Hi Craig - Yes machining it is tricky but my machinist has a large 5 axis mill but using that takes $$$. My thoughts at the moment is that the pressed metal steel is the way to go for the project at hand. But I keep looking at alum casting :) Peter
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Hi Craig - Yes machining it is tricky but my machinist has a large 5 axis mill but using that takes $$$. My thoughts at the moment is that the pressed metal steel is the way to go for the project at hand. But I keep looking at alum casting :) Peter
Hi peteeng,
when I first thought about casting I was going to cast the trunnion stub axles and table in one piece. That sure does put the 'cat amongst the pigeons' when you go to machine it. If you were doing
it yourself and did not account your time, that would be fine.......but all the large 5 axis machines around Christchurch cost $200/hr and more.
In order to keep the budget under three figures you pretty much have to assemble the trunnion out of smaller parts that you can make yourself or your budget will be entirely consumed by some tricky-dicky machining,
at least that if what I found.
Craig
Hi all - Today I sorted out how I will drive Epoch5's gantry. The 16mm ballscrews have a 15mm stack height delta to the bearings, always a juggle. But I'm happy with the solution. I shall cast the spacer using steel fibres so its damp... I've ordered the trunnion bearings so the build has begun!! I settled on using Unitised class 2 bearings from a Nissan Z. Using std double row (decided on 40mm) or matched tapered bearings was going to cost at least $80aud then I had to make the flange. These cost $46aud and I get the flange as a bonus!! They are matched tapered (43mm inside dia) so will adjust nicely.. Should run a million km.... I may use the same bearing for the rotary platter. When I get these two modelled up I can detail the trunnion then decide on the rotary details. Peter
I also sent a suggestion list to F360 support. See what happens.
Hi peteeng,
the unitised wheel bearings are a practical solution, just cut a hole and bolt them in.
Not quite so easy as the platter bearing. You really need a pair and the further apart the better. A single bearing will rock a little even when done up tight.
You will no doubt have seen mechanics jack up a car, grab the wheel and 'feel' the play. You always expect some, say 0.5mm at the periphery of the wheel but less than 2mm or so.
Even when the axle nut is done up tightly, very tightly even, and the bearing is hard up against the 'crush sleeve' it assumes its 'preload state', but on a car axle that is just short of 'no movement'.
For CNC purposes you might adjust them closer than that, even tight. A wheel bearing is heavily loaded and spins fast whereas a CNC bearing is very much more lightly loaded and glacially paced by comparison.
A wheel bearing requires a little bit of clearance otherwise it would overheat whereas a CNC bearing will not.
If you used just one bearing the platter would rock, not by a great deal certainly, but some, and much more than you'd be willing to tolerate I'll be bound. If you had a pair of these bearings
say 150mm apart the 'rocking' would be very much reduced.
Craig
Hi Craig - I have 2 on order for the trunnion and maybe a pair of double race deep groove for the rotary. Not sure yet, will have a play with the Nissan bearing and see how it feels. Auto wheel bearings have clearance suitable for running warm as they expand and as you say CNC runs slow and won't get hot. Will know more in a weeks time.... I may have to put a candle under the trunnion. Do they call them trundles in NZ? Peter
Hi peteeng,
for twenty years I worked as a mechanic. Over that time I have fitted hundreds of wheel bearings. My recollection is that they all had a tiny bit of clearance even when the axle nut is torqued to
250Nm. Wheel bearings need it, but I don't think you'd be happy with it as a platter bearing on its own. A pair of them several inches apart...yes, that would be fine but adds to the construction and alignment challenge
and overall height.
As you've seen in the pics the gear reducers I used have two opposing taper bearings, approx 40mm ID and 150mm OD. The worm wheel stubs are a press fit into the ID of the bearing. The worm wheel is through bored
to 28mm (3 size platter reducer) and 32mm (4 size trunnion reducer). To date I have yet to detect any compliance in the bearings. My guess is therefore that properly adjusted opposed taper bearings of 40mm ID and about 120mm
apart offer adequate rigidity for machine such as ours. I suspect the 28mm diameter shaft of the platter to be the most compliant part, not that I can say I seen or measured it.
