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EMCO FB2 mill Z-axis nut stripped

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Y C Lui28/06/2022 16:22:54
83 forum posts
35 photos

The head of the mill suddenly could not be moved upward during use today. It didn't take me long to find that the problem was due to the thread of the Z axis nut partially stripped. I removed the stripped threads with a boring bar and the mill is now working again but it probably will not last long. Would like to hear from FB2 owners your solutions .

Joseph Noci 128/06/2022 18:32:11
1099 forum posts
1317 photos

How 'partially' stripped? If the head would not move while the leadscrew turns, then the nut is stripped properly..How did you 'fix' this by boring out stripped threads?

I have 3 FB2's - One is cnc'd with ballscrews, but the other two have the original Z nuts in, one still OK after 32 years, the other after 21 years.

I do however have a spare set of XYZ nuts, obtained from EMCO 2 years ago - they still have stock..

The Z nut is a simple one to make though - if you can find a piece of 7075 aluminium bar stock that will work just fine.

The thread will need a inner pointing tool ground , but that is not to difficult I would think.The thread is quite short in that nut so the tool need not be very long. When I obtained the second FB2 (pre-owned) , it was imperial and I purchased metric leadscrews and nuts and dials (2001) and swapped out so I have a set of new imperial screws and nuts in the cupboard...I did investigate shipping costs to the UK for another fellow interested in some other FB2 parts, and from Namibia it was horrendous - near 170 pounds to the greater London area!

Y C Lui28/06/2022 18:56:01
83 forum posts
35 photos
Posted by Joseph Noci 1 on 28/06/2022 18:32:11:

How 'partially' stripped? If the head would not move while the leadscrew turns, then the nut is stripped properly..How did you 'fix' this by boring out stripped threads?

 

I haven't taken any pictures but the nut has two slots cut half way across the body at the upper end. These two slots are meant to be for backlash control. By compressing these slots, the threads in that portion will shift relative to each other so the backlash can be eliminated or reduced but that's IN THEORY. The same design is used in the cross-slide nut of my Emco Compact 8 lathe. This design does not work for me. The compression of the mentioned slot resulted in the threads in that portion totally stripped but the rest are OK.

It appears that I will need to make spare nuts out of 7075 aluminium. That will not be too difficult but I would like to know if there are other ideas.

Edited By Y C Lui on 28/06/2022 18:57:17

Joseph Noci 128/06/2022 20:23:16
1099 forum posts
1317 photos

Still not sure I understand - I know the nut well, so understand how it works, but fail to understand how if the lower half's threads are not stripped that the head would still not move. Was the leadscrew turning while the head remained stationary? If so then the lock-screw in the side of the slide block into the Z nut was not preventing the z-nut from turning. Or would the leadscrew not turn, ie, jammed?

If the upper half threads are stripped while the lower half are good, then the compression nut on top of the slide block was probably way overtightened - a number of times...

The design works and works well if not abused, so it should work for you as well!

I frequent various EMCO ( lathe and Mill) groups ( groups.io, etc) and no-one has complained of the Z nut design being unfit for task..

Y C Lui28/06/2022 20:50:17
83 forum posts
35 photos
Posted by Joseph Noci 1 on 28/06/2022 20:23:16:

Still not sure I understand - I know the nut well, so understand how it works, but fail to understand how if the lower half's threads are not stripped that the head would still not move.

The head could move, but just downward, not upward above after a certain point. The threads parted from the nut coiled around the lead screw preventing the nut to move upward over that point.

The compression of the slot is not entirely controlled by the locking nut but the weight of the head. I have never over tightened that nut and as a matter of fact, it often got lose by itself and I could not figure out why.

Joseph Noci 129/06/2022 08:32:08
1099 forum posts
1317 photos

Was the upper half of the nut, the part above the compression slits, still attached to the lower half, ie, was it only that the threads are stripped from the upper half, or had the upper half parted from the lower half?

Are there signs of rubbing tween leadscrew and the crimp nut interior ( item 4 below) at all? If not it should be impossible for item 4 to unscrew, unless the z nut somehow rotates- that could occur if item 22 is not well tightened..

Do you recall what the hole in the side of the nut, where item 22 impinges, looks like at the moment? Has it elongated vertically? If so, then all the head weight is on item 4, compressing the z nut slits all the time and wearing the upper thread half prematurely. However, that that should even more prevent item 4 from unscrewing as it is under tension all the time. Or has the hole elongated horizontally? If so, the nut would turn a little each time Z is wound, and that would cause loosening of item 4

fb2-z_nut.jpg

fb2-z_nut1.jpg

Graham Meek29/06/2022 11:14:10
512 forum posts
314 photos

The last FB2 I owned was 26 years old, I had never changed this nut or adjusted the backlash setting. The weight of the head always meant the backlash was taken up.

