Tuesday, June 11, 2019

A Webster Update

I've completed the next part on the Webster engine I've started building.  The first part was almost two weeks ago, May 30.  That's a long time to make a plate with seven holes in it, but there have been some extenuating circumstances.  The Game of the Other Thrones kept me out of the shop a couple of days itself.  

Top side: 

The part fought me in some ways, too.  There's an obvious rectangle of four holes on the left, with a pair of smaller holes mixed in with them.  The smaller ones get tapped 6-32 and one of them fought me bitterly.  It's a long story centered on breaking the tap.  The Click Of Doom happened when I started to back the tap out, which I guess means the chip had clogged the tap.  I was tapping on the mill's table, where the holes were drilled, and had a 1/8 dowel pin in the drill chuck on the mill which went into the gap on the top of the wrench holding the tap.  I'd advance the tap 1/2 turn then lower the pin to keep the tap very close to perfectly vertical, and I must have not backed the tap out often enough.  I'm fairly sure I had not backed it completely out yet and this was going to be the first.

Well, that's probably TMI, but I need to tap a lot of 6-32 blind holes in this thing and I'm pretty sure the only taps I've ever broken were 6-32.   I've also read it's the hardest common thread for tapping; something about the ratio of the depth of the thread to root diameter.  I need to buy some more 6-32 taps - I only have one left.  Maybe a gross will get me through.

Bottom side:

The rectangular pattern holes and the big one in the upper left in this view get a countersink that required I buy a new set of 82 degree countersinks; everything I own is 90 degree.  It's not an optical delusion that the bottom looks like a better finish.  I should have noticed that and machined it with that side up.  I'll see if some finish work can pretty it up. 

The next parts are modeled and ready to machine.  These are two side plates made from 5/16" thick aluminum. 

The faint green outline represents the sheet they'll be cut from, the two parts are in blue and the red lines are the tool paths for cutting out the two parts from one small plate. 

I have another piece of metal ready to work on, the cylinder, which is cold rolled steel.  I've done a little work on that, but barely scratched the surface (if you'll pardon the pun).  Not quite sure which part gets the attention to get completed first.  There are arguments in favor of both. 


  1. What surface finish is spec'd for the cylinder bore? I doubt if the piston has rings, so question #2 is: What the piston-to-bore clearance spec?

    I've lost track of the times I was 90% finished with a part, and then realized it would have looked nicer if I'd started with the other side up!

    1. The piston does have rings. There's a polymer called Viton that is used in small engines like this.

      One of the gurus says he switched to Viton from cast iron rings, saying the effort required for the metal rings wasn't worth it.

      In engines like this, the cylinder is generally cut and then the piston lapped to fit. The bore is 7/8" - and I say that instead of 0.875" deliberately. 0.875 is my goal. If it comes out another size, I'll adapt.

    2. Never would have thought to use Viton for a ring, but then I'm used to dealing with slightly higher cylinder pressures in the engines I work with.

      What does it use for "valve timing"? Is it a "ported" engine like a two-stroke? Is it a two-stroke?

      I tried Googling for the engine, but couldn't find an easy, concise description of it.

  2. SiG, you may have better success with thread-forming taps for those 6-32 holes. They are essentially chipless, starting with a larger tap drill and upsetting the metal to form the threads. They aren't cheap... I have exactly one, for repairing 10-32 threads in rack rails, figuring I'm better off putting the metal back where it is supposed to be instead of cutting it away.


    1. I've been thinking of those, but I've never used one. Those McMaster taps aren't a lot more than a good spiral flute tap.

      I saw that for big holes (over 1/2" I think) they have sets of taps, each one of which cuts proportional less thread. Called serial taps. Hard to find, though, and I haven't seen any as small as #6.

      Then I tell myself "it shouldn't be this hard!" ;-)

  3. 6-32 is the weakest screw, proportionally, in the small machine screw world. Hate dealing with them. Whenever feasible, I would update them to 8-32, or something with more tpi if the diameter couldn't be increased. A pox on designers who specify the damn thing. It instantly identifies someone who has little real-world experience in wrenching. Did I mention that I hate anything threaded 6-32? The screws break more often than the taps break. Don't get me started on idiots that use it for adjustment or jacking screws. Arrgghh!

    1. Thanks! I've never had a screw itself break, but that's a really ugly thought.

      I'm thinking about going to 8-32 everywhere on this engine. If you look on YouTube for videos of the Webster, you find a lot of variations. Making screw size changes shouldn't be that big of a deal.