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2 Attachment(s)
Rotating Ballnut - design ideas
I've been thinking about how to attach bearings and a timing pulley to a ballnut such that it can be rotated with a stepper motor, instead of rotating the screw.
I've already done this on my router with an M20 screw since the long X-axis means that screw whip and inertia would have limited the speed significantly:
Attachment 3913
This is, I think, the most promising design I've come up with for rotating a ballnut. The chosen ballnut is the standard RM2510 ballnut from linearmotionbearings eBay seller (among others). The diameter of the ballnut and screw would makes angular contact bearings quite costly. At first I drew it with the ballnut inside the bearings, however the following design allows for smaller 6007 bearings and also gives the option of putting 2 ballnuts in to eliminate backlash. One ballnut attached to either side of the shaft.
Here it is:
(I'll draw it in 3D if necessary)
Attachment 3914
Black is ballnut/screw/bearings
Red is shaft, steel or aluminium - probably aluminium since lower moment of inertia.
Orange is thingy that fits on the end of the shaft and attaches to the ballnut via the 6 holes in the ballnut.
Green is some aluminum plate to hold every thing. One plate will be extended to accommodate the stepper motor.
Brown is timing pulley. I could machine the pulley on to the shaft, however that means making a new shaft if I get the number of teeth wrong.
Blue is a collar bolted to the aluminium plate to preload the bearings.
Any comments/ideas/criticism welcome!
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Re: Rotating Ballnut - design ideas
Have a look at my Harrison conversion thread for pics of how I done it -http://www.mycncuk.com/forums/showth...ill-Conversion
Everything is big, but that's only because it needs a big screw. I should still have the solidworks files somewhere, which I'll have a look for shortly.
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1 Attachment(s)
Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
m_c
Have a look at my Harrison conversion thread for pics of how I done it -
http://www.mycncuk.com/forums/showth...ill-Conversion
Everything is big, but that's only because it needs a big screw. I should still have the solidworks files somewhere, which I'll have a look for shortly.
Nice design, if you could find the solidworks file that would be much appreciated. I had drawn something similar previously, but with two angular contact bearings. I've now redrawn it based on yours with a single 5207 bearing. It's going to cost about £32 for two of those bearings, so quite a lot more than the previous design. It also means the pulley has to be at least 60T (assuming XL pitch), which is getting a bit large if I need a bigger pulley on the stepper motor. I've drawn two versions with the pulleys mounted using different methods.
Attachment 3927
I'm not sure if where the force from the tension of the timing belt is ok?
The problem for me with the above design is making the thread / nut. It's not something I've had much luck with before. Unless I spend £20 on a die and can buy the nuts:
http://cgi.ebay.co.uk/35mm-35-x-1-5-...item1c1a2cb180
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Re: Rotating Ballnut - design ideas
The main reason I ended up with my design, was it was the best way I could find to fit everything into the available space, plus I was aiming for a pretty large ratio, so pulley size wasn't really an issue. Everything was made from hot rolled steel, due to it having to carry the full weight of the knee.
http://www.mycncuk.com/forums/asset....8&d=1302383608
That was my 3rd revision of the design. Prior ones used two single row angular contact bearings, but they were too tall to fit in the available space.
Once I get the Conect lathe up and running, and get the Triumph manouvered into place, I'm hoping to get back to the milling machine.
If you want the actual solidworks files, PM me your email, and I'll send them over.
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
m_c
The main reason I ended up with my design, was it was the best way I could find to fit everything into the available space, plus I was aiming for a pretty large ratio, so pulley size wasn't really an issue. Everything was made from hot rolled steel, due to it having to carry the full weight of the knee.
I see, so out of the designs I've posted so far which do you think is the best option? Space is no problem at all for me.
I'm just redrawing the latest one to use a 40mm bore angular contact bearing since that means I can bolt a sort of collar on to the end of the shaft so that I don't need to do any thread cutting.
