Machsupport Forum
General CNC Chat => Show"N"Tell ( Your Machines) => Topic started by: stringkilla on July 23, 2006, 09:48:25 AM
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I recently upgraded my mini with imperial lead screws. I'm now thinking of converting to cnc, but one thing has me vexed. The Z axis is rack and pinion. How will the difference between rack, and lead screw, affect the Mach configurations? Also what size motors to use to move the Z axis. All that cast iron is heavy. Any assistance will be greatly appreciated.
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Each axis in Mach is configured individually so it doesnt matter how it is operated, just work out steps per unit and off you go. eg if one turn of the motor moves the z axiss 1 inch then with a full stepping driver and a 200 step/rev motor you will enter 200 steps/unit, if you have a half stepping driver then it will be 400 etc etc. Your biggest problem with the rack may be backlash.
Hood
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Is there an easy way to manage the back lash? Will standard steppers lift the Z carriage?
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Mach has backlash compensation but it doesnt work very well with steppers and constant velocity mode. You can use it with exact stop and it seems to work well also it works well with servo motors from what I have heard.
Not sure how you could deal with the backlash, maybe some sort of counterweight or gas spring. Also dont really know what weight you are talking about with your mill but I would imagine it wouldnt be that great and should easily be handled by a smallish stepper, you can always have some reduction gearing which will increase the torque of the motor if you do run into problems. Steppers come in lots of sizes and you can get some quite big ones, 1800 oz/in is common but TBH I dont think you will need anything as big as that to work your Z Axis.
Hood
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I highly recommend the Ron Steel conversion. 200oz motors will do all the mill is capable of.
http://www.stirlingsteele.com/millplans.html
Here an older pic of mine, hasn't changed much since this was taken. I has Ron's belt drive kit, Driven by a Hobbycnc board, was running it with Turbocnc but the old laptop died and I've got a replacement that is Mach capable. I've added an auxilary high speed spindle to the side of it as well.
(http://www.outdoorsunlimited.net/~chucksmy/Amillwide.JPG)
Here's a review I did of his belt drive kit that has a few close up pics of stuff.
http://www.outdoorsunlimited.net/~chucksmy/Mini-Mill/Mini-Mill_Belt_Drive_Upgrade.html
Oh, the backlash comp works fine in Mach, in addition you can tweak your lead screw steps to dial things in. But keep in mind that even though you can compensate for backlash, it doesn't solve the backlash problem. If you are climb milling with a heavy cut, it will want to pull the work for the distance of your backlash. You can run the gibs a little tigher for light cuts and get by. Best thing is to use anti-backlash adjustable ball nuts or at the very least acme lead screw with a home made anti-backlash nut. Roton will custom fit your ball nuts to your ball screws for for the least amount of backlash fwiw, still not as good as anti-backlash nuts.
:) :) :)
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not sure abou
I recently upgraded my mini with imperial lead screws. I'm now thinking of converting to cnc, but one thing has me vexed. The Z axis is rack and pinion. How will the difference between rack, and lead screw, affect the Mach configurations? Also what size motors to use to move the Z axis. All that cast iron is heavy. Any assistance will be greatly appreciated.
hi-
you can leave the rack/pinion in place while the leadscrew drives the z-axis. for example, in our conversion, you leave the rack/pinion in place so that you can use both cnc and manual operation. if you go cnc only, it make make sense to remove it. when you go to confugure mach, you enter in the "steps per unit" (http://www.fignoggle.com/calculators/stepcalc.php) for the leadscrew. hope this helps!
disclaimer: we also sell plans, so please factor that into our reply. :)