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View Full Version : Z-axis design - help sizing materials please!



Jonathan
20-08-2010, 07:56 PM
I'm trying to design a Z-axis for the CNC router I'm building to have 400mm travel. My current thinking is to use two of the 20mm linear rails from Zapp with two bearings on each rail...not the supported rails. I think the deflection of the rails themselves should be fine with this size?

The rails will be mounted to 15x160x600 alu plate so the bearings are 200mm apart and the rails spaced at 150mm (ish). I then intend to use another 15x160x600 aluminium plate to mount the router. This works out at over 50 in aluminium...ouch

Is this going to be strong enough? Using the 'gantry stiffnessv4' spreadsheet I've found on the forum and putting in these numbers I get a deflection of 0.58mm with 100N which is clearly unacceptable.

That's made me think instead of using aluminium plate for the router/bearing mount thingy I should use say an I section. I'm currently thinking of using 6x160x600 aluminium and fixing some 2"x0.5"x600mm 'strips' onto the edges of the aluminium plate to make an I cross section. I would mill a 6mm slot in the strips to fit the main sheet. According to the spreadsheet this reduces the deflection to 0.027mm which I think is fine....it's also a lot cheaper!

So, which option is the best - am I missing something here?

Thanks in advance!

irving2008
20-08-2010, 08:57 PM
For a wood router 100N is much to big... I take it this is the horizontal cutting force you are refering to... for general wood cutting 5 - 10N is more reasonable, 100N is more steel-bashing territory...

Jonathan
20-08-2010, 09:19 PM
I was intending to use this for milling aluminium, steel would be nice but I understand that might be pushing it. I choose the 100N figure as a worst case...
Sorry I should have put 'spindle' not router in the first post.

Jonathan
20-08-2010, 09:30 PM
Just done a quick model to clarify the plan:

2807

routercnc
20-08-2010, 10:45 PM
Hi Jonathan,

I posted that gantry spreadsheet so am pleased someone is trying it out. However, it is not configured to do the calc you are trying directly, unless you are moving things around and doing some extra calcs. Can you tell me a bit more about which of the above pieces you are modelling in the spreadsheet and how you are doing it? I think there are moments which are not being considered with your approach. Don't forget that the cutting is taking place some distance away from the Z axis which will lead to moments in 2 directions.

Maybe even post the spreadsheet with it set to your parameters (you need to zip it to attach it).

How far down is it from some reference point on this Z axis down to the cutting tool?

Are you doing the calcs on the rail, the I beam or the plate etc?

If you are really looking at 100N, then can I suggest using profile rails instead of unsupported rail. I think this will always be a limiting factor no matter what sections you use elsewhere. I bought a used pair of 15mm profile rails and carraiges, about 400mm long, from Ebay China for around 80 if memory serves.

Jonathan
20-08-2010, 11:02 PM
I have another look at profile rails. Is it possible to machine the standard hardened rail? I'm thinking if I drill holes into the rail at say 100mm intervals and use them to support it then that should be pretty much as good as the proper stuff? Certainly a lot better than nothing.
What about using one or two of these:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4485512
Perhaps I could use two mounted perpendicular to each other to better take care of the forces in both directions.

Those numbers I got with your spreadsheet are a bit of a bodge. I just entered 1200mm for the width (to simulatie the I section bit) and put a low value for the supports - I figured that'd be near enough reality! I was more trying to compare the different cross sections than get absolute figures for deflection. I think I'll modify the spreadsheet to better fit my needs. By the way thanks for making it!

I've tried using the spreadsheed for the 20mm rails aswell and figured that they would be fine.

Jonathan
20-08-2010, 11:04 PM
Argh, just set fire to the microwave whilst i was posting that!

routercnc
20-08-2010, 11:21 PM
Hi Jonathan,

Hope the microwave is OK - this CNC stuff can draw your concentration!

I still can't quite picture how you are using the spreadsheet, but if you are happy with your calcs then that's fine. If you have any more detailed dimensions and layouts then post them up and I'll try to help further if you like.

I briefly looked over those IGUS bearings. 21 and the low moment capability were concerns, but I'll study them a bit more and get back to you. Someone else might have used them and be able to comment? I'm a big fan of profile rail and over that short a span (400mm), and in say a 15mm size, are potentially affordable.

