Why isn't the Z axis spring loaded?

[blackhawk]

I have always wondered why Shopbot did not setup the Z axis to be spring loaded like X and Y. Does anyone know why the Z axis is not this way?

I just replaced my Z rack and pinion gear over the weekend. When I put everything back together, I could notice certain areas in the Z where there was minor slop between the gear mesh. In other areas, there was no slop and everything was tight. I had to balance the location of my Z motor to where no area was too loose and no area was too tight. The Z rack is mounted with VHB tape to a non-machined surface so I can see why there would be some irregularities. It just seems like a spring loaded Z motor would solve that problem, just like on X and Y.

I just wonder if there is a good reason that Shopbot didn't do this?

Below is an old discussion where Paco had created a spring loaded Z axis. Take a look at the picture in the 3rd post from the top. Has anyone else done something similar?

http://www.talkshopbot.com/forum/showthread.php?t=9004&highlight=paco+loose

[blackhawk]

I'll give this one bump. No one has ever considered this before?

[jTr]

I've got a one year old PRSAlpha - No spring loaded motors at all. Evidently a slow evolution in design. As I assess it, the 3M automotive adhesive tape used to adhere gear rails to frame has about 1/16" of "cushion" or "spring" inherent in its makeup. I simply apply firm upward pressure on the motor to firm up the gear as I periodically adjust / maintain, and that cushion acts in place of a mechanical spring.

When my machine arrived, one rack did not do well in transit, and I had to remove, scrape factory tape, and apply new tape as I was assembling my new machine - no fun:(

Perhaps an application of new tape would take care of the irregular spots under the rail that you described? Putty knife, rags and a can of acetone can help ease the process, but that tape is stubborn...

Otherwise, you certainly could develop a spring system as previously discussed if you feel more comfortable with a mechanical version.

jeff

[steve_g]

I assume (there’s that word) that the theory is gravity keeps the rack and pinion properly aligned and that the counter balance springs keep it in a reasonable range… However , if there are snug and loose spots in the Z travel it shoots that idea! Also, knowing how much pull an up cut bit exerts I assume (again) that the same amount of down force is generated by a down spiral bit. If a down cut bit can mess with built in play, then it goes to reason that a spring loaded mesh is necessary. Interesting…

SG

[gerryv]

Any chance there is a correlation between the number of teeth on your pinion and number of teeth between the loose spots on the rack? This could suggest a bad pinion or stepper shaft issue.

Is it possible to lay a straightedge along the loose areas on the top and bottom of the rack to check if it is "wavey", although that won't do much in the case of localized tooth wear.

[blackhawk]

The rack, pinion gear, and VHB tape are brand new. I just installed everything at the same time. The play that I get is small about .001 to .002" when you push up and down on the router vertically. Probably 2 or 3 times better than what I had before. But if a spring loaded system eliminates all the play, just wonder why Shopbot doesn't do this.

[bleeth]

Brad:

In a related discussion I had with Ted a few years ago about errors in fine veneer inlay work he suggested hooking up a bungee chord. The minor differences that added up over the size of the piece making what was essentially a marquetry piece were the result of motor backlash when operating, which the standard motors with tapered hobbs reduce greatly but don't eliminate. Alpha's have the same issue. Of course, this is just a super-quick solution to a proper spring, or a system that eliminates the backlash issue, like Servo's or belted gearboxes.

[mrgadget]

Maybe this will help as it's what I did to solve this. I recently went thru the process of aligning my z axis perpendicular to the table. In the course of doing this by adjusting the concentrics I inadvertantly moved or shifted ,if you will, the whole z axis away from the motor. This caused some vertical play in the z axis because the pinion gear on the motor was not fully engaged into the rack. My solution?....loosen the bolts on the motor and slide it over so the pinion gear was fully engaged. No more sloppity slop! Hope this helps if you have a PRT.

[blackhawk]

Well, I finally decided to try out my idea for a spring loaded Z. I used a 1/2" thick piece of aluminum plate to create my motor mount. I attached it through the Z tower with a 3/8" bolt and a large fender washer between the tower and plate. I then used a standard Shopbot spring and turnbuckle to apply the force. Last week, I did several 3D carvings for a customer. In all about 10 hours of machining time with the spring loaded Z and everything worked great. When I jog up and down, everything is smooth and there is no play throughout my travel.