Machining aluminum crankcases for model airplane engines questions

[timo_gross]

Just an odd comment about boring... I.E. parallel bores, not my posts.
"If its out by 0.002 inches over 4 inches is that too much?" - YES!
Many thing a lathe is really accurate and useful if you bore into a workpiece held in the chuck. - Not so. The travel of the tool describing a straight line through the workpiece relies totally upon the accuracy of the alignment of tailstock travel or saddle (for saddle mounted tools) to headstock (quill)... 1 degree misalignment gives you a 2 degree included angle taper.... NOT good for maintaining compression in an infernal combustion engine, hydraulic cylinder, etc..
BUT a tool mounted in the Quill, that traverses a workpiece mounted on the saddle or tailstock will always produce a parallel bore. - you just have to set the alignment of the workpiece to the alignment of the quill/mainshaft. I.E. a rotating tool describes a true circle, which will produce a "true" cylinder when traversed through a workpiece in a straight line.
Whatever anyone else shows on U-tube, you cannot escape geometry. (And "Modern" production methods - that I was taught in the 1960s...).
Hope that helps you get parallel bores on your aero engines? Makes a big difference to compression and running-in. (friction/wear) and lifetime of bearings.
Cheers.
K2

You can explain it to me, but you cannot understand it for me. Having trouble to follow. ..... trying to explain it to myself.
Set the compound to an angle, keep the saddle stationary and move the boringbar (mounted on compound) into the workpiece that rotates in the chuck. It results in a taper. (done that on purpose.... to get a taper)
Now I swap. I put the boring bar into the lathe chuck, and the workpiece into the toolpost. What do I get?
A boring bar with enough clearance should still make a round hole ? When an Endmill is used it will make some sort of an elypitical hole?( from what I guess)

Greetings Timo

 

[Steamchick]

Correct:
A boring bar with enough clearance will make a round hole. It is rotating in a circle. - This circle does not change size. so it makes a parallel hole/bore as the workpiece passes it, along the line that the workpiece is set at.
A workpiece in the lathe chuck and boring bar or end mill in the tool-post/on the cross-slide/in the tailstock, (etc.) will make a tapered circular bore, the taper designated by the misalignment of the bed (motion) to the main shaft. (if 0.001" in 1 foot will make that same taper!). A circular bore because it is rotating in a circle, not an ellipse.
does this help?
Sorry my words are confusing.
K2

 

[jimbo7636]

Just an odd comment about boring... I.E. parallel bores, not my posts.
"If its out by 0.002 inches over 4 inches is that too much?" - YES!
Many thing a lathe is really accurate and useful if you bore into a workpiece held in the chuck. - Not so. The travel of the tool describing a straight line through the workpiece relies totally upon the accuracy of the alignment of tailstock travel or saddle (for saddle mounted tools) to headstock (quill)... 1 degree misalignment gives you a 2 degree included angle taper.... NOT good for maintaining compression in an infernal combustion engine, hydraulic cylinder, etc..
BUT a tool mounted in the Quill, that traverses a workpiece mounted on the saddle or tailstock will always produce a parallel bore. - you just have to set the alignment of the workpiece to the alignment of the quill/mainshaft. I.E. a rotating tool describes a true circle, which will produce a "true" cylinder when traversed through a workpiece in a straight line.
Whatever anyone else shows on U-tube, you cannot escape geometry. (And "Modern" production methods - that I was taught in the 1960s...).
Hope that helps you get parallel bores on your aero engines? Makes a big difference to compression and running-in. (friction/wear) and lifetime of bearings.
Cheers.
K2

Yes I guess that for the ML Midge crankcase Im not sure if I can trust my tail stock or that the drill might not wander. So I think I will follow the advice of the "The Machinist's bed side reader" and drill, bore, then ream. I also agree that boring wouldnt work if the lathe was not aligned but I have put in some time to resolve that issue and I will continue to also watch and see if the tail stock is also aligned. So I will probably experiment with both approaches and see what I can determine.

