My 2nd Build. Some advice please

[dudz]

Ok. Already had great success with my homebuilt cnc/router for milling billet. Now I have moved to NZ and cant get my machine for a fair while so planning on building another. Same Spec just bigger table, X and Y axis.

Planning on a 650x650x150 cutting area

This time I plan to use Hiwin linear rail instead of 25mm supported round rail. What size Linear should I be going with ? The bigger the better I suppose for rigidity ?

I have been looking at ready made Z axis :

https://www.aliexpress.com/item/4000...7%237558%23136

Are these any good ? thought it might save me some time and money to buy it already assembled. Might do this for the Y axis also (If they do the length).
Or are there better suppliers that anyone know of please ?

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[peteeng]

Hi Did - For most machines 20mm will be fine. If its a mill you should use high preload cars. 25mm has the advantage of being higher and will closer match the ballscrew bearing height. The supported rails and cars are a bit higher then the sq rails and cars for same size rail. Are you going with the AL extrusion construction? Peter

[dudz]

Thanks. Yes alu extrusion again. Its easy that way. You would have to give me a link to a product (if you would be so kind) that describes it visually. Not sure on the quality of this Chinese ready built stuff.

[peteeng]

Look up BST motion - https://bstmotion.aliexpress.com/store/314742 they have a house brand and Hiwin. I like the way you have built the machine in a box very stiff. You do need to specify "high preload" for the cars. Normally they are supplied in "zero preload" which will get small wobbles as they wear over time especially on a mill.

The rail stiffness is a function of its foundation more then the rail itself. So you will only be as stiff as the extrusion. You have taken advantage of the supported rail in your design, with square rail you will need something under it. If you keep to your first concept then round is the go... at least on the table rails...

You also have long bearings on the bottom of the Z axis rails. That was a very good move as these are the highest loaded bearings in that type of machine as they take the impact loads from the tool. In square rail you can get long bearings but using high preload is the go to resist the tool loads better. Peter

[dudz]

Look up BST motion - https://bstmotion.aliexpress.com/store/314742 they have a house brand and Hiwin. I like the way you have built the machine in a box very stiff. You do need to specify "high preload" for the cars. Normally they are supplied in "zero preload" which will get small wobbles as they wear over time especially on a mill.

The rail stiffness is a function of its foundation more then the rail itself. So you will only be as stiff as the extrusion. You have taken advantage of the supported rail in your design, with square rail you will need something under it. If you keep to your first concept then round is the go... at least on the table rails...

You also have long bearings on the bottom of the Z axis rails. That was a very good move as these are the highest loaded bearings in that type of machine as they take the impact loads from the tool. In square rail you can get long bearings but using high preload is the go to resist the tool loads better. Peter

Thats great Peter thanks... They dont seem to have the built units. So the using 25mm rails do I need to put shims on the Ballscrew nut to get the correct height ?. I know I did with 25mm round supported rail

If I put a 20mm slab of billet under the table on top of the alu profile then I could mount the linear rail on top ? or would 25mm round be as strong ?

[joeavaerage]

Hi,
I used 15mm rails on my mini-mill which have proven to be entirely adequate but they are bolted down to solid
cast iron beds. I agree with Peter, its more about what you screw the rail to than the size of the rail.

My new build mill will use 20mm THK new old stock rails on VERY HEAVY SECTION grey cast iron beds.
The rigidity of the beds and frame are more important than the rails.

Craig

[peteeng]

Hi Dud - This depends on the size of the support bearings you select. Which depends on the size of screw you select. You could use 12, 16 or 20mm screws?? The screw size depends on buckling resistance (load and length) and whirl resistance (length and speed)....Each bearing ht is different. Seems silly that the support bearing people don't talk to the linear guys and create a joint bearing series where the ht matches. Peter

[joeavaerage]

Hi,
I have soucred parts for my original mini-mill and my new build mill, both second hand and new old stock.
Well worth looking at to save budget for servos/frame/beds/electronics/spindle etc.

https://www.ebay.com/str/industrialpartsshop

Craig

PS Where abouts in NZ, I'm Chistchurch based.

