Problem- Do I need mechancial synchronizing dual belt drive?

[kiwibum]

I'm building a CNC router with belt drive x & y axis, looking at using dual closed loop belts and stepper motors, one on each side to drive the gantry along the x axis (normally longer axis but will be the shorter one on this machine). I already have the linear slide rails for X (18" THK SR20), the Y (26" IKO WLW15) axis and two 170 in-oz steppers with encoders to drive the gantry along the X axis. The steppers will move the gantry by reduction belt drives to the main closed loop belts to allow the use of the smaller motors and changing drive ratios. Both steppers will be driven via gecko drives (the new G450) and slaved via Mach3. Currently looking at using GT2 5mm pitch PowerGrip 15mm wide belts and pulleys.

My question is:
Do I need a mechanical means to ensure synchronization of the dual belt drive so there is no skewing of the gantry due to lost/missed steps?

I see some designs are doing this. With the belts orientated for horizontal drive shafts mechanical synchronization is cheap and simple by using one long shaft connecting pulleys at the same end of both belts. But I've been considering vertical orientation of motor shafts and belts so debris falls away from the belt rather than getting caught in the teeth as is possible when mounted horizontally. By mounting vertically I'm hoping less debris will go between the belt and pulley thus reducing any precision errors in cutting (such as wave lines of the cut surface). Only way to mechanically sync for vertical is via another belt and set of pulleys, from what I can determine. This adds to cost of the project so I'm wondering if it's really necessary since my X axis is short. I haven't tested the encoders on the steppers but I'm assuming I may be able to use them to help measure any missed/lost steps.

Any guidance on this is appreciated.
Thanks
Steve

[eSilviu]

What happend when router is stopped, and motors are not powered?! It's possible that someone to pull a belt (just to see if it moves, or by mistake etc.). I think that you will not have 90degree between X and Y axis - not good al all.

you could synchronize both motors at the start of work (solution are many), but how repetable is that?

It's no need for synchronization during work, if motors are "always ON" (Kcam and others can power-off stepper if it's not used for 5sec for example - you don't want this to happend). Both steppers/servo could be commanded with the same signals, and this way it will be synchronized.

[appicnc]

Hello Steve
I have a belt drive homebuilt router, the X axis belts are flat, most debris seems to fall off or is shaken off from the start stop motion, the router blows a fair breeze which helps, I have recently machined some blue foam, the bit I was using cut it like spider web, it did build up and some was compressed by the drive pulley on the downwind side, but again it just shook(sp??) off, the Y axis belt is vertical and is flat against the gantry and is protected by the gantry.

I wouldn't worry but if you do, you could mount the two motors vertically, I would still run a belt between the two motors to hold them in sync, If you are interested I'll post some phots of the crud around the pulleys and you can see.

Ed

[kiwibum]

Originally Posted by eSilviu What happend when router is stopped, and motors are not powered?! It's possible that someone to pull a belt (just to see if it moves, or by mistake etc.). I think that you will not have 90degree between X and Y axis - not good al all. you could synchronize both motors at the start of work (solution are many), but how repetable is that? It's no need for synchronization during work, if motors are "always ON" (Kcam and others can power-off stepper if it's not used for 5sec for example - you don't want this to happend). Both steppers/servo could be commanded with the same signals, and this way it will be synchronized.

Hi eSilviu
I didn't think about it being moved when powered down, I hadn't considered these things, thank you for this. This is my first CNC router build, I have a prebuilt X3 CNC mill but it's in storage at the moment.

[kiwibum]

Originally Posted by appicnc Hello Steve I wouldn't worry but if you do, you could mount the two motors vertically, I would still run a belt between the two motors to hold them in sync, If you are interested I'll post some photos of the crud around the pulleys and you can see. Ed

Hi Ed
Thanks for this, yes I would be interested in seeing photos of your router. Another reason I was considering vertical mounting is I have access to a set of bellow covers but the fit will be a little tight with the belt mounted horizontally (flat) and protecting the rails as well. Plus with covers it will be more difficult to see debris on the belts. The whole router will be in a polycabonate clear case to prevent prying fingers so it will get dusty in there. I will have a vacuum on it as well to keep the debris down but since I haven't run a CNC router before I'm unsure of the problem debris will cause on belts other than what I've read. Thanks again for your reply.

