Redesigning a Z Axis

[BillSN]

Hi all! I am new here and only a few months into the CNC world. I am an EE by day and have alot of manual machining/metal working experience.

Anyway, I bought a cheap small CNC router on Ebay awhile ago (whole package was around $1400) just to get my feet wet and to learn on.

I have been cutting like mad and come to a few conclusions regarding the design of the machine I bought. While the X and Y are well designed and stable, the Z axis is dreadful. The whole mounting delfects in both the X and Y direction approximately 2mm. It also warbles as it is moves in either direction. I bought this machine to mill circuit boards so this amount of play is unacceptable.

I am going to redesign the Z slide. I am looking for advice from some you. I know the play is comming from the linear bearings, the 0.375" guide rods are stable and do not noticablly deflect.

If you look in the picture you can see that the Z mounting or slide (what is the correct term?) consists of two small aluminum blocks that house the linear bearings and the drive screw nut. These two blocks are fastend to the spindle mounting plate to form the slide.

The two problems I can find are: 1 the slide design is too weak, it is only supported on one side by the thin mounting plate and relies on the bearing tolerance to keep it rigid. Secondly the bearings are not all that good.

So here is my plan. I have been using Autocad for years so I will design a new slide/mounting in Autocad and have a local machineshop mill it for me.

I am thinking about using one solid block with 3 bearings per side to increase bearing/guide contact area, swap to a Acme screw with an accordion coupling. What do you all think?

Update: I tore down the machine and discovered that the z axis "linear bearings" where nothing more than some bronze bushings with horendous tollerances! I have ordered some proper ball bushings along with a block of Al. I hope to get this thing finished quickly.

Can someone recomend the proper way to seat these "ball bushings" in the slide housing? I am planning on just tapping them into a hole sized to the OD of the bearing sleeve.

[YZF]

Hi Bill,

I would consider just using two bushings per side. I think the center one in a 3 bushing per side configuration would not be doing much except perhaps add a tiny bit of stiffness in the middle of your block. If you go with the block design you would need to place the bushings pretty accurately within the block, I can see that possibly being difficult, how were you thinking of doing that?

I believe guide rods used with recirculating ball bushings are usually fairly accurate hardened steel, is that what you've got in there? I also wonder how parallel those rods are...

Hope this helps,
Guy

[BillSN]

Thanks for the help! I will take your advice on using two per side.

I am planning on using these here: http://cgi.ebay.com/NB-Linear-System...742.m153.l1262

I checked the diameter of the rods using my dial calipers and they looked ok. I just checked in random spots. Do you think I should just buy a matched set of bearings and rods?

As far as inserting the ball bushings into the carrier block I have no idea what I am doing. This will be the first time I have ever used them. How do you normally install this type of bearing? I have a manual mill so I figured I would drill the holes in the block and use a reamer to get the right ID to press fit them in. Ultimatley anything will be better thay what I had before.

This machine is more or less a learning device for me. I plan on buying a more proffessional type machine in the future. Something that can do thick plastics and Al. Thanks for your input it is much appreciated!

[Switcher]

I've seen guys do something like the image below (top view of alum. plate).

Might want drill both top/bottom plate at same time?


.

[YZF]

Hi Bill,

Have you seen these style blocks with bushings?
http://cgi.ebay.com/ws/eBayISAPI.dll...m=130039257696
They are probably easier to work with. All you need then is a flat surface.

I've seen it done the way you're thinking but it will require high accuracy in the placement of the bushings. I'm building a machine right now where the Z axis is a modified linear pneumatic actuator and it's a solid block of aluminum with the bushings built in just the way you described. Have you tried NB's web site? They may have application notes with details about how to mount the bushings.

Your two bushings will have to be very accurately co-linear and the two sides will need to be exactly parallel and the right distance. You'll need to get the diameter just right as well.

The other way I've seen these used is that the bushings are installed in a clamp, just imaging a slot machined in the side of your block and bolts used to clamp the bushing in place. (I hope this makes sense)

Yet another option is to make the holes slightly larger and then pot the bushings in place with epoxy, I don't have any experience with that personally but I know it does work... The bushings then end up exactly at the right place...

If the current guide rods aren't hardened steel the hardened steel balls may chew them up, if they're not accurate the stage may bind... I guess you can always try the rods you have and go from there...

Guy

[YZF]

Switcher's picture is the clamping design I was talking about in my post, a picture is worth a thousand words

[BillSN]

Thank you all for your help! I tore the thing down last night and designed the block I want to use in Autocad last night.

This morning I had a few worries that here I am deisgning a very preceison fix to something that more than likely does not warrant it. I put the z axis back together while taking some measurments. A Chauchat was built with more accuracy! If I flip one of the slides upside down, it gets stuck. There is now way I can use the rails or the entre gantry for that matter.

I think Im dealing with putting a diamond on a gumball machine setting.

Sooo, I went to one of my companies facilities (Oil field OEM) and got some bushings made from a bronze alloy we use for inlays/bushings on various oil filed machinery. I basically turned down each bearings OD/ID, I had to custom fit all four. After some clanking and banging I got the thing back together and it works pretty good. I can no longer feel any enplay or delfection wich is a huge step.

Ultimately I am going to use this machine for some xmas ornaments and other wood working projects and thats about it. I am already looking for a new machine Thanks again guys for all of your help!