Author Topic: Are there plans for the potty mill engine in imperial measurements?  (Read 4139 times)

Offline Chris Leverett

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I would really like to build the pottymill horizontal engine for my next project but would prefer to build from some 'non-metric' plans. This would be only my third engine so I do not need anything else working against me. I have always worked in thousandths and all of my gauges are imperial also. Forgive me if this is the wrong place to ask this but I am new here and do not yet know my way around. This is also my first post here so....Hi everyone!
Chris

Offline b.lindsey

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Hi Chris and welcome to MEM. When you can, please post an introduction in the introduce yourself section so that everyone will see it and give you a proper welcome. There are two versions of Stew's mill engine in the plans and drawings area, one done by Stew, and another set redrawn by Julius. Unfortunately both are done in metric, Maybe someone is aware of an imperial set, but a quick look didn't turn them up here.

Bill

Offline sbwhart

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Hi Chris and Welcome

Thanks for your interest in my little engine, I'm afraid all the plans are in metric but they do translate into imperial sizes quite well some of our metrically challenged friends across the big pond have built the engine using UNF thread sizes. The engine uses mostly M3 threads so instead you could use 5 or 6BA for the M2 sizes use 8 or 9BA. I used standard imperial bar stock bought from Model Engineer suppliers so where it calls up 6mm bar use 1/4" and so on. To convert from metric to imperial just divide by 25.4 and then use the nearest standard imperial size, you will find it converts very conveniently. Just down load the drawing from her or send me a PM with your email address and I'll send you a set, pencil in all the imperial sizes on the drawing before you start, the design is very flexible and tolerant to errors so you should be fine.

Hope this helps

Stew

   
A little bit of clearance never got in the way

Offline kev

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Welcome to the forum. Whilst I understand your preference to stay with what you know its a very useful skill in this world to be able to move between imperial and metric and visversa with ease so maybe a good time to start for you :)

Offline Chris Leverett

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Thanks for the replies guys. Too bad about the plans but I may just do as suggested and translate them over myself. I will try to go over to the introduction section and post something and I would like to show my last engine in the proper forum also.

Chris

Offline tinglett

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I chose a "mix and match" approach for dealing with imperial conversion for this engine.  I'd be happy to send you photos of the prints that I've marked up, and I really do intend to finish my build log with a list of some of the conversions I thought were important as well as things that could have been improved in my work.

But meanwhile, it sounds like you are eager to get going, and you should be because this is a very nice engine!   Throughout the build I often used my electronic caliper to switch between imperial and metric.  It's no big deal to measure a length in mm that way.  I only have a single 0-1" mic and I never used it for this build (my work just isn't that good -- yet).

The important things are rod sizes and threads.  You don't want to do silly things like turn down a 0.125 inch rod to 3.00 mm.  You are probably going over the plans already to decide if you have the material, so that's a good time to pick what you'll use.  Mark up the prints as you make decisions.   When I thought a measurement might matter, or if I did a conversion anyway in order to run the dials on my lathe/mill, I'd always pencil it onto the prints.  In general I tried not to change the offset of a throw (crank or eccentric) or the shape of the valve.  And that was mainly because I didn't want to try to figure out how to correct the valve timing that I was sure to mess up.  I did very little of this up-front.  I guess I was too eager to get going.  Doing this on-the-fly didn't slow me down as far as I recall.  I would think differently if the parts cost a lot of money (castings and such).

One thing I found incredibly useful was a imperial/metric drill chart.  I have this one hanging above my bench:  http://www.accuratebushing.com/pdf/ENG-FractionalChart.pdf (it has one typo -- 3/16" says 13/16" so smudge out the extra 1 digit).  This was incredibly useful for conversions, and choosing drills of course.  I could use a calculator, but the chart was better because I could skim up or down looking for the closest "standard" imperial size that matched the metric measurement.  It'll make your conversion work go very fast.

You'll discover that many of the measurements really are "imperial" anyway, probably coming from the history of the industry.  For example, Stew uses 6.3mm (1/4") square stock for the cross head slide bars.

I only had one issue with conversions (that I recall) and this was with the slide bar (part MP5).  I chose to use 10-32 threads which was very big for the part.  I believe 8-32 would have been more appropriate.  I'll keep thinking if there were others.  Hopefully I pointed those out in my build log -- but that's a mighty long thing to read through.

Enjoy the build!  I certainly am :).

Todd

Offline kvom

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Linear dimensions translate easily in the shop with a simple calculator.  Fasteners are very rarely critical, and the closest size imperial fastener can be substituted.

Offline gerritv

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I use a caliper with mm/inch button. When deciding what size stock to use (while anaylizing a set of drawings) I dial up what the drawing says, flick to Inch mode and mark down the converted size. Afterward I go through and decide what stock to use, adapting mating dimensions to suit the raw material.

With a chart the same approach works as well. One benefit I found was that I could find errors in the drawings plus thinking along the way what the best build and machining sequence would be. Essentially you end up looking at the drawings more closely.

