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Author Topic: Guillows kit 505 Messerschmitt Bf-109  (Read 3110 times)
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RolandD6
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« Reply #75 on: February 14, 2020, 12:48:33 AM »

As promised, here are a couple of quick sketches of my thrust line adjustment device. It is very compact and completely bullet proof. The adjustment will not change due to impact or due to any sideways force applied by the rubber motor.

Description of the Components

A: The prop shaft, usually 0.032" or 0.025" wire.

B: Brass washer. Can be flat but I have made and use domed washers machined from 1/8" dia. brass rod.

C: 3/16" diameter PTFE or Nylon sphere. So far I have only used PTFE but do have stocks of nylon. They can be purchased.

D: 2.5mm or 3/32" diameter metal tube. Can be aluminium, brass or stainless steel. I have changed over to SS because it is lighter than brass and tougher and cheaper than aluminium.

E: End bearing made from 1/8" PTFE rod.

F: Spherical socket machined from Delrin rod.

G: The nose block.

H: Carrier for thrust adjustment grub screws. Machines from BIC ballpoint pen body.

I: 1/8" long by 2-56 nylon grub screws. Three required. Could be purchased steel 2/56 or M2 grub screws.


Manufacturing Notes.

C:
A 3/16" sphere can be held by my lathes three jaw chuck. The jaws have circumferential grooves that hold the spheres nicely. I can also hold 4 mm dia. spheres. A through hole to accept the wire shaft is drilled first. The hole needs to be a bit bigger than the shaft diameter because the PTFE closes up when the drill is extracted. May also be true to some degree for nylon.
Next a hole for the metal tube is drilled. I have found a new sharp drill the same size as the tube works well. The PTFE will contract a bit which means it will grip the tube when the tube is forced into the hole.

D:
The tube need to be free of burrs that will interfere with the prop shaft or prevent easy entry in to part C and E. The bore of the tube should be significantly bigger than the diameter of the prop shaft so that the shaft is supported clear of the tube and to allow for any bend in the shaft.

E:
Essentially made the same way as part C except that the PTFE or nylon rod  is held in the usual way by the lathe three jaw chuck.

F:
I make these from 10mm diameter Delrin bar stock but anything bigger than 6.5 mm will do. A 3.5mm through hole of sufficient depth is drilled first and then the spherical socket is formed with a 3/16" ball end mill held in the lathe tail stock drill chuck. Ideally the depth should be carefully controlled so that the centre of the sphere aligns with a reference face, the front face if the flange thickness is controlled or the flange rear face and by extension the face of the nose block. My crude sketch is a bit off. The part that enters the nose block is machined to 5.5mm diameter using a 1mm wide parting-off tool. I made mine from a bit of regular tool bit stock. I machined the length of the bit that is inserted into nose block by successive plunge cuts and finally parted off to length.

H:
This is made from the body of a cheap BIC ball point pen. I found the interior bore of the BIC pen bodies are concentric with the exterior hex cross section. Some other brands are not concentric. You may be able to machine the  part that inserted into the nose block from the pen body but you need to be aware that the internal bore diameter is important. I make mine 4.5 mm diameter so that the three 2/56 grub screws will always clamp the metal tube in any position. In my case I drilled out the pen body to accept a suitable piece of styrene tube which was drilled out to 4.5mm. The part that inserts into the nose block is machined down on the outside to 5.5 mm diameter. Like for part F, a 1 mm part-off tool was used for all turning operations; perpendicular to the lathe axis for turning diameters and parting off; and at a slight angle for facing the open end.

Next three 2/56 holes were drilled and tapped as shown by the sketch.

I:
The grub screws were made by inserting a nylon screw into a 1/8" thick piece of flat steel that has a 2/56 tapped hole perpendicular to the bar face. Wind the end of the nylon screw into the steel until sufficient projects from the other side so that a screw driver slot can be cut in the projection with a razor saw. The grub screw is then parted off by using a single edge razor blade using the steel surface as a guide. The cut face should be perpendicular to the grub screw axis.

Controlling the maximum amount of angular adjustment.

The factors that control the range of angular adjustment are:

* The distance d1 from the sphere centre to the rear face of H;
* The diameter d2.
* The diameter of the metal tube D.

Hope that helps

Paul


 
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TheLurker
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« Reply #76 on: February 14, 2020, 03:24:09 AM »

That is very elegant indeed, but far beyond the manufacturing facilities of my dining room table which got me wondering if a simpler, knock-off version could be made without access to a lathe.  Something like the sketch.  You'd still need a drill and taps (which I have) but taps and dies are not too expensive and tapping holes in the bic shaft* is (just about) a job that can be done "in-hand".  The conical aperture in the block and button** can be formed by hand with rat-tail files. If it comes to it most of the "body" of the button can be cut off if the diameter doesn't allow for a wide enough cone.  Haven't sussed how to retain the bush for the prop-shaft in the adjustment collar, push fit washers perhaps.

