Ballscrews Vs Software

[KEYTEEM]

BEING A GREENBEAN to the whole cnc thing can i ask what might be a foolish
question????

does the software (mach3) control "all of" the motor speed (servo), and if
it does then does it really matter what tpi i get ,, 5tpi or 10tpi???

i tried using a spreadsheet calc for desired ipm feed with 33003 and 33004
motors from a search i found at cnczone ,,but ,,BEING A GREENBEAN
it basically has just confused me more , but if the software controls it
then why worry so much about tpi etc. i can see why 2/1 ballscrew PULLEYS
should be used though.

any input would be helpful , thanx
Den

[NC Cams]

Look at it this way:

You have a computer that deals in 1's and 0's, no more no less.

You have a LPT port or drive card that takes these 1's and 0's and turns them into an electical signal that tells the motor to turn and in what direction and how fast and for how long at any point in time via a control card of some type.

You now have a motor that works in volta and amps to put out so many RPM at a certain motor terminal voltage that is controlled by the card attachd to the computer .

You have gear train that takes motor RPM and steps it up or down depending on the ratio via the control card via the computer..

You have ball screw that moves so many inches for each revolution of the screw that is stipulated by the motor at the gear ratio selected that is driven by the card attached to the computer that is driven by the software.

Given these very basic and admittedly overly simplified details, how do YOU think that all this stuff gets inter-related/communicated from one mode to the othe unless SOMEBODY tells the computer what sort of gearing is invloved and what sort of screw TPI is being drivenr????

Once you grasp this over simplification, can you see why a fairly dumb computer needs to know what's going on between it and the end of the screw that's hooked to a table via god knows what in the way of screw TPI and gearing and motor speed constant????

Just like driving a car only different feedback and response mediums....

[KEYTEEM]

I might have asked incorrectly, what i mean is if the software is basically
telling the motor to spin a certain time/speed then how do i determine
what tpi i need when purchasing ballscrews for a conversion ?



thanks nc i just can't seem to figure if i need 5tpi or not

[miljnor]

I think what he was asking is why is the ratio so important.

Several reasons:
1. Mach3 can only output a certain amount of pulses per second (until grex is up and running, but that is more expensive)
2. Higher gear reduction might be important for an undersize motor to do the work of a real mill. (everyone out there probably wants their cheap DIY jobber to be able to run like a Yasada with 2000ipm, Aint going to happen but thats what we are hoping (at least I am! ))
3.PRICE you can only get certain sizes of ballscrews for a good amount of money. If you look at a manufacturing you will notice a certain size (lets say 5tpi) is cheaper than the ones bigger (by a significant margin) or the smaller ones (probably do to Supply and Demand).

These are the reasons I can think of. There are probably many more.

hope this helps.

[Mike Mattera]

More TPI is a more accurate screw. 5TPI is moving .200"/inch per rev. 10TPI screw moves .100"/inch per rev. Finer more accurate motion. It will also have to do with the rapid speed.

5TPI is a faster screw. It moves a greater distance for each rev.

These are the +'s & -'s that need to be figured into your software/motion controllers capability.

Mike

[ViperTX]

Well to truly understand....you have to sit down and design a system......pick a ballscrew with some tpi......then pick a stepper or servo where you have the torque curves.....then speculate on what sort of 'cut envelop' you have.....now calculate the amount (number of steps) that it will take you to travel x-number of inches......so, now you have figured out how long it will take you to get from point a to point z......that will be your IPM (inches per minute).....that should get you started.

[wizard]

Hopefully I can simplify a response here. There are two things you need to consider.

One is the resolution of your system. Say you have a stepper system and are resolving one step. That is the minimal rotation on the screw is one step and for grins we will say that equals 1.8 degs on the motor shaft. The motor then turns that shaft that much or an amount determined by a drive ratio. In any event lets say the leadscrew shaft turns 1.8 degrees, you should now be able to see that a machine with a high TPI count will move the motor less than one with a lower TPI count. The pitch of the lead screw determines (in part) the resolution of the system or it impacts the overall ratio of the system.

Another issue is the torgue being applied. A screw is nothing more than an inclined plane wraped around a rod. A high TPI leadscrew has a mechanical advantage that is greater than a low TPI leadscrew. At least in theory, at some point the balls become so small that mechanical advantage can be difficult to realize.

All that being said you are other wise right in that the CNC control should be able to adjust for any lead you may have on the leadscrew. There is an issue with resolution though if you go the wrong way, the CNC can not resolve at the work piece less than the mechanical system allows for. This applies equally to stepper based systems and resolver/encoder based systems. In a feed back system the CNC has a set number of "pulses" that can be seen per revolution of the leadscrew shaft. So if you have 1000 counts per revolution, the circle of revolution gets divided up into 1000 parts. With the overall system ratio (including the leasdscrew TPI) you can determine real world mechanical resolution.

So the leadscrew TPI works in conjunction with the rest of the ratios in the system to determine the distance moved per count. <<<<<<That is it in a nut shell>>>>>>

As a rough guide though, I generally want systems to resolve better than the mechanical accuracy needed. Of course like all things in life there are a lot of opinions here. But if you want to maintain accuracy of 0.001" you should set your system up to resolve below that, with 0.00025" resolution being good. That is one fourth the desired resolution. I've seen and worked on systems that go well beyond that though.

I hope that my geting ot the nutshell hasn't put you to sleep.

Dave

[KEYTEEM]

xcellent help ,thank you.
now how about motor speed as in how safe is it to run a servo
at 25 rpm when its rated at 0 to 4200rpm continuous?
mcg motors to be an example .
at 2.5ipm with 5tpi-i will have to spin the motor at 25rpm i believe.
with pulleys at 2/1
i need to mill aluminum also so i do need a little more than this.
do i need pulleys at 4/1 ?
is 25rpm ok for a servo motor ,will it harm brushes and or other parts
prematurly .
i don't mind xperimenting with drive pulley ratios after its setup with
screws but i dont want to have to redo screws to be safe , now
what i mean, they arent cheap

again thanx much for your help guys,
this site is awesome

[NC Cams]

Does it seem even seem remotely reasonable to use ONLY the 0 to 25 rpm range of your motor when you have a continuous 0 to 4200rpm motor speed range capability to work with???

The method (asside from the math) to determine the answer to your gearing question is outlined in post #6 above...