Thread: Accurate Strong Gantry
Regards Hybrid easy Servo's then I'm sat on the fence because while I know they work excellent I agree they are too expensive and Servo's can be bought for not much more money.! . . . BUT . . .And for some it's a very Big BUT.!! Servo's require more knowledge of setting up to get the best from them and can be nightmare to tune correctly, espicially if other electronics are not done correctly and noise is introduced has they are much less tolerant of noise in the system. (It's not uncommon for some one to turn on a dodgy radio or some noisey appliance and Servos take off at full speed.)
I have seen people, experienced people, build machines or retro fit Mills with servo's only to change them at a later date with steppers because they can't get them running correctly or reliably. . . . Reliably being the key word.!! . . . . Now I'm not saying don't use servo's but I just mean be aware that they are not simple and can turnout to be more trouble than there worth for the inexperienced.
Hybrid easy servo's don't have this issue as they are essentially steppers with encoders for checking/correcting position.
Regards the machine I Try to always keep in mind the three most important goals of a CNC machine which in my opinion is accuracy, repeatabilty and finish quality.
Accuracy and repeatabilty comes from component quality and suitabilty along with machine ridgidity and attention to build.
Finish quality comes from some of the above but mostly from Ridgidity and machine design and while all three are important aspects the quality of finish is what I'm always aiming to improve. Achieving the best best finish quality is mostly about lessening tool deflection and chip clearence.
So if you can keep the tool stickout distance from spindle to a minimum and have the shortest Z axis extension you'll get the best possible result and it's here I'd look to improve machine design and in my opinion you won't go wrong if you follow this design. (which I'm sure he stole from my Head. ) Just make a better, stronger Job and put proper spindle on it.
Last edited by JAZZCNC; 03-12-2014 at 11:15 AM.
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However I do not want to cut this line and leave it this way.-probably I will make gantry with this principle too.
Some draft are attached with the gantry base solution.
I prefer to save the expensive, vulnerable parts
balls crew rail , belt drive so I tried to hide them next, behind or under the frame -I belive this is the best place for them against the impact of
-work piece uploading
The other view is to apply the roof trusses principle where incredible small profile in a certain arrangement can form a very strong element.
In my dwg the three SQtube run along he perimeter tying to each other with reinforcement plates.
It is a pity I cannot check and compare the different version with software so only suppose it gives big torsion resistance to the frame
Last edited by vargai; 06-12-2014 at 11:16 PM.
Looks good but you might need a bit more triangulation. The thin gussets only work in one direction so ideally you need them in equal amounts in all 3 planes.
Why is there a bit gap in the middle?
What rails are you using? There are different types and some can handle 4 way loading and others cant so are best mounted on top of the rail. Espesially if you are using a heavy moving gantry.
Yes this is weak point of this design to achieve the straightness and parallelism
- correct more expensive method to machine it - can be done in one step w/o flipping the frame on the machine bed (drilling by DIY)
- DIY method -I am sure you will invent it
By the way I am wonder about the cost of the
epoxy system comparing to the machining.
If you have access to earn money in mean time with other job I think machining cost can compete with DIY cost Sum.
Last edited by vargai; 07-12-2014 at 02:23 PM.
The gap in the middle just represents the place of the single ball screw in case of narrower machine
regarding the rail I would use one like this which is regular type now
The problem is as you pointed to it at my case only the upper row of ball hold the load but the moment resistance is good that occurs at start-stop and direction change
I have no experience using it but suppose it works -the catalogue also mention it as possible mounting arrangement
Because of my 100kg+ heavy moving gantry I have drooped this design and go with fix gantry where the moving Z will be balanced
Last edited by vargai; 07-12-2014 at 03:25 PM.
Ok, I'm a bit confused. You keep saying fixed gantry but then you show a bed with rails on the side.
Is this the bit that will be moving? If so I would have said that it is not the best way to do it. Moving beds should have the rails underneath and at approx. third points or a quater in.
That way the rails are supporting the load better and are closer together so you can get away with one ballscrew.
As you have drawn it the bed can bend in the middle under load.
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