What's my Z speed limit?

[MDCochrane]

I'm cutting a 3D image into soft HDU at a very small step-over. I'd like to run the Z feedrate as fast as I can to avoid problems with ramping. Does anybody know the specifications on my old machine? How fast can I push the Z rate?

I've got a 15 year old table that the prior owner updated to a PRT and then updated it to 4G. I'm not sure what that means, but he said it was significant. He described it as a ShopBot 4G controller, 48V power supply, Geco 101 stepdrivers, Vexta 2 phase motors PK296A1A-SG3.6 .

Maybe somebody out there has something similar. Thanks!

Mike

[Brady Watson]

Mmmm...I'd say max 2.5-3.0 IPS on a 4G, but I'd be careful with that as it is very easy to lose steps. The torque falls off pretty quick faster than that. An Alpha can push 8 IPS, but either machine will hammer the Z at those speeds. This is of course assuming that a proper 3D roughing pass has been done.

I've learned that 3D takes as long as it takes...aim for quality, otherwise you'll be doing a lot of hand work after machining. I've also learned that haste makes waste, especially on the older tools because any and all slop in gearboxes and gantry is transmitted to the cut; not to mention the chassis of the machine. It's real easy to beat up an otherwise nice cutting machine by hammering on it.

Try tuning your VR for 3D work. Read the article that I wrote. Advised speeds at the end for 3D.

YMMV - your tool, do what you want.

-B

[MDCochrane]

Thank, Brady! I did read your fine article on ramping and adjusted the defaults as you recommended. It took a lot of the jerking out of the table and reduced the pitting I was getting.

I'm still getting some pitting from what is, apparently, unavoidable ramping. Looks like I need to reduce the X feed rate (rastering along the X, the Y moves are only 8% of the width of a half inch mill) to something slower than the Z's rate. Ho boy... that translates into 19 or 20 hours of runtime for the cut when surfacing a 4x8 sheet. This is why I'm trying to boost the Z as high as I can.

I'll keep experimenting. Thanks.

Mike