Cylindrical Bearing Equatorial Platforms

Chuck Shaw

A newtonian on a dobsonian mount sitting on an equatorial platform could be just about the best of all worlds for many observers. When you combine the larger aperture for the money that a newtonian provides, the ease of construction and use of dob type alt/az mount, and the the ability to have whatever you aim the scope at STAY in view for almost an hour at a time, that's a tough combination to beat! Even photography is possible using an equatorial platform if polar aligned properly and guided photography in declination is possible if performed reasonably close to the meridian.

The earliest designs for an equatorial platform I could find published in magazines were by Adrien Poncet (SKY & TELESCOPE, January 1977, pg 64). Poncet's design uses a pivot point and rollers/slides on a plane to define the motion of the platform. Alan Gee designed a platform that used a cylindrical bearing on one end and a single pivot on the other. Georges d'Autume improved these designs by eliminating the single pivot and introduced the concept of using a conical bearing to reduce the high loading on the Poncet based designs (a potential problem for larger scopes, but not smaller ones). Messieur d'Autume provided an excellent review of all these designs back in 1988 (SKY & TELESCOPE, September, 1988, pg 303). I have seen and used platforms based on this conical bearing design that were made by Andy Saulietis. I also got an ear full from Andy on the difficulty of machining conical bearings! There had to be a better way! That's when I decided to try using all cylindrical bearings, similar to what Alan Gee did on the north end of his design. Before I go on however, it might be wise to make sure everyone understands the concept of how a platform really works.

The best way to start is to think of a Ferris wheel. Each "basket" hanging from the wheel's rim stays in the same orientation as the wheel turns. They do this because each "basket" is able to turn on a bearing or axle that is parallel with the Ferris wheel's main axle. Each "basket" turns at the same rate, but in the opposite direction, on their axles as the main Ferris wheel does on its axle. (see figure 1)

Figure 1

OK, so now take that image in your mind and picture the earth turning on its axis. If you had a "basket" on the equator, hanging from an axle parallel to the earth's axis (which at the equator would also be parallel with the earth's surface), and turned it opposite to the direction the earth is turning, it would appear to rotate once each sidereal day. It would also be staying perfectly still with respect to the stars (for all practical purposes). So, if you set your telescope on this basket whatever you were aimed at would stay in view, at least till the basket rotated enough to cause the scope to fall over! Before that happens though, you would need to swing the basket back through vertical and maybe a bit to the other side and re-aim the scope

As you move away from the equator, the axle for your basket has to still be parallel to the earth's axis of rotation, but this forces it to no longer be parallel to the ground. In fact, it will be at the same angle as your latitude. Remember, it is approximately aimed at Polaris, just as the earth's axis is. For this reason, we will refer to this axle as the "Polar Axis". However, if you still just sit your scope on the basket, it will probably not only try to fall over from rotating, but it will be tilted north/south too! This is where an equatorial platform design comes into play!

Figure 2

So, when you are not at the equator, the idea is to make each "arm" that the basket (or platform) "hangs" from a different length, so the basket itself stays level. (See figure 2). That would work, but the arms and the axle (Polar axis) would be in the way of using the telescope. So, think of the arc that the ends of the basket's arms scribe. Now think of having a cylindrical disk that is the same diameter of that arc, and with its center on the polar axis. Obviously, the disk that would be on the north side will be larger in diameter that the one on the south side (in the northern hemisphere anyway!). If you rest these two disks on bearings, and then connect them with a flat platform that is parallel to the earth's surface, and then remove everything ABOVE that platform, what is left is an equatorial platform!! It will be able to turn on a "virtual" polar axis. The north/south tilt problem causing the scope to want to fall off the platform will be eliminated, but the rotation tilt problem will still exist. However, if you limit how much rotation you allow, it never becomes much of a problem. Most designs limit the rotation to approximately 7.5 degrees on either side of vertical. This allows a total of 15 degrees, which provides approximately one hour of tracking. Pretty clever huh? We are not through however.....