It had always been my intention to replace this nut with a Phos Bronze one, but I never did get around to it.

Having checked the Emco Holz & Hobby site they do not list this nut as a spare anymore, so the home manufacture of a new one would be a priority in my book.

If anyone has access to a nut perhaps they could supply a drawing to help other users, should they find themselves in this predicament.

Regards

Gray,

Kiwi Bloke29/06/2022 11:37:08
702 forum posts
1 photos

Original nut is, I believe, a zinc alloy, Mazak/Zamak, etc. 7075 has been mentioned as a nut material. Honest question: is this a wise choice? In the back of my mind, a small voice is saying that the stuff is abrasive, and wears cutting tools quickly. Quite likely mistaken, though.

Joseph Noci 129/06/2022 12:28:22
1099 forum posts
1317 photos
Posted by Kiwi Bloke on 29/06/2022 11:37:08:

Original nut is, I believe, a zinc alloy, Mazak/Zamak, etc. 7075 has been mentioned as a nut material. Honest question: is this a wise choice? In the back of my mind, a small voice is saying that the stuff is abrasive, and wears cutting tools quickly. Quite likely mistaken, though.

Nuts can be obtained from EMCO Austria still - pricey I guess.

7075 does not wear cutters much worse than any of the other Al grades really - it's all related to cutting speeds and feeds, and lubricant. Carbide drills in 7075 wear when speeds are high and no cooling occurs - the Al adheres and forces grow, chipping the cutting edge. There is around 0.2% Si and Cr in 7075, where its 'toughness' is derived, but I would not say it is abrasive. Be careful of choice of bronze - some are very hard - try for a high lead bronze perhaps.

But maybe we are being over-fussy here - the Z axis movement is very small generally, and much more infrequent WRT X and Y. Wear in Z is rather slow, if things are set up correctly and lubricated - As Graham indicated, no wear or required adjustment for 26 years, mine even longer. Any of the harder Al grades would probably be fine, as would probably be any bronze or brass even. The wear action is so slow...And as Graham also indicated, backlash should easily be taken up by the weight of the head anyway - if not, then the gib strip screws and slide block pinch bolts are probably way to tight.

SillyOldDuffer29/06/2022 18:00:51
Moderator
8903 forum posts
1999 photos

Could the threads have been stripped by over-correcting backlash and perhaps over-tightening the gibs as well in hope improving the machine's accuracy? Tightening the gibs puts extra pressure on the screw and nut, and so does squeezing the anti-backlash slot. If slots are closed up to the point backlash is very low, the nuts wear rapidly, especially if the slides are tight as well.

All my machines have squeeze slot anti-backlash nuts and I'm not impressed by them as a mechanism. Cheap and cheerful rather than precision. I suspect they're only intended to reduce backlash, not to eliminate it. I only tighten mine gently after backlash becomes persistently annoying; once in a blue moon.

Used to worry about backlash. Rarely matters in practice though - except when I forget to allow for it. Now my milling machine has a DRO, I hardly need to care about backlash at all. My lathe occasionally catches me out because it only has dials.

Dave

Joseph Noci 129/06/2022 20:44:43
1099 forum posts
1317 photos

I second most of what you said Dave. But I still believe the method is not bad. No good in a production setup maybe, but the FB2 is a hobby machine.

I spend a fair time in the workshop, but the FB2 and the lathes work maybe 5-8 hours/week and I find I adjust the backlash on the FB2 maybe every three or four years.

On the lathe the backlash is ignored anyway - as you say, with DRO it matters little. The FB2 also has DRO, but baclash on XY is important to me there. It helps prevent issues with normal/climb milling trying to pull the opposite axis into the cut.

Graham Meek29/06/2022 21:51:21
512 forum posts
314 photos

I made Phos Bronze main feedscrew nuts many years ago for the FB2, after the initial adjustment they were never touched. Despite the Hobby build of the FB2 it does handle work far outside its scope.

Somewhere in the back of my mind I think Emco specify 0.05mm backlash as the desired goal. It may have been on my Emco F3 that I read this. It does go to show there needs to be some backlash, or premature wear takes place as there is no room for the lubricant.

Regards

Gray,

Y C Lui30/06/2022 03:39:33
83 forum posts
35 photos
Posted by SillyOldDuffer on 29/06/2022 18:00:51:

Could the threads have been stripped by over-correcting backlash and perhaps over-tightening the gibs as well in hope improving the machine's accuracy? Tightening the gibs puts extra pressure on the screw and nut, and so does squeezing the anti-backlash slot. If slots are closed up to the point backlash is very low, the nuts wear rapidly, especially if the slides are tight as well

I would say thats likely to be the reason. The gibs as well as the other two bolts for Z axis play control are all on the tight side. When the head is lowered slowly, sometimes it would stop before suddenly fall by around 0.1mm before moving smoothly again. That zinc alloy nut always worries me so I have been exercising great care not to overload it. When I move the head upward, I always push the head upward with my hand at the same time in order to reduce the load on the Z nut.

derek hall 130/06/2022 06:56:09
235 forum posts

I need to inspect my Emco as despite following the setting up instructions I cannot get the "balance" right where the handle effectively locks and unlocks the head.