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Re: Rotating Ballnut - design ideas
Personally, if space wasn't an issue, I'd take your top design, replace one of the normal bearings with a pair of angular contact bearings, and put a lock nut onto the shaft to adjust play out the bearings. That way endfloat is adjustable, the normal bearing provides extra support for the belt tension, and it allows for smaller pulleys.
I got my bearing and locknut from www.simplybearings.co.uk (for locknuts, click bearings on the front page then scroll down).
As for thread cutting, sharp tool (I bought a HSS insert one from greenwood tools, but I see chronos/glanze have them listed now), slow speed, and remember don't disengage the leadscrew feed once you've made the first cut when doing metric! Reverse the lathe back to the start.
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1 Attachment(s)
Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
m_c
Personally, if space wasn't an issue, I'd take your top design, replace one of the normal bearings with a pair of angular contact bearings, and put a lock nut onto the shaft to adjust play out the bearings. That way endfloat is adjustable, the normal bearing provides extra support for the belt tension, and it allows for smaller pulleys.
By a pair of angular contact bearings are you referring to something like 2 of 7207, or a single 3207? Either way that's getting a bit expensive, plus cost of threading tool though I would use that for other things. I don't want to compromise on the design due to cost, but there are limits. Another thing with that design is since the bearing supports are separate pieces I've got to get the centre heights very accurate, and mount them on a good flat surface for the bearings to be concentric. Bear in mind I'm probably going to be machining this on my mini lathe, which isn't great.
How much end float can I expect in a 5207 bearing if I went for the other design? These are the cheapest I can find:
http://cgi.ebay.co.uk/5208-ZZ-DOUBLE...item53e20e2fe6
This is the other design modified for no thread cutting:
Attachment 3929
Thanks for the link to the lock nuts, I'll get them if necessary.
Decisions decisions...
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Re: Rotating Ballnut - design ideas
I could use the original design but with tapered roller bearings - 32007 or L68149.
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Re: Rotating Ballnut - design ideas
The bearing I used is a 3208 double row angular contact.
Normal bearings, which includes double row ones, aren't really suited to applications with lateral loads. Deep grove bearings are better, but still not ideal.
However, most common angular contact bearings are the same physical size as normal bearings, so you can always switch to angular contact at a later date.
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
m_c
The bearing I used is a 3208 double row angular contact.
Almost the same as I drew in post #7 then.
Quote:
Originally Posted by
m_c
Normal bearings, which includes double row ones, aren't really suited to applications with lateral loads.
Yes, however I looked up on the SKF site before using those and is say that the bearings I chose for the initial design are OK for half the radial load rating in the axial direction. That's 5100N, which is far higher than I will get on the router - unless I crash it, and the force is shared over 4 bearings so my thinking is that that might be the way to go as they're so cheap and, as you say, I can either replace with angular or the same if they wear out. I wouldn't be able to fit double row in though.
Is there a reason why you have not mentioned tapered roller bearings? They would fit well.
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Re: Rotating Ballnut - design ideas
Is the 50% figure for static or dynamic loading?
You only really need to use taper bearings when dealing with high loads. For the majority of applications, ball bearings work fine, they have less resistance, and are more tolerant should dirt get into them.
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
m_c
Is the 50% figure for static or dynamic loading?
Yes that's static, forgot to mention that! I can't find a rating for dynamic, just interesting explanations about how the bearing responds to axial load.
Quote:
Originally Posted by
m_c
... less resistance, and are more tolerant should dirt get into them.
Fair enough, those are both clearly important for this application.
According to the SKF site the axial internal clearence for a 40mm bore double row angular contact bearing is 11um. So that's like having 11um end float on the screw if I used the design with the single bearing. I'm not sure how bad that amount is - it doesn't sound like much. C2 clearance bearings only have 2um clearance.
