Frame again

Frame coming along nicely, and drivewheels are done…

Clamping second side to completed first side:

Both sides done set out to decide width:

Notice the (soon to be) supports across the bottom of the frame.  The drive sprocket teeth are shaved to a point, and drive wheels are fully assembled (except bearings):

Welding the frame

I am making the frame, and doing a little reshaping of the drive sprockets right now.  The frame is going pretty well.  My plan is to make 1 side, then make the other side on top of the first side, so they are as identical as possible.   Then, I will add the cross-members from one side to the other.

As for the drive sprockets, as mentioned in my last post, I am taking the corners off of them, so the treads clear as they come off.  I am just running the corners of the teeth, freehand, along the straight-cut router bit.  Doesn’t have to be perfect, just a little more clearance.

Drive Wheels done. Track time.

Finished the drivewheels.  Still need to get a few more bolts, but 2 works for now. They look pretty much awesome.

So, now that I have those, I can begin working on the tracks, since I can figure out the exact spacing, size, and shape of the overhanging treads, which the drive sprockets will turn.

I cut up a rubber doormat to simulate the rubber track, and made some test plywood treads and inner alignment blocks, which apparently are called “bells”.

I then countersunk some holes for wood screws which go through the tread, through the track, and into the bells.

Then I put it over a drive wheel to test how it rolls, if it catches etc.

It does slightly catch as the track is coming off the sprockets, so when I make the actual treads out of UHMW, I am going to router the corners off the ends of the treads, and round off the corners on the drive sprocket teeth.  Like so:

Drive Sprockets almost done

Ok, I finally made some really good progress on the drive sprockets.   I cut out all 4 (2 for each drive wheel).  I had to spend a little time making a homemade router table for my router, but once I finished that, I was in business.

So, after being cut out, due to the bending of jigsaw blade, they needed some cleanup done on them.   So I put a straight bit on the router, and just used it to shave the jagged edges, like this:

Then I put a chamfering bit on the router, which cuts a diagonal edge.  Before on the right, after on the left:

Here they are, they turned out great.

All I have left to do now, is drill the holes to bolt it to the drive wheels, and drill a 1-3/8″ hole in the center for the bearing to fit it.    Here’s the bearing I’m using:

Also, finished both hubs for the drive wheels, so they are almost done.   Once the drive wheels are finished, that is huge, cause I will be able to start working on the tracks, since I’ll know the exact spacing for the treads on the tracks.

Hull, frame, and a little luck

Sometimes, when building a project  like this, nothing works better than a little luck.

I have figured out the plans for the frame, although I don’t have a set width decided on yet.  And my plan before today’s epiphany was to buy some steel sheet metal for the sides and bottom of the hull, and weld it on, with either some home-done long bends, or really long seam welds.  Which are both totally fine, I can do such a thing.   BUT, if I can have some nice, factory done edges, and not have to pay for the steel, that’s a no brainer right?

Then, I remember I have this thin metal wardrobe closet thingy in my shop.  It currently has my shop rags in it, and it’s totally in the way, in the middle of the floor.    It is about the right height, and about the right depth to flip on it’s back, make some cuts, and weld my frame to the inside of it.   The only problem is it’s too wide, so I will need to taker a strip out of the middle of it, and weld it back together.

I have also decided on the steel for the frame tubing, I got some 3/4″ square tube .065 thickness.

Here’s the basic plan of attack :

Body, turret, and chassis

My plan is to first build the chasis, which will include the tub/hull, drive wheels, tracks, bogies, motors, batteries, electronic controls, etc.  This will be a totally self-contained system.  The tank should drive, and function almost completely with just the chasis in place.    This will come in handy for me to be able to test drive the tank, because I may not fit inside the tank when the body is on.

The chasis should look something like this, although the controls, batteries, motors etc are not shown:

Next,  the body.  The body will fit and bolt onto the chassis.  The body needs to be lightweight, yet sturdy.   This is also what really makes it look like a Sherman Tank.  At this point I’m considering either thin UHMW (1/8″ thick) or plywood.   The UHMW can actually be bent or curved if heated, and will hold that shape.  This feature alone could make it more useful than plywood.  I would probably need to make a sub-frame like structure to attach the panels to, and I’m not sure how to make the seems look good.   I’m still thinking about his, and will try a few small-scale tests with materials, to see which makes the most sense.

Finally, the turret.  This is the big gun, and the round part with a hatch, on top of a tank, that most people are familiar with.  This will attach to the body.   There are still many questions for how I will do this also, like how will the gun shoot, how will it all turn, etc etc.   I’m not too worried at this point, but I am keeping it in mind for now, in case it effects the construction of the other parts, or if I see something that I think could be easily modified to work as the turret, or any of it’s parts.

Bogies and Roller Wheels

Bogies are the sets of roller wheels, which provide the track with its means of suspension.  I have considered a few different designs, including the two below which are loosely based on the M3/M4 Sherman tank.  I think for simplicity I will go with the One Spring design, since often times simple is better.

