Volume 5, Number 3
March 2000
Journal of the Home Metal Shop Club of Houston
President - Keith Mitchell, V. Pres.- John Lilly, Treasurer - Gordon Lawson, Secretary - Dean Eicher Editor - Keith Mitchell: Email kmitchl@wt.net

Membership is open to all those interested in machining metal and tinkering with machines. The purpose of the club is to provide a forum for the exchanging of ideas and information. This includes, to a large degree, education in the art of machine tools and practices. There is a severe shortage of written information that a beginning hobbyist can use. This makes an organization such as this even more important. For membership information and forms, call or e-mail Keith Mitchell at the phone numbers shown at above.

Notes from the President

By: Keith Mitchell

The next meeting will be held in conjunction with the FIRST robotics regional competition. For those who missed the last meeting, this is a group of high school students who design and build robots from a kit of parts. They then have competitions to see whose robot can best perform certain task. Representatives spoke at the last club meeting enlisting our support for the regional competition. The competition is held March 16-18 at the Astro Arena. Several members have volunteered to help with this event. If you would like to volunteer please contact John Lilly. Rutland Tool has agreed to loan a demo Jet 9X20 lathe to be operated by our members during the competition.

I have attached an info sheet so everyone can find us at the Astroarena. There is a meeting room reserved for us. We have a tour of the pit area scheduled for 11:30. If you would like to tour the pit area please plan to be there a little before 11:30. The pit tour will be followed by a short Chips meeting. The finals of the competition start at 1:00.

The subject of remote members came up in the February meeting. Remote members are defined as those who have a desire to participate in our club but are not within commuting distance of Houston. I get several e-mails each month from people all over the US and some international who want to join our group. When this issue had been brought up in the past the position was that we post essentially everything we do to the website where it is free. Therefore, the only benefit to a paid membership is receiving the newsletter by mail and the opportunity to participate in club activities. No resolution was reached in the meeting. I propose that we appoint a committee to study the aspects of this issue and report back to the members with a proposal. The proposal will then be put before the members for discussion and a vote.

March Business Meeting Agenda Since we are meeting in conjunction with the FIRST Robotics competition, I recommend we bypass the business meeting for March. If there are items which need to be addressed, please let advise John Lilly at jhlilly@aol.com

February Meeting Minutes

By: Dean Eicher

Chips Meeting - 1:00 P.M. February 19, 2000, Collier Library Attendance - 37, Three first time attendees In the Chips Meeting, the following activities took place.

1. Lucia Sevcik - told us about the robotics competition at the Astroarena March 16-18..She was trying to recruit people with mechanical and machining skills to help with the competitions. For more information, check www.usfirst.org

2. Newsletter Articles - Joe Williams volunteered to write a newsletter article for the March issue, Dominic Mancuso for April, Bill Sperry for May, and Vance Burns for June..

3. Remote Members - Further discussions on remote members ranged from discontinuing them to encouraging them. Proposals to both increase and reduce dues for remote members were submitted. It was generally agreed that remote members could provide useful information, had the same mailing expense burden as local members, but missed the benefits of the monthly meeting.

4. Meeting Time - Further discussions on the meeting time resulted in no action being taken. Doug Blodgett strongly urged us to consider changing it to avoid a conflict with the locomotive club run day.

5. Feature Presentation - Tom Moore talked about screw threads and thread cutting. He mentioned British Standard Fine (BSF), Whitworth (BSW), and Association (BA), Original Swiss (Thury), Model Engineer (ME), British Standard Cycle (BSCycle), Unified National Fine (UNF), Coarse (UNC), Extra Fine (UNEF), Special (UNS), Miniature (UNM), ISO Fine (ISOF), Coarse (ISOC), Miniature (ISOM), ISO next choices (ISO2,3,etc.), just ISO (ISO), Metric, Metric Miniature, Progress Watch, Swiss Watch, Acme, Stub Acme, Standard Thread Insert (Helicoil), National Pipe Tapered (NPT), British Standard Tapered Pipe (BSP.Tr), and Model Engineer Pipe Thread (MEPT). Tom compiled a composite list of screw thread dimensions for the above thread series which included single depth of thread dimensions for both sharp pointed and form tools, feeding at both 90( and 1/2 the thread angle, for both internal and external threads! He also mentioned the 'Root-Flat' method and 'Crest-Flat' methods for screwcutting improved threads when a properly radiused cutting form tool is not available.

6. George King - brought a holder he made for a conventional pencil that attaches to a compass.

7. Dick Kostelnicek - brought a split cam for a steam engine that he machined while it was fastened to a boring head. The straight shank of the boring head was chucked up in the lathe and the cam offset was simply dialed in on the boring head.