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1 Attachment(s)
Re: Rotating Ballnut - design ideas
4th, and hopefully final, design:
Attachment 3934
That's one double row angular contact bearing to take the thrust load, and one standard bearing. Stepper will bolt on to the 15mm plate. I can use 4" aluminium bar for the bits on the other side, and 2.5" for the rest purely because those are the sizes I've found cheap!
Edit: Disregard that, not thinking... this one still does not preload the bearing properly. I'll add another angular contact bearing, but that's going to cost too much :sad::sad: Now I'm a bit stuck, perhaps a thrust bearing would do.
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Re: Rotating Ballnut - design ideas
If you're using a double row angular contact bearing, then it doesn't need preloaded, as they come preloaded.
Provided the orange and red bits tighten together onto the inner race, and the blue and green bits clamp the outer race, then that design will work fine.
The only purpose the second bearing has, is to provide support to the shaft to stop it being pulled/deflected by the stepper/belt.
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
m_c
If you're using a double row angular contact bearing, then it doesn't need preloaded, as they come preloaded.
Are you sure about that, I was worried about what I posted here:
Quote:
Originally Posted by
Jonathan
According to the SKF site the axial internal clearence for a 40mm bore double row angular contact bearing is 11um. So that's like having 11um end float on the screw if I used the design with the single bearing. I'm not sure how bad that amount is - it doesn't sound like much. C2 clearance bearings only have 2um clearance.
Here's the link:
http://www.skf.com/skf/productcatalo...ogue=1&lang=en
http://www.skf.com/images/cat/images/1/1_3/010_0303.jpg
On a related topic, do you think 2 ballnuts like I drew in #13 is sensible for a router? It should enable backlash, and endfloat from the above, to be eliminated I think.
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Re: Rotating Ballnut - design ideas
You'll have far bigger sources of play than 11 microns in a bearing!
Whether you use two ball nuts or nut, will depend entirely on how much play you're willing to allow (or money you're wanting to spend!), and there will also be an increase in friction using two preloaded ballnuts.
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
m_c
You'll have far bigger sources of play than 11 microns in a bearing!
I was thinking it's best to eliminate as much play as I can. In that case it's either the design #13 or #7. The latter, with one bearing, is easier to machine and cheaper, but less stable and bigger pulley. I'm worried about getting the centre height in the design with two bearings accurate.
Quote:
Originally Posted by
m_c
Whether you use two ball nuts or nut, will depend entirely on how much play you're willing to allow (or money you're wanting to spend!), and there will also be an increase in friction using two preloaded ballnuts.
I've worked out that linearmotionbearings only charges £18 per RM2510 ballnut (and about £31 per meter of the screw), so it's not much more to have a second nut. Friction is a good point since the whole idea of this is to make the machine run faster.
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Re: Rotating Ballnut - design ideas
It's always best to eliminate play, but there are costs to that, as you're discovering.
Most accurate way to machine bearing bores so they're in perfect alignment, is ideally do them in one go, but for that design wouldn't be too easy.
However, as the double row bearing gets clamped in position, you could machine it's bore to allow the bearing to move around a bit, so when you finally assemble it, the bearings get lined up via the shaft, and it's just a case of clamping the doublerow in place.
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Re: Rotating Ballnut - design ideas
Hi Jonathan,
If you would like some ideas or a rotating nut you can look at my mill build log. I have just uploaded some pictures showing the assembly of the rotating nut. I used 2 AC bearings and preloaded with a nut. I bought a double AC bearing from EBAY but was not happy with the amount of clearance between the balls and the race so I resorted to 2 seperate AC bearings. The nut is not a lock nut but the pulley below has a grub screw over the key. The nut stack is preloaded then clamped with the grub screw. If this causes problems the nut is thick enough to slit radially and add an M3 clamp screw. Hope this gives food for thought:rolleyes:
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Re: Rotating Ballnut - design ideas
Hi Andrew
Quote:
Originally Posted by
Andrew Wilding
If you would like some ideas or a rotating nut you can look at my mill build log.
