I've built this machine 100 different times in my head!!!

[JAZZCNC]

Ermm.!! . . . . Been looking at this thinking don't seem quite right.??? . . . I'm sure it's correct because you've obviously taken lots of time over it but I'll ask anyway just in case.!!

The bearing/rail height seems low.? from the dimensions on the drawing it appears to be about 16mm this is very low even for 15mm profiled rails and surely can't be right.? Do you have the bearings set in pockets.?

[Iwant1]

Thanks for noticing how small my profiled rails are. I mentioned earlier in the thread I didn't realise that they came in such low profile dimension. I got them for £70 from ebay for two slides and 4 rails as 15mm width and 550mm length. I've attached some pics to show their dimensions with respect to the bk/bf blocks. This is why I'm in a pickle over using 25mm plates for the z axis with 15mm machines out from each.

Do you think I should sell them on and get slightly larger rails.

[WandrinAndy]

I've attached some pics to show their dimensions with respect to the bk/bf blocks.

As a novice who has never seen any of these components in the flesh so to speak, those dimensioned pics have really helped me and reminded me that I need to get a digital caliper... thanks Iwant1... I'm following your build with interest btw, and want one too!

[JAZZCNC]

Do you think I should sell them on and get slightly larger rails.

I wouldn't go that far but they certainly are not ideal in the respect they make the pockets deep.!!

There is one thing I'm not to sure about and it's hard to tell from the pics, is that they don't look like the 4 row type bearings.? This makes them that bit less ideal and less able to stand the strain that a Z axis will come under.!! If 2 row type then I think for a Z axis then yes I'd sell them and get 4 row type more suited and making the job a bit easier.!!

[Jonathan]

Having 4-row profile rails is an advantage over 2-row - twice the balls = twice the surface area in contact with the rails which should get better stiffness, in addition to factors usually mentioned. However 2-row is still in a different league compared to round rails, so unless you can sell them and get better rails for the same price I think there's other places the money would be better spent on. You've got long rails, so use that to your advantage as if you're prepared to sacrifice some travel you can space the bearing blocks out more to reduce the load on each, so long as the piece they are mounted on is stiff. I got 760mm rails for my Z-axis, so if I ever get round to making it I'll keep 400mm travel and have the bearings 360mm between ends, which is slightly extreme but the rails were cheap so why not.

Lost of cnc machine parts are available in the SketchUp marketplace, which you get to from within SketchUp. All the rails, bearings, spindle, motors, ball nuts, etc are available.

The standard part libraries can be helpful, but I tend to draw things myself so I can be confident it is correct. I guess if I used SketchUp I might think differently.

[Iwant1]

The standard part libraries can be helpful, but I tend to draw things myself so I can be confident it is correct. I guess if I used SketchUp I might think differently.

Very true, many of the parts I used, such as the pulleys, timing belts, ball nuts, linear rails and slides were all over 50 times the size I needed and some not 100% right. I spent loads of time scaling the items as a whole and also separately in each axis so that they match what I had. The problem was in SketchUp, the scale function works as a percentage where as I wanted dimensions, so I had to resize, measure, resize again, measure again..... to fractions of mm's. Next time I'll make them up but then I suppose the same tedious task will be spent with my vernier calipers.

Regards the profiled rails, I think what I paid for these 'mini slides' was very good and I definitely won't be able to get larger ones for this price. I spent ages today trawling eBay for profile rails and carriages and found that I can get Thk and Nsk sets at similar length for around £150 with p&p. Problem is I had to constantly go to their manufacturer websites and check whether they got 2 or 4 rows of balls. Then you find out the balls also have 30,45, and 90 degree configurations. Plus I started mixing models like hgh, hgw, hsr, ssr, sr, lf, lu with the wrong manufactures.

I've seen Zapp is very reasonable with his Hiwin rails and carriages, which work out at £180 for a brand new set, hgh15ca and 500 rails, compared to £150 for similar spec second hand Thk/Nsk on eBay. I don't understand how there can only be £30 difference, are Hiwin a cheap brand compared to Thk and Nsk.

I'm also not sure what preload is suitable for my z axis, as gravity is always going to pull the carriages down especially when all components are connected to it. Does ZO, little preload, mean there is more play between rail and carriages then ZA, medium preload. I have no actual experience what any type of preload feels like, as I only have the second hand ones to go by, which you can blow from one side to the other.

Thanks
Adil

[Gary]

Personally i would use ZA pre load.
Also a heads up, Hiwin have just raised their prices by about 20%, so expect prices to go up by about 25% with most if not all distributors.

[Jonathan]

I spent ages today trawling eBay for profile rails and carriages and found that I can get Thk and Nsk sets at similar length for around £150 with p&p.

I've seen Zapp is very reasonable with his Hiwin rails and carriages, which work out at £180 for a brand new set, hgh15ca and 500 rails, compared to £150 for similar spec second hand Thk/Nsk on eBay. I don't understand how there can only be £30 difference, are Hiwin a cheap brand compared to Thk and Nsk.

That's because you only spent 'today' looking, you have to be patient to get the good deals - bidding not buyitnow. Yes I know, I used to think it's annoying when people say things like that, but it's worked for me in the end.

Does ZO, little preload, mean there is more play between rail and carriages then ZA, medium preload. I have no actual experience what any type of preload feels like, as I only have the second hand ones to go by, which you can blow from one side to the other.

If you can blow the carriage along, or it drops if you hold the rail vertically, then the pre-load is zero (Z0). When there is preload there is no clearence (play) between the rail and carriages, in fact the clearance is negative so it's an interference. This means that when the carriage is slid on to the rail the bearings distort slightly, so there is a force between the bearings and the rail - hence the name preload.

I'd consider having zero, or very low, pre-load a much bigger issue than only two rows of balls. You've got both...

P.S. Noticed there's two discussions going on here, the machine and sketchup - I can separate them if you want?

[Iwant1]

I'd consider having zero, or very low, pre-load a much bigger issue than only two rows of balls. You've got both...

P.S. Noticed there's two discussions going on here, the machine and sketchup - I can separate them if you want?

Thanks for the insight Jonathan, seems as though I made a bad purchase with my profile rails and have decided to get new ones with medium preload and 4 rows of balls.

I think the sketchup discussions have come to an end, and now will be mostly about my build.