milling machinerako.com/design/machinery/milling-machine/milling... · 2018-09-15 · milling...

Milling machineRetrofitting a Bostomatic mill with a Delta Tau controland modern spindle and servo motor drives.

Back in the 1990s we bought a Bostomaticmilling machine from a shop next door. Itcame from Litton, and had very little use sincea total rebuild in 1979. Litton machinists didnot like the control, SPC2. It used a 64kB DataGeneral Nova computer. We planned to retrofitthe mill with a modern control system with apersonal computer at its core.

This was a side-project to our paid consultantwork. The design effort was spotty and lastedyears. Eventually the machine got to the pointwhere all the axis worked well. In 2014 wescrapped the machine. It was an obsoletedesign, since it did not have a tool-changer. Italso lacked an enclosure and high-speedspindles. It did teach us a lot.

Milling machine - Product Design Rako.com/Design 1 of 18

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The design concept was to put all the controlelectronics in a 19-inch rack. Then we couldbuild a spare rack to try new things.

The original control cabinet was huge, and hada paper tape drive for programs.

The rack had four enclosures, with an interfaceenclosure added later, and by mistake.Components that were in that cabinet couldhave been housed in the servo or spindle driveenclosures. The picture above does not showthe rack-mount PC with a PMAC motioncontrol card.

The rack had to interface with this wiringcabinet on the side of the mill.


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In addition to the huge original control cabinet,the mill came with this gigantic "magnetics"cabinet. Both were replaced by the rack.

Block diagrams

The block diagram shows the rack and thewiring enclosure on the side of the machine.Also an operator panel with a pulse wheel tojog the machine. This was another mistake. Atfirst we attempted to design a custom interfacepanel, described below. That was an unneededdesign effort. The operator control panel we diduse was a purchased part from Delta Tau DataSystems, the same folks that made the PMACcontrol card we used in the PC.

A big part of the design is cable management.This drawing shows the interconnections in therack, and to the mill. It adds complexity to haveseparate rack enclosures, but improves service.


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Operator panel

The milling machine operator uses this panel toset up and control the machine. The designintent was that the operator could have thispanel on the table of the mill, as he set up thework-piece and tools. The panel has anemergency-off (EMO) mushroom button,program control, and jog wheel.

The thinking was to replicate a traditionalcontrol like a Fanuc, but use the PC keyboardand screen, which can be much cheaper andeasier to replace. What is nice is thedevelopment of wireless keyboards, that couldhave been used to enter numbers for a largescreen that is safely far away from the tool andits chips and coolant.

The Delta Tau operator panel needed these twocards as well.

The Delta-Tau panel was industrial quality.

A picture from 2014, when I sold or scrappedall the hardware.


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This was the user-panel we designed fromscratch. I had talked to machinists who hatedtouch-panels. They wanted real switches theycould feel as they watch the tool jog down. Inaddition, I put in three optical encoders so youcould use the CNC mill like a manual mill.

I got this far with the prototype. I glued plots ofthe layout onto a purchased cabinet, and usedthat to drill all the holes. I used LCD panelsinstead of the LEDs in the CAD drawing.

All this "cool-guy" stuff was way too much todesign for a small shop. At least we had thesense to give up and purchase the Delta Taupanel.

That custom operator panel needed not onlyschematics, but a design that could interfacewith the PMAC control card. We upgraded thatcard to dual-port memory (DPM) but that onlymade it more work and low-level programmingto realize this custom panel design.

The custom operator panel did not need amicroprocessor, but it did need a multitude oflatches and registers.


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More latches.

.

Even morelatches.

Some decodersto parallelize thepulse-wheelencoders, It wasgetting prettyugly by thispoint. It was niceto designsomething that

my machinist friends loved, but all this tookaway from the main design effort. Lessonlearned-- if somebody makes an operator panel,use it, get the machine working, and then do aredesign of the panel as a follow-on project.

Computer enclosure

The computer was an IMB PC clone in anindustrial rack-mount enclosure. It had to havean ISA buss to interface with the PMAC card.Yet another mistake. The computer could haveconnected to the PMAC card via a serial port.That would have let us put the PMAC card inone of the enclosures to simplify the cabling.

The inside of the computer enclosure was prettystandard, other than the addition of the PMACcard.


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The PMAC card was complicated but veryhigh-performance. It used a custom ASIC andan Analog Devices DSP chip to control 8 axis.We made two block diagrams (above) for thePMAC and the PMAC2 we upgraded to. Wealso made a layout drawing of the PMAC2 cardbelow, to remind us of where all the connectorswere, and what they did.

