Deflection in precision rod rails

**irving2008** · 31-08-2008

Just by way of interest for prospective builders there is always discussion about what rod should I use... 12mm or bigger...

Well here is the measured deflection of both 12 and 16mm rod (500mm long in SK type mounts) under 5kg load.

In this experiment the mounts were not clamped up nor were they bolted down so the deflection of the rails themselves will, in reality, be less than this although the MDF base will deflects some as does the MDF 'table'.

2 x 12mm rod = 0.25mm (theory .08mm for rigidly fixed ends to .3mm for unfixed ends)

2 x 16mm rod = 0.1mm (theory .025mm to .1mm as above)

(i.e. if the clamps aren't tight and the supports not bolted down the rod flexes more)

**Lee Roberts** · 01-09-2008

Whats on the dials as read outs mate?

**irving2008** · 01-09-2008

errrmmm... 0.25mm and 0.1mm :)

**Smiler** · 01-09-2008

Originally Posted by Kip

:rofl:

Pretty bendy stuff but average joe probably wouldn't be bothered (or know to be)

Quite. I think it depends on the intended purpose of the machine too. For instance, I'm not going to be bothered about even .25mm deflection when I'm routing a house sign. On the otherhand, I would like good precision if I'm milling a mold for a modelling project. I'd like to see the results of the same experiment done with the ends properly clamped and bolted down to a firmer surface than MDF. I bet a clamped rail bolted down to a steel base would fare better.

Since I'm running 1m unsupported 20mm rails on my machine I may well do just that.

**irving2008** · 01-09-2008

Originally Posted by Kip

:rofl:

Pretty bendy stuff but average joe probably wouldn't be bothered (or know to be)

I'm surprised how rigid that stuff actually is...

12mm probably good enough for a PCB miller on the X-axis supporting the table, with 16mm on the Y axis and 12mm on Z. I think 20mm is prob required for a lightweight ali miller on X and Y with 16mm on Z

Fully supported rail with open bearings is best but harder to work with...

**irving2008** · 01-09-2008

Originally Posted by Kip

It'd be fine for a machinable wax router...cheap as chips too.... How bendy are 30mm supported rails 1.64m long mounted top and bottom on 75mm 4mm wall thickness 356? If you like numbers ;)

not very... fully supported rails are very solid....

**Lee Roberts** · 01-09-2008

Originally Posted by irving2008

12mm probably good enough for a PCB miller on the X-axis supporting the table, with 16mm on the Y axis and 12mm on Z.

I'm about to start work on a mill i plan to use for PCB's and 12mm rail was the choice, i have rail to hand and so i can use other sizes at what point would we not see any deflection, with this info i could work out if the weights i plan to move would be a problem on 12mm rails ;-) keep up the good work !

CM

**irving2008** · 02-09-2008

there will always be deflection.... the question is at what point does the deflection become to small to be of interest.... clearly if you are aiming to mill Ali to .01mm tolerance you need something that wont deflect more than .005 under cutting load.... remembering that deflections combine... movements in gantry, table, etc all add up...

**irving2008** · 28-09-2008

Just to answer the point made previously, now the rails are fitted and bolted up tight with the table attached, the deflection of the centre of the table (16mm rails) under 4.5kg of load is <0.05mm (so that is the combined deflection of the rails, the MDF table and the MDF base the rails are mounted on).

**shulio** · 12-03-2009

I want to build table 1.2x0.8 meter.I will use supported 16mm rails on 1.2m long side.On 0.8 what have to use to have no deflection - supported rails 16mm or 20mm unsupported.