Twisted

Brummbär Video Log Update

A little more work was done this week. The good news is the road wheel axle hubs are tack welded and ready to go.

The bad news is I broke my ring roller. However, the good news is I broke my ring roller. Probably a blessing in disguise. It rolled some terrible rings. It put a twist in the Z axis. I am in the process of making a ring roller from scratch. I hope to have it done before the coming weekend. I also hope that I can flatten out the remaining 6 or 7 seven rings I rolled with the old roller and then run them through the new one and get the twist out. I hate to throw 20' of flatbar in the recycle bin. :(

I did play around with some of the rings I do have on some extra road wheels. Did not go all that well. The twist in the ring caused all manner of issues. I probably will not try tacking any more until the ring issue is resolved.

In the next couple weeks, I hope to have my ring issue resolved and the drive sprockets assembled.

All Stressed Out

Making a scale tank is not a new idea. However, making one without a frame is. I have yet to see anyone's scale project that didn't have a frame. Real tanks don't have a frame. The armor is thick enough to provide ample support for the vehicles weight and any stress that may be encountered. That's not to say that there are not "frames" inside a tank, i.e. support frame for the transmission, main gun, engine & etc. Most of these sub-frames merely provide attachment points for the various equipment in a tank. The big reason none of the other scale projects are frameless is mostly due to weight. A frame with a skin of plywood or sheet metal is significantly lighter than steel plate. But being hard headed, I decided to buck the trend and go the heavy metal route.

The big question was what was the minimum thickness the steel plate needed to be, but still provide adequate support? From I planning standpoint, I started out with 1/4" plate. I was pretty sure that if the whole skin was made from 1/4" plate, everything would be strong enough. That would put the weight of the body (outer skin only, not tracks, suspension, engine & etc) right at about 2000 lbs. Which means by the time I added everything else in, I was looking at about 5500 lbs. 5500 lbs is starting to push the limit of my suspension, hydraulics and tracks design. So in an effort to reduce weight, I asked for some help from an engineer who frequents the scaledtanks.com forum.

He took my CAD design and fed it into a program to evaluate stress on the body of the tank. We went through several different combinations of steel plate thickness before we came up with a winner. The combination I finally settled on was the floor of the tank in 1/4" plate and the rest of the tank in 1/8" plate. In addition, there was some angle iron added to specific joints to help reinforce high stress areas.

In the first picture, you can see the effect of a 2.5 G (61625 Newtons) strike on the left front road wheel axle hub.

The color scale shows the "buckling" of the plate. Blue, green and yellow are ok; red is bad. As can be seen, there is just one small area of red in the front corner of the right side sponson. The next two pics are of different angles.

So what does all of this mean? By using the thinner plate with some reinforcement in key welded joints, approximately 800 lbs and $600 in material costs will be shaved off of the project. I'll call that a victory any day of the week. :)

A thread on threads.

I have continued to cut out parts for the suspension. Almost all of the stuff being cut out of 1/8" plate is done. I am also trying to make my torch a little better at cutting out multiple parts. I have a cutting jig that works well for circles. However, cutting straight lines and cutting out multiple copies is not the easiest. The plasma torch does not have a drag tip so it has to stay a certain distance from the cutting surface. Also, the sides of the torch's head are angled making it harder to be perpendicular to a fence. So, I took a piece of 1" schedule 40 and made a doughnut for it to sit in. This keeps the tip at a consistent distance, gives my a perpendicular face to drag along a fence and allow the torch to sit in a consistent vertical position making the cuts more accurate. I would love to have a CNC table for it, but even a used one is big dollars.

However, I have spent most of my time battling threads. Each road wheel swing arm has a 1" diameter spindle and a 1" diameter axle that has to have threads cut into them. Plus each return roller (6 total) has a 1" diameter spindle. That is 38 total. I was having quite a problem initially. The round bar kept slipping in the vice. I then took and tack welded some pieces of flat bar to the round bar giving the vice enough flat surface to keep it from slipping. Even with the fix in place it is still a very tough go at it. It takes a lot of horse power (man power more accurately) to turn the die around the round bar thereby cutting the threads into it. I have five done and am seriously thinking about trying to find a machine shop that would take pity on me and give my a good price on cutting the rest. They would use a lathe and probably cut 20 in in the time it takes my to do one.

I have also had a couple of other smaller projects that have been a pretty big distraction. I made a friend at work a small waggon for his four wheeler to pull through the woods and I made my loving wife a new plant stand. Both projects were good for the tank project though. I used them to practice banding metal which I will have to do, in spades, on the tank.

Next up is cutting out the parts I need from 1/4" plate. Hopefully I have learned enough cutting on the 1/8" plate as mistakes made cutting 1/4" plate get expensive, fast.