Drilling hardened rail - I'd worry about stress relief from the holes causing deflection of the nicely ground straight section. You'd have to switch to open bearing pillow blocks as well. Don't know if it's worth the effort, over buying the real thing. Anyone tried this?

Jonathan
21-08-2010, 12:06 AM
Luckily the microwave survived - some pretty impressive flames though!

So do you think the price of those rails implies that they are probably not up to much?

16mm supported rail and bearings works out at 93.60+VAT from Zapp compared with 45+VAT for the 20mm unsupported.

The open bearing pillow blocks are only a few more, so that's fine. I hadn't thought of stress in the rails. If it does work I reckon it's worth the effort to save about 40!

I'm currently dimensioning the drawing - I'll post it as soon as I'm done. But basically the 4 bearings are currently spaced as a square, 100mm between centres - hence the rails are also 100mm apart. The rails are 600mm long (the above prices are for 700mm just to be safe) which leaves a nice 400mm or so travel.
Thanks for the help so far - much appreciated!

Jonathan
21-08-2010, 12:23 AM
Here's the dimensioned drawing:
2810

Ross77
21-08-2010, 12:40 AM
Hi Johnathan

personally I wouldn't go for that length of travel on unsupported rail. The calculation you need is for a cantilevered beam, just you have it mounted vertically. As a rule of thumb I look at third points, so for a cantilever of 400mm you would need a 1200mm rail with the mounts 800mm apart. (not very feasible)

If this is a router then I guess this will then be mounted on a y axis gantry and needs moving as well so overall weight will be an issue. Is there a reason you need such a long z or could you just change the bed height

Jonathan
21-08-2010, 01:03 AM
Hi Johnathan

personally I wouldn't go for that length of travel on unsupported rail. The calculation you need is for a cantilevered beam, just you have it mounted vertically. As a rule of thumb I look at third points, so for a cantilever of 400mm you would need a 1200mm rail with the mounts 800mm apart. (not very feasible)

600mm is about as much as is practical allready. I read somewhere a rule of 2:1, i.e. if the bearings are 200mm apart then it's acceptable to have the force (of the cutter) 400mm away.



If this is a router then I guess this will then be mounted on a y axis gantry and needs moving as well so overall weight will be an issue. Is there a reason you need such a long z or could you just change the bed height

Yes it's a gantry router. I can think of things I'd like to make which need that long a Z, maybe not quite 400mm but close enough.
With regards to accelerating the mass of big Z axis/gantry I should be ok as on the X-axis I'm using two 3Nm steppers @ 70V for the and spinning the nuts, not the screws. The nut is currently held between two tapered roller bearings with a pulley in between.

Ross77
21-08-2010, 01:19 AM
600mm is about as much as is practical allready. I read somewhere a rule of 2:1, i.e. if the bearings are 200mm apart then it's acceptable to have the force (of the cutter) 400mm away.



As far as I know thats for friction bearings like the igus or dovetails like a mill/lathe. and is to stop/reduce binding, not for resisting the cutting force.....

The thing to remember is that all machine design is a compromise, and as such it will be designed for a specific task, if you need 400mm of travel then you will have to put up flex and vibration at certian points and reduce feeds and speeds accordingly. :smile:

The other point is that it will only be as good as the weakest bit....theres no point in have expensive powerful motors and drives and then using unsupported rail... it just dosnt make sense.:thumbdown:

I hope I dont seem negative or put you off but rather offered some pointers to another route.:smile:

John S
21-08-2010, 01:24 AM
Did you save the transformer out the microwave ?

Jonathan
21-08-2010, 01:32 AM
As far as I know thats for friction bearings like the igus or dovetails like a mill/lathe. and is to stop/reduce binding, not for resisting the cutting force.....

I've just checked the Igus datasheet and that is indeed the case.


The other point is that it will only be as good as the weakest bit....theres no point in have expensive powerful motors and drives and then using unsupported rail... it just dosnt make sense.:thumbdown:

I've already got the expensive powerful motors and drivers on my milling machine. I intending to use those motors on the router and put some 1 Nm ones I've got lying around on the router.


I hope I dont seem negative or put you off but rather offered some pointers to another route.:smile:
On the contrary - you're not putting me off, yet!

Jonathan
21-08-2010, 01:34 AM
Did you save the transformer out the microwave ?

