Boring problem

[Andrew Williams 5Participant @andrewwilliams5]

This may well be a common effect on a light lathe (ML7 with little wear) but on boring a hole the tool always takes an extra cut when withdrawing it from a hole. The size of the cut is somewhat unpredictable so making boring a rather hit and miss affair. Am I missing a trick or is it a matter of tightening up gib strips etc?

[Andrew Williams 5Participant @andrewwilliams5]

Extra cut when withdrawing tool from bored hole

[pgk pgkParticipant @pgkpgk17461]

Most likely spring within the tool..always use the largest you can get in there +/- moving it from the face before withdrawing.

pgk

[Anonymous]

It's a fact of life, irrespective of the machine. As said it's due to deflection of the boring bar. If hole size is important take a spring cut or two before measuring. The shallower the DOC the less the effect. But shallow cuts often lead to chatter. On the lathe I find it's best to take a final cut to size and then move the boring bar clear before withdrawing. If another cut is needed I reset the boring bar using the cross slide dial.

Currently I'm boring my traction engine cylinders and I get a shallow cut on reversing the tool out. That's with a mill weighing 3500lbs, a Wohlhaupter UPA4 boring head and a 25mm boring bar, adapted for the 7/8" holes in the boring head. One just has to work around it.

Andrew

[colin hawesParticipant @colinhawes85982]

This can be aggravated by the tool not being sharp enough so that the rear of the tool is sharper than the leading edge and also if there is any slack in the carriage adjustment causing it to twist when the tool is extracted. Colin

[Pete RimmerParticipant @peterimmer30576]

Try making a boring pass then simply pushing against the tool post with the heel of your hand as you retract the tool. Chances are it won't even touch.

[Andrew Williams 5Participant @andrewwilliams5]

Thank you for the responses. As I guessed it is just a matter of a bit of play here, a bit of flexibility there and learning to live with it. The problem is that I have always had in mind a conversation with one of the elite of the model engineering world at, I think, the last open day held by Myford in Nottingham. He was firmly of the opinion that, with proper adjustment of the lathe then the problem would not arise.

[roy entwistleParticipant @royentwistle24699]

Andrew Wind your boring tool in slightly before you retract. This will remove any spring in the tool or twist in the toolholder or saddle.

[BazyleParticipant @bazyle]

Hello, you seem to be new to the forum. Great you can join us. I suggest you read back through all the posts for the last month. We were discussing this at length twice in that period, covering inside and outside cutting effects. Just below the title of the thread you will see a link you can click called 'latest posts' which then allows you to see the newest thread titles and work your way backwards.

[Kiwi BlokeParticipant @kiwibloke62605]

Unavoidable tool deflection is the reason for 'spring passes'. The boring bar itself bends under cutting loads. Try to minimize deflection by increasing rigidity – largest diameter bar possible, with shortest possible unsupported length, and a sharp tool.

However, particularly because of the M7's saddle guide design, a greater problem may be slight rotation of the saddle, as it's moved by the action of the rack-feed handwheel or the leadscrew. It will tend to rotate anti-clockwise (viewed from above) as the saddle is moved towards the tailstock. Try it with a DTI. This problem can be avoided by using the boring bar upside-down, and cutting the rear of the bore. Now, the rotation works in your favour, relieving the cut as the tool is withdrawn. It makes the feed-screw dial maths easier too. Chip clearance may also be better, and you can see what's happening a bit better. It's my standard technique.

Edited By Kiwi Bloke on 09/07/2020 22:43:13

[Ady1Participant @ady1]

Am I missing a trick or is it a matter of tightening up gib strips etc?

If the tool deflects then use less cutting pressure and a sharper tool when doing those final cuts

Use deflection for the hogging out part

[Kiwi BlokeParticipant @kiwibloke62605]

'… or is it a matter of tightening up gib strips etc?

If the tool deflects then use less cutting pressure and a sharper tool when doing those final cuts'

It's not that simple, and the question is about the final cut that may ruin the part, as the tool is withdrawn towards the tailstock.

