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GeorgeD
14-07-2010, 06:34 AM
99% of CNC machines I've seen have the motor affixed to the top of the Y axis,I've never liked the moto in this position because with the spindle motor also at the front of the Y axis I tend to believe that weight foward forces the front of the Z axis somewhat ie tilt,well thats my reckoning and I'm sticking to it. :tongue:


So my CNC when built,I've decided to place the Y axis motor at the back low down to even out that weight on the Z axis and it will be belt driven.

Here's a crude paintshop pic I have done to show how it will be placed...Substitute the Z in the pic as the Y is wongly placed :redface: if you think its going to be a problem? could the more qualified let me know please. :beer:

irving2008
14-07-2010, 09:02 AM
It'll work, the only issue is that you lose z-clearance and therefore the y-axis has to be higher up. By doing this the spindle/router mechanism has to drop further below the Y-rail to reach the work and therefore you introduce a larger torsion moment from the cutter tip to the y-axis... depending on cutting forces this could be quite large compared to the relatively small torsion moment added by the weight of a stepper on top of the z screw (which in reality is quite close to the y-rail).

Another, lesser, reason to consider is that the belt drive will be more susceptible to damage/clogging by chips thrown up from the cutter... unless you add a shield which therefore increases the static weight of the gantry.

routercnc
14-07-2010, 02:00 PM
Hi George,

Another popular option is to put the motor at the back, as you have done, but at the top driven by a similar belt arrangement. In moment terms, since the weight acts straight down, this is the same as you have drawn and still gives a 'balanced gantry'.

This avoids the problems mentioned by Irving, and still keeps the C of G further back than it would otherwise be, which might be kinder to your side rail bearings, depending on where they are.

I chose this arrangement for my mk2 router, see my post 'time for mk2 upgrade'.

You might also want to consider swapping the Z axis bearings and rails around, with the bearing fixed and the rails on the moving bit, so that there is always a bearing at the lower part of the stroke, giving better support. What you have drawn is the idea that I think most people have at first, and is fine, but the other way has some advantages. Again you can see this idea on my post. By the way, I take no credit for this idea - I read about it somewhere and have borrowed it!

GeorgeD
14-07-2010, 02:44 PM
Another, lesser, reason to consider is that the belt drive will be more susceptible to damage/clogging by chips thrown up from the cutter... unless you add a shield which therefore increases the static weight of the gantry.

How do Irving.

Weight won't be a problem weight till you see what I'm using for the Y axis.


t'll work, the only issue is that you lose z-clearance

Now you've lost me,Irving? I can only think you mean when the gantry is at homing on the Z back?

GeorgeD
14-07-2010, 03:31 PM
Hi RCNC

Whats the weight of that water cooled spindle you have on your machine?

Ross77
14-07-2010, 07:26 PM
Nothing really to add to the comments above and its all explained much better than I could of.:heehee:



t'll work, the only issue is that you lose z-clearance Now you've lost me,Irving? I can only think you mean when the gantry is at homing on the Z back?


He means that the clearance between the bed and the gantry will be reduced because the belt drive is in the way, so to get the same clearance the gantry will have to be raised and also the z axis will have to extend past the belts first so loosing valuable travel.



Weight won't be a problem weight till you see what I'm using for the Y axis


The weight isn't an issue as far as loading is concerned but more with moving it around, acceleration and deceleration.

With regard to the first question I personally wouldn't put the belt arrangement underneath like that purely from a damage point of view,if the machine goes out of control or you miscalculate the settings you could damage the lot, motor and lead screw! trust me it dose happen!:whistling:

I would also urge you to follow RCNC's advice on moving the orientation of the z axis rails and bearings around. a much better design IMO :smile:

routercnc
14-07-2010, 08:43 PM
Hi George,

I weighed it when it arrived, but can't find the value. I think it was about 4kg. Not sure . . .

GeorgeD
14-07-2010, 08:52 PM
Ok so we have agreed it will lose clearence and possible damage?

So what if I was to hire the gantry(Not a lot) and then protect the moving parts with a sheet of perspex ie enclose the belt and pullys.

I really want to do it this way but if you think the above mentioned will be a problem then ok I'll reverse the idea atop.

Ross77
15-07-2010, 02:08 AM
It sounds like you have your heart set on doing it this way so you have to at least try it and prove us wrong.........:rofl:

Tried and tested is the best route, I keep thinking of ways to beat the system but ultimately I always revert to linear rails, ball screws and servo motors :smile:


So what if I was to hire the gantry(Not a lot) and then protect the moving parts with a sheet of perspex ie enclose the belt and pullys.

I don't know of many people that want to "HIRE" a gantry but you defiantly wont protect them with perspex, only swarf ingress.:naughty:

GeorgeD
15-07-2010, 04:04 AM
Oh well,I'll get this one out the way and then try the London Bridge type gantry. :whistling:lol

Anyway its only a desktop machine nothing too elaborate at the moment because I need to get my head round the software and Cam prog before I go wrecking the expensive stuff.:eek: