Need Help!- Linear Shaft - 20mm 1200mm long - Much flex?

[guerd87]

Hello everyone.

Im just working out to buy some parts for my first CNC build. Its going to be a small fixed gantry design with an enclosure, using torsion boxes, similar to yukonho's build here:

http://www.cnczone.com/forums/showth...p?t=3059&pp=10

I have found a place locally that sells linear rails and fully supported rails. I would really like to buy fully supported rails but the cost of putting them through the whole machine is alot.

Im looking at these linear rails:

http://cgi.ebay.com.au/1200mm-long-2...item439d0b4473

with these flange bearings to go into the bed:

http://cgi.ebay.com.au/LMF20UU-FLANG...item439d518f83

Once these are mounted up will i see a whole lot of flex/movement from these bars over the full length? My machine needs to be accurate down to around .5mm up to 1mm max

Am i better to go and buy the supported rails for the X axis and then run just with this type of setup along my Y and Z?

I was thinking that 20mm rails wont have much flex over only 1200mm length, but i may be wrong

Any help is appreciated, looking to buy next week

cheers
John

[Jesse B]

What do you plan on cutting? I'd opt for the supported rails; the 20mm rails will most likely have substantial flex over a length of 4'.

[guerd87]

plan on cutting plywood, all variants of MDF upto 19mm, Pine and was hoping for light alloy finishing of cast parts until i get a chance to buy my mill late this year. I if I can get away with cutting the timber on the mill not using fully supported all through then I will just wait for my mill to do the alloy, so think that alloy is not necessary

I was trying to avoid skate bearings and right angle alloy all together, but I cant afford to use fixed on all the axis

Maybe:
Fully supported for the moving table (X axis?)
Fully supported Gantry movement (Y axis?)
Being that the Z axis will only be about 6" travel max then unsupported should be fine?

Since im going fully supported on X and Y could i possibly reduce the cost by using 16mm Fully supported rails instead of the 20mm? They are a bit cheaper for the rails and the bearings.

John

Edit: Just done all the calculations using 16mm fully fixed rails for X and Y and 20mm unsupported for Z and it came to $403.10 for all axis rails + bearings, this is a little more then i want to spend since i still have to budget in the $300 for Hobby CNC and the router, MDF etc

Whats more critical for fixed rails in a fixed gantry design? The table or the gantry slide? I might just have to use unsupported rails and support them through torsion box and use the skate bearing designs on at least 2 of the 3 axis

Hopefully this will not limit my machines capabilities too much though

[Phife]

Waaaay to long.. I have 30mm rods on my 5' ish X axis and it flexes too much for my liking, Have to limit speed because the machine bounces everywhere..

I have 20mm on my 2' Y and it flexs too. Not as much but not ridgid enough for my tastes.

[marchantdice]

As a manufacturer of CNC machines, (worldofcnc.com) the maximum length of diameter 20mm unsupported rail is 800mm, diameter 30mm will also flex at this length...
For machines requiring longer stroke we change our design, using Hiwin profile rails and carriages

[hemsworthlad]

Cant you use a open type bearing and support the rail your self by making some upstands and drill n tap the rail using your mill, over 1200mm say at 300mm cntr's only 3 per rail.?

(Opp's didnt read your 1st post properly, no mill yet!! Same applies just harder to do. Lol)