I'm having a bit of trouble with backlash on the X axis of my DIY-CNC router (http://www.cnczone.com/forums/showthread.php?t=43748). While I noted this recently in the thread, I don't expect everyone to follow it. So I wanted to post my request for input separately...
Here's the situation: My gantry is heavy - at least 100 LBS, perhaps 130 LBS? I'm running dual lead screws on the X axis with 425 in/oz steppers (Xylotex), 48V/12A power supply, with a G540 driver. While the setup isn't a perfect match, it seems to work well. The screws are ACME 1/2"-10 TPI single start from Enco (not precision, by an means). And the lead nuts are the AB type from Dumpster CNC. That said, I can rapid at 100 IPM, though the screws do whip a bit.
I've noticed that the backlash is getting worse with machine use - I can now shift either side of the gantry 1-2 mm, and have isolated the movement to the lead nuts, at the threads. I have a buildup of black, semi-greasy, soft plastic in the slots of the AB nut. I'm not seeing this on the Y or Z lead nuts. So I'm wondering if the problem is one of the following, or a combination of two or more:
- The mass of the gantry is too much for the delrin AB nuts to handle.
- The speed of the screw during rapids over the length of the axis is generating heat from friction, thus softening the delrin and increasing the rate of wear.
- The screws are cheap and are a bit rough (not polished).
- The screws are not well-aligned (need to verify this - it might be true of my machine).
That noted, I am planning to upgrade to faster screws. But I don't want to by parts that can't handle the mass of my gantry (no complaints about Dumpster's products - I like his stuff). So I'm requesting input on what multi-start screws & lead nuts would function "best" on my machine (for both the X & Y) - keeping cost within reason (I can't afford ball screws... $500 US, give or take, is my limit). I know this type of question comes up time & time again. I've been searching through the threads, but haven't found anything focused on screws/nuts for a heavy gantry... on the DIY router, at least. So I'm hoping this round of query will be a good reference for others. Thanks in advance!