The microwave itself didn't catch fire - sorry I exaggerated a little there. Still it was pretty big flames! So yes the microwave still works fine.
I do have a couple of microwave transformers. I did take the 2KV secondary out of one with the intention of rewinding it for my stepper motors, however I got a toroid cheap on eBay so I left it.

Ross77
21-08-2010, 01:43 AM
Did you save the transformer out the microwave ?


Do you want George to enter this thread????????? its been nice not seeing his name on every one:naughty:

Jonathan
21-08-2010, 01:48 AM
Based on a point load I get 0.07mm deflection with the two 20mm unsupported rails and the 150mm bearing spacing in the drawing. I'm not sure how to calculate the torsional deflection...this is getting a bit beyond what I've just done at school and I'm too tired to start reading up now so I'll leave it for tomorrow!


Do you want George to enter this thread????????? its been nice not seeing his name on every one:naughty:

I'm happy for anyone to make a valuable contribution :)

Ross77
21-08-2010, 02:05 AM
this is getting a bit beyond what I've just done at school and I'm too tired to start reading up now so I'll leave it for tomorrow!

Ditto. except I've had too much to drink, What point load and what end fixity are you using?

routercnc
21-08-2010, 09:25 AM
Hi Jonathan,

The drawing helped, thanks. Wow, alot of overhang.

You could design a lower 'floating bearing'. This is where the lowest bearing block has two or more alternative mounting positions on the Z axis part which holds the router. For long reach work you bolt it where it is now, and accept the deflection. For shorter reach work you bolt it further down, spacing the bearings out so that the loads are taken towards the ends of the unsupported beam, rather than towards the centre. You need a moveable limit switch with this idea.

Another thought, although it doesn't really work with your requirements, is to support the rail at the centre, as well as the ends. The bearing blocks then run in the 'upper part' above this centre support, and the 'lower part' below this centre support. Your rail deflection is instantly halved, but there's a compromise with Z range.

It looks like you are using an 'I' beam for the router mounting plate. You're only considering the bending in one or possible two directions, which will give you good results on the deflection for these load cases. But since the router or spindle will be mounted off this, and the cutting forces will be offset, there will be a twisting force (Y direction cutting) and this section choice will give you poor results, particular with that overhang. Overall it might be worse than the original plate, all loads considered.

For such an overhang, you might want to choose RHS for the part which attaches to the router/spindle. This is good for all the load cases you are requiring. Ideally any bolting into this section should go all way through with spacers inside to get the loads into the section, rather than locally deflecting the wall. The calcs all assume good load transfer.

As Ross has already suggested, I suspect you don't need to mill down 400mm into things, but rather you want to be able to machine to a depth of perhaps 30mm into things which are up to 400mm deep. If this is the case you could limit your Z travel to say 100mm. You then block up the workpiece with spacers to suit. This would be fairly stiff for all cuts, but give you options in terms of workpiece sizes.

Jonathan
21-08-2010, 12:20 PM
You could design a lower 'floating bearing'. This is where the lowest bearing block has two or more alternative mounting positions on the Z axis part which holds the router. For long reach work you bolt it where it is now, and accept the deflection. For shorter reach work you bolt it further down, spacing the bearings out so that the loads are taken towards the ends of the unsupported beam, rather than towards the centre. You need a moveable limit switch with this idea.

Yes! I was thinking that just hadn't got round to posting it...I'll allmost certainly do that. Maybe have 75mm or 400mm travel. Thinking about it I may as well have the lower bearing with several positions - it's only a few more holes after all.

With regards to supporting the rail at the centre I could do this when the Z range is limited, as above. It would make it take slightly longer to change between the two set-ups though.


It looks like you are using an 'I' beam for the router mounting plate. You're only considering the bending in one or possible two directions, which will give you good results on the deflection for these load cases. ...

For such an overhang, you might want to choose RHS for the part which attaches to the router/spindle... bolting into this section should go all way through with spacers inside to get the loads into the section, rather than locally deflecting the wall. The calcs all assume good load transfer.

Just to clarify I'm not using actual I beam, just 3 aluminium bits slotted and bolted together to make an I. I can't find RHS in the right dimensions so I guess I'd have to use a similar approach, just now with two 6mm aluminium plates instead of one. I'm concious that using RHS is going to further increase the horizontal distance from the router to the Y axis. Would it be ok to cut the front out of the RHS (to make a C shape) just where the spindle mounts. It's either that or get big enough RHS to fit the spindle inside?


