Seeking input on Turntables

[pomilian]

I'm in the process of scratchbuilding a 130 foot TT.
Using a stepper motor from an old 5.25 inch diskette drive and a few ICs for the motor drive.
I have a mockup for testing my plans that I can take some photos of in the next few days.

Paul O

[bobwhitten]

I've built 2 HOn3 turntables from scratch using the Hankscraft DC gearmotors. They are electronically controlled and line up exactly with the Code 70 HOn3 track. I choose the roundhouse track, select the end of the end of the TT (head in or back in), and press a momentary switch. The TT slowly turns, puts on the brake, and stops dead on the track to the roundhouse. The part I remember the most is the 7 month delay the TTs cost me in building my layout. There is a lot more than meets the eye when it comes to designing and building TTs.

The advice that you have received above is good advice. If you want an electric model, I strongly advise you to buy one. If a hand cranked model will do, there have been a few articles written in the Model Railroader showing the details.

I dressed up my pits by making round molds (silicon rubber) against plastic stonework and making castings with very hard plaster. The castings are seamless and finish out nicely - look like the real thing.

Bob Whitten
[e-mail]

[SpaceMouse]

QUOTE: Originally posted by pomilian

I'm in the process of scratchbuilding a 130 foot TT.
Using a stepper motor from an old 5.25 inch diskette drive and a few ICs for the motor drive.
I have a mockup for testing my plans that I can take some photos of in the next few days.

Paul O

I'd be interested in that. I"m about to pitch a bunch of 5.25" drives

[Anonymous]

Bob, that sounds cool! did a little search, it appears Hankscraft has a BIG array of products - do you remember which model exactly you used? And how does this gearmotor drive the bridge? (maybe you have a picture or diagram?)
Thanks, Rik

[Anonymous]

I have had miserable experience with an Atlas TT. It jams up with the slightest bit of crud in the works. [And is hard to take out to fix]. A turning track is probably better.
Bob

[WilmJunc]

I realize that "as packaged" the Walthers 90 footer with the accessory motor will not operate. However, with some fairly minor modifications, this model can be turned into a realistic looking, reliable performer for less than $50. I don't see the need for indexing, the prototypes did not have it.

Modifications examples: deep-six the wipers, shim the motor for better contact with the drive gear, use an inexpensive DC transformer to power the drive motor.

[xdford]

As for scratchbuilding a table, you could use a stable piece of timber cut to the length you want (thick ply should be good as a base), and drill a hole right in the middle of it so you can fit a thicker stereo jack to it , the male plug being on the table and the female socket embedded into the "floor".

This gives you a pivot point which conducts to your track from your power supply. All you need then is a reverse switch between the power supply and the bridge itself.

The sides would be easy, just sheet styrene glued to the side of the plywood, which you could emboss rivets on if you are really keen. The safety rails could be made from offcuts of flex track rails with holes drilled in the web of the rail perhaps using guitar string for "fencing" in some modes or again styrene strip as fence rails.

Personally I have not built such a table but I assisted a friend who did for his layout when I got him started on building it - I had a Heljan kit table whcih is still OK 25 years later. My web site, www.xdford.digitalzones.com or contact me off list should you feel the need [e-mail]

Regards

Trevor

[pomilian]

QUOTE: Originally posted by SpaceMouse

QUOTE: Originally posted by pomilian

I'm in the process of scratchbuilding a 130 foot TT.
Using a stepper motor from an old 5.25 inch diskette drive and a few ICs for the motor drive.
I have a mockup for testing my plans that I can take some photos of in the next few days.

Paul O

I'd be interested in that. I"m about to pitch a bunch of 5.25" drives

I haven't been able to figure how to post photos to this forum. If you're interested in some photos and a schematic, send me an email: [e-mail].

Any help on how to include photos in a reply would be appreciated.

Paul O

[Anonymous]

Paul, you could sign up (free) at www.photobucket.com and upload your pics there. Below each pic this site automatically gives its address complete with html-code, just copy that and paste it in a message when you post here. (use the 'preview' function to see whether it's actually working) Looking forward to see the pictures (though for the turntable i'm looking for something more basic than stepper motors but maybe you can convince me?)

[pomilian]

QUOTE: Originally posted by dothinker

Looking forward to see the pictures (though for the turntable i'm looking for something more basic than stepper motors but maybe you can convince me?)

Dothinker, thanks for the info. Now let's give it a try:


Stepper motor is mounted below the board. I cut an insulating ring in the board, and have wipers going to the track.


