Cylinder

[1Kenny]

How do you guys get such a small bore smooth? Also what kind of cutting tools do you use? :?

Thanks,

Kenny

 

[rake60]

When I'm finishing a a small bore I take 3 finish cuts.
Try that in steel and you'll end up with torn ugly mess.
In brass, aluminum or cast iron it will smooth it out.
In most cases the actual diameter of the bore doesn't matter at this point.
You'll be making the piston to fit it. If you get a perfect finish .020" under
the print size, leave it alone!
Another thing is your boring bar has to be centered. I bought this cheap
boring bar set on eBay for less than $10

The holder for them was made on the lathe they'd be used on.
I clamped a block if steel in the tool post holder and a drill bit in the chuck
of the lathe. Drilled it .030" undersize then reamed it to size with an
end mill in the chuck. A saw cut and it was a perfectly on center holder.

 

[tattoomike68]

After boring a little trick I have done at work for years on small bores is to take a chop stick or dowel, saw a slot in the end to hold some emory cloth wrapped around the stick. spin the part slowly in the lathe while rubbing the bore in and out kind of fast and you can get a nice cross hatch finnish just like the engine shops do.

Its kind of cheap and dirty but you can use it in the lathe , mill or drillpress. Plus you can make them any size you need.

 

[olcowhand]

After boring a little trick I have done at work for years on small bores is to take a chop stick or dowel, saw a slot in the end to hold some emory cloth wrapped around the stick. spin the part slowly in the lathe while rubbing the bore in and out kind of fast and you can get a nice cross hatch finnish just like the engine shops do.

Its kind of cheap and dirty but you can use it in the lathe , mill or drillpress. Plus you can make them any size you need.

I've used the same trick on brake cylinders except I chuck the dowel in a hand drill. Works great!

 

[gt2ride]

I have been using the dowel rod also. I just did not know it was the right thing to do. I have bought a lapping set with the copper barel and laping coumpond.

 

[1Kenny]

Thanks so much for the tips. I do have the boring bit set and just have not thought about useing them to make small cylinders. It makes sense to use a split end rod to hone.

 

[rake60]

The shop I work for makes some critical high pressure cylinders.
No tool marks are permitted, and their too deep to reach to hand polish.
Honestly, they use a 2X4 with sandpaper stapled to the end of it, followed
by another 2X4 with Scotch-Brite stapled to it.
Maybe a little crude, but it gets the required result.

The only cylinders I've ever lapped were on my flame licker models.
I make the piston for a snug but movable fit to the cylinder then apply
my favorite lapping compound. Plain Crest toothpaste.
Then I'd use an electric drill chucked to one end of the crankshaft to run
the engine until the fit is free sliding.
It's a simple approach, but it works.

 

[Philjoe5]

While looking for information about fitting pistons to cylinders I found this forum. I'm fitting an aluminum piston in a cast iron cylinder. When I make my last infeed while boring the cylinder, I take at least 5-6 passes without changing the setting on the crosslide. I still get a slight taper in the bore. I found that with a little metal polish on the piston I can eventually force the piston through. Then I can work the piston back and forth with a little more metal polish. When I clean out aall the residue the piston moves easily in the cylinder and seems to make a good seal. Question: Is the metal polish removing aluminum and making the piston fit, or am I removing the taper in the cast iron cylinder?

 

[tattoomike68]

My guess would be that if you are polishing the smaller part of the bore then you are losing some of the taper and the piston is losing a little diameter.

 

[rake60]

What is the lathe your cutting it on?
Are the gibbs adjusted properly? If they are you might try this.

If the taper is consistent, try bumping it straight.
To make a simple example, if you have a 3" long bore tapering .003 tight.
Mark your boring bar in 4 equal sections over the 3" length.
For the example of .003 taper over 3" that would be approx 3/4" between
the marks. As the bar feed in at the first mark gently bump in out .0005
(1/2 a thousandth) at the second mark bump it out another half, at the
third mark the final half. You've bumped the tool outward .0015 over the
travel through the bore that will take a .003 taper out. It's a practice in
finesse.

 

[rake60]

I have the same lathe here.
Your number reads .0005 Did you mean 5 thousandths?
Did it always cut with that much of a taper?

When I first got the lathe it had a wicked taper. A little investigation
found that the flat way on the back side had a rough spot on it about
4" back from the head box. I worked it smooth with a flat piece of
fine stone and the taper went from .004 over 3 inches to .001

Another thing to check would be the chuck mounting thread.
If there's a burr, dirt or a chip toward the front of the thread the chuck
run out. It wouldn't be enough to notice by eye, but an indicator might
show it. I like the China made 9 X 20 lathes, but when you buy one
your basically getting a kit. Once you get them dialed in their a great
little machine.

 

[rake60]

My home inspector (me) would call .0005 over 2.625" Dead Nuts!
These engines aren't that fussy.
If it's getting tight at a half taper the fit is too close.
A stuff to stuff fit in a model engine won't run long.
Take another .002 to .003 off that piston and it will be fine.

 

[1Kenny]

The Dong Gou 12X36 I bought in 1992 has close to 3000 hours on it. I had it in my motorcycle shop several years and hardly a day went by that it wasn't used. Everyone in the shop used it. A few times we would run it 7-10 hours at a time. We would prop the gear box lid up and put a fan under it to keep the belts and gear box cooled down. I keep it well oiled. Have had the head out of it twice to check the head bearings and clean the sump area. The bearings are still fine, just adjusted the end play. It has a slight wear area up by the chuck as that is where I run it the most. Also it has the same motor it came with. It is the seventh lathe I have had after high school. I have nothing but good to say about it.