[AndreasT]

Hello!

I´m planning on converting my z-axis to linear rails due to my Z-axis are sloppy/big friction and i think it´s main stopper for better surface finish.
Have planned to use Hiwin linear rails (25mm) with carriages HGH25CA type.
My question is what do you think about preload for this carriages? I´m using servo 850oz/inc.
There is 3-levels of preload Z0, ZA and ZB. ZB are only recommended for 20mm>rails.
What is the general opinion on converting the Z-slide to linear rail, can i expect better/stiffer system?

BR/Andreas

[TiagoSantos]

It has been done before, but not that common. I think if you have the means to do it (a big enough milling machine!) or are willing to pay someone to do it, it would be great. I am very tempted to try it myself, but it's a bit of a big job, and since I can't afford to pay someone to do it, I'd be scared of messing up and ruining the column!

I'm not sure that it would affect your surface finish though, I don't think the head is bouncing up and down while cutting, I would think the weight on the head would more than prevent that.

I've heard some opinions that unless you have reduced your backlash to less than 0.001", the dovetails and the friction are actually helpful, as it keeps the table/head from bouncing around within the backlash range. With linear ways, there is so little friction and effort needed to move the axis that it becomes even more important to have as little backlash as possible.

Sorry, no idea about the preload..

[AndreasT]

The surface bottom surface finish is really god, the problem is when side milling. I have less than 0.001" backlash on x and y axis and about 0.002" on Z-axis.

I have noticed the head/endmill actually "twist around the column" and problably due to gibs deformation. The surface finish is rather good for benchtop machine but i want it further improved thereby my investigation regarding this. I´m planning to do this at the same time i fill the column/base with epoxigranite since I can measure quite a deflection in the column when applying force on this.

I dont think you need a big milling machine to this. I planning to making a drilling jig and attach it on the stock Z-slide to drill the holes for the linear rails. Since the dovetails already is "flat" the surface don´t need to be machined. I will the mill a new slide and mount the carriages. Carefully measuring and mockup and i think i can align/mount the rails good enough and the tram the column by shims.

[bozidar22]

I used SBC linear bearing with the #3 heavest preloaded. That is what the SBC catalog called for with milling and grinding machines.

My column is a custom welded steel tube

I'm happy with the finnish.

[AndreasT]

Hi, do you have pictures of the installation and some parts you made?
What SBC size/type do you use?

[bozidar22]

I used 30mm rails

here's a picture in aluminum and a link to my Youtube page.


22BOZIDAR's channel - YouTube

[LUCKY13]

In no way would I expect the dovetails to be flat or straight. Infact it is almost a given that they will be out a few thou somewhere along there travel..



This is enough to cause major binding in a good preloaded rail system once installed.



IMHO making a new gib and scraping it to fit the system would be well worth the effort of learning how to do it. A small surface plate would be all thats needed for a gib of this size and its a skill that will help you with many other projects down the road. Also the connection of the head to the slide where it rotates can be a problem which can be from not being flat as it should be, or the screw nut connection having problems which is documented in many of the RF-45 CNC changeovers. You would be surprized at just how much a good gib fitted to a machine will help. Infact one that doesnt fit good will not even ajust right on the Z axis and leave you with slack.

By all means look at the small things first before going down a major mod road such as adding rails. Rails are good in many ways, but they must be done right.
Not trying to talk you out of adding rails, but aproaching the problem with some determination and looking into the complete system may save you some grief.


If you do go with rails prepair the mating surface or you will have problems.

Jess

[109jb]

I agree with Jess. Although a linear rail Z-axis is nice, it seems like a lot of work along with a lot of expense when you could get tighter and smoother action than you have now by simply scraping the existing dovetails. Although controversial, you may even be able to get better result by just lapping the ways.

[AndreasT]

Hi!

Thanks for yours input. I will look into this area first before going further with my plan.
Do you have info regarding how to make a new gib/scraping it. What material should it be? Brass/Steel?

[D.L]

I'd only go with ball rails if you plan a major design change to the mill, such as adapting the head of another mill, or making it articulate.
Derek