Help with cutting

[TTalma]

I spent most of the 4th weekend getting my lathe set up and running (my 9" sb homeshop). (I'll post some pics soon).

I didn't think I got a tool holder but in the box of parts and metal I got with the lathe was a lantern style holder with some armstrong tool holders for the tool bits (or whatever they are called).

There were a few bits in the box that were sharp and looked to be in good shape. So I mounted everything chucked in a piece of steel and started tunring.

I had no problem get things round with the lathe, I used my calipers and over 3" if there was a tapper it was to small for my calipers to measure so I'm very happy about that!. I'm using a Serpentine belt and the lathe works nice and smooth, and I didn't have any slipping.

The problem I did have was that the turning is not smooth. Looked more like I scratched round rather than cut it. Nothing like the pictures I see heer and elsewhere on the web where the surface is pretty smooth right off the lathe.

I thought my problem was the bits so I bought a few cheap carbide bits at Harbor freight. I figured they wouldn't last long but should cut good at first. They seemed to cut fine but again there was a scratched surface left.

I repeated this with aluminum and brass with the same results.

I did alot of searching on the internet and found out how to grind the bits. I compared these to the bits I have and they looked the same and passed the sharpness tests.

But short of showing that the cutter has to be in the centerline of the stock (which it was) I could'nt find anything that shows the angle the bit should be to the stock. I aslo couldn't find anything telling what the cutting should be like.

I was getting ribbons of steel and not taking a very heavy cut so I am not sure what I was doing wrong. I also tried all 3 speeds on the lathe with the same results.

So to sum it all up could someone point me in the right direction on how to set up the bits and how heavy a cut and how to basicly cut with the lathe.

Thanks -Tim

 

[treeax]

There could be allot of different reason why it is not cutting nice. What kind of steel are you using? Different steels need different speeds and feeds. Are your speeds and feed right? Chip breaker on the tool bit? Tool bit is sharpen right? Tool bit center of work peace? and so on.

Maybe try to take a bigger cut,.020-.025 cut ,.040-.050 over diameter and make sure your speeds are right, if the chuck stalls take a smaller cut .o15-.020 and so on. You can tell if your cutting right is by the color of the chips and surface finish. Different steels, different colors. If you don't have a machinist hand book(The bible for me) go and buy one. It tells you all you need about machining, tool and die, mould making. Its about $1oo Canadian. Or buy the apprenticeship book, it goes step by step with the set up of the lathe, surface grinder, Mill. You don't need to search the internet just buy these books!

 

[PlantationPete]

sounds to me like your feed rate is too fast. With HSS you want lower rpms, with carbide you want faster rpms but slow your feed rate down and let the tool take a slower cut. Give that a try and see what happens.

If it doesn't work next time you post, take a picture of your setup and let us know what rpms you're running and what your feed rate is, it will help diagnose your troubles.

The books are an invaluable resource but to a novice it can be likened to trying to read chinese without knowing how to read chinese. The apprentice book would be good another book called "How to Run A Lathe." Look that one up, it would serve you well.

Also get on Youtube and look up "turning" or "lathe" videos. You're bound to come across something that will give you a good idea of what you should be doing. Seeing is easier than reading.

 

[Clive603]

Tim

Sounds like you are suffering from "not quite rights" rather than "wrongs". Often due to mixing individually good advice from various sources into the wrong contexts. PM me and I'll send you a PDF that many folk have found useful to pull everything together into a self consistent set of practical recommendations. Even those who have chosen for follow a different guru (there are lots of effective ways of bringing a novice up to speed) say its been useful.

Obvious potential errors are:-
1) Tip radius on the tool. A sharp point leaves thread like scratches so, clearly the tool has to cut over a width greater than the feed if the result is to be reasonably smooth. Typically 1/64 to 1/32 radius for ordinary cutting tools. Finishing tool needs a wider flat bit in contact with the work to smooth it out, say 2 or 3 times the feed per rev and a bit more radius on entry getting on for 1/16 will do.

2) Cutting speeds:- something around 65 ft/min will do as a starter reference. Go a bit under for iron, steel et al and a bit over for alloy. Divide the diameter in inches by 4 and multiply by the RPM to get cutting speed in ft/min (well near enuf). Choose your nearest. Avoid stainless steel, copper and pure aluminium until you get better at this game. Get some nice free cutting steel to practice on. Stuff found by or given to novices is almost always "difficult".

