How to Build an R/C Mini Tank

 

2nd Edition

 

By Kyle Liberg

 

Contents:

 

Parts and Materials

Tools

Chassis Building

Servo Modifications

Making a Hub

Final Assembly

 

Introduction

 

Please note that depending on the type and level of skills you have, this instructional could be used exactly or many different substitutes and changes could be easily made in the design. That is someone with good fabricating skill may only want to get some ideas from these plans, while other people may want to follow them exactly.

 

††††††††† These instructions are written for someone who is new to RC and may cover a lot of information that will seem very elementary to more the advanced hobbyists. I am also trying to be as thorough as possible for the same reason, so donít be intimidated by the length of the instructions.

††††††††† Be aware that the modifications to be done to the 2 servos for this project will most certainly void any warranty on them.

Finally, please use common sense and caution when working with any power tools (or glues). So donít come crying to me if you cut off a finger or glue your hands together.

 

Materials and Tools Explanation

 

Parts and other materials:

 

Chassis Material- In theory you can really build the chassis from anything you want. If you had the money you could built it from 24K gold if you wanted to.

I used plexi-glass (also know as acrylic glass or plastic) for my first and second models, because I liked the look of it. For this (third) model I used wood since itís a lot easier to work with. The choice is entirely yours, though. Just be certain what ever material you choose, you have the tools and skill to work with it.

As far as the thickness of the material goes, it should be a ľ of an inch. A 2 foot by 2 foot sheet will provide more than enough material. If you can find a smaller piece, a 1 foot by 1 foot should also provide enough material (and extra for any mistakes).

Now is the question of where to find the material for the chassis. Places like Menards or Home Depot should be able to offer you a good selection of both wood and plexi-glass sheets. Hobby shops also sometimes carry a selection of thin plywood, but expect to pay more for a smaller piece.

 

Glue- You will need glue for holding together the parts of the chassis. For wood there are many types of glue that will work well. I know that Elmerís Probond works very well, but there are plenty of other good choices out there too.

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For the plexi-glass chassis on my first two tanks I used Devconís Two Ton Epoxy. Like-wise there are also many other glues out there that would bond plexi-glass equally well.

 

If youíre not sure whether a certain glue will work with your selected material, just take some small pieces of scrap and test glue them to see how well they hold.

 

Tamiyaís Tracked Vehicle Kit- This kit from Tamiyaís educational series will obviously provide a lot of vital parts. You can find it at places like Tower Hobbies or your local hobby shop. I know that Edmund Scientifics carries it too (they charge more). You can Googe ďTamiya tracked vehicle chassisĒ too, to find more places that sell it.

 

A radio system- If you donít already have a radio, receiver, and two servos you will need a system that has all of those. I would recommend a Futaba AM system. Other brand systems would work fine too, so long as they are for surface vehicles. For this purpose I would also recommend it being a two stick radio, opposed to a pistol style radio. You certainly can use a pistol style if you want (I use one with mine), but the stick controller would be a little better suited for the driving style a tank requires. Here are some good, base level systems (see More Information on Radio Systems for an explanation of terms):

 

        Futaba 2DR AM radio

        HiTec Aggressor Ranger 2N

        Airtronics Avenger AV2X

 

 

Batteries for the Radio System- In order to run both the transmitting and receiving parts of the radio system, you will of course need batteries. AA alkaline batteries will work fine, but rechargeable replacements will save you money in the long run.

 

Fasteners- Many parts of the tank need screws to attach them to other parts. Most of the screws you need will come with the tracked vehicle kit or the servos. You will get 8 additional #2 by 3/8 inch sheet metal screws. I used hex heads, but these also come in a Phillips head type too. If the servos you are using donít have screws with them for what ever reason, you will need 8 more screws of the same size.

††††††††† If you would like to make a body or a lid for the tank, you will also need something to fasten that down too. For quick access to the batteries, I would recommend using some strips of adhesive backed Velcro. You can find it at many craft and sewing stores such as Hobby Lobby. Small screws will work too, but will make it slower to open the tank up.

 

You will need:

Eight #2 x 3/8Ē sheet metal screws

(16 if your servos donít have screws with them)

 

 

Internal antenna wrap- You need something to hold the antenna wire of the receiver. Something like a piece of cardboard or plastic will work fine. For mine I used the back panel of the package the above screws came in.

 

Optional Materials:

 

Paint- If you like, you can paint your tank. Make sure that the paint will stick to the material you choose, be it wood or plexi-glass.

