BMC-20 Box Way Gib Adjustment

[fasto]

Well, I took the X-axis drive apart, and took off the table to replace the oil metering valves. It wasn't 1 hour of work , more like all day. I've put everything back together, and have a question about adjusting the gib & the keeper plates.

When I took the keeper plates off, they seemed to be bolted tight to the table. That's how I replaced them, bolted tight. Well, the table can't be moved with them bolted tight. I had to loosen the bolts quite a bit to be able to move the table by hand. There were no shims installed when I took them off, nor did I see shims called out in the assembly drawings. It sems as though I will need to shim them, as I can't see any other method of adjustment.

OK, assuming I get the keeper plates sorted out I'll need to adjust the gib. I have set gibs on dovetail way manual machines before by disconnecting the leadscrew and adjusting the gib for smooth movement without binding. My understanding is that I should start adjusting the box was gibs in that manner - that's how I have the gib adjusted at the moment. Then, I should monitor servo current while the table is moving back & forth, then tighten the gib until the servo current increases dramatically, then loosen the gib just a touch. This gets the axis moving just short of binding up which is desired for minimum free play.

If this be correct, I will need to figure out how to monitor servo current on the Fanuc triple servo drive. Presumably one of the pins on it provides a signal proportional to current but I didn't reverse engineer that section.

Cliff, could you check which pin provides that in your book? Many thanks, in advance.
--
Aaron

[bloke]

Adjusting gibs on a small machine like this is best done by feel and by how much the table can be moved front to back on the X-slide. If you loosen the gib screw on the thin end of the gib and jog the table, it can pull the gib in damaging it and causing a lot of hard work to get it out.

Loosen the four keeps, then gently wind the gib in until it is about to contact. Put a 0.0001" dial gauge on the saddle with the stylus on the table. Pull and push the table to see how much deflection there is and adjust the gib until this just reaches zero.

The keeps will need to be either shimmed to increase the gap and stop the pinching or have the Turcite gently scraped to achieve the same. When assembled, it should be nice and smooth to turn the screw by hand with no tight spots. If you have the gibs and keeps too tight, you'll get uneven backlash with the most appearing at the unsupprted end of the screw.

Also, the axis motors are quite small and don't like to be pulling hard on a tight axis just to hold position.
Oh, and while we're on it, make sure you don't overtighten the belt for the axis as it can snap the motor shaft if you go silly tight.

[fasto]

Hi Cliff, great advice as usual. I slacked off the gib a bit (the keepers were already loose), and put an indicator on it. I pulled & pushed on the table attempting to rotate it parallel to the ground (which I think would be the C axis). I adjusted the gib for two or three tenths of movement at one end, which gives five tenths at the other end. I'll see how this works out.
I looked into shimming the keepers. Actually, I tightened them, one cap screw at a time, while checking torque to turn the ballscrew (I have an inch-pound torque wrench). I got them all tightened with only a very small increase in torque required to turn the screw by hand. I did not need to add shims. I can't imagine what was going on the other day.

Anyway, it's all back together, all the oilers are working, X gibs are adjusted though I might fine-tune a bit more, most of the 21 years of gunk are cleaned, and we're about ready to go!

Actually, I cut a simple part the other day before I took the table off. I am pretty pleased that it's almost done. I just have to fix the graphics screen (found a source for the flyback!) and add a toolchanger disable interlock so it doesn't crash itself again. Oh, and put the telescoping way covers back on. Oh, and clean the chip tray and coolant pan. Oh, and move it to it's destination in the shop.

Many thanks, good sir!

[fasto]

It's running! Well, it's been running a while now.
I got a flyback for the EGA graphic monitor, pretty cool that 1989 technology was showing the parts & toolpaths.
I made a bunch of parts. They're super dense polyurethane foam inserts for a roadcase that fits one of my products. They came out great. I'll be showing the product off at a trade show next week, so the timing is fantastic.
Still need to do a few more things, put some parts back on, move the machine to a better location, level it, etc.

Ugh, perhaps clean it a bit more?

Oh, the "modifications" to the enclosure were by the previous owner. The made HUGE openings, and new sliding covers, because they did a lot of shafting work. The openings make servicing the machine a lot easier .

[bloke]

Well done, fella! Looks like a top job well done.