Jim.Thompson

I now have the two main bearings and the front seal. (The bearings are the 6203 C3 type and the seal is genuine Stihl).
I'm ready to go looking for some aluminium stock rod to machine the prop driver.
What sizes is the one you picture in post # 39 Zagnut? It looks like I should start with 50 mm diameter stock.
I will start by machining a drilling guide out of some 8 or 10 mm thick steel plate. This is because I will not have a means to set up a drilling jig to drill the 6 prop retaining holes while the hub is still in the lathe chuck. I will rely on the guide, with it's 10mm centre hole, to drill the six holes in a drill press prior to hand cutting the threads. I assume from the chart above that 5mm machine screws will do the job, is that ok?

ZAGNUT

My Feedback: (1)

my hub is 41mm in diameter and is a pretty good fit for 22" props, you could go a bit larger.

yes, M5 screws are pretty much the standard. grade 12.9 is the best way to go and even those can get sheared if not kept tightened properly. the 34mm pattern with M6 screws would be much more robust but might be too big for the prop you go with....you could always do both patterns in the hub and decide which to use later.

Jim.Thompson

Thanks again Zagnut. I hope to start the machining next week.
What size do I machine the steel 19 mm mandrel for a shrink fit of the aluminium alloy hub?
By that I mean, what size difference (oversize), is the mandrel compared to the hub?

ZAGNUT

My Feedback: (1)

general rule is to go over size by 1/1000 of your bore diameter...1 micron per mm or 0.001" per inch.

so about 0.02mm should be good. heating your hub to 150C over ambient will make it around 0.05mm too large which should be a nice slide on fit. removal will be the hard part as you are now heating the steel as well which has half the thermal expansion as alum....so better to err on the loose side. if your hub has gotten a good heat soak at the above temp and it's not a drop on fit you might want to polish the mandrel a hair and then try again.

also, make sure the bore size you go with will accommodate the socket needed to tighten the crank nut. i chose 19mm out of convenience and then had to shave a socket down to fit....

Jim.Thompson

Got that, thanks Zagnut.

Jim.Thompson

I've started the machining work on our local Mens Shed lathe. It is an old Australian made lathe, made back in the Aus manufacturing heyday and is a suitable size for the kind of work I will need it for. I made my very first engine part; the bearing retainer and crankcase end cap. I just need to find some high tensile bolts to replace the soft ones I have in it temporarily. I also started to machine the intake manifold, but could not find a parting tool to cut the part off the bar stock.

w8ye

My Feedback: (16)

Looks like an old prewar South Bend lathe common instuitional lathe of the era in the USA

bcchi

Athol Idaho.
No right where it is.LOL
BCCHI

Jim.Thompson

I finished machining the intake manifold yesterday. It's only second part to made on a lathe. The finish machining is a bit rough, but it will plenty good enough for the task. It's ready for drilling and tapping for the retaining grub screws and the carburettor bolts.
I need to find some suitable material to use as an insulator between the carb and the manifold.

Jim.Thompson

I am about to choose the prop retaining screw circle diameter and begin on the drilling guide and prop hub.
Is there any reason I should stick to one of the standard sizes as per the chart in post # 47?
I'm thinking that the 35 diameter will suit my prop and counterbore size. If possible, I'll go for 6 x 6 mm screws if wall thickness allows (which I think it will). The retaining nut on the engine shaft required a 17 mm socket which is around 25 mm outside diameter. This determines to a large degree, the smallest circle size.
The prop has arrived and will suit a hub up to 52 mm diameter anyway.

Jim.Thompson

I am about to start the machining of the prop hub. This is a drawing; the dimensions do not snap accurately, so they require rounding out in practice. LibreCad!
It's simple, but a bit dodgy. I have it on my hit list to learn another CAD program. I do have AutoCad installed on my other operating system, but have yet to get it to install properly.
Maybe learn FreeCad, open source, and can be used with this operating system - Linux Mint 17 - Which I love! Prop hub 3.png (107.5 KB)

ZAGNUT

My Feedback: (1)

only reason to stick with one of the standard prop screw patterns is borrowing props from other modellers. for your engine size that would mean the 29mm xM5 pattern but that sounds like it won't work with the bore in your hub. larger pattern with M6 screws will be far more robust and less likely to shear screws...but then that might be a good feature to have in a ground strike, easier to replace a prop and screws that to straighten the crank.

every time i look at my 066 and imagine it on a plane i leave the stock carb boot in place and make up some kind of bracket from the firewall or engine mount to hold the carb.

for what you're doing a piece of 10mm G10 would work for the insulator. another material is that phenolic panel stuff often used as dividers in public restrooms....i imagine that might be easy to find at a recycling/junk yard.