That did get my attention - I was going to post but hadn't finished thinking!
Quote:
Originally Posted by
Andrew Wilding
I have just uploaded some pictures showing the assembly of the rotating nut. I used 2 AC bearings and preloaded with a nut. I bought a double AC bearing from EBAY but was not happy with the amount of clearance between the balls and the race so I resorted to 2 seperate AC bearings.
It is interesting what you say about the bearings. I was indeed intending to get them from eBay, two of these to be precise:
http://cgi.ebay.co.uk/5208-ZZ-DOUBLE...item53e20e2fe6
I'm hoping if I press fit the bearing onto the shaft that should expand the inner ring, and tighten the bearing up nicely. Similarly with the outer ring, except if I follow m_c's excellent suggestion of boring it slightly oversize and clamping it then that's not going to happen. Did you get the single row angular contact bearings on eBay, and what size are they?
The problem I've got is that I don't have the constant force applied to my bearings due to gravity, unlike on the various milling machines that have a rotating nut.
I'm really not at all sure what to do now. I've said already two single row angular contact bearings is a lot of money...and yet it looks like that is certainly the best way to do it. Having said that there's someone on CNC zone who has done it with two standard deep groove bearings.
If I could get away with a 16mm. 10mm pitch, ballscrew then I would save so much on the screw, that I could easily afford the cheaper smaller bearings. However I would prefer to have the 25mm screw just in case it fails completely and I have to spin the screw.
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Re: Rotating Ballnut - design ideas
Here is a rambling collection of my thoughts some maybe relevant most probably not!
The bearings I purchased were from Bolton Bearings and had a 30mm bore (too small for you?) and were about 8 quid each. As I am sure you are aware that bearing is not an ac bearing and probably not a deep groove bearing. Deep groove can take some axial load but a normal bearing is quite limited in this respect. The bearing shown will not have any preload but as you say you could acheive this by pressing the bearing onto the shaft (nut). You will need a fairly close tolerence shaft to control the fit and you will need to haver a fair idea of the clearance in the bearing to work out how much of an interference fit you will need. If you are pressing up to a shoulder than you will not be able to remove the bearing. Mine are a push fit (couple of tenths interference) which can be removed without wrecking the bearing. Compressing externally would give you more control and would allow you dismantle the assy.
The real solution is to try to avoid using a rotating nut, particulary when you are talking about 40mm bore bearings and all the associated pulleys etc that go with it. I expect this is a last resort as you have already looked at other design solutions.
I would be surprised if you are getting anwhere near the design axial loads of a suitable AC or deep groove 40mm bore bearing in your arrangement. The load figures may be based on L10 life rather than ultimate limit, or the bearing you were looking at is not designed for axial loads.
The worst thing for bearing life is not having enough load and the balls sliding rather than rolling and so preload is sometimes used for this reason as much as combatting lash.
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Re: Rotating Ballnut - design ideas
I've machined plenty of parts with bearings before, so I'm not worried about getting the required accuracy. The lathe I've just bought should help there too :)
30mm is too small I think, on the drawing currently I've bored the shaft 27mm which does not leave much for the bearing.
How do you tell if the bearing is classified as 'deep groove', or is it somewhat arbitrary? The standard bearings I had selected were the ones with a bigger outer diameter (72mm), though not the biggest. I've got 2 good quality, FAG and SKF if I recall correctly, 45x85mm bearings.
I'm not entertaining a belt or rack and pinion drive so the only other option as far as I'm aware is to tension the ballscrew sufficiently to stop whipping. That does not help with the inertia of the screw though, which for 2000mm is similar in magnitude to the other forces.
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Re: Rotating Ballnut - design ideas
deep groove is usually in the description of the bearing. On closer inspection of the diagram that you linked the bearing you have shown probably is a deep groove. These are the norm but still not ideal for use in an application where relative to radial load the axial load is significant.