If you draw the virtual polar axis through the center of gravity of your scope, and then play with the spacing of the two virtual cylinders a bit to keep the size of the platform connecting them the same size as the normal base board size of your dob mount, the effort to rotate the entire platform and scope is minimized, which means you only need a very small motor to drive the whole thing. (see figure 3). The actual CG of the system will be a bit below the CG of the scope due to the added weight of the rockerbox and groundboard. Thats OK though, plan for the polar axis to go thru the center of the altitude axis and the scope will never be top heavy and it will still be close enough to be easily driven.

Figure 3

There are a number of ways to drive the platform. The simplest concept is just to turn one or more of the bearings into a roller that is connected to a motor. While simple in concept, this option is slightly more complicated to build. You have to think of materials for the roller to avoid/minimize slippage/wear (stainless rollers and aluminum sectors are a good combination), and you need a clutch to disengage the drive motor to roll the platform back to its beginning of travel or a way to lift the sector off the drive rollers to move it back to the starting point. Balance is also more critical since friction is the only thing keeping the sector/roller from slipping. I made and am using a drive system like this now for my 14.5"f/5 scope and a cookbook 245 ccd camera. It works really well. Slightly less sophisticated, but MUCH easier to build and more forgiving of errors in balance, is a tangent arm drive. This option has a drive screw that has a nut on it, which grabs a "tangent arm" attached to the platform. The linear motion of the nut along the drive screw is turned into rotational motion, The nut has a tang attached to it that grabs the tangent arm. This tang has a vertical slot to allow for the motion of the tangent arm. This design is only really accurate near the mid travel point of the platform when the drive screw nut is actually tangent to the arc the tangent arm describes. In addition, all the extra pieces each introduce a bit of "slop" into the drive system, which also hurts tracking accuracy. However, for visual work, including using high power, the accuracy is more than adequate. For Piggyback photography it is also just fine. For guided photography or ccd prime focus, the mechanical slop limits the exposure durations to about 15 seconds (which is still OK for ccd ops where you stack the shots afterwards). Another way to attach the drive screw to the tangent arm is by a wire/chain that is bent around a "sector" instead of just a single tangent arm pin. This results in a dramatic increase in accuracy at very little additional complexity. Its problem is that you can no longer just lift the upper platform off of the lower platform since they are connected by the wire/chain. Both of these options also require a clutch between the motor and the drive screw. However, using plastic gears like in radio control cars, and pivoting the motor to disengage them, provides a simple, cheap clutch option. The bottom line is there are a lot of ways to impart the required motion to the platform!

I suggest first building the tangent arm drive. Its simple and its accuracy is more than enough for even very high power visual work when the platform is polar aligned and running at the right speed. Then work to minimize the mechanical "slop" in the system to be able to do photography, then consider a driven roller system that replaces one or two of the bearings, then go to stepper motors for the motors......meanwhile you can be using and enjoying the platform as you build upgrades to it!!!

Speaking of speed, there are also a number of options on how to drive a platform. I am a big fan of having everything battery powered so I do not have to stay near a 110v outlet (or need an inverter, etc.). However, if you do not mind these things, ac synchronous motors will solve the problem easily for you (more later on how to decide what speed). If you favor battery operations as I do, then you must decide on either a DC stepper motor or a regular DC motor. A stepper motor is very accurate. It only moves a measured amount each time its windings are energized. Then the next set of windings are energized and it moves again, and so on. Reasonably simple circuits have been described in many articles to drive stepper motors and this is really the right way to do it. However, remember the steps have to be fairly small and fast in order to NOT be seen when using the scope visually. This is more important for the driven roller option than the tangent arm option. This constraint, in turn, suggests gearing down the stepper motor to allow it to turn faster. I suggest at least 20 steps/second or faster for a stepper motor to keep the vibrations from the steps from being objectionable, with the faster the better. My current platform I use for photography runs at 40 steps/second. Circuit designs that "half step" (make the size of the steps smaller) also really help the smoothness and precision.

