Reamer size question

I'm building a Stirling engine and have a 1/8" drill rod that needs to slide freely back and forth in a 1/8" hole in brass that is 1/2" long while remaining as air tight as possible. I reamed it with a 1/8" reamer and it, not surprisingly, doesn't fit. Any recommendation on what size of oversize reamer I should use?

Thanks,

Mark

.1255 drill .120

72Zorad said:

I'm building a Stirling engine and have a 1/8" drill rod that needs to slide freely back and forth in a 1/8" hole in brass that is 1/2" long while remaining as air tight as possible. I reamed it with a 1/8" reamer and it, not surprisingly, doesn't fit. Any recommendation on what size of oversize reamer I should use?

Thanks,

Mark

Click to expand...

Might be easier to lathe polish your rod to fit the hole. Reamers are tricky. I find they do not always produce the size hole they should. And drill rod has an O. D. tolerance, you know. Did you mike it?

Larry

Get a reamer that's about .0002 oversize as the brass will most likely want to shrink up a little bit.
Or.. as said above... polish the rod to fit the hole, that would be my first choice if it's a low quantity run job.

Thanks all. I only have two to make. At the risk of opening myself to numerous 5th grade snarky remarks..... how does one polish a rod? And, I'm really curious how you would maintain dimensions over the length of it by polishing.

Mark

Drill and ream a plug in the same material about 3/8 inch thick. Use a 600 grit or finer sandpaper strip folded to about 1/4 wide, spin, and sand the first 3/8 of an inch or so from the tip in until your test piece just fits. Renew your sandpaper often and lite oil will keep the grit clean longer. Work your way in small sections at a time. You must completely clean the sanding grit off each time you check the size or you will go over. Use a little clean lite oil each time you check also to avoid causing burrs in the brass. If it goes on but has tight spots, leave them and use finer sand paper to clean up later by testing on your real part.

There is a better method but it takes learned skill. That is a spin with a long axes stroke ( i.e. masturbation stroke)that allows a little over a progressive full rotation between each stroke. (Kind of like thatching). The trick here is to not reduce the diameter inside the ends of the shaft ends thereby unknowingly over sanding.

Don't forget, it's very important to clean all grit off the parts before checking.

This lapping/sanding can take a fair amount of time if done right to get to size so have patience. If you have a .0001 micrometer that can help you to find and isolate high spots to selectively work on. I sometimes use a felt pin to mark those high spots.

72Zorad said:

Thanks all. I only have two to make. At the risk of opening myself to numerous 5th grade snarky remarks..... how does one polish a rod? And, I'm really curious how you would maintain dimensions over the length of it by polishing.

Mark

Click to expand...

Start with precision ground rod (drill rod) and fine emery / wet and dry in the lathe - used in sequence 800 W&D will get you a near mirror polish.

Limy Sami said:

Start with precision ground rod (drill rod) and fine emery / wet and dry in the lathe - used in sequence 800 W&D will get you a near mirror polish.

Click to expand...

Thanks' for the add-on got me in the middle of and edit...

Thanks so much everyone. The explanation makes perfect sense and I'm confident that I'll get the results I'm after. Might take a couple tries but I'll get there

Mark

72Zorad said:

how does one polish a rod? And, I'm really curious how you would maintain dimensions over the length of it by polishing.
Mark

Click to expand...

For your lathe attach a piece of pliable material (i.e., block of rubber or other firm plastic) with abrasive material (bolted at each end), i.e. emery or rouge cloth
or even Scotchbrite w/wo abrasive, to a length of square bar stock that you would clamp into a QCTP toolholder. You could feed this into the rod with your compound
in small increments making sure abrasion is consistent. Power feed this along your workpiece. Read about safety precautions if doing this by hand; there are PM threads about this, some recent,
i.e. right and wrong ways to apply a strip of sanding belt.

Nobody thought to mention that you want to run the RPM of your 1/8" rod as fast as you can. I would use my Levin lathe that goes about 5000 RPM.

And I usually use a flat ground piece of steel with a piece of 320 grit silicon carbide paper wrapped around it. The 320 is a general purpose grit for this work, but you can get finer stuff if you want. It is a compromise between speed of removal and finish. The longer your rod, the wider your steel block, but not as wide as the rod is long. Of course, with a thin rod, you have to avoid bending while polishing. New paper cuts faster than used. You start with new paper and then you find that the used portions of the paper will cut slower and leave a higher final polish.

Larry

Take a carbide insert and burnish the cutting edges of the reamer so it cuts a nats bigger. Proven, works and is simple + fast.

An 1/8" rod spinning at high rpm can be easily damaged and you could injure yourself. Pulling on the rod with emory cloth can cause it to bend and it can bend 90 deg. instantly. Be careful you could loose some fingers or worse.

victornet.com has an .0126 reamer for $5. part # RM112S-126