Joint Strike Minisumo

[slurp]

I can't find the P-Clips I was looking for, there are more substantial than these pictured but they make for easier building than machining from scratch... two would be required per motor to stop them swiveling.



The better solution would be the T-Bar Chassis if it's sized to fit within the the wheel. A rib across the back will help with stiffness of the shape, this sort of fabrication isn't always easy to do at home (accurately) and would usually be expensive to buy.

I think there's another way but I'll have to give it a little more thought and will need to sketch it to make sure it works...

regards,
colin

[Rick Brooks]

Which leads me to the question of how heavy can or should the wheels be? I can fabricate them out of Delrin or aluminum or brass, and by varying the thickness of the wheel wall we can get something that is quite heavy or something that is quite light.

- Bob

I'm a little slow at responding, but I'd like to get in a few comments on wheel weight. I did some experimentation on ExSpurt's wheels/tires a few months ago. Since ExSpurt was very light, I decided that adding heavy wheels was a good idea, so I made up a set of steel wheels. Four of ExSpurt's aluminum wheels/tires normally weigh 50 to 75 grams. Four of the steel wheels/tires came in at 170 grams.

The heavy steel wheels did not improve static "push" because the tires just spun anyway. What surprised me was the drastic loss of acceleration and speed. ExSpurt looked like it had lost half of it's batteries. After a few runs, the motors were getting very hot...that never happened before! The extra power to drive the wheels was really killing performance.

I went back to aluminum wheels and put lead in the chassis.

Rick Brooks

[slurp]

The heavy steel wheels did not improve static "push" because the tires just spun anyway. What surprised me was the drastic loss of acceleration and speed. ExSpurt looked like it had lost half of it's batteries. After a few runs, the motors were getting very hot...that never happened before! The extra power to drive the wheels was really killing performance.

That sounds like a stunning proof on the theories based upon moment of inertia. When car manufacturers go for performance you'll see light weight alloy wheels and reduced tyre depth, this knocks down the un-sprung weight improving the suspension but also the reduced moment of inertia has it's effect on acceleration - less effort to get it up to speed!

I went back to aluminum wheels and put lead in the chassis.

I generally advise against use of lead, I know it's dense stuff but it's a killer or at least can give you and your kids some serious problems. If you're molding then the fume when melting isn't nice nor is the dust if it gets mounted near rubbing surfaces. The Environment Protection Agency in the US has some details starting here:- http://www.epa.gov/lead/

Lead dust from rubbing surfaces is the main reason I advocate banning lead from competition.

best regards,
colin

[sap1975]

Well… I see no reason to re-invent the wheel

Thanks for the input Rick i guess that will save us a lot of work.
And thanks Colin for the quick brush up on the theory. Unlike the “traction theory” i believe this to be much more applicable to our project. And i know that every time one of us mentions lead you inform us of the hazards on using it but. Gold isn’t really an option cost-wise and not only is depleted uranium very hard to come by but it’s (as far as i know) almost as hard to work with as tungsten which isn’t really an option for the very same reason. Not to mention the potential hazard of using depleted uranium in itself i guess we could use copper which will “only” take up about a 1/5 more space but what would you suggest as a real alternative to lead?
The thing is that getting hold off and using just about any material denser that lead is a task in itself.

[slurp]

I guess it's all a trade off in terms of performance, lead no longer has an agreed safe level of exposure - you'll see its removal from electronic components and processes but also the mass withdrawal of consumer products when it's found (numerous toys recently).

Do you really need that little bit more in density? Is using lead really that important?

I don't think there's anything so close, Gold / Silver / Platinum are all expensive as you've said... mercury is toxic too & it's a real pain getting it to stay in one place.

I'd suggest sticking with Brass (8600 kg/m^3) or Steel (7800 kg/m^3) as copper becomes more expensive may be not so common in useful sizes.

Looking at the minisumo foot print, 100x100mm, you'll get to 500g in 18.5mm of Aluminium, 6.4mm of steel, 5.8mm of brass or 4.4mm of lead... alternatively a 50x50x5mm ballast plate would give 141g in lead, 107g in Brass or 97g in Steel.... 34g if you really want aluminium.

I guess there's a small performance trade off but I don't worry about contamination with the 1.6mm steel plate I've got. I can profile it to suit the location on the robot and stack them on a couple of screws to build a suitable block....

Where we are influencing a new generation of Engineers, I think we should discourage the quick and dirty solutions - a small cost for a cleaner brighter future! <<Rant Over>>

Best regards,
Colin

[sap1975]

We all have different things that ring our bells and i personally believe that if everyone tried to respect that just a little bit harder the world would be a better place.

Let me put it this way. If it’s that important to you (and i kinda sense it is) I will promise you that I will make an effort to avoid using it

[slurp]

i personally believe that if everyone tried to respect that just a little bit harder the world would be a better place.

I couldn't agree more.... or as Bill & Ted put it "Be excellent to each other."

I feel that the New Years resolution will be about ranting less and trying to inspire more...

There is much that we can do, with little effort, that can make a big difference to the people and world around us - it's just that it might not be the obvious or easiest solution. Now training people to thing in a different way, there's a challenge.

regards,
colin

[sap1975]

I got my sides milled and my motors have arrived.
Finally i can get on with some bracketry and the baseplate.

