How (not) to machine a cylinder

[Enough!Participant @enough]

Interesting day in the shop. I was machining the bore of a cylinder (iron casting) for a model stationary steam engine. I took a fair bit of time setting it up the way I wanted so that I could machine some other surfaces too at the same setting. Then I grabbed a boring bar out of my tool drawer, decided that it would be a bit flimsy over this length and took a larger diameter one instead (these are the types with replaceable carbide inserts). Had to extend it from the toolholder a bit to get the length (then had to extend it again when I realised that, although the bar was long enough, part of the casting would impact the topslide before I got there).

So, all ready to go I start boring, break through the cast-iron skin and bore the diameter out from the as-cast 0.85" to about 0.960" of the 1.000" that I want to end up with. Everything is going really nicely – finish is good and no problems, no funny noises. Decide on a diameter check ( about the third I'd done) before I start creeping up on the final diameter. So I move the carriage back a way to get in with a mic and notice ……. that the blxxdy boring bar doesn't have a carbide-insert installed.

Huh? It's been cutting on the corner of the holder itself – which actually looks in pretty good shape!

I blame the glasses.

[Enough!Participant @enough]

Hmmm ….

[Henry ArtistParticipant @henryartist43508]

Ooops!

Are the glasses filled with beer or vodka?

[CirclipParticipant @circlip]

Shuda gone to Specsavers.

Regards Ian.

[HopperParticipant @hopper]

Oh well, at least you know your boring bar is made of some darned good steel.

[KWILParticipant @kwil]

Perhaps the boring bar is also carbide! Carbide bars are very stiff which can be useful.

[Enough!Participant @enough]

Posted by KWIL on 18/04/2017 14:35:19:

Perhaps the boring bar is also carbide! Carbide bars are very stiff which can be useful.

Well, it wasn't a selling point when I bought it (CTC) but who knows? *** At least I know what to do when I'm out of inserts on a Sunday.

I've actually decided to re-state the whole thing as ' roughing ' cuts followed by finishing cuts (with an insert).

I think it probably says a lot for the quality of the PMR casting actually …. highly machinable with a thin skin.

*** it's actually described thus:

• Tool Holder made of High Density Forged Alloy Steel
• Hardened to HRC40, Ground and Black Oxidated (sic) or Silver Coated depending on Size

[Raymond Sanderson 2Participant @raymondsanderson2]

Maybe an insert was there but loose screw and both new the bar could handle the job well enough. Bit like leaving a 1st year apprentice to do a tradesman job.

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