Scaled up Cracker

This will be my build log for a scaled up Cracker.

I had every intention of making 2 identical Crackers for my son and daughter. Then I saw xo18thfa's Nina build and I decided to make a scaled up version of the Cracker. I am scaling up to 1/16th for every 1mm. I will still make the smaller Crackers, I just want to make all the mistakes on the bigger one first. ;D

First challenge was how to edge mill the base plate. My vise jaws are not that tall and I was getting chatter. I clamped 2 123 blocks to each side of the plate and butted up to the vise to help.

I also learned a valuable lesson: Use a smaller cutter at higher rpms will give a better cut than a larger cutter at low rpms when you use a good cutter! I have a 7/16ths data flute cnc cutter as part of a craigslist buy. Run at 1600 rpms this thing went right through with no chatter and excellent finish. My 3/4 end mill from the ubiquitous Chinese 2 and 4 flute end mill set just made a mess.

I think that with my 6x26 mill going above 1/2 is a waste. The bigger end mills take some big bites but have poor finish and hammer the work. It could also be that they are dull compared to the good bit. I will be buying more of this brand if I can find it. I also think that always going to the biggest cutter has held me back from accuracy and good finish. That little cutter proved that!


Next step was to make the ends square and to size. The y in the 6x26 is fairly small and with the 5" vise in the middle slots of the table I have no room for a 4" y cut! So I took the fixed jaw face off and mounted it behind the original position and used a parallel that was the same height as the fixed jaw without the jaw face. Worked great! I did have to space the plate out to get enough y travel so I used a 123 block again. Handy buggers!


Here are some pics of the scale difference:




This may take a while but I will update as I can. Next project is how to turn the wheels accurately. They are all faced. Do I make a stub mandrel? The axle is 3/16ths so it would be a small axle with the treads still to form.

I can hold in a 3 jaw and machine face features and boss for grub screw, then mount on mandrel.

I have an emergency 5c collet I can use but how do I turn down the OD to size? I can mount the blank in a 4 jaw and machine one side then the other but I have very little to grab on to and clocking in the other side would be tight.

Oh well. This is one of the reasons I love this hobby so much; no outside thoughts are allowed to intrude on shop time as it is all I can do just to make the small stuff work!

Later,

Sean

Great! It's nice to see so many locomotives popping up here, and even better when they're Crackers.


When I turned the wheels for my two, I added a boss to the inside face, to have a place for securing setscrews. I used the same boss to hold the wheels while facing them, then set the tool angle to cut the wheel flange, and the compound angle to cut the tapered 'tire' surface. I drilled and reamed each wheel at the same time it was in the collet for tapering, so the only runout would be on the two faces or boss, and not on any of the rolling surfaces.

I guess you've already cut your wheels to the drawing, and all my babble is useless.

Vernon said:

Great! It's nice to see so many locomotives popping up here, and even better when they're Crackers.


When I turned the wheels for my two, I added a boss to the inside face, to have a place for securing setscrews. I used the same boss to hold the wheels while facing them, then set the tool angle to cut the wheel flange, and the compound angle to cut the tapered 'tire' surface. I drilled and reamed each wheel at the same time it was in the collet for tapering, so the only runout would be on the two faces or boss, and not on any of the rolling surfaces.

I guess you've already cut your wheels to the drawing, and all my babble is useless.

Click to expand...

That one busted me too Some of Bog's friction-turning ideas help if you've boxed yourself into a corner with the wheels, but in the end I think I ended up fastening gears or collars to them to replicate the inner hub I didn't put there.

Thanks guys!

I haven't done anything more stock could take care of!

As for Vernon's idea, I think that will work. I will turn the boss for the grub screw a convenient size and use the collet chuck.

Thanks guys!

Sean

Very helpful post Sean.

Couldn't you have done this earlier? Like before last Saturday!!! ;D

I did my Toy Loco frames then and had fun with a 'big' mill. I'd wondered at the time whether a smaller end mill might not have been better. Well I have a couple more sheet metal parts to do...I'll give it a go.

I also like the pics using the 123 blocks.

Thanks Zee!

I always tend to go for the big guns. I have a roughing shell mill I got as part of a trade. Thing is 2.5 dia. It will hog out the side of a piece of 3/8 plate but it is useless to me for facing a large path which is what I wanted to do!

I have tried fly cutters and using Bogs' cutter geometry have had very nice results in finish but accuracy was ok. Seems the hammering was causing issues, work lifting, ect...

I recently received Rudy Kouhoupts Shop Wisdom vol. 1 via inter-library loan and I noticed that all his parts seem to have the tell tale marks of multiple passes with an end mill and a small one to boot! So I tried it and it worked perfectly! I even made some "test cuts" at .125 doc full width of the mill and it worked fine (buried the mill though :big. Seems the smaller cutters were actually cutting and less force was transmitted to the work. Imagine a cutter, well, cutting!

These are the lessons of the shop and I will have them forever. Between that and here I feel like I am "stealing the trade" to quote my favorite author...


Can't wait to see your latest progress Zee... Little Loco's rule!

Sean

Oh, I can't wait to see this project unfold. A larger size "Cracker" is going to be a gas. I like what has taken shape so far.

BC1
Jim

Seanol said:

Thanks guys!

I haven't done anything more stock could take care of!

