Trying to reuse rack and pinion

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Trying to reuse rack and pinion

Post by corfab »

I bought a old 8'x 30' cybermation plasma table that I'm going use parts of to build a new plasma table. one of the part I want to reuse is the rack and pinion. The table was originally set up with servo motor but I'm going to go with steppers.I have around 35 ft of of rack.

The problem I have is trying to identify the specs of the rack and pi pinion. is the a way to measure and identify what pinion I need to buy that fits both my stepper motor and the original rack?

Is it a good idea to use a rack that was originally set up for use with old servo motors?

Is this rack giving the condition goingbto be accurate enough once it's cleaned up?

Also I have the old d.c. servo motors, encoders and what seams to be the 40:1 belt driven gear reduction. I have a bunch of motors and 3 gear reduction all are currently working. Are these worth anything? Or can I reuse the gear reduction?

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Re: Trying to reuse rack and pinion

Post by tcaudle »

The important factors in the rack and pinion are the |Pitch" (distance between teeth) and th epressure angle (two choices: 14deg and 20 deg) so here is the things to consider:

The pinion |speed" (distance it travels in one revolution) is the Pitch Diameter (DP) times PI (3.1416) DP is actually the diameter of the gear to the Root of the teeth so its slightly less than the actual overall diameter/ That begin said, for estimation and sizing its okay to just measure the diameter and multiply 3.142 or even X 3 for quick rough numbers. So a 1" pinion rolls about 3 inches per one revolution. to get final max speeds you simple multiply that times the max motor RPM. so those DC servos will probably do 3000 RPM and direct drive you would hit 9000 IPM . Somewhere close to the escape velocity of the earth !. The other down side is you LOSE torque from the motor no matter how slow you actually turn the motor and with that you lose RESOLUTION.

So it obvious that some form of reduction between the motor shaft and the pinion is needed . In the case of the servo it takes the form of a gear head . Doing some quick math you could get about 990 IPM with a 10:1 ratio gearhead and 1" pinion. Any change in the pinion diameter effects the speeds with a 3:1 facto . Example" A 3 1/2" pinion diameter gives you 5:1 step up instead of 3.14 :1/

Lets look at steppers. They are lower RPM with more average torque than same size servos. A stepper of the same size (34 mount) will have about 1000 oz-in of torque at low RPM. It loses torque with RPM so you design around not pushing them above about 60% max RPM for cutting and 75% for rapids.

Most steppers have RPM ranges (max) in the 800 (big 34 motors ) to 1200 for smaller 23 frame motors) Use 500 for the 34 and 650 for the 23 under load.

Since steppers lose torque with RPM you want the most torque at the highest RPM you can get. In short the "golden ratio:" is what ever provides one inch of linear motion for 1 revolution of the motor. On a 1" pinion that is 3:1 reduction to cancel out the 1:3 step up from the diameter X PI number. #;1 is easy to get with belts and pulleys. Higher than that usually means two stage belt-pulley setups OR gearheads.

So there you have it. The rack is what it is and the challenge is to ge the gearing correct so you optimize the motor and speeds.

Some of it depends on the type cutting you will do. Plasma is done from a few IPM or really thick material up to almost 450 or really thing stuff.
Most cutting is done in the 100 to 300 IPM range .

Routing needs high torque (to shove bits tough dense material and lower IPM . Mot of that cutting is done in the 100 IPM and down range.

Count the number of teeth in 1" of the rack . divided 1" byt the number of teeth to get PITCH. A lot of rack is sold by the teeth per inch and the pitch. So a number like "20-20" *20 TPI - 20 deg pitch) is real common for US bult tables back in the 90's and early 3000's.

Bigger motors will not go faster . They WOULD let you get better acceleration and let you change the ratios some but just bigger motors do litle to help anything but torque.

Your existing motor sand gearheads are probably not something you wan tot try and use with modern controllers. While you MIGHT could use the motors you need to regress to a pure DC servo controller that uses a conventional digital rotary encoder (Like the Gecko G320 series) . On encoder runs of over 10ft you start to have problems with noise from plasma so you need shielded encoder cables OR differential drivers/receivers .

One other note about servos> They use a "PID " feedback loop that has to be "Tuned" to the motor. its operation voltage. and the dynamics of the load its moving. It can be a 3 pot set of adjustments (like the Geckos have) up to the laptop and USB cable method you do with the Clearpath self contained BLDC servos. I would advised against that choice unless you want to wade though a 130 p;age manual and learn a whole lot of new terms.

Pure DC PM motors like those MCD you have are largely obsolete and hard to find if you need a new one. Only thing more scarce than those are the "Smart" motors use on the a lot of tables.

Well , enough for today's lesson on Motors and rack and pinion!
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Re: Trying to reuse rack and pinion

Post by weldguy »

Awesome and informative post Tom, thank you :Like
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