The View Up Here

Random scribblings about kites, photography, machining, and anything else

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Posts Tagged ‘Lathe’

Kite Repairs

Posted by Tom Benedict on 17/12/2010

So my Dopero is fixed.  I had this irrational fear it would take weeks to months to get my big red kite back in the air, but it only took a couple of days.

Part of the worry was that I would have to get some exotic parts or fabric things made of pure unobtanium or something from a store that only sold to people who made kites.  Part was that I know I procrastinate to insane degrees at time, and ran a very real risk of letting my kite fall into that category.  (Some might call this “patience” as in  “I have the patience to let my CNC mill sit idle for several years while I get up the gumption to admit I need a new controller!”  I call it procrastination, pure and simple.)  But part of the reason, at least, is that this thing is roughly the size of a small airplane, and my house is roughly not even in the same category as a hangar.

So I set it up at work instead.

I really did.  For some reason my office is the largest one in our building.  That would make me feel like I’m the CEO or Director or something, but no, I’m about as low on the pecking order as you can get.  It all started when half of the technical library was moved into the main library, and space was opened up for two offices.  I was moved in there along with one other technician.  After a couple of years of this the rest of the technical library was moved into the new technical library, so my available office space doubled.

Something strange happened about this time.  Some people in the company got concerned that giving us too much space would lead to inflated head syndrome.  So in a bold move to keep them from putting more people in my office, I arranged all my furniture into an 8’x8′ square like a little fort.  I put my chair in the middle and pretended I was up in a tree.  My office!  It worked, but I think people thought I was weird.  A couple of years later the other tech left the company, and then I found myself sitting in my little fort all alone in the corner of this great big ROOM!

Of course it didn’t last.  Someone else figured this out and moved in enough furniture to house at least three more people.  If you arranged it a little better it would handle four.  Four empty little forts ringing this big room.  Ever since then, whenever we get interns (which we seem to do a lot) they wind up sharing the tree fort office with me.  I like it.  It’s fun.   Those are the good times.  But when the interns all go their separate ways and return to their regularly schedule life, the tree fort starts to feel a little empty, like it is now.

But it’s still pretty handy when you actually need it.  My Dopero is 6′ high by 9′ wide.  This doesn’t seem like it’s too big until you try to set it up in a house and turn it around to get to the other side while trying to fix busted things.  In my office I could basically pick it up, carry it to the middle of the room, spin in place, and put it back down.  It worked like a charm!  I finally got to see the extent of the damage the Kite Killing Kiawe Tree did to it and start to plan my repairs.

Brooks Leffler and Jim Powers gave me some good advice on how to go about fixing the kite, so I took their advice and planned it out.  I had a bent ferrule, which I fixed using the small lathe at work.  This sounds extreme, but it’s a really easy way to fix bent round things:  Chuck up the bent round thing in the lathe chuck, disengage the gears, and spin it around.  The free end will wobble around since the thing is bent, so rotate the spindle until the bent part points up.  Push it down.  Keep doing this until there’s no wobble left.

This is a powerful trick.  At one point we had a disaster at work that bent some precision guide shafts.  I watched our machinist do this to the shafts until the run-out was less than 0.001″ across the entire 12″ length.  This impressed me.  A lot.  I would love to say I held my kite spar to this same tolerance, but I never even came close.  I figure I got it to better than 0.050″ over a 36″ length, but I didn’t measure it.  I just eyeballed it.  As far as I can tell it’s straighter than the other one that didn’t get bent.  Go figure.

The other damage was that the bow lines ripped out of the upper and lower sails.  Brooks told me to sew some #200 braided Dacron onto the pocket and run the bow line through the loop.  Keep in mind I’m an absolute ditz at sewing, and that my sewing machine can’t actually be set to the right bobbin and thread tensions to sew ripstop.  (Believe me, I tried.  Over and over and over.  I know I growled a lot and my cats hated me for a while.  But I never could make a kite on that machine.)  So even though this seemed simple in theory, in execution it was going to be something of a feat.

Some time in the middle of this whole process, when I was taking trash and recycling to the transfer station, I saw that someone had dumped a Singer Millenium Series sewing machine.  “GREAT!” I thought, “Now I can sew ripstop!”  I threw it in my car, drove to work, and rubbed my hands with glee!  I knew it might need adjustment or something, but hey, I do machining.  How hard can it be to service a sewing machine?  (Yes, I’m aware this is an incredibly naive statement on par with, “It’s just a nuclear reactor.  How hard could this be?”  Actually, I think sewing machines are more complicated than reactors…)  I figured I’d fire it up, run some cotton denim through it to find out what the deal was, and fix whatever needed fixing.  It wasn’t until that evening that I realized the flaw in my plan:  It had no pedal, no power cord, no nothing.  Unless I could get it to absorb AC power through mind power or some sort of osmosis, it wasn’t going to be any more useful than a doorstop.  Ah well…  Project for another day.  (See?  I really do procrastinate.)

