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Gear ratios and steppers

Posted by Tierod 
Gear ratios and steppers
January 13, 2018 10:42PM
Hi everyone,
I’m stuck, I’m trying to figure out the proper gear ratios for an indexing chuck using a nema 17 1.8 degree stepper
I’m mounting the big gear (125mm printed) on the back of main shaft on a mini lathe.
I’m okay designing and printing both gears but my issue is figuring out exact ratios to match the 200 steps.
My goal is to turn the lathe chuck 360 degrees in minimum 1 degree increments.
I need the big gear this big to provide mech advantage and mount stepper. The stepper will be able to swivel away from gear.
I hope I’ve provided enough info. Any help is appreciated.
Re: Gear ratios and steppers
January 13, 2018 11:49PM
Don't worry about getting the gear ratio to exactly match 360 degrees. If you have a 5:1 gear reduction, and use 32 microstepping mode, each microstep will be .01125 degrees. This is more than accurate enough for indexing. Use a gear ratio that gives you the holding torque necessary.

I don't think NEMA 17 steppers are going to have enough torque to hold the work in place during the machining operation. The highest torque stepper I know of has 120 oz-in holding torque, more common are 60 oz-in motors. This NEMA 23 motor would be one I'd consider.

Edited 1 time(s). Last edit at 01/13/2018 11:50PM by etfrench.
Re: Gear ratios and steppers
January 14, 2018 12:04PM
thanks for the reply Etfrench

so the higher the ratio the more accurate and better mech advantage.
what ill do is see how small a tooth i can accurately print and go with whatever ratio that is.
i should be able to print gears with at least 10:1 ratios with 125mm diam big gear.
I'm going to try the nema 17 as I'm only drilling small holes in plastic and aluminum.
I'm saving the link you gave me, thanks!

should i start a new topic some where else to ask about using mega/ramps and software?

thank you,
Re: Gear ratios and steppers
January 14, 2018 01:01PM
Just use a worm gear or:a Nema17 with gearbox (worm or regular gears). Can be found in many places.
Dividers are built that way.
No need for a Nema23 and printed gears are close to worthless considering what is available and for cheap.

Edited 2 time(s). Last edit at 01/14/2018 02:56PM by MKSA.

"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)
Re: Gear ratios and steppers
January 14, 2018 02:42PM
You have a stepper motor with 200 steps per revolution and want 360 steps per revolution. Reading between the lines It sounds like you want to put a sheet of metal on the back of your lathe with a NEMA17 stepper bolted to it with a spur gears on the stepper and another to drive the chuck. What MKSA said is right, but don't drive a printed worm wheel with a printed a printed worm - that way lies sucky doom. It may be do-able with spur gears.

To get 1 degree per step on your lathe chuck the gear on the chuck needs to have 1.8 times as many teeth as the one on the stepper - e.g., 90 teeth on the chuck and 50 on the stepper. You will probably want to get a finer resolution but don't count fractional/microsteps beyond 1/2 or maybe 1/4 as the holding torque is not great. Without knowing what load you are putting on it I can't estimate but if it is something light I would use a 90 tooth 1 module ( about 90mm in diameter so fairly printable) gear to drive the chuck and a 20 tooth gear on the stepper: this combination will need 2.5 full steps or 5 half steps per degree. Consider using a steel HUCO spur gear such as the 101213020FAR instead of a 20 tooth printed gear - you may need to bore out the hub and drill for grub screws.

For the 90 tooth gear generate at module 1.0 and a 20 degree pressure angle. printed should be O.K. but depends on the cutting load.

Re: Gear ratios and steppers
January 14, 2018 04:13PM
I used GT2-3mm timing belts instead of spur gears on my Rose Engine. Large pulley is 180 teeth and the small pulley 20 teeth. It's driven by a 90 oz-in NEMA 17 motor. Cutting forces are quite a bit lower than they are on metal lathes. There are quite a few examples online of people printing module 1 gears to use on metal lathes, so I wouldn't worry too much about having them fail.

p.s. The diameter of a module 1 gear is the same as the tooth count.
Re: Gear ratios and steppers
January 16, 2018 10:05AM
Thank you for the information

I have a 20 tooth aluminum gt2 pulley so I’m going with Etfrench’s system
It’s most likely a bit more forgiving than a printed gear.

I have to say this, I’ve watch this forum about 4 years.
I’ve been helped myself so many times in my hobby as have others.
Thank you to all those who spend their time to help others!
Cheers to all!!
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