May 20, 2015

Cutting a Rack Gear on a Bridgeport

I considered buying a little metric rack gear for the lathe on SDP-SI, but it would have cost around $30.  Instead, I figured out how to machine one on the MITERS Bridgeport.  

The basic idea, which came from a thread on a machining forum which I can no longer seem to find, is this:  Put a single-point cutter in a fly cutter.  Angle the head of the mill such that the cutter points straight down.  Cut a tooth, then feed by the  pitch of the rack.  The forum-poster thought it would create a slightly incorrect tooth, because of the angle of the fly cutter.

Turns out, though, if the mill is angled between 90 degrees  and 90 minus the pressure angle of the gear, the teeth actually come out perfectly shaped.  Here's why:

This is what a single point cutter looks like when it has been swept around the axis of the mill spindle.  In this case, the mill is angled to 65 degrees, and the cutter is for a 20 degree pressure angle rack. 

The tooth profile that would be cut in this configuration is shown below.  Since the mill is angled to 25 degrees from horizontal, but the pressure angle is 20 degrees, the cutter removes an extra 5 degrees from one side of each tooth.  Simply rotating the head of the mill to between 70 degrees and 90 degrees means that the plane the tip of the cutter moves in doesn't actually interfere with the shape of the tooth being cut.  The result is a normal tooth shape, despite the funny arc the cutter moves in.

Here's what it looks like in practice:

I machined a .1" pitch rack out of some aluminum to make sure the process worked:

On to the actual rack, which is approximately a .5 module metric pitch.  The cutter was ground from a small brazed carbide lathe tool.  The rack was machined in three passes, to avoid loading the tool too much.  This took a very, very long time.

But I'm quite pleased with the results.  The rack feels like it meshes quite nicely with a (presumably) .5 module gear pulled off an old stepper motor.

Coming soon, making shiny hand wheels, and machining the spindle and headstock.

May 19, 2015

Compound Slide

School's over, so it's time to catch up on documentation.  The lathe isn't finished yet, but I've made a lot of progress.  Sadly, I won't have shop access this summer, so the lathe won't get finished until August or September.

The compound was made from two pieces, a dovetail slide which holds the ball nut, and a cone-shaped base.  The two pieces were silver-soldered together.

The base of the slide had a ring milled in it with a rotary table, so that the force clamping it down to the cross slide acts at a maximal radius:

The cone is clamped pulled down by a pair of set screws, the ends of which are turned to 45 degree cones:

The carriage, compound, and cross slide assembled on the bed of the lathe:

May 4, 2015

Cross Slide, Take 2

Man am I behind on documenting the mini lathe.

My original plan for the cross slide was to anodize it like Taig does, to prevent wear and galling on the aluminum-aluminum interface.  I decided instead of dealing with nasty chemicals and temperature control (required for hard anodizing), to just remake the part out of steel.  Steel machines much more slowly, but since I had already machined the part once, it went reasonably quickly.

Fortunately, Nick had a big chunk of steel which was very close to the size I needed:

The block was milled to size and top and bottom surfaces fly cut:

I forgot to take more pictures of it, but it looks pretty much identical to the original aluminum one.

I also attached one of the ballscrews to the carriage:

Bearing holders for the ballscrew were milled on the MITERS CNC mill.  The ballscrew is retained radially by a single ball bearing, and axially by a pair of needle thrust bearings (not shown here):

Prepare yourself for the making the compound slide and machining a rack gear on a bridgeport in upcoming episodes of Tiny Lathe.

April 19, 2015

The Longboard Has Gone Too Long Without a Motor

As previously indicated,  my longboard (can it be called that?  It's tiny) recently sprouted a battery pack and motor.

I started out with this board.  Nick crufted the deck out of a dumpster a while back, and I got an extremely cheap set of wheels and trucks for it.  The construction is interesting.  The deck is made from a 1/8" thick sheet of wood laminated on each side with a very thin layer of aluminum.  The resulting deck is extremely light and flexible, which makes the board really smooth out bumps (which are rather abundant on Cambridge roads and sidewalks).

Most electric longboards out there are stiff, because you generally don't want your batteries and electronics bending.  Alternatively, some electric boards have a small battery at one end, motor and controller at the other end, and nothing in the middle, allowing the board to be flexible.  This requires a full-sized longboard and/or a small battery though.  I wanted to keep the flexibility of this board, even though to get reasonable range the entire underside would need to be packed with battery.

My solution was the bendypack, which is the only part of this build that's anything new.  The rest of the electrical system is typical diy-electric longboard stuff, with a HobbyKing motor, motor controller, and remote control system.

The key to the flexible battery pack is its corrugated enclosure.  The corrugations allow the pack to flex at the gaps between rows of cells.  The inter-cell connections run against the deck, which is very close to the neutral bending axis of the board and battery assembly.  This means that the wires barely have to stretch as the pack flexes.

To make the corrugated casing, I cnc milled a foam mold on the MITERS cnc mill.

The mold was milled in three parts, because of the size limitation of the mill:

Two layers of fiberglass were vacuum bagged over the mold:

The foam was dissolved out with acetone. It left some color behind though:

The case fits 18 26650 A123 26650 cells.  These are arranged in a 6s3p configuration, for a nominal 19.8 volt, 6.6 amp hour pack.

Cells were soldered together with grounding strap to minimize the thickness of the connections:

The pack was insulated from the deck with a sheet of rubber.  Around the edge is a much softer synthetic foam, to act like a gasket and seal off the pack against the bottom of the deck.

