CNC mill questions - thrust bearings, leadscrew mounting, general questions

[tonofsteel]

Hello,

I am currently working on building my first CNC machine, and I need a bit of direction here at this point. I am not sure how exactly to mount the leadscrews at each end. I know that thrust bearings are needed as well as regular ball bearings, but how they all fit into the bearing block I am a little unsure. I have seen some pictures months ago, but cannot seem to find them. Does anyone have any pictures of assembling a bearing block?

As well how do i grind/mount the leadscrew at each bearing block? Does anyone have links or pictures to some detailed information on these subjects?

I built a foundry last summer so I can pour custom pieces of aluminum, and some of the components of the mill are going to be poured. This is going to be similar to some other ones on the net where people have poured their own linear bearings, and uprights etc. I plan on using cheaper pipe, probably black gas pipe, or the varients others are using. How much error will this introduce? I know its not a good idea, but the mill is designed so these can be changed to something better in the future if needed.

Last question: For driving the mill I have done a bit of reading on this forum, and the 270 oz-in. steppers I was going to purchase apparantly are too small for the types of work I plan on doing. So I am upgrading to 1000 oz-in steppers, with a 3/4 6tpi leadscrew. I plan on using the mill for foam cutting, and aluminum work. Would this combination be sufficient? The total length of the mill is planned to be 3ft, and width will be 2.5 ft. Would the 3/4 leadscrew bend alot under the pressures from milling aluminum?

Thank you,
Darren

[jeffs555]

Most designs dont use separate thrust bearings. They use back to back angular contact bearings which handle both axial and radial loads. Many people just use regular deep groove radial bearings which are not designed for high axial loads, but seem to work fine in low usage home applications. Many people have the ends of the screw turned down smooth to accept the bearings, but many just get bearings with an inside diameter the same as the thread diameter and use the rod as is.

[stewi]

In general the mill motor does the cutting job and not the X,Y Z motors.
There are no thrust bearings required. However, the Z axis carries the load of the mill motor and the weight of the head. In order to reduce this load you may want to consider counter balance.
Your combination of 3/4 lead screw and steel gas pipes is a bit odd. When spending all the money on the 3/4 lead screw you should be using some more precise linear track and bearings as well, than your suggested gas pipes.
The step up from 270 oz to 1,000 oz holding torque appears to be a very costly one. I would rather go with a lower pitch lead screw or even threaded rod, which will move your axis at lower linear speed but you can do this at a much lower cost. Your stepper motors will do 1.5 to 2 revolution per seconds in full step mode, try faster and your motors will loose steps or stop. From here you can calculate the maximum linear speed you get on each axis.

[louieatienza]

Originally Posted by stewi In general the mill motor does the cutting job and not the X,Y Z motors. There are no thrust bearings required. However, the Z axis carries the load of the mill motor and the weight of the head. In order to reduce this load you may want to consider counter balance. Your combination of 3/4 lead screw and steel gas pipes is a bit odd. When spending all the money on the 3/4 lead screw you should be using some more precise linear track and bearings as well, than your suggested gas pipes. The step up from 270 oz to 1,000 oz holding torque appears to be a very costly one. I would rather go with a lower pitch lead screw or even threaded rod, which will move your axis at lower linear speed but you can do this at a much lower cost. Your stepper motors will do 1.5 to 2 revolution per seconds in full step mode, try faster and your motors will loose steps or stop. From here you can calculate the maximum linear speed you get on each axis.

Especially where high forces are involved, but in general, it's probably not a good idea to support the screw shaft via the motor shaft and motor bearing. Also 1000 in-oz is a LOT of motor; for comparison I cut aluminum with 425 in-oz steppers, running 1/2"-8, 8 start ACME screws and delrin AB nuts, and cheap radial BBs on both ends, with a very slight preload. I use LoveJoy couplers for stepper-to-screw transmission. If your motor alignment is not 100% exact, then you'll prematurely wear the bearings in your stepper. Also AFAIK the bearings in the stepper are NOT angular contact and not designed to absorb thrust; that should be the job of your fixed end bearing blocks.

You might want to check out the cold-rolled-steel rail system from CNC Router Parts, many people use them here and it's a very ridgid system. Even with gas pipe, since it's not smooth, it won't be as accurate; though you have to decide what kind of accuracy you want. If you insist on round rails, then you might want to look into water-hardening drill rod, which is a bit cheaper than oil-hardening, ground accurately, and you could use it without heat-treating. Easy to drill and tap as well. I haven't tried using them with Frelon sleeves, yet.. buy my machine uses drill rod and skate bearing on the z axis...

[jsheerin]

Originally Posted by stewi In general the mill motor does the cutting job and not the X,Y Z motors. There are no thrust bearings required.

This is not really correct. Leaving aside the issue of cutting, any time you change the velocity of the axes, you have to accelerate or decelerate a mass. This requires force input. If you don't have any thrust bearings to resist this force, something else in the drive train must resist the force, and drive motors are not intended to do that. In fact some motors specifically warn that you must isolate them from axial loading. You do need some kind of thrust bearings for each axis in almost all machines. The type of thrust bearings used can vary with the design of the machine.

For diy machines like a lot of people here build, a lot of people have used needle roller thrust bearings on each side of a block and a radial ball bearing in the middle of the block. I use that on my router. Here's a link showing how that is typically done: CNCRouterParts This is typically a lot cheaper than angular thrust bearings and perfectly adequate for our machines.

Alternatively, here's a drawing of a ball screw bearing block I found from a quick search:
5 Bears - Y axis ballscrew

[annyW32f]

thank you for these information

[louieatienza]

Originally Posted by jsheerin This is not really correct. Leaving aside the issue of cutting, any time you change the velocity of the axes, you have to accelerate or decelerate a mass. This requires force input. If you don't have any thrust bearings to resist this force, something else in the drive train must resist the force, and drive motors are not intended to do that. In fact some motors specifically warn that you must isolate them from axial loading. You do need some kind of thrust bearings for each axis in almost all machines. The type of thrust bearings used can vary with the design of the machine. For diy machines like a lot of people here build, a lot of people have used needle roller thrust bearings on each side of a block and a radial ball bearing in the middle of the block. I use that on my router. Here's a link showing how that is typically done: CNCRouterParts This is typically a lot cheaper than angular thrust bearings and perfectly adequate for our machines. Alternatively, here's a drawing of a ball screw bearing block I found from a quick search: 5 Bears - Y axis ballscrew

Swede's build.... one very nice machine!

[jsheerin]

I didn't even realize someone restarted a 6 year old thread. I should learn to read these things closer before I waste my time posting in them.

[tonofsteel]

I abandoned the build after the comments on using cheap pipe/parts and I built a CNC foam cutter instead of a milling machine to use for lost foam aluminum casting.

So your post was not a waste of time, I still lurk here and it was very helpful, thank you!

I am looking into building a different machine and ran into this question again.