Webster Build Questions and Progress

[minh-thanh]

Here is a picture I took at x4 magnification on my phone, 19.7mm diameter cylinder 60mm long with a 2x10 mm HSS tool bar
With a "straight" cylinder you will see they are almost touching everywhere
"Straight" : in quotes because I am not sure if they are 100% straight but good enough .
If there is a taper they will not touch at the BDC position of the piston
No need to be mirror like, just smooth
----

 

[Jasonb]

Have a read of this thread of mine starting with machining the cylinder, it then goes onto lapping it, making the piston and lapping the piston into the cylinder. This is what I do for IC engines with no rings.

https://www.model-engineer.co.uk/forums/topic/jasons-firefly-46-build/#post-93931

 

[Danuzzo]

Here is a picture I took at x4 magnification on my phone, 19.7mm diameter cylinder 60mm long with a 2x10 mm HSS tool bar
With a "straight" cylinder you will see they are almost touching everywhere
"Straight" : in quotes because I am not sure if they are 100% straight but good enough .
If there is a taper they will not touch at the BDC position of the piston
No need to be mirror like, just smooth
----
[Photo removed]...

Thank you. Will put a small lathe tool in mine later to see if I can duplicate what you are showing. Will report back accordingly.

 

[Danuzzo]

Have a read of this thread of mine starting with machining the cylinder, it then goes onto lapping it, making the piston and lapping the piston into the cylinder. This is what I do for IC engines with no rings.

https://www.model-engineer.co.uk/forums/topic/jasons-firefly-46-build/#post-93931

Thank you. I like Ramon's lapping tool. Is there any instruction or diagram available? In the meantime, I intended to fabricate one similar to the one on https://www.adriansmodelaeroengines.com/catalog/main.php?cat_id=178.

 

[Jasonb]

This thread of Ramons should give you some weekend reading.

https://www.modelenginemaker.com/index.php/topic,1908.0.html

 

[minh-thanh]

.

just a simple way to see if the cylinder surface needs lapping.
@Jasonb has given a lot of information about lapping ( ), watch and practice
As I read you are not using rings so you don't need a certain exact diameter , if 25mm diameter is not good then lapping to 25.05 or 25.08 .. and making the piston fit the cylinder diameter
It's a challenge And if you succeed with ringless piston on this engine then other type engines are no problem for you .

 

[Danuzzo]

just a simple way to see if the cylinder surface needs lapping.
@Jasonb has given a lot of information about lapping ( ), watch and practice
As I read you are not using rings so you don't need a certain exact diameter , if 25mm diameter is not good then lapping to 25.05 or 25.08 .. and making the piston fit the cylinder diameter
It's a challenge And if you succeed with ringless piston on this engine then other type engines are no problem for you .

I tried the lathe tool bit "trick". I like it. My current diameter is .872". I think I will be able to get those lines out by going to .873". Finished the cylinder today, except the lapping. I figure that with the cylinder lapped to .873", if it does not work out, I can always latter lap it to .875", and make a new piston with rings per the plans.

 

[Danuzzo]

I like Jasonb's recommendation or silicon carbide as a lapping medium. I have decided to go with a lapping tool similar to the one on the Adrians Model Aeroengines for which a link was provided earlier today.
Now I have to figure the grit I need. I am thinking 600-800. But, I am inviting other suggestions. I only intend to remove about .001 to get the surface scratch free.

 

[minh-thanh]

I am thinking 600-800.

Hard to answer.
It depends on the tool marks, big or small, deep or shallow
..on the surface
Try with 600, if you find the abrasive process is too long then reduce to 400. If the process is ok for you then you start at 600

 

[Danuzzo]

Well I have been looking for silicon carbide in 600-800 grit and have not found any at a suitable price with shipping. I did find a Zirconia Alumina combined with Aluminum Oxide in 600-800 grit. I am wondering if this is a suitable replacement for silicon carbide??

 

[Jasonb]

Look on ebay for lapidary powder (stone tumbling/polishing) it is not expensive you only need buy very small amounts $5 for 50grams inc shipping will last a lifetime. Also the woodworking guys use it for lapping tools.

Ideally work through the grits as I showed in the link to my Firefly otherwise you will be there all day if you just use the finest finishing grit

 

[Richard Hed]

Well I have been looking for silicon carbide in 600-800 grit and have not found any at a suitable price with shipping. I did find a Zirconia Alumina combined with Aluminum Oxide in 600-800 grit. I am wondering if this is a suitable replacement for silicon carbide??

You might try looking up carbides used for telescope making.

 

[Danuzzo]

Look on ebay for lapidary powder (stone tumbling/polishing) it is not expensive you only need buy very small amounts $5 for 50grams inc shipping will last a lifetime. Also the woodworking guys use it for lapping tools.

Ideally work through the grits as I showed in the link to my Firefly otherwise you will be there all day if you just use the finest finishing grit

I saw your grits were 320, 600, and 1000. I was able to find and order 400 and 600 grit rock tumbler silicon carbide on Ebay. I would hope that the 400 is close enough to the 320 to suffice. However, I could not find 1000 grit. They did have 1200 grit from the same vendor. Would that be too big a jump from 600? Or, better yet, will 600 be enough for purposes of a cast iron piston in a mild steel cylinder?

 

[Jasonb]

400 and 600 will probably do the job

 

[Danuzzo]

Will not be receiving the silicon carbide till later this week. Made the cylinder lap very similar in design to the one in Adrians Model Aero Engines linked above and here (https://www.adriansmodelaeroengines.com/catalog/main.php?cat_id=178). Here is a photo (not the best photo).

The lapping portion is about 1.680" long, and .871 diameter. I did use a taper tap with a 1/4-28 thread. The taper starts about 1.250" into the thread (this is where the screw engages enough to start increasing the diameter of the lapping portion), which is approximately 3/4 the length of the lapping portion.

In the instructions linked above (Adrian's), if I understand correctly, the cylinder is inserted with the bottom end of the cylinder first and lapped that way, as he describes. I am having difficulty grasping the concept, and here is the reason. When I tighten the screw in the lap, the port end (tailstock end of lathe) of the lap is of a greater diameter than the chuck side end of the lapping portion. For example, I can adjust the lap to .875" at the tailstock end and it measures .8735" at the chuck end. I did think this would be remedied by starting the taper of the thread so far into the thread. But, I am also thinking that it may not matter. So, the question is whether this is an issue? Should I use a design like Ramon's linked above where the adjusting screw is in the middle of the lapping portion? Would this cause a larger I.D. (taper) at he closed end of the cylinder?

Another question to help me understand the concept, how far does the piston in the Webster travel in relation the location of the intake/exhaust port?