The principle of the lead screw drive for the Y-axis is as follows:
- The drive motor should not be subjected to any axial nor radial forces.
- The lead screw should be supported radially at both ends
- The lead screw should be held axially without any possibility of moving back and forth.
- The carriage should be driven by the lead screw without any play.
This detail shows the motor end of the Y-drive. There is a ball bearing (not visible) holding the lead screw radially.
The cylindrical part in the center is what is called a helical coupler. It accepts the 1/4” motor shaft at one end and the 1/2” lead screw at the other. The ends are split and tightened with clamping screws.
The remarkable feature of this coupler is the narrow helical slit in the center. This allows for slight misalignment between the motor shaft and the lead screw and also disengages any possible axial forces between the lead screw and the motor. However, the coupler is stiff with regard to the rotation.
Now, we have taken care of driving the lead screw, see next picture.
T h e r e a r e t w o ty p e s o f b e a ri n g s at th e o th e r e nd of the lead screw. A ball bearing holds the screw radially (in small rectangular plate).
The plate on the left is bolted to 8020 extrusions located behind the Y-axis bearing plate.
The lead screw is fitted with two clamp collars and two sets of washer-needle bearing-washer assemblies. The collars force both needle bearings against the mounting plate and keep the lead screw from moving axially.
The lead screw assembly is now complete. See below.
The last member of the Y-axis drive is the antibacklash nut. The nut type shown here has slits at one end and a strong spring forces the split end against the screw even if the screw or nut show some wear.
You also see a similar nut for the Z-axis.
The two lead screws are different types. The Z-axis screw has an ACME thread with a square profile, a pitch of 10 TPI and has 5 starts giving it an effective pitch 2 TPI.
The Y-axis screw is a rolled screw with a smooth rounded thread profile. It has a pitch of 8 TPI and 4 starts for an effective pitch of 2 TPI.
That means that both screws move their ”payload” 1/2 inch per revolution.
The small wood pieces are the cams that activate the limit and home switches.
Limit switches are important safety devices. The drive electronics monitors these switches and cuts the power to the motors when any switch is activated.
There are two switches for the X and Y axis respectively but only one for the Z-axis (limiting the up position).
T h e X a n d Y a xi s h a v e a n e xt r a s witch each, called a home switch which is activated earlier than the limit switch. Home switches can be used to instruct the machine to move each axis to a ”home” position.
There are other switches used sometimes, for example, for sending the router to a position for a tool change (not implemented in my design).
The router mount fastens to the 8020 profile T-slots. That allows to set it at different heights depending on the job and the tools used.
The mount shown here is for Bosch EV1617 and also fits other routers with a 3.5” body diameter. I made adapters for my Makita router (3.22” diameter) and for my Dremel tool.