x axis setup

[ger21]

Originally Posted by amishx64 Good point though, with out them being linked, there would be pretty severe consequences.

The majority of dual motor driven gantry machines built here, are slaved in software only, with no mechanical connection, and work perfectly fine.

[amishx64]

Originally Posted by ger21 The majority of dual motor driven gantry machines built here, are slaved in software only, with no mechanical connection, and work perfectly fine.

Just to justify what I meant, I was referring to pminmo's theoretical extreme part conditions.

I have seen these machines on the forum and do realize that.

Anyway, good luck on your build bluc.

[pminmo]

Now, if your gantry has some flex and could easily rack maybe .1 or so, wouldn't that cancel (wrong word) out the inefficiency? As the two sides aren't really fighting each other, just moving a little differently.

Think of it this way, it's a similar issue to leadscrew whip. You're creating a vibration as it moves side to side much like whip. It's not necessarily big, but it's inefficient.

I'm curious, how much of a performance hit are we talking here? 5%? 10%?

How fast will a car go with a small block 327 chevy engine.. (similar question) when you don't know the specifics. It all depends on motors, screws, nuts, drivers, bearings. Some screws are linear and accurate, some are not. Some systems have antibacklash nuts, some don't. I'm not trying to avoid an answer, but I don't think you can put a number on it.

Now, connect the two motors together with a belt, and they'll be fighting each other constantly, which would be a much greater performance hit.

Same concept, but that is more tightly coupled so just move back out of focus some to apply the same principle.

Different people build from the same set of plans with different results because person A pays attention to detail uses a digital caliper and every detail is right on. Person B, uses yardstick that has 1/16" marks as the finest resolution of scale and gets a completely different acting machine.

So the right answer to the original question depends on all the components the builder has selected, what quality they are and how well that person builds. And most importantly how well the axis tracks without racking.

[ger21]

Originally Posted by pminmo I'm not trying to avoid an answer, but I don't think you can put a number on it.

But can you agree that in most cases, (for a fairly well built machine) it's a rather small number?

So the right answer to the original question depends on all the components the builder has selected, what quality they are and how well that person builds. And most importantly how well the axis tracks without racking.

I think it's safe to say that most home built gantries (at least the wooden ones) will exhibit enough racking that they'll benefit from two motors. Especially when they are over 3ft.

[pminmo]

This one of mine is wood and over 3', no racking at all. One screw one 280 ozin in motor.

[ger21]

If you push on one side, it won't move at all?? Actually, I guess if you can tie the two sides together under the table, you can eliminate a lot of the racking.

Looking even closer, I can see why you wouldn't have any racking. Are those 1" shafts, or bigger?

[pminmo]

Originally Posted by ger21 If you push on one side, it won't move at all??

Nope, not even a hint of racking

Actually, I guess if you can tie the two sides together under the table, you can eliminate a lot of the racking.

They are connected by a piece of laminated strips of red oak that serves as the base for the gantry. The laminations serve to stiffen and resist warping.

Looking even closer, I can see why you wouldn't have any racking. Are those 1" shafts, or bigger?

1" 1/4" diameter 46" long unsupported. The base, gantry, shaft holding, motor mouts is all wood. I took the easy way out on the Z axis, it's a instant Z from cnconabudget.com One piece of emt pipe and and all thread are the only metal in addition to 4 straps on the 1" shaft holders. But the money is in the precision shafting and bearings and screws where IMHO it should be, thats where you get your precision and performance. But even there the long shafts and bearings cost about $150 (used) and the 24" 3/4" shafts and 4 pillow blocks (used) were less than $100 to my door. It does hanve new Roton screws, the long axis is antibacklash, and the 24" axis is non backlash.

[revwarguy]

To answer the original question:

I built one of these, a 24x48. I chose to go with one stepper and the long belt. Why? It is *much* simpler, and simple is good. If I ever want to change that, it is easy to do with only the cost of the belt sacrificed.

However, I am very happy with the result (see www.liming.org/cnc for more.)

The two rods do not rack, and they never get out of sync. I turn one of them all the time, and the other moves right along with it. There are other things to be concerned about (see above) but this is not one of them.

Good luck, bluc, on your build!

[bluc]

Yea am building same one and have looked at your build log fair few time I am a bit dissapointed as I cant find the nessasary channel for the gantry here in Australia. Trying to find someone to get some advice on an alternate grade. If cant get aluminium might go to light gauge mild steel or last resort wood.

[pminmo]

Originally Posted by revwarguy To answer the original question: I built one of these, a 24x48. I chose to go with one stepper and the long belt. Why? It is *much* simpler, and simple is good. If I ever want to change that, it is easy to do with only the cost of the belt sacrificed. However, I am very happy with the result (see www.liming.org/cnc for more.) The two rods do not rack, and they never get out of sync. I turn one of them all the time, and the other moves right along with it. There are other things to be concerned about (see above) but this is not one of them. Good luck, bluc, on your build!

An elegant solution that solves both issues, racking and two motors!