Some more thought (sorry if all this has been covered in other threads)
Moment of inertia is proportional to diameter squared so I am surprised that there is much difference between the shaft and nut? I am sure you have done the sums.
Is the moment of inertia significant when compared to the mass of the 'carriage' (when calculating acceleration)?
If you have to tension the shaft to raise its critical frequency then will have to start considering beefy shaft support bearings, I can now see why the rotating nut idea is appealing!
a 2000mm leadscrew poses a lot of design challenges even when cost is not an issue (which I assume is, if you were as poor as I was when I was a student!) have you considered other methods with feedback or your real accuracy requirement? One solution maybe to use a rack and pinion/belt and calibrate the travel using a dial indicator and blocks of a known dimension. The smaller the increments the better idea you would have of variation along its length. Even a crude survey (100mm steps?) would give you significant improvement in accuracy. Obviously this would require a decent home switch.
I look forward to seeing your solutions. This is not an easy project too keep in budget but will be satisfying when cracked.
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
Andrew Wilding
Moment of inertia is proportional to diameter squared so I am surprised that there is much difference between the shaft and nut? I am sure you have done the sums
The problem is mass is proportional to the radius squared, so you end up with 4th power. Hollow ballscrew would be nice!
Quote:
Originally Posted by
Andrew Wilding
Some more thought (sorry if all this has been covered in other threads)
Keep thinking! It's good to have it all in one place.
Quote:
Originally Posted by
Andrew Wilding
Is the moment of inertia significant when compared to the mass of the 'carriage' (when calculating acceleration)?
Yes it is, I'll post the numbers when I've verified them.
Quote:
Originally Posted by
Andrew Wilding
If you have to tension the shaft to raise its critical frequency then will have to start considering beefy shaft support bearings, I can now see why the rotating nut idea is appealing!
Just what I was thinking. I suppose tapered roller bearings would do the trick, but then you're adding friction.
Quote:
Originally Posted by
Andrew Wilding
For a rotating nut have you considered how much the timing belt tension would deflect the nut on a 2000mm length <25mm shaft and the effect of out of balance forces of the nut on the shaft?
Surely the bearings are going to take the radial load and stop the screw deflecting, or am I misunderstanding you? So as long as the ballnut is held on centre I should be ok. That could be an issue if the flange on the ballnut isn't concentric. Another issue I've found is that at sufficiently high rpm with a standard ballnut centripetal force will stop the balls rolling properly.
Quote:
Originally Posted by
Andrew Wilding
a 2000mm leadscrew poses a lot of design challenges even when cost is not an issue (which I assume is,...calibrate the travel using a dial indicator and blocks of a known dimension....survey (100mm steps?) would give you significant improvement in accuracy.
I think that method of measurement would have a cumulative error, which is especially significantly over this distance. It could maybe be done with a digital calliper and carefully fixing to consecutive points along the bed. Theoretically the process could be automated.
Quote:
Originally Posted by
Andrew Wilding
Obviously this would require a decent home switch.
Easily done with a cheap laser pointer I reckon.
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Re: Rotating Ballnut - design ideas
Jonathan,
You can ignore the stuff about critical frequecy/deflection of the screw with the spinning nut. I had a dim moment and forgot you were supporting the nut with bearings. Its late or at least that is my excuse!:cry:
Agreed with the 4th power bit but that would make the nut inertia even larger than the screw. How does moment of inerta or rotating bits compare with mass of linear bits when you look at a good old newton second law equation?
going tp bed now.
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Re: Rotating Ballnut - design ideas
If I was to spin the screw the unsupported length would be 1800mm and overall length about 1900mm, so the calculations below are based on that.
J = m*pitch^2/(2*pi)^2
J = 45*0.01^2/4/pi^2 = 0.114 g-m^2
For the screw:
J = 0.5*m*r^2
Steel --> 7850Kg/m^3
Therefore:
J = 0.5*7.85*pi*r^4*l
So for mine:
J = 0.5*7.85*pi*0.0125^4*1.9 = 0.572 g-m^2
That makes the moment of inertia of the screw 5 times that of the gantry.
