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[ukrobotics]

Greetings all,

Feels like ages since I last posted on this forum. Since starting uni I just have not had the time or money to put into the robots like I used to.

This was by no means a choice, but a consquence of the changes in circumstances surrounding my every day life.

For me, building a robot relied on being able to spend an hour or so a day, tinkering away in my workshop to create somthing cool at at the end.

Living 200miles away from home meant I didnt have this workshop access, and when the holidays came up I was just too busy with other things to be able to work all day every day on bot building to make up for lost time.

Dont worry, this is not another post to say Im quitting; quite the oppsite infact. Its just a post to let you all know im still alive and that sooner or later I will build another robot and get back into the roboteering hobby.

Currently Im working on a project restoring and retrofitting a 20 year old 2.5ton bridgeport CNC mill.

Hopefully it wont be too long before I have the thing running. Despite the vast scale of the thing, I was able to apply most of what Id learnt from building robots to make this work.

I have a blog of my progress here for those of you who are interested. http://www.ukcnc.org/forum/viewtopic.php?t=4http://www.ukcnc.org/forum/viewtopic.php?t=4

(If you choose to join the forum, please post somthing so I can tell you apart from the spambots :-))

The other thing I wanted to do was ask you all for a bit of advice on another mechanical project I will be starting soon.

Next month our village is holding the annual Whacky Races event where we close off the road from the top of the hill by the church down to the village hall.

Entrants have to build a gravity powered kart (no other forms of propultion allowed) which will be raced all day in a series of time trials.

Winner is the one with the fastest time.

I need to build a kart to enter in this race and I would welcome any design tips that would help me build a winning kart.

My idea is to build somthing as low and light as possible whilst keeping it quite wide to increase stability. The hill is quite steep so i dont think extra weight will be needed to provide additional inertia.

I would welcome any sugestions on materials to use, what type of chassis, brakes steering etc. I suppose it needs to be similar to the robots in many ways .. light as possible but still strong enough to survive a crash, and more importantly in this case, minimal ammounts of friction and wind resistance.

I suppose wheels are a important consideration. Would wide tyres cause noticable frictional losses ? Last year I used road bike wheels but Id prefer not to use these again as they have to be supported from both sides which makes the chassis much bulkier and heavier than it otherwise need be.


Any suggestions will be much appreciated.


Dom

[jamesb]

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if you do not need to go around corners, have solid ali or steel wheels running on the rims, with as big a diameter as possible.
Making it heavier helps.
but then this all goes out of the window if you have corners as lighter with grip is better.
good luck with it.

[woody]

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http://www.stotfoldscouts.org.uk/soapbox/instruct.htmhttp://www.stotfoldscouts.org.uk/soapbox/instruct.htm

[swanaldo]

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lol

[jamesb]

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is that the new renault F1 car?

[ukrobotics]

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There is one corner at the bottom of the hill but it is quite substantial.

I will try to upload some footage from last year to give you a better idea of the course.

[killercarrot]

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Lots of stuff on the redbull site.. I saw one a few years back which suspended the driver inside a cage shaped like a red bull.. absolutley mad
http://www.gravityengine.co.uk/redbull/http://www.gravityengine.co.uk/redbull/

[grafix]

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Dom, I am seriously impressed. Very good work. As you know, we both have the same mills and I love my TNC 151 controller.

First of all, I will point out that the mill only uses single phase. I had better explain that. ALL the various different parts are run from single phases ie:- the Spindle motor is on phase one, the servos on another and the TNC on the other. The only thing that uses all three phases is the spindle brake. The spindle drive upon braking momentarily puts all 3 phases onto the motor.

I did read your blog but not in great detail. Did you get your manual? If not give me a call and you can borrow mine and copy it.

I hope you get as much fun out of your machine as I get out of mine. However, I now have another mill as well! Greedy me. lol.

Mike.

[ukrobotics]

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Mike, thanks for the words of encouragement. I saw you had posted and was thinking to myself, oh no .. hes going to have a go at me for not using the machine in its original configuration

Im not too sure about the power thing. On the old bosch drives there were power inputs on the PCB labled 415v.

