Z axis linear bearings for MDF router

[martinw]

This may be in the wrong thread. If so apologises.

Plenty of MDF framed routers use pipe and skateboard bearings for the Z axis travel. This seems to work well.

Very few of the MDF framed constructors use linear bearings like these for their Z axis..
http://www.vxb.com/page/bearings/PRO...Systems/kit995

Now, I can see that dust might be a problem without some means of preventing it getting in, unlike the skateboard way which is "self cleaning", but, apart from that, is there a reason for not using them?

I'm curious, and any opinions are very welcome,

Best wishes,

Martin

[ger21]

Alignment can be tricky, but I used them on mine, and although I haven't cut anything yet, they seem to work great.
http://www.cnczone.com/forums/attach...3&d=1136858794

http://www.cnczone.com/forums/attach...4&d=1136859518

[Oldmanandhistoy]

Hi Martin,

I used them on a plywood machine for over a year and had no problems. The dust was taken care of by the wipers and I did not use lubricant. The only reason I stopped using them was because I built a bigger machine and used linear rails and blocks for more rigidity. To align them accurately I made a ply template and a hand router.

John

[martinw]

Dear Gerry.

I think your craftmenship is in a completely different league compared to mine!

I'm talking wonky drill-press, "bodge it with filler" stuff here. Maybe a bit better on a good day....

BTW, that is a great looking machine.

Thanks for the advice,


Best wishes,

Martin

[martinw]

Originally Posted by Oldmanandhistoy Hi Martin, I used them on a plywood machine for over a year and had no problems. The dust was taken care of by the wipers and I did not use lubricant. The only reason I stopped using them was because I built a bigger machine and used linear rails and blocks for more rigidity. To align them accurately I made a ply template and a hand router. John

Dear John,

Thank-you for that.

I'm glad that I'm not alone in thinking of trying it on a timber/MDF gizmo.

Best wishes,

Martin

[ger21]

Originally Posted by martinw Dear Gerry. I think your craftmenship is in a completely different league compared to mine! I'm talking wonky drill-press, "bodge it with filler" stuff here. Maybe a bit better on a good day....

All the holes for the rails and bearings were drilled on a standard drill press. Double face tape the parts together and use a fence to keep everything aligned.

[martinw]

Dear Gerry,

Thanks for that. Could you tell me what the shaft spacing is? It looks about 6 1/2".

Best wishes

Martin

[ger21]

Originally Posted by martinw Dear Gerry, Thanks for that. Could you tell me what the shaft spacing is? It looks about 6 1/2". Best wishes Martin

I can check tonight, but 6 1/2 is pretty close. If I recall correctly, the plates on top are about 8" wide.

[anoel]

I used them for all of the axis on my cnc router...

I would not recommend unsupported bearings on any axis greater than 12" And really a Z axis is ok because the load is usually static or within an very restricted amount of movenment.

So for the Z I give them a thumbs up... X&Y greater than 12" a big thumbs down unless you went with grossly large rails...

[martinw]

Originally Posted by anoel I used them for all of the axis on my cnc router... I would not recommend unsupported bearings on any axis greater than 12" And really a Z axis is ok because the load is usually static or within an very restricted amount of movenment. So for the Z I give them a thumbs up... X&Y greater than 12" a big thumbs down unless you went with grossly large rails...

Dear Nathan,

I was only thinking about using 20mm dia. shafts and linear bearings for the Z axis. The shafts might be about 15 inches long between their top and bottom mounting plates. The sliding carriage that would take the spindle might be about 8-10 inches tall , and the linear bearings themselves would, to a certain extent, reduce the tendency of the shafts to bend. I have not done the deflection calculations yet, but my guess is that the two shafts would not deflect by very much. I will check.

Best wishes

Martin

[martinw]

Dear Nathan,

Thanks.

Sorry for asking people more questions..

If you put a single 20mm dia. shaft, 15" long, on its side and put a 45 lb load at mid span, I think the deflection of the shaft would be about 0.3 thou inches. I'm not sure why I chose 45lb.

There are two shafts, and the spindle slide to some extent will reduce deflection because the linear bearings help keep the shaft straight. My guess is that the flexure of the MDF to which the shafts and spindle slide are mounted may be the weaker components.

I'm scratching my head about the actual forces involved. Common sense would suggest that the shafts do not have to take any great vertical loads as a column, but the bending loads will depend on cutter forces, and also the lever arm distance from the spindle to the shafts. Obviously it is a very good idea to keep the spindle to shaft distance as short as possible.

Has anybody got any idea what lateral cutter forces might be for non-aggressive cuts in MDF for a small Joe 2006 type machine? OK.. not a very well framed question... (and probably impossible to answer).

Best wishes

Martin

[dertsap]

with 20 mm shaft i highly doubt you have anything to worry about ,
for the z on my last setup i had 1/2 ss stock and brass bushings that i made and it worked like a charm