Single Point a 5 40 UNC thread

[Keith LongParticipant @keithlong89920]

I wonder if the problem is often bed wear in the lathe.

Due to the stick-out of the workpiece from the chuck and the necessary overhang of the boring bar, at the start of the cut the carriage will probably be sitting on a less often used part of the bed, so a closer fit for the gibs etc. As the cut progresses the carriage moves into the often used – and hence more likely worn – area of the bed. This allows a bit more clearance for the gibs and the possibility of slight twisting of the carriage on the bed as a result of the pressure acting on the cutting tip of the boring bar. It would only take very slight movement of the carriage to produce the necessary taper a often folk seem to be talking of a very few thou. – just enough to stop a piston fitting for example. The slight twisting of the carriage would be magnified by the length of the boring bar and hence the taper.

A between centres boring operation should not suffer from this or to a much less marked degree I think.

Just a thought, please feel free to disagree or rubbish the suggestion.

[NealebParticipant @nealeb]

Unfortunately, not all lathes are set up to turn absolutely parallel! Witness the number of times this is discussed on this forum. However, the OP says that it was bored with a boring head on a mill. Theoretically, this should give a parallel bore – not necessarily exactly perpendicular to the table, depending on mill setup, but “should” be parallel. However, my own experience with less-than-perfect machines is that this does not always happen due to the tool deflection gradually increasing with depth for various reasons. Classic vertical mill problem with quite a lot of overhang of tool tip from its support. Ideally, line-boring with a bar supported at both ends should give the best result but us amateurs end up doing our best with what’s available!

[JasonBModerator @jasonb]

Well I can turn a parallel OD but do sometimes get a tapered bore, I put it down to additional flex due to the distance the cutting edge is from the toolpost.

When turning an OD I seldom get any material removed when I wind the tool back to take the next cut unless I’m taking heavy cuts. But when boring the tool will usually leave the tell tale spiral on the bore as it is wound out quickly.

The only thing I can see that is different is that I may have 100mm from the toolpost when boring but 10mm when turning.

That cylinder I showed above had some taper but I know how to deal with it, either by using the brake cylinder hone or in this case as it was a bit more pronounced I used the Emery method. Does not seem to be a problem, this is the engine quickly assembled and as you can see there is a good bounce when it comes up to compression. I should say that for this video I had not yet fitted the Viton O ring, no gaskets fitted, no springs on the valves and I even had the valves in the wrong way round so they had not been lapped into their respective seats. Squeaks a bit as the air get spast my fingers which are covering the plug and valve access holes.

I don’t tend to bore cylinders on the mill as it is hard to feed consistantly so prefer to use the lathe either with the part in the chuck if it’s a small engine or bolted to the cross slide if large and use a between ctrs bar. The later does not produce a taper so again it suggest the problem is specific to using boring bars in the toolpost.

[Andy StopfordParticipant @andystopford50521]

Also, boring vertically, the hole tends to fill up with swarf which can make the boring bar jump leaving a narrow section which may be hard to detect if limited headroom makes a trial fitting of the piston difficult.

I had this problem recently with a ‘Minnie’ cylinder – a wooden lap/hone (I’m not sure which term is more appropriate here) sorted it out.

[brickyParticipant @bricky]

I have just honed the cylinder for opposed pistons at 112mm long this was a boring challenge,After boring ,I made a D bit to true the cylinder and then useing a length of brush shaft turned to be a close fit in the cylinder ,I drilled into the middle and used a steel tapered rod to expand the shaft and covered in grinding paste I lapped it true.

Frank

[David CoulingParticipant @davidcouling56621]

Thanks guys for all of the useful advice. I’ve been working on the cylinder with a wheel cylinder hone and thankfully its almost there.

Thanks for adding the pictures Jason – good idea to use a dowel or plastic pipe.

Appreciate you all taking the time to respond.

Cheers,

David

[JasonB]

On JasonB Said:

Well I can turn a parallel OD but do sometimes get a tapered bore, I put it down to additional flex due to the distance the cutting edge is from the toolpost.

The boring tool will of course flex more than a tool for turning an OD. So it is essential to take a couple of spring passes before making critical bore measurements. But when using a boring bar on, say, a 100mm long part the tool stick out, depth of cut and feedrate rate will be exactly the same at 5mm into the bore as at 95mm into the bore. So what changes to make it cut less deep at 95mm versus 5mm?

I bored the cylinder and liners for my engines on the horizontal mill as it wasn’t practical to use the faceplate or cross slide on the lathe. Far too much faffing about to get everything aligned and square, especially on the faceplate with a very heavy casting. Manual feed was a bit tedious on the horizontal mill, but centring on the bore was simply a case of moving, and then locking, the X and Z axes. A complication was two bores that needed to be accurately spaced, ideally within a thou or two. I had a lot of problems with chatter due to the length of the boring bar, but the bores were parallel to within a few tenths, over 5.5″ as best as I could measure with an internal micrometer:

When I bored the seperate liners I had less trouble with chatter as the boring bar didn’t need to be long enough to avoid hitting the cylinder mounting flange.

Andrew

[Andrew Johnston]

On Andrew Johnston Said:

But when using a boring bar on, say, a 100mm long part the tool stick out, depth of cut and feedrate rate will be exactly the same at 5mm into the bore as at 95mm into the bore. So what changes to make it cut less deep at 95mm versus 5mm?

Andrew

This has been the question that usually comes up. I don’t have the equipment to measure what may only be a 0.001″ difference over 4″ to be able to see if the “taper” is constant or more of a trumpet shape, I can just about measure the 0.001″ difference.

But you mentioning “spring” is also what is usually said about boring so even if a lathe turns parallel on OD there is something that stops a boring bar turning parallel on it’s first pass and the fact there is “spring” suggests flexing somewhere.

That bore in CI bar was drilled 22mm and then bored out to 24mm with a 16mm insert bar held in a 4-way toolpost so should not really be a lot of flexing of the bar.  Maybe a fixed steady would have helped if it is the work moving and not the bar, but probably not as bore was smaller at the chuck end.

[HopperParticipant @hopper]

It is often the nature of boring bars to produce tapered holes. Common procedure among even seasoned professionals is to take several final “spring passes” without adjusting the depth of cut until the boring bar makes a full pass without cutting at all.  This is said to take the “spring” out of the long projection of the slim boring bar. Simple, but it works.

 

[Anonymous]

The question of springing and boring tapered are seperate issues; there’s no causal relationship. There’s still no explanation of why a boring bar should inherently bore tapered. Out of curiosity I’ve just measured the bore on the rear wheel hub on one of my traction engines. The hub was bored in the lathe with a 16mm boring bar sticking out 5″. As near as I can tell opposite ends are within a thou of each other. At least half of that can be ascribed to the lathe setup.

Andrew

[Keith Long]

On Keith Long Said:

…the slight twisting of the carriage would be magnified by the length of the boring bar and hence the taper.

Good point, that may well be a possible cause. I expect it would apply to lathes with flat shears, but possibly less so for those with primatic shears as these should resist twisting forces.

Andrew

[JasonBModerator @jasonb]

That’s about what I was able to measure on that bore, 90mm long and the dial bore gauge was showing 0.02mm difference one end to the other. Still enough of a taper to jam the piston solid as it approached the head end.

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