There is a simpler option that sacrifices only a small bit of accuracy, but is still more than adequate for high power visual work. Simply run the platform off of a simple DC motor. You vary the voltage to vary the speed of the motor. As the battery wears down, you increase the voltage (a simple pot in series with the motor) to speed it back up. There is a very simple dc to dc converter circuit based on a Radio Shack Variable Voltage Regulator that you can use to automatically maintain the dc output voltage constant (see figure 6). This is actually accurate enough for some photography! Due to the simplicity and lower cost of using a DC motor, I suggest you start with this option and later experiment with adding the DC to DC converter. Then you can upgrade later to a stepper motor if you feel you need it.

All this "theory" is all well and good, but how do you bring this to life? Well the rest of this article will try to lead you through the different steps in building a platform............................

The first step is to measure the height of the CG of your scope. The dimension is from the center of the altitude bearings to the top of the ground board. If you intend to just sit the entire scope and dob mount on top of the platform, measure the height from the center of the altitude bearings all the way to the ground. Also measure the size of the ground board. This will determine the spacing between the cylindrical bearing sectors. You also need to determine the latitude that the platform will be primarily used at. If you go north or south of that latitude it will still work, but will need to be shimmed to keep the virtual polar axis aligned. 10 degrees of shimming will cause no problems. A good suggestion is to round off the latitude to make cutting the pieces a bit easier. While not required, it may make setting up table saws, etc. a bit easier...... The polar alignment procedure will insure the platform has its polar axis aimed correctly. Also note, the actual CG of the scope and the rockerbox will be somewhat below the altitude axis (due to the weight of the rockerbox and top of the platform). For now, disregard this since being pretty close will still work, and having the CG below the polar axis is better for stability.

Lay this out in a scale drawing, similar to figure 3. Be reasonably careful, but perfectly exact dimensions are not really required at this stage. For your drawing, assume the base board is 3/4 inch plywood, and the sectors extend 2 to 2.5 inches below the bottom of the plywood. Now you can measure the radius of both the north sector and the south sector from your drawing. If you are better at math than scale drawings, you can use the three figures at the end of the article to calculate the sizes of the platform and sectors. They were developed by Ed Grafton when he laid out the plans for his platform for his 18" dob (or you can just use the dimensions Ed came up with for his platform if your scope is about the same size!)

The sectors themselves, whether wood or aluminum, will be attached to two triangular cross section blocks. The cross section shape will need to match the latitude the platform is being built for. You need to scribe the arc that the sectors have on the blank pieces. For wooden sectors, a router with a "jig" to have it cut the arc works well. For metal, scribe the arc and cut it out with a hacksaw (or much better, a bandsaw, if you can get access to one!)

Attach the sectors to the support blocks, and then to the base board. If the sectors are metal, they will need to be trued. In order to do this, you need to build a jig to hold the base board as shown in figure 4. The 1.5 " conduit is along the virtual polar axis. Make the 3/4 " thick plywood jig of a size to hold the conduit the right distance from the base board. Attach the conduit to the jig with "U" clamps. Cut the conduit to a length such that it can be tightly wedged into a door jamb or garage door opening. Keep it from wandering around with two wood blocks with 1.5" holes in them tacked to the door jam and floor. The jig needs to keep the base board square, so add at least one triangular "brace" between the jig and the baseboard. All this sounds complicated, but its really simple. If more than one platform is being made, it is shared work too, since the jig is reusable (just make sure the CENTER of the conduit is along the Polar axis. The jig will have to be offset from center to make the conduit ride along the center (This is IMPORTANT and really easy to forget to do!!) If the U-bolts do not hold the conduit in place (i.e. no "play") well enough, add wood blocks beside the conduit to better locate it on the jig. Angle iron or angle aluminum is even better than wood blocks. Add a clamp on the conduit below the jig to keep the jig at the right height (use a BIG fender washer between the jig and the clamp).

