[Texasbowhunter]

I have stumbled upon several various sets of linear rails by different Mfgs. that has come on the market reasonably priced...
These Mfgs. include THK and Rexroth along with NSK
Are the THK as good of a rail and block as say the Hiwin...Or the Rexroth vs the NSK Are any of these three comparable to what seems the highly sought after Hiwin?
I can get these a bit more than the China versions which seems to be readily available.
I read on the packaging that the THK we made in Japan and under the impression they were quality blocks and rails?

Along with an addition question,
Example: if I was ordering a 1000mm rail I would assume that the rail from one end to another would be 1000mm
then If I ordered a 1000mm screw would the screw measure 1000mm from the outer side of the BF and BK blocks?
or
Measure from one end to the other
So If I wanted my OVAL screw to measure the from the outer side of the BK/BF to be the same as my 1000mm rail should I add? And if so how much?
Thanks
Paul

[joeavaerage]

Hi,
THK, NSK and Bosch Rexroth are long established and highly respected manufacturers....no doubt you'll see that reflected in the prices!

Hiwin is also very good. I have very high respect for many Taiwanese manufacturers, good quality at (more) affordable prices.

One problem to watch is that there are dozens of Chinese manufacturers making Hiwin knock-offs. They look identical, color, size even down to the same part
numbers as Hiwin....but not Hiwin.......just cheap Chinese knock-offs. All over AliExpress, Amazon, Ebay etc. Avoid like the plague!
Once you find a supplier of genuine Hiwin products you'll find the prices are much higher than you'll have seen on AliExpress....even if they are somewhat less than
THK etc they are not cheap.

Craig

[joeavaerage]

Hi,
the max travel for a ballscrew is the length of the threaded portion, less the length of the nut. You are wise to reduce that by 20mm, or 10mm at each end. You don't want to run the nut right to
the end. Look for the screw length spec, not the overall length.

Craig

[ger21]

Are the THK as good of a rail and block as say the Hiwin...Or the Rexroth vs the NSK Are any of these three comparable to what seems the highly sought after Hiwin?

I'd rank them all above Hiwin. In reality, if you compare apples to apples, there shouldn't be any discernible difference between them.

[joeavaerage]

Hi,
I think Hiwin's perceived popularity is because they are so cheap, when in fact its the knock-offs that are cheap.
Genuine Hiwin is nearly the same cost as THK....and on the basis of longevity in the market and demonstrable product excellence
THK is still the leading manufacturer.

Craig

[Texasbowhunter]

guys this is great news since I have a source of getting a set of 30's relatively reasonable...

Let me ask this if I may would getting the 30's be an over kill for a 36x24 hobbyist machine? if the price of a set of China 20's were the cost of a set of these THK 30's would you get them for this size of machine with the hope of trying to produce a very respectable well made machine...
paul

[joeavaerage]

Hi,

Let me ask this if I may would getting the 30's be an over kill for a 36x24 hobbyist machine?

Probably, but does it matter, if you can get good quality parts cheap then that makes them highly suitable in my book. If they are too big and therefore difficult to incorporate into your
design, perhaps then getting more appropriately sized rails/cars even if they cost more is indicated. 30mm rails/cars should be feasible though.

My first mini-mill had 15mm rails, and my current mill (350mm x 350mm x 350mm travels, servo driven 25m/min rapids) has 20mm rails. I would have preferred 25's or 30's
but these were new old stock heavy duty THK rails and cars....three sets including 6 rails and 12 cars for $700USD including shipping to New Zealand.
I got them and have been very happy with them.

It is very possible to overdo the ballscrews. The rotational momentum increases as the fourth power of diameter. Thus with outsize ballscrews it may mean you need a ridiculously oversized (expensive)
servo to spin them at respectable accelerations. You are advised to stick close to the recommended size ballscrews, aside from anything else they cost a fortune. You want to buy once
and once only....none of this getting another set because I made a mistake with the first set.

Craig

[ger21]

Let me ask this if I may would getting the 30's be an over kill for a 36x24 hobbyist machine

Yes, but it doesn't really matter. The only issue is that they are massive.

[peteeng]

Hi Paul - When comparing rails and ballscrews always check the grade of the product. For instance rails come in 5 accuracy classes. Ballscrews are similar. The normal rail is "C". So check that the rails are similar in class & what preload they are. Precision drives the cost... Peter

Hiwin have normal, high, precision, super precision and ultra precision classes.

