Kelly
Where circling the precipice of the rabbit hole.
This was my area (motion control) and also my hobby, so I’ll try not to ramble on.
I don’t think it’ll be 30 (the lead) in the pic you can see an Acme screw that’s for sale
from another manufacturer. I went to the millright site and couldn’t find the
exact specifications.
It’s a ½ inch diameter with a pitch of 0.2 and 5 starts.
Nomenclature:
• Pitch is the distance between the adjacent threads.
• Lead is the distance that a nut will turn with one rotation, and;
• Start is how many starting (or thread entry) points at either end.
So, for a leadscrew, with a pitch of 2 mm, if there is only one start to the screw then the lead is the same as the pitch. However, if there are four starts to the screw, then the lead will be 8 mm. If there are two starts to the screw, then the lead will be 4 mm. And so on.
Just to put things into context and to give a big picture overview.
You machine is a type called moving gantry and is made from Al extrusions.
It’s powered by a Atmega2560 using the millright fork of GRBL.
I didn’t see what type of Stepper driver they are using, but it’ll be the
minimum entry level. Maybe something like the TB series which use a chip
that was originally used to drive dot matrix printers.
It’s the minimum entry level machine, and for milling foam patterns it’s quite acceptable.
The minute you want to machine metal or use very small\Large diameter tools you’ll run
up against machine design limitations.
moving gantry aluminum construction, GRBL, Drives, motion system ect.
It’s very easy to over-capitalize a machine like this, by buying all the cnc bling
and still have mediocre results.
I’m not saying the you should not do any modifications, just some where they
will provide good value for the money spent.
For example replacing the drives (if they are the tb series) will offer a performance
upgrade, in the pic you can see a Leadshine DMM 522 drive, This drive is the ‘bees knees’
for your steppers they are excellent drives and they are not very expensive offering
good value for money. Don’t by the knock offs. There are plenty out there.
In the larger pic you can set the side of the driver where there is a table
to set the micro stepping value.
A standard 1.8 degree stepper motor takes 200 pulses to step the motor 1 REV.
however you can set the drive to 400,800 ect.
For your machine you would set 2000 pulses per rev.
This would equate to (with a 10 mm single pitch lead screw) 0.005 mm resolution.
However, we live in the real physical universe with real drives and motion components
so this is a theoretical number, it’ll be more like in the order off 0.025 for your motion system.
If I remember correctly GRBL can produce 30khz steps
How fast can it go? (grbl.org) At 30khz per channel this equates to roughly 4.5 M/min. Running on the 8 bit controller
GRBL can run on a ESP32 where I’ve seen figures quoted as high as 100K this is overoptimistic though and it will be more like 70khz.
It’s already a long thread, so I’ll try to address at least one issue.
By looking at the cut on the periphery of the air cleaner cover, it’s obvious that the tram
of the tool is changing (this is causing your tool to dig in), this can be to some degree a
little slop in the Z Axis (backlash), but in this case I believe it’s the gantry flexing in Z and
rotating in Y caused by the cutting forces needed to cut the polycarbonate.
You could beef up the gantry by making it stiffer and replace the wheels with linears
or if it’s a life and death situation and you have to make the part right now with the machine
you have right now, then the option would be to use lube (reduces the cutting forces a lot)
or run the machine with much reduced feedrates.
Something to note is, that the coefficient of linear expansion for Aluminium to twice
that of steel.
The manufacture of bi-metalic strips as used in thermostats operate on this principle.
As the two dissimilar metals heat up a bend is induced in the strip closing or opening
a contact.
Dave