"Rarius" <rar### [at] rariuscouk> wrote:
> Here is a plain text post with a updated image of my CNC design.
>
> Rarius

Hey, someone stole the drilling tool!

Attachment worked fine this time, thanks.

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"Rarius" <rar### [at] rariuscouk> wrote:

>
> The drive spindles are 12mm trapezoidal with a 3mm pitch
> running in bronze nuts. The rails are 12mm, 16mm and 20mm ground shafts
> running through linear bearings.

Some nice components! Especially the linear bearings with the recirculating
balls. I'm not familiar with 'trapezoidal' threads; from the photo at the
website, they look kind of like 'Acme' threads here in the 'States.
>
> I bought a job lot from eBay. It included four spindles, three pairs of



You lucky guy!! I'm jealous.

Ken W.

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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.

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.

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

-cshake

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"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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Cousin Ricky nous illumina en ce 2009-03-08 00:24 -->

> I love large corporations that practice what they preach.  Good thing 
> that Web browsers are maintaining, uh... (subtracts in head), 19 years 
> of backwards compatibility, and keep tabs on each other so that, say, 
> Opera knows how MSHTML formats its attachments.  Alas, povray.org's 
> software doesn't.  (Just continue using plain text and avoid the whole 
> mess.)
> 
> 
MSHTML makes me think of MicroSoftHTML, or a microsoft propriatary extention 
over the HTML specification.
Well, not even compatible with IE7 here...

-- 
Alain
-------------------------------------------------
If you're ever about to be mugged by a couple of clowns, don't hesitate - go for 
the juggler.

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