The pack was screwed to the deck with a bunch of 2-56 screws around its perimeter.  These screws were tapped directly into the bottom of the deck.  The material of the casing is very thin, so the screw heads alone would have pulled through the fiberglass.  I 3d printed a series of oddly-shaped washers which were glued to the casing to distribute the load.  The washers aren't shown here:

The motor-wheel-pulley system is nothing too special.  The 42 tooth HTD-5mm pulley on the back wheel was downloaded from this thread on endless-sphere and 3d printed.  Sounds a bit sketchy, but there are 6 M5 bolts going through it to hold the wheel hub, and the load is spread across a bunch of teeth.

The motor pulley was CNC milled out of some square 7075 bar stock:

I much prefer split collar style clamping mechanisms to set screws, so I manually machined some slots in the pulley, and drilled and tapped a hole for the clamping screw.

The super cheap trucks had no square faces or cylindrical surfaces, so I stuck if in the lathe and turned a small section down.  We don't have a steady rest for the MITERS lathe, so I used the old drill chuck, which has smooth jaws, with lots of oil:

I CNC milled a two-part motor mount, with about a centimeter of adjustment room for tensioning the drive belt.  The motor is a SK3 5065 236 Kv, left over from a project of Nancy's a couple summers ago:

The mount was temporarily fixed in place with a big set screw, and tack welded to the truck by Mike.  The 9mm wide HTD 5 timing belt was borrowed from Ava's motorized ripstik.

I shoved an RC airplane controller (160 Amp dlux HV) and rc receiver into a 3d printed case, and glued that to the battery pack.  I don't really want this to be the permanent solution, but it works well enough that I probably won't change it until something breaks

Right now the "switch" is just the battery's deans connector.  Also not ideal, and it will get replaced with a proper switch eventually.

Belly up:

The board is geared for a no-load top speed of just over 22 mph, so realistically can probably get to about 20.  I very rarely max it out though - with a board this short, it's just too easy to hit a rock and go flying.  I haven't measured the range exactly, but I'd estimate it at 7 or 8 miles, although this will depend on how hilly and windy it is.  Riding back and forth from East Campus to MITERS 3 times (.7 miles each way) drained about 3.5 amp hours from the battery pack.  In terms of power, I've watt-metered the board at a bit over a kilowatt peak out of the battery pack during acceleration.

March 29, 2015


A new innovation in silly electric vehicle battery technology:

More to come soon.

March 11, 2015

New Cross Slide

Since I only have one of the original taig cross slides, but want both a cross slide and compound on the mini lathe, I had to make a new cross slide from scratch.  Also, since the little ballscrews I'm using are right-handed, rather than left-handed like cross feed leadscrews normally are, the whole system has to be rethought a bit.  To get the correct motion, the handwheel that moves the cross slide will be fixed to the cross slide, rather than the carriage.  Basically the same way compounds are normally set up.

As with most projects, I started out with a nice brick of aluminum.  The original cross slide is on top for reference:

It's significantly bigger than the original for a couple reasons.  First, by mounting the handwheel on the cross slide, you lose travel distance towards the spindle, since the hand wheel would collide with the carriage.  Also, I just wanted a bit more travel than a stock Taig has.

Why isn't it steel, or better yet, cast iron?  Well, aluminum was good enough for Taig... I may try hard-anodizing it like the original to improve its wear resistance.  Also I don't have any cast iron bricks lying around.

I faced the brick to size, and cut a strip of brass to make a gib from.  The bar I cut from had some nasty internal stresses, so the piece warped like crazy when I slit it off.  I was able to machine it down to size and get rid of the warp, though:

Here's the squarely-milled jib and cross slide.  A few holes were drilled and tapped to fix the two together, and then I milled the dovetails:

Preparing to mill the dovetails:

Turned out pretty nicely.  Tapped holes along the side were added for adjusting the gib.

I lapped the surface the same way as last time to get it to slide smoothly:

Next episode, I'll be adding the ballscrew and hand wheel to the cross slide.

March 1, 2015

Taig Mini Lathe

Back over the summer, while exploring some dusty top shelves at MITERS, I found what appeared to be they ways, carriage, and cross slide from a tiny lathe.  No one seemed to recognize them (and they were covered in a healthy layer of dust), so I claimed the parts as my own.  The steel dovetail ways were a bit rusty, but everything seemed in good shape otherwise.

The design of this thing is unusual.  The tailstock is lever-actuated, the carriage is cast aluminum with machined surfaces, and the cross slide looks like an anodized aluminum extrusion.

While looking at shiny machined things on some corner of the internet, I found a photo of a workpiece in a small lathe, and I recognized the distinct carriage and way assembly.  Elsewhere on the site, the lathe was identified as a Taig machine.  Turns out there's a pretty large community of people who use (and heavily modify) these machines.  Here's an example of a blog dedicated Taig mill and lathe projects.  

All the parts for the lathe are readily purchasable, and they are actually really reasonably priced.  But that would be no fun, so I'm going to basically build everything from scratch.  My basic plans are:

  • Make it metric!  Why?  Because metric is great, and I also happen to have a pair of sweet 1mm pitch metric ball screws I'll be using on the cross slide and the compound.  I haven't yet figured out the carriage rack situation though.  
  • Make a headstock and spindle.
  • Make a chuck from scratch.  Depending on how adventurous I'm feeling I may make both an independent 4-jaw (definitely easier to make) or a 3-jaw scroll chuck.
  • Brushless motor drive.  I'm deciding between a SK3 and a fancy Moog inrunner I have lying around.  The Moog will be quieter and shinier, but is definitely less power dense.
  • Make a new cross slide, and turn the old one into a compound slide.
  • Add a leadscrew for the carriage, for at least one speed of auto feed.  I don't care about threading, but a slow auto feed speed for finishing passes would be nice.
  • Make a mini quick change toolpost.

To start out, I removed most of the rust with steel wool, and then lapped the ways with oil and fine polishing compound.

In the next episode, I'll make a new cross slide, and ball screw couple it to the carriage.