Gerrit
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Offline tinglett

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And on kvom's point with fasteners I found myself getting out the "bolts" (from microfasteners or place like that) and having a look to see what size looks nice.  I tended to err toward smaller sizes because it made the engine look bigger.

When threading rod, I would use whatever matched it best.   Since the rod I selected was imperial, this was pretty straightforward.  Although I kept thinking #5 would be nice for 1/8" rod, but doesn't seem to be common in the known universe.  Well, the universe I know so far :)

Todd

Offline tinglett

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Notes for an imperial build of the pottymill horizontal engine

Since I'm done with my build I thought I should record some notes on what changes I made for this engine for an imperial build.  In general I continued to use lengths and widths in mm, adjusting somewhat for common barstock sizes.  For example, for 6.3mm square barstock for the cross head slide bars, I used 1/4" since that is nearly identical.  Similarly, I chose rod diameters that matched what I could purchase.   As I chose parts to build, I tried to always look forward so I could bore a hole first using my imperial tools, and then turn whatever fits the hole as a later part.  When operating my dials I had to convert mm to inches.  I would write the conversions onto a paper copy of the print in case I needed to align parts later.

Here are some specifics that I jotted on my prints.  There is a lot of info here so hopefully I didn't screw up.  This isn't any fun at all to read, and you need the prints anyway, so hopefully if you are building this engine in imperial you can skim down and find the part and see if this jibes up with what you were planning to do.  I think you'll find most of these are obvious choices.  The main thing is that once you've made a choice you need to cross check what other part it impacts.  I try to point that out for each part below.

Cylinder Front Cover:  I replaced M8 with 5/16-24 threads and used a 3/16" piston rod so drilled accordingly. I used clearance holes for 4-40 screws.
Cylinder Back Cover: I used clearance holes for 4-40 screws.
Piston Rod Guide:  M8 replaced with 5/16-24 threads.  I used 3/8" hex brass as being approx 10mm.
Cylinder: M3 cover screws replaced with #4-40 screws.  4mm air ports used a #21 drill.  3mm air ports used a #31 drill.  M2 valve box screws replaced with #2-56 screws.  I turned larger diameter aluminum down to 32mm external diameter, but 1.25" should work fine (I didn't have any handy).  I believe I bored the cylinder to 15/16", but I would bet 7/8" would work just fine, too.
Valve Chest:  clearance holes for 2-56 screws to attach to the cylinder.  M2 replaced by 2-56 for the valve rod guide.  3mm air ports were #31 drilled.  4mm input air port was #21 drilled.  I tapped 3-48 screws for the air connection (2-56 would be fine, too).  The main 6mm bore was drilled/reamed to 0.249" and I turned the valve piston to fit.  The wider 7mm counterbore for valve guide stuffing box was drilled with a J bit.
Valve Chest End Closure:  turned to fit the 0.249 valve chest bore.  I got the right size by test fitting and made from 5/16 aluminum rod.
Air Connection:  I used clearance holes for 3-48 screws.  The 2.5mm air passage was made using a #39 drill.  I drilled/tapped the input for 10-32 so I could use quick fittings from LMS that are this size.
Valve Guide Stuffing Box: I turned the outside to fit the J-drilled hole in the valve chest which is 0.276, but I did test fits to get it snug (but not press fit -- I could pry it out).  I drilled the cup using a B drill (0.238).
Valve Rod Guide:  I turned the 6mm end to fit the B-drilled hole in the cup side of the stuffing box.  I drilled 2-56 clearance holes for screws.
Piston Valve:  I turned it from 0.250" rod test fitting to my under-reamed 0.249 valve chest.  I drilled/tapped 4-40 threads and made a 0.125" counterbore for a 0.125" valve rod.  These are very tight measurements -- in my first attempt I recall the 4-40 weakened the part too much as I drilled too deep.

Piston Rod:  I made this from 3/16" rod which matched the hole in the front cylinder cover.  I threaded the piston end with 6-32 and the cross head end with 8-32.
Piston:  I tapped 6-32 and counterbored 3/16" diameter for a good fit with the piston rod.  I turned the piston to fit the cylinder by test fits.  I got the packing groove a little deep on mine, but that may be better than too shallow.
Connecting rod:  I turned this from 1/4 x 3/4" bar stock.  I drilled the crank end to 5/16" and used a filing button to file the end to 1/2 inch diameter as the button was made from 1/2" rod.  I drilled tapped the holes in the other end for 2-56 screws.
Little End Bearing:  I made from 5/16 brass rod (fits the conrod) and drilled with a D bit (0.246).
Bearing Brasses and cap:  I drilled #43 clearance holes for 2-56 screws to replace the 2.5mm holes.  I assembled and drilled/reamed the 6mm through the brasses to an over-reamed 0.251"
Cross Head:  I made a note that the overall length of the cross head is 19mm (14mm+R5) giving a shoulder of 3mm.  I drilled/tapped M4 as 8-32 to fit the piston rod.  I drilled the 6mm with a D drill (I had initially had B but scratched it out.  B should work, but then the pin, thrust washer, and conrod bearing must match)
Cross Head Pin:  I turned this down from 0.250 stock to 0.246 to fit the D-drilled hole in the cross head.  I threaded the ends with 10-32, but in hindsight I'd recommend 8-32.
Slide Bar:  Use 1/4" square bar stock.  Drill/tap the M4 to match the cross head pin.  I used 10-32 but would recommend 8-32 to fit the 1/4" square bar stock.
Thrust Washer:  Use 3/8" brass rod and drill with D to fit the cross head pin.
Slide Bars:  Use 1/4" square bar stock.  I used 2-56 screws so made #43 clearance holes.
Slide Bar Spacers:  Use 1/4" rod and drill #43 clearance holes for 2-56 screws.
Valve Rod Guide:  Use 1/4" square bar stock.  Drill #29 for extra clearance for the valve rod.  Drill #43 for clearance for the 2-56 screw.
Slide Bar Pillars: I used 2-56 screws for attaching the valve guides.  I used 4-40 screws for attaching to the base plate.  I made them from 5/8 brass rod.

