Problem- Z axis termination

[jrsaw27]

I have looked and looked and looked. Pictures forums and more pictures. My question is how does the z axis end? At the top is the stepper, the rod in the middle, and I have a piece of angle iron at the end of the rod. Do I put an impression for the rod to sit in, a collar on the rod? certainly the coupler for the shaft and rod wont hold 15 pounds as it would just be suspended! Pictures of your machine would be GREATLY appreciated. How do you keep the Z axis rod from falling out?

[ssutton]

This may not seem crystal clear but if you study it a few minutes you will get the picture (No pun intended). The shaft coupler that I am using will NOT support a hanging load, it is the spiral split type, so I am using a thrust bearing to hold the router weight and cutting forces. I am also using a standard bearing in the mount plate to handle rotary loads and keep the acme rod centered on the thrust bearing. The bottom of the acme rod is just simply captured by a zero backlash acme nut from Dumpster CNC.

Hope that helps,

Scott

[CarveOne]

Originally Posted by ssutton This may not seem crystal clear but if you study it a few minutes you will get the picture (No pun intended). The shaft coupler that I am using will NOT support a hanging load, it is the spiral split type, so I am using a thrust bearing to hold the router weight and cutting forces. I am also using a standard bearing in the mount plate to handle rotary loads and keep the acme rod centered on the thrust bearing. The bottom of the acme rod is just simply captured by a zero backlash acme nut from Dumpster CNC. Hope that helps, ScottAttachment 139203

When your router bit is plunging into fairly hard materials like oak or aluminum there is upward pressure transmitted into the motor shaft and the internal motor bearing is taking the load forces. I would recommend installing another thrust bearing and shaft collar under the aluminum plate as well. The lower end of the Z ACME rod can still "float" but for accuracy reasons it needs to be constrained by at least a simple hole in a Delrin block, if not by a bearing block.

CarveOne

[tjskcnc]

Anatomy of a Z-axis:
CNCRouterParts

[jrsaw27]

Thanks much, that is just what i was searching for I just didnt know what to call it. Thrust Bearings. I dont have a collar so I am using cheapo nuts with a set screw on either side of the plate with a thrust bearing between the nuts and the plate. Does that sound right for the setup? And last questions; should these be tight to the point that if it were disconnected it would still sit there and spin? Also how do I connect the bearing to the rod, is there a set scew? Does it need another part or threads? Pinch it on the plate with the nuts, weld them on? If they slide on the rod, they slide off right? Sounds really simple but this is the first time Ive heard of a thrust bearing.

[CarveOne]

Needle thrust bearings have a flat washer on each side of them and the needles (cylindrical bearings) are captive in a ring. You just place a washer/bearing/washer between the collar and the plate and another set on the other side of the plate then snug the collars against the washers. The collars prevent the shaft from falling through the plate if it is disconnected from the motor shaft. This captive assembly is what prevents the motor shaft from absorbing vertical thrust loads that could possibly damage a bearing in the motor.

The thrust washer set and shaft collars are available from CNCrouterparts and other places. There are other ways to arrange this, like having a set on top of the plate and another set below the lower bearing block.

The thrust bearings just slide over the ACME rod. They take the vertical loads on the rod from the weight of the router and mounts plus the resistance to plunging into materials being cut.

The ball bearings in the upper and lower bearing blocks take the side loads from cutting in all horizontal directions. These bearings are usually set into pockets in the bearing blocks and the blocks are held rigid with screws or bolts. You need both types of bearings.

For the collars, use clamping collars with set screws to tighten it around the shaft without making dimples on the shaft threads. The dimples will cause an interference fit when you need to replace a ball bearing.

Many DIY designs don't use thrust washers but they still work, just not with the best accuracy. My first machine was built from purchased plans and only had one ball bearing at the top and no thrust washers. The lower end of the ACME rod wobbles around without restraints. There are no alignment issues that cause motor stalling though. For a rank beginner it served its purpose.


CarveOne