Author Topic: Benson engine  (Read 13775 times)

Offline kvom

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Re: Benson engine
« Reply #75 on: March 18, 2017, 06:03:46 PM »
1/8" would look a bity chuncky, I only used 1/8" on my 1.5times one. I have also done a couple of other similar sized engines and use 3/32" or 2.5mm on them and its about the right proportion with the  piston rod which is generally larger than the valve rod.

What about a split bush or even a drill chuck held in the lathe chuck to hold the 3/32" stock

Drill chuck won't work as the eccentric rods are ~3" long.  But I do have one I can experiment with on a short length of stock pending the collet purchase.   A bushing could also work, but there are plenty of other parts to work on in the meantime.

Offline kvom

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Re: Benson engine
« Reply #76 on: March 18, 2017, 08:02:41 PM »
Today's project is the forks at the end of the eccentric rods.  Start with some 3/8" brass rod and, on the lathe, drill 1/2" deep with a #43 drill (3-48 tap).  Then on the manual mill 2 side flats 1/4" long and 1/4" apart.



Then on the CNC mill, drill cross holes for 2-56 clearance and mill the profile.



Back to the manual mill to mill the center slot 5/32" wide and 3/16" deep.



Then to the lathe to turn down the spigot and part off.



Finally to the tap stand.  Finished result:


Offline kvom

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Re: Benson engine
« Reply #77 on: March 19, 2017, 08:24:55 PM »
Some more unfocused work for today.

First I parted off 5" of 1/4" drill rod that will serve as the crankshaft.  Rather than tuning down from larger stock to leave the mounting for the flywheel, my plan is to turn/drill/glue that mount to the crankshaft.  The flywheels I bought at Cabin Fever have 7/16" center holes with the ability to tighten onto the shaft with clamp screws.  Therefore, no key needed to retain the flywheel.  I'll wait to attach the mount to the shaft at assembly time, and as well I'll wait to fit the crank at the other end once that part is made.



Next I revisited the issue of providing the end of a 3/32" eccentric rod narrowed to 1/16'.  I mounted my tapping chuck on the lathe with a short length of 3/32 rod, and was successful in drilling a 1/16" hole.



I inserted a length of 1/16" drill rod in the hole and secured it with Loctite 620.  I'll let it cure for a day and then see if I can thread the end 3-48 without cutting into the smaller rod.



Last part of the day is the crosshead.  I decided to make it from brass rather than steel, but I doubt it matters.  Starting with some 1/2" diameter brass rod, I drilled and tapped 2-56 for the set screw.

[/img]http://www.pbase.com/kvom/image/165149104/large.jpg[/img]

Then turned down the spigot.



Then over to the Bridgeport to machine the sides.  I was originally going to do this part on CNC but decided this way would be faster.  In reality it was about the same amount of time.



Then drill and tap the sides for 3-48 and then drill/ream the top for the 1/8" piston rod.



Then all that was left was to part off on the lathe and sand off the nib from parting.




Offline crueby

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Re: Benson engine
« Reply #78 on: March 19, 2017, 08:55:33 PM »
Interesting clamp on the flywheel, is one half of the clamp machined as part of the wheel hub?

Offline kvom

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Re: Benson engine
« Reply #79 on: March 19, 2017, 09:54:44 PM »
The clamp is one piece that's a press fit into the actual bore of the flywheel.

Offline crueby

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Re: Benson engine
« Reply #80 on: March 19, 2017, 10:08:10 PM »
The clamp is one piece that's a press fit into the actual bore of the flywheel.
So the end where the clamping jaws are, is that saw-cut to form the two halves, like a collet?

Offline kvom

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Re: Benson engine
« Reply #81 on: March 19, 2017, 11:14:31 PM »
Yes.  The cuts go about halfway through the bore.

Offline kvom

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Re: Benson engine
« Reply #82 on: March 20, 2017, 09:19:45 PM »
After letting the trial eccentric rod cure overnight, I mounted it in the tapping chuck on the lathe once more and applied the 3-48 die.  No cut through with the threads, and nothing loosened up, so that's good to go.  Screwed into the strap for fit.



Then I went back to the half finished eccentric, drilled and tapped for a 2-56 set screw, turned the center groove, and parted off.



With the three parts assembled, the eccentric is retained by the 1/16" nib riding in the groove.  Rotation is a bit stiff but should wear in.


Offline crueby

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Re: Benson engine
« Reply #83 on: March 20, 2017, 09:49:36 PM »
Aha - was wondering how you were going to retain it on the eccentric, using the end of the shaft is a neat way to do it. Could even scribe in a line where the top/bottom joint would be to get the full split follower look?

Offline kvom

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Re: Benson engine
« Reply #84 on: March 20, 2017, 10:28:55 PM »
The book recommends scribing a part line as well as dummy bolts.

Offline kvom

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Re: Benson engine
« Reply #85 on: March 20, 2017, 10:31:16 PM »
In preparing the stock for the base, I noticed that I was not getting the sides square.  Checked the vise tram on the Bridgeport and the fixed jaw was off 50 thou from one end to the other.  Good reminder to check tram more often.

Offline kvom

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Re: Benson engine
« Reply #86 on: March 21, 2017, 09:52:32 PM »
I decided to get to work on the base today.  This is a part where having the casting is an advantage.  First step was making a semi-temporary fixture on some quarter inch aluminum.  Ran a face mill over it and drilled and tapped 5 mounting holes 5-40, plus 1/2 and 1/4 reamed holes for locating later.



Then machined the base itself.  I used a large facemill to face as well as profile the boss for the cylinder.  Then drilled all the necessary holes.



Next, screwed the base to the fixture, located the 0,0 coord from a ground pin in the 1/2" hole, and milled the outer profile.



I still need to tap the mounting holes for the riser on which the column and pump sit.  As well I want to mold the outer profile with a corner rounding bit.  I did not drill any holes for the cylinder mount;  I'll wait to see if I make the cylinder flange as per plans, or possibly slightly larger.

Offline Dave Otto

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Re: Benson engine
« Reply #87 on: March 22, 2017, 12:10:37 AM »
Hey Kirk

As long as you had the CNC fired up why not surface in a nice radius around that outside edge?

I have been enjoying following along with your build and it is looking great!


Dave

Offline kvom

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Re: Benson engine
« Reply #88 on: March 22, 2017, 12:17:19 AM »
Dave,  I do plan to do so.  Got a corner rounding bit on standby alert.

Offline Dave Otto

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Re: Benson engine
« Reply #89 on: March 22, 2017, 12:24:25 AM »
Not sure what CAM you are using; can you do a contour or (water line) tool path with a ball end mill?

Dave