.249" diameter acme screw?

[Macht_Tot]

I have a couple ebay'ed Acme threaded rods here that I want to try making a linear motion table with (with some pillow blocks etc). Specs as given are:

double start Acme thread with a 4mm pitch. The screw O.D. is 0.375", 10 1/2" thread length and 0.249" diameter bearing and drive journals.

.249" diameter? I tried some .25" bearings I had lying around and sure enough feels like about .001" of play in there. Where am I going to find .249" bore bb's for these? Seems like an odd non-standard size. Or is it common? I haven't been able to find any bb's with a .249" bore

Come to think of it, the .25" bore shaft couplers will be wrong too as there will be wobble from the .001" difference, no?

[ger21]

Actually, the coupler would only wobble .0005, or half of .001.
Do you really need better than .001 radially for the screw?

[Macht_Tot]

Probably not you're right, but it just doesn't 'seem' right knowing it's not snug. I broke out the 10d & macro tube and took some close ups (images too big to include here):

Gap closeup:
http://www.dirtychrome.com/shared/gapzoom.jpg

The screw itself:
http://www.dirtychrome.com/shared/screwandnut.jpg

And here's a little animated gif of me twisting the bearing on the shaft:


Admitedly some of that is play in the bearing itself but you can see quite a bit of freedom there between the bearing sleave and the shaft.

Doesn't seem like there's any .249" bore bearings out there unless anyone knows of some? Or maybe I'm being too anal here The bearings fit prefectly on the motor shafts, nice and tight zero play between sleave & shaft.

regards,
Todd
NY

[pepo]

Loctite or knurl.

[pepo]

Loctite it or knurl it or thread it and use a jam nut or sleeve them up bigger or turn them down smaller or use a sleeve bearing. If you want to design,well.............design.

[Macht_Tot]

Thanks Pepo those are all great suggestions in lou of .249" bearings which doesn't seem possible. I wonder why these shafts were made .249... must be screws out of OEM stock destined for a custom application.

[RICHARD ZASTROW]

Maybe that's why they were for sale? lol If you have a resource, you could plate them up to size or use the old solder over size & turn down to size trick.

[Macht_Tot]

lol yeah likely isn't it. So looks like turning the shafts down to 6mm would be the next most common bearing size. Glad I made friends with the guy who owns the machinist place across the street from me

[Andre' B]

Are you sure that the bearing ID is 0.250 and not a 6.5mm (0.2559")?

In that pic it looks like a lot more then 0.001 differenc in diameter.

[Macht_Tot]

Got my "bargain" calipers from KBC Tools the other day, bearings are definately .25". The shaft however shows .246"/6.22mm.

Going through my box o' bearings from taken apart printers and the like, I find 23 identical NBK 6mm inner/15mm outer bearings... w00t! These calipers sure are handy... pardon me as I go measure random household items for fun I wonder what the diameter of my cat's tail is....

[dertsap]

Originally Posted by Macht_Tot I wonder what the diameter of my cat's tail is....

i'm sure while your checking the dia of his tail he'll offer his claws to be checked as well

[Mcgyver]

I'd go the next size up in bearing ID bearing turn a sleeve to accurately fit shaft and bearing - turn id & od at one setup.

MAcht, it would surprise me if even the kbc cheapos were out 4 or 5 thou but when it counts use a quality micrometer not a caliper . get a mic and small hold gauges and figure out what you got there or go ask your machinist friend to measure them - if the spec on the shaft is .249 and its .246, send it back. any markings on the bearing?