Linear bearing help please.

[Oldmanandhistoy]

Hi all,

I’m hoping some one could help with a linear bearing I am hoping to use in a Z axis.

I have attached a drawing; the important details are Mpo = 95 Nm, Mro = 108 Nm and Myo = 80 Nm. If other details are needed please let me know.

I have a length of rail and two bearing blocks. I am hoping to be able to mill aluminium but the machine is going to be designed to primary cut timber products. The gantry will be aluminium and the rest will be steel.

My question is; can I use just two bearing blocks for my Z axis or will I need four?

Thanks in advance for any help given it is very much appreciated.

John

[JerryFlyGuy]

It's going to depend greatly on how much load you put through these trucks. If you cut alum w/ a 1/32" end mill and a 1/32 DOC at 1 IPM the loading is going to be alot different than if you do it w/ a 5/8" end mill and a 1/2" DOC at 50IPM. Out of principle I'd say use four, it's a safe guess that it'd be stiffer than two

Jerry

[Oldmanandhistoy]

Thanks Jerry for the advice, I probably should make more effort to work it out for myself but was hoping it may be a simple question for a knowledgeable member with experience of these types of linear bearings and was hoping the spec’s of these would be more than enough for light aluminium work and medium to heavy timber work.

The truth is I needed to know quickly as another rail is up for grabs on Ebay with less than a day to go. I will hopefully win the rail and then do the maths and if I don’t need the extra rail and block will either keep them for another project or put them back on Ebay.

Thanks again for your time,

John

[JerryFlyGuy]

If they are cheap enough I'd just buy them and use them, its good insurance w/ little extra weight/cost, whats the negative to using too many carriages?

Jerry

[Oldmanandhistoy]

Originally Posted by JerryFlyGuy If they are cheap enough I'd just buy them and use them, its good insurance w/ little extra weight/cost, whats the negative to using too many carriages? Jerry

Only negative is the extra cost so will go with four block.

Thanks,
John

[ger21]

In your original question, did you intend on using just one rail with 2 blocks?

[Oldmanandhistoy]

Originally Posted by ger21 In your original question, did you intend on using just one rail with 2 blocks?

Hi ger21,

The idea was to cut a rail into two pieces and use one block per rail.

John

[Oldmanandhistoy]

Please ignore this thread for now; I am working on a drawing that will show the lay out of my possible Z axis. Am hoping with a drawing and as much information as I can give some one could help do the maths as it will be a very helpful tutorial for me to calculate how all the forces react to each other for future designs.

John

[Oldmanandhistoy]

Hi again,

Well I’ve been and done what I should have done in the first place, more information.

I have attached a drawing of the possible lay out of the Z axis showing the blocks that are in question in green.

The spindle is a Kress 950W (1.3 hp) variable speed from 8000-28000 rpm. The cutting area of the machine will be 1000mm (3’ 3 1/4 “) X 500mm (19 ¾”) Y. It will primarily be used for cutting timber products but I would like to cut some aluminium from time to time. With this in mind speed will not be an issue while milling aluminium and light cuts will be fine but I don’t want a job to take for ever.

So let’s say the largest end mill used will be ¼” two flutes. Aluminium grade 6082 T6 (I hope that is a good grade for machine building ) ). Forgetting about the rigidity of the machine for now can anybody give me some idea of the max depths of cut I could take with the limited power of my spindle and at what feed speed? Once I know this how do I work out the cutting force required at A in my drawing?
The next problem I see is how to translate the force at A to the bearing blocks .Due to the lever effect of the distance between point A and the centre of the blocks B in the drawing = 142mm (5 9/16 “) ?
If I can get help with these two points I think I will be ok to work out the rest time will tell ).

Any help with information or helpful links would be very much appreciated.

John

[JerryFlyGuy]

If your concidering cutting alum, then the two blocks is the rout to take.

The Mpo is really what your concerned about, it's a moment force. If you know the distance from the tip of the tool to the center of the truck, and the force that the steppers are delivering to the tool, you can calculate how much load at what moment is being applied, pretty simple really once you know that stuff..

Jerry

[Oldmanandhistoy]

Thanks again Jerry,

I did actually acquire another rail and two more trucks this evening so will be using four in total.

I am now more interested in how all the figures work together. Could you give an example of how to do the calculations please?

John

[Oldmanandhistoy]

Failing an example does anyone know an online source for help with these types of calculations?

John