linear shaft specs?

Are the round shafts typically undersize? I need to get rid of some slop in my X axis carriage and the shaft I am using measures 7.91mm with a IM8UU bearing.

No. If you have 8mm linear shaft they should be as close to 8mm as possible. Standard tolerancefor commercial rod look to be within .02mm, so .09 is quite a bit out of spec. Now if you got some random linear shaft from ebay or China, or China on ebay then your tolerance could be anything.

My guess is though, since it is so far out of normally expected tolerance, that you don't have 8mm linear shafts. You probably have 5/16" linear shaft which would be 7.9375mm. That would be almost within the .02 mm tolerance if you're generous with rounding/decimal point precision.

If you'd be a first time customer, you can get $150 of product free (just pay s/h) from Misumi until the 31st of this month. Might be a chance for you to upgrade your linear shaft to something better for cheap.

Thanks for the Misumi tip. I have been wanting to purchase a ball spline for my new machine design but have been choking on the price. With that discount it looks like I'm in for a cost of 75 bucks less shipping. That I can deal with.
The rods are on a Robo 3D printer I purchased and your right. It looks like they are 5/16". I picked up some chromed 8mm's from a reputable seller on Ebay. That should be the last tweak I need to get this machine up to par.

Glad to hear you going to build your own bill I'm sure it'll be much better than Robo printer you purchased. When you first started posting said you didn't have the time but I'm sure the results of the Robo aren't that good. I'm sure you could have built something real quick with aluminum extrusions and fully supported linear rails nothing fancy and not lot of design work other than a few dimensions and pencil and paper pretty fast. But like I said I'm glad you're delving into it and going to try to drive from a ball spline. But if you ever do want two heads that's going to pose a problem unless you have two ball splines. You can make heavy things move fast but the frame has to be strong enough to do it obviously you own CNC machines so you know this

Edited 1 time(s). Last edit at 03/28/2014 02:04PM by cnc dick.

Thanks for the feed back CNC. As you may recall the Robo was purchased so I could get my feet wet before starting from scratch. I knew it would need some work (maybe not this much) but it looks like after adding some reinforcements here and there and a complete alignment along with the correct shafts, quality, metric threaded rod, and an E3D it should print well. I still think I will be ahead, time wise by picking the Robo. I have a disease unfortuntaly and that is I cant just put something simple together. I always want to make every aspect the best it can be. I say that with no level of pride. Its really a curse.
Once I get all my scribbles down in CAD I will send you a pic of the design for my machine. Basically X and Y are above. The bed only moves in Z. the ball spline drives the X axis and all that will be moving is the extruder (or spindle) mounted on a linear rail bearing so not much weight. I know extrusion technology will have to catch up to my goal which is high speed printing but I am trying to prepare.
Currently I am building a fixture (attachment) to test various steppers, pulley diameters/pitches and different masses with data acquisition.The diameter of the ball spline bearing forces the use of a 20 groove GT2 or larger. I'm hoping to find that a good balance between speed and accuracy/precision can be had with the 20 groove or bigger. Why is it that some designs use very large pulleys (36t)? I don't understand that
Edit; I attached a quick wire frame sketch to give you an idea