DeskProto user forum

(copied to the forum from an email conversation).

I want to do some milling of a miniature train wheel, about 3.75" in diameter. When I write the NC file, the g-code makes the A-axis rotates at the same rate in degrees per minute as the mill travels in inches per minute. When I try to compensate, I receive an error message that I'm only allowed to travel up to 60 inches per minute.

I have a Microproto DSLS 3000 mill with the 4th axis and I am demo-ing your product- I want to buy it, but this is frustrating. I would like to spin the A-axis at about 2000-3000 degrees per minute. 3000 degrees per minute is what Mach3 mill tells me it is jogging when I do a rapid traverse.

Please advise the best course of action.

Adam

That is a difficult question for me, as the answer is machine dependant.

What DeskProto sends to the machine is a G-code file, and in G-codes the only definition for speed is linear (mm/sec, inch/min or alike) in the F command. Which makes sense, as that speed is what matters during machining.
A number of CNC controllers indeed execute the G-code files as intended: for each rotation move they calculate the distance between cutter and rotation axis and then calculate the rotation speed needed to achieve the linear speed in the F command.
However many controllers cannot do this, and various solutions are present to set the rotation speed.

So my question to you is to find how your Mach3 controller sets the rotation speed. Or, as alternative, to find a sample G-code file that runs OK on your machine in this situation. When I know which codes Mach3 needs then I can see if I can get DeskProto generate such codes.

I know that we have many users with a Mach3 driven machine, also with rotation axis, so it should be possible. Perhaps you just need to change some setting in the Mach 3 Preferences.

Lex.

I think this might be an answer- but I have not yet had an opportunity to examine this for myself and test it to be true or not.
www.machsupport.com/forum/index.php/topic,6937.0.html

Do you have any further input?

Adam
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Copied from the MachSupport link mentioned above:
In Mach3, try this:
Config -> toolpath -> [tick the box] Use diameter for feedrate
Then go to the Settings page and look for "rotational diameter": put in there the approx diameter.
Now, it will spin the A at whatever is needed to keep the XY at the requested feedrate.
(if not, adjust the rotational diameter value to speed up or slow down the rotational axis).

This might indeed very well be the solution.
It perfectly matches with my last email on the subject of the distance between cutter and rotation axis being the important factor. This solution offered by Mach3 (just choose a distance to be used for the speed calculations) is not perfect but will work OK. No further input: just try and see if it works for you.

Lex.

This method DOES work.

Adam.

DeskProto V7 supports a setting for rotary feedrate called "inverse time feedrate", which solves this problem. this setting is enabled in the Mach3/Mach4 postprocessor of DeskProto V7.

Lex.

One more setting in Mach3 needs to be correct to get the desired speed (also when using the Inverse Time feedrate):

In Config > Motor Tuning the correct number of ‘Steps per’ needs to be set for each axis, to make the machine travel the correct distance. For a rotary axis this value needs to be in Steps per Degree.
The settings for 'Velocity’ and ‘Acceleration’ in that same dialog may cause a speed problem: these two settings need to be entered in DEGREES per minute, even though the header says In’s or mm’s per min.

So for the rotation axes these settings need to be completely different from the settings for the three linear axes (generally speaking much higher). The Velocity value sets the maximum velocity, so when that is too low your rotation axis will move slowly, no matter what feedrate you set.

Thanks to Paul for the extensive information that he sent me on this subject.

Lex.

For Mach3 users Paul has one more observation:
On the Mach3 Settings Tab you can set a Rotation Radius for the A-axis (default 0.0). This value appears to influence Mach3’s graphical display of the toolpath, without any effect on the actual feedrate.
Thanks Paul !

Anyone who has more information on this setting feel free to enlighten us.