Figure 4

The electric drill should be held in place securely. Use something like a "Work Mate" to clamp the drill. Rotate the Baseboard/Jig back and forth while just touching the sanding disk. Do not try to take too big a bite with the metal sanding disk since it will deflect and you will end up with a surface that is not square. If this does happen, the platform will still work just fine, the bearings will just not ride on as much surface of the sector. Go slow and enjoy watching the fine finish appear and observe the motion of the platform around the polar axis (conduit). Make sure you are wearing glasses or protective goggles while grinding!!! You will be amazed at the precision grinding that will be the result even using such a crude setup!! This is a bit more work than just cutting them out of wood with a router. I think the increased durability is worth the extra effort, but you be the judge. Both options work well as long as the sectors are smooth.

With the upper portion of the platform completed, start work on the ground board. The bearings can be mounted using wooden blocks or angle aluminum. I use 2" aluminum angle. You will have to "whittle" on the shape to get the tangent bearings to be in the same plane as the sectors, and then do a bit more whittling to get the axles of the thrust bearings to cant inward towards the polar axis. Go slow and compare often and the job is easy. If you are using wooden sectors, try using the concave scrap leftover as a mount for the the rollers. It already has the approximate shape you will need to align the rollers, just cut the "bottom" off at the same angle as your latitude. You can also cut a concave wooden piece that matches the metal convex sectors and mount the aluminum angle on it to get the roller orientation right.

When all 4 bearing holders are ready to be mounted, place the north bearings in place (they will be wider apart since that sector has a slightly larger diameter). Then place the south bearings in place. Move the bearings in/out till you have the same amount of rotation in each direction. You will have to play with the exact location of the bearing assemblies to make sure that all 8 bearings stay in contact with their respective sectors at all times. Again, it sounds harder than it is. The secret again is to go slow and be patient. Remember, the four bearings that ride against the south sides of the two sectors MUST have their axles aimed inward at the polar axis!!! Otherwise they will not roll correctly!! The four bearings that roll against the edges of the two sectors have their axles parallel with the polar axis.

The ground board has 3 feet. Two are on the north side, the 3rd is centered on the south side. Three feet assure that the platform will not "rock". When setting the platform up and polar aligning, it will be important to level the platform in N/S tilt, more about this later. Putting adjustable feet on using T-nuts and 3/8 carriage bolts is a good idea and can be added if desired. Put a wooden disk on the end of the carriage bolt as a "foot" so it won't sink into the ground. Add a small bubble level that is ajustable with shims so that after the first really good polar alignment has been done, you can adjust the levels so they indicate level. Then, for subsequent coarse polar alignments you only really have to worry about azimuth alignment!

The tangent arm is next. A 1/4" lag bolt with its head cut off will do nicely. Either measure off of your drawing or use the grinding "Jig" to measure what the radius the tangent arm describes. Then calculate the circumference of the circle the tangent arm makes. As an approximation, use 24 hours for the rotation rate for the platform. You can then divide the circumference of the tangent arm's circle (in inches) by 1440 to get inches per minute that the tangent arm must travel. I use 16 tpi "all thread" rod as a drive screw. That means the rod must turn 16 times to move the tangent arm 1 inch. So, multiply the required speed of the tangent arm in inches per minute, by the number of threads per inch, to get how fast the threaded rod must rotate per minute. For average sized telescopes it will be somewhere about 2 rpm.

Hold the threaded rod between two bearings per figure 5. Attach the bearings to the rod with nuts jammed against the bearings. A small wooden box that has holes drilled in the ends the same size as the bearing outer races makes a nice drive box. Extra pieces of aluminum angle can also be used to hold the end pieces in place. Use large fender washers on the inside of the drive box to keep the threaded rod and bearings in place and react the pushing on the platform the drive screw must perform. The length of the travel of the drive screw must be adequate to swing the platform through 15 degrees (i.e. 1 hr of travel). Use gears found at hobby shops for electric cars to couple the drive screw to the motor. Two nuts on either side of the gear on the drive screw will hold it in place. Mount the motor on a hinge to be able to swing it into mesh with the gear on the drive screw, and to swing it out of the way to disengage it during rewind. Hold it in mesh with a small spring.