As for Vernon's idea, I think that will work. I will turn the boss for the grub screw a convenient size and use the collet chuck.

Thanks guys!

Sean

Click to expand...

Vernon's idea is what I did for Nina. It allows you to all the machining steps in one set-up in the lathe chuck and not have to do a stub mandrel.

Bob,
I agree. I was not too keen on making the rim on a 3/16ths mandrel!

When I do the smaller Crackers I will use the same idea. May even cut all wheels at the same time.

Nothing says progress like wheels on a chassis!

Sean

Seanol said:

Bob,
I agree. I was not too keen on making the rim on a 3/16ths mandrel!

When I do the smaller Crackers I will use the same idea. May even cut all wheels at the same time.

Nothing says progress like wheels on a chassis!

Sean

Click to expand...

I turned some 1.125" dia wheels on a .187" mandrel. I would not have dared anything bigger.

Guys,
Busy week so little garage time. I am going to my first Steam Up this weekend so all is not lost! ;D

As for my Cracker I am scaling up a bit and I wonder if I scale up the engine if the dimensions will work out. I have:

From a .315 bore to a .500 bore (from 8mm to 12.7mm)
From a .217 stroke to a .348 stroke (from 5.5mm to 8.8mm)
Flywheel from 1.181 to 1.875 (from 30mm to 47.6mm)
Port size from .188 to .1875 (from 3mm to 4.7mm)

I just want to make sure an oscillator will scale up reliably or do I need to find a plan for the proper size oscillator?

Thanks for looking,

Sean

Hi Sean. Oscillators scale just fine. You should be good-to-go with these numbers.

I heard a long time ago that where there is metal-on-metal contact the metals should be dis-similar. The cylinder and port face is a prime example. For the Nina build I plan to use bronze for the cylinder and brass for the port face. I don't know if that is true

Bob,
Thanks for that.

I am going to build the cylinder and port face of brass but the piston out of stainless.

My understanding is that brass on brass is ok? ???

If not I can always make it steel. Of that I have plenty!

Anyone need any AR plate? :big: :big: :big: :big: :big:

Sean

It may be. The dis-similar metals thing could be an old wives tale. I guess experiment and see. It will probably be fine.

Bob,
On dissimilar metals you are correct as far as I know.

Brass seems to be exempt from that as I have seen many smaller engines built with brass pistons and cylinders.

I don't know how well it will work on steam though.

See you tomorrow,
Sean

Hi Sean

Good start on the Cracker Thm:

As to the brass-on-brass - that works fine IMHO, as long as you give the engine a drop of oil on the port face before running. The biggest issue on an oscillating engine is the spring pressure that keeps the cylinder and block faces together. The spring pressure should _just_ be high enough to prevent the two parts to blow apart at normal operating steam pressure. I think there may be many people who have built oscillating engines and had poor performance from them because of too strong spring pressures.

Regards, Arnold

Arnold,
Thanks. I will try to make the spring light enough to reduce friction and strong enough to seal.

Your build was also one that inspired. Thank you for that.

Sean

Brass on brass has been working fine in my little Cracker. I suppose it could wear more, but that'll be a while and I can always make new parts if needed.

What a weekend!

I went to my first steam up on Saturday and had a blast! Learned a lot about steam engines and their operation. Got a tour of an amazing shop from one of the members (7.5 gauge is unbelievable!) and in general learned more in one day than I have in a long time. Thanks to xo18thfa for the introductions and lessons!

Sunday was the entire family sick in bed! A long day...

Today, shop work had to be done! Zee is on his sixth page and making parts like a CNC! (J/K Zee!)

So I started on the wheels.

First, I want to let everyone who worries about posting that your stuff is valuable. I can't remember where I saw this version of chucking a part but it was an Ah Ha moment! I was placing large parallels against the chuck and it was very squirrley.
Instead I did this:



Worked much better. I turned a step to make the wheel blank fit a 3/4 collet. Mounted the collet chuck and turned down to OD before making the tread. Turned the front tread on each wheel without moving the cross slide and did the same thing to the side. Rounded the outer edge and I have 4 wheels that seem to be dead on:



I now needed to drill the axle hole. I center drilled and drilled 7/32 through. Then used my dbit reamer for an on size .250 hole. I took a spare .250 endmill and ground the shank down (after mic'ing it to make sure it was on size). Put on back and side clearance and it cut very well.



I did have one small mishap. I was wearing my watch in the shop and while turning the side to depth I had a 6 (or is it a 9 :big land under my watch band. Ouch!


That is all for now,

Sean

P.S. sorry about the pic quality. Kids gravity tested the digi camera and gravity won!

Seanol said:

What a weekend!

I went to my first steam up on Saturday and had a blast! Learned a lot about steam engines and their operation. Got a tour of an amazing shop from one of the members (7.5 gauge is unbelievable!) and in general learned more in one day than I have in a long time. Thanks to xo18thfa for the introductions and lessons!

Sunday was the entire family sick in bed! A long day...

Today, shop work had to be done! Zee is on his sixth page and making parts like a CNC! (J/K Zee!)

Click to expand...

Nice wheels Sean. I hope the family gets well soon.

Was that meet near Las Vegas? Once in a while I get out there on business.

Don't be setting me up for comparison! Even if it's just counting posts :big: (And thanks for the compliment.)