Problem slot milling in lathe

Hi all!

I wanted to cut a woodruff key half moon into a shaft for my Myford. It was a new countershaft that I was machining. I had a woodruff cutter of the proper size so I went about it thus..

1) Put the shaft horizontally in my myford vise on a myford vertical milling slide. The shaft extended towards the lathe operator and the area to be cut was not hanging out of the milling slide vise by more than couple of inches at most. The shaft was 0.750 in dia.

2) the woodruff cutter was tightened in the lathe chuck and did not have excessive overhang out of the chuck. The milling slide was adjusted so that the milling cutter would be lowered onto the vertical centerline of the shaft where the keyway was desired.

3) With a fairly slow speed, and after liberally dosing the job with cutting oil, I raised the slide so that the cutter started to do its work. Initially it progressed well with the slot being formed cleanly, but it got harder as the work progressed. The shaft material was 0.750 Precision ground mild steel. Then all of a sudden the cutter jumped out the slot which must have been 3/16 of an inch deep and proceeded to climb out of the slot.

I managed to stop the lathe quickly and despite the rather nasty gouges where the teeth of the cutter marked the surface of the rod, I proceeded to try any finish the job by repositioning work/slide and shaft. The same thing happened. Since then I have tried again on different shafts, but using the same cutter it makes me very nervous and the same results. What am I doing wrong? The cutter is part of a set of Chinese origin.

Your knowledgeable advice would be greatly appreciated.

My apologies, I dont have any pictures to share of this debacle. The vise was on the slide which was between the lathe centerline and the operator. The portion to be cut was hanging out the front of the vise by at most 1.5 to 2 inches.

Best regards from Canada!

Sounds like you didn't lock the slide gibs, the only gibs not locked should have been the vertical slide as you were down feeding onto the cutter if I understand you correctly.

Emgee

You don't say where along the "couple of inches hanging out the vice" you were making the slot but if at the end then move it back towards the vice so the edge of the cutter is no more than 1/8" from the side of the vice, it all helps with rigidity and minimal overhang.

What moved? The vice? Vertical slide?
I suspect though as already said the slides should be very securely locked.
i think this is mainly a backlash issue. I bet you fed in towards the centre to position the slot. I.e pushing the slide in. The normal cutting tool in turning will be forcing the slide out against the thrust bearing on the screw. Your cutter does the opposite and will pull it in away from the thrust bearing. So you are effectively suddenly giving a setup with little intrinsic rigidity a climb milling situation , and all the backlash will be taken up, thus once any inevitable snatch starts it will rapidly increase

perhaps a way forward is to make it a slightly elliptical slot by starting a cut at the back and feed in FPS out from the lathe centre a trace then go back, increase the depth and repeat . You will still be climb milling but the cut will be of a significantly reduced length rather than the full periphery of the cutter. Best to experiment on an unimportant bit of stock

Edited By Zan on 30/11/2020 18:56:47

Thanks All for the comments and suggestions. I think improving the ridigity is a good idea. I will lock down all the unused axises (axiii?). Zan's suspicion is likely any accurate assessment.. can climb milling be avoided by reversing the spindle or ?

Gary

The vice did not move, the shaft got pulled away from the thrust bearing and towards the centerline of the lathe. the cutter climbed out of the slot, jumped on top of the shaft and made some gouges on the top of it.

Gary

Any ideas as to why the cutting slowed and required much more downforce on the milling slide vertical feed to continue cutting?

Sounds like you are climb milling. I'm assuming that the cross slide was being fed inwards and the lathe revolving in the normal direction with the milling taking place on top.

I suggest you either feed in the opposite direction or cut underneath the shaft.

Are you attempting to cut to full depth in one hit? Maybe think about several passes.

regards Martin

There should be no "feed" with a woodruff cutter you just plunge in sideways ( or vertically in this case on the lathe) unlike using a milling cutter and have to use full engagement. Any movement either climb or conventionally and you won't get the "half moon" shaped cut for the key.

No Spindle can't be reversed as cutter's teeth only work in one direction.

Edited By JasonB on 30/11/2020 20:29:43

Just to note that reversing the spindle won't work as the cutter would then not cut.

Rob

My apologies. I should have read the post more closely but I was pre-disposed to think it was a slot being cut by the thread title. Absolutely unused slides should be locked.

If the shaft would fit I would be inclined to mount it vertically on an angle plate as a more rigid set up ideally cutting on the oposite side to the operator so cutting forces are towards the bed and the shaft is packed off the boring table at the correct hight. Lock the saddle and there is very little that can move. Depth is read directly off the cross slide micrometer. Vertical slides are not the most rigid of animals at the best of times.

regards Martin

Remove the vice and lay the round bar in a T slot on the vertical slide slide and clamp it down with T bolts and clamps or bits of flat bar with a hole drilled each end to match T slots. This reduces overhang and flex.

Lock all slideways not in use by nipping up gib screws.

Reduce amount job sticks out past slide to a minimum.

Slide the cutter as far into the chuck as possible.

Run your cutter about 400 rpm.

It's digging and getting harder as you go deeper because the length of cut is getting longer and more teeth are in contact.

Also check your headstock bearings are tight.

Hi Gary, Martin has just described exactly the way I was thinking the best way to do it and yes packing or a good machine jack between the end of the shaft and the cross slide will be important. You can always mount a mirror the opposite side to see the progression of the cut. Cutting or drilling for that matter, outside of vice jaws, is not a good way of doing things, as you will have a lever effect. If you do need to go this method, then support under the cutter position or at the end of the shaft away from the vice should be used, especially when using round bar. The reason it became harder the deeper you were cutting is probably due to the increasing area of cutting, i.e. a progressively larger arc, thus causing more drag and load on the cutter.

Regards Nick.

@Gary Clarke.

I have ML7 and I have done plenty of milling on it.

Despite of what people say, once enough thought is given, very reasonable milling cuts can be achieved on this lathe.

My current record is a beautiful, clean 14 mm deep cut done with 10 mm wide circular disc cutter of diameter 63mm held between centers. It was done with single pass on 45 HRc steel bar 40mm diameter, to make a slot across it, with an aid of vertical slide.

My guess is that holding your cutter in chuck is a cause of problem as long as lathe bearings are fine.

Old chucks are tending to be bell-mouthed and cutter is also kept far from bearing. Not good.

You should buy MT2 mill holding chuck with ER 10 or ER 32 sets of collets and for smaller mills use Myford collet system which is very good too.

Always drive your job into a mill in such a way that mill is not "dragging" it on due to feedscrew backlash and that cutting force is taken by machine headstock rather than bearing caps.

Always lock unused slides as others have pointed out.

Thanks everyone! Lots of suggestions here.. and improvements to my technique!

Gary