I will estimate J for the ballnut assembly.
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2 Attachment(s)
Re: Rotating Ballnut - design ideas
Not much to add to the actual design here, just a 3D drawing which clarifies the bearing mounts:
Attachment 3950
Solid Edge got a bit carried away with the reflections here:
Attachment 3949
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
Andrew Wilding
You can ignore the stuff about critical frequecy/deflection of the screw with the spinning nut. I had a dim moment and forgot you were supporting the nut with bearings. Its late or at least that is my excuse!:cry:
Interestingly I've since found that when you rotate the ballnut the critical frequency still has an applies - the end fixity is just better which, in my case, gives 1250rpm. So 12500mm/min. I think I'll be lucky to get that much out of my motors anyway so I'm not going to worry.
I just got prices for the screws:
2 of RM1610-2000mm with nut and end machined(both ends machined same as A type )
238 usd
1 of RM1610-900mm with nut and end machined(standard end machining)
72 usd
Sub-total
310 usd
Air express shipping
127 usd
Total
437 usd
-----------------------
Offer tow:
2 of RM2510-2000mm with nut and end machined(both ends machined same as A type )
269 usd
1 of RM1610-900mm with nut and end machined(standard end machinings)
72 usd
Sub-total
341 usd
Air express shipping
168 usd
Total
509usd
I'll go for the second option - the price difference is not as much as I expected. The smaller screw is for the Y-axis. I've decided to get both ends of the big screws machined to fit the BK type support ... that should enable me to tension the screw using the nut on either end.
I have also acquired a pair of 7206 FAG bearings cheaply, so I'm hoping I can use them instead of 7207. The obvious problem is that the bearing bore is only 30mm, and with the screw passing through it being 25mm the shaft will be very thin - about 1.5mm.
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Re: Rotating Ballnut - design ideas
The ballscrews arrived today - I paid for them on 08/06 and I requested different end machining (longer portion for pulley and same machining on both ends of screw) so I think that's good service. He also charged me $447 in total even though I ordered the 25mm screws, so $72 less than originally quoted. The ballnuts backdrive easily and they seem nice and smooth.
I will start working on the rotating ballnut mount soon. I'm currently doing one last 3D drawing. I will use 5/8" aluminium for the plates that hold the bearings with some aluminium 'posts' to hold the plates together parallel at a fixed distance.
I made a mistake with the bearings and bought open not shielded so I'm going to have to put some sort of enclosure round them. I got them on eBay, one SKF and three SBC 7207.
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
Jonathan
I made a mistake with the bearings and bought open not shielded so I'm going to have to put some sort of enclosure round them. I got them on eBay, one SKF and three SBC 7207.
Don't bust a gut searching for single row angular shielded, I don't think it exists :naughty:
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
Robin Hewitt
Don't bust a gut searching for single row angular shielded, I don't think it exists :naughty:
That explains a lot!
I've just estimated the moment of inertia of the rotating bits of the assembly, so pulleys shaft, ballnut and bearings. Surprisingly it comes to slightly more than the inertia of the 2085mm, 25mm diameter screw! That's annoying since one of my main reasons for doing this was because I thought this value would be much lower. I've not included the inertia of the bearings I would require to rotate the screw, and I suppose I would use pulleys with it anyway. The inertia of the ballnut and shaft is still about two thirds of that of the screw.
This is going to make a big difference to the rapid speeds I can get since the torque required to move the 50kg gantry, due to the low coefficient of friction of the bearings, is very small.
Now I'm going to try modelling it with the stepper at different ratios - not 1:1, and see what happens.
