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MachinistBlog.com

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

How to Grind and Use the ‘Contrary Finishing Tool’

This excellent article was written by Bob Bickerton, a new contributor to Machinistblog.com. It was originally published to the files section of the Yahoo 7x12minilathe discussion group.

The slowest feed rate on the Mini-lathe is about .004 in/revolution. This is fine for many applications but when you want a nice, smooth turned finish the slowest rate isn’t slow enough. You can set the knife tool to ‘rub’ and that helps sometimes. And, there have been modifications published that allow the slow feed to be reduced, but all require building something for the lathe. Here’s another option that only requires one to grind a HSS tool blank to a different shape. It’s a “Contrary Ground Finishing Tool” described by Frank Burns on page 58 of the Jul-Aug. 1997 issue of Home Shop Machinist magazine. Here’s how I do it.

I turn the OD of the work piece with the usual ‘knife tool’ until it’s about 10 thousandths oversize. Then I mount my ‘finishing tool’ and adjust the cross feed to just skim the surface (about a thou or so). Note the cross slide in feed dial reading. Measure the OD and set the cross slide to remove about half of the still oversize work piece. Measure the OD again, and if everything went well you should have removed half the oversize. If so, set the cross slide to remove the remainder and, presto, you have a nicely turned finish on specification size. If the first pass removed a little more or a little less than half, make an appropriate adjustment before the final cut.

Here’s a picture of some EMT conduit mounted in the chuck of my Cummins 7x12. EMT is welded steel tube and difficult to finish, at least for me. The welded seam can be harder than the surrounding area making it difficult to get a good, smooth finish. The front has been machined with my usual knife tool. The finish doesn’t look to bad but it is rough to the touch.

Here’s that same piece after using the ‘Finishing Tool.’ Hard to tell from the picture but the finish is very smooth to the touch. Because the tool cuts on the front it doesn’t need to be exactly on center height. You can’t cut up to a shoulder because of the geometry.

How to Grind and Use the ‘Contrary Finishing Tool’

Because of the high level of interest in this subject, I decided to show how I grind the tool and then show it in use. Here’s how I do it. Continue reading How to Grind and Use the ‘Contrary Finishing Tool’

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

How a rookie makes a flywheel

I ran across this on the Home Model Engine Machinist forum.  I think it is an ingenious, easy and almost fool-proof way to make a spoked flywheel with just a lathe and drill press.  The flywheel blank is turned as shown and the spokes screwed in.  The back is then cut away and the rim turned down a little bit so the spokes are flush.  You can see more pictures and the finished flywheel by visiting the link.

Use Friction Turning to Make Thin Disks and Flywheels on the Lathe

Friction Turning on a Mini-Lathe

Click for more pictures

This article describes how I made two round disks on my lathe out of sheet metal too thin to be held in a lathe chuck.  I also could not use a mandrel because one of the disks was not going to have a hole drilled through its center.  The two disks were made from .073-inch thick aluminum sheet metal and are about 5-inches in diameter.  They are the top and bottom cylinder plates for the Stirling engine I am building.

First, I want to give credit where it is due.   I learned this from “Bogstandard” who described it on the Home Model Engine Machinist Forum and illustrated it with lots of nice pictures.  He calls this method friction turning and uses it to make flywheels out of flat plates for models, which can be sometimes be easier and cheaper to obtain than a chunk of large diameter round stock.

With this method the work piece is held against the jaws of the lathe chuck by a live center in the tailstock.  This allows you to turn down the diameter of the entire length of the work piece.  The jaws are opened so their outside edges are a little less than the finished diameter of the work piece.  Bogstandard recommends that you put an appropriate size piece of round stock in the chuck, presumably so it helps support the middle of your workpiece.  I did not do that for this project and it still worked fine.

Continue reading Use Friction Turning to Make Thin Disks and Flywheels on the Lathe

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