In an effort to keep the screw length down I moved things around in the design and did a stack of calculations for different screw and motor sizes and then did a bit of pricing up. This spanned a couple of days and a lot of discussions with people that know far more about CNC than I currently do.
First up though is the table I produced to compare screw and motor combinations:
|#||Ballscrew||Length||Stepper||Nema||Supplier||Price||Max Speed||Limiting Factor|
|1||1610||1600||60BYG301B||23||CNC4YOU||£25.99||4500 (5300)||Critical Speed|
|2||1610||1600||60BYGH401-03||23||CNC4YOU||£38.99||4500 (7100)||Critical Speed|
|3||1610||1800||60BYGH401-03||23||CNC4YOU||£38.99||3500 (6700)||Critical Speed|
|5||2010||1800||86HS115-4208||34||CNC4YOU||£79.99||4700 (5600)||Critical Speed|
|6||2010||1800||86HS80-EC-1000||34||Zapp||£540.00||4700 (6700)||Critical Speed|
- Voltage set to 70V for all steppers
- Phase current set to rated current of stepper
- 2510 screws have a minor diameter of 19mm
- 2010 screws have a minor diameter of 16mm
- 1610 screws have a minor diameter of 12mm
- Speed is set for cutting
- Fixing is Supported-Supported
- Gantry weight of 35Kg
As well as the assumptions there are a couple of things that need to be explained about the data. The eye-watering price for the 86HS80-EC-1000 includes a top of the range driver. Zapp don’t appear to sell the motor separately and I didn’t fancy going looking for a price. Where the maximum speed was limited by the critical speed of the screw the number in brackets is the torque limit. For the bigger screws the limit is the torque of the motor which can only be overcome by using a larger motor. The critical speed on the screw can, however, be altered by better supporting the ends of the screw. In this table I have selected supported-supported for the ends of the screw but in reality the support I’ll be using is probably closer to fixed-supported. Additionally the critical speed calculation is apparently highly conservative, people I’ve spoken to have successfully run at twice the calculated critical speed for brief periods. There’s a good article here about end supports on screws. These calculations were done using this spreadsheet.
While the figures produced by the spreadsheet are good they don’t necessarily take into account the real world issues of build the machine and getting the parts. I’m most likely to go with the 2510 screws even though the 2010 appear to offer better performance simply because the supplier I’m planning on using doesn’t stock 2010 screws. There is an alternative supplier I could use for 2010 screws but they work out almost £80 more than my preferred supplier and they don’t have a reputation in the community (no one has bought from them as far as I can see).
Interestingly, looking at the figures, it appears that the 1610 screws would work well, for example row 3, and with smaller steppers but I’ve been advised that the real world would conspire against me if I went for this option. For a start there’s a chance the screws would be damaged in transit since they are quite long and slender, then it’s more difficult to mount then as even a slight misalignment would cause whipping. Finally, because they are long and thin, they may have a tendency to vibrate or cause resonances in other parts of the machine. All in all the advice was they would be hassle.