View Full Version : A couple of general design questions - ball-screw thread length, stub-shaft etc.

04-07-2013, 08:42 PM
I have several questions that I need to ask and a few of 'em may be pretty elementary, but some of the details are holding me back and I could really do with clearing them up before I start buying potentially expensive paper-weights...

I'm about to build a couple of axis akin to this example:


...but I'll be using TBR20 supported rail.

(I'm aware that I'll be spending money on motor-mounts/ballscrew brackets etc. that many of you could knock up in a matter of minutes on your gear - but I have no such tools to hand and would rather not make a pain in the arse of myself asking for favours/to-ing and fro-ing with dimensions, so I'm just gonna bite the bullet, buy the hardware I need and design the axis based on the available measurements)

So, the questions:

1. When you design your machines, do you buy your ball-screws at a ball-park length and then design the machine around that, or do you work everything out in advance and then make the purchase?

In this instance I have done the latter and am pondering this -

If I know that the distance between my floating and my fixed bearings is 400mm - do I specify my ball-screw thread at 400mm + the machined ends - or would I go for slightly under (say 399mm) just to be sure that I'm not 'squeezing it in' between my bearings?

2. End machining of the ball-screw. This looks reasonably straightforward to specify, but how long would you spec the 'F' length (stub-shaft) as a rule? Is standard + an extra 30mm overkill? Would standard give me enough length to reach the coupling housed in an MBA12C motor-mount like this one?


3. TBR20 supported rail. Am I right in thinking that the mounting holes are drilled 20cm apart with the first hole 10cm from the starting end - ie. mounting holes on a one metre length rail would be drilled at


How accurately are these mounting holes drilled (from the end) and what happens when a rail length such as 49cm is used - are you left with 19cm of unfixed rail at one end?

I'm sure that there are other questions I've omitted here, but this is a start for now!

Thanks for any help/advice.


01-01-2015, 11:15 PM
I'm absolutely no expert on this but I'm fitting my ball-screws at the moment so I'm sharing the LITTLE bit of experience I have.

1) I went the other way round and ordered it ball park and then worked around that but I think your way is better. I might stand corrected on this but there's normally a little circlip on the floating end of the screw that serves little purpose in my mind. If that is not used you do have a few mm to play with so rather err on the short side, the shaft will just sit a bit shallower in the floating bearing which I believe is not a problem.

2) I went for the standard machining and the bit that sticks out on the fixed end after the lock nut is in place is a bit short for my liking. I'm not familiar with that motor housing so I have no idea about the correct F length. Keep in mind that with most of these flexible couplings you do have a few mm to work with both on the motor and shaft side so you will likely be ok if you just work it out as best you can and order it that length. Assuming its 12 or 16mm dia skrew and you really want to play is super safe then go a bit on the long side and cut any excess off with an angle grinder or even a dremel tool with one of those clip-on cutting disks will cut it off in no time. It does not have to be cut off perfectly square.

3) I've never used the supported rail so I can't be of any help there but I think it might be risky to make assumptions around where those hole are on non standard or even standard lengths. These rail type products are often made in longer lengths an then the reseller cut them to your requirements so a lot depends on the guy who's doing the cutting :-) Those holes do seem quite far apart so why not just drill your own holes where you need them?

02-01-2015, 12:14 AM
I might stand corrected on this but there's normally a little circlip on the floating end of the screw that serves little purpose in my mind. If that is not used you do have a few mm to play with so rather err on the short side, the shaft will just sit a bit shallower in the floating bearing which I believe is not a problem.

Yes you are being corrected I'm afraid and the little circlip is very important. The floating end is just that floating so the bearing can be any where inside the block but the circlip stops the bearing falling off the shaft. And it does or will over time if you don't have the clip.


#1 Depends on your designing and drawing skills but me I always prefer to draw it accurately in Cad then have no surprises. This means I can safely order to size.

#2 At this short length then it won't be much of an issue but I'd keep it just long enough for my needs plus 2-3mm more. Normally I have mine machined to 30mm. BUT using the MBA12C I think you need standard machining other wise the coupler won't fit between motor and screw.? Thou like been said you can easily whip it off with cutting disc if required.

#3 Best thing to do with TBR20 is don't bother as they are rubbish.!! . . .There tends to be no accurecy to any of them whether that be the hole locations or the machining accurecy of the base.!
Again thou like been said if you must use them just drill your own holes.!

02-01-2015, 02:33 AM
Hi Mitch,

Cheers for your post - you'll be happy to know that my mill was built just under a year ago now (http://www.mycncuk.com/threads/7001-How-much-deflection-to-expect?p=53492#post53492)..!

Forgot I posted this topic TBH, but since it's been bumped back up I have a few observations of my own to add.

I fretted for a while about measurements, how precise tolerances needed to be etc. - it got to the point that it stopped the project going forward because I was over-thinking how things would piece together. Now that I've been through the process I can see that super-exact measurements aren't necessarily mandatory - sure getting things as close as you can should be your goal, but once all the bits are to hand you soon realise that the motor-shaft that fits into the coupler will have a bit of slide-room, as will the machined end of the screw - likewise, that floating bearing at the other end has plenty of room to slide around inside the support block. What I'm saying to the casual reader of this who might be worrying that things aren't going to fit if they're 0.1mm out, is design it knowing that there will be a bit of wiggle room, get your hardware machined according to how you've designed it to fit together, but rest assured that there's a bit of room for manoeuvre (which is also something you should aim to build in to your design too.)

Jazz warned me off the TBR20's a while ago, and on future builds I'd certainly go with profile rail, but you know, so far they've been alright - yep, the bearings need a bit of adjustment from time to time, but hey, twice a year ain't bad. To answer my own question - the TBRs came un-drilled - Clive S kindly drilled mine out for me on his Warco. Once again, worries about how far apart holes were spaced along the length proved meaningless - I spaced them sensibly on the rail mounts and then placed the holes on my alu plates accordingly. The clearance holes can be up to a mm bigger to give you that extra bit of wiggle room when lining stuff up later.

Cheers, and a happy New Year!


02-01-2015, 07:32 AM
@Wal, glad you got it sorted and thanks same to you.

@Jazz, thanks, very glad now I ventured an opinion. My floating bearings seems to have a almost like a bit of taper and it looks like the bearing can only be pressed out to one side which is why I made the assumption. Like they say: Assumption is the mother of all f%-ups. Did not figure out that it may wear over time and the bearing might float right thru the housing. I can see that will be a very bad thing so I'm going to fit the circlips! Like I said mine appears to have a bit of taper so does it matter if the bearing is pushed to the side where its sitting a bit loose in the housing? Also, on two of my floating bearing housings, the bearings sits very tight, to the point where the have to tapped out, should I make those holes a smidgion larger so that the bearing can actually "float" in the housing instead of the shaft floating inside the bearing?