 

Double sided tape- Not to be confused with servo tape. This is like ordinary scotch tape, yet both sides of it are sticky. You can find it at many office supply stores, greeting card outlets, and other stores that carry stationary type materials. Itís very useful for holding parts together temporarily for cutting out identical parts.

 

Servo tape- Having some of this to secure some of the parts in the tank would be a very good idea, but technically not necessary. You can find servo tape at hobby shops that carry RC stuff.

 

Adhesive Velcro strips- This would be good for securing a lid on the tank, or holding the batteries in place. Anything you want to stick down, but be able to remove quickly later. You can find this stuff at many arts and craft type stores such as Hobby Lobby.

 

Extra material and fasteners for a body or a lid- You may want extra material for making a body, or just a lid. Youíll also need some sort of fastener to hold it on, whether Velcro or screws.

 

Tools

 

Saw- You will naturally need something to cut out the parts with. A power saw is nice to have, but a hand saw will work fine too. Make sure the blade type (teeth per inch number) and cutting speed are suited to the material youíre using. Plastics tend to melt at the high speeds many wood saws operate at.

 

A drill and set of bits- There are 16 holes that need to be made in the chassis, so having a range of small sized drill bits and drill to use them is necessary. You could even use a simple hand chuck for making the holes. Thatís all I used for the 8 screw holes that hold the servos. Dremel offers a nice set that has all the sizes youíll need and more for about $6.

 

You will need at least:

3/32 inch

1/8 inch

 

Having one larger bit (1/4 inch or larger) would also be good for removing material on the drive wheels.

 

 

Sandpaper- This will come in handy for smoothing out any rough edges on parts or enlarging holes gradually. If you use plexi-glass it is a good idea to rough up gluing surfaces for better strength.

 

An accurate ruler- It should have lines on it that mark at least 1/16 of an inch. Try to mark and cut everything as accurate as you can.

 

Something to mark cuts with- Just a plain old #2 pencil will work if youíre using wood as your chassis material. For plexi-glass or other types of plastics, a hobby knife (x-acto knife) or sharp scribe will work better. Running a pencil over a scribe mark on plastic will help to make the line more visible still.

 

Optional tools:

 

Rotary tool (Dremel) - These come in very handy for a lot building projects. With the wide variety of attachments available for them, you could literally build the entire tank with this tool alone. There are even attachments that will turn one into a mini saber saw.

††††††††† For this project it would be particularly helpful for shortening hub screws (with a metal cutoff wheel) and making a recess in the back panel for the servos (with a router bit and attachment). This is an entirely optional tool, though.

 

Mini Square- This would be useful for making sure parts cut and glued at accurate 90 degree angles.

 

Letís Begin

 

When it comes right down to it there are only three main components to be built. First of course is the chassis for the tank. Thatís basically just a box with some holes and slots in it. Second are the modifications to the servos to make them act as gear boxes instead of limited travel arms. Last are the drive hubs which attach to the servos. All the rest of the work is just putting existing together and adjusting them to fit.

 

Making the Chassis Pieces:

 

Part Dimensions:

 

 

Left and Right Sides (mirror dimensions of each other):

 

Both sides start out as rectangles that are 6 and 1/4 inches long by 1 and 1/2 inches tall. Once both rectangles are the proper size it would be a good idea to stick them together with double sided tape to make the next few cuts at the same time. Make sure the pieces are lined up straight before they get stuck together.

Now you are going to cut out a long slot at the back for the servo to fit into. The slot in the back needs to be 13/16 of an inch wide by 2 and ľ inch long. It needs to be 1/8 of an inch up from the bottom. Once the slot is cut, make sure it is big enough by sliding one of your servos into it. If it doesnít quite fit you can take some sandpaper or a file to carefully enlarge the slot. Donít worry if the slot looks a little too long (or tall), this will allow you to make some slight adjustments of the servo to adjust track tension.

Next a 1/8 inch hole needs to be made Ĺ an inch up from the bottom and 3/8 of an inch in from the front. This will be where the front axle goes.

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Bottom Piece:

 

With the sides completed we now come to the bottom piece of the tank. The bottom needs to be 6 and ľ inches long by 2 and 1/2 inches wide. After you have cut out a rectangle to these dimensions, you may want to cut the front and back edges at a 45 degree angle to increase clearance. The bottom edge will be shortened, but the top should remain 6 and ľ inches from end to end. Making these angles is optional, though.

