How precise is Haydn diagonal rods when assembled
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I just picked 12 ball studs at random and measured them, and I'm afraid that my conclusion matches David Crocker's: I build the sets of arms so that they match within 0.05mm, and typically match within 0.03mm, however the ball studs have not been manufactured as accurately as the arms. The ball studs are manufactured in an machine shop in China, and I have been very thankful that this particular machine shop has given me very good service – I've dealt with others who were obviously not as good.
When I'm building the arms, I use G25 rated chrome plated ball bearings in my jigs, not the ball studs themselves.
This makes it easy to get the arms into and out of the jigs without varying the tension.The 12 I just measured had the following diameters:
9.43
9.48
9.51
9.53
9.54 (three)
9.55 (two)
9.57
9.59 (two)
Nine were in the 0.07mm range from 9.53 to 9.59.I also measured the distance from next to the M3 threaded stud to the tip of the ball on the same 12 ball studs. This isn't as precise as measuring the diameters, but I suspect this is more important. The results were more uniform:
12.46
12.59 (nine)
12.62 (two)
Nine were very close to 12.59, and two more were in the 0.04mm range from 12.59 to 12.62. There was only one outlier.I expect that it is more important to have this distance consistent than the diameter.
So far, I've built almost a dozen printers using these and my experience has been that all of the printers have calibrated and printed well, so I'd guess that even though this isn't ideal, practically it doesn't cause problems. That said, for $12 extra, one can get two sets of ball studs and select the ones which match the best.
If I remember correctly from what other people have reported, the microswitches we use for the endstops have 0.05mm or so of variation from one triggering to the next, and many other parts probably have comparable variations.
Next time I'm placing an order for more ball studs, I'll ask them what would be the cost/tradeoffs to make these more uniform.
I'm sorry these aren't perfect and I feel a bit embarrassed! -
Wise words sir, I wouldn't feel embarrassed in the slightest, your arm systems improve any delta printer they are attached to so the real world effect on printing is negligible. If this had a big effect I wouldn't be calibrating at 0.004mm deviation.
Maybe its just a case of as you said, selecting the best matching balls, to make up sets. It would be worth asking here of those who have a deep knowledge of the geometry involved, what is the most important factor the ball's diameter or the distance from the end of the ball to the mating surface where the thread emerges? Or a combination of both?
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I would say the important measurement would be from the mating surface to the centre of rotation of the Ball itself.
This would then reduce the effect of any inaccuracy of the diameter of the ball by a factor of 2 so we are now into the 35-40 micron error region which in real world terms obviously doesn't make that much difference.
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But as you said before Doug, thats not so easy to determine since the balls may not be round. Granted if they are round and a set of 12 is selected of all the same size, and therefore distance from ball's centre of rotation (which is known if they are round) and the mating surface, then bingo were golden. But are they round?
I think its probably best to supply a pair of balls matched to each rod (or arrange this yourself if you already have them, this enables the slight differences in rods to be evened out). I would also propose we organise a swap-shop so that existing users can make up a set and swap ones they don't need. Of course it doesn't matter if the 6 similar balls on the carriages are a different "size" than the 6 on the effector as long as they all create equal length rod/ball sets.
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I would say the important measurement would be from the mating surface to the centre of rotation of the Ball itself.
This would then reduce the effect of any inaccuracy of the diameter of the ball by a factor of 2 so we are now into the 35-40 micron error region which in real world terms obviously doesn't make that much difference.
It seems to me that it is not the case. Except the distance to the center of the ball matching surface and the distance from the surface to the center change also. I gave approximating formula above, it is a factor of 1.85 so total result would be of order ball diameter not radius. After all said I should confess that now I think that the difference in the ball diameter is not very detrimental to quality of print as there are many other imperfections in effector carriadges and so on of comparable intolerance and as many people used the Haydn's rod with very positive results it say for itself. Thank you all for very fruitfull discussion!
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I am wondering if getting hold of 12 precisely made ball bearings (3/8th inch I believe), and having them precision-tapped for m3 thread, might then provide a replacement for the current ball studs. If the range of movement is restricted configured like this, the addition of a small stack of carefully measured washers under the balls will simulate the stand-off we currently have, and provide some adjust-ability for total rod length, ball centres should be precisely the same if suitably high quality bearings are used.
To say the effect will be small and covered by other imperfections is true of some printers, but I spent a lot of time and money to make sure there are few if any other imperfections in my printer, so I am willing to see if we can find a reasonably simple workaround, to iron out this blip, which is cheap, easy and which doesn't place too much of a burden on Haydn who does an amazing job making these arms and doesn't charge a huge amount for them, for which I am very grateful. Perhaps if this approach works then that might be the way forward.
I presume I would need to carefully chose a steel (thinking chromed steel) to optimise its magnetic properties? Does hardening increase or reduce its magnetic attraction?
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I am wondering if getting hold of 12 precisely made ball bearings (3/8th inch I believe), and having them precision-tapped for m3 thread, might then provide a replacement for the current ball studs. If the range of movement is restricted configured like this, the addition of a small stack of carefully measured washers under the balls will simulate the stand-off we currently have, and provide some adjust-ability for total rod length, ball centres should be precisely the same if suitably high quality bearings are used.
To say the effect will be small and covered by other imperfections is true of some printers, but I spent a lot of time and money to make sure there are few if any other imperfections in my printer, so I am willing to see if we can find a reasonably simple workaround, to iron out this blip, which is cheap, easy and which doesn't place too much of a burden on Haydn who does an amazing job making these arms and doesn't charge a huge amount for them, for which I am very grateful. Perhaps if this approach works then that might be the way forward.
I presume I would need to carefully chose a steel (thinking chromed steel) to optimise its magnetic properties? Does hardening increase or reduce its magnetic attraction?