As for the spindle motor, it is a shunt wound dc motor which according to the data from contraves needs 400v on the armature to operate at the correct max speed.

While our machines may look similar, I think internally there could be differences.

Does your spindle motor have a 24v electromechanical (wheelchair style) brake as well?

Thanks for the offer of the manual but I do have one of my own. It was one of the first things I bought. Didnt come cheap, but the time it saved was priceless.

I already have my mind set on a VMC, but this one will have to pay for it so I have a good incentive to get this one done.

The good thing is that when I got this one home I found it wouldnt fit in my workshop so I had to find an alternative home. As a result, I now have space for about 6 VMCs with room left over for a cnc laser or waterjet .

I cant wait to cut out my first part, though if progress carrys on at the current rate that time might be as soon as 4 weeks from now!

Back on the subject of gravity karts, is it more advantagious to have the back wheels with separate axles (one per wheel) or a single one running between the two ?

With one axle I could get away with just two bearings, but then id have the weight of the extra length of axle, with independant axles id save weight on axle length but would gain it on two more bearings. Which one would have the frictional advantage? Would there even be a noticable difference between either config.

I guess the main reason im asking this question is because I want to go for a single rear axle congfig so when im not using it on the race day I can bolt a gurt big motor to it, but i dont want to include this option if itll give me a disadvantage when running on gravity power.


(Message edited by Ukrobotics on March 18, 2007)

[typhoon_driver]

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Funnily enough I investigated something similar to what you are planning a while ago, gave it up but here are some points that may be useful to you.

Remember that as you go around a corner, the outside wheel travels further than the inner wheel. This obviously causes problems with a single axle configuration. There are two ways to get around this,

1) Use a differential - expensive, heavy and not really viable

2) Use the same method as a go kart. A go kart has a single axle across the back. When it goes around a corner it is light enough and powerful enough to spin one of the wheels as it goes around the corner. This does mean that your tyres wear down quicker but the saving you make on a diff makes it worthwhile.

As for the axle itself, you can get a hold of some light aluminium hollow axles on ebay. I have just bought a solid steel one for the front of my new heavyweight for a fiver plus a few quid for delivery.

Good effort on the mill! Looks exceptionally impressive.

(Message edited by typhoon_driver on March 18, 2007)

[grafix]

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Yes Dom, it does. and if left for a while without running it does seize. The 400V is for the brakes as I said.

I looked at your pis. Its all the same stuff.

BTW, your old TNC 151 was a 4 axis controller. All you were missing as the drive units.

Mike.

[ukrobotics]

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Mike: The tnc was blown as was the spindle drive the price of getting those repared/replaced alone was equal to the cost of an entire retrofit; thus why i chose that option.

Its going to be good though as everything is being done properly. Removed the spindle motor and took it to a specialist and they cleaned out all the carbon dust, skimmed/undercut the commutator, replaced the brushes etc... That reminds me one more thing that definatly is 3 phase is the blower motor on the back of the spindle motor. They have made a capacitor based phase conversion circuit to make that work off single phase.

Thanks gary, I hadnt thought about that. When under power it would be fine, but as its gravity powered in the race i think i will go for separate axles. (one per wheel).

Last year I tried to make my kart skid steer. I figured with really narrow wheels with quite low friction I might be able to get away with it. No such luck though. As soon as i stopped one wheel, all the others stopped too!

[psychostorm]

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Dom, you may wish to use wind power in your favour. If you made the floor pane like an aerofoil, you could use the resulting lift to reduce your drag.
just a thought.

[jamesb]

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just a small point, on a properly set up gokart, the solid axle works by actually lifting the inside rear wheel, not by spinning it. the best way to get this effect in your soapbox is to use an inclined front kingpin, so that turning the steering wheel moves the wheels slightly up and down as they move side to side, lifting one off the floor. this allows you to wiggle your inside rear in the air, even when static, if you get it right. this lets you go around a corner with a solid rear axle.

[grafix]

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James, I do believe that MacLaren are looking for you.

Mike