Domagoj's Potty Mill

[Domagoj]

It's been more than a week since I introduced myself on this forum, so I suppose it's about time I start actually building something, right?
Like I said in my intro post, I decided on Stewart Hart's Potty Mill engine as my first build (Thank you Mr. Hart for making and providing the plans!). This is going to be a bit of a learning experience for me, since I just recently got a milling machine. I've had a lathe for a bit longer, but by no means I'm an expert, so if you see me doing something wrong, just holler. I'm bound to make mistakes (already did, but more on that in a bit), but hopefully I'll be able to fix them (with your help).

I'm still waiting for some stock to arrive, and considered starting to work on some elements I do have stock for, but I stumbled upon an unexpected find in the workshop. A few pieces of nice 3 mm aluminium plate that just begs to be turned into the base for the engine.

It has a very nice, but fragile brushed finish which I hope to preserve until the engine is complete. It has a protective plastic film, so it's easier to manipulate, but once the film is removed, you can't even look at it wrong or it will scratch, so the film stays for now. It seems to be machinable without tearing too much.
I cut the plate with a hack saw along the lines and cleaned up all the sides in the milling machine, starting with the rough cuts and then cleaning up the factory cut, which seems to be done on large scissors so it's not bad, but could be better. Machined edge certainly looks nicer.

I spot drilled all the mounting holes. DRO was very useful here. (Sidetrack: the milling machine is imperial, and I am thoroughly metric, so installing the DRO was the first thing I did after purchasing the machine. I just can't wrap my mind around it. Of course, during the installation, something got in the way and I installed only X and Y axis, while Z is still pending. I really should do that.)
Anyway, with the DRO and edge finder, this went smoothly. I had to do it in two parts since the vise jaws are a bit too narrow (105 mm) so I had to reposition the plate half way through. Edge find again, check against an earlier spot and drill some more.
After that I drilled 3 mm holes for the engine bits and 5 mm for mounting to the wooden base.

Honestly, I don't know why I decided to go with 5 mm for mounting to the wood, but at that moment 3 mm seemed too small. This could turn out to be my first mistake in this build, but for now I'll go with it, and if it turns out to be problematic I'll figure out the solution later.

While setting up for countersinking the holes (on the bottom side), I had an idea to chamfer the edges of the base plate. A quick try on an offcut looked nice, so I went with it. Thickness of only 3 mm doesn't provide for a very secure workholding when part of it has to stick out, but the procedure was uneventful.

My 4 mm parallels were either too short or too long, so I had to stack some thin ones under them to lift the edge above the vise jaws. It actually worked.


Countersinking the 3 mm holes on the underside:

This tool seems to like slow speeds and a bit of a pressure. It starts to chatter at about 600 RPM, but at 300-400 works nice for such a cheap thing.

And that's it. It took me about two days (couple of hours each day) to get here, but I'm happy I have something done.

[propforward]

Very nice - looks like you're off to a fine start.

[Flyboy Jim]

Good start.  I'm looking forward to following along.

The use of a shop made Fixture Plate and some shop made clamps might of held your base more securely for machining. I use mine a lot.

Jim

[scc]

I'm all set too....            Terry

[Admiral_dk]

You are off to a good start 

It starts to chatter at about 600 RPM, but at 300-400 works nice for such a cheap thing.

Back in Tech-School in my youth, we were told to run a countersink tool @ the slowest the drill press would go or between 50 and 200 RPM, but preferably around 90-150 RPM (that was probably in steel). I tried them to do something similar to what you did some months ago, just on plastic - this has to be done at much higher RPM's - but the same happens as you discovered - too low or high RPM's and you get a poor finish with them.

Best wishes

Per

[Domagoj]

The use of a shop made Fixture Plate and some shop made clamps might of held your base more securely for machining. I use mine a lot.

Yeah, it does look like a useful tool. I'll certainly look into making one. Do you have some pictures of yours?

Back in Tech-School...

I have no formal education when it comes to machining, and I'm learning as I go, so it's no surprise that there are some bits of knowledge that have long since been established, but are new to me.
I am glad that my tool is behaving similar to others, meaning it's not total junk. This being a hobby and all, I usually can't afford expensive tools (even though the few that I do have I absolutely love and recognize their worth) so my usual suppliers are ebay and aliexpress, which unfortunately are not known for stocking highest grade.

