Mini-Lathe Repair

Ron,

That being the case - you have had a good finish since changing the gears then the probability is this is not related to bearings or gear material / vibration charachteristics. I think another look at tool, speed or feed is the next logical thing.

Paul.

A couple of pictures below of the finish I am getting, I had to enlarge the images so they may not be the best.

First picture is 20mm free cutting steel, brand new carbide insert, .005" cut, speed 1600 rpm, didnt measure the feed but it was good.

Second picture is 20mm 6082 alu, HSS knife tool freshly sharpened and dressed on a diamond card, 003" finishing cut, speed 800 rpm.

They both appeared to cut really well but not the finish, I have just noticed that despite the different material and different cutters they look very similar.

Looks like something I made!

Some suggestions:

• the tool is too high or otherwise rubbing rather than cutting,
• tool overhang is too big causing slight chatter,
• swarf is failing to clear and getting trapped between the edge and the work.
• lathe is vibrating - I suppose the bearings might have been damaged during installation, but your posts don't suggest you're a cack-handed brute with a big hammer! So I doubt the bearings are shot, but you might have been unlucky. Try holding an oily finger against a smooth rod while the lathe is running - you might be able to feel any untoward vibration. Also, put a mug of water on the bed and bounce a torch beam off it on to the ceiling. Then run the lathe up and down the speed range. The amount of ripple seen on the ceiling may be obviously excessive or become much worse at particular speeds. If so, may indicate a problem in the motor, drive belt, gear train, or a resonance due to a mounting problem. (Anyone know a better way of detecting vibration?)

Might help to post some close-ups of the tools as well.

Dave

For carbide, if it is a CCMT insert you want the depth of cut to be at least 2/3 the tip radius.

You can take smaller cuts with CCGT

Neil

Ron

Are you using the tool slide or carriage wheel to advance the cuts?

I note you have been playing with the gib strips

Try locking down whichever axis you aren't using to see what effect that has - if you are advancing with the tool slide, lock the carriage, if you can, and vice versa.

Also try winding on your next depth of cut, and then locking the cross slide before advancing the cut.

Just takes some of the variables out of the equation.

The torn finish on the low carbon steel is exactly what I'd expect for an insert with too low a DOC and feedrate. Similarly for the 6082 I'd say tool geometry is the problem.

Andrew

Andrew, the insert is a CCMT and I know it goes against all the theory but I have cut .005" with the same settings on low carbon steel previously and not had an issue. I will try a .020" cut, the material is 20mm, what settings do you suggest and I will give it a try.

The knife tool I used on the 6082 has 5 degree angles all round except I increased the top rake to 10 degrees for cutting aluminium, again it has not been a problem previously and always produced a good finish.

Ron

Ron if your tools have not changed and you got a good finish after changing the gears could it be something to do with the new belt/pulleys?

Are they a good fit on the shafts and running true? May also be worth playing with belt tension a little either side of where you have it now.

Having said that it does look more of a tool issue. On the steel the brighter bands are where the tool is cutting but then it is getting pushed off the work or the work is getting deflected and that gives the poor areas. The burrs on the aluminium also point to the tool not cutting well. Try a facing cut with the CCMT yo check it is on ctr height then a deeper cut along the length say 0.25mm

Edited By JasonB on 25/01/2019 08:19:30

Hi Jason, the new pulleys are not fitted just yet, I have just received the 3mm tool steel so I can now cut the motor pulley keyway and get them fitted. I have also done as you suggested and removed the current drive belt to see if the gears had a tight spot but they seem fine.

I just know something has changed but as yet I cant fathom out what. I accept that I am probably not using the insert cutter at its best settings but my old faithful knife tool would always give me a superb finish on aluminium but you can see from the pics what I,m getting now.

Ron

Our posts crossed, I did check the CCMT is on centre and it is, I will try a 0.25mm cut and see how it goes. Do you think the 1600 rpm is ok or should I dial it up..?