Valve Rod:  Use 0.125" rod.  Thread 4-40 on one end to fit the piston valve.
Eccentric Rod:  Use 0.125" rod.  Thread 4-40 on both ends.
Valve Rod Coupling:  Turn from 1/4" rod.  I threaded for a 6-32 set screw (much bigger than M2.5).  Drill the 3mm hole as 0.125 to fit the piston rod.  I turned and threaded the M3 end for 4-40 threads.  I turned this to 0.125 (big for 4-40) and threaded only the end for 4-40.  The threads stopped short so the eccentric rod coupling could freely hang on the unthreaded 0.125 section.
Eccentric Rod Coupling:  Tapped for 4-40 to accept the eccentric rod threads.  Drill 0.125 to fit on the valve rod coupling.
Eccentric Assembly:  I used metric measurements except for the offset shaft hole which needed to match the crankshaft, and I tapped for a 6-32 set screw.  Make  "outer" parts first and turn each inner part to fit an already-made outer part.  The offset is important for timing so I kept it at 3.5mm (0.138").

Bearing Stands:  Use 5/16" x 1.5" bar stock.  Drill the 14mm hole as 9/16.
Main Bearings:  Turn 5/8 brass down to fit the bearing stands.  I went for a press fit, though loctite is fine.  I chose to drill the 8mm as 5/16.  If you are making the crankshaft from parts, make this fit the rod you are using for the crankshaft.  5/16 is probably right and overreaming would be a good idea.
Crank Shaft Assembly:  I turned this from a single chunk of bar stock and turned it to fit the main bearings.  The crank needs to fit the conrod bearing brasses and 0.25" is about right.

Flywheel:  I made mine from scratch from 5" diameter steel.  The hub needs to fit the crank shaft, of course.  I used a 6-32 set screw at an angle.

Base Assembly:  I used 1/4 thick aluminum.  I converted all the mm measurements to inches and wrote them on the print.

Hope that helps!  It's a great engine.  Build it and you'll see :)

Todd

Offline b.lindsey

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Re: Are there plans for the potty mill engine in imperial measurements?
« Reply #10 on: May 23, 2015, 07:41:45 PM »
Thanks for recording and posting these Todd. It will be a handy reference once this build gets to the top of the list.

Bill

Offline GordonL

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Re: Are there plans for the potty mill engine in imperial measurements?
« Reply #11 on: May 23, 2015, 08:16:57 PM »
I built the engine and I think that I have the drawings in inches. I will look through my stuff next week when I get a chance. If I made drawings I should have some copies with notes about my errors in conversion.

Offline GordonL

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Re: Are there plans for the potty mill engine in imperial measurements?
« Reply #12 on: May 28, 2015, 12:35:35 PM »
I did make the engine and made CAD drawings in inch sizes. Unfortunately sheet 2 seems to have been erased and all I have is a blank sheet. That sheet has many of the small parts so it is a necessary part of the set. Sorry.

Offline Chris Leverett

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Re: Are there plans for the potty mill engine in imperial measurements?
« Reply #13 on: June 03, 2015, 08:26:09 PM »
Thanks guys, I too have electronic calipers with the inch/metric feature. But all my gauges are imperial so changing those out would be too costly. I was just leary  of using a chart to convert the measurements. I didn't know if it would work out well having never done something like that before. I was just thinking about rounding the measurement of one part up and rounding its corresponding part down and they not fitting.
Chris

Offline tinglett

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Re: Are there plans for the potty mill engine in imperial measurements?
« Reply #14 on: June 03, 2015, 10:48:09 PM »
Chris,

Yeah, that is true.  When rounding, in particular for imperial rod or thread sizes, you need to catch the mating uses of it.  Something could be missed.  When I'd convert anything, I would pencil it in the plan.  Then at least later on I'd know what I dialed when I made the part -- in case it mattered later when I'm making a mate.

Maybe it's best to comb through and convert the whole plan up front, but it's a little engine and nothing is crazy-expensive if you make a mistake.  But it would be frustrating.  The only thing that bugged me was making the cross head pin with 10-32 as it was a bit big.  It would have been easy enough to re-do, but it was "good enough."

Todd