As Ross has already suggested, I suspect you don't need to mill down 400mm into things, but rather you want to be able to machine to a depth of perhaps 30mm into things which are up to 400mm deep. If this is the case you could limit your Z travel to say 100mm. You then block up the workpiece with spacers to suit. This would be fairly stiff for all cuts, but give you options in terms of workpiece sizes.

Unfortunately that's not the case otherwise yes, I would just use blocks. One project I have in mind which needs a long Z axis is making wind turbine blades. That would, or course, be wood I should be OK.

Thanks for the help. I'll have another look for what steel RHS I can get hold of and modify the drawing accordingly.

irving2008
21-08-2010, 12:49 PM
Unfortunately that's not the case otherwise yes, I would just use blocks. One project I have in mind which needs a long Z axis is making wind turbine blades. That would, or course, be wood I should be OK.

Forgive my stupidity, by why do you need all that z-movement for a wind turbine blade? If you're thinking of cutting them vertically 'on the hub' so to speak I think you run the risk of, as a minimum, the blade deflecting, or worse snapping off... anything that high a length to width ratio probably needs milling horizontally and adequately supported... or have i got the wrong end of the stick (or should that be blade?) :lol:

Jonathan
21-08-2010, 12:57 PM
Forgive my stupidity, by why do you need all that z-movement for a wind turbine blade? If you're thinking of cutting them vertically 'on the hub' so to speak I think you run the risk of, as a minimum, the blade deflecting, or worse snapping off... anything that high a length to width ratio probably needs milling horizontally and adequately supported... or have i got the wrong end of the stick (or should that be blade?) :lol:

Yes you got the wrong end of the stick/blade there! :lol: I am intending to mill them horizontally. Near the hub, due to the twist on the blade, the Z distance is quite large. I'm intending to make a set of 3.2 meter (diameter) blades for a variable pitch wind turbine I've made. There are other things I want the big Z travel for, I just can't remember them at the moment!

I've found some 160x80mm steel box section. There's 5mm, 6.3 and 8mm wall thickness available. I think the 5mm should be ok. No idea how much it costs or if I can buy less than a 7.5 meter length....

http://www.steel-shop.co.uk/contentfiles/file/SteelShop%20Catalogue(1).pdf
(page 7 - I live very near to them but I've never actually been)

Robin Hewitt
21-08-2010, 01:04 PM
The rails are 600mm long (the above prices are for 700mm just to be safe) which leaves a nice 400mm or so travel.

Do you really need 40cm travel? If you have some specific task in mind that needs it, do it, if not you are seriously weakening the machine for no good reason.

If you are not stuck with 40cm, why not bring the gantry down a foot, then increase the separation on the pillow blocks so you can centre support the rail?

irving2008
21-08-2010, 01:35 PM
Yes you got the wrong end of the stick/blade there! :lol: I am intending to mill them horizontally. Near the hub, due to the twist on the blade, the Z distance is quite large. I'm intending to make a set of 3.2 meter (diameter) blades for a variable pitch wind turbine I've made. There are other things I want the big Z travel for, I just can't remember them at the moment!

I've found some 160x80mm steel box section. There's 5mm, 6.3 and 8mm wall thickness available. I think the 5mm should be ok. No idea how much it costs or if I can buy less than a 7.5 meter length....

http://www.steel-shop.co.uk/contentfiles/file/SteelShop%20Catalogue(1).pdf
(page 7 - I live very near to them but I've never actually been)
Ah ok i see now... thats some serious machining you're planning! what material will these be made of (I know you said wood, but what type?). So each blade is 1.6m long approx by what at the root? the aspect ratio of a blade like that would be maybe 12:1 or more so the biggest cross-dimension is 135mm but I dont see that needs a 400mm travel. What am I missing?

Jonathan
21-08-2010, 03:32 PM
Ah ok i see now... thats some serious machining you're planning! what material will these be made of (I know you said wood, but what type?). So each blade is 1.6m long approx by what at the root? the aspect ratio of a blade like that would be maybe 12:1 or more so the biggest cross-dimension is 135mm but I dont see that needs a 400mm travel. What am I missing?