Craig
Afternoon All - I'm impatiently waiting for the bearings so I can start detailing Epoch5. But I also want to make it half the weight (or close to) that of Frankie. So I worked through the usual material densities and stiffness's and find that F17 formply is a formidable material. If I look at equal weight basis then I need to use 4mm aluminium sheet or 1.5mm steel sheet. If I calculate the in-plane stiffness of these they are basically equal. A 100mm wide strip of F17 is 0.000029mm/N 4mm AL is 0.000033mm/N and steel is the same as AL. So to halve the weight I need to use 2mm thick AL which I can work at in my workshop or get 1.5mm steel laser cut. Now if I look at F17-17mm in bending it has a rigidity of 818,800mm4GPa. The 4mm AL is 37,300mm4GPa and the 1.5mm steel is 5625mm4GPa... So the F17 is the clear winner, in bending its 818/37= 22x stiffer then AL... Plus its easy to edge glue and screw into a stiff connection. The 2mm or 4mm AL I'd have to make brackets not difficult but its something to do... Now back to the half weight target. Unfortunately they don't make 8mm formply.. But 12mm pine ply is just over half the sheet weight. So seems 12mm BB ply achieves the weight target but not the stiffness target. So I looked at a laminate of 0.8mm AL each side of 12mm pine ply which gives the same stiffness as the F17 formply. But F17 is $34sqm and the laminate is $199sqm plus labour to make... Every time you try to loss weight it costs more!! There is a falcata ply that is very light but does not take edge screws well. Its pine skins with balsa core... So seems I shall use 12mm BB ply for primary areas with doublers at thru screws and 9mm for secondary structures... I also have to paint the ply but then I'll use epoxy or vinyl ester resin so its waterproofed and better glued together. Onward Peter
A 'trick' with balsa and screws is to soak the balsa with thin CA glue around the screw area which hardens it with little weight penalty - used in the RC aeromodelling field all of the time. May be worth further investigation.
Thanks fellow Pete - I don't haver CA glue but I can do the same with thin epoxy and wait overnight. But falcata is not really stiff enough. I went to the shed to do a screw test in 9mm ply and just looking at it gives me the insecurities.... so I think the main parts that are big and the screw is going thru it will be 9mm but the secondary bits that I edge screw into will be 17mm and I shall sculpt the webs etc in an engineering truss manner and try for a 50% reduction.... I rebuilt the E5 base into V14 using 9mm and dropped the weight from 21kg to 11kg so that was encouraging....will have to do some simulation to compare the two as well....Peter
something like this--- still 11kg Peter
Hi All - Been too hot to work in the shed and we had a little threat of a flood with the Qld cyclone so I've been busy prepping for that. But I have done some more on Epoch. Styled it a a bit and light weighted it. Happy with its progress. The saddle will be a fibreglass/carbon fibre hybrid part that look's like our aluminium saddle. I'm tossing up whether to go the std steel Z axis or making it composite. First get the trunnion sorted then the rest can follow. Hopefully the bearings arrive monday Tuesday then I can finish the "bottom end" Peter
Morning All- Getting closer, by V20 should be polished....Peter
Hi All - I've started mustering bits for Epoch5. I de-commissioned Brevis 2 for its rails and cars and set up a project table. I've been wanting to make a cabinet for a router for a while. Last machine Franky I tried but the gantry motor kept getting in the way. Maybe Epoch will have it. Peter
Morning all - I have been looking at the drive/rail car height difference problem. One way to solve it is to use a 16mm ballscrew assembly and a 30mm rail assembly. But the cost difference 20mm rails to 30mm rails is quite a bit. I usually use a spacer on the gantry cars to make up the 15mm delta. Some designs mount the screw on edge to the side but my style of wall means then I'd have to make two spacers vs one... Seems I'm at a stale mate so its use the usual spacer and combine that with the nut drive.. Peter
Afternoon All and Sundry - The bearings arrived just now. Very exciting; plus they have a lip seal! So 2 bearings, housing and seals all in one for $49AUD how good is that! I may have to change out the grease its quite thick and sticky... Peter
Hi peteeng,
clamp the two inner races together as tightly as you can as they would be in a car and see if there is any play between the inner and outer, I would expect some being a wheel-bearing.
If you are going to use these as trunnion bearings then the speed will be very low, a few tens of degrees per minute. Don't bother trying to change the grease, you will inevitably damage the
rubbing seal and thereafter it will leak. A little bit of viscous drag will do absolutely no harm on a trunnion axis.
Craig
Hi Craig et al out there - I did plan on clamping the inners and see what drag (torque) it has. If it has significant torque I'll swap the grease or get some oil into the assm. Hmmm. By significant I mean a good amount of motor torque. That's the next thing to calculate the motor sizes needed. I hope its not big because as you say I'll damage the seal getting it off then it will cost $50 to get a new seal!! The rotary is Dia500 and the max part height at the moment is 300mm so if it was heavy wood that would be 60kg it has to swing. The trunnion at the moment has a 150mm swing. So I have to work thru those numbers and check what size motor to use and the gearbox ratios etc etc.... I've modelled the bearings and placed them into V17. plus I have to get the geometric relationships between the platter and top end right yet... But I feel like I'm into details vs general stuff which is a good thing...