8. Joe Williams - brought some 5C collet holders.

9. Bob Lang - brought a small Rudy Kouhoupt design steam engine that he made and ran off a tank of compressed air.

10. Doug Blodgett - brought machined castings for a steam engine blow-down valve.

11. Tom Moore - brought a commercial between centers type boring bar.

Foundry Group Notes

By Keith Mitchell

Art Voltz presented his results from searching for Petrobond. Budget Casting Supply seems to be the best source. They advertise both premix Petrobond and a Petrobond kit which contains, the Petrobond resin, catalyst, and oil. The Petrobond kit makes most sense for us since shipping the premix is expensive and there are multiple sources for graded sand in Houston. The Budget Casting Supply website states that it has been difficult to obtain satisfactory results with the Petrobond kits without a professional muller. Tom Moore has a muller and has agreed to mull the materials. It was agreed that Keith Mitchell would order the kit and obtain sand for a trial run with the Petrobond kit.

Bill Kimbrough brought the furnace he donated to the club. Billy Hobbs took the furnace to his shop and will investigate what needs to be done to make it servicable. Our first pour is still scheduled for April 22.

Delta Band Saw Blade Tensioning Knob Revision

By J.R. Williams


The revision to the blade tension adjustment is a welcome addition for an "Old Man" whose wrist joints are showing the effects of time. The crank is far easier to turn than the original knob. The crank assembly speeds up the blade changing procedure. BASE The base unit is made from a section of brass bar stock 1-3/4 inch in diameter with a slot for the crank arm. The slot is cut with a 3/8 inch diameter end mill and is 3/8 inch deep. The three holes (A) for attaching to the units knob are for 10-32 SHCS on a 1-1/4 inch diameter bolt circle. The holes are counter sunk. The other hole (B, on the same bolt circle, is to attach the arm to the base and is drilled and tapped for a No. 8-32 screw.

HANDLE The handle is made from a section of plastic (or aluminum) round stock turned to provide a tapered section 0.500 inch in diameter on the small end and 0.650 inch diameter on the large end. The center hole is drilled with a No. 9 drill through and counter bored (C) for the head, 5/16 inch in diameter by 0.230 deep.

ARM The arm was made from a section of 3/8 inch square brass bar stock and is 3 inches long. The ends are machined to a 3/16 radius. The attaching screw hole (H) is drilled to clear a No. 8-32 SHFH machine screw and counter sunk for the head. The other hole (I) for the handle is tapped for a No. 10-32 machine screw but only 0.300 deep as the screw bottoms in the holes threads to lock it in place on assembly.

BEARING BASE The Bearing Base is made from 3/4 inch thick aluminum stock and is attached to the saws' upper guard with two 1/4-20 FHSH machine screws in the holes (F). The guide section and bearing (E) consists of a 3/8 inch bore "oilite " bronze bearing that is pressed in a 1/2 inch bore. The bearing is 3/4 to 7/8 inches long. The outer section (D) is machined to a 9/16 inch radius and stopped in the middle to provide a 1/2 inch wide support section.

The final assembly preparation consists of machining a relief section in the Band Saws' knob, 1-3/4 inch diameter by 0.025 deep, using the base as a guide to drill holes in the knob for the No. 10-32 machine screws. The base unit is connected to the upper guard with counter sunk flat head screws as clearance is at a premium in this area as it is adjacent to the outer rim of the upper wheel.

Stepper Motors Need Not Be Totally Mysterious!

By: Jan Rowland

Keith has asked me to do a kinda follow-up to the November Home-Brew article about stepper-motor fundamentals and controls-I took that to mean that I should compose this with an eye toward those bits of such information which would be 1)interesting to the HMSC membership, and 2)useful to those of us who brew our own "CNC" gear. Whew! Being academically-challenged, that is a task! Actually, it is that which caused me to be academically-challenged which got me on the path of self-made CNC tinkery: I am very, very lazy! Thus, I couldn't make it in real life via conventional means, so I have always sought ways to ease the pain of doing real work, and making machines do mundane, repetitive jobs seemed the way to go. It has always been entertaining, and continuously very informative; and I am most happy to do my best to share, although I hasten with an apology to those who already know way more than I in the following topic:

As I have said in my previous HMSC Newsletter article, there are two routes customarily used to move those portions in CNC gear which are traditionally moved by hand, say, by turning a handwheel, etc: "Closed Loop" and "Open Loop". The latter method is the one which is achieved with stepper-motors, with no feedback to the controlling computer, which would be "closing the loop". With the Steppers, assuming all is properly engineered, this feedback is not needed; thus, this system is considerably simpler to design and build, and much less expensive to build, oz-in. for oz-in.