Ash would be nice, maybe pine ... depends what I can get hold of really! Either way it's not a hard material, and anything I make which is a hard material won't be more than say 75mm tall so I think the making two of the bearings moveable and supporting in the middle is the way to go. Doing it that way would surely be sturdier than if I'd designed for 75mm in the first place - it will just take a bit longer to set up.

Here's one of my blade designs. This one was optimised for less Z axis travel but as you can see it's still quite a lot. The calculations show the root at an even steeper angle, and hence need a good Z travel. I agree this blade won't need 400mm, but I really don't want to be faffing trying to squeeze the blade into the available space!

2816
2815
2814
2813

The blade there is a 0.7m blade I made for a smaller with turbine. Plywood isn't so good...

Do you think the 160x80x5mm box section will be sufficient?

Jonathan
21-08-2010, 03:56 PM
I've edited the drawing to include holes to mount the bearings further down when I want only 75mm travel:
2818

The thing floating around is a support for the middle of the rail. I've designed it in two parts since that means I can fix it on without removing the bearings from the rails.

I've not put the box section in yet. It's just occured to me that mounting the bearings to box section sturdily is going to be a lot more difficult! Would fixing the bearings to just one side be ok, or should I put a spacer inside the box section and use a long bolt?

Ross77
21-08-2010, 07:32 PM
Having now seen it fit so well on the mill I think you would be better for off building a VMC, but optimised for wood, 400mm z axis on linear rails bolted to a column would be be no problem. The other way to deal with the large Z axis you need is to fit a 4th axis and then rotate the blade to the tool, I've seen furniture machinces work like this.

Are you going to be making a lot of these blades? if not it seems like you are building the machine to make a part but limiting other useage (hope that makes sense)

nice work so far, is that your workshop?

Jonathan
21-08-2010, 08:25 PM
Having now seen it fit so well on the mill I think you would be better for off building a VMC, but optimised for wood, 400mm z axis on linear rails bolted to a column would be be no problem. The other way to deal with the large Z axis you need is to fit a 4th axis and then rotate the blade to the tool, I've seen furniture machinces work like this.

Erm, it doesn't really fit so well on the mill. To do that 0.7m blade I had to do half of the length at a time on each side - so a total of 4 setups which is less than ideal since getting it aligned is a pain.

4th axis...yes! I had that in mind. It's one reason I want a large Z travel to be able to fit a 4th axis, either directly to the Z axis or onto the base.


Are you going to be making a lot of these blades?
Hopefully - I'm not sure. I would like to make them to sell however I'm not sure how much of a market there is.



is that your workshop?

Yep :smile: I've got that 'CLARKE CMD1225C' milling machine which I've converted to CNC, plus a Sieg C3 mini-lathe and pillar drill, bandsaw, wood lathe plus the big router we've been discussing all in a 20'x10' workshop! Space is limited to say the least. The workshop was originally intended for me and my dad, but I've kinda taken over a bit...

Here's my latest plan for the Z-axis:

2819

I've changed it so that the rails move and the bearing are stationary. This significantly reduces the weight / cost. The supports in the middle of the rail limit the travel to 75mm, so I'd remove them when I need the full travel. I guess I'll mount the stepper motor on the top of the slide - shame about the added moving mass there.
The aluminium plates in that drawing are 15mm. I'm thinking I could make the bigger one 20mm thick, to better resist torsion, then *if necessary* add side pieces like in the previous design.

So, do you think I should go ahead with this design, or spend another whole day designing :lol:?

Swarfing
21-08-2010, 08:44 PM
Jonathan
Have you considered to make the bed rails adjustable at each corner? this way you could keep the 'Z' shorter and do the machining in stages? It would make t machine very flexible for different uses.Spacers or a wind up/ down mechanism could be done fairly easily.

Jonathan
21-08-2010, 08:59 PM
I had considered making it so that I can bolt the Z axis to the Y axis at different heights which, if I understand you correctly, would have the same effect as your idea. I don't like this idea so much since it means I've got to zero the axis to the part multiple times. From my experience with the smaller blades this is definitely not something I want to be doing.


Earlier in this thread I asked about drilling holes in the standard round 20mm rail to enable me to add supports. 'routercnc' said that this could stress the rails causing them to be deformed. Is this view shared by others? Not to say routercnc is wrong, I just want to be sure that this option isn't viable before forgetting it. If I used the adjustable bearings then any slight imperfection shouldn't be too big an issue?

routercnc
21-08-2010, 10:44 PM
Hi Jonathan,

Great project you have there, interesting stuff.