Over a 1000 pages of manuals, scary.


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AC distribution enclosure

The ac distribution enclosure was complex. Ithad a emergency-off lockout. If the EMObutton was pressed, it cut off ac power to thewhole machine. The enclosure also had a3-phase step-down transformer to make 120volts for the servo amplifiers.

The ac enclosure started life as an AutoCAD3D drawing. Using CAD tools in advancedways was another goal of the project. Very fewmachine designers had adopted 3D at the time..

.In addition to the step-down transformer, thecabinet had a large EMI filter. There were3-phase breakers for the spindle and servoenclosures, as well as single-phase power forthe computer, the lube pump, and the floodcoolant pump. The enclosure had solid-staterelays (SSR) to operate the pumps underprogram control.


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.The 3-phase transformer was a custom orderfrom Shape Electronics. The ac enclosure alsohad a small transformer to power the lube andcoolant pumps.

The top and bottom covers of the enclosurewere perforated for air flow, so convectivecooling was adequate.

.The heat sink for the two SSRs is over-designedand oriented for convective cooling.

The schematic of the enclosure shows the EMO(emergency off) lockout, as well as thetransformer and SSRs. While this schematiccalls out Bulgin connectors, we eventually usedconventional ac connectors to save money.

The ac enclosure required several wireharnesses. Since a goal of the design effort wasto document everything, there are cabledrawings with dimensions and a BOM.


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This harness drawing is only a single wire.

Some of the internal cables in the ac enclosurewere simple, like these two. With a drawing foreach cable, it would be possible to build andidentical enclosure without tearing apart theexisting enclosure. This was in keeping with thegoals of having an evolutionary deign thatcould be manufactured for other machines.

Even the simplest jumper wires needed adrawing,

.The ac enclosureBOM (bill ofmaterial) costwas 1500 dollarsback in the1990s. All thoselittle parts add

up. Some things like the EMI filter and atransformer came from a salvage yard so didnot have a valid cost to put on the BOM.

Changes always happen in a design. That iswhy it is important to be meticulous in rattlingthe changes through the whole documentpackage. I had a boss that changed the name ofa product for no reason, It required 40 ECOs(engineering change orders.) That's dumb. So isnot documenting real engineering changes.Only now, 20 years later, do I see there are twoT1s in the master schematic.


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Interface enclosure

This was the interface enclosure added at thelast minute. An employee convinced me it wasneeded since the two PMAC breakout cardsneeded a separate power supply. I said "yes"without really looking at the problem. We couldhave fit this stuff in the spindle or servoenclosures. I put an "X" through the blockdrawing when I realized this years later.

The interface enclosure had a pretty simpleschematic. All it did was take a ribbon cablefrom the PMAC motion controller card in thePC and break it out to two accessory cards.There was also a power supply that fed theaccessory cards..

The major point of this milling machineredesign was the enormous space savings.Taking a giant magnetics cabinet and a clunkyoperator cabinet and replacing it with a rackand a small portable panel the machinist coulduse as he hunched over the spinning tool.

Seeing this much empty space in a rackenclosure should have set off alarm bells. Thiscould have been a 2U rack, or even a 1U, butbetter to put this stuff into other enclosures.


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Servo enclosure

The servo enclosure contained the threeAdvanced Motion Controls FET amplifiers thatran the dc servomotors on the mill. Weover-designed the heatsinks, in the hope that afan would not be needed for any realisticmachine operations. Block diagram below.

This early CAD drawing shows the step-downtransformer inside the servo enclosure. Therewere no heat sinks for the three amplifiers.

This later AutoCAD drawing shows the servoenclosure as it was built, with large heatsinksfor the amplifiers, a huge noisy fan, and aseparate fan and heatsink for the rectifiers. Thestorage caps are behind the circuit breakerpanel. it was stupid to keep the small heatsink,the rectifier could be mounted on one of thelarge heatsinks. So could the power supplyneeded by the PMAC breakout boards. As itwas, we added an interface enclosure.


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.

A SolidWorks CAD model done after theinterface enclosure was added. It corrects thedesign and eliminates that interface enclosure.It puts the power supply and breakout cards thatwere in the interface enclosure into thisenclosure. The rectifier and power supply aremounted on the Z-axis heatsink.

It was also a mistake to have three expensive dccircuit breakers on this cabinet, as well as acircuit breaker in the ac enclosure for the servoenclosure.

The 2001 schematic shows the power supplyfor the amplifiers, as well as the cables forwhen there was an interface enclosure with thebreakout boards for the PMAC controller..

