Discouraged Feeds on a Lathe?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 11/04/2017 07:28:31:

So my Warco280 must be a good machine as it has the separate feed shaft, I'm happy now it has been confirmed to be better quality than a Myford

.

The design concept is certainly better

… I am in no position to comment on the relative quality.

MichaelG.

[BazyleParticipant @bazyle]

To what extent is that deliberate or just a consequence of the gearing needed to get the feed up to the slide and the designer didn't give it any thought?

I've always wondered at the hardinge separate motor as variations in load would potentially show up or is it just too good a lathe to be affected.

[JasonBModerator @jasonb]

Posted by Bazyle on 11/04/2017 09:20:06:

To what extent is that deliberate or just a consequence of the gearing needed to get the feed up to the slide and the designer didn't give it any thought?

Thats the bit I find strange, the gearing inside the apron that takes the feed shafts rotation to the carrage or cross slide should be more or less the same and just the difference in pitch of the cross slide leadscrew would be the only real difference. Somehow they must have got some very different gear ratios in there to get the wide difference of 8:1 on the metric machines and the more useable 3:1 of the imperial.

The ideal would be no difference between the cross and lengthways feeds. Unless they deliberately want you to put less load onto the cross slide screw and nut by limiting the feed rate as they are smaller dia screws?

Regarding lathes having a separate feed shaft, it is probably more to do with size than quality as you don't often see them on small lathes but they start to show up as things get bigger like around the size of Bantams, M250s, etc

[SillyOldDufferModerator @sillyoldduffer]

Well experimenting with different cut depths and feed rates didn't get me anywhere this morning.

I took facing cuts from 0.1 to 1.0mm deep at 0.035, 0.017 and the 'illegal' 0.008 rate. The photos are very dull so I've not bothered to upload them. Depth of cut made very little difference. Reducing the feed rate does improve finish, but not dramatically.

One thing: watching cutting carefully, I noticed that obvious 'tree-rings' occur whenever swarf gets caught under the tool. There's an slight 'tick' and a ring appears. Clearing the swarf continually with a toothbrush improved the finish: I suppose what's really needed for best results is a flood of coolant.

Just for laughs I tried taking 4mm off the diameter with a triangular carbide insert. The single deep cut was no problem at all for the lathe, but golly the swarf was HOT. No flowers please, I'm only slightly burnt. The finish was better than I got facing with HSS.

I'm starting to think that for rough use tool settings aren't all that critical. There's probably more to it than I've learned today – Jason's pics on the forum prove he gets a better finish than I do despite me having a similar lathe.

Dave

[Neil WyattModerator @neilwyatt]

Posted by not done it yet on 10/04/2017 20:09:24:

It's my change-gear mentality, where there's no clear line between screwcutting and threading.

Ahem!

​

Gearbox or change wheels aside, I thought screwcutting and threading have no line between them – at all!

On the S7 you have a reversible cluster to swap between pitches useful for screwcutting and pitches for fine feed.

But I appreciate what you mean

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 10/04/2017 20:09:24:

It's my change-gear mentality, where there's no clear line between screwcutting and threading.

Ahem!

​

Gearbox or change wheels aside, I thought screwcutting and threading have no line between them – at all!

.

On reflection … I think I do have something useful to add to this discussion:

Surely; in the context of the machine in question] the "line" is between screwcutting [which uses the leadscew] and the fine feeds [which use a separate key-wayed shaft]. … the loadings on these two drive systems must be very different.

MichaelG.

[Martin ConnellyParticipant @martinconnelly55370]

The Smart and Brown model M has a selector that feeds either the leadscrew or a keyed shaft for powered feeds or screwcutting. The leadscrew drive has a shear pin in it so that if it is overloaded you have to replace the shear pin and in theory you could carry on threading without losing your position in the thread. On the feed system for powered feeds there is a slipping clutch in the apron which will prevent overloading the drive system but would totally mess up screw cutting if a rapid feed rate was used to cut a thread. So in this case there is a definite difference in screw cutting compared to powered feed.

With CNC the difference between powered feed and screwcutting is only a matter of synchronization with the spindle position when threading.

Martin C.

[SillyOldDufferModerator @sillyoldduffer]

I'd always assumed that the purpose of a separate keyed drive for self-act was simply to reduce wear and tear on the leadscrew and nut, thread cutting being an infrequent precision operation compared with common as muck turning.

My lathe can be set up to cut threads with the self-act, and to make finishing cuts with the leadscrew. I'm not sure there's any advantage in doing that though?

My old mini-lathe happily used the leadscrew for both operations, mostly to keep the cost down I would guess but also because there isn't much room to squeeze another drive in on a small lathe.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 13/04/2017 11:50:43:

I'd always assumed that the purpose of a separate keyed drive for self-act was simply to reduce wear and tear on the leadscrew and nut, thread cutting being an infrequent precision operation compared with common as muck turning.

.

A very reasonable assumption.

MichaelG.

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