tevans9129
Well-Known Member
Getting ready to open the bore in the PMR #3 cylinder and being ignorant on these matters, is it best to ream the bore or bore it? My mill does not have a power Z-axis.
Ream or bore?
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I like reamers but cost of boring is cheaper. FYI All of my work over 3" was done with boring
Dave
Hi,
Always bore, never ream
Boring produces an axially true hole, a reamer bends and follows the (curved) path of the existing hole
All the best,
Ian
I might be a bit old-fashioned, but I would say it is better to bore it in the lathe if you can.
I would suggest boring.
Either on the mill, or using a between centres boring bar in the lathe, with the cylinder mounted on the cross slide.
Either on the mill, or using a between centres boring bar in the lathe, with the cylinder mounted on the cross slide.
Another question - once bored, is it acceptable to then finish grind the ID while still on the lathe and then follow with a hone for final finish?
There should be no need to grind the bore.
The last cut should be at a fine feed rate.
Any honing should ideally be left until assembly with the piston, to achieve the required fit.
Don't try to 'lap' the piston into the bore. You'll never get all the abrasive out of the piston and it will wear the bore when running.
The last cut should be at a fine feed rate.
Any honing should ideally be left until assembly with the piston, to achieve the required fit.
Don't try to 'lap' the piston into the bore. You'll never get all the abrasive out of the piston and it will wear the bore when running.
vederstein
Must do dumb things....
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Honestly, for practical purposes, it really doesn't matter. This engine is so forgiving you'll have a runner with either method.
...Ved.
...Ved.
Tug40
Well-Known Member
I chucked the cylinder on a 4 jaw chuck & bored it on my lathe.
Honed it slightly when fitting the piston.
Honed it slightly when fitting the piston.
The true answer - from a manufacturing and technical point of view - is "Bore" as Peter Twissell says. A fixed body with a tool describing a circle - passing along the axis - will give a truely aligned and circular bore. Anything else may or may not be as good. But for models, it is true that some designs can be "forgiving" - I.E. they can manage a very slight taper, eccentricity or whatever, as they are never going to need to do the thousands of hours of hard labour that the original enegines did. Also, it depends on whether you have an hydraulic seal requirement (like a brake cylinder or suspension strut on your car) or have piston rings, or just a groove or 6 for a labyrinth seal. Again, is it a compression engine or steam? - that makes a big difference as to what you can get away with. Tighter tolerances needed on single acting IC pistons - for sealing - than maybe with double-acting steam engines? (You can't see the piston blow-by leaks inside the steam cylinder, but can detect the loss of compression on the IC engine!).
Also, please understand: any cut (like drilling, reaming) with a multi edged cutter in a hole has a microscopic tendency to "wobble" and cut a surface not unlike the outside of a 20p or 50p piece: with 1 more "Point" than the number of cutting edges. In fact this devious cutting process can be utilised to cut square holes with a 3-fluted tool, hexagonal holes with a 5-fluted tool, etc. So the reamer will produce one more (partial flat" surface than the mumber of cutting edges. But the surface variation may not be a problem (or visible) on your 1/2" bore steam engine. - But may be significant on your 50,000rpm aero- engine 2-stroke! Especially a 1" bore over-square engine with slipper pistons.
Here's one from Mass production manufacturing: Car engine blocks used to be "just clamped" and bored at low temperatures (bags of cold coolant). But "Modern" practice is to have the machining coolant and parts at 85C (Or similar engine operating or thermostat opening temperature) and clamped to exactly simulate the clamped condition of the block when builty with the head, crank, sump frame etc. fitted. This is so that the distotion caused by assembly and normal running temperature is set into the cylinder-metal before boring. So the bore is "true-er" at operating conditions and reduced running-in is required... which means a prolonged lifetime. When I was 13, we re-bored car engine blocks typically at 20~30,000miles... where I worked. Nowadays, no-one expects to need that! 150,000~250,000 miles is common for today's cars without a re-bore. We worked to toloerances of a few tenths of thou' when grinding crank journals - using the vernier on the barrel of the micrometer - the mic kept in your overall pocket for "constant temperature" in the hot or cold workshop... Today - with air guaging, lazers, etc. and CNC feedback for constant measurement, the crank journals are ground to fractions of the tolerances we used. Bearing shells (broached), Pistons (diamond tool turned to od "clever" shapes), piston pins, etc. are manufactured then measured and sorted into grades at assembly - as the Designed tolerances of assembly (microns) are smaller than the variation of the latest manufacturing processes. So don't bother trying that at home - unless home has a workshop that makes Formula 1 engines!
But enjoy what you can do! - I do!
