Help! Excessive machine marks!

More….

Edit: rotated photos.

Edited By SillyOldDuffer on 27/08/2023 14:13:56

Edited By SillyOldDuffer on 27/08/2023 14:14:41

Just one: When you finish the cut, back the tool out slightly before returning to your start position. A lot of the marks look like you're winding the tool back over the work without doing this, giving the coarse spiral marks.

It might be your chuck!

I had a lathe with similar problems and it took me forever to pin down the problem! In old lathes you sometimes get a problem where the jaws are worn leading to the part not being held properly, this is commonly referred to as "bell mouthed jaws". Now your lathe doesn't appear to be old, and neither was mine. Unfortunately it seems some of the Chinese chucks suffer from poor quality control.

Remove the chuck from the lathe and clamp a piece of round bar in it. Hold the chuck up to the light and look through the back opening. If you can see any slivers of light between the jaws and the bar you have problems.

Another test is to clamp a piece of round bar stock in the chuck and run a dial indicator over it, then push on the bar stock to see if there is significant movement.

A thought - I noticed that these bolts which hold the compound in place are very lose inside the channels - although when tightened the compound seems tight and rigid, I wondered if when under pressure from the tool contact it was moving?
Would it be worth trying to get better fitting bolts - they are nearly 2mm smaller than the channel.

This video by Blondihacks explains the subject quite well:

“Metal Lathe Tutorial 12 : Surface Finish”:

**LINK**

Stuart

I wouldn't have thought the t-nuts are a problem if they are tight.

However I would bet my life savings you have a rigidity problem somewhere.

Wouldn’t mind betting that the saddle gibs need adjusting if you are marking the material on the way back.

I'd be interested to see if there is any spindle play.

Find a decent thickness bar about 300mm long and hold in the chuck. Grab hold of the end of the bar and see if it feels like it is moving as you push and pull on it.

Thanks everyone. I will look into your suggestions in the week as I am now committed to family time!

Thanks again.

Margaret, your facing looks much better.

I'm not sure whether anyone has mentioned it, but when turning, even if your tool is sharp and set-up is good, if you move the carriage towards the headstock too quickly at a slow spindle speed it will be difficult to avoid the lines you can see on your part.

Try upping your spindle speed and, if this doesn't help, decreasing your travel speed as well.

It may sound too simple for most, but after watching some experienced machinists with chatter, I noticed the chips dancing around on toolhiolder, toolpost etc. So I would throw chips on toolholder, toolpost, crossslide etc, then cut as usual, and see if you can see the chips dancing from the originating source like the crosslide base or higher up, and investigate those parts. Just a suggestion if nothing else works.

Edited By Chris Mate on 27/08/2023 18:42:02

Putting my head above the parapet here!

I personally find that in some cases, a ground HSS tool will do the trick (after experimentation) when having problems with a specific job. Part and parcel of this hobby is getting solutions to issues, and often this means going back to basics.

In this case, if you have tried all the basics (speed, feed, chuck, gibs, technique etc ) detailed by others and still have a problem, essentailly it must be either the material itself or what is cutting it. You appear to be using different sized stock in your various photos, so fair to assume it's not just one dodgy bar.

Basic grinding of HSS tools should form part of your skill set in time. Indexable tips are wonderful in many ways, but will not cover every eventuality in the hobby environment. In time you will come across tasks that require a special one-off tool ( e.g. a form tool ) which is simply not available commercially, so you have to make something suitable.

Perhaps look into the possibilty of starting to grind HSS, you do't need to go mad - just learning the very basic techniques and a lot of practice are all you need. There are masses of instructional videos / online text / books that show you how. May save you a lot of frustration in the future.

I'm not trying to come across as condescending, but simply stating a personal opinion that I feel might help you with your problem.

Regards Mike

One of the first thing that we were taught was how to use both hands, one taking over from the other, to provide a steady feed rate.

Like Mike Hurley, I prefer HSS.

As said, whatever the tool, the cutting edge must be on centre height. If all else fails, and you cannot make a Centre Height Gauge (Look in my Albums, if they are still accessible, to see the sort of thing ) You just set the edge of the tool so that it contacts the underside of the blade (Which has been set with the underside of the blade at centre height. )

For brass, I would suggest, on HSS, no top rake, and small front and side clearances.

On a carbide tip, unless the tool shank is held at an angle, these are preset for you.

How do you find centre height?

Take slow steady facing cuts across the end of a bar. When the tool has been set to Centre Height, there will be no "pip" in the middle, of the bar.

The cutting edge of the tool will be at centre height, so set your gauge to that so that it cannot move.

This will ensure that in future, any tools can be set to centre height.

If use a sharp tool set on centre height, the finish is determined by the rate at which you feed the tool in or along.

Too fast and you produce somethging like a screwthread. Slow and steady should produce a nice smooth finish

If you could measure your feed rate, you should be aiming for 0.002 - 0.004" (0.050 - 0.1 mm ) per rev.

Carbide tips usually have a small radius which improves the finish The radius is likely to be greater than the feed/rev, producing a series of overlapping radii, for a smooth finish.

As said, things need to be rigid. clamped firmly, with minimum tool overhang, and with gibs giving absoluterly minimal slop (just enough clearance to slide easily but with a slight drag )

A tangential turning tool, uses HSS toolbits make (two designs using 1/8" have been published in MEW )or buy, The latter means buying from Eccentric Engineering in Australia. This uses 1/4" HSS toolbits, and should come with a grinding jig.

They can be used for facing or turning, but have to be set to centre height using a gauge, (unless you are either very lucky at guessing, or have time to spare to experiment)

Most of my facing and turning is done using a tangential tool.

HTH

Howard

Photo 1 looks lie excessive feed rate. Photo 2 looks like irregular feed rate. Photo 3, not sure. Brass works best with a very sharp HSS tool with zero top rake. Look at Clickspring's You Tube videos.

My views differ, for a beginner who has never ground HSS or even owns a grinder going for in this case a CCGT insert which is known to have good geometry for non ferrous metal totally eliminates the cutting tool from the equation so the problem lies elsewhere if it does not work.

.

You astonish me, Howard

I have not yet made the tangential toolholder for my small lathes; but my understanding of the modus operandi is/was that it takes a slicing cut [like with a graver, or a skew wood-turning tool] and therefore tip-height should normally be above centre.

MichaelG.

Does not astonish me, they cut like conventional tools just the jig and way the tool is held makes sharpening simple

How would you face without leaving a pip if tip were above ctr?

Edited By JasonB on 30/08/2023 09:45:14

Unless I misunderstand, the Tangential tool, by virtue of the angles in the toolholder shank and the grinding jig, it cuts. VERY well in my experience.

Shop made tool holders use a 12 degree angle offset, in two planes, and a 30 degree angle on the diagonal end of the toolbit. (Hence the "Diamond" description. )

(The Eccentric Engineering jig seems to be 33 degrees )

As Jason says, the tip shoulkd not be above centre height, otherwise the tool will rub rather than cut, and if it does actually cut at all, will not cut the below the tool; hence the pip when facing.

A tool cuts because the force acting on the minute area of the tip produces an enormous pressure which shears the metal. An above centre tool presents a much larger area to the workpiece and will not cut but rub. The best that you will get get will be a burnishing action.

Dare I suggest that the graver cuts because its action is very much akin to that of a tangential tool, and pressures at the point of contact.

Howard

One lives and learns

MichaelG.

.

P.S. My reference to ‘above centre height’ was intended to be specific to turning ‘rods’ not to facing.

MichaelG.

Edited By Michael Gilligan on 31/08/2023 17:30:30