gib locks would still work fine. The only thing you might find is you need the gib locks just lightly done up to give a bit of drag.

I have ball screws for my mill to CNC it, I would fit them now but need to wait until I sort out the motors etc to machine them to suit. It would be the best of both worlds having them with no real backlash and no need to adjusting for backlash. It would also allow climb milling.

Dave

I believe the issue is to do with the table being able to move simply by being pushed.

On an acme screw if you grabbed the table and pushed or pulled it it would cause a great deal of friction on the screw threads where the nut contacts, which would prohibit rotation of the screw and thereby prevent the table moving.

With ball screws the same action would cause the table to slide and the ball screw to turn due to the lower friction amount and the fact that balls spin.

The problem this creates is that if you're milling in certain directions the cutter can grab the work and "pull" the table along, much like if you dig the cutter in on a hand held wood router and it pulls itself along at high speed.

I'm not sure how true this is or not, however, I'd certainly be inclined to believe it can happen. There's no way you can slide on a nut on an acme screw simply by "pushing" it, however on a ball screw the same action will cause the nut to move back and forth with ease.

[Jonny]

It wouldnt be a problem climb milling, i always put tension on the gib strips so it dont pull up to the backlash.

Chester want £244 for just a elevation gearbox, the whole Super Lux mill new was only £930 8 years ago!
Figured cheaper to go ball screw and nut £100 to £150, spend a week adapting etc with another motor or steeper and two pulleys.

[WillieL]

I thought the whole reason behind using ball screws was to eliminate all backlash and the reason they are chosen for CNC machines? 

Applying drag to the table via the gibs seems counter-productive to me. Much like driving a car with one foot pressing on the accelerator and the other pressing the brake at the same time. 

[DMIOM]

I thought the whole reason behind using ball screws was to eliminate all backlash and the reason they are chosen for CNC machines? 

Applying drag to the table via the gibs seems counter-productive to me. Much like driving a car with one foot pressing on the accelerator and the other pressing the brake at the same time. 

Willie,

you may want/need to apply some drag - not if you're fully CNC, but if you're trying to use a ball-screw equipped machine without the CNC kit to drive and hold the axis, as summarised by others above.

On my manual (still ACME) mill, unless I'm taking really big cuts or really need to hold tight tolerances, I can get away with just winding the handles to the correct location and leave it there, without locking X and/or Y - courtesy of the significant force needed to back-drive an ACME/trapezoidal screw.

On a ball screw machine, cutting forces - or even just vibration - can cause a ballscrew axis to walk - effectively the screw is back-driven just like an Archimedes Drill. A CNC machine will either have this steadied by the holding current in a stepper or actively kept in place by the encoder closed loop on a servo.

Concerning the situation where you are using ballscrews on a manual machine:

1/ Applying a little drag to a free-running manual ballscrew axis might be useful to get a more controlled cut (I sometimes use a second hand to apply a little drag to the cross or compound slide on the lathe), esecially to stop it being pulled away if you're climb cutting.

2/ If you didn't have your hands on an axis, I think you'd need more drag or to lock it, as any significant machining would be likely to cause the untended axis to move - at least it did for me!.  I only have 3 full axes on my ballscrew-equipped CNC mill but at one time had a jury-rigged 4th. 'A' axis which had its own motor but used the 'Y' axis control, so the program moved into XY position, then stopped and I threw a switch/relay which changed the Y axis drive from the Y motor to the A motor.  On one occaision, I forgot to lock the Y axis before press cycle start again, and the cutting forces back-drove the Y axis ballscrew/motor....

Dave

p.s. and regarding driving with one foot on the accelerator and one foot on the brake, that's how John Haugland taught me to use left foot braking in a rally car!

[WillieL]

p.s. and regarding driving with one foot on the accelerator and one foot on the brake, that's how John Haugland taught me to use left foot braking in a rally car!

But do you apply both at the same time?   

Rally car racing isn't a fair comparison anyway because one doesn't normally "drive" a milling machine like they had stolen it. 

Seriously though, that is an excellent explanation.  But I guess DavoJ is right. If the factories are shipping manual machines with ball screws - there must be a market for them.

Now.... what was the original post about again?      

 

[Jonny]

But do you apply both at the same time?   

Rally car racing isn't a fair comparison anyway because one doesn't normally "drive" a milling machine like they had stolen it. 

Its a tool at the end of the day and tools break stolen or not.
Both at the same time Will, try it works great with power feeds.

[Powder Keg]

My parts arrived today\o/ Now I can get back to work. I think I need to get a N-04 lock nut to replace the metric nut that I have. Looks like I better get back to work...

~Wes~

[Powder Keg]

After work we threaded the end of the shaft on the CNC at work for the metric locknut:o) That way I didn't have to get a new nut. I didn't get a chance to key it though:o( Maybe Monday?

~Wes~