Accurate Strong Gantry

[JAZZCNC]

I am seriously thinking of the moving bed option now it is just 5-700 mm longer space and can forget many trouble i.e the dual motor, heavier spindle
My goal is a bit better an more accurate machine than a router so called semi industrial so within a 4000 -4500 GBP budget I would like to reach the best performance.

If you have the space then Fixed Gantry is the way to go without any doubt but the principle still applies regards motors and moving mass. There is nothing wrong with regular steppers if sized correctly and if used with quality Digital drives and external motion control card, which you would 100% require for servo's, then they won't lose position and will hold high accurecy without any encoders.

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.

[vargai]

you won't go wrong if you follow this design. (which I'm sure he stole from my Head. )

Bloody dual screw again! Just kidding.
I am really happy to switched on the right track. Thank you.
Just interesting : he says at 6:37-in steel @9000rpm with HSS -can it be real?

[JAZZCNC]

Just interesting : he says at 6:37-in steel @9000rpm with HSS -can it be real?

Yes but didn't mention how long the tool lasted did he.!.

[vargai]

Now i am looking back at your gantry design. What size are the profiles, why 4, why are not connected each other? Whats your idea there?

I will go on with the Fix gantry system because it is properly fit to my goal -the moving bridge is a big advantage and present for me- so I am not restricted with a fix bridge height.
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
-chips
-coolant
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

[Ross77]

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.

[Boyan Silyavski]

Hi Vargai,
so you will go with moving gantry right? The way you have drawn the rails looks ok, but how you plan to straighten them and make them parallel?

[vargai]

Hi Vargai,
so you will go with moving gantry right?

I will go on with the Fix gantry system because it is properly fit to my goal -the moving bridge is a big advantage and present for me- so I am not restricted with a fix bridge height.
However I do not want to cut this line and leave it this way.-probably at a later time I will make gantry with this principle too.

The base frame version just idea for future project where the purpose and load let me use this principle.
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.

[vargai]

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. Especially if you are using a heavy moving gantry.

Yes some corrective action should be done .

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

[Boyan Silyavski]

The Hiwin HGR are double row bearing blocks and there is no difference how you mount them, so don't worry about that.

For a router HGR20 is enough and even a bit overkill, depending on the design. So no worry here also.

[Ross77]

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.