BMC20 & Ultimax II Backlash Comp

[fasto]

I hope you've enjoyed my saga with the BMC20.

Before putting all the chip shields back on, I was doing some final checks to make sure the gibs & such were set nicely. I ran into something funny....

First, the Y axis. It's perfect. Shoving the table around results in 1-2 tenths of motion, so the gibs are adjusted nicely. Checking for lost motion, which I did by putting a tenths indicator on the table, jogging towards the indicator a couple thou, then jogging away and noteing how much the control's position indication changed before the indicator moved resulted in about 4 tenths of "motion" from the control before the indicator moved. Perfect. (Remember that it's got 38,000 hours!)

The X axis is a different story. Again, checking the gibs by moving the table showed 1-2 tenths of motion at one end and 4 tenths at the other. Good enough for a 20 year old machine.
The lost motion check is another story. The same procedure, jogging the axis towards a tenths indicator, then jogging away, revealed that the axis moves smoothly towards the indicator, then when I reverse the jog wheel's rotation (to move the axis away) the axis jumps TOWARDS the indicator about 1.0 to 1.2 thou before moving smoothly away.
When I invert the directions, ie, move away from the indicator then reverse to move towards the indicator, I get the same jump in the wrong direction before movement in the correct direction starts. I slackened the gibs & keeper plates with no change in results. I indicated the end of the ballscrew, and it doesn't move lengthwise enough to deflect the indicator at all.
Then, I touched the timing belt while the axis was stopped. It's got a lot of miniscule motion going on even though the axis is stationary - it feels like vibration rather than actual movement. I presume this is typical, as the Y and Z axes do the same.

While touching the timing belt I repeated the jog one way then jog the other, and I can feel the servo turning the wrong way for an instant on axis direction reversal before turning the correct way. It does this consistently. Thus, I don't think I have a mechanical lost motion issue.

It sounds like (1) the servo needs tuning; or (2) the backlash comp is set wildly incorrectly; or (3) the axis acceleration is set wrong; or (4) something else that I am not familiar with.

OK, my question: How do I set the backlash comp? I'd like to check what it is, at least. It's running Ultimax II V8.60. I tried all the secret screens that I know (102, 488, 642) and I did see axis acceleration values on 102 but not backlash comp. It was updated from V7.something (I have the old EPROM set), and the dual-axis cards have four, 8-position dip switches on them, and one 4-position dip switch. I haven't seen cards like this before:


Of note, the machine was run for quite some time with the auto lube system inoperable. Well, there was one metering unit that worked on the Z ways. To compensate for this, the previous owners had upped the volume pumped by the auto lube pump to the highest value possible. This had no effect, of course. Presumably running with relatively dry ways required the backlash comp to be adjusted much higher? You'd have though they'd just fix the auto oiler.

[bloke]

As you look at the board above, the backlash is set in two ways.
The S3 switch does the Y axis on the X&Y dual axis and the S5 does X.
On the Z&Spin card, S5 does Z.

The switches add backlash comp from right to left as follows:
sw8 0.000025"
sw7 0.00005"
sw6 0.0001"
sw5 0.0002"
sw4 0.0004"
sw3 0.0008"
sw2 0.0016"
sw1 -------

So if you find you have 0.00055" of backlash, use switches 4,6&7.

It is also worth noting that backlash can also be entered on the leadscrew map. The best way of setting it up is to make the reverse map the same as the forward map then setting the backlash on the switches.

To get to the map, calibrate the machine and go to toolchanger diagnostics. Enter 642 enter.
As ever, note down as it is before changing it just in case you don't like it.

[parkerbender]

Bloke,

I am firstly posting to express my admiration and appreciation of your understanding of these hurcos, and secondly, to make it easier for me to find this thread, when I need this information for my slv-40!

Thanks again!

-Parker

[fasto]

Hi Bloke, many thanks!

One question, do you know what Position 1 switch does? You marked it with "----".
Reason I ask, my X-axis has this position 1 switch CLOSED, while Y & Z have it OPEN.

Also, my leadscrew map is all 0, forwards & backwards. Inside the electrical cabinet there is a sticker that says "Laser Compensation" and then has a bunch of numbers:
Xaxis:
LS .0007683$ (yes a dollar sign)
BL .003349
Motor Gap N/A

The other axes have similar numbers. Can I back-calculate the leadscrew map from these?

Thanks,
Aaron

[bloke]

The SW1 setting I left marked ---- is for the backlash sign. Off will compensate for ordinary backlash, On will compensate for negative backlash. This is about the silliest thing anyone could include on a control. Just leave it turned off as it does nothing of any use (unless you are planning on taking your machine into space where the lack of gravity may cause problems)!

The length comp works by using switches S2 for Y, S4 for X and S4 for Z.
Measure the travel over 20" using a stack of slips. Note the error on the display (say, 20.0015") enter it on the switch for that axis (0.0015 equates to SW 3,4,5 & 6) Then add the sign bit on SW1 - On will shorten the travel, off will lengthen it. in this case, you need it to be off to make the table travel further.

When done, you can store the switch settings in the leadscrew map by going into the map page and pressing "leadscrew map defaults".

[fasto]

Originally Posted by bloke Off will compensate for ordinary backlash, On will compensate for negative backlash.

Evidently this is the reason for the odd Xaxis behavior that I saw, the control is compensating for negative backlash. I'll adjust it and see what happens.

Thanks for the note about leadscrew comp, I'll see what I've got for slip gauges (except we call 'em "Jo Blocks"). What was that about two countries separated by a common language?

Oh, I presume that if my axis travels are less than 20" I can just scale the needed adjustments? Say, if I check over 10" and I find I need 0.0005 I can scale by two making the needed value 0.001" over 20"? (My Y is only 14" of travel).

Thanks again,
Aaron

[bloke]

Yep, you can scale the figures to get the correct comp for the other axes.

[kulibin]

Originally Posted by bloke Yep, you can scale the figures to get the correct comp for the other axes.

Hello. Excuse that I get and for that that not so on English. I too have bought recently for a hobby similar CNC in a faulty and bad condition. Back she has earned couple of weeks and there were problems. As on (HURCO BMC25 Board CRP2 Ver. 8.61 very ancient model) to work with the big operating programs. It can work in an additional load mode. At loading of the big program through RS232 loses touch when there are 10 % of free memory. At CNC 47Kb operative memory and additional FLASH 4xDS1235. On 128Kb. What format of the program is loaded through Floppy disk? What at floppy volume? (1.44 or 720) At program record in CNC on FDD, disk on PC it is not read. At CNC there is no system DOS.

[bloke]

The NC through the RS232 will load until 10% free then it will stop. As you run the program it will dump out the beginning and start to load again until it has 10% free and will carry on like this until the program is completely loaded.

The floppy has 720Kb capacity but cannot be used for NC file loading.

[JWC]

I found that replacing my Y axis drive belt solved my backlash problems on my BMC20. They do wear out over time.

[kulibin]

Originally Posted by bloke The NC through the RS232 will load until 10% free then it will stop. As you run the program it will dump out the beginning and start to load again until it has 10% free and will carry on like this until the program is completely loaded. The floppy has 720Kb capacity but cannot be used for NC file loading.

Hello. I have BMC25 Ultimax 2. How it is possible to keep parametres CNC on a diskette?
Where it is possible to take numbers and the description of parametres? I wish to increase a range of movement Z Axis. And to calibrate all Axis. I will be very grateful to you for any information.