Hi Ron

Rather than dial the speeds up I would try them quite a bit lower , for say 20 mm aluminium with a HSS tool try 250-400 rpm , and for the freecut steel the finish you got in the photo is okay for roughing down with an indexable tool but to make sure of a fine finish on steeI I always use a sharp HSS tool with a decent thin cutting oil , this works even on difficult stuff like EN3B and the speed for 20 mm dia would again be 250-400 rpm , a lot of novices think the answer to bad finishes or tools not cutting is to whack the speeds up when they would be better off trying lower speeds ,but too high speeds even on aluminium can quickly knock the edge off HSS tools and as said before if you want to get a good finish on a light cut with indexable tools you need the ones with proper sharp edges , the less sharp variety will just push off on a light cut and give the sort of finish in your pic, the lower speeds will be kinder to the lathe as well , the faster you run it the more it will tend to vibrate compounding problems with finish.

regards Geoff

Edited By Kettrinboy on 25/01/2019 09:13:30

Thanks guys for all the advice, I am away shortly for the weekend but I will have a play when I get back Monday

Ron

It depends upon the nose radius of the insert, but for the steel I'd be using be running at 1200rpm, DOC at least 10 thou and a minimum feedrate of 4 thou per rev. I'd be using more top rake on the HSS tool, say 15 to 20 degrees.

Are we sure the materials are the same, ie, cut off the same bar as before?

Andrew

Thanks Andrew,

The bar is supplied as the same EN1A but it is from two different orders a few months apart so unlikely it is from the same bar, I think I see where you are coming from.

Ron

In theory that shouldn't be a problem. But the tolerances on the constituents of most alloys is pretty wide. And who knows what else is in the mix. I've certainly had problems with poor finish on a material type which from previous purchases has turned beautifully.

Andrew

I am away from home at the moment but before I left yesterday and out of interest I tried the CCMT 060204 insert on the 20mm 6082. I ran it at 1000 rpm and to my surprise it cut beautifully leaving an excellent finish. I was of the opinion that the insert type was general purpose and not a first choice for aluminium so wasnt expecting much but it would appear that engineering is full of surprises.

I know the feeling - I hate surprises!

However, the insert isn't a totally bad choice for Aluminium - the blurb says: "Used for a wide range of materials including soft and exotic alloys".

The professionals are far better placed to avoid surprises. They have hefty machines in good nick, know all about the materials they're using and can get expert advice about cutting tools, coolants, and optimum metal removal. Another advantage - they almost always use the right tool for the job.

Much less black and white in my domestic situation, which is why I bought 'Fifty Shades of Grey' in hope of improving my technique. I'm semi-skilled, using materials and tools I don't quite understand, making the best of small machines with limited capability, and often bodging improvising to get the job done. No wonder results are erratic and diagnosing faults difficult!

The good news is that experience helps enormously - the more you learn the less difficult it gets to deliver satisfaction. At the present rate of progress I should be quite good at it by about 2040...

Dave

New Poly V drive belt and pulleys fitted at long last.

It will be a bit of trial and error re the belt tension, it obviously needs enough for decent cuts but at the same time able to slip in a stall or a crash (not that I plan on doing that of course).

At the moment I have set it up so that I can twist the belt through 90 degrees quite easily so its not too tight, whether that is somewhere near or not I will see.

Thanks once again to Jason for the idea in the first place.

Edited By JasonB on 03/02/2019 10:17:45

Back home after a week away so this morning I tried out the new drive, the pulleys run nice and true and are a good fit. I gave the belt a touch more slack but I,m guessing really at the balance between having enough grip for drive and the safety of belt slippage in the event of a crash or stall.

I took a number of 1.0 mm cuts on alu and a 0.5 mm cuts with steel and it was fine, I know some take heavier cuts on mini-lathes though I think that is heavy enough but thats just my thinking. I did consider taking increasingly heavier cuts until I stalled the lathe to see at what point the belt slipped but decided against it. I dont think it a good idea to stall a machine so I will hope and trust that the slack I have in the belt adjustment does its job should I have ever have a lock up.

Hi

I am looking for some one in the Uk who can repair Electrics on Mini Lath. It sounds like relay chattering. Starts then stops. Any help appreciated. Thank you!
Regatds

Godfrey