The tool will always deflect: nothing has infinite rigidity. Even if you were to bore, back-off feed, retract, return tool to previous in-feed position, bore again, and repeat the sequence a few times (ie several 'spring cuts' ), you would in all probability find that, after all that, withdrawing the tool towards the tailstock, without backing-off the feed, would result in a light cut, ruining your day. The saddle guides, particularly of Myfords, are short, compared to their separation, and, whilst, as has been suggested, gib strip adjustment will help, it won't eliminate the phenomenon, particularly when the saddle's fixed guide has worn a bit.

The use of 'upside-down' boring tools is a standard technique, but it seems to be less well-known than it deserves to be. It has advantages – try it, if it's new to you.

Also, use boring tools of a suitable shape (ie with the 'corner' leading) because these will tend to deflect less and can face the end of a blind bore. GHT's writings explain it, IIRC.

[Sam StonesParticipant @samstones42903]

Hi Andrew (Williams 1)

The trouble with certain Myford saddles (I had a 1946 ML7 and worked on a Super7 in the late 50's), and the way the saddle can rotate (very) slightly) also relates to the narrow guide principle (NGP) and the wear on the saddle's shorter contact length with the back edge of the front shear.

There was a modification in Model Engineer by J A Radford of NZ (which I never got around to executing), that switched to using the rear face of the back shear.

Although I haven’t bothered to read further, starting with ‘Chipswitheverything’

**LINK** that hints at where and why the existing design is as it is.

For a better understanding of the NGP see here …

**LINK**

I'm sure there's other stuff on the subject, including Jack Radford's ME article, but needs digging for.

Hope that helps.

Sam

Edited By Sam Stones on 10/07/2020 04:05:09

[Sam StonesParticipant @samstones42903]

Egg on my face with my first link. It's not about the saddle, but the top-slide. Apologies all round.

I was intent on finding J A Radford's 'Saddle modification'.

By the way, if it weren't for old age and health issues, I'd buy another Myford in the blink of an eye. It would however, be a Super7.

Sam

[HopperParticipant @hopper]

Posted by Sam Stones on 10/07/2020 04:03:42:

Hi Andrew (Williams 1)

The trouble with certain Myford saddles (I had a 1946 ML7 and worked on a Super7 in the late 50's), and the way the saddle can rotate (very) slightly) also relates to the narrow guide principle (NGP) and the wear on the saddle's shorter contact length with the back edge of the front shear.

There was a modification in Model Engineer by J A Radford of NZ (which I never got around to executing), that switched to using the rear face of the back shear.

Although I haven’t bothered to read further, starting with ‘Chipswitheverything’

**LINK** that hints at where and why the existing design is as it is.

For a better understanding of the NGP see here …

**LINK**

I'm sure there's other stuff on the subject, including Jack Radford's ME article, but needs digging for.

Hope that helps.

Sam

Edited By Sam Stones on 10/07/2020 04:05:09

See MEW Issue 287 for a modernised, much simplified method of doing Radford's Wide Guide modification, without needing a milling machine to do the job.

A strip of gauge plate is attached to the rear guide surface of the saddle and the narrow guide is left unmachined and no longer contacting the bed. Same set up as post-1972 factory procedure. Radford, and others', method of machining away the original narrow guide is unnecessarily complicated.

 

But it;s probably not going to make much difference to cutting on the back cut. That is pretty much normal on any lathe due to the necessary oil clearances and spring in the boring bar. Standard procedure is to retract the tool on the return travel.

Edited By Hopper on 10/07/2020 08:34:23

[jann westParticipant @jannwest71382]

as everyone has already said, its a tool retraction issue, and completely expected. You often get the same effect when turning. The various solutions provided are all good … I hadn't thought of pressing on the toolpost when retracting (which is quite clever … and means you don't need to fiddle with your cross slide settings … which is always annoying).

Two solutions not mentioned … JA Radcliff designed a retractable tool holder, which solves this problem when screwcutting … and would also be suitable in this circumstance if you inverted the tool and cut on the back face of the hole (which may also probably provide an improved finish).

Alternatively the use of a DRO makes the retracting and unretraction a little less fiddly than working with dials … and would be worthwhile if this was a frequent activity.

[Anonymous]

Posted by Andrew Williams 1 on 09/07/2020 20:43:10:

…….a conversation with one of the elite of the model engineering world…..

The most important lesson to learn is that there is a difference between experts and ex-spurts.

