[pikolo]

There are several variables to your questions.

What Software are you using?
What motor drive are you using?
What is the runout of your spindle/router?
Are the parts uniform? i.e. is the 49.4 x 49.7 square and uniform, or does it vary and the part ends up trapezoidal shaped?
Is the error repeatable or does error vary?

True, it would seem that a non-concentric pulley would seem to introduce a variance in position, but the variance should be repeatable over the range of movement.
Many possibilities exist.

-=Doug

Hello i would like to asj somethibg else...the first problen was solved it was a faulty stretched belt.But i stikl need more accuracy tha 0.2 mm... i am thinking of changing the belts with this kind of lead screw http://m.ebay.co.uk/itm/3D-Printer-8mm-Lead-Screw-Rod-300mm-4-Start-Z-Axis-30cm-Linear-Rail-Bar-Shaft-/191382736803?nav=SEARCH&varId=490496660274 and an antibackshlash nut. Whats your opinion on this?thanks!if others can help please do!

[pikolo]

ah ok !So i will stick with the ball screws and see what happens....

[A_Camera]

Hello i would like to asj somethibg else...the first problen was solved it was a faulty stretched belt.But i stikl need more accuracy tha 0.2 mm... i am thinking of changing the belts with this kind of lead screw 3D Printer 8mm Lead Screw Rod 300mm - 4 Start Z Axis 30cm Linear Rail Bar Shaft | eBay and an antibackshlash nut. Whats your opinion on this?thanks!if others can help please do!

You will not be happy with that. Get a proper ball screw and you will never look back.

[Mac.CNC]

Hi!
I tend to disagree with the ball screw recommendation, because of the experience.
There's nothing wrong with the original Momus design, using properly tensioned cevlar reinvorced belts on the x and y - the beauty is that there can't be any backlash.
The 2nd X stepper motor is a must to make this work, which isn't in the original manual, unfortunately...

With that mod, the long version that I've been running for a few years now has no problems holding 0.001" precision on the X and Y.
The machine I've built back then should be pretty worn out by now, but it isn't - and I've done nothing special to it, except for keeping the rails clean - using WD-40.
So far, several hundred board foot of poplar boards were profiled into smaller parts without any adjustments and it still holds up to cut engravings into jewelry without any issues..

Long story short, stick to the belts - it really works.
--
Mac

[A_Camera]

Hi!
I tend to disagree with the ball screw recommendation, because of the experience.
There's nothing wrong with the original Momus design, using properly tensioned cevlar reinvorced belts on the x and y - the beauty is that there can't be any backlash.
The 2nd X stepper motor is a must to make this work, which isn't in the original manual, unfortunately...

With that mod, the long version that I've been running for a few years now has no problems holding 0.001" precision on the X and Y.
The machine I've built back then should be pretty worn out by now, but it isn't - and I've done nothing special to it, except for keeping the rails clean - using WD-40.
So far, several hundred board foot of poplar boards were profiled into smaller parts without any adjustments and it still holds up to cut engravings into jewelry without any issues..

Long story short, stick to the belts - it really works.
--
Mac

0.001" = 0.0254mm

Assuming it is per meter... that is good for wood or engraving, but with ball screws you can get better than that.

Another thing... don't assume that there is no back lash, unless you have especially made your pulleys for your belts. I think cheap Chinese pulleys have always backlash and I think if you have a long belt you will always have tension caused backlash unless your belt is very very strong. The greatest advantage of belt drive is that it is quiet and can be very fast. Also that it needs no lubrication and I don't think it needs that much care at all. Ball screws must be protected, regularly greased and cleaned and makes more noise, but no doubt that they are more accurate, unless you are using really high quality things. Never the less, if the goal is 0.2mm or better accuracy than a belt drive may also be just as good. 0.2mm is a quite low set requirement.

[pikolo]

So after a while i have thought of a different design for the momus...I want to buy cnc rails supported 1200 mm or maybe 1800 and also move tha gantry with one leadscrew ...i dont know if this design is better and that is something that maybe you can help me with..I want it to be better in accuracy and more ridgit ....please tell me your opinion on thisi ope you can understand what i want to do by looking at the pictures...tha plan is not yet completed but you can get an idea..Does this change woth the trouble making it >?

[louieatienza]

So after a while i have thought of a different design for the momus...I want to buy cnc rails supported 1200 mm or maybe 1800 and also move tha gantry with one leadscrew ...i dont know if this design is better and that is something that maybe you can help me with..I want it to be better in accuracy and more ridgit ....please tell me your opinion on thisi ope you can understand what i want to do by looking at the pictures...tha plan is not yet completed but you can get an idea..Does this change woth the trouble making it >?

At this point it is no longer a Momus design... The Momus gets its rigidity from the side walls also being torsion boxes, and you lose the benefit of that by going with the gantry legs as drawn. Especially if your intent is making guitar parts. Also those linear bearings are probably not as good at preventing the gantry from racking, or rotating... I think it's better to utilize the portal design of the Momus but then drive two leadscrews via timing belt and pulleys.

[pikolo]

so the gantry going out of tune isnt a problem you say,,,i really am great full for your help i will have to think alot about it !

[plcamp]

so the gantry going out of tune isnt a problem you say,,,i really am great full for your help i will have to think alot about it !

I know the decision process at the beginning of a project can sometimes be overwhelming, however at the end of the day the decision is yours as to the construction method.

You simply have to consider the information that is available, make an executive decision and move forward.

[louieatienza]

After going through some pretty cheap steppers, I can say this is one area that leaves room for improvement with a little extra outlay in investment. Check out the steppers on the GeckoDrive website, made by Lin Engineering, and are high quality with better finish, wire and insulation and better bearings. Check eBay for Parker, Kollmorgen, Portescap, Vexta, Sanyo Denki, even Leadshine - all superior to some of the stuff out there now. Even brand new, Sanyo Denki and Vexta motors aren't that much more.

[pikolo]

I see but what happens if a stepper misses a step...? itruely dont know which is the better option in the end?

[plcamp]

I see but what happens if a stepper misses a step...? itruely dont know which is the better option in the end?

If the gantry remains free missed steps shouldn't be a problem. I have never encountered this problem.