Sunday, March 29, 2015

Threaded Pot

Some things seem simple but take a whole lot of time.  Threading the new on-board potentiometer was one of them.  After deliberating with Nancy we decided that a max speed of 3 mph is good for the on-board control.  The new 1K pot is larger in diameter than the old one.  I had to drill out the SBT frame for it to fit.  I also had to thread the shaft of the pot to 1/4-28 for the wheel.   For hours I tried everything but the die was not catching it would just spin on the ground down champher I had made.  Finally after a nap I remembered something Robb asked, was it a split or solid die?  Well it was solid, I split it, first by removing all the teeth on my hack saw, then with a dremel.  I now had an open die.  Sure enough It was working great, cutting at 28tpi and an oversized shaft first try.   Holding the tip of the potentiometer in a vice grip, and then the bench vice I was able to cut enough threads to mount the wheel in the right spot.
 It did take chasing the threads a few (about an hour) dozen times to get the diameter down to the size needed for the threaded wheel to fit.  It is hand tight until the last three threads, then it has to be worked with a tool.   Tomorrow I will cut off the extra pot length and start wiring the machine.

Saturday, March 21, 2015

Electrical Planning

I set up the controller on the work bench today.

I am trying to set up a good range on the onboard control.  The 5k pot the machine had would go from 0 to 12 MPH in in a 1/3rd of a turn.  Much too fast for that form of control.

Here is some data I gathered from trials...
Start angle, Max speed, Resistor, Pot, Thought
120, 11.8, 0, 5k,  way too fast & narrow range
0, 9.8, 0, 0, what happens with nothing is connected
90, 2, 3.3K, 1K, max speed is too slow, good range
0, 6.2, 2.2K, 1K, good max speed but slow speed is 2.4 mph if start angle was 10 degrees this would be perfect
90, 6, 2.2K 2.5K, this is good, slow end starts late, not sure how to manage.
10, 3, 3K, 1K,  good but slow
110, 4, 2.68, 2.5K  spot on with huge low end deadband

OK I am doing this the silly way.  I have a meter... I should measure across PH and PW and get a table of speeds for each resistance.  With the 5k pot.  My electrical minded brother said I did it wrong. I kept the pot hooked up to the source when I measured, which skewed my readings.  I threw out the suspect data, and after talking with him and realizing that I had similar results as him we did have similar results: 1k deadband at bottom 3k of range and 1k of deadband at top.  Look at the poly lines not the linear ones.  The chart is messy because the experiment was poor.


Note that the controller works backwards, more resistance is full go, no resistances is stop.  Not safe in our mind since we have a tether that can separate (infinite resistance = fasted speed and no way to stop.)

So for on-board control I want a max speed of 4mph, that is 2K Then I need to make up the rest with 3K of fixed resistors. At least that is the theory. I am going to try 3k fixed with 1k pot & 2.5k pot to see which I like better.  I will also try 2.5k fixed with both.  But first dinner....

OK I am back... Added some new values to the table above in Blue
2.68k fixed with a 2.5k pot has a max speed of 4 and 110 degrees of dead band at the zero end.
3k fixed with a 1k pot has a max speed of 3 and about 10 degrees of dead band at zero.
I need to order a pot that will fit in the machine online anyway, I am thinking of getting one each of 1K, 1.5K, 2K, and 2.5K  Then I can play around a bit more to dial in what I want.  I am leaning towards the 3K 1K setup with 3mph max as a good option.  Note that 2.68K & 1K did not stop at the low end, a 2.68 and 1.5 might.

Note:  the pacesetter is really supposed to be center off.  I found out after wiring up the whole machines that I should have worked with both directions when setting the resistor and pot.  I was able to get it to work with what I got but top speed in forward is 4mph and 2 in reverse.  Forward has no deadband, reverse has about 30 degrees.

Saturday, March 07, 2015

Rub the belly

When I see the storage battery truck like this I think of little grey rolling on his back so I can rub his belly.
I started work on the SBT again this afternoon.  I had stopped a while back when I ruined 3 drill bits trying to drill through some bondo.  I used the same hole, just opening up the back side with the dremel and a die grinder bit, that let the bondo flake away and I could finish installing the mudflaps.  I was working on the machine upright, but one of the rivets would not catch.  So I turned it over to see what was going on easier.  Too many thicknesses of metal, I changed rivet sizes to 1/8th inch and it worked fine.  Next I installed the drive axles to check clearance to the mudflaps.  This was the reminder that I failed to purchase a nibbling tool... yet.   Nancy ordered one off ebay today.  

Also ordered today is the wheels, axles and bearings for the two tank cars.  The wheels and axles are coming from mr-motion.net bearings are ebay.  

This mornings task was working on the flatcar boards for the tanks.  Dialing in the height of the blades and learning how to get consistently good work.  I was having trouble that the way I was pushing the boards I was making a taper.  With practice, and lighter cuts I was able to remove the taper and make all the boards consistently 11 to 13 mm thick.  Yes a bit of a range there... but I can deal.  For the math nerds that is 1-3/4 thick scale boards... scale 2x6 boards.  

I am waiting to work on the tankers until my axles come.
I am waiting on the SBT until the nibbler comes.

Next job is to make a stand for the table saw and jointer.

Sunday, March 01, 2015

nozzle and bands

This weekend I spent the time to install the top nozzle and the bands on the tanks.

To start I wandered around the big box construction store picking up items that would make a good cap.  After reviewing options such as sofa glider pucks, electrical covers, and PVC plumbing parts I settled on a 3" PVC cap and a short section of nozzle.   To get this was a 10 foot section of pipe, and two caps.  Then it was back to the store for a 3.25 inch hole saw.  The saw cost more than the pipe.

I started by removing three staves and putting a two by four curved to fit inside as a backer, glued in with gorilla glue and held with nails.

I cut off a six inch section of pipe, and used that to hold the cap and cut it down to a half inch.  Then a whole lot of hand sanding to remove the lettering on top.  I then cut off sections of pipe to use as a nozzle.

This afternoon was setting up and cutting the bands and drilling for studs with two nuts.  The studs started off as one inch long 10-32 bolts cut with the dremel.