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Boring head finish

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Nick Welburn21/05/2021 20:41:30
81 forum posts

As I posted before my Stuart 10v cylinder is a little rough inside and it’s also tapering about .2mm end to end. When I machined it I used a 6mm carbide cutter on the lathe and did it by hand.
My new boring head has arrived for the mini mill. Would I get a better cut from that? Less spring in the tool? It is viable to mount it up and bore it 0.2mm e2e? Or is this a fools errand?

Nigel Graham 221/05/2021 21:50:09
1767 forum posts
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It is hard to achieve a really good finish by hand-feed. Could you not have used fine feed on the lathe?

Also, that taper is usually from the boring-tool springing as you suggest, and a couple of spring cuts can often cure that. It's best to use the largest boring tool the cylinder can accommodate, if necessary using a smaller one for the initial cuts.

It can be set up for re-machining but being so close to finished size, will need a lot of care to ensure that 0.1mm each side does not become 0.2mm from one. A boring-head won't necessarily give you a better result as the tool can still spring; and you'd need consider which would be the more rigid machine, as well.

The best result if your lathe has a T-slotted cross-slide, is probably from a between-centres boring-bar with fine feed, though it may involve a lot of setting-up.

Nick Welburn21/05/2021 22:11:50
81 forum posts

I’m very much a beginner until now I only had the one boring bar and I’ve only just discovered the results from the fine feed. An offset bore I suspect is preferable to taper bore.

Andrew Johnston21/05/2021 22:20:37
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Posted by Nigel Graham 2 on 21/05/2021 21:50:09:

A boring-head won't necessarily give you a better result as the tool can still spring...............

Exactly, before rushing off to use a boring head the question that should be asked, and resolved, is why the bore is tapered when using the lathe? A difference of 0.2mm is huge. I might expect a thou or two difference due to tool spring, but no more, and after a couple of spring passes you should be able to achieve well under a thou. Tool spring should stay pretty constant throughout the cut, so I suspect the taper is caused by something else. Like feeding with the top slide when it is not set parallel to the lathe axis?

Andrew

JasonB22/05/2021 07:01:59
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Once again asharp tool makes a lot of difference, if you were using HSS than a touch up with a diamond slip before the last couple of cuts and the spring cuts is best particularly if you started cutting from the cored hole with the same cutter. Also lock top slide and cross slide to help eliminate play in the ways.

As Andrew says 0.2mm is a lot, you can correct a slight taper with the cylinder hones or better still lapping but that is too much

The Stuart 10 series could take a slightly overbore cylinder if you want to set it up again in the lathe hand have another go as there are no piston rings, not sure what diameter material they supply for the piston so you may need to source a replacement for that.

Martin Connelly22/05/2021 08:52:57
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Andrew, I agree, why would tool spring cause tapering? The spring forces at the cutting point are the same when starting the cut as when finishing. A lot of people seem to think this is a thing but I have never understood why. Bed wear, poor lathe setup, the workpiece flexing (least likely with anything substantial and well secured) or material not being homogenous are all I can think of to cause tapering.

Martin C

Edited By Martin Connelly on 22/05/2021 08:56:19

not done it yet22/05/2021 09:01:07
6433 forum posts
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Wondering here, when OP says ‘by hand’ and ‘beginner’ if the boring was undertaken using the compound slide? That could easily account for the taper.

For on-size fettling I would now use a reamer, methinks.

Nick Welburn22/05/2021 09:49:18
81 forum posts

Ok- I’m not sure what the compound slide is. I used a 6mm boring bar held in the tool post and wound it in my hand using the wheels on the lathe.
I’ve found I get a much better finish using the power feed instead of by hand,

I think what is being said here is is the hand slide not parallel to the bed? Interesting possibility. So if I move the whole head instead of the slide then the bed and tool then have to be parallel.

