Parting Problems - Backlash ?

Grrr. Just in case anyone ever has the same problem and is as daft as i am, the tension can be adjusted.

The axle for the middle pulley (top one in picture above) is mounted in a vertical slot with a t-nut behind, so it can be shoved up a bit. The slot was covered by a washer and I hadn't noticed it until I started looking at which pulleys would be easiest to change to alter the speed range.

I initially had problems parting off with my WM250. All the symtons mentioned above.

I then sought instruction and found Tubalcains videos on YouTube. I followed his advice and now part off with spectacular success.

I use carbide inserts just below centre.

Key points, not too fast, about 1/3rd normal macining speed. Lock the saddle and power crossfeed as slow as possible using selectors ( not gear changes or low range ). Minimum overhang, both tool and topslide. Lubricate harder/tougher metals.

Incidently l also used Tubalcains advice to cut my first course (16TPI) thread in stainless steel.

This might only be a contributing factor to the problem, but the "surface feet per minute" (SFM) cutting speed of your parting operation will vary with the ever decreasing radius, as the cutting tool moves in towards the centre of the bar.

What is a sensible calculated cutting speed for your material at the outside of the bar may be completely wrong by the time you get near the middle of the bar.
If the SFM for the tool and material goes too low then it will start creating the wrong shape chips and it could clog the slot, overheat the tool, or start rubbing rather than cutting. If this causes metal to start adhering to the tip of the cutter, then as it is in a narrow slot it can cause it to jam and snap.

Modern CNC machines are smart enough to continuously adjust the spindle speed during a parting operation to keep the SFM a constant.

On a manual lathe you might want to consider emulating this by pausing the parting cut half-way through, withdraw the tool, then increasing the RPM to finish the parting-off operation.

The other side of this coin is feed rate. This also affects proper chip production.
Check the shape and colour of the chips, and see how they change as you do your cut.
That should tell you what's going on.

No need to stop and change speeds on a 290, just wind up the wick on the variable speed as you get nearer the centre

Edited By JasonB on 29/01/2016 16:24:03

I'v seen the same done on a Chipmaster, as the tool went in, the operator just turned the speed control wheel, 4" bar in no time flat.

Ian S C