Figure 5

The nut that travels along the drive screw should be a "coupling" for the all thread. Attach a flat plate (Tang) to the coupling with two "U Clamps". Make the plate long enough to reach down between two "runners". The slot will keep the Tang (and coupling) from turning with the drive screw. Cut a slot in the plate for the tangent arm to fit through. Don't make the slot too big, the slop will show up in the eyepiece! However, the thicker the plate material, the looser the slot must be since the tangent arm will be at a slight angle at the ends of the travel and will bind. A strong spring attached between the tangent arm and the plate to always hold the tangent arm against one side of the slot will eliminate the play. You need a slot instead of just a hole since the tangent arm describes an arc that has it lower at mid travel than at the ends of travel. If the coupling is too loose on the threaded rod, there are at least two solutions. One is to pack the threads in the coupling with a mixture of talcum powder and epoxy, and coat the drive screw with PAM, or silicone spray (WD-40 is NOT good enough). Then slowly screw the drive screw into the coupling and let the epoxy harden. The Talc/epoxy mixture will have made snug fitting threads for you and the lubricant will not allow the epoxy to bond to the drive screw. An alternative is to drill and tap two small (4-40) holes into the coupling at each end. Insert a nylon machine screw into the holes and gently tighten the nylon screws to eliminate any play. Both methods work wonders in eliminating play which will find its way into the eyepiece view.

So lets review what we have completed so far. The ground board should be completed by now with the 4 sets of two bearings each mounted and aligned. The drive screw box and motor can be mounted (make sure the box is not too close to the platform else the platform corners will hit at the end of the travel). The base board should have the two sectors mounted on their mounting blocks and the tangent arm (a 1/4" lag screw with the head cut off) attached. When the baseboard is set on the ground board and the tangent arm is inserted through the slot in the drive screw tang, attach a strong spring to hold the tangent arm over against one side of the slot to remove any "play" in the assembly. Very gently tighten the two 4-40 nylon machine screws in the coupling nut to take out play in the threads but not so tight as to add too much friction.

You can use the baseboard as the "ground board" for your dob mount, or you can simply set the whole thing on top of the platform. If you use the platform for the ground board for the dob mount instead of just sitting the scope on the platform, you may want to put a couple of long 1/4x20 bolts and T-nuts between the base board and ground board on the east and west sides to hold them together when you transport the whole thing. Don't forget to remove them before trying to run the platform though or it will stall!

The motor controller can be as simple as a battery and a pot hooked in series with a DC motor. In fact, I carry a "spare" emergency controller in my parts box made of these components and have loaned it to friends that have had problems in the field. So, start with something like that to get running as soon as possible. When you are ready, build a DC to DC converter like shown in figure 6. All parts can be purchased at Radio Shack. Hold all the parts in place with a small piece of perforated circuit board and put it all into one of the small experiment boxes (also found at Radio Shack).

To use the platform is simplicity itself. Align the virtual polar axis with the earth's polar axis and turn it on. Adjust the motor speed to eliminate any RA rate error and enjoy! Whoa you say, how do you align the two "virtual axes" of the earth and the platform? Well, that's not so hard as you might think. Initially you want to spend some time doing it, and then you need to add two more small pieces of hardware. One is a level (the round ones are the best, ACE hardware carries them for about $2.00). The 2nd think is a small cheap compass. Once you get the platform initially aligned, glue the level to the ground board (the south side is best I think). This will make sure the polar axis is aligned in N/S tilt. It is important to note that when properly aligned, the base board will be level only at the "center" position. The ground board may never be level, depending on the spacing, etc. of the bearing assemblies. So, when attaching the level, you may have to shim it. The compass will allow you to repeat the E/W azimuth alignment. Using the compass and level will get you close enough for almost all visual work, and allow you to do it in 30 seconds. Be careful the compass is not too close to any metal parts! If closer alignment is desired, it will also get you to a very close staring point. Remember though, if you move to a different location, the compass and level may not be accurate for that location...! As an aside, I also use the TELRAD on my scope. I have pins on the alt and az bearings that lock the scope's optical axis with the platform's virtual polar axis. Then I just move the whole thing around till the pole is in the right place in the TELRAD reticle. (This is only accurate if the platform has replaced the Dob's ground board).