[appicnc]

Hello Steve,

This is my third machine and first to use belts, first was designed by a " professional " engineer that used long ballscrews, speed and crabbing were problems, second used acme screws, again speed and homemade parts were " issues ".

The third was belt on the X & Y, ballscrew on the Z, using Mach3 I can rapid on the X & Y at an indicated 15000mm per min', and 8000mm on the Z, I am using Nema 34's, 1800in oz on the X & Y, and 1200in oz on the Z, all go through 2:1 reductions, the table is restrained, I am a builder ( house ) and am over 50, I am not of a jockey type build!!!, I can push the X axis when powered up with a lot of effort.

I have started to design a 4th machine that will be basically the same as the 3rd but with a cleaner X axix belt drive, I made a mistake with the parts for the X axis drive and was to lazy to fix it, I am not happy with how it looks but is works well, BTW, the new machine will be a nominal 3200mm by 1600mm.

As you can see in the photos, the machine is in a 25mm thick MDF box, two of the three windows are glazed with a single sheet of perspex, the third also has a sheet of 6mm laminated glass, it is in a seperate room in my garage, with the door to the room and the two windows closed the Trition router can be heard but you can talk in the rest of the garage without shouting, it needs to have a dust extraction system installed, in the works for the new machine.

You can see some swarf on the belt / pulley in one photo but generally it falls off, I would normally not be cutting anything as messy as the blue foam and would clean it out after use.

You might notice the Z axis is missing, in the photo showing the box, you can see some blackness on the spoil board. What happened was, when I machined the step in the ballscrew for the angular contact bearing, I machined the shaft 6 thou' over size, I didn't know this as I was using a vernier and it wasn't that accurate, I then pressed the bearing into the holder AND onto the oversized shaft ( another no no, I have since learned the bearing should be pressed onto either the shaft or housing, not both ) that caused the bearing to fail, the motor must be more powerful when pushing the router down ( makes sense ) so Mach 3 pushed the bit into the spoilboard AND the machine base, I can't source the bearings I need on a Saturday so It is apart waiting to be re-built early next week.

FYI, The T Rex was cut from 9mm MDF at 1100mm per min' in one pass, the limiting factor was the router not the machine, if the cut depth was decreased the machine could go faster, it was a 6mm bit, when cutting only 3mm deep it would cut at 3000mm PM.

I know some have had problems with belts stretching and my machine is < 6months old but so far I have not had any of that type of problem they are quiet, easy to use and appear to be accurate, certainly for CNC routers they appear to be the best solution.



Ed

[appicnc]

Hello Steve,

Sorry the photos were to big, here they are.

Ed

[kiwibum]

Hey Ed
Thanks for such a quick reply with the photos that's awesome. What will be different on your knew X axis? Are those 20mm T5 belts and pulleys you are using? How did you choose your size/type of belt? I'm currently working and building this in Panama (hence my X3 in storage in NZ) and have to order everything from the US and pick it up there when I visit my girlfriend. So I have to get it all right first time since I can't just get things next day when I like/need them.

Interesting regarding the noise, I'm making a separate spindle with treadmill motor drive with the aim of better life in bearings and less noise than hand router. The aim is to also share the ER20 collects I have between mill and router using a ER20 collect chuck shank I bought to make the spindle.

I have a ball-screw Z-axis already built I bought on ebay. I chose the belt design to reduce costs and because I don't have the tooling or equipment here to accurately mount drive screws correctly. I'm hoping the belts will allow a little more tolerance errors in mounting motors etc than required with screws. Which is why I'm also making the base out of t-slots aluminum as it allows me a little more flexibility bolting it up but my router size is determined by the lengths of t-slot I can fit in my luggage:-).

Thanks again for your reply. Steve.

[Mike Nash]

Hi Steve,

You might also want to check out the ideas presented here:

http://www.cnczone.com/forums/showthread.php?t=51485

[kiwibum]

Hi Mike
Thanks for reminding me about that, I read that a while ago and forgot about it, it's a great idea. I think I'll go with mounting the belts horizontally and connected with a drive shaft to keep them synchronized. Last night I figured out how I can make them fit under the bellow covers ok with the belt flat (horizontally mounted) and mechanically it was getting complex to add more belts etc for vertical mounting.

Thanks again for your replies, always interested in more advise/ideas though, and have a fantastic day.
Steve