K2
So consider your application, skills, and tooling ability (Money?), and based on tolerances you can achieve, select the most appropriate results = to leave yourself "Happy without regrets". I select Models and jobs I can do well, rather than a challenge to make something that needs tolerances beyound the capability of my tooling and skills. (e.g. a taper between the head and bed of your old lathe probably is beyong most people to improve - so don't try and make something that needs better tolerances than your tools).
Also, please understand: any cut (like drilling, reaming) with a multi edged cutter in a hole has a microscopic tendency to "wobble" and cut a surface not unlike the outside of a 20p or 50p piece: with 1 more "Point" than the number of cutting edges. In fact this devious cutting process can be utilised to cut square holes with a 3-fluted tool, hexagonal holes with a 5-fluted tool, etc. So the reamer will produce one more (partial flat" surface than the mumber of cutting edges. But the surface variation may not be a problem (or visible) on your 1/2" bore steam engine. - But may be significant on your 50,000rpm aero- engine 2-stroke! Especially a 1" bore over-square engine with slipper pistons.
Here's one from Mass production manufacturing: Car engine blocks used to be "just clamped" and bored at low temperatures (bags of cold coolant). But "Modern" practice is to have the machining coolant and parts at 85C (Or similar engine operating or thermostat opening temperature) and clamped to exactly simulate the clamped condition of the block when builty with the head, crank, sump frame etc. fitted. This is so that the distotion caused by assembly and normal running temperature is set into the cylinder-metal before boring. So the bore is "true-er" at operating conditions and reduced running-in is required... which means a prolonged lifetime. When I was 13, we re-bored car engine blocks typically at 20~30,000miles... where I worked. Nowadays, no-one expects to need that! 150,000~250,000 miles is common for today's cars without a re-bore. We worked to toloerances of a few tenths of thou' when grinding crank journals - using the vernier on the barrel of the micrometer - the mic kept in your overall pocket for "constant temperature" in the hot or cold workshop... Today - with air guaging, lazers, etc. and CNC feedback for constant measurement, the crank journals are ground to fractions of the tolerances we used. Bearing shells (broached), Pistons (diamond tool turned to od "clever" shapes), piston pins, etc. are manufactured then measured and sorted into grades at assembly - as the Designed tolerances of assembly (microns) are smaller than the variation of the latest manufacturing processes. So don't bother trying that at home - unless home has a workshop that makes Formula 1 engines!
But enjoy what you can do! - I do!
K2
So consider your application, skills, and tooling ability (Money?), and based on tolerances you can achieve, select the most appropriate results = to leave yourself "Happy without regrets". I select Models and jobs I can do well, rather than a challenge to make something that needs tolerances beyound the capability of my tooling and skills. (e.g. a taper between the head and bed of your old lathe probably is beyong most people to improve - so don't try and make something that needs better tolerances than your tools).
tevans9129
Well-Known Member
Thanks for the comments Dave. I also like reamers and I have quite a collection but they are less than .500. I decided to bore while I have it in the fixture.
tevans9129
Well-Known Member
Tell me, it is a small knee mill and especially when the Curt vise and 8" RT are on the bed. I did not notice it 20 years ago be boy have times changed.
tevans9129
Well-Known Member
Thanks for the suggestion Lan, I did bore.
tevans9129
Well-Known Member
Thanks Charles, I did bore but on the mill. Used 400 paper to polish a bit.
tevans9129
Well-Known Member
Thanks Peter, I did bore on the mill as it was set up for other tasks.
tevans9129
Well-Known Member
That is a pleasing thought Ved.
The truest bores (like me!) tell you the same story repeatedly. I agree you have done the best job by boring (in principle).
Reamers can give a very repeatable size, but the best circles are described by boring (.. e.g. people like me are called bores because we always go round and round describing circles....).
Enjoy!
K2
Reamers can give a very repeatable size, but the best circles are described by boring (.. e.g. people like me are called bores because we always go round and round describing circles....).
Enjoy!
K2
I typically will bore close to size then use reamer.
With reamer I hold +- 0.0002 on thousan of bores for setting 6203 ball bearings.
Dave
With reamer I hold +- 0.0002 on thousan of bores for setting 6203 ball bearings.
Dave
I have a complete set of chucking reamers, and that is how I finish the bore on all of my engines. I think boring is more accurate as long as you take enough spring passes to account for any deflection of the boring bar. All of my engines (about 20 steam and about 20 i.c. ) have reamed cylinders and they all seem to work fine with no lack of compression. I generally give my cylinders a short lapping session with the appropriate sized Acrolap and 600 grit compound after reaming. This cleans up any machining marks and leaves a nice finish ready for the piston.---Brian
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