JasonB22/05/2021 10:11:27
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If feeding by hand use the big wheel on the front of the main carriage as this will give a more parallel cut, you will see the handwheel moving as you use power feed. Compound or topslide is the one that feeds the part that the toolpost sits on and that can be swivelled to different angles.

 

Edited By JasonB on 22/05/2021 10:13:43

JasonB22/05/2021 10:13:15
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Posted by Martin Connelly on 22/05/2021 08:52:57:

Andrew, I agree, why would tool spring cause tapering? The spring forces at the cutting point are the same when starting the cut as when finishing. A lot of people seem to think this is a thing but I have never understood why. Bed wear, poor lathe setup, the workpiece flexing (least likely with anything substantial and well secured) or material not being homogenous are all I can think of to cause tapering.

Martin C

Edited By Martin Connelly on 22/05/2021 08:56:19

I wonder if most people just measure each end and assume a taper as they don't have the tools or ability to take multiple fine readings along the length to determine the exact shape of the cut

Nick Welburn22/05/2021 11:06:35
81 forum posts

Cheers again! I’m learning a whole lotta stuff from this forum. Thank you for answering my very much beginners questions.

Andrew Johnston22/05/2021 11:35:36
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Posted by Martin Connelly on 22/05/2021 08:52:57:

........why would tool spring cause tapering? .....A lot of people seem to think this is a thing but I have never understood why.

It's just one of the great model engineering myths. smile

Nick: Compound slide is the same as the top slide; the slide on top of the cross slide than can be swivelled for taper turning or screwcutting.

I've just had a quick look at the rear wheel hubs for my traction engines, which were bored in the lathe:

rear_hub_boring.jpg

Tool is a Glanze 16mm boring bar and stick out is around 130mm. I measured both ends and as near as I can tell both readings are 1.629". I don't see why only measuring at both ends should lead to assumptions of tapering. Apart from a very short lead at the start the bore ought to be parallel, if the tool is being moved parallel, as the cutting forces should stay constant.

Andrew

Nigel McBurney 122/05/2021 12:40:40
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Been thinking why would you get an 8 thou tapered bore in cast iron over the short length of cylinder,the lathe would have to be horribly worn to turn that taper,the obvious is use of the topslide rather than the bed,though Nick states that he got a better finish when using the the power feed so one has to assume that the saddle is being used. For boring an iron casting I would use carbide for roughing and hss for finishing , I find carbide inserts tend to push the metal off rather than cut it, If using carbide for the whole job then use a new insert for the finish cuts,small iron castings can have hard spots which can chip carbide inserts on roughing cuts.so if the lathe bed is in good condition then something must be moving, ie the tool insert in the boring bar,the boring bar in the holder ,the tool holder moving ,the top slide not fully clamped, to stop rotation ,the cross slide moving away from the cut due to any vibration,Make sure the tool is on centre or a few thou above centre,boring tools below centre will cause problems ie rubbing & not cutting cleanly and not too obvious to the beginner. Good practice is to use power feeds though a good turner very often finds it quicker to feed by hand over short lengths, for the novice feeding by hand allows one to get a "feel" for how the tool is cutting,if it appears to be hard work feeding the tool then check the cutting edge and cutting rakes and clearances,also honing tools, L have seen novices hone the tool too hard and remove the cutting clearance. My first turning at work was on a Lorch plain lathe,that taught you how turn the slide handles smoothly at a constant feed and get a good finish,and keep the toolbits sharp.

Clive Foster22/05/2021 12:45:07
2880 forum posts
104 photos

Not convinced that taper bores due to boring bar spring is a complete myth.

Given the basic quality, age and less than stellar condition of lathes commonly affordable by relative beginners in the past its pretty certain that bed wear would be greater near the chuck. Flat edge beds, whether dovetail or Myford style, generate greater inaccuracy for any given amount of wear than the prism guide style used on professional machines. They are also harder to set just so for best results on a worn bed. Setting involves a few tricks too which are not as commonly promulgated as they should be.