From the outside, unmounted, most steppers look exactly like any brushless motor might. (In fact, there are Industry Standard Case-Sizes which are common to both types). But there is a considerable difference, and I hope to explain this sufficiently that the reader will at least have a mental grasp of how a stepper functions: Figure 1 shows a rotor which is here a simple bar-magnet with both "N" and "S" ends. There are four electro-magnets about this, on 90 locations, and the polarities of the ends of those windings are shown, assuming a particular electrical polarity to each. You can see that the rotor is being repulsed by like magnetic-polarities at either end, and on one side, giving the rotor no choice but to try to move to the left, as depicted by the arrows. In Figure 2, you can see that this has happened, and the rotor has continued on due to inertia, to an "average" position so that its poles are centered between opposite poles of the electromagnets.
 

Now, if the electrical polarities of these windings are inverted, the rotor will snap again, to the next position. Thus, by alternately inverting the polarities of the windings, the rotor can be made to rotate at a constant rate. But this rotor has, as you can see, only two possible steady-state positions, NE-SW, or NW-SE. Not very fine resolution! But if the rotor is made with many teeth like a pinion-gear might be, each with alternate magnetic poles, and the windings are likewise made with teeth, but the total number of these teeth is not exactly the same as the number of teeth on the rotor, then, that rotor will find a point at rest where as many teeth as possible face opposing polarities on the stator around it, and as few as possible are not directly-facing opposing polarities. This way, when the polarities of the stator-windings are electrically changed, the rotor steps to the next-most-satisfactory position.

Before you stop reading in frustration, I will hasten to admit that, no, there are not 200 different windings in a 200-step-per-turn stepper-motor! These windings are common to groups of five teeth (I think!), and are arranged in a parallel-series combination so that, electrically-speaking, there are actually only two different windings, such as you have when windings in Fig. 1 (or 2) A and B are in series, and form one winding, and C and D are in series, forming the second winding. Thus, you have only two circuits for motor-current. But let's go back a couple of decades in electrical State of the Art before we go on: Refer to Fig. 3. Here, you see a unipolar DC supply, that is, a single battery. The two double-throw switches inside the heavy boxes are actually solid-state circuitry in real life, but are so drawn here for explanatory-simplicity. You can see that there are effectively two windings in the motor, but these are center-tapped, and the center-taps are the (-) common, via the white wire, through the resistor "R". Thus, when a switch is changed, the magnetic-polarity changes in that center-tapped winding. Note, this motor has 6 leads. Also note that only one-half of either winding is being used to carry current, switches in either position. In other words, one-half of the copper is unused at any time. But you have only one DC power-supply to build or buy. Now, look at Fig. 4: Here, you have two DC supplies ("batteries" in the schematic). But you have no center-taps, and only three wires to the motor-windings (in real-life, both ends of both windings, a total of 4 wires, are actually brought out to the driver-circuitry; here, the draftsman was lazy, so, to save time, drew the concept-circuit as simple as possible for himself). By tracing the circuit with a pencil, you can see that a similar swapping of magnetic-polarity of either winding happens when either switch 1 or switch 2 is operated. Now, as you can see, all the copper is conducting current at any instant. There is less wasted-heat, less wasted copper, less wasted physical space, less wiring from the motor, all at the expense of a somewhat more complex electronic driver circuit, here, shown as double-throw switches for simplicity.

For a time, a little over twenty years back, I requested all the literature I could get manufacturers of stepper-motors and their drivers to send me, so long as I didn't have to pay the exorbitant postage. At first, I read it all, cover to cover, and was arrogant enough early-on to believe I had taught myself enough about stepper-technology to dare to build some home-brew CNC gear. I did, and it worked. Didn't I say, before, that the reason it did was simply that I was too stupid to realize an ignorant fool such as I couldn't do that, so I went ahead and did, and it worked, anyway? It can happen! Back then, driver-circuitry for size-34 and smaller stepper-motors was simple enough that I could design better, myself, and did-and I built my own drivers for some of the first little machines I built. Only once, did I have opportunity to build something using size-42 and a MO-172 motor (big as a 1-hp. 3- tool-motor!), and the driver for those was a bit too serious for me, so I bought them, ready-made. Then, just 15 years back, those very-high-power stepper drivers were very expensive! Since the customer was not fiscally disadvantaged, this worked OK. But lately, I have had occasion to build two new CNC lathes, and I have decided to beef up some parts, including the size-34 motors for the lead-screws. Fortunately, Superior Electric, now "Motors and Motor-Controls Division of Warner/Dana", has begun to make a new series "KM" of size-34 motors which have over twice the torque of the old standard MO-92 motors. Those older MO-92 motors had tapped windings for use with unipolar drivers, whereas the KML-92 motors have untapped windings, and require bipolar drives. Oh, and the old unipolar drives were the "R/L type", that is, in a compromise to keep the inductive time-constant down sufficiently that the stepping-rate of those old unipolar designs was acceptable, a power-resistor was wired in series with the +commons to the motor-windings (center-taps). (R=Resistance, and L=Inductance) Due to electrical-engineering details such as that might begin to imply, and which I need not nor want to get into here, more energy is wasted as heat from those series-resistors than the motors actually convert into useful mechanical energy! But the new Bipolar Chopper drives chop the DC to the motor-coils, and thereby modulate the duty-cycle, so that the average current is no higher than the nameplate rating. But such a drive is a tad more electrically complex, and it makes no more sense for me to design/build my own bipolar chopper drivers than for a new-car buyer to build his own engine in his home-shop, and buy that new car sans motor. So, for a couple $hundred more, I buy myself more time to spend doing the mechanical work. And, to be realistic, even if I thought I could build my own chopper drives efficiently, the parts-cost, alone, would probably exceed the unit-price of the ready-made/tested/warranted drives! To recap a bit: I feel it is worth the extra money you might imagine you thus-spend to buy something that works-for-sure, and for which you spend no time building anything electrical.