Bottom view of the TT track. It rotates on a pair of HO trucks.

Circuit for the stepper motor: http://aaroncake.net/circuits/stepper.htm

Paul O

[jwmurrayjr]

QUOTE: I don't see the need for indexing, the prototypes did not have it.

True and with good motor control (like DCC or a fairly good DC power pack) you can line up the bridge tracks with the roundhouse tracks without a problem. But you do need to be able to see the track (I use a walk-around throttle).

[Anonymous]

Go with the Atlas! Build your bridge out of any bridge girds you can find. Now get foam 1" board and cut round slightly smaller than Atlas TT. Mount Atlas just so that the flat pit doesn`t bind. Mount built bridge so ends are above 1" foam. Mount on whatever looks good to you for turning pedistal. Make bridge ends look like thier riding on the ring rail. Use another 1" fom board to form pit upper wall. Paint Atlas pit bottom to suit rest of pit structure.
LATER!

[Anonymous]

use the record player idea,but substitute the atlas turntable for the record player and then just get the atlas turntable motor.that's what I'm doing on my shelf layout that I built on some pink slab insulation.

[Anonymous]

Hmmn this thread keeps bringing me more questions than answers... Before all questions of which manufacturers to use (and where to look for them?), what is the advantage of a a stepper motor over a properly geared DC motor?

Or is it that with a stepper motor you dont need a big gear ratio? But surely you do need _some_ gear? Or would you attach the axel of the stepper directly to the tt bridge center?

puzzled, Rik

[selector]

Unless I am mistaken, a stepper has fixed speeds to it. Speed one, speed twice as fast, speed six times as fast...or something like that. My DC TT motor can absolutely crawl, silently to boot, and I can line up the bridge rails and the bay rails with apparently enviable accuracy. The only problem comes with the occasional side-slip as the loco traverses the gap...the bridge will slip to one side by three or four degrees, and that is enough to cause problems that anyone can imagine. That can be cured with removable pins placed in receptacles permanently placed around the ring of the pit. They needn't be large and obtrusive, even something like a shirt sleeve button embedded flush with the surface, and all but one button hole covered over. If necessary, cut the button in half so that it can be brought closer to the bridge rails at the pit edge. In fact, you can notch the pit lip with a drill bit, and place the button in the notch.

[pomilian]

QUOTE: Originally posted by selector

Unless I am mistaken, a stepper has fixed speeds to it.

If the control circuit for the stepper motor is driven by a variable frequency oscillator it can step an any speed. The stepper motor I used from a 5.25" diskette drive moves in 1.8 degree steps. One drawback to a stepper is that it "steps"; so there is some unprototypical motion (which can be minimized with a slight drag on the bridge track.
I attached the bridge directly to the shaft of the motor. (See photos a few posts back) The stepper is under the PC board that supplies power to the track.

A DC gearmotor would be smoother turning, but I couldn't beat the price of the stepper motor (from a scrapped diskette drive).

Paul O

[Anonymous]

You didn't specify about what length and type you want(steel or wooden). I am in a similar situation with a very small N shelf type layout. I just want to say that in my collection of MRR since Jan. 1958, there are a quite a few article on various types of TT's. also electrifying these and indexing possibilities. I have looked at Micro Engineering Parts for a Steel Bridge. If you need more help contact me.
Bob

[Anonymous]

Instead of using motors for running turntable, my suggestion is a manual crankshaft menthod [8]

[edkowal]

As Paul O (Pomilian) has mentioned, a stepper motor is called that because it moves in discrete steps, rather than a continuous motion, like most motors we are familiar with. Common values for the size of the step are 100 and 200 steps per revolution ( 3.6 and 1.8 degrees per step. ) What this means is that you can control the position of the driven axle with some precision. Each time you pulse a stepper motor, it will move exactly one step.

But how do you control how fast, or which direction? You control how fast the motor spins by how fast you send pulses. And how far the motor spins by how many pulses you send. Direction is a little trickier. Which direction the motor spins is controlled by which pulse gets to the motor first. What ????

In a common stepper motor design, there are a number of coils in the body of the motor (the stator), and the rotor of the motor is a permanent magnet. Energize one of the coils in the stator, the rotor is attracted to that coil. If you've got, for instance four coils in the motor and you energize them in the order 1,2,3,4,1... the motor turns one direction. Energize them in the order 1,4,3,2,1,4... and the motor turns the other way. Such a motor, though will have only four steps per revolution. Not very handy for a turntable to have only four steps per revolution, although it would be fun to watch, once or twice. (Don't use one of your better engines!!)

