Last night, after messing for 30 minutes with a caliper, issuing
commands, etc I have figured out that the screw pitch of the
ballscrews, is 2.5mm. I thought originally that it was 10 TPI. So the
precise value of INPUT_SCALE is 40640. Amusingly, my own estimate of
it (before I guessed that it was a metric step) was 40629, a close
Nope, it's used on pretty much all machines. No ballscrew is perfect,
and by measuring and mapping the error the control can correct for it
and make the machine that much more accurate. The ballscrews typically
come with a pitch error chart and table from the factory that you enter
into the control. I presume your machine likely doesn't have this
anymore, so you'll need to measure the error.
A good dial indicator will get the job done, just in tedious 1" steps.
Your axis travels aren't too long, so it shouldn't be that bad. You have
to get out the dial indicator anyway to measure the backlash.
FWIW, I've never done this on any of my machines. Depends on what you make,
nearly all my work +/- .002 is fine. Bores would be an exception, but the
accuracy here comes from the boring bar, not the CNC. Now, if you intend to
make production molds or tooling dies, all bets are off.
Certainly you can live without doing pitch error mapping, but for the
modest amount of time it takes to do one time, don't you think the
increased accuracy is worth it? Back when I did CNC service I replaced a
few crashed ballscrews and each time spent the time to enter the pitch
error table provided with the new ballscrew into the Fanuc control.
All you need is a good dial test indicator and a stack of gage blocks.
The bigger a stack you can collect, the longer a section of the screw
you can map. You align a straightedge with the axis to be measured, and
place a stop at the end. A big machinist's square is good. Then, you
indicate and take a reference off the crossbar of the square, and then
move the table and insert gage blocks and read the difference on the
dial. Once you get the hang of it, it actually goes pretty fast.
Coming up with 18" of gage blocks may not be trivial, however.
Yes, on my machine. It is Bridgeport brand, but was made in England.
I tried to very carefully calculate desired input scale by using the
depth part of a caliper to measure the moves of the table.
The number seemed odd, until I decided to try a metric conversion.
I hope that I will be OK without it. I will se how it goes.
Backlash is what could give you the most problem. It is usually caused
by bad thrust bearings but a worn ball nut will also cause it too. IF
you want to get into axis compensation, cross compensation between the x
and z would be the thing to look at. If your table ways are worn the
table moves in an arc rather than in a straight line. The way to
measure this is with a straignt edge that an indicator mounted on the
table rides on. Ideally it should not move but in most cases it will
increase the measurement when the indicator moves off of the center line
of the machine. All machines that the table moves beyond the bottom
ways will eventually wear this way. YOu cannot measure this by putting
an indicator on the quill and moving the table x back and forth.
When you get your cnc mill running fine I have a job for you. retrofit
a nice OM VBM for me. Its been sitting in the corner waiting for some
loving attention. :) Every time I start on it something else comes up.
I'm finishing up rebuilding the transmission on the Niles vertical
boring mill and last Thursday the 15 HP spindle motor blew out on the
Monarch CNC vmc. The rewind shop had the thing done Monday night and I
reinstalled it this morning.
I could figure out backlash, by powering the mill, making sure that
servo motors are holding, and then by pushing the table back and forth
against a dial indicator, right?
I am not losing sleep over wear. Someone told me that the machine does
not have a lot of wear, it has a nice lube system and the ways look
good, so I believe that. I do not want to increase accuracy beyond
which the machine was designed for. I think that I can get under
0.001" and that is all I want.
Sounds fun, but I wonder, were there any overloads on the motor?
This works, but not my preferred method. Put a small part in the vice and
machine all four sides with a smiple Gcode program. measure part and adjust
backlash values to make it on target dimension.
The easiest way to figure out the backlash is to put a dial on the
table. Then increment the table in .0001 steps and watch the indicator
move. Then reverse the increment movements and see how many it takes
before the indicator moves.
To check the backlash in the nut attach an indicator on the table and
put the tip on the ballscrew where the balls run. Move the table from
end to end and watch the indicator. Any wear or backlash in the nut or
wear in the screw will show up as movement on the indicator. Ideally the
indicator should not move at all.
The problem on the motor was that the field winding shorted out to
ground and was blowing the field fuse. I seem to remember that one time
way back it blew the field fuse but its been running for a long time
since then. The one job we did before it blew was deep pocketing some
steel plates with a insert cutter at high speed. The vibration probably
moved the field coil a little and took it out to lunch. The rest of the
internals of the motor looked fine. The commutator had almost no wear
but I put new brushes in it. The bearings were in good shape so we left
them in. I did notice that the bolts for the housing end bells were too
long and just bottoming out when tightened so I went with a little bit
shorter bolt. The previous rebuilder didn't pull the grease seals off
the motor so all the grease pumped into the grease fittings did no good
and just came out the other plug opening. The machine was up and
running since Tuesday and catching up on the parts backlog.
Hmm ... reminds me of the way a particular type of machine like
an ultra-precise milling machine -- called a "jig borer" IIRC is set up.
Start with a 'V' the length of the axis with the narrow end
down. It needs to be truly parallel to the axis. (I guess that good
angle iron will be a good start if you don't have a 'V' machined into
the cast iron.)
Then add a dial indicator to read position at one end of the V.
Then there is a set used with it -- a micrometer thimble at
2-1/2" long fully closed (IIRC) and 1" travel, and a set of bars with
domed ends and lengths of 1", 2" 4" 6" 8" and perhaps 12" (two of each
and two of the micrometers (color coded one for each axis) to provide
precise positioning for both X and Y axes).
All of these have collars near the ends to support them in the
'V' at the same height so the centers touch.
So -- you can set up any position within the accuracy of the
dial indicator and the micrometer thimbles.
I've got a boxed set from a Pratt & Whitney jig borer. However,
I can't find a set on eBay currently. This set was picked up at a
hamfest about thirty years ago or so.
The original purpose of the jig borer was to manufacture drill
jigs to allow accurate drilling. But for a lot of work, it became more
efficient to use the jig borer to directly make the holes in the
workpiece, skipping over the jig.
And the way of measuring everything from a common datum was
adapted into NC (and later CNC) machining.
Absolute overkill for iggy's application.
Setting backlash would be as far as worth going.
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.