Lathework for Beginners

[Ron LadenParticipant @ronladen17547]

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

[SillyOldDufferModerator @sillyoldduffer]

Posted by Ron Laden on 16/07/2018 12:48:51:

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

[Anonymous]

Posted by SillyOldDuffer on 16/07/2018 13:38:20:

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

…………….

Perhaps someone who has done it will advise please?

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

[Ron LadenParticipant @ronladen17547]

Thanks Andrew, the advice is much appreciated.

Ron

[Ron LadenParticipant @ronladen17547]

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

[Martin ConnellyParticipant @martinconnelly55370]

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&quot. 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

[Anonymous]

Posted by Ron Laden on 18/07/2018 09:00:58:

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

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

[Ron LadenParticipant @ronladen17547]

Point taken Andrew

[Ron LadenParticipant @ronladen17547]

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

[JasonBModerator @jasonb]

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

[Ron LadenParticipant @ronladen17547]

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

[Ron LadenParticipant @ronladen17547]

Well I dont think Nylon 6 is the easiest material to work with, I am still experimenting but mixed results so far.

Andrews advice of cut slow with high DOC and feeds is certainly true. Its no use taking fine cuts with a low feed rate, it does not work. Its also true that the swarf just wont break and you have an ever increasing ball of it which tends to wrap around and hide the job. Parting off is also fun, it cuts ok but as yet I cant stop the outer edge of the cut having a thin raised wafer of swarf still attached. I found the easiest and neatest way to remove it is to use a slow speed and the tip of a Stanley knife.

All good fun.

 

Edited By Ron Laden on 22/07/2018 10:07:40

[Ian S CParticipant @iansc]

Had a Nylon job the other day, doing repairs on one of our machines, just needed to shorten a Nylon chain guide 3" diametre by 1/.4", the block was 3" long, put it in the 3 jaw chuck, and using a fairly general purpose tool(the one that happened to be in the lathe), lined up and took the 1/4" of in one cut at about 350 rpm. A clean cut, no tangles.

Ian S C

[Ron LadenParticipant @ronladen17547]

The parting tool in my HSS set is 3.2mm with a straight cutting edge, is there any advantage in putting a slight angle to the cutting edge. Parting off some aluminium this morning and I was getting a bit of chatter, the tool is sharp with a good edge and wondered if a slight angle would help.

Also using the correct speed or near as damn it for turning and facing a given size of material is that speed good for parting or should the speed be adjusted when parting..?

Ron

[Tim StevensParticipant @timstevens64731]

Hello Ron

In my experience, adding an angle to the business end of a parting tool causes the tool to deviate sideways. This can result in a conical face on the workpiece, if the angle is minor and the blade is stiff, or, complete failure and damage to the tool as well as the job.

Others may have different results – let's see …

Cheers, Tim

[Neil WyattModerator @neilwyatt]

I think the best simple modification to a HSS parting tool is LBSC's fish-tail shape. Round the corners and put a notch in the end. It will tend to produce swarf that folds in on itself and doesn't jam, and it does an excellent job of side to side finishing cuts for crankshaft journals and the like.

Neil

[HopperParticipant @hopper]

Posted by Ron Laden on 19/09/2018 10:37:00:

Also using the correct speed or near as damn it for turning and facing a given size of material is that speed good for parting or should the speed be adjusted when parting..?

Ron

Go slower when parting. Maybe half the speed or so. If chatter persists, slow down some more.

Generous spraying of WD40 onto the cutting edge as you go helps when parting aluminium.

Ordinary machine oil works OK for steel, and proper cutting oil even better.

[Ron LadenParticipant @ronladen17547]

Thanks guys that is helpful, I was obviously parting with too high a speed, I didnt reduce the speed from the speed I had been turning with.

The aluminium parts are 20mm diameter and the 3.2mm parting tool seemed heavy for that size, but it is the only parting tool I currently have. I have just ordered a 1.6mm HSS tool with holder which I hope is more suited to smaller jobs.

Ron

[BazyleParticipant @bazyle]

The other big problem with parting is being bold enough to keep feeding in so the tool keeps cutting rather than doing a bit then rubbing and losing its edge. Take a moment to work out what rate of turning the handle keeps the cut biting at a couple of thou minimum per rev but not too fast that it digs in. Don't be too proud to finish off with a hacksaw as the initial cut guides the blade. Also if the part has a hole down it the breakthrough is a danger area for snatching the tool. Tighter gibs helps here.

Sometimes you have to use a 'wide' tool and accept that more metal ends in swarf. Last year I was parting off 1mm discs with a 1.5mm tool and could have cried but they were only 5/16 dia so I bit my lip and took the hit to my wallet.

[Ron LadenParticipant @ronladen17547]

Thanks Bazyle, I have found that the 3.2mm tool works well as a grooving tool. I tried it on a sample yesterday in prep for turning the buffers. I didnt take too heavy a cut but it worked fine in elongating the grooves at the different diameters of the buffer ram and ram body.

[Bob McParticipant @bobmc91481]

I'm glad someone mentioned parting problems again, I have had many problems with this myself and although I can part with reasonable results I can't say I have had an easy time, in the end I purchased a new smaller width hss blade with holder from a well known supplier, this is a 1/16 inch blade which I thought would make the cuts easier.

I was disappointed again, the problem in the end is the blade holder supplied,..I looked at the one I had previously as well and I assumed they would both have been made so that fitting it in a normal toolpost set squarely would give the correct clearance angles at the sides of the blade, to my amazement I found that the holders are not made with an angled side face but are just at 90 deg so that there is no clearance at one side of the blade and double the clearance at the other; this causes the tool to move over and dig in to one side.

I made a new toolholder so that there would be clearance either side of the blade…the results were very satisfying.. has anyone come across this before…? or is this obvious to the more experienced.. I am only too willing to learn..

Bob.

[JasonBModerator @jasonb]

I've not had problems with the 1.5mm type parting tools, this lot in 40mm dia steel went without a glitch, infact I made two such parts without even touching up the tool

[Ron LadenParticipant @ronladen17547]

Well I,m hoping the 1.5mm tool and holder I have just ordered will be ok.

Its a 8mm holder said to be suited to mini-lathes so fingers crossed.

[MWParticipant @mw27036]

I have heard that ideally on a parting tool you want the sharpest corner/edge to be on the right side, so that the stock you're cutting off is going to have the clean face, rather than whats left behind. So that it kinda has a right skew. 

So that geometry kind of resembles a wood turners knife/parting tool. 

Michael W

 

 

Edited By Michael-w on 20/09/2018 14:11:44

[Anonymous]

I use a straight 3mm insert blade on the manual lathe for parting off. I'm fairly cautious on speed, around 300-400rpm, but I do use power crossfeed and a minimum of 4 thou per rev, but usually 6 to 8 thou per rev.

However on the repetition lathe I use a slanted HSS toolbit:

I've never noticed it wandering, even when the parted off part has fallen away and I'm just removing the pip. For scale the collet is 1/4" so the blade is 1/8" x 5/16" and the overhang is about 3/4". I part off at the same speed I turn, about 1000rpm. Seems to work well, although the lathe is in the brick outhouse class. About the same footprint as a Myford ML7 but weighs three to four times as much.

Andrew

[Author]

Posts