Lathework for Beginners

Thanks Jason. I wondered if it just bolted on, I couldn't think how else it would fit but I think it is bigger in diameter than the 'back plate' the chucks bolt to so I'm wondering what sort of a struggle it is going to be to get them in

I might sharpen a bit of HSS and see if I can scratch a better mark than I will get with my scribe.

Morning guys,

I was up early this morning and had a couple of very pleasant hours on the minilathe.

Nothing too exciting but its so nice producing parts for the loco without having to rely on other people to get parts made. I made 4 chassis tie bars from 3/8" steel bar machined and faced to length and then tapped M4 each end. I then started on 4 suspension spring locators in 1/2" alu bar.

However yesterday was not so good, one of the steel loco wheels has some surface marks on the outer face, nothing too much but I thought I would take a couple of thou off the face to tidy it up. The wheel is a plain disc 82mm diameter across the tyre and it is pressed onto the axle which made it convenient for holding in the 3 jaw.

Using a new HSS knife tool I set about facing the wheel, I thought a 2 thou cut should be about right with maybe a second cut if needed. Being new to this I obviously didnt realise that the tool went blunt almost immediately and was just rubbing not cutting. I added another 2 thou but that just left rough marks on the surface so I stopped. On checking the tool sure enough it had lost its edge and had a rubbing mark down from the tip.

What did I do wrong...?

Was the speed wrong, I had it at 600rpm

I was cutting it dry as I thought with very light cuts it would be ok.

I had set the tool angle to the job so I dont think there was an issue there.

The same tool I sharpened and it was fine on both steel and alu parts I made this morning..?

Regards

Ron

Around 100rpm would be more like the speed for a HSS tool on steel of that diameter.

Mike

Waaaaaay too fast. At the wheel periphery 600rpm corresponds to about 500fpm; that would be pretty sporty even for carbide insert tooling. A general rule of thumb for low carbon steel and HSS tooling is cut around 100fpm. So you need to be running at more like 120rpm.

Holding the wheel by the axle means that the wheel is not able to resist cutting forces very well. Although a 2 thou depth of cut should be ok, you may run into chatter problems.

Andrew

Thanks Mike and Andrew.

Well a lesson learnt there.

Thanks again

Ron

Useful rule of thumb for cutting speed in steel with HSS is 10000 / diameter of work in mm. Easier than working out feet per minute and will get you in the zone. Multiply answer by 2 or 3 for carbide. 10000/86 = 120rpm.

Learning the appropriate tool geometry, cutting speed, depth and feed for different materials takes time. It's not overly critical but...

Andrew's point about work-holding is also important. Considerable forces are applied when you cut metal, and, given the slightest opportunity, the work will bend and spring back. Best solution is to hold the work rigidly. Where that's not possible, lighter cuts.

Dave

Thanks Dave, the 10000 divided by diameter is easy to remember, that will be stuck in my head now.

regards

Ron

I have seen recommendations for staying with HSS tooling on the minlathe whilst others seem to use carbide just as often as HSS if not more.

To add to my selection of tooling I got myself and 8mm index tipped tool from Arceuro with a set of the 060-204 tips.

Despite my gouty hand at the moment I gave the new tool a try this morning turning a piece of 10mm steel bar. Just a simple test but it seemed to work fine. I have read that the tipped tools need to work harder so I ran the bar at 1600 rpm, dont know if that was about correct for 10mm bar and a tipped tool but I guessed at it. I gave a finishing cut of 2 thou and it seemed to be a decent finish, probably not quite as good as HSS but ok.

Would appreciate thoughts on the tipped tools and minlathes.

Ron

It's more respectable guide than it might first appear.

All materials have a recommended cutting speed which is determined by a combination of chip formation and how hot the tool gets. It happens that mild steel and HSS cut best at about 30metres per minute.

So RPM = 30 / pi * diameter (in metres)

By converting metres to mm and taking pi = 3, you get the easy sum:

rpm = 10000 / diameter(mm)

Other common materials cut a bit faster or a bit slower than steel and the sweet spot for a particular job can usually be found nearby by experiment. Multiply rpm by 3 to 6 if using carbide rather than HSS.

A professional shop would find it worthwhile doing a more sophisticated calculation, but for amateur use the shortcut is good enough. It's unusual for an amateur to maximise production rates. We live within the limits of our skills and machines, and have time to tweak for best results from what we have.

Dave

Edited By SillyOldDuffer on 15/07/2018 13:31:55

Thanks Dave,

I need to make some guide rollers for the loco using 1 inch Nylon 6 bar, I have read that a very sharp HSS tool with a 1/16" tip radius makes a good tool for the job. What about speed..? I can experiment of course but wondered what would be a good starting point.

Ron

Not cut Nylon myself. I'd guess high-speed with a sharp knife-like tool.

Cutting steel it's the tool tip that gets hot, loses it's edge and rubs. With plastics it's the material that gets hot, deforms, sticks and behaves badly. You may have to experiment, I'd start by slicing off the plastic quickly and letting smallish chips carry off most of the heat.

Perhaps someone who has done it will advise please?

Dave

In all cases it's the material that initially gets hot due to deformation as a result of chip formation. The problem with plastics is that they go soft, and even melt, at low temperatures. And you don't really want to be using flood coolant, as many plastics will absorb it to some extent.

The trick to machining plastics is sharp tooling with high rake angles, and uncoated, and ideally polished, if using carbide. Superficially one would think high speeds would be great for plastic, but in reality slow speeds are needed to prevent melting, say low carbon steel/HSS type speeds. To compensate large DOC and feedrates can be used, say 10 to 20 thou per rev or per tooth.

Nylon machines well, but it's near darn impossible to get the swarf to break. So you end up with an ever increasing ball of swarf that would put a birds nest to shame. The best thing is to stop and remove the ball of swarf at regular intervals. If it gets caught up the results can be spectacular. If the ball of swarf rubs on the work it can get hot enough to melt, equals a right mess.

To summarise, cut slow with high DOC and feeds.

Andrew

Thanks Andrew, the advice is much appreciated.

Ron

Hi guys,

I found a few charts on line that give a basic list of cutting speeds for most of the materials we are likely to use. One or two of them list speeds for both HSS and tipped tools which is handy. I was going to print one off and mount it above the lathe as a beginners guide when I wondered is there a chart you guys would recommend.

Ron

I have cut quite a lot of various plastics with a 6mm button insert, they have a nice sharp edge and I use coated ones as that is what I tend to have. Finish is very good. Like parting off the secret is to not let things rub, go at it with good feed rates. The swarf flies off in a single ribbon like it is shooting out of a hose when cutting is going well. Think of an arc with a radius about 400mm (16". I have a shop vac and find the best thing to do is set the hose up near the tool point to suck the ribbon straight into the vac as it is produced.

Martin C

On the whole I don't bother with charts. The speeds are mostly not that critical within a few percent. And finish is also determined by a host of other parameters including depth of cut, feedrate, quality of tooling, rigidity of the machine and the material itself. I've got the basic numbers in my head and if needs be I do a quick mental calculation and that gets me close enough. In the end it's experience that counts and you only get that by experimenting.

Andrew

Point taken Andrew

I have just discovered that my minilathe is slower in reverse than forward, 2575 rpm forward but only 1800 in reverse, is this normal..?

Ron

Yep, the dc motors don't go as fast backwards

Thanks Jason, well you learn something everyday, I never knew that.