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Don't forget to plan how you'll measure

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magu

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#1
Just a PSA on my dumb move to hopefully save someone else down the line:

I am starting the construction of a small "cox" style engine from a casting. Last night I excitedly put the casting in the lathe and proceeded to begin boring out the journal when the main shaft bearing will go. Everything went well until I went "DOH!" I can't measure it.... The whole in question is recessed beyond the reach of my calipers, and I do not own a set of bore or pin gages to measure the diameter with. I was also set on machining two bores and two faces all in the same setup.

The finished bore should be 0.687-0.688" and will have a bronze bushing pressed in. Fortunately, there is enough stock in the casting and thickness in the bushing that this number can have an "ish" added on as the bushing hasn't been started yet. I ended up opening the bore until I could just barely tap in a cutter I have with a 5/8" (mic'ed at 0.624") shank. I then gently removed the cutter and cut blind until the dial on my compound said I had taken off another 0.030" radially (0.060" diametric + 0.625"=0.685") This should be acceptable, but if needed will leave me a few thou to ream or hone out.

If I were to do it again, I would turn a stepped pin with diameters of ~0.665", ~0.675", 0.680" and finally 0.685" I then could have mic'ed the pin and known where I was based on when I hit each step. For the last two I would be able to creep up on them until I had a tight running fit and be close enough for my purposes.

Below is a picture of the casting in the setup, and a really bad picture looking down the bore after I started cleaning up the as cast surface.

MVIMG_20171128_190445.jpg

MVIMG_20171128_190458.jpg
 

ch2co

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#2
I think that I speak for many of us here. Been there, done that.
Only on a different part of course.
Sometimes it isn't savable, sometimes its just a pain in the .... (neck)
 

David S

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#3
Magu that is a great reminder. After a few "opps" like you have encountered I make sure I review the steps ahead of time and yes I will often make a plug gauge in order to size a critical feature.

David
 

Doubleeboy

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#4
We have all had this problem at one time or another most likely. One of the reasons I have had a second lathe is so that I can leave a project on the main lathe and do simple work on the other. One of those simple jobs has been to turn down quickie home made gage pins out of scrap rod. on a critical bore I might have 3 or four pins in half thou increments so I can nail the bore as accurate as possible. I know you can set up a mill to do turning by holding stock in quill collet or chuck and clamping a turning tool in vise to do the turning and go that route too. Might want to consider a set of telescoping gages, the Chinese ones will do the job and are cheap.
 

mikey

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#5
If I were to do it again, I would turn a stepped pin with diameters of ~0.665", ~0.675", 0.680" and finally 0.685" I then could have mic'ed the pin and known where I was based on when I hit each step. For the last two I would be able to creep up on them until I had a tight running fit and be close enough for my purposes.
As previously stated, if you do it again, buy a set of telescoping gauges and learn to use them. Stepped pins are okay but when tolerances are tight and especially in a casting that gives you only one shot at getting it right, telescoping gauges are the cheapest alternative. A bore gauge is more accurate but can sometimes be a hassle to use with the part in the lathe. When used with skill, telescoping gauges can accurately measure in the tenths ... whether you can bore to those tolerances is another thing.
 

magu

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#6
As previously stated, if you do it again, buy a set of telescoping gauges and learn to use them.

Thanks Mikey and others. I know how to use both and have done so before, it was more "don't be a dummy like me and put the cart ahead of the horse" the but about stepped pins is how I would have approached it without owning specialty measuring tools.
 
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