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Help! => Specific Engine Help => Topic started by: warrenmaker on May 10, 2018, 09:12:40 PM

Title: Stuart #4 Reversing eccentrics
Post by: warrenmaker on May 10, 2018, 09:12:40 PM
Hi Guys, my first real post.

Can someone please help this newbie with machining the eccentric cams for a reversing kit on a Stuart #4 please. A single cam is a piece of cake, but this sucker has 2 ! 
The drawing calls for them to be turned at 24 deg from the horizontal . Is this for timing?   And how the hell do you do it.   :headscratch:

Thanks in advance,
Title: Re: Stuart #4 Reversing eccentrics
Post by: steamer on May 10, 2018, 09:17:45 PM
Hi Warren,

Welcome to the forum!   Can you post a picture of the drawing....just the eccentric.

I'm sure we can help you.

When you get a chance, post a introduction in the "Welcome" section so people know who you are.

Thanks!

And again...welcome to the forum!

Dave
Title: Re: Stuart #4 Reversing eccentrics
Post by: Gas_mantle on May 10, 2018, 09:25:37 PM
Hi Warren, welcome to the forum.

I can't help with your question as such but I'm interested to see what others say as I'm making a no4 as a part castings / part barstock model.

Can you post photos of your engine ?
Title: Re: Stuart #4 Reversing eccentrics
Post by: Dan Rowe on May 10, 2018, 10:07:43 PM
Warren, the 240 is the angle of advance which as you suspected is for timing the engine. Two eccentrics on the same block of metal will have 3 center points. The obvious one is the hole for the crankshaft. The other two are the centers of the eccentrics they will be 240 on either side of the specified horizontal and at a radius from the center of half the valve travel.

If you are not following this I can make a sketch.

Dan

Edit: there is a right and left hand way to make one of these make sure the forward and reverse eccentric are in the correct location. I made two the same for a Shay once and that was a mistake.
Title: Re: Stuart #4 Reversing eccentrics
Post by: warrenmaker on May 10, 2018, 11:46:03 PM
Hi Dan, A sketch would be hugely appreciated please. I hate to admit I am totally lost as to how to go about machining the eccentrics to get them timed to 24 deg.
 :help:
Thanks again for your assistance.
Title: Re: Stuart #4 Reversing eccentrics
Post by: crueby on May 10, 2018, 11:51:13 PM
I am not familiar with the nbr 4, is there room to make two single eccentrics and mount the back to back, setting the offset with each ones set screw? It would take up a little more room on the crankshaft, but if there is space it is much simpler to do.
Title: Re: Stuart #4 Reversing eccentrics
Post by: warrenmaker on May 11, 2018, 12:08:11 AM
pics of drawing and the casting as supplied
Title: Re: Stuart #4 Reversing eccentrics
Post by: Dan Rowe on May 11, 2018, 01:28:44 AM
Warren, that drawing has almost all the necessary parts it only lacks the distance along line A measured from the center of the crank hole to the center of the eccentric. I see that 5/32" is noted as the throw that should be half the valve travel and the distance from the center of the crank to the center of the eccentrics.

Ignore what I said about right and left hand that is already set in the casting.

Dan
Title: Re: Stuart #4 Reversing eccentrics
Post by: Dan Rowe on May 11, 2018, 01:57:35 AM
Warren,
To be sure I checked my copy of "Stephenson's Valve Gear" by Don Ashton and the eccentric as drawn is the position when the piston is at top dead center. If you are asking machine advise I would use the four jaw chuck to machine the boss with the set screw first then set out the other two center points and use the four jaw chuck to machine each eccentric.

Dan
Title: Re: Stuart #4 Reversing eccentrics
Post by: steamer on May 11, 2018, 02:12:50 AM
Ok    I did a bit of figuring....Here's my idea on it.

Make a turning fixture   First pic and picture 2

Face and bore the hub to 7/16
Mount up on the fixture and get the first eccentric running as little as possible .   Turn to size with light cuts.    Picture 3

Now dismount, and turn around and put back on the 7/16 mandrel.    Now rotate the finished OD until it comes tangent with the flat plate.   Lock it down   Picture 4

Now turn the second eccentric , and the result should be the geometry you need

Assumption.   The eccentricity is 0.21875  for a 0.4375 throw.     It's not real clear in the drawing, but it's kinda implied

Dave
Title: Re: Stuart #4 Reversing eccentrics
Post by: Jo on May 11, 2018, 06:51:44 AM
Its the same problem as machining the eccentric for the No5A which starts here: http://www.modelenginemaker.com/index.php/topic,6912.msg158413.html#msg158413

Jo
Title: Re: Stuart #4 Reversing eccentrics
Post by: warrenmaker on May 12, 2018, 12:09:28 AM
Thanks for the help guys, I "think" I got this now. Time will tell. I am going to shelv this engine for a while as I have to wait for a new casting as there was a problem with a blow out in the cylinder.

I bought it second hand at a flea market and noticed a floor in the bore, but thought I could machine it out. It machined into a cavity.  :embarassed:

I also bought a new Stuart #7A kit so I will make a start on that in the meantime.
Title: Re: Stuart #4 Reversing eccentrics
Post by: Jasonb on May 12, 2018, 07:17:30 AM
The other way to make it is to forget the casting and use a bit of Steel bar that is easy to hold, face it off and mark the two eccentric centres. Then locate each in turn with the part in the 4-jaw to machine them which is no different to turning a single eccentric. Then back in a 3 jaw to drill and bore then form the shoulder. Finally saw or part off, drill and tap for grub screw and hold on a bit of metal to face off the opposite end
Title: Re: Stuart #4 Reversing eccentrics
Post by: Mcgyver on June 23, 2018, 11:58:57 PM
I know you said you've put it aside, but this my help you later or the next guy making one

Its a tricky without knowing the trick :)  I made a mandrel as shown.  face the casting, drill and ream.  There are two parts to the mandrel, one held in the chuck and then a bolt to clamp the casting to a shoulder.  They key is both parts have an accurately turned register that picks up the bore the casting.

Basically the part has three centres, so needs three OD turning sessions for each one.  Calculate where they are as X/Y coordinates and make a tiny centre mark in the mill.  Bolt the fixture together with the casting, hold the fixture and in the four jaw and with a wiggler and indicator dial in the appropriate centre, turn, and repeat.

I've shown the resulting eccentric, this one had the added complexity of being split so it could mount in the middle of a multi cylinder crank.  Its nice to use the orignal cast iron as it machines well and makes a good bearing surface

(https://i.imgur.com/gTNYTWL.jpg)

(https://i.imgur.com/vyGjgQl.jpg)

(https://i.imgur.com/zCh3BHS.jpg)





Title: Re: Stuart #4 Reversing eccentrics
Post by: Ian S C on June 24, 2018, 01:08:21 PM
You might be able to rescue the cylinder, bore it out as far as you can, make a liner for the cylinder, install the liner with the bore unfinished, bore out to size after assembly, and if it's a little smaller than the original size who cares.
If it's a bronze cylinder you might find a bronze bush about the right size, save making a lot of bronze chips.
Ian S C