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Now one last thing needs to be done to the bottom piece. There needs to be four 3/32 inch holes drilled into so the road wheel brackets can be attached with screws. To mark the location of these holes you simply take the bracket (the grey plastic piece with five holes on one side and two on the other) and center it on the bottom, keeping it flush with the edge. Now take your pencil and trace the two holes where the screws will go. Take the plate off and drill the 3/32 inch holes at the center of the circles you just made. Repeat for the other side.

 

Front, Back, and Middle Panels:

 

All youíve got to do now is cut out some rectangles for the front, back and middle of the chassis. The parts should all end up being 2 inches wide by 1 and Ĺ inches tall. The front piece is just a plain 2 inch by 1 and Ĺ inch rectangle.

The piece to be placed in the middle is the same size as the front (2 x 1 Ĺ), but needs a small slot made at the bottom (for the servo wires to go through). It should be center from left to right and is 3/8 of an inch wide by 3/16 of an inch tall. This should be the right size for the servo wires (and plugs) to fit through.

 

The panel that goes into the back needs to have a recess made in it so that the servos fit right. There are several ways to do this.

 

The first way is to build it from three separate pieces. All you need to make it this way is a saw. Cut two pieces 1Ĺ inches by Ĺ of an inch. Cut the third piece 1Ĺ inches by 1 inch. This third piece will be the one the slot is cut into. The slot will be ľ of an inch wide by 1/8 of an inch deep. Start by making a cut 3/8 of an inch up from the bottom and continue to make 1/8 of an inch deep cuts until the slot it ľ of an inch wide.

Once all three parts are made, glue them all together so that they measure the same size as the front and middle panels (1 Ĺ inches tall by 2 inches wide). Place them on flat surface to thoroughly dry. If you place a piece of wax paper under it, you can avoid gluing the part to your workbench.

 

The other way to make this slot is to take a small router or adapted rotary tool and simply make a 1 inch by ľ inch wide by 1/8 inch deep slot in the piece. This slot will be 3/8 of an inch up from the bottom and Ĺ inch in from both sides.

 

Assembly of the Chassis

 

††††††††† Now that all the pieces for the chassis have been cut out, itís time to assemble them to form the chassis of the tank. Take the bottom, left and right sides, and the front and back pieces (the plain piece and the piece with a recess in it). DONíT glue the middle section in place yet! First set them next to each other on the bottom piece to make sure they all fit together. After youíre sure that the pieces are the right size you can glue them together, making sure they are set as flush as square as possible. Using clamps or weights to hold the pieces together while the glue dries is a good idea.

††††††††† After the glue has thoroughly dried, you can now take you servos and the middle chassis piece and find the proper location for it to be glued. Push one of your servos into the side slots and then insert the middle piece in front of it. Adjust the servo so that edge of the back tab is flush with the back of chassis. Now move the middle section forward or back so that the edge of the front tab is lined up with the front of the piece.

††††††† Once you sure the middle piece is in a good position, you can mark the location, pull the servo out and mark the same distance on the other side. Now glue the middle section in place at the locations you marked, making sure that it sits straight left and right as well as up and down (square with the bottom).

††††††††† Wait for this piece to dry before drilling the holes for the screws.

 

Holes for Attaching Servos

 

Now all we need to do to the chassis is make some holes into the sides of the middle and back pieces to put screws into. Take your servo and slide it into the slot in the side. In order to put the servo in you may need to tilt it slightly (wire side going in first). Hopefully the tabs will still line up with the edges of the middle and back sections. Now mark the location of the holes in the tabs and pull the servo out again.

††††††††† Now you have a good idea of where the screw holes need to be placed. If the holes donít quite sit center (left and right) on the edge, try to move them over a bit so they do sit center. The holes in the tabs should be large enough to allow for a little movement of the small screws. You will want to avoid splitting the wood, so keeping as much material on both sides of the hole is a good idea.

††††††††† Once youíre sure that the holes are marked right, you can drill the holes. They need to be Ĺ of an inch deep and 3/32 of an inch in diameter. Drilling a smaller hole first (called a pilot hole) is also a good idea to help keep the hole straight and avoid splitting the wood.

Once all holes are the right size, you can take four screws that came with the servos and test fit one of the servos. Donít over tighten the screws! They only need to be lightly snug.

 

Making Shims

Now to get both servos to fit in at the same time youíll need some shims. There should be one set (four total) of rubber shims that come with the servos. You just need to make one more set of four shims to put the servos a little farther apart. I made these shims out of a popsicle stick. Any other material that it about 1/8 of an inch thick will work too. They need about the same size as the tabs that are on the servos. You could simply trace around the tab to get the size of the shim and location of the holes in it.