Cheers,
Lurk
PS - I particularly like your sneaky method for making grub screws.


*Drill*** and tap the holes in an uncut pen shaft then slice the adjuster off.
**I'm think especially of the small nose-buttons that VMC sell.
***A pin chuck would do the job, albeit slowly.
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RolandD6
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« Reply #77 on: February 14, 2020, 05:12:05 AM »

Hi Lurk

Your suggestion may work but I think you need to consider one modification and that is to lengthen the tube so that it nests in the apex of the cone in the button.

A short piece of tube as you have drawn is likely to tilt and bind the prop shaft when the grub screws are tightened. Not much pressure is needed from the grub screws but the pressure points where they touch the tube will likely impose a rotational moment on the tube.

Lengthening the tube so that it nests in the cone will minimise any attempt at rotation. It will probably help to radius the edge of the tube so that it does not dig into the plastic button.

Worth a try at minimal cost and effort.

Paul
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TheLurker
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« Reply #78 on: February 14, 2020, 06:52:42 AM »

Quote from: RolandD6
Your suggestion may work but I think you need to consider one modification...
Ah.  Schoolboy error.  Obvious, now that it's been pointed out. Smiley

I shall see if I can cram the making of a prototype into my busy schedule.


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strat-o
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« Reply #79 on: February 14, 2020, 09:15:34 AM »

Lurk, what do you have in mind for a conical aperture borer?  Hmm, maybe if the button was placed in a standard-sized hole as in a nose block that could be clamped down, you could find a suitable conical metal spike and heat it?

Marlin
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TheLurker
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« Reply #80 on: February 14, 2020, 11:52:56 AM »

Quote from: strat-o
Lurk, what do you have in mind for a conical aperture borer?
If you hack most / all of the barrel off the small nose-buttons that VMC sell you wouldn't need much more than an M5/M6 drill bit used as a countersink which would give a nice socket for the extended bush carrying the prop-shaft to sit in.

Paul, sorry about the thread hijack. I'll shut up now.
Lurk.
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RolandD6
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« Reply #81 on: February 14, 2020, 02:15:19 PM »

  No problem Lurk

Alternatives for the PTFE sphere are:

* a Peck style button reversed with part of the hole drilled out to accept the tube;

* a suitable plastic bead drilled like the above for the tube;

and so on.

The socket at the front of the nose block could be plywood with the half sphere hole formed with a spherical diamond grinding bit. This is an approach I have tried experimentally to find an alternative approach if I ever need to give up my lathe.

So far I have not had much luck squeezing short pieces of PTFE tube into the ends of the metal tube to provide two point support for the prop shaft. Little pieces of brass tube can be soldered or super glued successfully.

A very important aspect of the concept is the relationship between the diameter of the grub screws, the diameter of the metal tube, and the diameter d2. Get it wrong and the metal tube will not be held reliably.

Paul
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OZPAF
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« Reply #82 on: February 14, 2020, 11:13:46 PM »

Thanks for all the info Paul. That is as Lurk mentioned - an elegant Engineering approach. It has a distinct advantage over many other adjustment designs I've seen in that all the side and vertical thrust loads are taken along the axis of grub screws and do not rely on friction between a screw head/washer and a back plate. Not to mention the well spaced bearings for the shaft and the spherical guide blocks.  With care the boring of the PTFE spheres may be possible with a good drill press and a machinist's vice.

Lurk your proposal looks  promising as well but you would need to ensure you left enough bearing area for the shaft in the front bearing.

John

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RolandD6
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« Reply #83 on: February 15, 2020, 01:30:38 AM »


...  With care the boring of the PTFE spheres may be possible with a good drill press and a machinist's vice...


Hi John

I have already set my self up to do this.

I have a jig for the 3/16" sphere. It is a piece of steel drilled part way though 3/16" dia. and the rest of the way about 1/16' dia. There is a side grub screw to clamp the sphere. I suspect it may not work for PTFE spheres but should for nylon spheres because nylon is a harder material. I have yet to use it because at present I have plenty of pre-drilled PTFE spheres and the lathe is still available.

I plan to use an old 3/16" drill to centre the jig relative to the bench drill chuck axis.

I do not have an alternative way of making the rear PTFE bearing but have experimented successfully with brass sleeves. I also modified a small tube cutter to swage down the diameter of the metal tube by replacing the original cutting wheel with a blunt wheel. It works but the brass tube needs periodic annealing otherwise it will crack. It all depends upon the amount of diameter reduction.

Paul
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« Reply #84 on: February 21, 2020, 06:35:07 PM »

Thanks Paul

John
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