There's no easy way to commit to an own design, and you'll no doubt end up tweaking whatever you make anyway. Making these CNC machines is a mix of previous experience, guesswork, copying parts of existing designs, understanding forces, making calculations, and fabrication skill. You've obviously thought about the problem, the risks, and the compromises, so it's probably time to give it a go.

My concern on drilling the hardened rails is that I don't know if they are stress relieved as part of the hardening process. If not then drilling a hole may allow the stresses contained inside to distort the rail. The bearings won't help you to 'make it straight again' between the span. But I'm really interested in a second opinion on this as well. I'm going to post this question seperately to put it to a wider audience.

Jonathan
21-08-2010, 11:18 PM
There's no easy way to commit to an own design, and you'll no doubt end up tweaking whatever you make anyway. Making these CNC machines is a mix of previous experience, guesswork, copying parts of existing designs, understanding forces, making calculations, and fabrication skill. You've obviously thought about the problem, the risks, and the compromises, so it's probably time to give it a go.

Yes I tend to agree. I think I'll wait a bit to see the what the conclusion is on drilling these rails first. Is there any loss in buying the rails and open bearings and if the drilling doesn't work still using the open bearings unsupported?

If I do go down this route which bearings are a better bet...the adjustable or fixed ones?

I could always support the rails underneath without actually fixing them (hmm loctite!?). That would at least stop them deflecting in one direction.

FatFreddie
22-08-2010, 01:25 AM
Here's my latest plan for the Z-axis:


How about putting the bearings either side of the centre rail support? That way you increase stiffness but still get reasonable travel.

Jonathan
22-08-2010, 01:39 AM
How about putting the bearings either side of the centre rail support? That way you increase stiffness but still get reasonable travel.

I think that would increase the length of the rails too much. Either way I've just found this on eBay, don't know how I missed it before:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=250493259973&_trksid=p2759.l1259

I've emailed the seller to list 600mm - that should be cheap enough :)

Ross77
22-08-2010, 02:53 AM
The postage is the same as the product..........120 would get you a nice linear rail system from gary at zapp or if you are patient then wait for S/H linear rails not unsupported round rail crap for that price....

routercnc
22-08-2010, 10:25 AM
I bought my 300mm Z axis profile rails, and my 700mm Y axis profile rails from Korea. FA system:
http://stores.ebay.co.uk/FA-SYSTEM_LM-system_W0QQ_fsubZ275192619QQ_sidZ834395939QQ_trksi dZp4634Q2ec0Q2em322
And very pleased with them. Wouldn't contemplate buying these things new because of the price. Remember to allow for import tax, handling etc. and a 2-3week wait if you go down this route. I got lucky on the 700mm rails (import wise), but the 300mm rails cost around 20 for import etc.

If you go with open bearings on an unsupported rail (or supported) remember that under certain load conditions they will 'open up' to some degree. Thanks to Ross for pointing out that obvious fact to me some time ago! There's alot to think about, or blissfully ignore!

Swarfing
22-08-2010, 12:05 PM
Jonathan

I used unsupported rail on my build but it is 30mm in diameter, i wanted to go for the truck type and profile but cost was a factor. If i could i would have gone for this option. At some point i will change it to accommodate it but not now. My advise is if you can stretch go for the right thing in the first place. It will save in the long run. I have another use for my parts at a later date so that is a saving for me and my design allows for that change as an upgrade. IT could cost you double by skimping because if it does not work out then those parts are wasted?

Jonathan
22-08-2010, 12:19 PM
The postage is the same as the product..........120 would get you a nice linear rail system from gary at zapp or if you are patient then wait for S/H linear rails not supported round rail crap for that price....

Is gary going to stock NSK SH rails then or have I misinterpereted the 'S/H'?

I sent a message to the seller and got this:



Dear Jonathan,
2 SBR16-600mm supported rails +4 blocks:
92 usd
Air shipping cost is included.
===============
2 SBR20-600mm supported rails +4 blocks
109 usd
Air shippping cost is included.
Thanks!
Chai


So that's 70 for the 20mm or 60 for the 16mm. I may as well get the 20mm for the sake of 10. Are you saying these rails wont be any good quality? They're the same part numbers and dimensions as the ones at Zapp...they look the same as far as I can tell.