Jim.Thompson

The socket required a 25mm counterbore. I might be able to use the 34 m pattern with 5 mm screws. Not sure yet, have yet to draw it out. Otherwise, it will be the 35 mm pattern and 6mm screws I guess.
I've seen that stuff at the recyclers. Sometimes used as electrical power boards too.

ZAGNUT

My Feedback: (1)

yep, same stuff, the toilet dividers usually have formica on both sides. very easy to machine, sand and polish when compared to G10/FR4. not as strong but plenty good for this application, same as the molded phenolic blocks used on other saw/trimmer engines. lots of uses around the shop when you need a hard, flat and heat resistant surface that's not conductive.

av8tor1977

My Feedback: (6)

What is the actual name of the material?? I don't think when I go to the lumber yard and request "toilet divider" material that they are going to know just what I mean.....

Also, how thick is it? Do you laminate it to get the thickness you want?

AV8TOR

Jim.Thompson

If I have interpreted correctly, the product is called Paper Phenolic.
I have never seen it used as Zagnut described, but I know it is used in electrical insulation boards and the like. Power box boards and similar.
It may be the same as printed circuit board, but I'm not sure.
Thickness? I've seen it in 8 - 10 mm thick. I'm going into my local recyclers today and will see if I can find some.

ZAGNUT

My Feedback: (1)

the toilet stall stuff is 10-15mm thick and usually faced both sides with formica or the like. core is dark red to brown. vandal proof, burn proof and easy to clean.

i believe in the states you would call it micarta, garolite or just paper phenolic. i have a small slab that is around 40mm thick and unfaced, most likely from some electrical installation.

corian might also make a good insulator but would be heavy.

Jim.Thompson

I found a power box board at the recyclers today for $2. It is around 6 mm thick, so I will two layers of it. It is branded "Formica" and described as insulating board. I will know if it is phenolic when I go to saw and grind it. It has a characteristic smell when worked. Strange to say, I made rigging blocks for the yacht I built long ago from it.

Jim.Thompson

The prop hub machining has been started. The counterbore is done to 25.025mm. I've removed it from the chuck and machined the mandrel to 25.050 mm.
This is the first time I have ever used a snap guage to measure internal bore diameters.
I'm ready to heatshrink the hub onto the mandrel. I'll get to tomorrow or the next day.
The drilling circle for the prop will have to be 34 mm or 35mm. I will probably make it 34 x 6 studs @ 5mm.

w8ye

My Feedback: (16)

Don't forget to have a radius to the inside corners or you will create stress points

Jim.Thompson

The only corner that could apply to is the end of the 25 mm counterbore and that is already done. It has to accommodate a washer anyway, so don't know how I could have done it and still have a washer seat properly?

I've successfully mounted the hub on the mandrel and machined the taper for the crankshaft. All looks good, I had a small amount of difficulty getting it off, but it's ok. Just took heat and a puller with some small amount of torque on the spanner to get it moving.
In my eagerness, did forget to cut the key slot, but can chuck it up again and cut that tomorrow.
I plan to machine up the drilling guide to a 34 mm hole circle and 5mm bolts. Some stock is in the chuck of the lathe already and partially machined down to size. I will machine a spigot on one end to fit into the 25.025mm counterbore of the prop hub and a 10 mm hole through it to take a centreing bolt to drill the prop.
An alternative idea is to machine a 10 mm spigot on the other end to fit the prop instead of a 10 mm bolt hole. This might be the better way.
After it is made, I'll use a drill press to drill the 6 holes for the 5 mm machine screws in the prop hub. Drill one, cut the thread and screw in the bolt, then drill the rest of them.

ZAGNUT

My Feedback: (1)

with the amount of beef there i wouldn't worry about not having a chamfer. if you were necking the hub down in the center and using something nasty like 7075 then it would start being important.

i like the idea with the reverse prop spigot. a "bald" prop washer would be interesting.

and what size of hex and thread is the crank nut that it needs such a huge socket?

Jim.Thompson

That is what I thought. This is large hunk of machined aluminium.
Bald prop washer? What is that?
The thread is 10 mm. The nut that fits it is 17 mm. The 17mm socket that fits the nut is 25 mm diameter. Just under actually, probably 24. 8 mm or so. Hence the 25 mm counterbore.

The drilling guide will look something like the attached drawing.

ZAGNUT

My Feedback: (1)

misread, i thought you were talking about putting the 10mm spigot on the actual prop washer instead of in the hub....probably not a great idea anyways.

Jim.Thompson

Since the posting the previous post, I now realise that the larger spigot only needs to be 3 mm long.