(I won’t show pictures since they’ll look exactly the same as Bob’s)

I’ll do the baseplate in brass to try to get the weight up and i considered doing the bracket in brass to but i can’t seem to find a suitable piece to mill it from. (Getting hold of small quantities of various metals is pretty hard around here, i get most of my stuff from a metal recycling facility kind of place)
I can’t wait to see this thing go. And find out if runs like expected or i should have listened closer to the words on theory

[JonHylands]

You might consider something along the lines of what I'm doing with Seeker 2x, for the motor mount. It is fairly simple to machine, and will hold the motors far better than a simple pressure clip...

Seeker2x-MotorMount-01.jpg (24.67 KiB) Viewed 3081 times

This shows the motor mount, with the motor installed. I use a couple short 2mm machine screws to hold the motor in place (as it is designed to be).

Seeker2x-MotorMount-02.jpg (24.88 KiB) Viewed 3076 times

This shows the back side. I need to redo this mount, and make it 1mm longer, so there is some space between the left and right motor wires. You can also see the two #4-40 mounting holes, which solidly mount (and align) the wheels with the chassis.

This motor mount is set up to handle a wheel diameter of 23mm, and a total tire diameter of 25mm (one the polyurethane tire is added).

- Jon

[slurp]

It's always better to go for an engineered solution such as yours where the tools are available.... I guess I've been working on the buget too much

This soultion gives a number of advantages. Number one being that you use the mounting provied by the motor manufacturer and reduce the torque arm acting along the motor/gearbox. You've got some easy fixing to the chassis and the ability to place a large mass where it will best increase your traction and act as an anchor to the motor with it's moment of inertia resisting the initial start-up torque.

While there is some work turning and milling this component, it will be easier to make accurately than the folded plate depicted earlier. The problem with plate folding it always the bend radius, and therefore material used, leading to setting out issues if holes are pre-drilled.

regards,
colin

[JonHylands]

The motor mount in aluminum weighs in at 16 grams. Switching to brass, it should be about 50 grams. The final version will be a touch heavier (maybe 1-2 grams), because I'm going to make it 1mm longer.

It took about 2 hours to machine that piece, from a solid piece of 1" diameter aluminum rod.

Step by step machining instructions (note that this is for the one I made - I need to make a couple changes to this in order to add 1mm to the length):

Code: Select all

Motor Mount
- start with 1" diameter rod
- turn to 0.945" outside diameter
- face it both ends to 1.615" long
- center drill outer end, 0.236" diameter, 1/4" deep
- center drill/bore inner end to 0.551" diameter for 1.575" deep (will need to use long boring arm)
- turn down outer side to 0.708" outside diameter, for length of 0.866"
- bevel outer corner
- bore inner hole out to 0.708" diameter for 0.120" depth

- mount in mill vise horizontally, mill bottom flat (take off 0.0787" material)
- remount in mill vise horizontally, using flat
- mill front flat 0.1575" deep, 0.590" from inner edge
- mill front slot, 0.4724" wide, 0.590" from inner edge
- remount in mill vise vertically, outer side up
- drill motor mount holes, 0.3740" apart, 2mm drill bit, centered on motor hole
- remount in mill vise horizontally, bottom facing up
- drill mount holes #4-40 tap drill (on center line)
   - centerline is 0.3150" from front edge
   - first hole is 0.1968" from inner edge
   - second hole is 0.5905" from inner edge

- Jon

[sap1975]

John-
That looks like a really nice piece of work. And quite a clever mount i might add.
Unfortunately i don’t have a lathe and i fear that trying to make that on my mill would cause me an unreasonable amount of grief. It did however get me thinking… I’ll get back to that though.

I can tell from reading your build blog on trossenrobotics that 2X is coming along nicely.

Anyways thanks for sharing. Especially the machining instructions.

I am however trying to get the highest reasonable quality parts and since i don’t have a lathe Bob has been kind enough to start a part swapping thing with me. Basically he is doing the rims for me and i did the sides for him.

[JonHylands]

What kind of mill do you have? With a Sherline, you can rotate the headstock 90 degrees, and do simple lathe work on it...

- Jon

[sap1975]

I have a european version of the Sieg X2 “minimill” called Rotwerk EFM 200 DS with a cncfusion ballscrew conversion kit. To do angles the entire Z axis column rotates and it will only 45 degrees. I have been considering getting a 2” MT3 chuck and try to do it vertically but so far i don’t have anything that will accept anything larger than a 16mm round stock. (Hey that should do!!! ) but then again i would still need the turning tools and since i’m a bit strapped for cash (Christmas and all) it will have to wait a little.

[sap1975]

I just did the basic baseplate and is beginning to look like something. Not much but i wanted to show it to you guy’s anyway.
The two sides the baseplate and the motors come in at around 240 grams add the two 41 gram wheels and were up to 322 grams. (this is before adding “rubber”)

So i’m guessing were just about right on the money regarding weight. Worst case scenario i will need to take some of the baseplate.

I won’t do the front and rear edges until i get the to cover done since i expect a certain amount of fideling to make everything fit together just right.