Hi all, I'm a newb to cnc milling and have several questions. I recently picked up a used sherline 5400 cnc mill on ebay for cheap. When I got the mill unpacked and setup I realized I didn't have any documentation on the steppers or the control board so it took quite some research and trial and error to get it configured in Mach3. Once I got the correct steps per figured out and the correct pins configured I was able to run some tests. Initially, on simple parts I designed in AutoCAD, then imported into MeshCAM for tool paths, everything seemed to be good.

Then I noticed the Z axis was "dropping" for lack of a better term. What was happening was the leadscrew was getting pulled up into the stepper housing slightly and then on the down movement the screw/spindle assembly would drop back down the distance it had been drawn up in. Obviously that was unacceptable. I machined a washer to go between the top threads of the leadscrew and the bottom of the sleeve where it mounts in the stepper housing which fixed that problem.

Next I tried a much more complex part which is somewhat indicative of what I intend to mill. This part had 30,000 lines of code generated from MeshCAM. I noticed once it got to the parallel finishing passes that the machine had obviously lost steps at some point. I discovered by reading that it was the driver board (old CNCgeeker/univelop TA8435) and that there was apparently a hardware problem with that board. So I ordered a geckodrive g540. I got it all wired up, configured etc. and continued to run tests.

Now I've found that I believe the table itself may be causing some step loss. If I grab the x table and pull on it I get movement back and forth in the x. The y seems solid. This mill seems to be an older model and either an older cnc kit on it or an after market kit. The stepper housings aren't like the modern sherline stepper mounts. They're rectangular boxes and the coupling between the steppers (vexta 2 phase 5v 1 amp steppers) and the leadscrew is done via a finned nut attached to the stepper shaft with a set screw, then a plastic collar of some sort which receives the finned nut and finally another finned nut on the leadscrew with a set screw. I initially discovered that the x axis leadscrew had threaded itself out of the stepper mount so I took it off and got it seated properly. The movement seems to be in the table and I guess the next step is to take the table apart and figure out why it's sloppy.

All this leads me to my real question. I had originally intended on buying a taig setup from deepgroove1 w/ the 4th axis. When this little sherline popped up on ebay I thought it was cheap enough that I could at least learn with it and get some things done. It's becoming apparent now that I'm probably going to end up spending as much or more on getting this sherline setup than it would've cost to just buy a complete 4 axis taig. I intend to cut fairly precise and complex shapes with the mill. If I was just cutting out rectangles with holes, the sherline would probably work fine with some final tweaking/tightening. I noticed yesterday though that my washer solution on the z seems to be slowly failing. I think the leadscrew is wearing the washer out so I'm getting movement in the z again. Probably the only way to fix it properly would be to buy new replacement leadscrews and housings and stronger steppers etc. Which would, again, be bringing the cost of the setup up to the cost of a brand new taig setup and that's without even buying the 4th axis rotary table yet.

So should I cut my losses and try to sell the sherline and just buy the taig or should I use the sherline and upgrade everything to try to get it tight? If I would've known how sloppy this mill is mechanically I wouldn't have bought it. It did say it was sold as is with no warranty or guarantee but it didn't say anything about the components being shot or flawed. I'm not going to try to get my money back or return it. I don't think I have any right to based on what the auction said.

If it helps, my intended use is milling brass parts for model master patterns to make molds. I need a fairly high resolution of performance. My goal is to be able to get very fine detail on very small parts (from 1/8" x 1" x 1" down). I obviously can't do that with slipping leadscrews and missed steps. So can the sherline be straightened up to handle high resolution/detail work or should I offload it? Would a brand new taig be able to handle high detail work? Sorry for such a wordy post, but if you've read this far you can see there's quite a few factors involved and I wanted to try to be thorough with my explanation.

Any advice is greatly appreciated.