Split feed screw nuts - are people happy

Probably one of the easiest things a guy can do to improve backlash.
Are people happy with the results.

Split nuts drastically improve the backlash on both the X and Y axis. My mill originally came with the "split nut on each. After 28 years the backlash on the X axis grew to 019". The lash on the Y axis grew to .031". I was able to bring them both down to .003 - .005" by splitting the original nuts, facing them and reinstalling them.

A "professional" machine re-builder once bought an antique Hardinge Cataract lathe from me. He scraped all over the slide rest, including surfaces that should not be scraped. Then he explained that he fixed the loose feed nuts by sawing them lengthwise and hitting them with a hammer. I think he left the worn out feed screws alone. He did not want to spend the money to buy NOS feed screws and nuts that I had. What a disgusting way to treat a nice lathe.

So that is the mental image I have when I see someone ask about "split feed screw nuts."

Maybe you are talking about a Bridgeport mill, but you did not actually say. Hardinge came out with two-piece feed nuts in 1960, for most of their lathe applications, and they seem to work OK.

Larry

L Vanice said:

Maybe you are talking about a Bridgeport mill, but you did not actually say. Hardinge came out with two-piece feed nuts in 1960, for most of their lathe applications, and they seem to work OK.

Larry

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Sorry. It's for a Bridgeport mill.

My 1958 bridgeport had over .100 on X and about .40 on Y after splitting I got X down to .003 and Y down to .004 I definitely recommend it.

New nuts are only 30 bucks last time I bought some from BP they are already spilt and brand new and since you can replace them without pulling the table just get some new ones. Splitting leaves you with a tight table out on the ends.

I will call Hardinge Brothers on Monday and ask for a price on new ones. Over
the weekend I will split the ones I have.

Something I noticed before but got reminded today. The spline type gear inside
the saddle sure gets exposed to all kinds of chips. I'm going to think of a way
to protect it before I put everything back together. Maybe some kind of dirt
flap on that vertical square hole in the back of the knee. It needs to be made
sanitary.

You guys splitting your nuts are really making things worse in the long run, just to get a warm and fuzzy feeling that your machine had less backlash. Anyone who knows about machines knows that backlash really is not a problem. By splitting the nuts you are effectively reducing the surface area in contact by half. That means that now the nut and screw will wear out twice as fast. All for less perceived back lash? I don't think so Holmes. I predict that those who have split their nuts will see the backlash start to reappear quickly.

--Doozer

Doozer said:

Anyone who knows about machines knows that backlash really is not a problem.

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Untill you HAVE to climb mill

Doozer said:

By splitting the nuts you are effectively reducing the surface area in contact by half.

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No. The nut actually only runs on a very small contact patch due to non linear wear in both the nut and screw. That point moves depending on where in the travel you are. Only under high tensile/compressive loads does a nut/bolt deform enough to get more threads engaged. A BP leadscrew wont ever see that kind of load (it would buckle while pushing for example).

Doozer said:

I predict that those who have split their nuts will see the backlash start to reappear quickly.

Click to expand...

In another 20years or so possibly... Actually at the duty cycle I use it vs the original owners it'll surely be another 80-100.

On a light machine (Bridgeport) if you want to climb mill you snug the table locks a bit. That is how it is done. If you have a heavy mill (Kearney & Trecker or Cincinnati) they have backlash eliminator mechanisms as factory equipment that allows easy climb milling.

By splitting the nuts you are effectively reducing the surface area in contact by half. I stand by this assertion. I'm not going to explain this one. If you can't see it, you can't see it.

--Doozer

Beaver mills were approximately the same size as Bridgeports and had backlash elimination built in, with nuts also spaced about 1ft apart. It isn't rocket surgery, just good design. Snugging up the table locks a bit is a good way to increase wear on gibs and leadscrews...

Doozer said:

By splitting the nuts you are effectively reducing the surface area in contact by half. I stand by this assertion. I'm not going to explain this one. If you can't see it, you can't see it.