Evening All - To put Epoch into the Milli thread context. Milli started out with the intent to use composites and castings to make a mill. With my failure to make a castable, machinable high modulus material (except carbon fibre) this presented a dilemma. So for a bespoke machine now I'd pick machined Aluminium or Steel (Billet or laminated). Mixed and matched accordingly. Thick plates say 12,16 and 20mm and edge bolted. This has proved to be a very stiff and reliable method in plywood and the concept would scale to a metal plate build easily. Machined parts can be made to form and fit tolerances so they self assemble accurately. So If I where to make a 5 axis mill like Epoch it would be a metal plate build as described. The Trunnion would be cast aluminium and finish machined.
The Milli thread spent a lot of time looking at machine configs - I would choose the high rail design like Franky and Epoch5 or the Mori Seiki M1 config.. This depends on the "width" of the required machine. I would not go high rail/gantry if the width was narrow. I choose aluminium if I were to make the machine, as I can cut AL on my router(s) steel offers greater stiffness but with a heaver machine...
This brings me back to composites. Making plate carbonfibre is quite straightforward. So it is technically possible to make CF plate and make edge bolted CF plate builds. I'd have to look at the costing of the materials to see what the benefit over billet AL is. Clearly it would be a lot damper then AL or ST. But I'd have to take that on as machinists will not take on CF in their machines. Which sort of brings me back to why Milli started. Since others won't machine CF I'll have to have a mill and do it myself!! So maybe Epoch is around so I can do edge drilling and profiling in one setup... Hmmm
Now if its a production machine and say they are made in 10 unit runs I'd seriously look at carbon fibre or engineering grout.... These require moulds but that's where the benefits start to happen.... Peter
Hi peteeng,
with regards to the trunnion table. You'll want a torque at the table of at least 100Nm. Despite being five axis whats the bet the majority of the work on the machine will be regular three axis.
Unless you have a hydraulic or pneumatic brake then the trunnion gearbox/motor combination needs to be able to resist any cutting forces that would cause the trunnion to rotate. That is why the available torque needs be as high
as I have suggested, to resist the trunnion back driving the gearbox and thereby rotating the trunnion with inevitable loss of Z axis precision on your three axis tool path.
My trunnion has a 19.5:1 reduction and powered by a 2.4Nm (rated) servo for an available torque at the trunnion of 46.8Nm. I've found it is adequate but only adequate. If I were to have the freedom to choose
I would have chosen a 39:1, or a 50:1 reduction. My choice was influenced by availability as I was buying new old stock, but also I wanted a high speed output for my presumed continuous rotation four axis
tool paths. Since then the fourth axis has morphed into a trunnion where I would rather have greater torque authority but would be entirely happy with less speed.
I would recommend a 50:1 or 100:1 reduction, most probably a harmonic drive if you can find an affordable one with a modest motor, say 2Nm. Whether it be a servo or a stepper is less important.
Torque authority is much more important than speed for a trunnion. Assuming you have something like 100Nm available at the trunnion, who cares about bearing drag?...it'll be miniscule by comparison to
the available torque.
A related issue is how you Home such a trunnion. I have found that it is necessary to Home to within 20 arc seconds, and preferably to better than 5 arc seconds. This requires some thinking about.
The most likely and cost effective solution is a regular home switch, be it mechanical, optical or whatever, followed by Index Homing. This would require the motor have an Index output and that your control
system supports Index Homing.
That is a huge conundrum. If you were of the opinion, or better yet, had pre-orders for ten or twenty machines, the choices you get to make a vastly different if you have the prospect of building just one.Quote:
Now if its a production machine and say they are made in 10 unit runs I'd seriously look at carbon fibre or engineering grout.... These require moulds but that's where the benefits start to happen....
I have often contemplated if I could replicate my machine or some production ready facsimile thereof....could I sell them?, and if so in what sort of volume?. If you were building dozens of them then the choices
become much MUCH wider. My own conclusion is that I could not sell them in the sort of volume that would be required for me 'to go full steam ahead', as much as I would like to have it otherwise.
Craig
Hi Craig - Epoch is for plastic, foam and timber so the pushback is much less then metal. But I was looking at 50:1 anyhow. Once I get through the calcs I will know more. I have a harmonic drive catalogue here and in the marketing txt it says no backlash but in the tech specs some have 5arcmin and some have 10arcmin.. That's disappointing as precision planetaries have 10arcmins... I did squish the bearing this morning and it got better the harder it was squished. I used an M8 bolt and some large washers but I'm bending the washers so need to make a better rig... But its looking good so far...Peter