[pippin88]

Rails are hard to oversize, apart from the simple issue of how much space they take up.

Ball screws - bigger is not better - need to be appropriately sized.

Hiwin are probably the cheapest and most readily available of the recognised brand name linear rails, or ateast were in years gone by. There has been a big increase in availability of Chinese brands in recent years. Can't speak to quality / accuracy.

[Texasbowhunter]

Not to keep beating a dead horse but can we discuss ball screw length one more time?
If the ball screw is advertised as 1000mm does that mean the threaded portion to be 1000mm?
So, with the BK/BF mounts were attached then a 1000mm linear rail would sit inside...Right? theoretically
Thanks
Paul
Just attempting to ensure I order the correct length...

[pippin88]

That depends on the advertiser.

You should only buy from a specialist seller (not a store seller kitchen spatulas as well).
A decent store will make it very clear on the dimensions and/or will want a sketch from you.

Ball screw length consists of:
Overall length
Threaded / Screw portion
End machining

The usable travel = threaded length minus the length of the ball nut (and a bit extra - you do NOT want to run a ball nut off the screw accidentally).

Linear rail usable travel = rail length minus the bearing spacing (outside face of one bearing car to outside face of the other bearing car)
So this depends on your design because you choose the spacing.

[joeavaerage]

Hi,

If the ball screw is advertised as 1000mm does that mean the threaded portion to be 1000mm?

Usually no, 1000mm means 1000mm overall. Typically 200mm-300mm is taken up with the end machining leaving 700mm-800mm of thread.
The actual travel will be the length of the thread LESS the length of the ballnut LESS 10mm or so.....no need to run to the very end of the thread.
This is where a double nut screw may have less travel than a single nut design. Double nut screws are preferred because the two nuts can be very precisely ground
to produce exactly the amount of preload without having to worry about critical ballsizes etc, but a double nut might be 120mm long whereas a single nut might be
60mm long. Therefore the single nut design will have a little more travel than the double nut for the same length of thread.

Craig

[Texasbowhunter]

Thanks guys
Appreciate this info
Paul

[phomann]

Hi,
I’ve always been happy with BST Automation on AliExpress.

Fred is very responsive. I send him dxf drawings of the ballscrews including the machining of the ends and they machine them up.

BST AUTOMATION Store
https://a.aliexpress.com/_mNJ4dlw

Cheers
Peter


Sent from my iPhone using Tapatalk Pro

[joeavaerage]

Hi,

Linear rail usable travel = rail length minus the bearing spacing (outside face of one bearing car to outside face of the other bearing car)
So this depends on your design because you choose the spacing.

pippin is 100% correct, although I phrase it differently:
The length of the rail is the travel distance PLUS the distance (between centres) between the cars PLUS the length of one car PLUS 10mm so you don't run off the end.
maximizing the distance between the cars improves the stability of the axis, but also therefore requires longer rails and the machined surface to mount the rails to.
If you draw a force diagram there will be commonly one axis (at least) where the applied machining forces is as great or even exceeds the distance between cars, resulting
in instability of that axis and potential loss of accuracy.

The attached pic shows my Y axis. The cars are 200mm between centres and the travel is 350mm, the length of the cars is 80mm, and the rails 650.

With the Y axis centered the Z axis is right in the middle of the Y axis saddle, stable and accurate. Note that when the Y axis is at the extreme of the travel the Z axis
is now 'outside' the footprint formed by the cars, which will sort of cause the Y axis saddle to 'tumble'. This is not ideal, but there is a balance to be struck. The rails available
to me were 650mm long and I could not make the beds longer than 700mm or they would not fit in the heat treatment oven. Thus I had to make the best compromise
that I could. I wanted as much travel as I could, but did not want to introduce a situation where the axis was too unstable.

So when pippin says 'So this depends on your design because you choose the spacing.', he somewhat understates it. This is one of the most critical design decisions you'll
have to make.......space the cars widely for best stability but reduced travel or increased final size....OR....space the cars closer together for maximum travel for a given size but at the
expense of stability.

It is a very VERY good idea to draw some diagrams to establish which, if any, of your axes do this, and then make a well thought choice, maybe supported by calculation,
on what arrangement suits you best.

Craig