newbie first build questions

[Lance]

Hello all,

Well I took the plunge and bought an Economy casting kit. Been doing fine so far until the cylinder and hopper.
The first problem was boring the cylinder. Tried first with the work in the lathe and the boring head in the tailstock. To much chatter, so I honed the chatter marks out and rethought things, and ended up with the boring head in the mill, with much better results.
My first question. The bore spec is 1.125. How far should I bore before using a 1 1/8 chucking reamer?

The second problem is locating the cylinder in the hopper. The holes are egged shaped on both ends. I know the dimension from the bottom up to the center line, and the dimension side to side for centerline, but with an egg shaped hole, how do I find center to start boring for the cylinder sleeve?? I've looked and thought about it for a while now, but can't see the forest for the trees. Don't want to blow it on such an important placement.
Thanks in advance
Lance

 

[Lance]

Now that I have reread my post, I figured out the second part of my problem. Edge find on both axis's and and measure up and over zero my dro, then change over to the boring bar. Guess I was staring at that oblong hole to long.:wall:

 

[jasonh]

Lance:

>The bore spec is 1.125. How far should I bore before using a 1 1/8 chucking reamer?

Rule of thumb - reamers can reasonably remove about 0.5 - 1.5% of their OD.
Smaller reamers towards 0.5%, larger reamers towards 1.5%

So 1.125 - (1.5/100 * 1.125) = 1.108125
So bore to 1.11

BTW- I think most people bore cylinders to within a few thousands of the desired size and then hone/lap carefully to get a nice surface finish and a parallel bore. The precise ID is not so important because the piston will be turned to fit the cylinder. It's more important to get a good seal.

>how do I find center to start boring for the cylinder sleeve??

You need a reference surface from which to measure things. I haven't seen your plans, but that flat bit on the bottom of the casting looks like a mounting flange. The plans may call for the cylinder to run parallel to and at a defined distance from that mounting surface.

Just looking at the picture I would think.
a) mill the mounting flange entirely flat- have that be a reference surface.
b) mount the casting on an right angle plate (reference surface against the plate)
c) measure the position of the cylinder axis wrt the mounting plate reference surface.
d) left/right position at center of mounting plate (or whatever the plan calls for)
e) bore the hole. Presumably there is is enough machining allowance in the casting to get a complete round with no hole-idays.

Hope this helps.

Jason H.

 

[jasonh]

btw - the setup for boring the cylinder in a the mill looks rather unsupported- and your bore is likely tapered. Did you try a steady rest when you had it in the lathe? That might take care of your chatter problem. -jasonh

 

[Jasonb]

The normal way to bore the liner is to use a boring bar held in the toolpost and use the lathe powerfeed to cut the bore which gives a more rigid setup and the even feed rate will give a better finish. I just hone the bored surface, never ream. Use of the fixed steady will also help keep things rigid.

Regarding the cylinder casting, plug the hole with a rough shaped piece of hardwood and then you have a surface to mark out on.

There is quiet a good build series for an Economy on Model Engineering Website.

http://modelengineeringwebsite.com/Economy_engine_1.html

J

 

[Lance]

Many thanks to both of you. Going back to the lathe, but just ordered a 1/2 boring bar with holder. The boring bar and head in the picture are 3/8 and have way to much flex, and produce to much chatter. Unfortunatly the chinese curse is still following me as I have not found a steady rest that will fit this machine. The bed is solid in between the ways, and no way to mount one.
At least if I screw up the cylinder it's black pipe, cheap and easy to replace.
Thanks again
Lance