3) Feeds and cuts:- around 3 to 6 thou per rev should be about right and cuts 25 to 75 thou deep will serve. Although my machines could handle more I pretty much always came down in 50 thou cuts on my 9" and 10" South-Bends unless the metal was very tough or the work delicate.

When you are getting going its important to be able to repeat your set-ups. Not quite right cases are notoriously sensitive to small changes. Irritatingly when you do get things nailed down to a bomb proof technique its often hard to tell what was different in the days when a fly walking across the ceiling next door seemed enough to upset things.

With the lantern post and Armstong holders you use the bent to the right holder for most turning work and the bent to the left one for facing. This keeps things nicely away from the chuck. So far as I can see the main purpose of the straight holder is to drive up spares sales by promoting collisions between the chuck or work and the compound.

Its notoriously difficult for a novice to get a standard lantern back to exactly the same position or to make known small changes. Replace the rocker with a solid washer of a suitable thickness to put the tool tip on centre height at about 1/2" extension. Better system is to make a fixed block for a more solid tool mount. Idea is to leave the holder mounted as near permanently as you can and simply change tool-bits. Clearly with the holder fixed the bit ends up in the same place when set to centre height. With the aid of some simple templates its easy to ensure that you always put the same angles on the tool-bit. Much easier to keep track of WTHIGO. I advocate setting the compound about 30 degrees angled off, handle to the right, relative to the cross-slide. Its a bit more rigid against cutting forces, puts the handle out of the way and makes thread cutting easier (but that's in your future). If you make a square, single slot block for your Armstrong holder set the compound (top) slide angle so that you can use a fence against the side of the compound to put the block back in position each time with the tool bit perpendicular to the work. Make another one for the left bent holder to do facing along with a pair having angled bases for the parting tool holder and boring tool holders to produce a simple quick change system.

Whist you are getting up to speed consider nipping up gibs on the compound to lock it down. Extra rigidity helps.

Last point. You will get a range of different takes on WTHIGO because one of the joys (and frustrations) of machining its that there is rarely one "perfectly right" answer to any reasonably general problem. Any advice here will be good in its context and will help you find the way that works best for you. For now that is. Next year you'll probably do things a bit differently having more experience under your belt.

Clive

 

[vraptor]

One thing you can do is to listen to your lathe. That will tell you a lot about how your tool is cutting. You should hear a kind of metallic hiss if the cutter is operating smoothly. It's sort of hard to describe the sound, but you'll know it when you hear it. One thing you do NOT want to hear is a grinding or groaning sound as the tool makes it's way through the stock. That tells you that the tool is jist moving metal rather than cutting it.

Your cutting tools should also have a tiny radius at the tip. The softer the metal, the larger than radius can be. If there is no radius, you will get a rough finish, even if the tool is cutting smoothly. I don't know what shape your tools have, but a good experiment to try is to just make a plain round-nose tool of HSS. You don't have to be fancy with rake and chip breakers, just a simple flat top with a 60 degree angle and a small radius at the tip. These are very forgiving of set-up angles and they do a good job of making a smooth cut if you've ground it properly. If your cuts aren't clean, then it's something else.

 

[peterson]

I'm a fellow novice. Not much to add to Clive's advice, except to restate the need for a rigid setup. Make sure nothing is moving but the work piece, and the cutting movement of the bit. On my first cut, the cross slide had some unintended play in it, making for a poor cut.

This fellow does some dangerous things. Good information on set up, however.
http://techtv.mit.edu/collections/ehs-videos/videos

Google books has a copy of machinery's handbook, though it is from 1914. Not really an issue, as some of the lathes around here are from 1914 too. = )

good luck,
tom

 

[Mike C.]

" Replace the rocker with a solid washer of a suitable thickness to put the tool tip on centre height at about 1/2" extension."

Better yet, learn to center the cutter. That way you can easily find center no matter what type of toolpost system you use, without having to go for a height guage or a reference mark. Just like learning to grind your bits, it's part of the job. Learn ow to set your bit to the proper location by eye. It will take some time, but you'll catch it quickly. I am a big proponent of Armstrong holders and HSS cutters on these older, slower lathes. I started with Armstrong, I understand them, I learned their weaknesses and virtures and find their virtues far outweigh the weaknesses of other toolpost types. The problem with Amrstrong holders is the operators, not the tooling.

The straight holders are actually quite handy for threading and anything else where you don't get up close to the chuck or need to work closely between shoulders. They can also be used easily for counterboring, short internal tapers, and short boring work if you grind up a little boring bar. Just another trick in the bag. Same for angled cutoff blades, left and right.