 

[Steamchick]

Hi Jimbo, As I am only here to advise, not dictate, and there are many different ways to do many jobs, I can only repeat the geometrical edict. I was taught this at 12 years old, but geometry doesn't change.
True parallel bores are made with a rotating tool traversing a straight line through the job. Tapered bores are made with every tool that is stationary, traversing a rotating job along some line that is not the perfect axis of rotation.
If your lathe is set-up and retains alignment within your required tolerances, then the taper developed will be OK for you. Many DTI clock-perfect machines for hobby use deform during cutting loads, so are not quite perfect in the finished job. Industrial strength machines fair better. My lathes all deform to a degree, usually adequate, but not for precise boring for pistons.
For jobs I have made, I stopped using the faceplate or chuck to mount jobs where the bore will carry a piston without rings, because the inevitable taper could be detected by the piston being just OK at one end and tightening or loosening at the other. Honing is a non-controlled "adjustment" in more cases than you would think.
So I once learned my lesson because I had to make a boring bar and use the milling machine to re-bore the cylinder for an oversize piston. My boyhood tutor was right! I was stupid thinking I could get away with boring with the job in the headstock of the lathe.... Hence my advice to you.
A reamer will make a parallel bore, as long as it can transition far enough that the initial taper (if it has one) has passed through and clear of the bore. Some reamers are only parallel, some taper for the first third of the length. Check yours.
Enjoy!
K2

 

[jimbo7636]

Hi Jimbo, As I am only here to advise, not dictate, and there are many different ways to do many jobs, I can only repeat the geometrical edict. I was taught this at 12 years old, but geometry doesn't change.
True parallel bores are made with a rotating tool traversing a straight line through the job. Tapered bores are made with every tool that is stationary, traversing a rotating job along some line that is not the perfect axis of rotation.
If your lathe is set-up and retains alignment within your required tolerances, then the taper developed will be OK for you. Many DTI clock-perfect machines for hobby use deform during cutting loads, so are not quite perfect in the finished job. Industrial strength machines fair better. My lathes all deform to a degree, usually adequate, but not for precise boring for pistons.
For jobs I have made, I stopped using the faceplate or chuck to mount jobs where the bore will carry a piston without rings, because the inevitable taper could be detected by the piston being just OK at one end and tightening or loosening at the other. Honing is a non-controlled "adjustment" in more cases than you would think.
So I once learned my lesson because I had to make a boring bar and use the milling machine to re-bore the cylinder for an oversize piston. My boyhood tutor was right! I was stupid thinking I could get away with boring with the job in the headstock of the lathe.... Hence my advice to you.
A reamer will make a parallel bore, as long as it can transition far enough that the initial taper (if it has one) has passed through and clear of the bore. Some reamers are only parallel, some taper for the first third of the length. Check yours.
Enjoy!
K2

it sounds like a lathe that is not stiff enough would be a problem. however, I would also believe the milling machine (vertical I assume) has similar alignment and/or flexibility issues. perhaps the milling machines your refer to are stiffer and easier to align than lathes. but the relative motion of a tool and workpiece cause cutting so whether the tool is moving or the workpiece is moving should not be the difference. again I do appreciate the input and I will be making a few crankcases to test the setups on both the lathe and perhaps the mill. All that being said I did find this description in George Thomas model engineers workshop manual, which I believe is what you are trying to say as well. He spends most of his time on option A which is boring in the lathe.

 

[Jens Eirik Skogstad]

When I look at your attached picture, it seems that the engine is quite small..
My opinion is to lap the bearing to fit the shaft tightly, which ensures proper fit and surface finish of the bearing.

Minh-Thanh..
Bearings made of aluminum or bronze should not be lapped with a lapping tool because the abrasive grains will settle into the material which is soft leading to abnormal wear on the crankshaft. Best to ream bearings and let run-in do it better than we do with tools.

I'm using adjustable reamer to adjust the correct size to fit the crankshaft. And I has both adjustable and fixed reamer.

 

[minh-thanh]

Maybe...I'm a very lucky person
I have never had a shaft wear out due to abrasive grit, even though I regularly use the lapping method, with quite a few aluminum, brass cylinders and bearings.
It always gives the results I want and the engines is always running

 

[Steamchick]

Hi Jimbo.
"Exactly". Well done on your research. Sorry my explanation was a bit clumsy.
K2

 

[jimbo7636]

When I look at your attached picture, it seems that the engine is quite small..
My opinion is to lap the bearing to fit the shaft tightly, which ensures proper fit and surface finish of the bearing.

thanks for the info. I do plan on lapping the crankshaft but I havent seen recommendations to lap the crankcase.

 

[Vietti]

If you are worried about the abrasive embedding in the Al, try Timesaver. It is supposed to break down and that's why its called timesaver so you don't have to disassemble the parts to clean out the spent abrasive

 

[f2cf1g]

Try metal polish.

 

[minh-thanh]

thanks for the info. I do plan on lapping the crankshaft but I havent seen recommendations to lap the crankcase.