The as-built design version of the powerschematic. It shows the circuit breakers, theconnections to the dc MOSFET amplifiers, andthe connectors on the back of the enclosure togo to the motors.

After seeing that the interface enclosure couldbe eliminated, we drew up new schematics,showing the PMAC breakout boards inside thisenclosure, also with a power supply for thebreakout cards.


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This schematic shows design intent for havingthe breakout boards inside this enclosure.

The ac distribution design intent.

This shows a large cable from the machinegoing to the breakout board for encoders etc.

.These ribbon cables arefor the operator interfacepanel.

Spindle enclosure

The spindle enclosure was simple, it just held asmal inverter purchased to drive one of thespindle motors. A D-sub went to the PMAC.

The block diagram is pretty basic for thisenclosure. Three-phase ac comes in from aconnector and connects to the inverter. Thisshows a large inverter driving all three motors.


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The enclosure used a 25-pin D-sub to take allthe signals from the inverter "just in case". Inreality, all the inveter needed was analog speedand direction, and a fault out signal..

This is the old Vee-Arc inverter that wasreplaced by the inverter inside the enclosure.We sold it on eBay to offset the cost of theproject. Note the brake resistors on this inverter,so the spindles would stop faster. That wasplanned for the new inverter as well.

That large Vee-arc panel is replaced by this oneinverter, but admittedly sized just for one of thespindle motors.

Machinists told me the three spindles made itseem like you could make three parts are once,but it was a bad idea. It was so hard to set allthe tools just right, it was almost impossible tohold the close specs this machine was designedfor.

A front view of the enclosure before theinverter panel got mounted on the front. Itshows how much space was left over, easilyenough for a second inverter if needed.

The mistake of adding an extra interfaceenclosure rattled thought the whole design.Thankfully, this enclosure was not affected.


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Aftermath

.The machine ended up in my side yard under atarp for 7 years. I got a rigger to take it for free,for the scrap value. A machinist who boughtmy Monarch 10EE toolroom lathe told me notto regret letting the Bostomatic get all rusty. Hepointed out it was a slow obsolete machine, andagreed with my friend that the three spindleslooked like a good idea but really were not.

I sold off the tooling and electronics on eBay,Craigslist, and the Silicon Valley ElectronicsFlea Market. I was delighted to have a fellowengineer want the control cabinets. I sold themfor pennies on the dollar so I did not have tomove them to Florida.

I don't regret for a second doing this project. Ittaught me a lot and I still love machine control.I have worked on semiconductor machinery asa consultant, and this project was just as muchfun.

Now I am looking into building one of those kitCNCs. Things are so much easier now. You canbuy a controller and drives and everything elsefor very low cost. That is a future project.

To do it over:

I got the mill to run all the axis and a spindle,but never made any significant chips. In all, itwas a bridge too far for a small shop that hadreal paying projects to work on. I could havebought a used Haas for what I spent on thisproject, not even accounting for my time.

So the first mistake was buying this old mill,even though it was from a machine shop nextdoor to mine, and I got it for almost scrap valuewith the money I made selling the operatorcabinet, the magnetics cabinet, and the Vee-arcspindle inverter. At least it was already a CNC,so we didn't have to engineer in ballscrews andthe things needed to convert a manual mill.

The next error was trying to make the redesignform-fit-and-function from the first crack. I sawthis same error when I worked for oqo palmtopcomputer a few years later.

There is a reason electrical engineers call aprototype a breadboard. It used to be an actualbreadboard, with stuff nailed to it and wired inhelter-skelter as it got developed.

Rather than fall in love with the rack-mountinterchangeable design, I should have leaned a4x8 sheet of plywood next to that wiringcabinet on the side of the mill. Then screw inthree servo amplifiers, no heatsink, they willthermally fault when they get too hot. Then Iwould know how big to make the heatsinks.

Rather than that complex expensive acdistribution enclosure, I should have gone toBell Electric and got a little sub-panel withthree breakers, one for the spindle, one for theservos, and one for the computer and lubepumps.


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My mentor, Big John Massa, went to Ohio StateUniversity. He said they purposely did not putin sidewalks between some new buildings.Instead, they let the students walk on the grasswhere they needed to. Then, the school just putdown sidewalks wherever the grass was wornaway. As a guy that grew up in Cleveland, Ihave to love that Ohio ingenuity.

The same goes for this cabinet. With all the"guts" nailed to a sheet of plywood, I wouldhave been able to see where the cables went.Then I would have known how to route andpartition them.

