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Boring an Engine Cylinder

Which technique should I use? advice please

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John Wood128/11/2009 13:32:24
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116 forum posts
Hi all
 
I am building the Pioneer petrol engine from a set of castings by the Engineers Emporium and have come to the main air cooled cylinder. This is about 2.75" in diameter and 4.5" long and made of cast iron.  I have faced the ends to dimension but now need to bore out for the piston and am not sure how to proceed.
 
I have a Soba boring head which I would like to use for this job in the milling machine, but the longest tool in my set is 3" so, should I look out for a longer one or even make one myself or would I be better doing it another way?
 
I don't have a between centres boring bar which, I understand is the traditional method, and in any case I would have some difficulty mounting the cylinder on the cross slide of the lathe. I presume you would need to remove the bar each time you take a measurement and, how difficult would it be to make fine adjustments to the cutter depth?
 
Then I could mount the cylinder to a faceplate, if I could find a Keats Angle plate large enough - this is also a problem if I mounted it to the milling table - and use a long boring tool mounted in the toolpost but I would be a bit concerned about parallelism due to flexing.
 
I would be grateful for any advice please so that I can get a bit more sleep!
 
Thanks guys
 
John
 
PS. My Wife says it's me that's boring - nice!
 

mgj28/11/2009 14:42:47
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Can you not get it in the 4 jaw with a fixed steady?.
 
If you are worried about the overhang while turning a register for the steady to run on,  make up el cheapo quick plug, jam it in and then centre trill for tailstock support. (assuming there is a cored hole. If not so much the better). Get it all initally aligned using a surface gauge, or whatever kind of indicator suits/you normally use before centre drilling of course.
 
Turn a register, and another one near the jaws (just clean that one up) so when you reverse it  to turn the other end register, you have a true running track on which to run the clock. (Unless you want to run your clock on the bore itself, and set up off that which is another option)
 
Set up the fixed steady and turn to a little under size so you have honing allowance. You'll need a fairly serious boring bar - I have made one up that is 1" in dia, and thats what I'd use for  that sort of job, and I'd still cut on the infeed and the out to take out any spring in the tool, and twice in and out at the same setting getting near the end. (apologies to grannies)
 
Oh and set up a stop if you have to use the external jaws on the 4 jaw - just so you don't pile into them with the cutter. Pity you set length accurately at this early stage; would have been better to have done it after honing/lapping to size/parallel and taken out any taper on entry which is what usually happens.
 
The mill with a long stout bar is an option, and certainly it can be done, but it will be a pain without power downfeed (which you may well have?) Still you'll have to turn some kind of OD so you might as well put it in the 4 jaw but once - unless you like cutting ODs with a reversed tool in a boring head. Can be done of course, but doesn't count as entertainment in my book.
 
Boring with between centres bar. I've never got it to work well in reasonable sizes. It may well be OK (don't know) on small gunmetal cylinders etc when one is stretching a Myford to its limits!! Every time I have tried to do a bore of any size its been a recipe for chatter, even with x-slides locked etc. Maybe others have a better way of setting up between centres for boring?
 
----------
Parallel due to flexing. No, once you are cutting in a prepared bore all the forces remain equal. The leverage on the tip, WRT to the point of contact doesn't change. What you can/will get is spring in the tool. So you set .005, it springs and cuts .003. You remeasure, and see its a few thou undersisize on cut. so you add that next time, and bingo you find you have an oversize hole. See comment about avoiding spring above)
 
If its tapered its because the lathe is not set straight! See earlier long thread about setting lather straight, or hone it after boring with a barrel lap.
 
 

Edited By meyrick griffith-jones on 28/11/2009 14:48:07

Edited By meyrick griffith-jones on 28/11/2009 14:49:29

JasonB28/11/2009 16:41:59
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Its not hard to make a between ctrs bar out of a bit of say 1" bar, cross drill oor a 1/4" HSS toolbit and retain with a grub screw, you can stop teh 1/4" hole short and thread the remainder so a screw can be used to advance the tool bit. Thats how I did the cylinder for my Hit and Miss engine.
 
The faceplate is another option, you can just use any angle iron and some clamps, bit like this cylinder thats about the same size as yours. I used a bit of 5/8 bar and machined the end to take a carbide insert to give me a stiff boring bar.
 
Jason
John Wood101/12/2009 15:54:12
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116 forum posts
Many thanks for both of the above.
 
Without powered downfeed and taking everything else into account I won't use the mill. Also, being un-adventurous I think I will skip the 'between centres' method as well in this particular instance so will opt for holding in the 4-jaw as recommended. I take the point about machining to length before boring, never though of that but will add it to my knowledge for the next one. I have a plug already made up which I used when facing the ends so that can be pressed into service again for preparing the registers.
 
Thanks Jason for the info on mounting to a faceplate, I'm as guilty as the next man for thinking I have to chuck money at a problem (locate and purchase a Keats angle plate) when it's not really necessary so that's a great help.
 
Right then, I shall get on with it now and see how I get on.
 
Thanks again
 
John

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