What really matters is the bearing's surface finish after reaming or boring
With smaller diameter and longer - the smaller the boring tool, may be it will be a problem
If I can't get a smooth surface - good fit to the shaft I will lapping
Having a good bearing finish and a good fit to the shaft ensures the engine suck the right amount of fuel - instead of having to open the needle to make it easier for the engine suck fuel and then I would have other problems.

 

[Mechanicboy]

What really matters is the bearing's surface finish after reaming or boring
With smaller diameter and longer - the smaller the boring tool, may be it will be a problem
If I can't get a smooth surface - good fit to the shaft I will lapping
Having a good bearing finish and a good fit to the shaft ensures the engine suck the right amount of fuel - instead of having to open the needle to make it easier for the engine suck fuel and then I would have other problems.

In the smaller bore and longer, use D- bit reamer after the hole is drilled.

The oil in fuel is "sealer" to preventing the false air to be sucked into the crankcase in the two stroke engine via bearing/crankshaft.

 

[Mechanicboy]

thanks for the info. I do plan on lapping the crankshaft but I havent seen recommendations to lap the crankcase.

I has build own model engines, all bearings is renamed with adjustable reamer to adapt the crankshaft.
Running-in the engine make better than we can do it with the tools.
It's oil who is lubricate and keep tight against air leakage and remove heat by friction.

 

[Mechanicboy]

It's not necessarily to lap the bearings after boring and reaming the bearings. See at the surfaces is smooth.

 

[Steamchick]

Maybe I was taught in a different school to some of the opinions here, but this is what I know:
The reamer should not give a surface that needs lapping. Honing/lapping are processes that use abrasives to grind-off the peaks from a "pointed tool" type of surface. The reamer is a single long edge that finely scrapes the surface so no grooves/peaks exist for lapping.
Metal polish is almost the finest grinding paste you can get. But still grinding paste! As is tooth paste/polish! Sugar and salt have been used, because they are water soluble afterwards... Jewellers and metallurgists use aerosol cans of the finest diamond powder as polish to get mirror finishes... but it is still grinding paste and must be washed-off after polishing.
But Do Not lap or hone after reaming. The reamer is incredibly precise and gives a smooth finish On-size, so why would you want to mess that up?
K2

 

[Steamchick]

Hi Jens,
Actually, I prefer the one like Sydney Harbour Bridge... as it looks much stiffer as a bearing cap. I know it makes the nut difficult to access - I should make long studs an nuts with thick collars to compensate: or just have the flats much higher...
But these models are more cosmetic than strong anyway, o it does not really matter. Love your work!
K2

 

[minh-thanh]

Maybe...I need to relearn how to make my own bearings and cylinders, especially with small diameters and small engines

 

[jimbo7636]

If you are worried about the abrasive embedding in the Al, try Timesaver. It is supposed to break down and that's why its called timesaver so you don't have to disassemble the parts to clean out the spent abrasive

im focused on the positional accuracy of the hole. it seems that drill, bore, then ream would be the most conservative approach. I believe the reaming would address the surface finish

 

[Steamchick]

Or, if you buy matching drill (undersize) and reamer, then simply Drill then Ream on the same set-up. That was what I was taught 58 years ago.. I doubt the technology has changed with time.
But be sure the drill is the right amount undersize - as specified in the "reamer tables" - if you can find any? And remember reamers can be parallel, for blind holes, or have a tapered lead for passing fully though. Tapered for the first 1/3rd.
They can also be straight-cut, or helical, with the helical cut the reverse of the cutting direction, so the reamer is fighting to back-out of the hole, not drag itself in via the cut. The requires steady but firm, controlled continuous feed as the reamer rotates in the fixed bore.
Finally, reamers (as with all multi-edged cutters in circular holes) will "mathematically" try and make a shape like a 20p of 50p coin, by oscillating across the centre of the hole as they cut. In fact they are trying to "walk" around the hole, so one edge bites, stops, and the remaining edges translate about the one fixed edge momentarily, when forces change and the cut opposite digs-in and the tool walks around that edge as a fulcrum.... This oscillation produces an oversized hole, but with the minimum contact circle being OK, yet having tiny gaps all around the minimum contact points. There will be n + 1 "flats" making the hole (n = number of cutting edges on the reamer) and the same number of enlarged corners... But the "flats" are actually curves of radius = diameter of reamer. Just like the 20p or 50p, the diameter is the same all around, but made of curves and corners, not a circle!
But using a Milling machine or good drilling machine is a good way to ensure a true hole is made. (Very slow speed!).
Enjoy.