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MachinistBlog.com

Plans, projects and how-to's for home machinists

Cutting Glass on the Mini-Lathe

Diamond blade cutting a test tube on a mini-latheI needed to cut a piece of glass tube to a certain length and this is how I did it using my mini-lathe, an inexpensive diamond blade, some masking tape and WD-40.

The Stirling Engine I’m building uses a glass tube for the power cylinder.  It calls for one with an outside diameter of 16 mm and a length of 26 mm (1.02-inches), although those dimensions are not critical.  I thought I’d have to order a tube with the right diameter but I got lucky and quickly found a package of three 16 mm test tubes in the science kit section of our local Hobby Lobby store.  They cost $3.99.

I found all kinds of techniques on the Internet for cutting glass tubes and bottles.  One, which I’ll call the flaming string method, didn’t look like it would create the smooth even edge that I wanted.  Another video showed how to cut tubing by scoring it part way around with a carbide tool, putting a drop of water on the scratch and then pressing a red hot glass rod into the scratch.  That seemed to neatly break the tube, but I didn’t have a glass rod that I could heat up.  Others suggested cutting it with a diamond blade and I happened to have one of those.

The blade that I used was from a set of Dremel-size diamond blades that I bought a while ago at my local Harbor Freight for only three or four dollars.  They came with a mandrel which I was able to clamp in one of the tool holders for my quick-change tool post.

The test tube fit perfectly, and I mean perfectly, in the center hole of my three-jaw chuck.  Any bigger and it would not have fit.  To ensure that the tube wouldn’t slip while cutting I put some masking tape around it and just barely tightened the jaws.  The test tube had a lip on it that would interfere with the piston so the first photo shows me cutting it off.

I set the lathe in high gear and turned the speed control to almost maximum and then very, very slowly advanced the cross slide.  Water is usually used as a lubricant and coolant when cutting glass with a diamond blade but I didn’t want my lathe to get rusty, so I used a squirt bottle filled with WD-40.  It didn’t require much and afterward a paper towel soaked most of it up.

To be safe, I wore a face shield and I recommend that you do the same.

The cut edge was pretty smooth but I tried to make it even better by using the side of the blade to grind the edge.  The second cut I made left an even smoother edge so I left it alone.  I have a small diamond sharpener that I use to hone cutting edges and if I were to do this again I would probably try to use it like a lathe file to slightly round the edge of the rim that will be exposed when the engine is assembled.

Photo 2 Finished glass tube for the power cylinder

A Screwed Up Part and Some Ramblings about Using and Buying Drill Rod

Hub for Jan Ridders' LTD Stirling EngineMy last post described how I made the flywheel hub for Jan Ridders’ simple LTD Stirling engine.  It was pretty easy to make, which is good because I screwed up and now I have to make another one.  The hub slides on to the crankshaft and is held in place with a set screw.  The shaft is suppose to be 2.5 mm in diameter which is pretty close to 3/32-inch.  But I drilled the hub’s center hole for 1/8-inch drill rod because that’s what I had on hand.  I later realized that the slightly bigger shaft could cause problems when I make and assemble the crank webs and the graphite crankshaft bushings.  It would probably work, but I don’t want to lead anyone down the wrong path.  So I ordered some 3/32-inch drill rod and I am going to remake the hub.

By the way, I like working with W-1 drill rod and I keep an assortment of different diameters on hand.  It’s fairly inexpensive and manufactured to close tolerances for size, straightness, surface finish, and uniformity.  Regular metal stock can sometimes be a little over or undersized, which can cause problems and extra work.  But drill rod is ground and then polished to within .001-inch or better of its nominal size.  It’s great if you need to make a shaft or a pin.  You may be able to just buy the diameter you need, cut it to length, machine the ends and be good to go.  It already comes with a nice finish on it and it doesn’t take a lot of work to polish it to an even shinier finish if you need one.  It is also fairly resistance to rust.

Drill rod is also known as tool steel and in the UK it is called silver rod.  There are different kinds of drill rod but they are all tough, high-strength steels with good wear resistance and good machinability.  There is something else about drill rod that makes it very useful.  It is designed to be hardened, although it is often used without hardening.  Drill rod is classified by the way it is cooled during the hardening process.  I use W-1, which is the least expensive and it is hardened by heating it to a certain temperature and then cooled by plunging it into water.  A-2 and D-2 are air hardened, O-1 is oil hardened and S-7 can be air or oil hardened.

I’ve got one more thing to say about drill rod.  Buy it from Enco if you are in the U.S.  They seem to sell W-1 drill rod for less than anyone else.  Not just a few cents less, but a lot less.  Plus you can usually get it with free shipping.  If you can find it for less elsewhere please let me know.

Flywheel Hub Construction

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Begin by turning the shoulder the CDs will fit on.

Here’s how I made the hub for Jan Ridders’ New Simpler LTD Stirling Engine.  I started with a piece of 1.25-inch diameter aluminum and turned down a shoulder to fit the CDs that are used for the flywheel.  This part has only two critical dimensions and this is one of them.  The CDs need to fit well so the flywheel will have a minimal amount of runout.  The other critical dimension is the center hole.  It needs to be drilled or reamed so it will fit well on the crankshaft and the hub won’t wobble when it turns.  I recommend drilling the hole to fit a crankshaft made from 3/32-inch drill rod (more about this in my next post).

It is also important to cut a square shoulder so the disks will fit tight against it.  I use indexable carbide inserts and they have a very small cutting radius.  So I was able to cheat and cut just a little deeper into the corner of the shoulder so the radius left by the insert would be a little below the surface.  The radius ground on HSS bits is usually bigger so they might require you to clean up the corner with a parting or grooving tool, or a pointed bit.  If you don’t know what I mean then see the bottom of this page where they show and explain how to turn a shoulder.

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The next step was to turn down the diameter that will be drilled and tapped later for the set screw used to hold the hub on the crankshaft. I also faced it down to the correct length and center drilled it.

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Here's how the CDs will fit on the hub. They are used as a flywheel.

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The hub is parted off.

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The flange (is that the correct term?) is too thick and the parting tool left it convex-shaped. So I flipped it around to face it down to the correct thickness. You can't see it, but I wrapped a piece of aluminum tape around the part in the chuck so it wouldn't get marred by the jaws.

Usual Disclaimer: I’m not the most experienced machinist and there is almost always more than one way to do something.  Please leave a comment if you can teach us all a better or different way of making this part.