Rack - Mount and align

[harryn]

Hi, This question relates to how to mount and align to racks on the same axis.

System idea
- X axis with 2 racks (one on each side)
- Stepper motor on each side (not connected mechanically, just with stepper driver ( convenience / space / cost reasons )
- Drivers obtain their signals from 1 Mach 3 parallel port output, but split using a Kreutz hardare signal splitter. (1 pulse driving 2 motors)

I am trying to figure out how to mount and align the 2 racks to each other.

The mechmate site discussed the idea of mouting rack with double sided tape. This might be ok, but it seems like there should be holes drilled sideways through the rack for screw mount to the system frame. Do the racks normally come with holes drilled for screw mount, or do people really tape them down ?

I assume I can have someone like std steel drill holes if needed, I just wondered if this was "normal" or a special.

No matter what, it seems like the gear on the rack would need to be aligned to each other - not just for distance from each other to be straight and square, but so that the teeth on the pinion are alll starting from a common 0 point. Is there a common method for doing such an alignment ?

Put another way, if you were to set two stepper motors mounted to the gantry with the associated pinions / gearing, it is unlikely that this would be a perfect alignment. It might not matter when there is no power, but once the signals start, they will likely not be exactly straight, perhaps off as much as 1/2 of a tooth, or approx. 0.020 inch.

I suppose there might be some kind of method with the software or micro stepping, (Mach might have this) but I am running a hardware signal splitter (kreutz design) so this is not so easy.

Am I making a mountain out of a mole hill since with micro stepping the motors can be turned in approx 0.001 - 0.003 in increments anyway ?

Suggestions ?

Thanks

[lgalla]

Harry, The Mechmate rack is glued with double sided tape but there is at least a bolt near each end.Personally,I don't like the method.I would go with larger rack and drill and tap.I think rack is generally hap hazard mounted and the spring loaded pinion takes up the slack.I would try to mount the rack as carefully as the linear guides.Don't like the idea of the spring to reduce backlash.
Larry

[fatal-exception]

Dosen't sound all that 'sound' to me. Double sided tape usually sticks pretty well, so parallel adjustment might be a little difficult after it's bonded. I would slot the surface that the rack it mounting to (or oversize the holes) and use bolts and nuts for a permanent solution.

I don't think that having the pitches of the racks perfectly aligned is required. When you setup the machine, just make sure that the pinions are seated properly in the racks before powering up the drivers. I'm pretty sure that the racks could be off by 1/2 pitch and there will be no problem, might even be better to stagger them for smoother energy transfer? Both pinions are going at the same speed, they both have to travel down the rack at the same rate, regardless of how far out of phase they are. Make sense?

Paul

[Gerald_D]

Originally Posted by fatal-exception . . . . I don't think that having the pitches of the racks perfectly aligned is required. . . .

Not going to discuss rack mounting or alignment because I am biased

However, aligning the pitches of the two sides of the gantry is interesting . . .

The stepper motors will switch on at a full-step position, not at a micro-step position. So, if you want your gantry to be square at switch on, then you might want something finer than a motor's 1/200th step. Most times you can simply rotate the pinion so that a different tooth engages with the rack. If you have a prime number of pinion teeth (17,19,23,29,31,37) you can get a very fine vernier effect.

Anyway, to summarise, no need to synchronise racks......rather fiddle with different gear meshes if the need does arise for fine tuning.

[Mike Nash]

If you use a clamp style pinion gear rather than a set screw style, just power up with one pinion loose and then tighten it while the gantry is square. If you do use a set screw, but don't have a flat, same deal, but probably not a good idea.

All of the burning tables I've seen use a spring loaded pinion. Some of these would need a rather large crane to lift the gantry off the rails. One even had a seat for the operator to ride on, and yet the servo motors were tiny.

[harryn]

Hi

Larry - Thanks for the inputs on rack mount - looks like I might need to consider having them drilled. I am also still studying the spit pinion concept. Eventually I will narrow down to a concept and finish this build. Probably will end up rack and pinion because there is no way I can afford 9 ft long precision screws / mounts for a wood router setup, and the belt idea I had (using a very high quality belt as a rack) ran into a glitch.


Paul - that is an interesting comment about the idea of purposely setting the gears 1/2 step off from each other.

Gerald - thanks for that widipedia link. I have used verniers but never gave much thought to how they actually work. I am still thinking about how to use it to do a precision alignment - interesting concept.

I need to study the driver / micro step question in a bit more depth. I am planning to use high inductance unipolar stepper motors run in 1/16th mode with Linistepper drivers. The break out board with the package signals the stepper to run full time in 1/16th mode. I need to check if this is really "instantly", or just "very soon". Probably will still not really know the total system answer anyway even with this info.

Perhaps what makes the whole question a bit more exciting, is that the X table will be removable - not really by desire, but to deal with a general lack of space in the garage. Each time I want to use the system, it will need to have the X table inserted into the router, pinions, hold downs, etc. The advantage, is that I think I will end up with 4 x 8 ft range, and it takes up relatively little floor space.

The downside, is the need to do at least some re - alignment each time. This is what really pushes the question of how much fiddling will be needed each time for both sides of the X table racks.

[harryn]

Mike - Thanks for the note. I missed your very useful comment the first time.

Harry

[DSL PWR]

I feel bolting the rack on is a waste of time. All I did on my table is tack it on with my welder. It's not like you are replacing it that often!