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Boring problem

Extra cut when withdrawing tool from bored hole

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Barrie Lever10/07/2020 17:05:58
651 forum posts
75 photos
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

Yes an Ex is a 'has been' and a spurt is a 'drip under pressure !!'

That Elite member of the Model Engineering World must have been dreaming if he thought there was a cure for the spring back problem, it occurs pretty well all the time, it is just a matter of how much force is required to get the deflection.

Even happens in heavy industrial machines as pointed out.

This reinforces my argument that small hobby machine tools need very good tools to get the desired results.

B.

Ketan Swali10/07/2020 20:12:13
1269 forum posts
105 photos
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 110/07/2020 20:36:32
3 forum posts

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 mart10/07/2020 20:39:53
1825 forum posts
148 photos

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.

Sam Stones10/07/2020 20:45:05
avatar
759 forum posts
303 photos

Goodonya Katan.

Although down here we are nine hours ahead of most of you people, the following notes are beginning to lag behind.

However, having overlooked a third issue mentioned in the NGP link and mentioned more than once in this forum, is that the saddle etc. is driven by an offset force from the apron, whether it be the leadscrew or the rack and pinion. The tool will ‘lift’ after a (forward) surfacing cut (or dig in after boring), thus illustrating that the saddle indeed rotates, albeit slightly.

After more than 60 years of regular use (not all mine), by the time I sold the ML7 it wasn’t even necessary to test with a dial indicator. There was also obvious bed wear closer to the chuck than the mid-section and tailstock end that no amount of gib adjustment could alleviate.

Irrespective of the dimensions of the workpiece and tooling (e.g. boring bar), the old saddle of my ML7 was clearly worn.

Aside from the stress/strain pathway (tool-tip to workpiece), where deflections accumulate, I commend a ‘glance’ at the principle formulae (particularly cantilevers) of beam theory where the dimensions are raised to some power, i.e. cubed and indeed to the fourth power.

Keeping these in mind for both tooling and workpiece, may bring a clearer understanding of how stuff bends, twists, and tries to get out of the way.

I might add – ‘Know your machine’.

Enjoy your hobby, I did

Sam

Edited By Sam Stones on 10/07/2020 20:46:25

not done it yet10/07/2020 21:48:45
4739 forum posts
16 photos

One way of checking the carriage is, or is not, the culprit might be to bore using the top slide - with the carriage and cross slides locked down. OK, it would be hand-fed - most use the carriage feed, when boring, because it will be move parallel to the work-piece without the careful setting up for the top slide (which would normally be jammed tight at the end of its travel).

The gibs can be set tight-ish for a test. Only a good check if the top slide is in good condition, of course.

Andrew Johnston10/07/2020 23:07:14
avatar
5553 forum posts
650 photos
Posted by Andrew Williams 1 on 10/07/2020 20:36:32:

......whenever possible use an hss tool newly sharpened rather than an insert tool.

That's not my experience. I've just finished boring the cylinder castings for my traction engines. The bores are parallel and round to within 4 tenths of a thou over 5.25", but I've had all sorts of trouble with chatter causing a carp finish, around 6 to 7µm Ra at best. I tried several tools, two boring bars and different speeds and feeds. In terms of chatter I'd rank the toolbits as follows, starting with the worst:

Sharp insert CCGT

Home ground HSS

CCMT insert

Once the HSS toolbit started to chatter that was it, each successive pass (even a spring pass) just replicated the chatter marks. With the CCMT insert shallow, or spring, passes reduced the chatter marks rather than simply following them.

The cylinder boring was done on the horizontal mill, but when boring on the lathe I use CCMT inserts as standard, and CCGT on aluminium. Oddly I rarely have a problem with chatter on the lathe and get a much better finish that I did on the horizontal mill. The key to reducing chatter when boring on the lathe is to increase the feedrate. On the horizontal mill it appeared to be the opposite, ie, reduce the feedrate.

Dunno if the above makes me an expert, an ex-spurt or just plain awkward? Ketan? smile

Andrew

Ketan Swali11/07/2020 10:05:17
1269 forum posts
105 photos
Posted by Andrew Johnston on 10/07/2020 23:07:14:

Dunno if the above makes me an expert, an ex-spurt or just plain awkward? Ketan? smile

Andrew

I dont know what that makes you either Andrew. smiley

Ketan at ARC

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