I think you CAN measure the forces expected.
Using Tim's method you end up with a larger, more expensive motor then you need. Maybe you don't care if you are using it for a CNC machine that already weighs 2 tons. what is an extra 6 pounds?
One of the things I'm working on is a robot arm. There is a motor in the base that lifts the entire arm then there is an "elbow" motor. If the elbow motor weighs more than needed it reduces the payload the hand can lift. Because motors are lifting other motors you want all (but the first) motor to be as small and light as possible but also as powerful as possible. You end up with large motors in the shoulder joint and small ones in the fingers. Pretty much the way human arms work. You start the design at the hand, make it as strong as needed to lift the objects that need to be moved. But make it no stronger. Next select the wrist motors and work backwards to the base. When you find that you need a $1000 motor at the base you start over thinking harder about weight reduction of the fingers, wrist and so on.
The motors all come with torque curves. They tend to produce less torque at higher speeds so the first design decision you have to make for a CNC machine is "how fast". If you think 50 inch per minute (IPS) jog speed is fast enough then you can select a motor. Conversely if you happen to have a motor, even a tiny one will work but you speed will be determined by the motor. So any motor can work but you are best off by FIRST deciding on a speed. 100 IPS is very fast 50 is not bad for a low cost mill.
How to measure the force required. You might wrape a string around a hand wheel and pull the string with a fish scale and not the force on the string. Multiply by the radius of the hand wheel.
A slightly more sophisticated method uses weights on the sting. Drop the weight. Add weights until the hand wheel moves at the speed you want (say 50 IPS on the table max speed) Then just double the torque and buy a motor that has the required torque at that speed (look at the speed/torque graph.
Or simply buy a motor based on price and live with whatever (random) speed it can do, with a $50 per motor budget it will likely work
If you decide to replace the lead screw with a ball screw then you can use some math rather then experiment with strings and weights. Ball screws have almost zero friction so the theoretical equations apply. They have so little friction you can push the milling table and spin the motor. In the case of ball screws the motor power is only used to accelerate the table, not to overcome friction in the screw for Newton's F=ma applies. But for norm lead screws MOST of the motor power goes to screw friction and you have to measure, not calculate it.
ONE more complication. You can of course double the the torque of a motor if you gear it down 2:1. This is easy to do with a simple belt drive. The belt drive allows you to move the motor out of the way too.
It would be wasteful not to run motor at it's full rated current. They are even spec'd for a "peak" current that can by applied for short periods
Motors are typically rated for an 80C temperature. Going higher will damage the motor. I would say that unless you install a temperature gauge, epoxy it to the housing you need to maybe limit temper to about 60C so there is some margin. (But in a high performance design you might just install the temperature sensor, they are cheap enough.)
60C which is a good practical limit feels pretty warm, you can stand to live your fingers on it for a short time but you would not want to hold a 60C motor in your hand for more than a minute. At 80C your natural reaction iis to take your hand off of the motor. You would not burn your hand if you did but 80C is to hot to hold in your hand, 60C is bordering on that.
Yes you can run them "cold" but this means yo spent twice as much money as needed on the motor and the driver and the power supply plus the weight is about double what it could have been.
Bottom line: there are two ways to design this (1) just use whatever you happen to have and live with whatever performance you get or (2) design on the performance you want, measure the force needed to get that performance. Decide if you want to use a direct or belt drive then buy a motor about double the size the measurement would indicate. Then size the driver and power supply based on the motor.
It is really easy to meause torque. With a weight and string wrapped around a wheel.
On Thu, Mar 22, 2018 at 5:16 AM, old_toolmaker@yahoo.com [7x12minilathe] <7x12minilathe@yahoogroups.com> wrote:
Hi Chris,
That is simple enough. How does one size a stepper for a given purpose? Especially when one has no way to measure the forces expected?
I have understood how steppers work and hold position for years. They were on the first CNC mill I bought back in the 1980's. The downside is the heat they produce. Everything has pros and cons.
Dick
Chris Albertson
Redondo Beach, California
__._,_.___
Posted by: Chris Albertson <albertson.chris@gmail.com>
| Reply via web post | • | Reply to sender | • | Reply to group | • | Start a New Topic | • | Messages in this topic (32) |
Have you tried the highest rated email app?
With 4.5 stars in iTunes, the Yahoo Mail app is the highest rated email app on the market. What are you waiting for? Now you can access all your inboxes (Gmail, Outlook, AOL and more) in one place. Never delete an email again with 1000GB of free cloud storage.
.
__,_._,___
No comments:
Post a Comment