Thread: Lathe Mysterious Noise

My Harrison M250 has worked fine for the last couple of years with a VFD.
Yesterday, it developed a noticeable 'knock', that varies with speed.

Today, an investigation shows that the knock appears to be close to half spindle speed (after a short period observing chalk marks on the chuck at jog speed).
Found that the knock goes away if I disconnect the banjo gears, and that it appears only in the forward banjo selection.
Took the cover off the headstock and inspected the gears while operating at jog speed, nothing seen.

Anyone else had a similar situation, or can offer a suggestion of the next level of investigation ?

John.

Could it be swarf stuck down in the bottom of the gear teeth? Happens on my lathe from time to time.

Graham.

Graham,
Thanks for the suggestion, the same thought had crossed my mind.
Should be able to get back to the lathe tonight.
One relief is that there was plenty of oil high up in the feed gallery to the spindle bearings, operating with the VFD means the input shaft is frequently running at reduced speed with a risk of inadequate oil fling.
John

Probably way off beam but after I set fire to the chip tray in my lathe (a sphincter puckering moment if ever there was one) it developed a knock which ended up being caused by the drive belt having been softened in one short section by the heat. Is it possible that the belt is wearing?

Well, that was interesting.
The banjo set has a 44T steel on the headstock output driving an 88T fibre, the half-spindle speed knock points to the 88T.
Chalk marks and close observation soon narrows it down to a small group of teeth on the 88T.
Inspections reveals nothing obvious, all teeth good shape without any obvious inclusions.
Rotating the 44T into different positions shows the problem lies only with the 88T.

A few moments scraping teeth did nothing, with the suspicion growing that the fibre teeth may have swelled.
Got a stiff brush and cleaned all the oil/grease off the area in anticipation of doing a detailed tooth inspection.
The culprit is soon revealed - a tiny metallic reflection laying below the oil layer.
A sharp pointer, it is removed.

A test shows silence (love the VFD).
Just amazing that such a tiny contaminant could make a noise obvious while turning steel.
John