Wobblycogs CNC Mk1

[Wobblycogs]

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 mm	Stepper	Nema	Supplier	Price	Max Speed mm/min	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

Assuptions
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.

Cheers

[JAZZCNC]

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.

[Jonathan]

I think I'm just starting to see how the multitude of factors play off against each other.

Which is precisely why I linked you to the spreadsheet instead of just telling you the answer , but it seems that's what you're after so...

So, to all those who know a lot more about this than I do, what would you do?

I'd use RM1610 and rotate the ballnut instead of the screw. From using the spreadsheet it should be now be obvious why it is such a big advantage to not rotate the screw.
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.

[JAZZCNC]

The highest it seems reliable at with 3Nm motors is 12m/min (so about the same as Jazz's but with substantially smaller motors),

Not much difference really because I have single motor turning 2 screws and you have 2 motors, plus 34's are stronger and will hold more torque higher up the curve.!!

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.??

[Jonathan]

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.??

I agree - clearly the only reason to do it is if it works out cheaper or if you especially need the higher speed and (perhaps more importantly) acceleration. Personally I would do it even if the cost is slightly greater since the factor of safety will be much greater, so you will have a more stable system. Perhaps ask Chai for the price separately for two RM1610-1800mm and RM2510-1800mm so we can compare the relative cost more precisely?