Perfect combo of rails and ball screw

Hello

So I've a question for all you clever fellows here:

As you can see from the two attached screenshots taken from my Solidworks drawing assembly showing the X and Y beds, rails and ballscrew, I've always have to lift the rails from the bed to match up the height of the mount for the ballscrew, her with a 16 mm screw. Have any of you come over the perfect combo of rails and ball screw that eliminates this problem? I could of course put a spacer on top of the carriage,but rather not.

Terje

Attachment 483173
Attachment 483175

Are you saying that the ball screw nut is too wide to fit in there? Or are you concerned about the timing belt pulley? If it's the latter, it doesn't need to be in between the bed and base - it can mount outboard from there. If it's the former, you can either shop for a ball screw with a slimmer nut, carve down the nut flange a little, make a groove in the base plate for the nut to run in, or use spacers to elevate the trucks.

What size rails have you modelled?

Often people use bigger rails (than required for load)

Rather than raising the whole rail, you can just have spacers on top of the bearing cars

Yes the ballnut is to high and also both the end mounts, they are actually precise 30mm to high, so a groove want do it.
My rails are 23mm high, so probably have to buy a bigger rail and a smaller nut, but bigger rails are expensive.
I see that I can probably reset the end mounts 10mm down into the bottom plate and the same with the nut in the top plate, så that mean that I will have to shim the carriages with 10 mm.

I would think that I need at least a 16mm ball screw on a 800mm long rail.

I'd spend a few days thinking about the reasons that dictate the location of each and every component.Asking why every aspect of their location is constrained and thinking about what might be possible if some of the clearances were reduced and also how the machine is to be serviced and assembled.For example,if there were a slot completely through the base plate for the ballnut to run in-would that make it possible to get the bolts beneath the table to allow the table to be slid off the ballnut?With the ballnut in the centre of the table there could still be a substantial linking section at either end of the baseplate and the whole assembly could be reduced in height.The typical ballnut flange may have six holes for bolts,but do you have to use all of them,or can part of the flange be cut away?Question everything and create arguments for the alternatives before honestly assessing the merits of all of the choices.

Hi Routalot - I too have contemplated this issue. It surprises me that the bearing manufacturers do not make a matched series as this would make sense. I too despise spacers and seek a minimal parts count. So 1) spacers on the carriages or a well in the foundation is the solution if the same "plane" of part is required... Peter

Thanks both out you.

First I’m not sure how cutting a slit true my plate from end to end for the ball nut will affect the stiffness and twisting of my plate, I don’t think it will be positive, may-bee if I could splice the slit together from the underside with som metal straps, but I would not trust it to be flat anymore.

Second I can not agree more with Peteeng, surly the companies like THT, Hiwin, Misumi and so on must have had some taught’s about this as they sell all five parts in this setup, rails, carries, ball screw, ball nut and end supports.

I will send them all an email and ask what there’s take on this is.
Terje

The thing you're trying to build looks a lot like an automation actuator. These have ball screws, linear rails, and ball bearing trucks all mounted in a rigid enclosure. The companies that make them, like NSK and Parker-Daedal, have worked out all the issues you're worrying about. Here's an example on Ebay: https://www.ebay.com/itm/273100398220 I've built a CNC router by mounting some of them in a steel frame; it was a lot easier than starting from scratch.

Thanks, I’ll look at those

Hi,
when I made my first mini-mill I encountered the exact problem you describe. I use the phrase 'difference in stack height between ballscrew assembly and linear rail/car assembly.'

I went to the scrap yard and got some cast iron elevator weights, they are about 1000mm x 210mm x 125mm, and cost $90NZD ($60USD) each. I then milled the required axis beds
on the Bridgeport clone we had at work. The cast iron of the weights was pretty poor, after all they only have to be heavy, but none the less I ended up with some nice beds.

My new build mill, about a year old now and is much bigger. The difference in stack height is 60mm. I'm using 32mm BNFN THK C5 ballscrews and HSR20 THK linear rails/.cars. I got the axis beds cast in grey iron.
They are 700mm x 240mm x 110mm and weigh 115kg each. In fact they were cast a little larger and machined down to size. Not a cheap excerise, but then again they are the heart of the machine.

I think you just may have to consider either casting axis beds, OR milling a channel in a 50-60mm slab of steel. Around here laser cut steel works out about $10NZD/ kg cut, not too bad.

Craig

Yes you are probably right, but not som many scrap yards here in Norway, they run a tight scheme on recycling, and they done away with every do-it yourself places/workshops in the big cites, and as a layman one can hardly bye any steel by the metre here, so sad.

I've seen some cast iron surface plates on ebay which I have asked about, so maybe I can use them as tables.
I've also had this crazy idea of using a Siegfried welding table as the base for the build, but it will never be cost effective as they start at $2000, but it would be fun then.

I've also sent emails to four of the major companies that deals in automations product, so let's wait and see what's will come back.

So with the base in a Siegmund 1200x800 mm welding table and some genuine Siegmund parts, box 200x100x100 and 1000x200x100mm, all steel S355J2+N 13 mm thickness.
The table weight is 169 kg, with 16mm holes in a grid of 50mm
Bolt it all together with some strong 16mm bolts and some loctite and you should have a good base.

But then it was the pricetag!

Hi,
the problem with a surface plate, be it cast iron or granite still requires a groove machined down the middle of it.

Norway is a super advanced country with a huge oil industry....and your trying to sell me the idea that there are no steel suppliers, foundries
and machine shops? I don't buy it.

Try emailing some of the profile cutting companies in Norway, I found five with a Google search, and find out just how cheap a piece of steel say 250 x 500 x60
can be.

Craig

If you can locate a machine shop within driving range I think they'll cover what you need. But if you don't want to order a 6m/12m piece of steel you might need to adjust your plan to the dimension they have in stock.

But you need to decide of you are source parts that are flat/square suitable right away or if you need machining before assembly.

Look at other threads for design inspiration..

https://www.google.com/search?q=weld...obile&ie=UTF-8

Thanks, a good tread