I would still keep the rack, but it would not beall these separate enclosures. It would have therack-mount PC, but underneath would be onebig enclosure that was hard-wired into themachine. That one big enclosure would have aregular Square-D sub-panel. I would not designa custom step-down transformer, I would justbuy a standard one. Better yet, maybe a 120Vdc power supply for all three servo amps.

.It is good not to build electronics onto the mill,the vibration kills things. I saw that with earlyBridgeport CNCs, where the large capacitorswould break out of their solder joints.

I did not realize that I really needed the step-uptransformer you can see under the oldmagnetics cabinet in the picture above. Thespindle motors needed 240 volts, and SiliconValley has 208V.

Motors will burn up on under-voltage, evenwhen powered by an inverter. The inverterchanges the frequency of the ac down from60Hz. This was just simple ignorance on mypart, but at least I never sold off the transformeruntil I scrapped the whole machine.

No custom operator panel, so no need for thedual-port RAM option on the PMAC card. Sothen easy to mount the PMAC remotely, anduse a modern PC motherboard.

Using the PMAC card as an ISA-bus plug incard was a huge mistake. It meant long delicateribbon cables had to snake out of the PC andinto the enclosures. The PMAC only needed theISA buss for power and to connect to a modemchip in the card. It worked just as well whenyou fed it power via a connector andinstructions with a 2-wire serial cableconnection.

The whole point of the PMAC was to offloadthe PC, which you dare not depend on fortimings in machine control. All the PC sent thePMAC card were short ASCII commands, verysimilar to G-codes. They got buffered on thePMAC, so it was a very easy, non-critical thingto "feed" the PMAC over a serial port, and nodifference whatsoever to using its internalmodem.

Rather than buy separate power supplies for thecard, we could have brought a Molex dc powerconnector out of the PC. That has 5V and +/- 12volts, which would have run the PMAC2 andall the breakout cards.


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The ISA bus computer was obsolete even backwhen we designed this system The shop was allPCI bus machines by then. We had to search fora compatible motherboard and enclosure for anold ISA bus motherboard.

To do it over, I would just lean a mid-tower PCagainst the plywood breadboard and use it thatway until things were working. Then it wouldbe time to look at an industrial PC, but thatwould be a modern-at-the-time PCI busmachine.

I could see using a laptop in this day and age.Things are so non-critical with the PMAC serialport, a USB-to-serial converter cable wouldwork just fine, and we could have upgraded thecomputer with no effect on the milling machinecontrol.

I would not make the EMO circuit kill the mainac power. Modern machine designers knowbetter. Even back in 1995, I could see an axiswould stop faster if commanded to stop by theamplifier, rather than have to drain all the inputcapacitors while things still moved.

I did not brush up on my control theory. Theamplifier worked in four different modes, andcould take in the tachometers on the motors. Itworked, but I am not sure it would have workedbetter with an explicit velocity loop.

The servo amps had inputs for limit switches.The BostoMatic used industrial limit switchesthat had both a home switch, and then a limit.The home switch needed to go to the PMAC,while the limit could have gone directory to theservo amp. I now think that would be moredependable, rather than having the PMAC cardshut down the amplifier. Instead, the ampwould fault and PMAC could figure things out.

Rather than use some handy extrusion to makegiant servo amp heatsinks, I should have triedmuch smaller ones, convectively cooled.

Perhaps the biggest mistake wasmismanagement. By insisting on all thoseseperate enclosures, I made my employee spendmost of his time making cables. He is a brilliantengineer, and I should have used him to figureout all the PMAC documentation and the PLCprogramming to get the machine to home andoperate the lube and flood coolant pumps. Itwould have been much more fun for him toscrew hardware to a sheet of plywood, wire itpoint-to-point, and start playing with themachine. We could have gone in afterwards anddone all the harness drawings and cablediagrams.

I also was not good about keeping the blockdiagrams current to the way the machine wasactually getting built. This is a cardinal sin inengineering, and it shows how making cabledrawings can interfere with the much moreimportant task of overall design. Tactics shouldnot interfere with strategy.

We did do some things right. Getting the servoamplifiers with the isolated option wasessential. That way the amplifier case can beconnected to chassis ground and the same forthe negative output of the three-phase bridgerectifier. That also meant I needed a three-phasestep-down transformer that had isolation,another thing I managed to get right.

We also were right not to spend 1000 dollars ona 10hp spindle drive for all three motors.Instead, we got a 3hp drive that ran a singlespindle just fine.

Experience is a mean teacher. It gives the testbefore the lesson. At least I learned the lesson.


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