Poppin Help

[student_Machinist]

I did as Frazer said for the tool, I always use HSS tools, I find they give me a better finish. Cast iron should be turned pretty slowly actually, it's quite a small diameter we're talking about though. I'm lucky as I can get right down to 34 rpm in my lathe - that's for really big stuff though, I think for the pistons I used the slowest non-back gear speed which was about 200rpm I think. As Frazer says, slowest feed - again I'm quite lucky as my lathe has a screw cutting gearbox and at the finest feed you can hardly see the carriage moving.

Sorry if you've already said but how did you make your bore? I struggled the first couple of times then used a method that Jan Ridders suggested, using an adjustable reamer after boring to keep it round rather than trying to lap and bringing the unknown into it.

Bob's trouble shooting is logical, I'd do this before going any further if I were you. If any of the rod is out of alignment though, just enlarge the holes a touch - people get hung up on having close fits, which is all well and good but if you can't hold the geometric tolerances it will all just bind up. Give it a bit of clearance so it can move a bit and find it's own place. As long as there isn't a massive amount of play in it, it won't affect anything, it will just reduce friction and stop the binding, unless of course it's way out.

That is the trickiest thing about 'poppin', some of the clearances on the drawing are very small - I found myself looking at things thinking, "that's not critical" but then thinking more about them "oh yes it is because it will foul on this, that or the other" !

Keep plugging away, you'll get there.

Nick

I agree with the hss, its all i use Im just not sure what angle to grind it at for cast. How would i go about adjusting the reamer to size while keeping the blades parallel? I made the bore with a boring bar then lapped it with a bit of wood i turned down with a slit (about 3/4 of the diameter deep)down its length and some cutting compound to get it smooth. Ill probably try running an adjustable reamer through the bore and making a new piston

 

[NickG]

It should have a low rake angle like for Brass - about 2 degrees with a small rad on it. With the reamer I just added a little over the size of the bore, maybe 1/64" and with the lathe spinning on a really slow speed I used it as a 'floating' reamer. That is supported it and fed it in with the tailstock centre - whilst holding it with a large tap wrench or letting the tap wrench come up against the lathe bed. Use plenty of oil and if necessary, repeat adjusting the reamer up again. The bore size really doesn't matter as long as you machine the piston to suit, obviously so long as you don't let it go too large and break out.

The way you did it is what I tried first, the wood with a slit and I wrapped grit paper around it - I've tried it a couple of times now and not had success so that's when I switched to the reamer method and it worked for me on both Flame Lickers - nice, smooth, parallel, round bore.

On my poppin, machining the graphite for the piston was easier to get the close fit than with cast iron - it was infact a tiny bit tight at first but a few spins over once assembled and it found it's own perfect size. Because graphite is much lighter than CI I was able to make the piston like this meaning there was no yoke and screw to seal which was also easier.

This method might work for cast iron, but when I was trouble shooting with my Jan Ridders flame gulper I reverted to the yoke type piston to eliminate another possible variable. In the end, I think it was the fact that the adjustable reamer gave me the nice bore and it was so much easier to make a good fitting piston to suit, even in cast iron. Once you get it to a tight fit so it's just starting to go into the cylinder, just take extra passes with no extra cut on. That's how I did it anyway. If you aren't confident of your lathe giving a good finish, you could find something hard and flat and wrap grit paper around it at this stage to gradually get the piston down to size. I just found my lathe was good enough though.

I know some people, once they get the piston a tight ish fit in the cylinder, use some sort of fine compound and lap the piston and cylinder together which sounds a good method, but as I said, mine worked with no lapping.

 

[student_Machinist]

thanks for the info Nick
Ill give the reamer a try. Also do know what kind of place I could get blocks of graphite from?

 

[NickG]

No problem. There were a couple of places I was pointed towards: http://www.bengs-modellbau.com/material/semifinishedproducts/index.php Trouble is it's in germany so it will cost you relatively high for a small amount.

In the end a kind memmber from another forum posted me some graphite. Are you in the UK? I'll see what I've got left and if there's enough I'll send some on.

Nick

 

[student_Machinist]

thanks for the offer mate but im in New Zealand and it cost a fortune to send stuff here :-\

 

[NickG]

Ah, yeah it could do :-\ Other sources of the stuff would be from large old motor brushes I think. If not, it'll work well with cast iron, just needs more care and attention when turning the piston.

Nick