Compensate for more backlash in the middle of the screw on an old mill with G-code

[flyingpickles]

Hello I have converted an old mill into an very basic CNC-mill. The thing is that the screw for the table have different backlash on different places. A lot in the middle because it has been more used here and not so much at the ends of the screw. I am making a G-code that will cut out around 10 pieces from a pipe but I wonder if it is possible with a some command in the G-code to tell the controller to compensate for the backlash and change the value for it along with the movement of the table? I think that I will measure the backlash for the ten places and then just insert the values in the G-code instead of manually calculate al the values for X and add and subtract for each movement. I know the fact that I can compensate for the backlash in the settings in the software (USB CNC from planet CNC) but I can only insert one value here, not different values for different places...

thankful for help!

best regards: flyingpickles.

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[deadlykitten]

hi pickles use uni-directional positioning

I have converted an old mill into an very basic CNC

isn't this harder then replacing / refresing the screw ? kindly !

[flyingpickles]

hi pickles use uni-directional positioning



isn't this harder then replacing / refresing the screw ? kindly !

Hello and thanks for the answer. I googeled that but I still dont know what that is. Do you mean that I shall install some kind of fysical mesure equipment on the mill that tell the software what position the table has?

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[deadlykitten]

hi pickels, uni-directional positioning involves positioning only among x+y+ or x+y- or whatever combination, but never positioning in more then 1 direction ( of each axis )

for example, if you are at x0y0, and you wish to position at location x10y10, then, if positioning is among :
... x+y+, you may go directly at x10y10
... x+y-, you should go at x10y15, and after that to x10y10
... x-y+, you should go at x15y0, and after that to x10y10
... x-y-, you should go at x15y15, and after that to x10y10

please find attached a file with such documentation for okuma cnc

imagine milling on a classical mill that has a lot of backlash : you would always use "uni-directional positioning", so to keep the backlash always "behind or in front" of the nut; if you should reverse the direction with 30mm, you would reverse more, like 40mm, then reverse again, so to avoid positioning errors caused by backlash, so to keep positioning as acurate as possible

classical mills are used this way ( especially those with a lot of wear )

Do you mean that I shall install some kind of fysical mesure equipment on the mill that tell the software what position the table has

seems to be a good idea, but, if you wish to put an abso-scale, consider also to replace / refresh the screw

r e f r e s h t h e s c r e w; or is it hard to remove it ? if you will do it, you may end up with a nice mill

---

if you can't implement backlash compensation into the cnc, then what about implementing it inside the software that generates the g-code ?

even so, i guess you are aware that you can not change directions while the tool is cutting, because the nut will be floating; also no climb milling, no helix; only drilling, facing, uni-directional pockets, etc ; not bad at all, may be more then enough for many common operations ( roughing a die ? etc )

kindly

[flyingpickles]

Hello again, yes that would work just fine fore some kinds of operations, the thing is that I want to cut some different patterns and as you said it wont work if I would do like helix cutting and so. But I have heard that there is a command that you can use for compensating the backlash and if you or some one else know what this could be I would be glad

best regards: flyingpickles

[deadlykitten]

I want to cut some different patterns and as you said it wont work if I would do like helix cutting and so

well, actually there is a chance to make it work, but partially, very limited


climb milling requires cuation when used on cncs, because it increases the wear, increases the play between nut and screw, because the cutting forces are sending the nut way ahead the comanded position

but, if you master it, then you can climb-mill at a low cutting force ( like light finishes ), that won't send the nut ( aggresively ) into a floating position


so, if you have a lot of backlash, but the machine is heavy, you may be able to cut light patterns that don't move the nut this means using the machine at a very low load ( = increased cycle time ), but you are very limited in available patterns : changing direction inside material, climb milling, some opened pockets, but no helix etc


i can't tune a controller, i can't compensate the controller for backlash, but i suggest creating the G-code with compensation within it

sorry for repeating, but why don't you refresh that screw ?

I have heard that there is a command that you can use for compensating the backlash

if you wish to tune that cnc, you may need to check a different forum; and i am afraid that you don't need a command, but a fine tune of each portion of the screw

i know that what you wish to do is possible on an Okuma cnc; those machines are calibrated ( before shipment ) for backlash compensation at each 30mm or less, and also they have functions to manage backlash ( after shipment ); this comes in handy when increased accuracy machining is needed, or when there is some wear on the ballscrew

also, it is possible to check how much nut floating does an operation input into the cnc, and what zone of the ball screw is affected; this is prevention kindly

[samco]

Best is to get rid of backlash...

Although you have a different controller - linuxcnc does screw mapping with a different settings for + vs -. so you could map the backlash over the whole screw both directions.

LinuxCNC Documentation Wiki: Screw Compensation
INI Configuration