[pikolo]

Hello friends!I am in need of your help if this is possible.I have built a momus cnc that want to use it for luthiery projects and general woodworking. I have some accuracy problems.I am trying to cut a 50x50 mm square and it cuts 49.4 x 49.7,The steps per unit if changes will cut it correct but then on a bigger item they will cut it oversize....What can i do to improve the accuracy(the repeatability is good). the pulleys i have are offcentered is this causing the problem?I cant find pulleys with 8 mm bore xl and also 10 teeth ...the ones i used where opened in a lathe from a 6 mm bore to 8 mm but are 0.2-0.4 offcentered.....Also the cables tha i use for the motors ares simple cables not ground ones....any thoughts?Maybe if i use another type of belts and pulleys?

[vtxstar]

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

[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]

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?

[awerby]

Looking at the design of that machine: momus CNC | benchtop DIY router plans | home page it's not surprising that it was not very accurate. The idea of using a long timing belt instead of a leadscrew isn't that great for CNC cutting machines, although it seems to be a popular way of driving 3D printers. But printers don't have to contend with cutting forces. Replacing that system with leadscrews should help, although it's hard to say how much, since no specifications about the accuracy of those screws were included in that ad.

[pikolo]

Looking at the design of that machine: momus CNC | benchtop DIY router plans | home page it's not surprising that it was not very accurate. The idea of using a long timing belt instead of a leadscrew isn't that great for CNC cutting machines, although it seems to be a popular way of driving 3D printers. But printers don't have to contend with cutting forces. Replacing that system with leadscrews should help, although it's hard to say how much, since no specifications about the accuracy of those screws were included in that ad.

I use one of these screws in the z axis i can say it is good quality and steel.the thing that i dont know for sure is if it is good diameter for cnc...ot has a lead of 8 mm per turn but it is 8mm thick.will it be ok for a cnc like this?or i need biggwe in diameter?i will also use diy antibackshlash nuts.Another question i would like to ask is if i can run a 4th stepper with the same dm controller..?or i will need to purchase another controller for the same axis?i want to put another moter on the other side of y axis(now it is driven by one)

[louieatienza]

You can find a precision ground ACME screw or trapezoidal screw that is just as accurate as a ballscrew. The problem is they can almost cost as much if you buy new, and if you use a fine pitch you will be limited in speed.

The effective lead of any screw is the pitch times the number of leads. Yes even ballscrews can have multiple leads. A typical rolled ballscrew has an accuracy grade of C7 which is about 50 microns or about .002" over 300mm/1ft. But they will also have a typical backlash of about the same amount because of the clearance needed to overcome any variation in pitch that would cause the scrw to otherwise bind. The better solution for rolled ballscrews would be a double nut that is preloaded. A C5 precision ground ballscrew is half that over the same length, and a C3 half that of a C5. Also because of the more precise process, the ballnuts of precision ground ballscrews can use other ways to achieve anti-backlash, like skewing the ball races to achieve a preload. They also make a double nut for higher load capacities. Note that anything below C3 is pretty rare to find, and will cost way more.

[awerby]

8mm seems a bit thin for a leadscrew. I'd suggest 12mm at minimum, and thicker would be better if they are longer than about 300mm. Are your anti-backlash nuts springloaded? Just be aware that if the spring force is exceeded, then you get all that inaccuracy back and more. To run dual motors, you'd need an extra driver, but not necessarily another controller (never heard of the dm, sorry.) Usually you'd use the extra motor on the longest axis, which by convention is called "X".

[pikolo]

thanks awerby!The problem is that i cant find a 4 start lead screw with 12 mm diameter in Greece or on ebay....the ones i find are one start and have 2 mm lead the most i think...the nut is spring loaded delrin nut (two nuts jointed together and forced apart with a spring) i think when the spring force go away i will build another one ....i think this should hold for a year maybe?At this pont the machine is on +- 0.2 mm accuracy but i want to use it in building musical instruments and accuracy is needed...if you have any more helpful advice please tell me!

