"CShake" <cshake+pov### [at] gmailcom> wrote in message 
news:49b3f716$1@news.povray.org...
> Rarius wrote:
>> I think the problem of "racking" on the X axis is more of a worry. 
>> Although not shown on the image I have decided to add a cable system like 
>> that used on large draftsmans drawing boards to stop any rotational 
>> movement as the carriage moves backwards and forwards along the X axis.
>>
>> Rarius
>
> Yeah, that could be an issue. Since you've got two sliding rails and a 
> third driving rail, your system is kinematically over constrained, so any 
> racking between the two sides would put a lot more frictional load on the 
> drive motor, possibly causing harm. This can be helped by doing what you 
> said with extra cables, or another workaround is using thinner bearings 
> spaced farther apart, instead of a single long sleeve bearing. That way 
> any rotation can move the individual bearing races a little bit out of 
> alignment, instead of putting a huge torque side load on each end of a 
> long bearing.

I would have liked to move the X bearings further apart, but that quickly 
starts to eat into the available work area. If I had been buying the rails 
new I would have bought much longer ones, but as I bought them second hand I 
am stuck with what I have.

> How are you keeping z vertical? If x is the first motor, I'm slightly 
> worried about it bending around y, especially if you have a large load on 
> the cutter - it could bend against the long thin rails.

Just to clarify: In my design, the X and Y  lie in the horizontal plane. X 
is the front/back axis, with the motor on the far right. Y is the side to 
side axis and Z is the up/down axis.

The Z axis is held vertical by the two vertical  rods that drop through two 
pairs of linear bearings. I agree that three would be better but I'll see if 
the two can cope. I can always rebuild later.  It is lifted by the two 
vertical spindles that are both driven by the stepper motor.

The sideways force on the router bit should be reasonably small as I will be 
using small bits and low feedrates.

> Another thing - the timing belt pulleys on the z axis aren't centered on 
> the threaded rods?

Thanks for pointing that out. A typo crept into the script during an edit 
lat last night! Fixed now.

Rarius

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