[Flyboy Jim]

The use of a shop made Fixture Plate and some shop made clamps might of held your base more securely for machining. I use mine a lot.

Yeah, it does look like a useful tool. I'll certainly look into making one. Do you have some pictures of yours?

I have a fixture plate for both my mill table and rotary table. Mine were purchased from Sherline for my Sherline mill and RT. If I were doing it again, I would of made mine myself and made both of them bigger. Maybe Chris (Crueby) will be along and post a picture of his. The unpainted clamps I made in my shop.





Here's a fixture plate I made for holding the Flywheels for my P & W build:





Fixture plates and jigs are a whole branch of machining and, being pretty new at machining myself, I've learned a lot about them by looking at the ones others have made. I've found that sometimes making a fixture or jig to hold a part takes a lot more time than machining the part itself. 

Jim

[crueby]

Hi Jim - as requested, here is the tooling plate I had made a couple years ago. This is a bottom view, showing the center button that fits the center hole of the rotary table. There are two rows where it will fit, so I can center the plate or offset it for long parts like con-rods that I want to work on the end of.

And here is a top view of it in use:

Simple to make, made mine from a piece of steel flat stock, aluminum would work too though the holes could wear quicker in that. Drilled a grid of holes for the hold-down clamps, threaded a bunch (and thread more as I need them, it got tiring to do! )
Chris

[Domagoj]

Those look great.
You guys convinced me, so my To Do list just got a bit longer.

[crueby]

Tooling plates like that are very handy - when milling down the sides of parts, I stick a bit of card stock, like a 3x5 card piece, under the part so the mill cutter doesn't have to touch the plate and wreck the surface.

[Flyboy Jim]

Those look great.
You guys convinced me, so my To Do list just got a bit longer.

 

Plus .............shop made fixtures, jigs and tools are really rewarding to make and get used over and over. You won't regret the time spent.

Jim

[Flyboy Jim]

Hi Jim - as requested, here is the tooling plate I had made a couple years ago. This is a bottom view, showing the center button that fits the center hole of the rotary table. There are two rows where it will fit, so I can center the plate or offset it for long parts like con-rods that I want to work on the end of.

Chris.......... your tooling plate for your RT is way more useful than mine for sure. But at least I paid a lot more money for mine!  As you can see from a previous picture, I could barely clamp the Coke Bottle frame to my plate. Your's would of been easy. The moveable Center Button really makes it useful.

Jim

[crueby]

The rectangular plate is definitely versatile, in some setups I do need the riser block on the head to get it to clear. Also I have the larger mill base with longer table travel.

[Domagoj]

Some progress on this front. A bit of flat steel stock arrived the other day and I got to work.
The base plate is not particularly exciting, so after that I decided to start on something that actually looks like an engine part - the connecting rod.
It also means I got to use the four jaw chuck, which has been sitting in a drawer unused for quite a while. I just haven't had much opportunity to employ it, so after the chuck change, I quickly (and perhaps a bit inaccurately) chucked up the flat bar. The sides are rounded from rolling and I should have cleaned them up before putting in the lathe. It would have made turning it easier, but that's another lesson learned.

Marked up rough features and started removing the material. Doing interrupted cuts with carbide is probably not the best thing to do, so I went slowly. I did eventually reached the tapered part.

I had some trouble with the carbide insert and the left side where the taper ends and meats the rounded part didn't end up looking all that nice.

The 8 mm hole was drilled, and the small M2.5 threads made.

The rounded end needs some cleaning up. I was thinking of doing it in the milling machine, but just couldn't figure out a safe workholding that would allow me to rotate the part (I don't have a rotary table), so I turned a small aluminum hat/plug/button with an 8 mm and a 12 mm diameters which I used as a guide for grinding.



I did some minor polishing, but I think I'll come back to that. There are still some tool marks that I would like to get rid of as much as possible. I should probably have done more sanding on the tapers while the part was in the lathe, but it's a bit late now. A dremel and a polishing wheel will have to do. I know it's just cosmetic, but this is not a case of function over form, so I think I'll have to do some more polishing.