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2 Attachment(s)
Re: Rotating Ballnut - design ideas
I have at last started making the rotating ballnut assembly. I've started with one stepper motor mount, it's 100x245mm:
Attachment 4207
Attachment 4206
The reason for having such a long cutout for the motor is to allow me to use any reasonable size pulleys without changing the belt, which will obviously be difficult. I CNC milled all except the bearing bore which was bored on the lathe. I had to remove the gap bed on the lathe to do it - the join was painted over so it looks like this might be the first time it was removed! The six holes closest to the bearing will be used to push a ring against the outer ring of the bearing to preload it. The 4 bigger holes are for posts which link this to the other bearing mount.
I'll make some more bits when I've found my caliper :whistling:
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13 Attachment(s)
Re: Rotating Ballnut - design ideas
I've now made all 4 bearing/stepper mounts and one shaft. I will machine the pulley (30 tooth) directly on to the shaft.
When I machined the shaft on the lathe I got only small pieces of swarf, not the long stringy stuff you normally get - see picture below of drilling it for what I mean. It was almost like cast aluminium...but it's not. Must be a strange grade?
Lots of photos, hopefully pretty self explanatory:
Attachment 4213
Attachment 4220
Attachment 4212
Attachment 4208
Attachment 4216
Attachment 4219
Attachment 4215
Attachment 4209
Attachment 4218
It got 0.01mm bigger after drilling...so now 34.99mm which fits the bearing nicely:
Attachment 4214
Attachment 4217
Attachment 4211
Attachment 4210
(Hopefully nobody without broadband is trying to view this!)
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Re: Rotating Ballnut - design ideas
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
wiatroda
Very nicely done
Thanks :). I hope it works well after all this... I modified irving's spreadsheet to include pulleys and used it to model this setup. Looks like, with the 3Nm motors I'll get at best 7500mm/min rapid and 2000mm/min with 50N cutting force. Both of those are a lot lower than the prediction for the Y-axis, 18100mm/min and 8200mm/min. All of those figures are with a 3x safety factor on the torque, so fingers crossed it might be better!
I'm currently making a mandrel from 35mm steel bar to hold the shaft in the rotary table/4th-axis to mill the pulley teeth on to it.
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Re: Rotating Ballnut - design ideas
Quote:
Originally Posted by
Jonathan
Thanks :) .... mill the pulley teeth on to it.
What belt profile? How wide pulley going to be ? I had to buy 3M HTD pulley recently , but I would give a try to cut my own .
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Re: Rotating Ballnut - design ideas
I'm using XL belts, purely because I already have lots of XL timing pulleys which will enable me to experiment with the ratios. HTD would be better in theory.
I will use a form tool to cut the pulleys. The profile is a simple trapezium which I have ground on to a piece of HSS. You can make HTD pulleys accurately with a ballnose cutter ... or some people use a drill but I think that's a bit crude.
If you do decide to try it you might find my program here useful:
http://www.mycncuk.com/forums/showth...ating-programs
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2 Attachment(s)
Re: Rotating Ballnut - design ideas
Today I cut the pulley teeth into the first shaft, and whilst that was running made the second shaft.
Pictures:
Attachment 4226
The belt fits (yay!):
Attachment 4227
I will cut the pulley teeth into the second shaft tomorrow, then I just need to make the part that connects the shaft to the ballnut, and the ring that pre-loads the bearings...and maybe some bits to act as bearing seals.
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1 Attachment(s)
Re: Rotating Ballnut - design ideas
I have now cut the second pulley, and made the remaining parts for one of the assemblies except the bearing pressure plate. I immediately stuck it in the milling machine vice, put a stepper on and tested it:
Attachment 4228
Will post a video tomorrow. With me just holding the 2092mm ballscrew in one hand it did 14m/min. Almost certainly more - I didn't try.
Might be while until I finish off the other one since the tip on my parting tool became an Unexpected Flying Object. It might still be ok, but my lathe is buried in so much swarf I can't find it. It took me 5 mins to find the 60mm diameter part I had parted off, let alone a tiny tip...maybe I'll tidy it tomorrow.
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Re: Rotating Ballnut - design ideas