[awerby]

I wasn't talking about the spring losing its force over time, but about it being overpowered by the resistance of the material you're cutting. If you have a spring with 1000 grams of power to resist compression but it runs into a cutting force of 1250 grams in the other direction, what do you think is going to happen?

You don't need a 4-start screw, you just need a relatively fast one. But try to find one that specifies how accurate it is - cheap screws can be shockingly imprecise.

[pikolo]

I wasn't talking about the spring losing its force over time, but about it being overpowered by the resistance of the material you're cutting. If you have a spring with 1000 grams of power to resist compression but it runs into a cutting force of 1250 grams in the other direction, what do you think is going to happen?

You don't need a 4-start screw, you just need a relatively fast one. But try to find one that specifies how accurate it is - cheap screws can be shockingly imprecise.

Ohhh thanks alot!Now i see what you meant!I only have access on ebay to buy the screw...maybe a china ball screw is better?

[pikolo]

My friends thanks a lot for your answers! My mommus cnc currently has an accuracy of + - 0.2mm. I want to use it for luthiery work and this accuracy is not very good! In advance i want i to cut a bigger surface so as to curve neck for guitars. At this momment i am thinking of a possible way to drive the gantry with only one motor but from the center (in some machines i have seen it ) i dont know buti have to ask is there any good modification to momus that will not need alot of extra work and also allow me to attach lead screws ?or ballscrews!~?

[louieatienza]

My friends thanks a lot for your answers! My mommus cnc currently has an accuracy of + - 0.2mm. I want to use it for luthiery work and this accuracy is not very good! In advance i want i to cut a bigger surface so as to curve neck for guitars. At this momment i am thinking of a possible way to drive the gantry with only one motor but from the center (in some machines i have seen it ) i dont know buti have to ask is there any good modification to momus that will not need alot of extra work and also allow me to attach lead screws ?or ballscrews!~?

I would say if you want to carve necks, then there are some cutting forces to contend with where a screw can have an advantage. I think which ever way you go, it will need work. Luckily for you, YOU HAVE A CNC of which you can cut parts with! You have to design you parts within the accuracy of your machine but it's not out of the realm of possibility. And likely you're going to have to do the leg work.

If you use ballscrews, my biggest concern would be, being the machine is enclosed, ballnut contamination with dust. You'd have to ask for matching sweeps, but they don't really work as well. They make bellows for the screws but they take away some travel. Maintenance, mainly preventive, is key.

As far as tolerances. The instructions quote a tolerance of about .005", which is due partially to the mechanical resolution of the drive system, but also the components and materials. You have to ask yourself if investing in screws with more than double the suggested tolerance than the other parts in the machine are worth it? Likely you won't see much advantage unless you also upgrade to linear rails. Also remember the fixed and free end bearing blocks needed, some form of alignment and shock absorbing couplers for the motors with the requisite proper motor mounts... Your Momus will no longer be. You may want to investigate getting higher quality pulleys and belts with a more accurate tooth profile designed for more precise automation, like GT3...

ACME screws can have a tolerance from .004"-.009"/ft. It may sound like a lot but in practice I found the tolerances are not that bad. Delrin anti-backlash leadnuts can be had rather inexpensively from vendors like DumpsterCNC, the quality is very good, and they last a long time. I've had the same nuts on my machine for over 6 years, with no appreciable signs of wear or lash. You can "tweak" the settings in Mach3 for more accurate results, but I find them fine as-is. Buy them from a reputable source; they have to be precision ACME screw. A 2-start, 4tpi (as in turns per in) would be a good choice and balance of speed and resolution (1/2"-8, 2 start or 1/2"-8, .250" lead is what it would be called out as from a supplier.) You'll have lower rapids, but better torque and resolution at cutting speeds.

[pikolo]

So i have ordered ballscrews and now am thinking about the rails....shouls i just buy these CNC Set 20x 800mm 4x Linearwagen Linearführung Linear Guide Rail 3D Fräse | eBay and place them where the momus cnc had its rails or just find a way to make the cold rolled steel straight >? at this time they have a 0.35 mm bow in the 800 mm distance (tried it with a string tensioned and feeler gauges go under it )