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Wells Index 847 - X axis ball screw conversion?

GregSY

Diamond
Joined
Jan 1, 2005
Location
Houston
After owning my 847 about 25 years, I decided to peek at the X axis Acme leadscrew/nut. I've always kept it well oiled but never paid much attention to it. I find it is considerably worn. Not shot but with a definite amount of slop. The screw itself looked nice...no visible wear.

I also looked at the bearing....there is a deep groove ball bearing on the right side and the left side is a double-stacked ball bearing which takes the thrusts. See pic if anyone wants to know what they are.

A new nut is around $450, and a new leadscrew is around $1500 from Wells.

I ordered a new generic nut from Roton ($60 plus shipping) and pressed it into the old nut. New bearings ($125 and $7).

On reassembly, the whole affair is much tighter, as expected, but it does have some places along the travel where the nut wants to move radially (runout) as the screw is shoving it. This implies the screw is running out...but that is odd as there are also places where the nut runs dead true with no runout. Normally things don't behave that way.

So anyway, I'm wondering if it would be a good idea to replace the screw and nut with a ballscrew setup. What are the pros and cons?

This mill is 100% manual operation, other than a power feed, and there are zero 'CNC' plans for it.
 

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After owning my 847 about 25 years, I decided to peek at the X axis Acme leadscrew/nut. I've always kept it well oiled but never paid much attention to it. I find it is considerably worn. Not shot but with a definite amount of slop. The screw itself looked nice...no visible wear.

I also looked at the bearing....there is a deep groove ball bearing on the right side and the left side is a double-stacked ball bearing which takes the thrusts. See pic if anyone wants to know what they are.

A new nut is around $450, and a new leadscrew is around $1500 from Wells.

I ordered a new generic nut from Roton ($60 plus shipping) and pressed it into the old nut. New bearings ($125 and $7).

On reassembly, the whole affair is much tighter, as expected, but it does have some places along the travel where the nut wants to move radially (runout) as the screw is shoving it. This implies the screw is running out...but that is odd as there are also places where the nut runs dead true with no runout. Normally things don't behave that way.

So anyway, I'm wondering if it would be a good idea to replace the screw and nut with a ballscrew setup. What are the pros and cons?

This mill is 100% manual operation, other than a power feed, and there are zero 'CNC' plans for it.
If were mine I would pull the screw and nut and on the bench run the nut one end to the other. That way you can identify the exact threads that are problematic .Can also check for bends.Then you can make a better decision on what to do. Won't cost any thing but time. If the screw is bent that can be addressed using the old nut split in half as a pusher to protect the threads. The problem may be something on the threads themselves.
As for ball screws don't know if I would do that . They back drive a lot easier, so you may have to get in the habit of always locking it. Although the zero back lash is a nice feature. It's only money but since you already bought the new screw I'd give it one more chance after all it lasted 25 years.
 
Shop around, think there are sources that sell quality Acme screws where you would only need to machine the ends to your needs. If your machine has a DRO then it becomes less important to buy high accuracy screws.
Would not go the ball screw route.
Cutting forces will drive the table and make it necessary to lock the slide to hold any move.
My NC Deckel has hand wheels , ( pull out to engage) buy they have friction devices which helps to hold position and actually make hand moves stiffer than a manual machine of a similar size.
Cheers Ross
 
Shop around, think there are sources that sell quality Acme screws where you would only need to machine the ends to your needs. If your machine has a DRO then it becomes less important to buy high accuracy screws.
Would not go the ball screw route.
Cutting forces will drive the table and make it necessary to lock the slide to hold any move.
My NC Deckel has hand wheels , ( pull out to engage) buy they have friction devices which helps to hold position and actually make hand moves stiffer than a manual machine of a similar size.
Cheers Ross
Good advice from Alfa GTA. What you would gain in accuracy you would lose in stiffness and the ability of the table to resist cutting forces.

Regards Tyrone.
 
Thanks, that is what I needed to know.

Acme is a weird world...the screws seem fairly cheap but the nuts are kinda expensive. Roton seems to offer the best value.
 
Yeah but more than I want to tangle with given my level of retardation and lack of tooling. Hell...the bronze these days alone is probably $100.

I'm pondering making a new screw and notice that the Wells Index 1.125"-5 Acme thread is a less common size. I could go to 1-1/4"-5 for less money, actually. Plus....McMaster sells a 1-1/4-5 screw in Class 2C 'precision' thread while the 1-1/8 is only Class 2G. Any opinions on that?
 
As long as you have the clearance don’t see a down side.
In theory a larger screw should wear at a slower rate. May have a bit more resistance to moving, betting you can’t tell the difference.
Some design choices are aimed at maintaining a proprietary part.( factory only replacement source)
Cheers Ross
 








 
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