Don't think this way is worth of efforts. The threads are not precise at all. They are loose and depend on many things as quality of threading, torque applied when assembled etc.
To make the final assembly precise we need some sofisticated measuring device, optical I think. A much simpler to make ball stud binning as relative sizes have matter, IMO. -
I'll try matching balls I ordered an extra 6 with my pcb effector so I have 24 to choose from now. If this works then I'll stick with that. I'll have to take it apart to fit the pcb effect tor anyway.
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the critical Dimension (Or the one that would IMHO make the most difference) is the distance from the threaded side on the mounting base and the centre of the ball just in case I didn't make it clear above (I Re-read my post and it confused me lol could have been taken as the circumference to the center)
Again as you say Simon very difficult to measure accurately.
Doug
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the critical Dimension (Or the one that would IMHO make the most difference) is the distance from the threaded side on the mounting base and the centre of the ball just in case I didn't make it clear above (I Re-read my post and it confused me lol could have been taken as the circumference to the center)
Again as you say Simon very difficult to measure accurately.
Doug
Both the dimension you mentionned of and the ball diameter are critical as the former defines the distance from linear rails to the center of the ball and the later changes the rod length from center to center. The rod axe is not coinside with the ball stud rotational symmetry axe.
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As a proxy for that I'll run 10 calibrations (not saving with m500 in between) then I will try to make two sets of 6 balls, which have the closest matching dimensions in both diameter and distance from mating surface to end of ball using one set on the carriages and one on the effector. I'm hoping this is valid, since these are things that I/we can measure easily. I will then keep my existing setup and reassemble it. Running another 10 calibrations to see if there is any effect. I trust my sensor to be accurate to 0.01mm with deviation of 0.007mm so the sensor should not be a factor in the outcome.
I might then get my rods measured precisely and see if I can use the slightly larger or smaller balls by diameter and "length" to even out any differences in rod length and repeat the process.
Then I'm hopefully going to have a pcb effector to try out, but with the optimum set of balls/rods.
Can anyone see any flaw (excepting measuring the ball centres which is not practical for me) in this methodology?
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Very interesting experiment! Too many uncertainties in your arrangement (reassembling another set of ball studs is the biggest one in my opinion but there are others of course) to make full conclusion about it validity. If the result will be negative it don't tell very much but a positive one will surely give a clue.
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I think that would be as precise as you can get without resorted to something like interferometry in a Lab to accurately profile each ball stud.
Could probably get it done if you knew a friendly engineering Professor at a uni (Or a research student/Fellow) who would be up for a challenge.
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I know an ex-CERN particle physicist who now works at IBM?
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that may a good contact for this
I will have to see if anyone at the advanced manufacturing site near me would be interested ( Boeing/Maclaren amongst other )
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Today I got my SmartEffector and Duet electronics at last, very happy. I have received another lot of 12 ball studs with it and made measurement as I did with the first lot of ball studs received with Haydn's rods. The results are in table below
Two lots are alredy statistics
so some conclusions.
1. Ball diameters are concentrated around 2 values: 9.4mm and 9.5mm, looks like 2 lots have been manufactured on 2 CNC with a little different setup or tool wearout.
2. Diameter measurements in different planes (A1-3, B1-3, C1-3 and D1-3) correlates very well with each other. So for binning purpose 1 measurement of diameter (A1 for example) is enough instead of tedious multiple measurements in various planes.Fortunately I can select 12 ball studs that match pretty well among 24 ones that I have now.
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I got my smart effector, and have only just got some 3/8 bearings to place at the ends of the rods so I can take them to be measured.
As such I haven't got far with trying out matching the balls and rods.
I'd be interested to hear if you get much difference calibration deviation if you go from your well matched set to maybe a random placement of balls?
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Just collected all parts needed to start assembling my Delta (honestly speaking I haven't got the Nimble adaptor yet but it should be here soon). So I'm afraid there will be much more hurdles I should overcome before I could make a comparision of calibration deviations for different ball stud sets, sorry.
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Okay so this afternoon. I went to my local engineering firm with 2x 9.51mm ball bearings (G100 best I could get but they measure 9.51 whichever way they were orientated) and got them to measure my six rods.
These are the total lengths including the ball bearings:
369.7
369.68
369.72
369.7
369.72
369.7I ran 12 sequential calibration G32 runs first with random sets of balls/rods:
0.008
0.008
0.009
0.007
0.010
0.008
0.005
0.009
0.007
0.010
0.005
0.006 Mean 0.008Then I removed all of the balls from the printer and measured all 24 balls that I have.
I made up sets of rods and balls as so, it was not possible to make exactly 3 pairs of the same length to 0.01mm accuracy, but I got to 0.02mm!:
Rods Total Length Rods+Balls
1,2 369.55
3,5 369.55
2,6 369.56Then 12 sequential calibration runs after installing the matched rod/ball sets. Nothing else was changed whatsoever.
0.013
0.005
0.007
0.008
0.006
0.007
0.008
0.008
0.012
0.013
0.006
0.009 Mean 0.009I will happily accept my method is flawed in the following ways, I measured the ball's diameters across the ball's equator so to speak, not averaging over 8 different planes of measurement, I did not measure the distance between the mating surface of the base of the ball stud to the end of the ball, as I have no accurate way of doing so.
However based on this whilst I have determined that my rods are remarkably accurate (0.04mm range - 369.68mm-369.72mm), the balls are not as accurate (range 0.13mm from 9.38mm-9.51mm), but matching them up does not seem to have any effect on the accuracy of the machine, as determined by calibration runs. I think once you reach the 0.00x range you are in the noise, and not reading a signal you are limited by sensor accuracy and accuracy of the rest of the printer.
