honing or lapping a cilinder

[erikverkade]

So hey there again.
A little while ago I decided to build a scaled up version of the ''boll aero 18'' converted to glow ignition. It seemed like a easy first project and apparently a little to easy because I'm 12 hours in and I'm pretty much done.

But I've run into a problem, The finish of the bore is to rough, It will run for 3 or 4 seconds and after that the piston will be worn down to the point that it doesn't seal enough anymore to run. I've made 3 pistons already in the hope that it would smooth the bore out a little but no matter how much I like turning stuff, it does get a little old at some point.

So here is my question: Do any of the more experienced folks here know of an easy and especially a cheap way to polish the bore? It's about 3/4 of an inch (19mm) and made out of mild steel. I don't have the guts to sand or polish it by hand because it has a beautiful 0,003 inch pinch and I don't want to mess that up as I tend to do.:hDe:

Kind regards, Erik (the newbie among you)

 

[Mechanicboy]

The cylinder must be tapered due heat of expansion hence the engine lost compression when engine got hot of working temperature. Also it is not fault with piston. I prefer lapping method to controll fit of piston to cylinder. Correct tolerance: Piston is loose in BDC and tight in TDC when the piston/cylinder is oiled. Tight fit when both is dry and free of oil. Lapped surface will be dull gray before the engine is in use.

 

[petertha]

This topic gets discussed quite often on HMEM, both in question form & inside project builds. Just search on 'lap' & 'hone'. But this link might be more closely related to your particular application.
http://www.modelenginemaker.com/index.php/topic,1908.0.html

 

[Mechanicboy]

Here is better to read.. http://modelenginenews.org/techniques/cylhone.html

 

[MachineTom]

How did you finish the cylinder when you built the engine? If ypu just bored it, that was half the job. After boring ot needs to be honed otherwise it acts like a file on the piston rings. A spring hone is the tool, also called a brake cylinder hone. No need to lap after that. Lapping could be used to correct any taper in the cylinder, but it needs to be honed first.

 

[Mechanicboy]

How did you finish the cylinder when you built the engine? If ypu just bored it, that was half the job. After boring ot needs to be honed otherwise it acts like a file on the piston rings. A spring hone is the tool, also called a brake cylinder hone. No need to lap after that. Lapping could be used to correct any taper in the cylinder, but it needs to be honed first.

Brake cylinder hone is not a good tool to create the model engine without piston ring.

Bore the cylinder with fine feeding first, then lap the cylinder with lapping tool to all tool marks by lathe work is disappeared. Do not make bellmouth in both ends of cylinder caused by lapping! Cylinder will be parallel first before the piston will create tapered cylinder in the last lapping.

Then make the piston and lap the piston. Wash the cylinder and piston clean, then check half of length of the piston is entered into the cylinder then stop to lap the piston (piston must stop about 50% into cylinder and no enter whole way into cylinder).

The last work to make cylinder tapered: Smear the piston and cylinder with very fine lapping paste or chrome polishing paste to example Autosol chrome polish or Tormek polishing paste for wood cutting tools. Put piston into cylinder while the piston is rotating in both way under light pressure until it will stop. Take out piston of cylinder and wash clean, check tolerance is correct: Piston is loose in BDC and tight in TDC with oil on piston/cylinder. Dry and no oiled piston/cylinder: Piston goes tight in cylinder. The surface will be dull grey and have honing marks due better to keep oil into the surface such as "hill and valley".

Running-in the engine wil improve correct fit of piston/cylinder.

 

[Chiptosser]

I wouldn't recommend anyone using a spring loaded (hone) Glaze breaker on anything, other than a brake cylinder.

This type of tool will follow the existing contour, it will not true a cylinder.

A true hone or rigid hone will true an out of round cylinder. It will also taper a cylinder.

If you are not familiar with using a hone of any type, practice on similar pieces to get the results that you desire.

 

[erikverkade]

Here is better to read.. http://modelenginenews.org/techniques/cylhone.html

I'd like to thank you all for your help. And I'l be taking a look at those from modelenginenews. Those look quit promising.

 

[royal-corsair]

have the cylinder "jig ground".

 

[erikverkade]

have the cylinder "jig ground".

Or I could just buy a compleet engine, then I won't have to bother at all.
The point is doing it yourself.

 

[Mechanicboy]

have the cylinder "jig ground".

Cylinder "jig ground"

We do not need cylinder "jig ground" to grind the cylinder..

Better explained lapping operation to read here..

http://easyweb.easynet.co.uk/~chrish/cancelled%20account/t-lap.htm

Erik,.. the text who is wrote in this link: At this stage, it is usual to finish mating the two parts by using metal polish and briefly using the piston to lap the bore directly.

Also you are creating tapered bore when you are lapping the piston into the cylinder in last stage. Remember: Allways lap the piston from BDC to TDC.

 

[Billitmotors]

G'day Eirckwerkade
Honeing is not that hard it is just time consuming.
Find or purchase a piece of brass a little bigger than the bore of your cylinder. set it in the chuck with a bit over one and a half to two times the length of your cylinder and machine it down to about .001" - .002" smaller than your bore so that it will be a neat slid fit into the cylinder. Centre drill the end and using a13/64" or a 5.1mm drill, drill into the end around one and a half inches. Using a 1/4"BSW or UNC taper tap wind it in about half way or until it is about two threads past the tapered section of the threads. It is worth making a few laps so that when you change your grit size you can use a new lap.
Using a hack split the lap axially down the centre for one and a half inches. and then run the tap back in to the same depth as before to get rid of any swarf or burrs, and then file any burs from the outside along your hacksaw cut. Also make a large washer to slide over the lap that stops the cylinder from hitting the revolving chuck as you traverse the cylinder along the lap. Also Make some sort of holder that the cylinder will fit into and you can attach to the cylinder that will protect your hands.
Purchase some fine grit diamond paste, you can use carborundum paste but the diamond paste is cheap on ebay and works much quicker. You don't need very much.
Get a piece of steel plate or a large piece of flat high speed steel smear a small amount onto the steel and press and roll the lap onto the steel with the paste between to charge the lap.
Loosly screw an allen headed screw onto the lap set your lap in the chuck with the large washer next to the chuck jaw and slide the cylinder onto the lap with the top of the cylinder next to the chuck jaws to preserve your taper.
Make sure that you cover your lathe bed completely. I put rag down first and cover that with plastic.
Set your lathe speed to very slow and turn it on, traverse the cylinder back and forward along the lap. You will need to keep tightening the allan screw ever so slightly as the lap and cylinder wear.
Your aim is to get an even mat finish with crisscross polish marks as well as what ever taper you want to have.
Rob.