The best way I have found to achieve a really accurate alignment, whether an equitorial platform or any other type of tracking mount is the two star drift alignment. I'll give you a quick description. Its a LOT easier to do than to describe, but once achieved, thats why you want to attach the level and compass (or lock the alt and az bearings) to achieve a "coarse" alignment, which will be plenty accurate enough for most casual visual observing.

Initially, set the platform down and place a level on top of the base board (or inside the dob mount's box if the platform has replaced the dob mount's ground board) with the platform at mid travel and level the base board (not necessarily the ground board) in both directions. Also roughly aim the platform along north/south, with the motor/tangent arm towards the north. Now look in your star charts for a star on or close to the celestial equator (declination = 0) that is near the eastern horizon, and aim your scope at it. Use as high a power as you have. Don't worry about a good image, you will probably be doing this before the optics have finished cooling anyway and will only get a swimming "blob". All you want is to be able to watch which direction it will drift. Now turn on the drive. If you have calculated the drive motor's rotation rate based on the geometry of your platform, adjust the motor to run at that speed. Now watch the star and see which direction it drifts. Use the pot in your motor control circuit to null the drift errors due to motor speed (RA errors). What is left is drift that is in declination (i.e. north south). If the star drifts to the north (if in doubt, nudge the scope towards polaris, stars will enter from the north in the eyepiece field of view), that tells you the south end of the platform is too low (the virtual axis is aimed above polaris). If the star drifts to the south, then the south end of the platform is too high. Re shim the platform and repeat the test again. Then find a star near the meridian, but also on the celestial equator. Again, watch which way it drifts in declination while you adjust the motor's speed, if required, to keep it in the center of the RA direction. If you are confused as to which direction is RA and which is Dec, turn off the motor and watch which direction the star goes. That is RA. Declination is perpendicular to that. If the star drifts to the north, then the virtual axis is pointing west of north. If it drifts to the south, the polar axis is aimed too far to the east. Rotate the platform to correct the error and re-perform the test. To make sure, recheck the star in the east, and then the one on the meridian again. When you have it, remember to attach the level and the compass. Any errors from then on are due to the motor running too fast or too slow. The Dc to Dc converter will automatically adjust the voltage for you as the battery slowly drains (at least till the battery gets to about 1.5 vdc above what the motor needs, then it cannot help any more). Most Dc motors I have used draw about 30-50 ma and are 12 v motors that I am running at about 3 vdc. I use 6vdc Gel Cells from a surplus store (rechargeable), or a normal lantern battery. A lantern battery lasted an entire week once at the Texas Star Party!!

If you want to do guided photography in both RA and Dec, you will need to stay within 10-15 degrees of the meridian to avoid field rotation if you make declination corrections. Declination corrections when looking south are made by adjusting the "altitude" of the telescope in the Dob Mount. You really need a small motor drive on the altitude axis to do this smooth enough for guided photography. If guiding in RA only or for unguided photography, the motion is fine and has no rotation problems anywhere in the sky. But unguided shots require very good alignment and a carefully adjusted motor speed. Piggyback photography is much less demanding and is a lot of fun! You might try that at first. Another method of photography that is fun with a platform is video. I made a mount for my 8mm camcorder to "look" into a 32mm erfle (2"). Set the camcorder focus at infinity. This is called afocal photography. I got some great shots of Jupiter and the Shoemaker-Levy comet impact sites using this method! I also recently purchased an inexpensive surplus survelience camera that has a removeable C-Mount lens. I can use this camera at "prime focus". Without the lenses in the optical path, and with the camera's increased low light level sensitivity, even some of the brighter deep sky objects are video targets now!! Combine the approach using a video camera with a laptop and a small portable frame grabber like a "Snappy ã " and you have a digital imaging system!!!!

