Re: Timing Pulleys - Chain & sprockets?

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "ballendo" <ballendo@y...>
>wrote:

Thank you Ballendo for this information. I stand corrected but I was
thinking about milling machines and not routers etc.
With all the information contained below I can't see how this is OFF
TOPIC for this list.
Perhaps one of the other moderators can explain.
Hell Tim you could even be an agent for Reynolds chain, at deep
discount of course. [ just joking ]

John S.

> Hello,
>
> The single biggest "problem" with chain drive is "chording". Here's
> what that means:
>
> Say you are gonna drive a gantry router using #25 roller chain. Not
> too long ago, microstepping was out of reach financially; so you

had

> 400 steps/rev from the motor(using halfstepping). Now the motion of

a

> chain drive is like a cable or belt drive in that the circumference
> of the final drive wheel determines a few things, resolution and
> travel per rev. being two. So you want to use as small a final
> sprocket as possible, to get the best resolution.
>
> Let's pretend you can find a 4T sprocket. Can you see that as the
> chain (or belt, or cable) "wraps" around this, it is no longer
> circular, but square? This means that the radius of the 4T sprocket
> is going to vary as it turns, which means that the distance and

speed

> traveled will not be constant, but rather "jumpy" or "jerky".
>
> (You can also think watching the end of the cord when you wind up

an

> extension cord around your hand and elbow-a 2 "tooth" sprocket- you
> pull a bunch, then a little, then a bunch, then a little...)
>
> Now most of the REAL routers which use chain drive have an output
> sprocket of 12T, so the "square" becomes a 12 sided polygon. Much
> closer to a circle, but still not as good as a belt, or cable, or
> screw drive. But it DOES work.
>
> Finally, most of these machines also used a 4:1 reduction before

the

> final output sprocket. So we have a pitch diameter of something
> like .966 (comes from 12 times .25, for #25 chain,divided by PI-
> 3.14159; with the chording taken into account). And .966 times PI
> equals 3.034, and we're gonna have 1600 steps to travel this

distance

> if we use a half step drive. Which means we get a resolution of
> about .002 (about 525 steps/inch).
>
> For a wood router/plastic fab/sign machine this is just fine, and

as

> I said, hundreds have been sold over the years. Nowadays you can

use

> a microstepping drive to improve things a great deal.
>
> So, No, it's not nuts... A chain drive machine with Gecko drives is
> an impressive combination. And cheap.
>
> BTW, a properly designed chain drive has no more backlash than a
> cable or belt drive. And it tolerates the shop environment rather
> well. Some will speak of chain stretch, but again proper setup,
> initiation/break-in and design reduce this to a non-issue for the
> types of machines where this drive makes sense..
>
> Hope this helps,
>
> Ballendo
>
> P.S. What you DO get in exchange for these tradeoffs is... Speed.
> Even a cheap 200 RPM unipolar drive will get you moving at 180
> inches/minute!
> When John Kleinbauer starts offering plans based on this, remember
> you saw it here first!<G>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "rawen2" <rawen2@y...>

wrote:

> > Mark,
> > I've wondered the same thing.
> > What's the downside, if any, to using small chain & sprockets vs.
> > using toothed belts and pulleys? Backlash? Cost? I'm sure

chains

> > would make more noise.
> > I saw a picture of a router table a while back that drove the
> gantry
> > with an open-ended stationary roller chain looped between two

idler

> > sprockets and the drive sprocket. All three sprockets were

mounted

> > on the gantry and the chain was stretched along the length of the
> > table.
> > Would appreciate hearing from anyone who's used a setup like

this.

> > I've never heard of or seen the X & Y motors on a mill driving

the

> > leadscrews with chain. I suppose that should give me a clue but

I

> > still wonder....
> > Ralph Wenzl
> >
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, Markwayne

<markwayne@m...>

> > wrote:
> > > Would it be nuts to use roller chain and sprockets in small
> sizes,
> > like
> > > 10 or 20.
> > >
> > > Mark
> > >