In the end I finally did take Brooks’s advice, and sewed some #200 braided dacron across the pockets.  I couldnt’ do it with the Singer or with my older sewing machine, so I did it by hand.  Hey, don’t knock hand sewing.  It’s how clothes were made for thousands of years.  The whole lock stitch machine only came about very recently by historical standards.  And even I can hand sew!  (Well, sort of.  It looks more like a really crude kid’s drawing of sewing rather than the real deal.  But it holds!)  It took me a while, and I had to wear my headset magnifier to see well enough to get the job done, but the job finally did get done.

I took my kites into work this morning, and when lunch time rolled around I pulled out my Dopero (nicknamed “Porco Rosso” after a character’s airplane in a Miyazaki movie by the same name), hooked it onto my line, and gave it a test flight.  It flew great.  As good as new.  You can’t beat that.  And if I can get a Singer pedal I might just have a new sewing machine sitting out in the garage.

– Tom

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Posted by Tom Benedict on 04/12/2009

I’ve owned a benchtop CNC mill since the middle of 2000.  The first real machine tool in my shop was my Taig lathe, so when it came time to find a CNC mill to go with it, the Taig mill was a natural choice.  The two tools have identical spindles and take much of the same tooling, so this was a money-saving measure at the time.  But in terms of transferring work from one tool to the other it has added benefits as well since the same chucks will fit the lathe spindle, the mill spindle, and the rotary table on the mill.  All in all, it’s a nice combination to have in the home, even if it is a little small by industry standards.

My shop has seen several moves over the years, the most recent being a move from Texas to Hawaii.  It wasn’t the kindest thing to do to my tools, but a set of good stout shipping crates kept things from going too horribly wrong.  Even so, the performance of the mill began to degrade once it arrived, and a little over a year ago it finally gave up the ghost for good.

Luckily none of this had anything to do with mechanical damage of any sort.  I’ve tested the tramming on my mill over the years, and unless I do something really stupid, like dig in a tool and keep driving one of the axes, it’s never had any problems of that sort.  I replaced the spindle motor on both the lathe and the mill with 1HP variable speed DC motors at one point, which was a good change for the lathe and a vast improvement over the mill’s stock 1/10HP AC motor.  Other upgrades came along as well, such as a relay box for the mill’s spindle and coolant, a quick change tool post system for the lathe, and a number of other things.

But once the mill was dead, none of that really mattered.  So it sat.  And I fumed.  And eventually I more or less walked away from it.

If it had died a violent death with smoke and loud noises and all the trimmings, I probably would’ve had an easier time of things.  Instead it died a slow death of having random position loss in various axes.  Parts started to come out wrong, and toward the end I couldn’t make a “there and back again” pair of moves in any of the axes and have it come back to the same place.  The thing still moved, but in essence it became useless as a machine tool.

Digging into the electronics indicated that there was a problem in the high voltage power supply.  But as with most problems, that was just the symptom rather than the cause.  Digging deeper, it became apparent that the mill’s electronics weren’t designed well in terms of heat extraction.  Over the years they had run a little too hot a little too often, and things were starting to fall apart.

Despite what people may believe about computers, heat doesn’t instantly kill electronics.  But it does reduce its expected lifetime.  Every electronic assembly carries with it an expected mean time between failures, or MTBF.  It’s been a while since I’ve done an MTBF search, but once upon a time most hard drives had an MTBF in the several tens of thousands of hours.  Running a hard drive hot wouldn’t instantly kill it, but it would reduce the expected time before failure for that individual device.  A well-treated drive might have an MTBF of 20,000 hours.  One that was in a computer with a busted cooling fan might have its TBF reduced by a factor of ten or more.

Which is essentially what happened with my mill controller.  The big electrolytic capacitors in the high voltage power supply had cooked.  And chances are the FETs in the motor drivers were cooked as well.  Everything was suspect.  So I either faced a whole string of test-and-replace operations, with the certainty that it would fail again unless I re-designed the case and cooling fan layout, or I could skip all that and replace the whole mess.

I chose the latter option.  But not having the money readily available, I couldn’t actually make good on the plan.  So the mill sat.  And sat.  And sat.

Then the miraculous happened and I got a bonus at work.  Ever since I first bought my lathe and mill, my wife and I have had an agreement:  Bonuses are bonuses.  Half goes to the family accounts, but the other half goes to the individual who earned the bonus so they can enjoy the fact that they were rewarded for good hard work.  This is how my mill was purchased in the first place, and this was exactly what I needed to get it back up and running again.

