Wobblycogs CNC Mk1

[Jonathan]

Hmm, just read the beginning of this thread again. You're after 1220mm travel on the X-axis but we're discussing buying 1800mm ballscrews, so did you suddenly decide to increase the travel?

300mm is a good width for the gantry, so to get 1220mm travel you could easily use 1550mm(ish) ballscrews. If you're slightly more imaginative with positioning the bearing mounts then you only need the travel plus the length of the ballnut and end-machining, which for RM1610 is 57mm for the ballnut and 76mm for the end machining (including 10mm extra for the pulley). That makes the required length 1220+57+76=1353mm. Call it 1400mm just to 'be safe' and suddenly the motor and ballscrew choice becomes a lot easier as the critical speed is much greater...

On my machine the rails are 2000mm with 2094mm ballscrews and 1700mm travel. The new rails I got are 2200mm, but I'm keeping the travel the same and using the extra 200mm to increase the bearing spacing (among other things!) and using the same ballscrews. This is a similar situation since the frame and rails will be a fair bit longer than the ballscrews.

If you're after more travel then you could get about 1650mm travel with the 1800mm ballscrews.

[njhussey]

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 :D

[wilfy]

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 :D

where the frig did u get them prices?

[Wobblycogs]

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.

[njhussey]

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!

Damn...should have put a markup on it lol ;)

[Wobblycogs]

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.

Cheers

[Jonathan]

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.

[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.