[crueby]

Off and running on the parts, nicely done!

[Domagoj]

It's been a busy month. Unfortunately, work related, not steam engine, but I finally managed to find a few hours and made a couple more bits.

Again, DRO to the rescue.
I've also hit a snag. Either I'm misinterpreting something or I'm completely wrong. It could also be a matter of error in the plans. Can somebody explain what I did wrong?


In the plans the hole between the two brass parts is 8 mm, but the axle it should go on is 6 mm. I don't see a bushing in the plans that would fill the gap. As of right now the hole I made is 6 mm, and I'm hesitant to make it 8 until I figure this out.
Any suggestions?

[Jo]

Is it a made up crank? If so you will probably find that the two ends where they press into the webs are 6mm and the centre section where the con rod goes is 8mm diameter.

Jo

[Jasonb]

No it's an error on the early drawings, the crank pin is the plain 6mm diameter but the hole in the big end should also be 6mm, see attached

As you have made the bigend which I assume has the larger 8mm hole you can either drill the crank webs 8mm and use a plain 8mm pin or do as Jo says and have a pin with a 10mm long 8mm diameter middle and turn the ends down to 6mm.

If you have made th ebig end bearings 6mm then just use a plain 6mm rod for the pin at 22mm long as per drawing

[Domagoj]

Jo, no, I haven't done the crank yet, so I'm flexible.
Jason, the hole on the bigend I made is 6 mm (I stopped at 6 to make sure and double check), so I'm still on track.

Thank you both.

[sbwhart]

Woops my error 

As Jason said no big problem the crank can be 6 or 8mm just make the bearing to suite.

Stew

[Domagoj]

Thank you for pitching in, Stew.

I managed to put in a few more hours into the build, so I started to work on piston and cross head assembly.
First on the order of business was to make a form tool. I didn't mean to make it ugly.

I had a CAD model where I drew several versions and decided on radius of 8 mm as pretty enough. I ground the HSS blank and while it was not the prettiest, I figured it's close enough. Unfortunately, once I turned the actual part it didn't match the intended geometry. I suppose it's not important, but is also just another reminder that I need more practice.

It's not nearly as rounded as I hoped for.
Anyway, I decided to carry on. I can always come back and redo the part from scratch, right? I removed the bulk of material in the center on lathe, in preparation for the milling machine that will clean up everything else

A quick thread tap. I have tapping function on the milling machine, where it has reversing buttons on the feed levers, but I'm a bit anxious to use that on such small threads. I've stripped some threads with it in the past since I really don't have a good feel of how it's coming along. The machine is too powerful for such small taps, so hand threading it is.  I promise, the hole is not that crooked, it's just the photo.

I also drilled out and reamed the cross holes.

You'll probably notice (as I did, but too late) that the cross holes are not centered. At first I thought that I made an error in finding the center, but it seems (as I will later find out) that my gibs are a bit too loose and I either bumped the wheel handle or it dropped on its own due to gravity and vibration. Tightening the gibs is on my to do list.
I'm not sure why I don't have the photos of the milling process, so you'll have to imagine a piece of metal in a vise on a milling machine. I also rounded it on the grinding wheel.
All this was done about two weeks ago, but I didn't get around to posting it, so now you get a double dose.

The following work is still warm from the lathe. I started with the piston rod, and perhaps the only interesting part about that is the threading. I have a nifty little thing for threading on lathe which sits on the tailstock and freely slides and rotates, keeping the axial alignment. I turned the chuck by hand, of course.

The piston itself, after parting, I reversed it and mounted it on the piston rod to clean up the parted side. This is where I discovered some wobble. I'm not sure where it came from, since the hole and the threads were done on the lathe and the piston rod is brand new piece of silver steel, and definitely not nearly as crooked as to cause that much wobble. In any case, somehow I managed to fiddle it to run true and clean it up.


This is where I had another one of those bright ideas. I was fiddling with the cross head and realized that I forgot to clean it up and break the edge, so I thought it would be fine to put it on the piston rod and chamfer it.

Of course, it caught and bent the piston rod, fortunately only slightly, and I was able to return it to the original shape. You wouldn't even know it if I hadn't told you. Admittedly, the cross head does look better now.


Everything together.