The main difference between the track and bogey system I’m using, versus from an M3/M4 Sherman, is I am using an inside, and an outside row of roller wheels.  These roller wheels will ride around 1 row of blocks on the inside of the track, which will keep the track from coming off.  An M3/M4 uses 2 rows of these alignment blocks, an inside and an outside, with just 1 row of roller wheels between those.     The reason to do it like I am, is having a few extra rolling wheels seems easier than making 2 sets of inner alignment blocks, where you need to do hundreds of them.  There are a few other reasons, but no one really cares.

Still not sure exactly what material to make the bogey pieces out of.  Leaning toward UHMW plastic, or possibly aluminum, trying to keep it fairly light.

Frame design

After building the frame in Blender3d, I have a real good idea of how it needs to look.  However, my 3d rendering is not to scale.  So I have to do a 2d side view of the frame to determine it’s proper dimensions.

For  the people that like “plans”, this is about as close as I get.   If the dimensions aren’t perfect, I can make up for it by moving the bogies (and roller wheels) up or down until everything is about the right shape.

So, I used my drawing from TrackMath, and modified it to figure out the frames dimensions.  I did make one slight change from  the 3d model, which was having a small flat part in the back, instead of a straight 45-degrees to the top rail.

The frame is of course, the thick blue line.   That’s pretty much that.  I know, nothing ground-breaking here, but a necessary step nevertheless.   And most importantly.. it means I can begin the frame construction soon!

I’m going to use square tube, probably like 3/4″ OD.  I will decide for sure once I feel the actual material, and get an idea of weight.

I also need to decide exactly how wide to make the frame.   I can’t think of a good way to do that, other than pretty much winging it.  Total width will be around 3ft, but this frame will fit inside the tracks, so about 2ft wide… I guess.

Motors, gearing, and speed.

I have done some very preliminary research into what I should use for the electric motors.   Thanks to the wonderful popularity of those silly motorized scooters, there actually a decent number of affordable electric motors.  I will need 2 motors of course, one for each side.  This way each track can go forward or reverse, independently of the other.

The motors I’m considering at this point are 24v, around 250W and run up to like 2500 rpm.  They even come with small pulleys, around 1″, already on them  (I will have a future post about belts and pulleys versus chains and sprockets).

So, I wanted to get an idea of the top speed the tank would have, if the motors were running at 2500 rpm.   I am figuring a gear ratio of 6:1 (1″ pulley on the motors, to a 6″ pulley on the drive wheels).  I will also have approximately 6″ drive wheels, which, when driving a track, you can pretend they are actually just wheels on the ground, and the numbers will work out close enough (there is a slight difference due to the thickness of the track, but that’s not a huge deal for now).

Anyways, here’s the equation to figure out top speed:

(RPM/gear ratio)  x  60  x  WheelDiameterInches x Pi
———————————————————————-
5,280 x 12
= 7.4 mph with a 1:6 gear ratio
=5.4 mph with a 1:8 gear ratio

I am very happy with those speed estimates.  If they came out to like 30 mph, I would know I’d need to gear it way down or get lower revolution motors… likewise if it came out to 1 mph I would need to gear it up and so on.   Also, when finally selecting the motors, if it’s a high torque motor, but turns way less revolutions, I will know I can use more even gearing, etc etc.

Installing the motors is obviously far off at this point, but these things need to be considered.  For instance, I am leaving myself enough room for a 6″ pulley on the drive wheels, when I construct the frame, axles, and the other associated parts.    That’s about it for today, sorry, no cool pictures, but at least we did some handy math you can use when making your own tank.

UHM…W, I call it. UHMW the wonder plastic.

I have decided to use UHMW, which is a hard plastic, to make the drive sprockets and track treads out of.  And other stuff later on.   You can get remnant/scrap pieces, ($1 per pound in my case), which is a very fair price… after all, it’s plastic, not lead.

Why UHMW?   It is a thermoplastic polyethylene.   It has the “highest impact strength of any thermoplastic presently made.”   It has very low friction (less than nylon and acetal, and is comparable to Teflon), is self-lubricating, and is highly resistant to abrasion. It is odorless, tasteless, and nontoxic.  It is also very resistant to water, moisture, most chemicals, UV radiation, and micro-organisms. *

Basically it’s super tough.   BUT, the other great feature, is you can cut it with most woodworking tools… about the only thing you don’t wanna do is sand it with sandpaper.    I was also able to get it in black, which is nice, will give the tracks a nice realistic look.

I got 1/2″ thick for the treads.  I decided I will cut them 1″ wide, and I think the 1/2″ will be perfect… I had originally said 1″ thick, but I think that is overkill… it would weigh twice as much, and would dig or catch more into soft ground, which should not be necessary.

For now, I cut out one drive sprocket, just need too clean up the edges a little:

* – Copied verbatim or paraphrased from http://en.wikipedia.org/wiki/Ultra_high_molecular_weight_polyethylene