I knew the controller I wanted: a Gecko G540.  It’s a beautifully engineered piece of electronics capable of driving four stepper motors at up to 48V at 3.5A.  My motors weren’t a great match for it, but I knew I could replace those later.  The G540 is just the driver electronics, though, so in addition I knew I’d need a power supply.  And a case.  And a cooling fan.  And an E-stop switch.  And a power switch.  And a whole host of other doodads that seem simple but add up fast.  In the end I skipped all that and picked up a G540-based controller from Keling.

But Keling is in China, so while the unit was shipping I had some time to think through everything else I wanted to do.  Since this constituted something of a “do over” from the standpoint of my mill, I made a list of annoyances, both major and minor, and started work.  The list is by no means finished, but here’s where it stands now:

  • Cleaner – At one point I switched to a coolant/lube that did something I’d never run into before.  Even better, I found there is a name for it: “varnishing”.  It had a tendency to cover everything in a nice layer of oil (a good thing for a machine tool!) and then dry out to a hard, tacky film.  This is other-worldly bad for tools.  I took the whole mill apart, cleaned everything to bare metal, and re-assembled it with good quality way oil on every moving part.  The goobery coolant/lube is now gone.
  • Storage – My mill work area has been horrid for storage.  So I ran an eight foot shelf across the top of the mill and lathe bench, and moved a number of tools that had previously lived underneath the mill bench to the shelf.  This keeps them clear of swarf, coolant, and other crud that seems to happen to everything below benchtop level.
  • Lighting – The lighting around the mill, and to a lesser extent around the lathe, has been gawdawful.  I mostly work in the evenings, but I can’t turn on the full lights in the shop once my kids are in bed because they can shine in their bedroom window.  I picked up some under-shelf kitchen LED lighting, and got a nice spotlight set up that points at the mill spindle.  These are switched from the same switch unit that powers on and off the mill controller.
  • Cables – The cables on the original mill motors were very short, and of uniform length.  Which makes sense from one standpoint, but was painful when trying to locate the mill electronics.  The one place that really made sense to mount the electronics was on the mill’s work surface.  Which is a terrible idea when it comes to keeping flying metal out of the cooling fan.  (Hm!  Now I know why there was such poor ventilation on the original controller!)  I installed longer cables that let me put the electronics on the shelf over the mill.
  • Computer – I’m still not where I want to be with this, which would be to run my mill using Mach3 controller software.  But I got a new installation of EMC2 on a faster computer, so my pulse stream to the mill’s motors should be cleaner.
  • Way covers – The mill moves things around using big stepper motors attached to lead screws.  If metal chips or coolant gets on the screws, it can seriously mess them up.  The way covers I had were basically plastic sheeting.  I replaced them with rubber accordion way covers from Little Machine Shop.  It’s a vast improvement that’s hard to appreciate unless you’d spent almost ten years using plastic sheeting.
  • New controller – The Keling G540 controller finally came.  I plugged everything in, set up EMC2 to use it, and away it sang!

One of the big drawbacks with the previous controller was that it did nothing to address something that happens with all stepper motors: midband resonance.  Every stepper motor on the planet acts like a wheel with a detent spring holding it in place.  To move the motor, you move where it wants to detent to by changing the configuration of the magnetic fields in the motor’s coils.  But it’s still acting like a shaft on a spring.  If you make the motor move to the next step by changing the configuration of the magnetic fields being generated by its coils, the motor will very quickly accelerate toward the new position, and decelerate once it hits it.  And it’ll bobble back and forth (very quickly and with very very small moves) kind of like a car with bad shock absorbers will bounce around if you jump on its bumper.  The real catch with this is that at a particular speed, you wind up hitting a harmonic of that bobble frequency, and the motor will resonate.  Typically this results in lost steps and lost position.  And the really unfortunate thing is that this happens at a low speed that you really do want to get past so it can run at high speed.

The G540, in addition to being a 10 microstep driver, also compensates for midband resonance.  So when I said I plugged everything in, set up EMC2 to use it, and away it sang, it really did sing.  The mill is quieter now, doesn’t hit that “growly” sound as it accelerates up to speed, and has no speeds at which it can’t really operate at all.  This is a massive contrast to the way things were.

My mill still isn’t 100% ready for prime time.  There is still some work to be done like adding home switches, installing relays and outlets in the Keling box for spindle control, getting the VFD output from the G540 to drive the Digispeed I’ve got on my mill’s spindle motor so I have closed-loop spindle speed control, and a whole host of others.  But much of that is in the category of “want” rather than “need”.  It can wait.  For now, I have a mill that’s almost ready to go.  I expect the first of the new parts will be coming off the mill this weekend.  Quite frankly, it’s overdue.

– Tom

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