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View Full Version : Cast Iron and Interference Fits



lynnl
03-30-2004, 01:23 PM
I need to make a new bushing for the leadscrew on my lathe. The original is CI and the attaching flange is broken. I'd thought of just making a replacement of bronze, but it has occured to me that I probably need to keep this as the weak link, to protect other more costly components in the system, and the tensile rating of the bronze is almost double that of CI.

So I'm now considering making the bushing itself of bronze and pressing it into a flange of CI. The flange will be a little over 1/4" thick and wall thickness as little as about 5/16" at the thinnest section. The OD of the bushing is .997 as I recall, and ID of .563". I'm concerned about whether the brittle CI would withstand the pressure of a press fit. This bushing fit will have to withstand the full thrust load of the leadscrew.

I'd appreciate any knowledgeable advice on the matter.

Carl
03-30-2004, 01:56 PM
I worked for a while in an automotive machine shop and we used to sleeve cast iron engine blocks. Some of the sleeves didn't have locating flanges on them and were fit quite tight, tight enough that we had to shrink them in dry ice before driving them in. We never had any problems with the blocks cracking.

dsergison
03-30-2004, 02:09 PM
cast iron doesn't take impact or distortion well, but if your carefull and support it while pressing it will withstand the press fit just fine. you can always make it a very light press fit and use locktite.

IMHO your worying too much. just do something and go on with life. Besides it will be a LONG time before you make the same mistake again, Right?

lynnl
03-30-2004, 02:15 PM
Oh I don't really worry about much of anything. I'm just viewing it as part of the learning process, and would like to do it right.

"...a LONG time before you make the same mistake again..."
HaHaHa! That's a good one. I see mistakes as learning opportunities. So why not take full advantage ...? http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif

SGW
03-30-2004, 05:09 PM
I'd be inclined to seriously consider Loctite 609, maybe pinning the bushing too.

If you do go with Loctite, don't make the bushing too tight a fit -- or you'll squeeze out all the Loctite when you push the sleeve in.


[This message has been edited by SGW (edited 03-30-2004).]

lynnl
03-30-2004, 05:36 PM
Really? I didn't realize Loctite (even the permanent formulations) offers that strong a bond. I'd perceived it to be analogous to using glue in a fitted joint like mortise & tenon in woodworking. There to hold the joint together rather than withstand the force.

Of course in my case I could provide a slight shoulder on the OD of the bushing which would be pressed into the flange from the back. The thrust force is really a 'pull'. Is "thrust" still the proper term? I understand that covers any force exerted axially. ..right?

gvasale
03-30-2004, 06:00 PM
I've thought of doing that too, but have contemplated using nylon or other machinable plastic screws for the weak point. Haven't done it yet. Trying to avoid breaking something first.

dsergison
03-30-2004, 06:17 PM
"There to hold the joint together rather than withstand the force." ????????

what manner of double talk is that? http://bbs.homeshopmachinist.net//smile.gif /kidding

locktite WILL turn a light press fit into a strong press fit.

Spin Doctor
03-30-2004, 06:27 PM
0.001" ought to do it just fine

Joel
03-30-2004, 07:20 PM
When you have to take something apart that was properly locktited together, you gain a lot more confidence in the product. http://bbs.homeshopmachinist.net//smile.gif
If you want to err on the side of caution, use a setscrew. Drill a shallow hole that is a snug fit for the tip to bear against.

wierdscience
03-30-2004, 10:12 PM
If you go for a press fit .001 would be the max I would go,red loctite 271 is potient stuff too,but my choice would be silver solder.