DC Power Supply? With either the power-wasting R/L driver or the more-modern Bipolar Chopper driver, you still have to supply amperes of current at 24 to 40 VDC for two size-34 steppers, or even two size-23 steppers such as MO-62. Little things, only 21/4" square flange, and maybe 3" long, but they want some amperes! A DC supply which can supply 24 VDC at, say, 8 or 10 amps, is a serious piece of electrical hardware! For stepper-motor drivers, either kind, well-regulated DC is not particularly necessary. In fact, I recently noted in the specs for a new drive I was perusing that "DC Required" was "24-40 VDC". That's another advantage to the new chopper designs: They automatically sense the average-current through their output-terminals to the motor, and, so long as the incoming DC will supply that average current, the chopper-drive automatically "chops" at the required duty-cycle so that desired average current flows, with 24 VDC in , or 40 VDC. That is, it would do you no good at all to supply regulated 24 VDC to the chopper drivers. For the reader not "into" electronic details: A DC power-supply can be as simple as a transformer connected to the AC line, followed by one, two, or four diodes, and a filter-capacitor. The resulting output would be "DC", but the actual voltage would depend upon the instantaneous AC-line voltage into it, the load-current, the condition of the parts and the temperature of the transformer, etc. But if you "followed" that DC with a regulator, you could keep this DC-out within a fraction of a percent, so long as the "raw DC-in" was at least 2 volts, or more, than the regulated output voltage. Such details are trivial to a person into electrical stuff, but such a detail can have a significant effect upon the reliability, actual function, and cost-to-build of such a project as discussed here.

Warner/Dana has been introducing new stepper-drives and DC supplies faster than the post-office has been delivering their junk-mail to me! For the two new CNC Lathes I am building at the moment, I have ordered and received two new units from them which contain power-supplies and two motor-drives, each. Oh, and they have opto-isolated inputs, meaning that they should be about as electrically-immune from EMI (electrical noise) as practical. Electrical noise? Yes -- with circuitry involving computers and like electronic gear, you can have "spurious inputs" from such sources as a large machine with a nasty magnetic contactor coming on, across the room; an AC unit with a poorly-wired AC-line-in, or even something as simple as a Mr. Coffee machine switching to the "keep it warm" mode, in the same room! Surely the reader has noticed those little "sparkles" on the TV-screen, now and then, even when the lights don't blink, and there is no lightening-storm in visible proximity? That is "electrical noise"! It can be harmless, or it can cause several steps (0.0005" each!) into that nearly-finished arbor you are turning, which has an OD tolerance measured in tenths!

Some further chat on that point might be of interest: The little CNC lathe I built at home in '84 for a very esoteric and specific task, turning pipe-organ drawknobs, has, by now, made about 18,000 knobs of "exotics" such as ebonies and rosewoods of several kinds, and even some Continental European Boxwood, which had been stored in an attic in England for the last 127 years-long very illegal to harvest, transport, sell, export, mention, or show!- In all that time, whenever I hear thunder, I switch off and go inside and hassle the wife. Because a lightning-strike on a power-line within miles can cause a "glitch" which can cause an unwanted step, or stall. This latter might not "ruin" a work-piece, but then, I have to "find" the 0,0 point again, once the lightning has gone away, and "cut air" until the cutting-tool gets back down to where it left-off. Oh, that is very, very rare, but it has, more than once, cause the utterance of untoward syntax!