Rack & Pinion Reduction or 1:1?

[cncjunky2011]

Hi Everyone,

Im trying to integrate a rack and pinion system into my machine and have a question regarding the gearing of the pinion.

I understand why reduction is needed and appreciate the difference it makes to the resolution and machine accuracy. I cant stop wondering however how bad the results would be if a 1:1 ratio was used, e.g. directly drive the pinion on the rack.

I have been looking around and very little information is available on R&P, I know Ahren sells kits but apart from that its a subject nowhere near as popular as other linear motion systems.

Anyway, I have 3 small nema 34’s (3NM) and would like some opinions on using a 1:1 pinion which has around 21 teeth (mod 1), the diameter of the pinion is around 20mm giving around 60mm if travel in 1 revolution (sounds biiiiiiiiiiiiiig I know).

I have found a system that uses direct drive from Marchant Dice in the UK (where Im from BTW), their video shows the machine operating at a very fast pace but this still offers no encouraging signs regarding accuracy. The video can be seen here: ‪Rack and Pinion.AVI‬‏ - YouTube

Does anyone have any experience they could share regarding direct drive or have any advice on any work arounds without having to gear down?
Cheers guys, any advice is much appreciated.

CNC J

[RomanLini]

At a first glance with a "pinion of about 20mm" and "small" Size34 steppers that might have 300 oz-in you will only be getting linear force of about 300*25.4mm/10mm = 762oz which is about 20kg. That seems really low for moving any decent sized machine as it needs to account for all cutting loads, accel forces, and still allow a decent safety margin (which I don't think it does).

A forum member called CarveOne has a very impressive rack and pinion driven machine in his thread;
http://www.cnczone.com/forums/cnc_wo...c_router.html?
and that might give you some ideas as he has written about the problems and issues of rack drive there.

[CarveOne]

Step motors produce the most torque at low rpms. Using a direct drive at 1:1 ratio just means that the motor rpm is higher for the same ipm feed rate and the torque is lower as a result. It will work, but you will need to keep the feed rate lower when cutting harder materials so that you have enough torque to do the job without stalling or losing steps.

3:1 is used for the NEMA 23 drives from CNCRouterParts because it is about optimum for most of the motors in that case size. A different ratio is needed for the NEMA 34 motors.

Other r&p drive assemblies are available from McMaster-Carr, and many commercial machines and some home-built machines use proprietary designs.

Direct drive could also be used with geared motors and keep things simple. More of the expense goes into the geared motors though.

I use CNCRP NEMA 23 r&p drives, Keling 495 in-oz motors, G203V drivers, 72vdc PSU, and can rapid at 1,400 ipm. Haven't tried pushing it any faster than that. I think it can go faster but it is going 2x faster than I ever expected anyway.

CarveOne

[jcoats]

On MechMate.com they normally suggest using a gear reduced stepper with a 7.2:1 ratio and 1" diametrical pitch spur gear to drive a rack.

Does that help?

[ger21]

Originally Posted by CarveOne Step motors produce the most torque at low rpms. Using a direct drive at 1:1 ratio just means that the motor rpm is higher for the same ipm feed rate and the torque is lower as a result.

You have that backwards. The stepper spins 3x faster with the 3:1 reduction vs direct drive.

Originally Posted by CarveOne 3:1 is used for the NEMA 23 drives from CNCRouterParts because it is about optimum for most of the motors in that case size. A different ratio is needed for the NEMA 34 motors.

I personally believe that a higher ratio than 3:1 is better. Higher ratios than 3:1 are difficult to achieve with off the shelf pulleys and belts. That's a major reason why 3:1 is so common.

You may get 1400 ipm rapids, but if you could use 5:1 or 6:1, you might only get 700 ipm, but you'd get much higher acceleration, which would result in better performance.

As far as a different ratio being need for Nema 34 motors, I don't buy that either. The ideal ratio is calculated from a combination of things. Mainly, you design a machine to meet specific performance goals. Once you have the machine designed, you can choose a motor to fit your needs, then calculate the ideal ratio to maximize performance of the specific motor for your specific machine.
Two different Nema 34 motors may need drastically different reduction ratios.


Back to the original question. It really depends on what your goals are. Both resolution, and performance. With 1:1, imo, both performance and resolution will be poor. It'll be capable of very high speeds, but the low acceleration probably won't allow you to reach them.

[CarveOne]

I usually do get it backwards when I do these mental contortions involving gearing ratios. Sorry about that.

CarveOne

[crane550]

I have settled on 4:1 myself with a R&P and 900oz Nema 34's. I have seen the exact same setup used with really good results. Just my .02c.

Alex

[yancho]

At high speeds and loads periodically varying turnover every small or big wheel, it is desirable to be mutually prime numbers or have the lowest common denominator, in which wear is more evenly distributed between your teeth.3 is the lowest common denominator.Do not forget the play in the gearing will affect the accuracy of movement.Yancho

[ssutton]

4 to 1 ratio here on my rack setup. I was concerned about this at first because of the calculated tooth contact on the small pulley, but I am very pleased with it and have not had any hint of belt jumping over teeth. My gantry is steel and aluminum and just over 60 inches wide. I am moving a bosch 2.25 HP router and a 4 inch dust collection hose. My steppers are only 265 oz in nema 23, but they have plenty of power to move at the speeds I use, I have rapids set at 400 ipm and I can cut under moderate load at 200 ipm. To this point I have not had a stepper stall when cutting. Ger21 has made me curious what others have acceleration set at in Mach. I have had mine all over the place, I can accelerate so hard that my rack springs start to unseat the pinion so I backed it off.

Scott

[ssutton]

Drive train on my machine;

[yancho]

For my previous post, this is only for couples gear,in rack step is calculated with 3.14.....-
М = 1, z = 21, L = 3.14 x21 = 65.94 mm. d = z x m = 21x1 = 21mm.Thanks