I hope you enjoy building and using an equitorial platform. Let me know how your platform building escapades go!!!

Chuck Shaw

cshaw@ghgcorp.com

Cylindrical Bearing Equitorial Platforms

Parts List/Approximate Costs

Ground Board

GROUND BOARD

       

Groundboard Base

20x25x3/4 BCX Plywood

1/8 sheet

$22.00

$ 2.75

Brg Braces/stiffeners

4x20x3/4 BCX Plywood

1/20 sheet

$22.00

$ 1.10

 

6x20x3/4 BCX Plywood

1/20

$22.00

$ 1.10

Wood Screws:

#8 x 1 1/4 Flathead

8

$ .04

$ .32

         

Feet/Leveling Screws

3x3x3/4 BCX Plywoood Scraps

2 pcs

n/a

n/a

 

3/8 T-nut

1

$ .25

$ .25

 

3/8 x 16 x 4" Hex Bolt

1

$ .35

$ .35

 

3/8 wing nut

1

$ .40

$ .40

 

3/8 Flat Washer

1

$ .03

$ .03

 

Wood Screws:

##8 x 1 1/4 Flathead

2

$ .04

$ .08

         

Bearings and Mounts

2" x 2" aluminum angle

4 3" pcs

$23.00

$ 3.85

 

Bearings (surplus)

8

$ .85

$ 6.80

 

Bushings (3/8 lockwashers)

16

$ .06

$ .96

 

Axle Bolts (3/8" x 1")

16

$ .14

$ 2.24

 

Spacers (3/8 Flatwashers)

16

$ .12

$ 1.92

 

Axle Bolt Nuts (3/8 hexnuts)

16

$ .06

$ .96

 

Wood screws:

#6 x 1" roundhead

8

$ .03

$ .24

         

Sub Total

     

$23.35

Sales Tax

     

$ 2.04

Ground board Sub Total

     

$25.39

Base Board

BASE BOARD

       

Baseboard

17.5 x 20 x 3/4 BCX Plywood

1/8 sheet

$22.00

$ 2.75

Sectors

2x4 Fir, 20" long (laminated)

4 pcs

 

$ 4.00

 

Epoxy (to laminate fir)

1 pint

 

$ 4.00

 

5/16" aluminum plate

5 lbs

$1.00/lb

$ 5.00

 

Wood Screws:

#8 x 2" Flathead

#8 x 1 1/4 Flathead

#6 x 1" Flathead

6

6

16

$ .05

$ .04

$ .03

$ .30

$ .24

$ .48

Tangent Arm

1/4" x 20 x 4.5" bolt

1

$ .12

$ .12

 

1/4" x 20 hex nuts

2

$ .05

$ .10

 

Wood Screws:

#6 x 1" Roundhead

2

$ .03

$ .06

 

5/16 Aluminum Scrap

1 pc

n/a

n/a

         

Sub Total

     

$17.05

Tax

     

$ 1.49

Baseboard Sub Total

     

$18.54

         

Travel Locks

Travel Locks

       
 

1/4 x 20 T-Nuts

2

$ .12

$ .24

 

1/4x20, 6" Carriage Bolts

2

$ .29

$ .58

 

1/4x20 Wing Nuts

2

$ .12

$ .24

 

1/4" x 1" Fender Washers

2

$ .14

$ .28

 

1/4 x 20 Hex Nuts

2

$ .05

$ .10

         

Sub Total

     

$ 1.44

Tax

     

