Gantry wheel not touching guide

[bbortnick]

Yikes. After being idle for several years, I am getting our Shopbot PRS Alpha going again. Today I removed all the v-wheel covers to clean and oil the felt. I found one of the V-wheels on the gantry is not riding on the rail, it's about 1/16" above it. The other 3 wheels seem fine. I don't believe these are adjustable (like the eccentrics on the xy car).
Reading page 3-4 on the setup guide, it seems to say to adjust the feet until the wheels are running on the guides. Is there another option?

[coryatjohn]

It sounds like your machine was either screwed up from day one or something happened to torque it. Did a semi crash through your shop lately?

I believe you can adjust the squareness (which is probably the issue) of your gantry by loosening the large bolts holding the cross member to the end pieces. You'll probably have to do it on both sides of the gantry to get enough adjustment. Once you have the wheels in the track, you'll need to check the squareness of the spindle or router to the table before tightening the bolts again. These bolts need to be torqued to the right setting so I suggest calling SB and ask them what that is.

[bbortnick]

:) No semi that I know of! I'll check the gantry squareness. Thanks for the tip!

[Brady Watson]

First, check that the machine is level. It doesn't take much to introduce twist into these frames and that can cause one wheel to sit up. Start with the 4 outside legs and level it. Then bring the center legs down until they just touch the floor, then go another 1/4-1/2 turn & you're good.

After doing this, drop the X motors out of sync with the racks so it rolls freely. Cruise it back and forth throughout the entire length of travel, with NO downforce and note how the wheels are riding on the rails. Be sure to check parallel between the rails to make sure they're not splayed in or out.

If all is good with that, get 2 short pieces of 2x4 and a pair of 36" clamps. Put the gantry beam so that it it over one of the 3x5" crossmembers. Apply a small amount of pressure with the clamps to both sides of the beam, using the 2x4s to pad the top of the beam to clear the Y rail. No king kong pressure...Then, loosen up the eight 10mm bolts at the side plate to beam and the gusset underneath on the side that has the wonky wheel. The pressure of the clamps will force the wheels down to the rail surface. When done, tighten it all back up again, remove the clamps and glide the beam back and forth until you are satisfied with the results. If not, repeat.

One more thing...it is possible that the weight of the spindle has caused the beam to rotate forward - resulting in ridges in the Y direction when you machine your table etc with a large bit. Put a big framing square on the table and up against the YZ car (AL blocks ideal surface) to verify that the gantry isn't tilted forward. If it IS, you'll have to very carefully loosen BOTH sides and with a helper, square it up while rotating the beam so it's square and then lock it into place. SB uses a jig to assemble these, so doing it in the field is a bit trickier - but I've done it myself so you can too. Good luck!

[coryatjohn]

As usual, Brady provides the actual procedure. Thanks!

[bbortnick]

Thanks, I have levelled the machine and now have the 4 gantry wheels riding nicely on the tracks and the machine is levelled (along with using the clamps on the gantry.) I cut a large square and the diagonals are equal. I did see that the yz-car is not perpendicular to the bed, the top is tilted to the front. Brady, you suggested above that with the 8 bolts on the two sides loose, you could adjust the yz car. I don't see how, though as the bolt holes appear to have no slop in them. What am I missing?
Thanks again for the help!

[bbortnick]

I measured from the z-car to the table, not the spindle. Should measure from the spindle?

[Brady Watson]

This is why I mention that 'one more thing part'....so you could rotate the gantry beam while you had it loosened up. Even though it seems like there's no slack in the side plate holes, there is - enough to probably rotate the beam front or back +/- at least 1/16", which translates to a lot with a large diameter bit. You will need to loosen up BOTH 8-bolts in the end plates AND the gussets underneath. This is where a helper comes in handy - or, use a floor jack or bottle jack under the spindle so you can rotate the beam back yourself a little at a time. Use your brain & don't mangle the spindle with the jack...right?

A few methods of squaring the YZ car in the XZ direction:

1) As long as your spoilboard is flat and doesn't look like Freddy Kruger's face, you can use a 2' framing square against the spoilboard and steel YZ car face. Since it's dark blue, it can be hard to see, so look carefully. The spindle front face is usually true enough to use as a square reference.

2) Use a tramming gauge (https://www.edgetechnologyproducts.com/pro-tram-system-01-000-09-000/) and adjust Z as required to get it dead on 0. Requires flat/clean spoilboard.

3) Use a dial indicator (https://www.amazon.com/Magnetic-Base-Adjustment-Indicator-0-0005/dp/B01JU18A0Q/ref=zg_bs_3012165011_sccl_5/132-2233038-7962138?pd_rd_i=B01JU18A0Q&psc=1) in a solid base and run the Z up and down in it's entire stroke, adjusting as needed until the indicator reads 0 from top to bottom. Requires something steel and flat to stick the magnet to. A 2' square can be used in a pinch if you are careful.

Since you've already gone through the motions getting that one wheel down, it should be no problem to do it all again, except this time, check the XZ tilt to make sure you're dead on. If you have a square long and true/reliable enough, you can also square up the front face of the gantry beam, but you should be OK with the YZ car steel plate.

Oh...(here we go again...)....one more thing....check the bottom V-roller bearings on the YZ car BEFORE YOU DO ANYTHING - you'll need the thin goofy wrench that came with the tool. Take your time and rock the wheel into position and make sure it isn't too tight...or you'll mash the upper Y rail into the beam so far there isn't any more adjustment to be had...Just until it touches + no more than 1/8 turn.

Good luck!!!

-B

[bbortnick]

The skinny wrench is MIA, will have to make one. Do you happen to know the bolt diameter, thickness of wrench? I was also thinking of facing the spoil board first to see how bad/good things are.
Thanks!
Barry

[bbortnick]

I surfaced the spoil board (with a 1.25" bit) and there is a small ridge on the back side of the pass. Worse than that previous surfacing before I levelled and had all 4 gantry wheels resting on the tracks - make sense, it was a front wheel that was too high.
So nothing rocket sciency about adjusting the Z axis - loosen the 24 bolts, and adjust spindle front to perpendicular to the table?

[MogulTx]

You can buy a tramming tool ( a right angle device that has a dial indicator on the end.) You chuck it up in the spindle, and slowly rotate it through 360 degrees and see how much "run out" there is.. ( what is the high and the low) and adjust spindle accordingly to drive down the high and bring up the low.

I have seen people do this by mounting an allen wrench into the spindle collet, and use a caliper to make a measurement down to the spoil board in multiple places. That is the CHEAP and PRACTICAL way of doing it.

If your spoilboard is a little wonky right now- go for some good averages and make your adjustments. firm it up and just SKIM the bed again with your flattening bit, and remeasure. If it is not acceptable to you, make a second adjustment and skim the board ever-so-lightly.

Let us know if this solves your difficulties.

Monty