Not sure if it's the nut in question that is wearing out or the fact i cannot get bolts 39 and assembly 40 and 41.

Anyone got a clearer procedure than in the manual?

As regarding the nut if it hasn't broken or stripped what is acceptable backlash?

Regards

Derek

Y C Lui30/06/2022 09:31:08
83 forum posts
35 photos
Posted by derek hall 1 on 30/06/2022 06:56:09:

As regarding the nut if it hasn't broken or stripped what is acceptable backlash?

 

My machine is equipped with DRO so backlash is not a concern. As of now, the Z hand wheel can turn freely over 0.12 mm when the head is locked. The same amount is seen on the cross-slide hand wheel of my Emco Compact 8 lathe.

Even without DRO, the weight of the head should be sufficient to take out any backlash but that's assuming that the head can move very freely. For that to happen, the gib and bolts 39 will need be on the lose side which can hurt accuracy along the X axis.

Edited By Y C Lui on 30/06/2022 09:33:39

Graham Meek30/06/2022 10:23:03
512 forum posts
314 photos

Derek,

I have in other posts on this forum outlined the way to go about the Slider adjustment. This cannot be done with the head in-situ. The Column assembly needs to come off the machine, the feedscrew removed and the column held while the slider is dealt with. The Gib adjusting screws need to be backed off, (19) this allows the fit between the slider and the column to be adjusted. It should move freely, without any side play from one end of the column to the other, but beware the keyway will foul towards the bottom of the travel.

The movement should be able to be carried out with hand pressure alone, any tighter and it is too tight. Then it is possible to adjust the Gig strip, and again after adjustment this should move with hand pressure alone.

With the slider at the top of the column and locked put the feedscrew back in and lock up the bolts, but not too tight as the area beneath the housing is not fully supported. I have known some users break this part.

Re-install the column and refit the head. (The head can be rested on the milling table to remove it with suitable packing).

I hope this helps,

Regards

Gray,

Kiwi Bloke30/06/2022 10:59:16
702 forum posts
1 photos

It's with some trepidation that I offer an opinion that differs from Graham Meek's. I believe that it is possible, preferable, even, to adjust the 'slider' and gib clearances with the head and column in situ.

The crucial thing is to ensure that the head can fall under its own weight. The feedscrew should never be trying to push the head downwards. The adjustments should be done in the order Graham specifies, with the feedscrew removed. The head isn't that heavy, and can be slid up and down the column without too much effort, and still with good feel, the process being helped by being able to stand behind the machine, at a suitable height (on a stool, for example). Ensure there's a block of wood, etc. between the table and the head, in case you lose grip!

Anxiety about x-axis inaccuracy can be investigated by 'clocking' possible rotation around the column, and attending to the gib adjustment.

It's not the most rigid design, so I believe that the adjustments need to be as tight as possible, to the point where the head is only just free to move under gravity's influence. It's for that reason, and for convenience, that I suggest doing this with the head and column in situ. Lock the head to the column whenever possible when cutting. Some folk have improved the clamping arrangement. I just use my impressive strength...

Edited By Kiwi Bloke on 30/06/2022 11:01:35

derek hall 101/07/2022 15:21:41
235 forum posts

Thanks for the info guys, sorry to hijack the thread Y C but felt it was related.

Kind regards

Derek

Y C Lui04/07/2022 04:00:48
83 forum posts
35 photos

Making a new nut is now on my to-do list. Seems doable and I have two questions :

1) What kind of material will be the best ? It seems that SAE660 bronze is a more standard material for lead screw nuts but I can get 663 only. Will that be OK ?

2) The lead screw seems to have trapezoidal threads but I am not sure about the angle. There are two types : 30 and 29 degrees with 30 being more common. Does that one degree difference matter in practice ?

Graham Meek04/07/2022 10:43:33
512 forum posts
314 photos

The Leadscrew is indeed Trapezoidal, by that definition it is 30 degrees included angle. This being the Continental standard. A 29 degree included angle means it is an Acme Thread.

Any Bronze is better than no Bronze at all. If I were making one and given the limited use this Axis gets compared with the X&Y Axis, then I would opt for Colphos 90, as this is much easier to machine. At a push I would substitute Aluminium, especially if you have not cut a Left Hand Trapezoidal thread before.

Just be aware that there will be wear in the centre of the Leadscrew, there always is.

Regards

Gray,

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