Boring or Reaming

After making two cast iron cylinders 60mm long by 18.5mm dia I bored them out to a fine finish using a HSS tool for this as advised on these forums. But if the lathe is not 100% true then the bore will have a taper, if it was float reamed then this must be a more accurate way of doing this. The hole dia on each end has a difference of .03 mm, so has a taper of .03 along 60mm, if it was float reamed would this of happened. Is .03 over 60mm acceptable on a lathe bearing in mind that its a China made lathe. When I bored the holes I did not reverse the saddle when it was with in .5 of size I used the DRO for the setting for the final cuts.

Who knows what is acceptable on a low-cost lathe? Depends on what you (or they) are willing to accept. Way to check is to chuck up a peice of 1" round about 4 to 6 inches long and take a fine test cut over the length of it. Measure both ends and observe teh amount of taper. I would want it to be less than one thou/.02mm at home. Mr Schlesinger would want a bit less than that on a quality industrial machine.

Is the hole smaller at the headstock end of the job? If so, the taper may be due to spring in the boring bar. Pressure tends to build up on the bar during the cut, pushing it ever so slightly away from the job surface. Best way to avoid this is to take repeated cuts at the end without adding any depth of cut on the cross slide. Do this before you get to the final size, then taking very fine cuts to sneak up on the final size, run the tool through the hole twice at each setting. When you get to size at the mouth of the hole but further inside is smaller, take another pass through, or even a couple.

If the hole is bigger at the headstock end of the job, you may have a small lathe bed to spindle misalignment. One way around this as you say is to bore to say .03 to .06mm or so undersize then ream to finish size.

With your existing tapered bores, you may be able to salvage them by honing, lapping or even reaming to final size to get rid of the taper. A carefully made expanding lap could do the job. I have done similar jobs using a brake cylinder hone, or even a piece of wooden dowel with a saw cut in the end to hold a strip of emery tape spun in the pistol drill and judiciously worked up and down the small end of the bore. I'm sure the purists will arrive shortly with pitchforks and sickles to disabuse you of that notion though.

If you want the holes round, getting them honed with a proper hone is fairly quick and simple to do and may not cost very much to have an experienced honing person to do the job for you. Reamers in that size are not cheap and the offset reamers that do make round holes will be very expensive just for the 2 cylinders that you are making.Home made laps can work well , but like everything care is needed and you really need a bore gauge or something similar if you want them to be a specific size or roundness.

Neil

The relevant Schlesinger test for the lathe is a test bar in the spindle and a maximum of 0.02mm out over 300mm measured with a DTI in the toolpost.

However, there are other things that could cause the taper as Hopper points out. Another thing that matters is measuring diameter at least a short way into the bore.

0.03mm taper over 60mm is probably OK for a steam engines, but not for an IC enging - except for those that demand a taper towards the top of the cylinder.

The best and way to bore a parallel cylinder and the usual approach (with things like Myfords, not just far-Eastern machines) is to mount it on the cross slide and hold the tool in the chuck or ideally between centres. If the spindle has no shake and the cross-slide is locked, a parallel and round bore is practically guaranteed.

Reaming is unlikely to have given you an adequate finish.

Neil

I would say that 0.03mm (about 1 thou) on a 60mm bore was pretty good going. Indeed measuring such and being sure is a bit of a problem. Then 'what is it for', how good does it need to be?

If just a bit shiny then a wood mandrel and fine wet n dry would do.

As said reaming is not a sure fire answer and liable to mishaps. As also said a lap is probably the way to go. But that is no easy road either, If a home brew job used by running back and forth in the lathe the lap will need to be about 3 times the bore length (not cheap). Then you will need some sort of plug gauges to tell when you have got close and when you have nearly gone too far.

Using a home brew lap is not hard and a fairly controlled process - so long as you don't let it get dry and seize.....

good luck.