I've just looked at the 'HGR20' profile rail from Zapp. It comes to 129 for 2x600mm rails and only two blocks. I simply cannot afford to spend that much on rails!



I bought my 300mm Z axis profile rails, and my 700mm Y axis profile rails from Korea. FA system:
http://stores.ebay.co.uk/FA-SYSTEM_L...34Q2ec0Q2em322
And very pleased with them. Wouldn't contemplate buying these things new because of the price.

There's a good set of 590mm listed, but still 135 plus the tax is too much. I don't wish to sound as if I'm neglecting all your good ideas purely due to cost, it's just even spending 70 on the Z-axis is a bit much for me but I can see that it's worth it to get supported rail!



If you go with open bearings on an unsupported rail (or supported) remember that under certain load conditions they will 'open up' to some degree. Thanks to Ross for pointing out that obvious fact to me some time ago! There's alot to think about, or blissfully ignore!

I see, thanks for pointing that out to me! I'm still interested in the drilling option but it hardly seems worth it for the Z-axis now.

If I decided to use the supported round rail for the Y axis (or just round rail supported with the holes etc) is 16mm going to be sufficient or should I go for 20mm?

Also, how to I work out the optimal spacing for the rails on the Y axis? Is the torque from the Y screw a significant factor here since that would suggest I should put the rails close together?

Thanks again for all the advice - there certainly is a lot to think about!!

Jonathan
22-08-2010, 12:38 PM
2e0poz

Yes I'm in the same situation with regards to cost. The profile rails are roughly twice as much as the 20mm supported rails.
Surely the supported rails mounted on a 15mm are not going to bend much at all? I'm well within the 860N load rating of a single bearing.

The Z-axis assembly will weigh roughly 13kg.

Jonathan
22-08-2010, 01:53 PM
Dear Jonathan,
The best price to you:

2 SBR20-600mm supported rails +4 blocks with
2 SBR20-900mm supported rails +4 blocks
238 usd
air shipping cost is included.
If you accept this best price to you,please let me know your paypal email. I will send the paypal invoice to you.
Thanks!
Chai

Hmm very tempting - the best price too :lol:

ecat
22-08-2010, 02:44 PM
Cost is the killer and it all comes down to where are you willing to make the compromise. From what I've picked up so far you chose your target tolerance and design so the inherent flex etc of the machine is better than 1/10 of you target. So, for 0.1mm tolerance you design for better than 0.01mm deflection, or the other way around, if you suspect an assembly will flex by 0.1mm then you should expect no better than 1mm tolerance.

Unsupported rail will flex but the bearings should be consistent in all directions (?). Is the flex significant to you?

Supported rail suffers from much less flexing but what about the opening in the base of the bearing? How much extra movement does this allow?

Profiled rail + blocks cost a lot more than the equivalent round rail set up but minimises the issues with both unsupported rail and open blocks and does so in a smaller footprint. Are the benefits worth while and the cost acceptable for your design?

Second hand parts? Just how second hand are and why were they removed? What would the cost be of replacing a single worn part? The true cost of second hand = price + testing + reassembly + replacement_part_costs - how_much_you_enjoy_the_process

It wouldn't be so bad if there where a cheap and easy upgrade path but that's not the way the critical parts are designed. Going from the 70 round rail set up to a 200 profiled set up involves changing all the widths and lengths and offsets and leaves you with a pile of used parts to sell or the idea that a second machine could be built someday.

Your compromise, your budget, your choice :)

Jonathan
22-08-2010, 03:13 PM
...0.1mm then you should expect no better than 1mm tolerance.

Maybe I've got to the stage where I should just try it and see and just accept that I may have to compromise on cutting depths to reduce the cutting forces.



Supported rail suffers from much less flexing but what about the opening in the base of the bearing? How much extra movement does this allow?

That's a good point. Looking at the drawing for the bearing blocks if the force is in the 'worst' direction with the supported rails then it's only supported on 2 of the little ball bearings compared to 4 on the unsupported ones.

With this in mind maybe I should mount the supported rails 'back to back' on the Z-axis aswell as the Y.

Second hand: true, you never know.


Your compromise, your budget, your choice :)

Indeed

I think I'll go for the 600 and 900 supported rails for 155. That's already much more than I was intending to spend and a much better design.
Has anyone here bought from that seller / can confirm that they're good?

