Friday, July 26, 2019

Webster Update #4 - Saying Uncle

In my last update, two weeks ago, I mentioned I'd start on the flywheel next.  To refresh everyone's mind, this is a piece of D2 tool steel which I found on eBay and bought, well, basically because it was cheap.  After trying to reduce the length of the piece, I said "uncle" and bought a cold rolled steel disk. 
The flywheel finished dimensions are 3.750" diameter, and 1.125 thick at the hub.  Like this:

Note that it says the material is either cast iron or cold rolled steel (CRS).  So off I went to the metals dealers to find a piece of CRS that could yield this.  Some shopping around, first at Online Metals and then on eBay, resulted in me finding that a 4" diameter by 2" long piece was going to cost about $55 with shipping.  Then I stumbled across an eBay seller selling a slightly smaller piece, 3-3/4 by 1-3/4, but saying it was D2 Tool Steel.  His price was $18.75, including shipping.  I figured that the important part was that flywheel has a density more like steel or iron than aluminum and the alloy doesn't really matter.  I took what's probably a better piece of steel for many uses at almost a third of the price as the 1018 CRS.
It came in slightly over the claimed 3-3/4", more like 3.8", so I cut it slightly oversized in diameter at 3.77, leaving some for future work.  The raw piece was now just about 1.78" thick and as you can see from the drawing, the thickest portion at the hub is 1.125 or 1-1/8".  I could part it off down to around 1.15 or 1.20 inch and save hours of facing off .010" thickness at a time.  

Parting on the lathe is one of those operations that is simple in concept but has lots of art embedded in the technique.  The cutoff tools are narrow strips of either tool steel (so-called High Speed Steel, HSS) or a tool holding a carbide metal cutter.  Left and right in this picture from that link. 

The main advantage of the HSS tools is that they're easily sharpened in the shop so if one should break, it can be ground back to shape and then sharpened.  I have HSS cutters in a couple of thicknesses and use a tool holder, as shown in both views above.  The tool in its holder is advanced into the slowly rotating work under lots of cutting oil and cuts a groove.  The tool wanders a little bit, so the groove is a couple of thousandths bigger than the tool, but there's a lot of drag. 

To keep the setup rigid, it's recommended to only leave as much cutter protruding from the tool holder to take off a small amount, so you stop every quarter of an inch or so in radius and extend the tool a quarter inch farther.  To cut to the center of a piece that's 3.770 in diameter, at some point I need to extend the tool 1.89".  That gets scary.

The first 1/4 to 1/2" in radius went fairly quickly but that D2 steel hardens as you work on it.  I'd switch to a hacksaw and extend the cut down a bit, then use the parting tool to make it the right thickness slot.  My technique was to spin the work at 100 RPM (minimum for the lathe) and hold the hack saw onto it.  After that first 1/2", I noticed the saw didn't seem to be cutting; metal dust wasn't piling up  when I used the hacksaw.  Inspection revealed the teeth were now worn down - completely gone in the middle.  The D2 ruined my blade, but blades are disposable and I have a backup hacksaw.  On the other hand, it's a backup because the other blade used to cut better.  Now what?  Time for my Ryobi reciprocating saw.  Unlike the hacksaw, I can't run the lathe while cutting, but I can move the chuck by hand.  I have a blade with carbide teeth for the Ryobi, so I tried that.  It sorta worked except it inadvertently (and probably inescapably) turns the disk into a lumpy thing like a cam lobe (or a four leaf clover), not a smooth cylinder as gets cut on a lathe.  Which means interrupted cuts and banging onto the cutting tool.  Which leads to the blade binding in the cut and causing the motor to either slow down until it breaks free and suddenly races very fast, or else stalls and makes me reset its overload sensor.

I got the cut quite a bit farther in, but was left with about a 3/4" stem holding the two chunks together.  My rate of cutting went from .050" (one rotation of the cross feed dial) in a few minutes to maybe .05" in half an hour.  The parting tool was then so far extended that it didn't fit on the cross slide.  I couldn't back it all the way out of the work.  
At this point I figured I need to finish this with the saw.  Sometime in the middle of the night, the background processor in my brain (which never stops working on problems) said, "why not a carbide hacksaw blade"?  "Do they even make them".  Yes!  They're on Amazon, and on Home Depot's website, so off I go to Home Depot.  None in stock.  Neither were there any in stock at True Value.

