Flat spots on 3" circles

[thephridge]

Hello Friends,

We mass cut 3" circles out of PCV and we are experiencing small flat spots on the two nodes on the Y axis. We have checked the hardware and everything seems in order. We can actually see the machine making small hesitations at those points. Any thoughts?

Thank you,

Phridge

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[ger21]

Backlash.

[thephridge]

Backlash seems to be the consensus. I've attached an image of where/how it occurs. It's very consistent. Our Z drive is ballscrew. X/Y are chain sprocket. I can actually see a slight hesitation in the X motion at the points but I cannot see/feel anything loose. Does the image suggest anywhere specific on the machine that needs attention?

[ger21]

Chain drive never going to be precise, or backlash free, as the chain will flex and stretch.
Is this based on a buildyourcnc machine?

[thephridge]

Yes. Its home built. We bought it to help produce product components for the items we build. It doesn't need to be super precise like 4 decimal places, but 2 would be good. I just figured that if we can get 90% of the way around the circle and have a good curve, we ought to be able to figure out how to do the whole thing and eliminate the flat spots. But maybe not. I've added an image of the machine.

[wizard]

Chain is really ugly when it comes to motion control. I would do timing belts before even considering chain. By the way timing belts are not ideal either for a router.

My guess here isn't so much backlash as tension changes from one side of the chain to the other as motion slows and reverse. You may have backlash in the system but i try to separate this problem from the issue of backlash. What you want to do is to look at how the chain is tensioned and see if it sags as direction is changed. If so tension the chain until it is tight.

Unfortunately chain does wear resulting in clearances in each link. This wear is really what chain stretch is and is why chain drive require frequent tensioning. Interestingly this clearance will not show up as backlash in a drive like this as the tension in the chain preloads each link, at least until the links wear loose again. Eventually wear in the chain ends up so bad it wont lay properly into the sprocket causing all sorts of weird motion problems. You might see this if tensioning the chain suddenly causes motion problems on an axis.

So in a nut shell you might be able to get improved operation through adjustments to the chain drive but whatever you do it will not last long. If you want to keep the machine you will have to upgrade the chain drive to something else.

By the way you can't dismiss backlash in other parts of the drive. Couplings, gearboxes sprockets and such can all be sources for backlash. A loose sprocket rocking on a key could cause the same sort of failure. In the end though chain drives are pretty much a maintenance nightmare.

[thephridge]

Wow! Thank you. That is a wealth of information. I've begun exploring the possibilities of replacing the chains/sprockets at least with belts and pulleys. It may play out that we are just able to buy a whole new gantry. Any ideas where to looks for such parts? There are lots of search results but I don't want to contact an organization unless they really know what they are talking about.

[wizard]

Wow! Thank you. That is a wealth of information.

That is what happens when you are as sick as a junkyard dog.

I've begun exploring the possibilities of replacing the chains/sprockets at least with belts and pulleys. It may play out that we are just able to buy a whole new gantry.

Belts and pullets are only some what better and in this case would require a major redesign. For belt dives you really want to be looking at wide belts in the 40 to 50 millimeter width ranges. Plus the belts should ride close to a frame member. From the pictures posted going to a belt drive may require a lot of drive refactoring.

Likewise I'm not sure a new gantry makes sense. It simply is hard to tell from the pictures if it is worth it. My guess is that you will end up redesigning/rebuilding the whole machine at which point you might as well build a new router.

Leadscrews are a good upgrade but i would look at Rack & Pinion to see if an easy fit with existing parts is possible. Im assuming you already have gear motors for the chain drive.

Any ideas where to looks for such parts?

There are hundreds of possibilities.

There are lots of search results but I don't want to contact an organization unless they really know what they are talking about.

Leadscrew manufactures are available as are distributors. The problem you will run into is that you need to know what you want. You can call a distributor and they will happily take your order, they will not design the machine or components for you. The exception is valued added suppliers that charge a good penny to help you out.

Ultimately if you cant machine the leadscrew ends yourself you will need technical drawings to specify leadscrew end machining. The exception here might be companies like Mitsumi that have the ability to do some customization work.

[thephridge]

Well. Its starting to sound like we should use it for a while. Its gets us by as it does basically what we want for now. But before long, I can see putting a nice new coat of good paint on it and selling it for what we have into it and buying a better model all together. We're making a 4 zone vacuum table for it so the value would be good for a hobbyist. From everything I've been reading, circles are the litmus test for cnc routers and that's mostly what we do. And small ones at that. LOL. Thanks so much Wiz. Your a huge help. And I hope you feel better soon.