$ .13

Travel Locks Sub Total

     

$ 1.57

Tangent Screw Drive

Tangent Screw Drive

       

Rod and Bearings

Bearings ( 3/8 ID, 7/8 OD)

2

$ .75

$ 1.50

 

3/8 x 16 Threaded Rod

18" (of 36")

$ 1.63

$ .82

 

3/8 Hex Nuts

4

$ .06

$ .24

Rewind Crank

1/4x20, 1.5" bolt

1

$ .10

$ .10

 

Nylon Spacer (1/4" ID, 1" Long)

1

$ .22

$ .22

 

1/4" Hex Nuts

2

$ .05

$ .10

 

5/16 Aluminum scrap

2" long pc

n/a

n/a

Bearing Thrust Plates

3/4" ID Fender Washers

2

$ .29

$ .58

 

Woodscrews:

#4 x 1/2" Roundhead

4

$ .05

$ .20

Box

3/4" clear Fir, 3" wide

18"

$ 2.00

$ 2.00

 

1/4" plywood, 4"x12"

1pc

$ 1.50

$ 1.50

 

Woodscrews:

#6 x 1.5" Flathead

#4 x 1.5" Flathead

8

7

$ .05

$ .04

$ .40

$ .28

Traveller

3/8 x 16 coupling

1

$ 2.00

$ 2.00

 

U-bolts

2

$ .65

$ 1.30

 

3/8 hex nuts

4

$ .06

$ .24

 

10 x 24 Machine Screws

2

$ .06

$ .12

 

10 x 24 Hex Nuts

2

$ .03

$ .06

 

10 x 24 flat washers

2

$ .02

$ .04

 

spring (rattle damper)

1

$ .50

$ .50

 

1/4 " aluminum plate

3" x 3.5"

$ 1.00

$ 1.00

         

Sub Total

     

$13.31

Tax

     

$ 1.16

Tangent Screw SubTotal

     

$14.47

Drive Motor, Clutch, and Gears

Drive Motor, Clutch, and Gears

       
 

2 RPM DC Motor

1

$ 2.95

$ 2.95

 

2" x 2" aluminum angle

3" long pc

$23.00

$ .96

 

10 x 24 machine screw

1

$ .06

$ .06

 

10 x 24 nyloc nut

1

$ .10

$ .10

 

Gear Set (Model Car)

2 gears

$ 7.85

$ 7.85

 

3/8 hex nuts

2

$ .06

$ .06

 

3/8 flat washers

2

$ .12

$ .24

 

Woodscrews:

#8 x 2" flathead

#6 x 1" roundhead

1

2

$ .05

$ .03

$ .05

$ .06

         

Sub Total

     

$12.39

Tax

     

$ 1.08

Motor/clutch subtotal

     

$13.47

Electrical System

Electrical System

       
 

Spade Connectors

7

$ .10

$ .70

 

On/Off Toggle Switch

1

$ 2.00

$ 2.00

 

25 Ohm Wire-wound Pot

1

$ 1.75

$ 1.75

 

10 Ohm Ballast Resistor

1

$ .50

$ .50

 

Piezo Horn

1

$ 2.50

$ 2.50

 

Micro Switch (limit switch)

1

$ .85

$ .85

 

Aluminum Plate/2x2 angle

2 small pcs

$ .64

$ .64

 

Alligator Clips

2

$ .25

$ .50

 

Wire

18"

$ .50

$ .50

 

Woodscrews:

#4 x 1/2" roundhead

2

$ .05

$ .10

         

Sub Total

     

$10.92

Tax

     

$ .88

Electrical Sys Sub Total

     

$10.92

SUMMARY

SUMMARY

 

Ground Board

$ 25.39

Base Board

$ 18.54

Travel Locks

$ 1.57

Tangent Drive Screw

$ 14.47

Drive Motor/Clutch/Gears

$ 13.47

Electrical System

$ 10.92

   

TOTAL:

$ 84.36