To provide 100 or 200 steps per revolution, stepper motors are designed slightly differently. It wouldn't be practical to have to energize 200 independent coils sequentially in a motor. You've still got a magnet as the rotor, but instead of just one south and one north pole, make the rotor with 100 each interleaved north and south poles. Now set up up a series of coils similarly to before, with cores that are about the same size as the small poles of the rotor. Magnetize one of the coils, and the nearest opposite magnetic pole of the rotor will be attracted to that coil. Magnetize the next coil and the rotor will turn in one direction such that a pole of the rotor is now closest to that coil. This is necessarily simplified, but I think you get the idea.

It's important to realize though, that stepper motors, per se, won't solve the problem that selector is concerned about. This is because of two things. Many stepper motor controllers don't continuously energize any of the coils on the motor unless it's moving. When none of the coils is energized, there's very little resistance to turning the rotor of the motor. You can energize the coils, which will cause the motor to hold it's position, but even in this case, you can turn the rotor with a sufficient force.

Secondly, if the motor gets moved unintentionally, or even if there's a slight glitch in the electronics, there's no way to control absolutely where the rotor is, the only thing you can control is if it's at the the next step or not. There's no command like, "move to step 127." So if the turntable does move accidently and becomes misaligned, the next time you send a train of 100 pulses to the motor, in order to turn the table exactly 180 degrees, it will do just that, and your table will be turned around, but still misaligned by the same amount it was before.

In order to be able to absolutely control where the table is, and where it stops, you've got to be able to tell where the rotor is pointing. This requires a device called a shaft encoder, and the logic to read it and control the motor appropriately. This is not elementary, and is why those kits for automatic turntables are so expensive. It can definitely be done by anyone who's willing to learn the ins and outs, but it's not a one evening, or one week task.

If you don't want automatic control, though, things are much simpler. A variable frequency oscillator will do the trick, as mentioned by Paul O. Going this route, it will still be possible for the table to side slip several degrees when a locomotive wheel bridges the gap. Unless you've already got a stepper motor, you won't be gaining anything over using a good regular DC motor.

To answer another question on this thread, the Hankscraft motor which was being talked about is just another name for the Switchmaster switch machine type motor. Hankscraft is the supplier of these motors to Builders in Scale. The original article in Model Railroader on this stall type of switch machine was written about Hankscraft motors. Only later on in the game were these motors available from a model rairoad supplier. Originally, they were just referred to as Hankscraft motors. The Builders In Scale website refers to them as Hankscraft Series 3440 DC gearhead motors.

-Ed

[bnnrailroad]

ereimer

QUOTE: if you look at the price of the Walthers turntable ($299.99 list , easy to find discounted for about $250) it's really not that bad for an assembled , motorized , indexed turntable .

I bought mine for $90 off eBay. Paid another $40 I think for the motor. It's not a bad turn table. I'm using a Backmann transformer for the positioning motor. It's one that doesn't have a reverse switch. It rotates from 12 o'clock to 10 o'clock for reverse and from 12 to 2 for forward. It works for me!

[Anonymous]

great info, Ed. I guess having electronics as a side-hobby is a big plus when you do model rr'ing!

"Unless you've already got a stepper motor, you won't be gaining anything over using a good regular DC motor." - that's good to hear!

Bob, it's HO scale, will be 105ft tt, no indexing. Is there any info on gear- or belt drives? The manufacterers I find on the web are all for heavy industrial use, for making a VERY reliable tt - in 1:1 scale...

Ray, and others, I find the Walthers 130ft tt just tooo X*%#! large... and 90ft is too short. Plus, re-inventing the wheel every now and then is part of the thrill of this hobby for me. (Though the Bowser is still on my radar...)

[bwftex]

I used an old Atlas to turn a TT that I built. Its crudely constructed but very strong, totally reliable and can be made to look as nice as you want. Here are some pictures and construction notes. http://s22.photobucket.com/albums/b317/bwftex/Turntable/
Bruce

[Anonymous]

that's a fantastic idea Bruce, thanks for sharing. Solves the whole drive question for once and for all, so I think I'll go your Neanderthal route! (with a perfectly round pit and centered shaft to boot, thanks to a carpenter friend)

[Anonymous]

My Atlas tt arrived today. It has some mechanical indexing / locking system (every 15 degrees the deck stops for a while) , which is not what I want. Now I'll have to construct part of the drive after all. Do all Atlas tt's have this or is it only the latest run?

Rik