I bought plain hacksaw blades, and a carbide reciprocating saw blade.  My lathe is rigid enough to cut on, but the thing shakes from the saw.  That's about 250 lbs of lathe?  I tried to take the disk out of the chuck and cut it in a vise on my woodworking bench.  It literally almost fell apart.  Drawer bottoms starting sliding out of place and stuff started falling out of drawers.  Everything on the bench started sliding off.  Back to the lathe.

With the lathe off, I cut it, used the flashlight to see progress and could see it was working.  Rotated and cut, rotated and cut, until it fell off.  Couldn't say exactly how long it took because of fooling around with the hacksaw blades and trying them.  Once I figured the tricks, it went within five minutes.  All told I spent nearly eight hours of work cutting off that thin chunk you see in my hand, below.  I broke one parting tool (but had a spare). 

Shakespeare said, "Parting is such sweet sorrow".  I'll just take out the word "sweet".  Better yet, I'll make that "parting is such a royal pain in the a**." 

Now I had a problem.  I had just spent about eight hours making a single cut.  To turn that 1.2" thick disk into the flywheel is going to involve removing about 4-1/2 cubic inches of that steel in very thin layers.  When I add in the cost of the new carbide blade for the Ryobi and a replacement parting blade, even ignoring the hacksaw blades (the one I ruined was old and they are consumables),  my savings on the D2 steel is gone.  One more ruined tool and I would have been better off getting the material they called for.  I thought it was time to say uncle and get the 1018 steel I should have gotten in the first place.  I found I could get a custom cut piece that was even cheaper, by having them cut it to 1-1/2" thick.  Then I got a 20% off coupon from Online Metals in the email. 

The new flywheel is in the lathe now.  Completely untouched.


  1. OUCH! Well, BTDT on a lot of similar things. Saved $10 in material costs, and either lost ten times that much in time, tools, or both.

    We have a metal supply place here that has a "Cut Offs and Scraps" bin where you can get some really good bargains.

  2. What drjim said, what, no metal shop or salvage/scrap yard near by?

    Even in socialist hell Gainesville, FL, we have at least 3 metal shops that I can get clean metal from. Or I can go to Svinga's in Ocala and have far too much fun.

    Svinga's is a new and salvage metal yard. Want a length of 2" cross-section hex Bronze rod 6' long? Seen it there... And, yes, someone bought it to make hand gonnes from. Three to be precise, and they worked, worked real well. Made pig hunts real interesting for a few years from what I heard. Gotta be weird, hunting feral hogs with hand-held matches and 2' long 3/4" diameter 14th century styled 'gonnes.'

    And you know you're at a serious metal dealer when the guy/gal helping you whips out one of those bazillion dollar hand-held spectrometer thingymabobs that shoot laser deathrays at the metal and can tell you pretty much the exact grade and composition of the metal, as long as the metal isn't plated.

    1. I've got to look more. There was a place at the other end of town, but I went there once and they were expensive. I can't say how much more than the online places because I was just getting started, so my needs were different and I didn't shop online as much. A quick check shows several scrap buyers in town, I just don't know if they sell. Some phone calls are in order.

  3. Early in my lathe-fooling-with times, I picked up some "easy to machine" pieces of stainless steel I didn't have very good cutting tools or knowledge then, and concluded that this material must have been left over from making the hinge pins on the doors of Hell. I gave it to the machinist at work.

    1. The very first piece of stainless I ever played with still has a broken tap in it. It was before I understood virtually anything about tapping, so-called free machining stainless and before I understood just about anything.

  4. That is a heck of a part off. The only thing I know about parting off is the old advice about "decrease the speed and increase the feed." Something that deep , though- I would be thinking a 1/8" minimum wide for the tool, and carbide tipped.

    1. The tool was 1/16 and HSS. My lathe is 1 HP and making interrupted cuts on this that were .001 or .002 would bog it down. I couldn't increase the feed.

      But a carbide parting tool is on my short list.

  5. @SiGb Reminds me of parting a grade 8 bolt on a 7x10 lathe. Another option is a thin cutting disk on an angle grinder. Maybe you could make a wire EDM power supply and tank, and move the part past the wire with your CNC mill.

    @Beans drop by Gainesville Hackerspace on a Tuesday presentation night and we can compare notes in meatspace.