 

Servo Modifications

 

††††††††††† Now that you have a chassis to put them on, you can modify the servos to work the way you need. To make the servos turn all the way around, two parts need to be cut off. First, of course, you need to open up the servos to get at the parts inside.

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Taking the top cover off you should see a bunch of little gears that connect from a little motor and wind their way to the output shaft. It would be a very good idea now to take notes as to where all the gears go.

If you take the output shaft, you should see a little notch sticking out of the side in between the spline and the gear. This is a stopper that allows the output shaft to only turn a limited distance. This needs to be cut off as flush with the shaft as possible.

††††††††† Now, looking under the output shaft, we see a little knob sticking up (brass on Futaba, plastic on Hitec). This is the pot (or potentiometer), which tells the servo when it has reached the right point. This knob needs to be cut off as low as possible with out damaging any other parts. With some servos (standard Futaba servos are like this) it is possible to push the circuit board out from the center piece of the case, and easily cut the knob off plenty short. Other brands (like Hitec) have the motor and pot glued into the case, and thus are harder to cut down.

Once the pot is cut down to the point it no longer can be moved by the output shaft, set the pot to a good mid point. You can just feel the travel and estimate where the center should be, or you can connect the servo to a receiver and battery to find a point where the motor stops running. Donít forget to center your radio trims before adjusting the servo pot neutral point.

††††††††† Once that is set you can put the servo back together. You did make notes as to where the gears go, didnít you? Repeat the process for the other servo.

 

Building the Drive Hubs

 

††††††††††† Now that we have a chassis and drive line, we just need to put on the drive wheels. We need hubs to mount the drive sprockets on, and if you bought two new servos for this project, then you should have all the parts you need to make them.

††††††††† First cut off the tube that sticks out of the center of the drive sprocket. You want it as flush with the inside of the wheel rim as possible. You can take a large drill bit to remove that last bit of orange plastic shaft that canít be reached with a saw. Donít drill too deep; you just want a fairly flat surface on the inside of the drive wheel.

††††††††† Now itís time to make the hub out of the servo arms. If youíre using a Futaba servo and arm, you will use the four armed horn. Each of the four arms needs to be shortened so that only the l hole remains. Leave as much material as possible.

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If youíre using a HiTec servo, you will be using the round servo horns that should come with those. No cutting needs to be done on these; you just need to find the holes that best line up with the holes on the drive wheel. The fit on these horns is not quite as good as the Futaba, but should still work. Two of the holes will be slightly closer together and the other two should be a little apart. Regardless, the larger holes on the drive wheel should allow for that horn to work.

Take the servo horn and attach it to the servo using the screw supplied with it. Donít over tighten the screw.

Now to attach the drive wheels to hubs. For this #2 size sheet metal screws should work very well. Ones that are 3/8 of an inch should not need any shortening (they come pretty close, though). Shortening the screws with a rotary tool, grinder, or other similar tool would look nicer, but is not necessary.

 

Assembling the Parts on the Chassis

 

Now that all custom part have been built all thatís left to do is to put all the components on the chassis.

First put the servos in the back, making sure to feed the wires on them through the slot in the middle section before putting the servos into the slots and screwing them in place. Donít forget to put the shims in place too.

Next youíll just assemble the tank in the same way you would with the kit, just using the new chassis to attach the components.

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Once the wheels and tracks are on you can put the receiver, battery, and switch in the front compartment. Since youíre anxious to give it a test drive, you can just set these parts inside for the time being. Later you will probably want to servo tape the receiver and switch in place to keep this area tidy and secure. A strip of adhesive Velcro could be used to hold the battery in place. You can find servo tape at any hobby stores that carry RC stuff, and the Velcro strips at sewing and craft stores like Hobby Lobby.

 

Making an Internal Antenna

 

All thatís left to do is to find a way of holding the antenna. What I used for making an internal antenna is the back side of the plastic package the sheet metal screws came in. I cut it off at the hinge and made a few evenly spaced holes (14) in the lip around it to feed the antenna wire through.

 

Finishing Touches

If you like, you can add more parts to your tank to make it look nicer. Having a lid and or body for the tank is not completely necessary, but will help to dirt and debris out of the tank. Again, Velcro can be a good way to secure a lid or body. Painting the tank is another option. You can put as much or as little detail into it as you like.

 

Help and more Information on RC:

For any questions concerning the project, please feel free to e-mail me at polaristb6@yahoo.com. Iíll try to answer your questions as soon, and as best as I can. You can also find help and more information on RC vehicles, on the bulletin boards at http://www.rcuniverse.com. There are many, many people on the boards that are willing to help.

 

 

 

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