--Doozer

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Hardinge/Bridgeport sells split nuts in bronze. You stand alone.

How is the fact that Bridgeport sells split nuts relevant to the other fact that thread contact area is decreased when you split a nut to reduce backlash??

--Doozer

Doozer is not standing alone, I will stand next to him when talking about “only half the thread contact on a split nut”. On the other topics of this thread, well, I thought I was happy enough with my worn BP. Did not use the cast steel dials and was careful of climb milling, etc. So, until recently, I lived with as-is/worn nuts and feedscrews, I thought I was happy. I couldn’t of been more wrong! I changed the X and Y feedscrews and nuts (one piece) with new and what a difference! Smoother cuts, now inside pocket milling is with minimal overshoot, for the moment no more tight travel at the ends, climb milling is more possible, and…I can’t say enough good about it! If you are thinking about changing the nuts and feedscrews it can be easily done without removing the table. As for BP’s design on the feed nuts and the yoke that holds them, I ain’t happy. The force of the feed direction on the nut-yoke assembly in one direction is good and up against a solid shoulder. In the other direction, all the force is on a little screw! Some of you that think a worn BP just needs feed nut adjusting, might be looking at a feed nut adjustment screw adjustment.

I'm not trying to be sarcastic here, so don't interpret it as such.
Splitting the nut allows both pieces to "squeeze" into each other, eliminating backlash by contacting the "outer" thread on its respective side? Is this a correct explanation?
How does the one piece nut function any differently?

Doozer said:

How is the fact that Bridgeport sells split nuts relevant to the other fact that thread contact area is decreased when you split a nut to reduce backlash??

--Doozer

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I don't know enough about the topic at hand to really discuss pros and cons, other than to say that splitting the nuts is cheap to free, and while the "right" way of doing it may be to replace feed nuts and screws, the cheap way to get a machine tight enough to do far more precision work - more than good enough for most hobbyists - is to split the nuts from what I have seen.

Add to that most hobbyists who end up with a bridgeport are pretty happy to find something beat up for less than a grand - myself included - and you have precisely the reason why this mod is so popular. And neither side will see the viewpoint of the other, lest it be viewed as weakness on the interwebs....

For those of us with mills without factory split nuts, can somebody please post a picture of a factory split nut and a nut split by an owner to reduce backlash? It would help me visualize to situation.

Thank you, John.

jeagan said:

For those of us with mills without factory split nuts, can somebody please post a picture of a factory split nut and a nut split by an owner to reduce backlash? It would help me visualize to situation.

Thank you, John.

Click to expand...

No need for that. A factory two piece set is just that. Put end to end it would be about the
length of a longer nut with a saw kerf on one side. The new one piece nuts have
a two saw kerfs on opposing sides. This should allow that one piece nut to compress
equally. I have seen drawings of nuts with four saw kerfs (two saw kerfs on each side)
but never heard of anybody talking about them on these forums.

The bottom line is this:
1. Split the nut if you don't want to buy new screws just yet. This will tighten
the screw system up but could lead to no free play at the extreme table
positions. If you have a 42 or 48 inch table then that is a rare event. I think
for the people which have a BP for a hobby the split idea is a good one because
it adds life to the old screws.

2. Buy new nuts and use with the old screws.

3. Buy new screws and nuts. This is the best way to go if you want everything
to be the way it should be.

If you are thinking about converting a BP to CNC then you can get a set of
ball screws. I have also heard of a way of converting a split nut system to act
like a ball screw, this idea came from a pretty knowledgeable guy.
Either way you can buy yourself some time by doing the split and shop around
for you CNC conversion.

Another issue is that the new nuts are bronze for the split system and brass
for the two kerf nut system. I would always opt for the kind of nut which caused
the least amount of wear to my precious screws. I suspect the bronze screw
would wear the screw faster.