Craig

[dudz]

Hi,
I have soucred parts for my original mini-mill and my new build mill, both second hand and new old stock.
Well worth looking at to save budget for servos/frame/beds/electronics/spindle etc.

https://www.ebay.com/str/industrialpartsshop

Craig

PS Where abouts in NZ, I'm Chistchurch based.

Craig

Dunedin Craig.

[dudz]

I might as well go with 25mm round supported rail then like I did on my first machine, and just build a better stronger support for the sliding table rails. This is cheaper. My first machine gave great results, just a little run-out of accuracy on the edges of the table.but i think it would have been better if I used 2x balscrews, in parallel and next to each rail rather than one in the middle ?

[joeavaerage]

Hi,

I might as well go with 25mm round supported rail then like I did on my first machine,

No! They are not a patch on square rails, they are cheaper but that is the only thing to reccommend them. Even the aluminum pedestal
on which the round rail sits is about as stiff as cooked spagetti.

My comment that rigidity being about the frame/bed on which the rails are mounted does not mean that you can use round
rails, after whats the point in spending all that time and money on a rigid frame/bed only then to thow it all away with round rails?

Craig

[dudz]

Hi,



No! They are not a patch on square rails, they are cheaper but that is the only thing to reccommend them. Even the aluminum pedestal
on which the round rail sits is about as stiff as cooked spagetti.

My comment that rigidity being about the frame/bed on which the rails are mounted does not mean that you can use round
rails, after whats the point in spending all that time and money on a rigid frame/bed only then to thow it all away with round rails?

Craig

Gotcha. Its been 5yrs since I built the first one. I need to get my head around what ballscrew matches which linear rail on height etc... Is there a table somewhere that show this ?

[joeavaerage]

Hi,
to my knowledge there is no such table and the stack height of linear rails and cars is invariably different to the clearance height
of any ballscrew, the difference in height must be accomodated by the mill bed.

Craig.

[peteeng]

Hi Dud - BST have drawings on their site for rails, bearings and support bearings. If there's not a dwg send them a note and they promptly send you one. The stack ht is a fiddle and machine builders deal with it in various ways. Spacers and milled grooves are the usual... Do you have CAD? This is the best way to sort your design.

Round rails have balls rolling on a concave surface (concave to concave contact), generally along the same path. This means they fatigue faster then square rails as the groove in a square rail is convex producing less surface stress. So a round rail has less capacity for similar size due to this effect. Also round rails can rotate on the shaft which in some applications is good. But for a machine tool the square rail is very good as it can support moments in any direction. And due to the better distributed contact they are statically stronger and have a longer life. Their standard life is usually 50km at full load (from memory)

You just have to correct the known issues with your prior machine and the next machine will be betterer. Each machine should be better then the last one

cheers Peter

[ger21]

I might as well go with 25mm round supported rail then like I did on my first machine, and just build a better stronger support for the sliding table rails.

Profile rails are at least 5x stronger, and far more rigid. There's no comparison between them and round supported rails.
There's a reason they are not used on ANY real machines, only lightweight hobby machines.

I need to get my head around what ballscrew matches which linear rail on height etc.

Machine components are not designed like an Erector set, where everything bolts together. Some machining will likely be required, or additional spacers and plates.

[dudz]

Thanks Pete for the detailed explanation. I remember after I drilled all the table holes to fit the ballscrew nut and rail bearings, that I thought it odd that I had to make a spacer. . Now I know its the norm. Yes I have CAD,CAM and Mach3.

Indeed, I intend to make this one better. Just gutted that I haven't got my other machine to mark out all the holes for this machine.

[peteeng]

Hi All - I think that round and square rails have their place. Its a cost to performance choice. Dud's first machine I think is an excellent example of getting the best out of the bits he used. The table stiffness is a bit down but that can be fixed. A good machine has a selection of balanced cost/performance & commensurate parts to its application. No value having precision expensive parts on a low tolerance bed etc. So the Maker has to work through all of those variables and end up with a machine. That's one reason its so interesting here to see so many different solution's to basically the same problem. Peter