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Hello to amateur luthiers in the forum!I am amateur luthier too and built a momus cnc so as to accompany me with the building.So it is a belt driven in y and x axis machine and a screw in z axis. The reason i am asking your help is that i have a problem with the accuracy of the machine... i managed to find that the steps per unit are 31.94888178913738 for 50.08 mm of movement in one revolution...i tried to cut a piece 50x50 mm and in the xaxis it cut 47.8mm and in the y axis 49.5mm so after that i tried to set it via mach3 steps per unit option.....i did this and i tried again...the result was to cut almost perfect 50x50mm square. After this i tried to cut a longer piece 24.7 mm and in the x axis it was off by 2 mm....i know that the pulleys i use are off centered cause i drilled the center hole with a drill press!(from 6 mm to 8)is this the problem? please reply me if you have any ideas?

I built a Momus based machine, only I used 8020 extrusions and the trucks from cncrouterparts for the rails and carriages. It is a great system, and the solid sides are much better than the flying gantry design I used for the first machine I built. Getting the settings right for belts, however, is quite a bit tougher than for the screws.

That having been said, once I had the settings dialed in I still would get seemingly random errors. Almost always I would discover some small place where the trucks were scraping or bumping somewhere causing lost steps. I also drive my gantry with two pulleys, one on each side. I can't remember if that was in the Momus design. If not, you should consider adding it. Racking can likewise cause some binding of the motion. It can be very subtle, but enough to miss a step.

A good test is to zero the machine to the home switches, zero your DRO's, move away to the farthest corner, and then home again. Your DRO's should still read zero. If they don't, you are losing steps somewhere.

dear rlrhett i think i am not loosing steps cause every time i run the same file it produces the same result again and again...i think that if i was losing steps it would produce a random result...i just tried to see what it iswrong and put a dial indicator show me what it was missing in certain movements..on y the movement waspretty accurate 0.01 -0.03 mm but in the x axis when themovement was back to front it was accurate but front to back it was loosing 0.3 mm...also the belt has backshlash and i dont jknow why no matter how much i stretch it...any thoughts?

So i set the steps per unit as what they should be ...then i cut a square 50mm x50 mm and in the x axis it was 50.05 and in the y axis 50.3....i guess i will have to live with it cause if i change this and try to calculate the steps via mach3 the machine may cut correctly the 50 x50 mm square but it will lose in bigger items...

Dear friends i still cant understand what is causing the problems maybe the off centered pulley?when i bought the pulleys they had a 6 mm bore and i had to open the hole in a lathe but after that they are off centered ..any ideas?

These problems are hard to diagnose. The fact that the x-axis is accurate in one direction and then off when it returns sounds like a physical problem like binding of the gantry or backlash. More, I couldn't tell you.

As best as I can tell there are few people on this board with home made CNC machines other than me, so I doubt any body here can offer more advice than you have already gotten. We don't know where you sourced your belts, what kind of pulleys you got, how you adjusted them, how are your driving them, etc. I could be wrong, but I think most people on this board with CNC machines don't use belts, did not design or build their own system, and did not source individual parts from different vendors. I am not sure that there is a lot of depth of experience here on solving those kind of problems. The truth is that you will get much better response on the Momus forum at Cnczone.

It's almost certainly the pulley.

Imagine the hole was way way off center - every full revolution of the pulley would result in the correct amount of travel but there would be a constant speed up and slow down of the velocity as the pulley rotated. During roughly half of the rotation, the movement per step would be more than expected and during the other half, less than expected. In one full revolution everything would be fine but, there would be a cyclic error during the revolution.

At least in my head, that's how it's working out but, since it's a toothed belt probably it's possible that I've got it wrong. The angular distance per rev will be correct no matter where the hole is drilled.

Do this to prove out this theory:

Take your steps/rev and divide by four.
Set up an indicator on an axis.
Make a program that moves the axis 1/4 rev per move.
Measure the actual distance traveled per 1/4 rev.
Compare.

If the off center pulley hole theory is correct, you should get the correct distance for one rev but the 1/4 rev results will not be exactly 1/4 the distance per rev.

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It was the pulley!!!!!!Andy Birko you were correct!!!!i changed the pulley with another one i had with a modification for the bore screw and it cut 49.6x49.6 the two pulleys were drilled opened in thesame lathe with tiny offcenter value....the one i had drilled in the drill press had an ofcenter of maybe 2 mm!!these that i have know have may be 0.2 mm i configured also the belt backshlash in the mach3 0.045mm in x and y too...Know the machine is muck more accurate!i would like to have a pulley 10teeth and 5.08mm pitch but with a 8mm bore that it is made like this from the factory so as to eliminate even the 0.2mm d oyou know any source?Anyway Andy Birko you are a life savior !!!