Swarfing
22-08-2010, 06:18 PM
Well i have just been out in the workshop to finish off the 'Z' of my small desktop machine and screwed it all up because i was interrupted. I cast and machined all the plate i needed, cut it all drilled and milled only to realise all the planes are wrong because i needed to stop and re-adjust my mill head. A point in question relevant here. You can spend a lot of time and effort to make as much as you can but not always effective. I will give up and now buy the appropriate parts. The cost of the time (which is the most precious cost) is what hurts most. The profile will cost more in the end but will make the job easier and quicker to get done. Better to wait a few weeks more and get it right than be in a hurry and see the cost mount up. Just think the time you have in the middle can be used else where?

routercnc
22-08-2010, 09:23 PM
I think I'll go for the 600 and 900 supported rails for 155. That's already much more than I was intending to spend and a much better design.
Has anyone here bought from that seller / can confirm that they're good?

I haven't bought supported rails from Chai, but I have bought a water cooled spindle and some ballscrew support blocks (BK12) from him and can vouch for his good communication and good service. One of the square nuts in the support block set had a poor thread, but a quick email to him had one on it's way over from China. Remember to allow for import tax, duty, handling etc. in case Royal Mail search your parcel, but even with these the price is still tempting.

Jonathan
22-08-2010, 09:26 PM
I haven't bought supported rails from Chai, but I have bought a water cooled spindle and some ballscrew support blocks (BK12) from him and can vouch for his good communication and good service. One of the square nuts in the support block set had a poor thread, but a quick email to him had one on it's way over from China. Remember to allow for import tax, duty, handling etc. in case Royal Mail search your parcel, but even with these the price is still tempting.

That sounds good. I did a search on his feedback for 'quality' and found lots of praise and only the odd bad one.
Do you think tax is likely? Did you get charged tax?
He's now listed the rails for me :)

phill05
22-08-2010, 09:41 PM
. The cost of the time (which is the most precious cost) is what hurts most.

Look at the bigger picture, look at what you have learnt this time, next time you should not do it wrong.

On another note I built a z axis a few weeks ago, after reading through all the different information on here and other forums I decided on 20mm x 340mm profiled rails fixed to back plate and carriages moving, just got it all powered up and I don't see any deflection/movement whatsoever, I am getting a perfect cut finish and this is on alley.
The rails and carriages were bought off e-bay as used and have no problems with them.

Phill

routercnc
22-08-2010, 09:49 PM
Did you get charged tax?


Listing my various imports:
Spindle no, bearings no, one set of profile rails no, one set of profile rails yes.

So possibly not, but allow for it.

Jonathan
22-08-2010, 09:52 PM
On another note I built a z axis a few weeks ago, after reading through all the different information on here and other forums I decided on 20mm x 340mm profiled rails fixed to back plate and carriages moving, just got it all powered up and I don't see any deflection/movement whatsoever, I am getting a perfect cut finish and this is on alley.
The rails and carriages were bought off e-bay as used and have no problems with them.
Phill

Ok you've convinced me that this is the way to go - thanks! I'll buy them asap. Have you got a picture of your Z axis as I'd be interested to see?

2e0poz:
I had a similar experience. I started out by trying to make the Z axis with plywood and the bearings on aluminium angle method. Oops. It was horribly bendy...still as phil05 says, I've learnt not to try that again and I've not really lost much. One good point is the backlash was <0.005mm!

Jonathan
22-08-2010, 09:54 PM
Listing my various imports:
Spindle no, bearings no, one set of profile rails no, one set of profile rails yes.

So possibly not, but allow for it.

How much were the rails that you got charged for? Customs only change if it's above 18 if I recall correctly, so it depends to what extent the seller fakes the value on the form since alas, no rail is that cheap!

phill05
22-08-2010, 09:54 PM
Just landed on e-bay 150483471899

Phill

Jonathan
22-08-2010, 10:02 PM
Those are tempting. I bet they will go up in price a lot though!
Would only one carriage per rail be sufficient for my application?

phill05
22-08-2010, 10:10 PM
As I said I have no problems with the single carriage per rail solid as a rock. I bought two sets of 340mm two rails & four carriages for just over 100 plus postage have a look here http://www.mycncuk.com/forums/showthread.php?2069-Re-build, will get some more images soon of it all together cutting metal.

Phill