Andrew

[Stuart BridgerParticipant @stuartbridger82290]

I have been using an insert boring bar that will cut effectively in both directions. So I have been taking my spring cuts by reversing the feed screw at the end and cutting on the way out. It may not be industry best practice but it works for me, great finish and spot on size.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Johnston on 10/07/2020 09:17:47:

Posted by Andrew Williams 1 on 09/07/2020 20:43:10:

…….a conversation with one of the elite of the model engineering world…..

The most important lesson to learn is that there is a difference between experts and ex-spurts.

Andrew

Even worse than ex-spurts are Fanboys! Possibly a fanboy might believe Myford lathes are magically immune to cutting tools behaving like a spring, when the awful truth is that tools bend on all machines, and boring bars are particularly long and slender. It's unlikely to be a lathe problem. The issue is fixed by the operator driving the machine to compensate, which isn't always easy. Deep narrow bores are more difficult to get right than short wide ones because long bendy boring bars tend to cut slight tapers. Longer bores are probably better tackled by mounting the job on the cross-slide and boring with a bar supported at both ends between chuck and tailstock which much reduces the bar's tendency to bend whilst cutting.

Anyhoo, although the main cause is tool spring, there's a second effect. On a back-stroke the other side of the cutting edge is applied. As the back edge takes less wear than the front it's likely to be sharper. Sharper or not, it's different, which can effect the finish, often for the better.

Dave

[Dave HalfordParticipant @davehalford22513]

Andrew,

Another phrase that describes your 'experts' advice is 'self aggrandisement' it makes sure you always feel inferior to him as he knows you can't avoid it.

[old martParticipant @oldmart]

To eliminate the possibility of saddle rotation some tests with a DTI could be tried. With a piece of straight metal in the toolpost, get it parallel with the axis of the bed when moving the saddle right to left. Then see if there is any deflection when reversing the direction. With flat ways of Myford design, there is a possibility of rotation which would be less with prismatic ways in other bed types.

[Former MemberParticipant @formermember32069]

[This posting has been removed]

[Ketan SwaliParticipant @ketanswali79440]

Posted by Andrew Williams 1 on 09/07/2020 20:43:10:

Thank you for the responses. As I guessed it is just a matter of a bit of play here, a bit of flexibility there and learning to live with it. The problem is that I have always had in mind a conversation with one of the elite of the model engineering world at, I think, the last open day held by Myford in Nottingham. He was firmly of the opinion that, with proper adjustment of the lathe then the problem would not arise.

As Hopper said, standard procedure is to retract the tool on the return travel. Some boring bars have more sping than others, depending on size/material of bar, and how far it sticks out from the tool post.

I too went to the last open day held by Myford in Nottingham. All the elite I met at that open day were/are respected people in the model engineering world, with a wealth of knowledge. If we are talking about the same event, and the same people, then it is possible that there was some misunderstanding in either the question you raised, or the response you received from those people with knowledge who attended. It would be wrong to refer to those people with knowledge as ex-sputs.

Ketan at ARC

[Andrew Williams 5Participant @andrewwilliams5]

Again thank you all for helpful comments. I have just done the thing that I should have done earlier and fixed a 200 mm long piece of 50 x 10 bar in a tool holder and mounted it parallel to the lathe bed with a clock in contact reading to 0.01mm. At 150 mm from the centre of the tool holder pushing on the bar as hard as I can with a thumb gives a maximum deflection of 0.025mm whereas at 40mm I can only manage 0.01mm deflection when pushing the bar with a spanner. A small movement of the carriage either way with the rack hand wheel does not give a defection Exceeding 0.002mm (the slightest deflection on the dial). So the conclusion? The lathe is in pretty good nick, try a boring tool upside down and cut on the rear of the hole, use a solid a tool as possible and, whenever possible use an hss tool newly sharpened rather than an insert tool. This latter point will no doubt raise eyebrows among some but my recent work using a tangential tool holder with an hss tool (plus the completion after a long gestation period of a decent tool grinder) has convinced me to ditch the chips for much work.

[old martParticipant @oldmart]

I have some left handed boring bars which have advantages using the lathe in reverse, but steps are needed first to lock threaded spindles. Left handed internal threading bars are good too, as you produce a RH thread cutting away from the chuck.

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