First thing I would check on that scratchy finish is the edge of the bit behind the point where it is cutting. Sounds like it may be rubbing after the cut. You'll see some powdery metal and/or a shiny spot where that is happening if that is the case.

Get on center, make sure the backside isn't dragging, get the depth of cut up and the feedrate down, and keep the speed correct. Simplified version of speed calcs is 500rpm for .500" in steel. 1,000rpm for 1/4" and 250rpm for an inch. Interpolate from there. .125" is 2000rpm, 125rpm for 2"... 62.5 for 4", which is within 5ft/min of the 68ft/min figure given above, but a lot faster to figure. Triple these numbers for aluminum, double for brass, half for stainless and titanium. Double all figures for carbide cutters.

Carbide is funny stuff, It has to be going fast enough to give a real nice finish. The faster you push it, the better the finish, until it melts... which is REAL fast. If you run too slow, carbide gives a bad finish and will be very likely to chip. If you ever stop the work with a carbide tool in contact, the cutter is chipped. That means stalling, turning by hand with the tool in contact, etc... turning backwards is instant carbide death, too.

Carbide is like trying to work with glass. It breaks at the slightest error. Learn to run the machine with the HSS and then try the carbide so you don't break it. You probably already have, unfortunately.

 

[TTalma]

Thanks for all the tips. A little more info. The metal I have, I am not sure what hardness or type it is. I have some steel, brass, and aluminum. And that's about all I know about it.

The metal came with the lathe there was a box of it that the seller said goes with the lathe. Most of the metal looks like test pieces for parts, as there is the same part a few times in various stages of completion. And all the cuts on them look nice an smooth.

The cutters that came with the lathe are HSS. There we're two boxes, one label sharp and one labeled dull. I used the ones from the sharp box. a couple are new and have the rubber stuff on them. I used one of these. I compared the angle and grind on it to a south bend lathe army manual I found on one of the PM members web site. They matched exactly, so I think think its an OE (Operator Error) issue, not one with the tooling.

Clive mentioned the set up angles etc. This might be where I'm going wrong. I just set the angle of the slide so it wouldn't be in the way. and what eve angle it ended up at is where I left it.

Unfortunatly I won't be able to get in the shop until probably this weekend. I will try to take some pics of my set up then. I will try the suggestions offered here, and let you know how it goes.

Thanks for the help, -Tim

 

[Clive603]

Sorry Tim I forgot to mention approach angles for the tool cutting edge.

General principle is that the tip trails the leading cutting edge on any tool that gives a respectable finish. Typically roughing tools used to shift metal fairly quickly and capable of giving an OK finish if you use a shallow cut, say 10 thou, have the leading edge at 60 to 70 degrees to the axis of the work-piece on lengthways cuts and equivalent geometry for facing. Finishing tools typically put the leading edge at around 45 degrees to the work piece followed by a parallel section at least 2 or 3 times the feed per rev. Trailing the tip ensures that cutting forces push the tool back against the slides and feed screws. If the tip leads forces will tend to pull the tool into cut, clearly bad news if you have any backlash or slack as an old lathe always does. Only exception to the trailing tip rule for normal tools is the knife tool where the edge is at 90 degrees to the work axis and parallel to the cut. Knife tools chew of metal fast but leave a bad surface. On a 9" South-Bend I'd reserve them for trimming flanges square.

Parting tools and grooving tools must of course go in square. But parting off in particular is a whole new world of problems. To quote W.A.Chapman in Workshop technology "When it comes to the apparently simple process of parting off theory and practice may need considerable manipulation to bring them into agreement."

Mike knows I was being a touch facietious concerning the straight Armstrong holder! It is of course desirable to learn how to set a tool on centre height without the assistance of a gauge. Hafta say its an art I've never mastered to sufficient reliable accuracy so I always use a gauge to check. I always found that when people are getting started its best to minimise the variables and have a set up where one thing at a time can be set and stay set. Hence the suggestion for bolted down holder and angles ground on the tools. For this sort of work its quite acceptable to bolt a holder up from flat and block stock material. The South-Bend Tee-nut size is just right to make from two pieces of flat stock too.

Had a steady parade of vac students and trainees though my optics research lab over the years. Always amazed me how different people found different things hard and easy. Had two one year who were absolute yin and yang, what one would struggle with the other would manage almost without being shown.

Clive