Extra cut when withdrawing tool from bored hole
|Andrew Williams 1||09/07/2020 10:08:02|
|3 forum posts|
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?
|pgk pgk||09/07/2020 13:29:19|
|2123 forum posts|
Most likely spring within the tool..always use the largest you can get in there +/- moving it from the face before withdrawing.
|Andrew Johnston||09/07/2020 13:47:22|
6054 forum posts
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.
|colin hawes||09/07/2020 18:23:56|
|542 forum posts|
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 Rimmer||09/07/2020 19:47:34|
|979 forum posts|
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 1||09/07/2020 20:43:10|
|3 forum posts|
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 entwistle||09/07/2020 20:50:36|
|1352 forum posts|
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.
5896 forum posts
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 Bloke||09/07/2020 22:40:42|
|537 forum posts|
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
4437 forum posts
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 Bloke||10/07/2020 02:12:13|
|537 forum posts|
'... 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 Stones||10/07/2020 04:03:42|
819 forum posts
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 ...
I'm sure there's other stuff on the subject, including Jack Radford's ME article, but needs digging for.
Hope that helps.
Edited By Sam Stones on 10/07/2020 04:05:09
|Sam Stones||10/07/2020 05:35:29|
819 forum posts
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.
5505 forum posts
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 west||10/07/2020 08:56:35|
|77 forum posts|
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.
|Andrew Johnston||10/07/2020 09:17:47|
6054 forum posts
The most important lesson to learn is that there is a difference between experts and ex-spurts.
|Stuart Bridger||10/07/2020 09:51:40|
|522 forum posts|
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.
7221 forum posts
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 Halford||10/07/2020 15:20:12|
|1506 forum posts|
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 mart||10/07/2020 15:31:02|
|3062 forum posts|
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.
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