Thread: Wobblycogs CNC Mk1
Wobbly.....steel prices for you
50 x 50 x 4mm box - £23.77 Each (7.6mtrs)
50 x 25 x 3 box - £13.25 Each (7.6mtrs)
50 x 5 flat - £6.99 Each (6.4mtrs)
VAT will (unfortunately) be on top of these prices. Anyway, once you've finished tweaking your design let me know and I'll firm up the prices :DNeil...
Thanks Neil, I never thought I'd say it but compared to what I thought I was going to have to pay at Metals4U VAT doesn't seem like such a hardship!
As an aside, I was talking to the wife about the build over the weekend and it occurred to me that box section steel is actually about the same price as wood. 7.6m of PAR pine would set you back about £19 if bought in 2.1m lengths and considerably more if you could source it as a single length.
Ok, so I stayed up to the wee small hours trying to figure out what size ballscrew and stepper I should be using on the X-axis but there's some things I don't think I'm understanding. I'd like this machine to be able to cut aluminium so my calculations are based around this. My thinking is that if it can cut aluminium then wood shouldn't be a problem (hope that's right).
In the post describing the motor calculation it says that for cutting aluminium a speed of 1800mm/min or better is desirable but the best I can achieve, even with the largest Nema 34 motor, seems to be about 1600mm/min. In this post though Jazz talks about cutting at around 700mm/min which is well within the realms of possibility. What speed should I enter for Max linear speed (B22) when cutting?
From trial and error I've found two setups that appear to work
Nema 24 (Zapp, SY57STH76) + RM1610
Nema 34 (Zapp, SY85STH80) + RM2510
I wasn't sure exactly what Screw Fixing (B19) meant so I left it as supported-supported. Would bearing blocks be considered fixed though?
What I don't understand though is why changing from a 1610 ballscrew to a 2510 nearly triples the total torque but doubling the mass of the gantry to 70kg makes only a small difference (in fact in the scenario with 24's is still ok with a super heavy gantry but the 34's are out of the safe range).
Am I doing something wrong here? The smaller motors and ballscrew seems to be a much better choice which feels totally counter intuitive to me.
The common bearing blocks are somewhere between supported-supported and fixed-supported since the bearings are not sufficiently spaced to truly count as supported:
Fixed-end, supported-end, free-end are ball-screw support conditions you need to understand in order to design the best actuator system for your equipment
You need to increase the motor voltage - 70-75V is common for Nema 23/24 motors and Nema 34 motors will work on the same, but can be operated from mains voltage with expensive drivers, which shouldn't be necessary here.
The reason the increasing the ballscrew diameter from 16mm to 25mm makes such a large difference is because the moment of inertia of the ballscrew is proportional to the radius to the power 4, so you need (25/16)^4=5.96 times more torque to accelerate it at a given rate. The gantry is driven via a huge mechanical advantage, and increasing it's mass only increases the inertia proportional to the mass (no power), so it doesn't make such a big difference.
Very interesting article, thanks. I'll leave it set to Supported-Supported to be on the safe side.
Should "Set Phase Current" be the rated current of the stepper? I'm sure that's going to be the winner of stupid question of the month :-).
I couldn't help feeling I was drowning in a sea of numbers comparing the various options so I put together a little table (hopefully others might find this useful as a starting point). I threw in a single calculation for a shorter X-Axis as well just to see how much of an effect that had.
# Ballscrew Length
Stepper Nema Supplier Price Max Speed
Limiting Factor 1 1610 1800 60BYG301B 23 CNC4YOU £25.99 3500 Critical Speed 2 1610 1600 60BYG301B 23 CNC4YOU £25.99 4500 Critical Speed 3 1610 1800 60BYGH401-03 23 CNC4YOU £38.99 3500 Critical Speed 4 2510 1800 60BYGH401-03 23 CNC4YOU £38.99 1900 Torque 5 2510 1800 86HS115-4208 34 CNC4YOU £79.99 4000 Torque 6 1610 1800 SY57STH76-3008B 23 Zapp £26.40 2750 Torque 7 2510 1800 SY85STH65-5904B 34 Zapp £51.79 1500 Torque 8 2510 1800 SY85STH80-5504B 34 Zapp £75.60 2100 Torque 9 2510 1800 SY85STH156-6204B 34 Zapp £108.17 2100 Torque
Voltage set to 70V for all steppers
Phase current set to rated current of stepper
2510 screws have a minor diameter of 19mm
1610 screws have a minor diameter of 12mm
Speed is set for cutting
Fixing is Supported-Supported (I'll be using Chai fixings)
Gantry weight of 35Kg
Looking at the table I think my best bet if I want to go for 1800mm on X is row 5 which gives me 4000mm/min when cutting (and 7000mm/min in rapids). Row 1 looks like a good cheaper option though with only slightly reduced speed but rapids would be the same as cutting.