Hence I find it reasonable to suppose that cutting forces could deflect things a bit as the saddle moves over the worn area.

Especially if there are various infelicities in the tooling such as imperfect shape, inadequate sharpness or errors in the height of the cutting edge creating excessive push out forces.

Most especially where relatively small bores are concerned boring tools need deceptively large amounts of heel clearance. I've more than once been too casual about shapes in the past and found that the cut was being made by the heel rather than the tip.

Even if the heel isn't actually cutting it can drag on the newly bored surface slowly forcing the tip out of cut as the bore gets deeper. Instant taper.

Something I've actually deliberately worked through in the past either to keep a stronger tool or because I didn't want to modify the one I had to hand which was fine for the finish cuts. Just not enough clearance for the first few cuts from the starter hole. So long as the last cut is right it matters little what goes on before.

I have a printed out, pictorial, crib sheet to remind me of heel clearance angles for smaller bores.

Obviously the cutting tip needs to be on centre and the lead in angles such that cut forces don't tend to draw the tool into cut or push it out of cut. Again with slender boring tools in small bores getting the shape and sharpness right so it behaves itself with a proper cut is important.

Mucking about rubbing a couple of tenths off each pass is not the way to go. Even razor sharp tools rarely behave themselves on hair fine cuts. I reckon that anything under 10 thou cut is asking for trouble due to the inevitable limitations of hand ground and honed tooling made by relatively inexperienced folk with relatively limited facilities. My experience is that a misbehaving tool tends to make a measurable taper.

The cure is to sharpen the thing properly rather than assume it was ok last time. I usually get bitten by one dragged out of the back of the tooling drawer for a special job that none of the general use collection can handle. Generally on a quick "Can you sort this?" job I don't want to do but can't politely turn down. Hurry-up and don't wanna are a potent combination.

It all gets much easier once up in the 16 mm - 3/4" (ish) bar size range. Even then excess stick out is a gremlin magnet. I've worked at 10 times diameter (and more) stick out when needs must but keeping under 5 times is far more restful.

As Geo. H Thomas was wont to say the common "mutant golf club" boring tool is something of an abomination. Expecting a neophyte to get good results from one is asking a lot. Quite apart from mechanical limitations getting a well shaped, sharp tip is hard. If only because of the difficulty of holding it up to the grinding wheel at the correct angles on the normal toolrest. My preference in smaller sizes is to use round HSS held at a slight angle to the lathe axis so you don't need to grind a neck behind the tip. take off just enough to give heel and to clearance with a short cutting edge.

Clive

Nick Welburn22/05/2021 12:58:12
81 forum posts

Just put a square on the cross slide... spot on it’s not set square ‘doh’!

Howard Lewis23/05/2021 14:09:55
5533 forum posts
13 photos

Hopefully, the taper is not the result of slightly different measurement techniques. Check, and double check!

Riding a hobby horse.

Does the lathe turn parallel on ODs?

If those cuts are tapered, maybe the bed is twisted. But to produce 0.02 mm taper over the short length of a cylinder, there would be a lot of twist! Whatever twist is present, it should be possible to reduce, if not eliminate it.

(Read Ian Bradley's "The Amateur's Workshop", or his "Myford 7 Series Manual" for the method. )

But if the problem is caused by wear, the same problem would be present when turning outside diameters.

Howard

Nick Welburn23/05/2021 14:37:20
81 forum posts

The lathe is a new Amadeal cj18 I think it’s fair to assume it’s not worn out. I’d assumed that there was a detent to hold the cross slide at 90degrees. Yesterday I stripped it down. Set the cross slide to 90deg and set the gibs. I think the reality here is the that the lathe has been ‘ assembled’ as opposed to built and ‘setup’.

The out of square was shown with a set square so I’d hazard the slide was more like 89.5 dogs than 90. I’ll buzz some thing down and see how we are set this eve hopefully

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