Sorry, but new user registrations have been temporarily disabled because we've been overwhelmed with spam users with IP addresses from Russia and that part of the world. This is true and not an alternative fact. 2/20/2017
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’
I want to tell you about Ralph Patterson’s free plans for a ball turning tool post that will fit a 7-by-whatever mini-lathe, and show you how he used it to fix a broken shower head. I’ve also included a YouTube video that shows a similar tool post being used to make a ball end for a tool handle. Near the end of the article you’ll find a link to download the plans for this tool, along with a link to where you can get Ralph’s other plans. And if you read carefully you’ll also find a link that leads to plans for a ball turning tool that will fit a 9×20 lathe.
This ball turning tool post is part of set of more than 25 free plans Ralph made for some very useful mini-lathe accessories and modifications. He also designed a boring bar holder, a quick change tool post, a leadscrew hand wheel, two versions of a tailstock lock, a carriage clamp based on Vicki Ford’s design, a die holder, a modification for slowing down the leadscrew feed rate, a file guide, a spindle indexer, a spindle crank, a height gage that uses a digital caliper, an adjustable tool rest for a bench grinder, and much more. In addition to drawing nice plans, Ralph also did something I think is very smart. He included photographs with most of them so that those like me, who don’t have a lot of experience reading prints, can easily see how the parts fit together and what the finished project will look like. He also includes a parts and material list with his more complicated designs, which is something I wish more people did.
The original swivel neck was made from plastic and it broke
Ralph is an excellent CAD drafter, skilled machinist and a gifted designer who knows a lot about mini-lathes. I’m very surprised that his designs and plans aren’t more widely known. (I intend to do something about that if he doesn’t mind).
His ball turner is adapted from one that Steve Bedair made for his 9×20 lathe. If you go to Steve’s web site you will not only find free plans for his design, but also photos showing how it is made and used, along with pictures of some of the things he has made with it. Besides things like ball ends for handles, this kind of tool post can also make beads on a straight rod, and with a little modification it can also do coves. They’ve also been used to make the lenses for optical center punches.
Ralph’s ball turner attaches to the top of the cross-slide with a couple of bolts and it doesn’t look like it will be hard to make. The most critical dimension is the height of the cutting bit and Steve Bedair has a nice picture that will show you how he measured it. The base has a recess that can probably be bored on a lathe if you have a 4-inch, 4-jaw chuck, otherwise you’ll need a mill and a boring head.
Ralph gives you a choice of four different tool bit holders that you can make. Three of them use carbide inserts and they’ll require you to mill a number of angles, some rather precisely. A forth version uses an HSS cutter made from a 3/16-inch drill bit. Both it and the HSS cutter look like they should be fairly simple to make.
If you would like to get in touch with Ralph or thank him for the plans then the best way to do that is to join the excellent 7×12 minilathe discussion group.
There’s a small error in the plans. The materials list says to use aluminum for the body and base, but the plans say to use steel. I asked Ralph about it and he said to use aluminum, or whatever you would like.
He also had this to say:
“If I were to make the tool again, there are a couple of improvements to consider. In order to move the cutter toward the center of rotation as the ball is formed from the straight round stock, it is necessary to loosen the 3 clamping screws, re-position the slide, and tighten again. For doing that operation easily, the clamping screws should be on the right side of the body. To make room for the clamp screw holes, the operating handle should be moved a little further down the side of the body.
Secondly, moving the tool holder manually and guessing at how much movement has been made, it would be nice to incorporate a screw into body that can be used as an external feed screw to regulate that movement. The screw could have a shoulder that engages a slot in the side of the sliding part, so that the tool could be moved both into and away from the work. Maybe making the slider longer at the base would enable a feed screw/slider to travel through the entire desired range of distance. Some thought is required.
Thirdly, the cutting tool sometimes needs moving to the left of the axis of the swivel screw of the tool. This would enable the cutter to move much closer to the axis of rotation of the workpiece when working on the chuck side of the ball. Maybe the tool holding slide could be made a half-inch lower in height to enable a cap piece that is adjustable from side-to-side to be mounted atop the post. A variety of cap shapes might be useful.
I am dangerous when thinking about these possibilities, instead of trying them out first.
The DRO uses an inexpensive digital dial indicator that’s held in place on the apron with powerful rare-earth magents. The stop clamps to the top of the cross-slide and presses against the indicator’s plunger, which measures the movement of the cutting bit.
This article describes how I added an easy-to-make DRO (Digital Readout) to the cross-slide of my 7×12 mini-lathe. It uses an inexpensive digital dial indicator that I bought from Harbor Freight for about $25. It works very well and you won’t have to drill any holes in your lathe or disassemble it to install it.
I’ve rarely seen a picture of a mini-lathe with a home-made DRO on it (or any kind of DRO), even though they seem to be fairly common on mini-mills. The ones for mills are usually made from inexpensive digital calipers or scales. I considered using them on the mini-lathe but rejected the idea because there is very little room to put them where they won’t be in the way. They will probably collect piles of swarf, the displays are also likely to be difficult to read and the buttons will probably be located where they will be awkward to push.
Using a digital dial indicator eliminates or minimizes those problems, although it is not a perfect solution. You’ll have to read the display and buttons upside down, which is not really much of a problem because they’re quite large and easy to read.
The indicator has a range of just 1-inch, but my design uses an adjustable bracket that will let you engage the DRO where ever you need it.
The DRO has two parts. The first is the indicator holder. It’s an aluminum bracket that attaches the dial indicator to the apron using very powerful rare-earth magnets. The magnets are located where they are unlikely to attract steel or iron chips. The second is a stop that clamps to the top of cross-slide and makes contact with the point of the indicator.
The stop can adjust two ways so it can always be made to press against the indicator’s probe, no matter the diameter of your work piece. You just slide it along the top of the cross-slide until it makes contact with the tip of the indicator, which is mounted on the apron. I thought there might be situations, like when working on a very large diameter workpiece, where the stop might not be able to contact the indicator. So I also added an adjustable rod to extend its reach. I now know it’s not needed, although it might if you adapt my design to another lathe.
The DRO does not interfere with the lathe’s controls and you can easily remove it in seconds if you want.
The DRO works very well and it has really improved my productivity. I no longer have to keep stopping to measure how much more metal I have to remove, or keep track of how much I’ve turned the cross-slide knob and then calculate how much more I need to cut.
Most of the time the DRO and the cross-slide dial are in complete agreement, or at least within five ten-thousandths (.0005), which is the resolution of the DI. When they disagree it’s usually because of backlash.
However, I was surprised to find that they would sometimes consistently disagree by forty-five thousandths, and it wasn’t due to backlash. I haven’t fully investigated the cause yet, but I’m pretty sure it’s going to turn out to be axial end play in the leadscrew. It’s a pretty common mini-lathe problem caused by a gap between the head of the leadscrew and the flange it is suppose to turn against. The most common fix is to make a small washer to fill that gap. It’s not really an issue and I’ve learned to trust the DRO.
This a prototype and not a finished design. I am hoping that others who are smarter and have better machining skills will think of ways to improve it and then share their ideas. That’s why there are no plans for it yet (a shortage of spare time and poor CAD skills also has something to do with it). So, until I get some plans drawn, I hope my photographs will allow you to make your own if you want to. Please let me know if you have any questions and I’ll do my best to answer them.
It has been more than a year and a half since I bought my 7×12 mini-lathe and I finally decided I needed to get serious about building or buying a cam-operated tailstock lock for it. A lever-operated lock is probably one of the most popular modifications for the mini-lathe because it increases your productivity by eliminating the need to find a wrench and turn a nut every time you want to move the tailstock.