So, to all those who know a lot more about this than I do, what would you do? Please feel free to tell me I'm massively over-spec'ing / that I should build a smaller machine / suggest other parts etc.
My advice is only build this length if you truely need it because over 1500mm your in funny territory. 16mm is too close for whip and vibration etc and has you've seen 25mm takes you to another level regards motors/drives/PSU and for no gain really.
Your Max speeds are a bit off mark thou and provided you use decent voltage you will easily get more than 4000mm/min cutting speeds. Don't get caught up in the critical speed figures has they are wide margin and in practice you can get much higher speeds provided the power is there.
I run twin 1500mm 20mm 5mm pitch geared 1:2 with 75v and 6NM 34's and can easily cut at 7mtr/min and rapid @ 11-12mtr/min. Going by the specs for a 5mm pitch screw then they are probably spinning close to twice the critical speed and have been doing for years without any trouble.
Now I'm not saying to do this has 10mm pitch is clearly better and less hassle. When I built my machine 10mm wasn't available from Chai or china and far too expensive in UK so this was my only option, But today thats not the case so go for 10mm.
Regards cutting Ali @ 1800mm/min then thats not realistic for DIY machine at any decent DOC(Really it should be removed from the sheet has it's misleading.!!) and would need a very very strong machine with a very powerful spindle. I rarely cut Ali much above 1200mm/min and 95%+ of time it's below 1K.
I you must have the length then IMO go for the 25mm/34 combo has it will give the least hassle and yes while in theory it won't quite reach the speeds 16mm Dia would allow it will still easily full-fill your needs.
The Following User Says Thank You to JAZZCNC For This Useful Post:
Thanks Jazz that's spot on what I needed and really helpful, I think I'm just starting to see how the multitude of factors play off against each other.
I suspect the speeds in that table are a bit low because I went for supported-supported and was quite conservative with the limit.
I'll have to get the calculator out and see what the financial damage is if I stick at 1800 on X :-). Whether I build it 1500 or 1800 I've got to get a load of stuff out of the workshop so I'm tempted to stay with 1800. I've also got a project in mind that would need more than I think I could reasonably do on 1500 (lol, two bad excuses)
I realized today that my frame design makes mounting the screw bearings really awkward so it might be back to the drawing board for that. My aim of getting the mechanical side of things done by the end of the year is looking wildly optimistic already
The X-axis on my machine is 1700mm travel, with RM2510-2094mm ballscrews and rotating nuts. The highest it seems reliable at with 3Nm motors is 12m/min (so about the same as Jazz's but with substantially smaller motors), although it will intermittently run at 15m/min still with no signs of vibration/whipping problems. Tenson here has RM1610-1500mm and rotating nuts, so you may wish to ask him how it is performing as that should be a good indicator of the feedrate that configuration would get with 1800mm.
If sticking to a conventional setup it's not so clear cut. RM2510 and Nema34 is probably the safer option, but your table suggests there's a significant chance that the RM1610 and 3Nm motors would outperform it quite significantly you could go for the latter as it is the cheapest option and if whipping is a problem change to a rotating nut at a later date and keep the same motors/drivers.
Don't see the point of going to all the expense and trouble of implementing a rotating nut design and all it's complexity's when the standard setup will still allow everything he needs anyway.? The extra expense of components and having someone (Namely You) machine the rotating nut assembly cancels out extra cost of 34's etc for what is affectively wasted speed has he'll never cut at those speeds and Rapids are only usefull in quite limited circumstances.!
If speed is required then yes I'd take the rotating nut assembly route has it clearly will allow higher feeds but if cutting below 7mtr/min which most materials are then why go to all the trouble.??
BUILD LOG: Wobblycogs CNC Mk2By Wobblycogs in forum DIY Router Build LogsReplies: 113Last Post: 10-02-2015, 12:37 PM