Muncaster's Joys Valve Engine

[smfr]

Thanks Steve! I have a bit of 7/8" bar coming from Enco for a boring bar, and some green wheels to grind some little carbide bits that came with some other boring bars (yes, I know the dust from grinding carbide is toxic!). Also got some 32 TPI grub screws from the hardware store, which should suffice to hold and adjust the bit (forgot dig through my taps to make sure i have taps for those  ).

Simon

[tvoght]

SImon,
  I just realized that using the between-centers boring bar, it'll be difficult to use a plug gauge as you intended. Have you an idea for gauging the diameter?

--Tim

[smfr]

SImon,
  I just realized that using the between-centers boring bar, it'll be difficult to use a plug gauge as you intended. Have you an idea for gauging the diameter?

Good point!

I was thinking that the between-centers boring bar was repeatable enough that I could just remove the tailstock and slide the carriage far enough to get into the bore, especially if I keep the boring bar held in the three-jaw chuck on the headstock end. Seems reasonable?

Simon

[tvoght]

That does sound reasonable. A little inconvenient, but yes I see how since the boring bar will have a center, it's safe to move the tailstock periodically for a check.

--Tim

[steamer]

Looking good Simon!.....Plus one for the between centers boring bar, and don't loose your mind if it comes out a thou or so over or under, the geometry is more important.

For drilling, put a plywood pad over the end of your drill chuck, and push the carriage with  the tailstock during the drilling operation.....like Jason said.   That will keep the forces in line during the drilling operation.

Dave

[Jasonb]

Yes you should be able to remove and replace the bar for testing. Or make a ring gauge that will fit over the bar to speed things up, just leave it at the end of the bar and slide into place when rhe lathe is stopped.

[Florian Eberhard]

Or make a ring gauge that will fit over the bar to speed things up,...

I was almost going to propose this but - someone's been faster than me. 

[smfr]

Good idea on the ring.

Now, how should I finish the bore? My plan was to use cast iron pistons with piston rings, but I suspect that this is an unusual piston size (1-3/8") and finding pre-made piston rings would be hard. I'm willing to try making my own (I found http://modelenginenews.org/techniques/piston_rings.html).

I have a cylinder hone, so could use that, or could make up a lap and lap the bore. I seem to recall that with cast iron piston rings, a perfect finish on the bore isn't essential because they'll bed down of their own accord, but is a honed finish a good starting point?

And how much should I bore the cylinder undersize for the different finishing options?
The boring bar material arrives today, so I might be able to make the bar this evening! 

Simon

[steamer]

Bringing an OD to size is pretty easy...in the home shop....but the same is not true on the ID.   I tend to make the bore first, and then fit the OD after.  Then if it's .001" over...no big deal.

I adjust the piston, or the tailstock quill to suite.

Dave

[smfr]

Bringing an OD to size is pretty easy...in the home shop....but the same is not true on the ID.   I tend to make the bore first, and then fit the OD after.  Then if it's .001" over...no big deal.

I adjust the piston, or the tailstock quill to suite.

Thanks Dave. Just to clarify, I haven't made the pistons yet. I'm just being anal and want my bore to be on size

One consideration I forgot was wear in the bore (this engine was originally designed for real work, and as such Muncaster made sure that worn parts could be easily replaced). Piston rings could eventually wear a step in the bore, which is possibly why Muncaster doesn't mention or show them in the original articles? In fact he doesn't talk about piston material or fitting at all, as far as I can see.

Simon

[Dave Otto]

Hi Simon

Debolt Machine has 1 3/8"X 3/32" cast iron  rings for $5.00 ea.

Great folks to deal with BTW.

Dave

http://www.deboltmachine.com/id4.html

[Jasonb]

Bore it to 1.374" and hone out the last 0.001", assuming you are getting a good finish off the tool.

Reeves also do 1 3/8" rings but its not hard to make your own.

J

[smfr]

I'm almost ready to do the boring now 

I made myself a ring gauge out of a bit of cast iron bar:



The first ring was cut undersize (but I can use it as an early warning), but the second ring is pretty spot-on:



This also gave me a chance to see how my big Chinese drills behave on cast iron. They cut it OK, but really want to go off-center, so I can see myself using a boring head to re-center things in between drills possibly.

The boring bar is about 10" of 7/8" 4140 hot-roll which was banana-shaped  , so a bit of fiddling with the steady rest was required to clean it up. To hold the boring bit I drilled through #8-32 tapping size, then drilled and and reamed for 3/16" to about 3/4 of the way through. The end of the hole was then tapped for the set screw. I then turned the bar 90deg, and drilled and tapped for the hold-down screw.



The toolbit is a bit of 3/16" carbide with a flat, which had a chipped end. The brown truck brought some green wheels yesterday, so I fitted those onto the grinder and used them to clean up the toolbit. However, I wasn't able to avoid some tiny chips on the cutting edge; is there a trick to getting a good edge on carbide? Can it be honed in the same way as HSS?

I should probably give this carbide bit a test before I use it in earnest. I suspect someone's going to tell me that I'd get a better finish from HSS 

Simon

[Jasonb]

I'd grind it to a profile more like a turning tool about 10deg front and 10deg rear, as you have it its more like a threading tool so will give a fine thread, also some top rake. Oh and HSS for the last few cuts.

[Florian Eberhard]

Hi Simon

Additional to jasons advice I would make a not too small radius to the cutting edge (in about 0.5mm or similar)
That gives a very smooth surface if combined with a fine feed.
Though if you are going to lap the bore with a lapping mandrel, the surface doesn't have to bee mirror-like before lapping.

Florian