How to get a better Finish

Have a look at Gadget Builder's site for this finishing tool:

http://www.gadgetbuilder.com/VerticalShearBit.html

Just realised that this is what Ed Duffner mentioned earlier on -

apologies to you, Ed.

Dave
The Emerald Isle

Edited By steamdave on 31/08/2015 12:06:45

No Problem Dave I should have done the extra bit and added some links.

Ed.

It might be interesting to compare the results with that tool with a raked tip carbide tool intended tor finishing stainless on the same material. Both very light cuts. Image scale is similar too. The raked tip one is twice life size on my monitor. Not sure about the other. I suspect it's a bit less.

The shaving tool

Raked carbide tip, TCGT type

Both show signs of micro tearing and mine shows another problem cause by slight vibrations form a wobbly pulley on the counter shaft, a sort of banding. Some of that will be down to head stock bearings as well. The marks are a few microns deep so it feels as smooth as glass. Doesn't look like it will though.

There are limitations what ever is used and the only solution is high end machines that are also in perfect condition. Mirror finish on more mundane gear needs something else - polishing or sometime planishing with a hefty ball ended metal spinning tool which is effectively cold working the the surface of work..

John

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Hello Dave,

A couple of points; firstly get yourself a copy of Lathework A Complete Course by Harold Hall This book assumes no previous experience and secondly If you've a model engineering society nearby, make contact and there will be a wealth of knowledge there to tap into. What you really need is to watch someone setting up and using the lathe, you'll learn more in ten minutes watching someone. We all get there in the end, but in the beginning its all a bit of a mystery.

Regards,

Bob Youldon

I tend to reduce the tool to work piece clearance when making a finishing pass

The section nearest to the chuck is with a larger clearance, the section the tool is in has the tool set at the angles shown and the beginning section with a setting in between. I can do better but just had a quick play to see the differences, 540 rpm, Sandvik tip with a small radius.

Bob

This is 0.45% carbon steel (1045) with an unground (yellow) CMMT insert . Even without a ground insert you can get a decent finish.

The skill is often to make sure you take a decent final cut rather than tickling it. This requires confidence in the use of your dials or DRO (if you have one - I don't). And use higher speeds rather than lower. This is often the opposite of what you might do with a HSS tool which may be why people struggle to get a good result with indexable tools.

Murray

For the best finish with HSS, I tend to go with sharp tool, slow speeds and light cuts. I haven't mastered carbide tips yet, but given I do OK with HSS, then not sure I'll get to it any time soon!

One thing about the speeds and feeds charts is that I don't think they apply to me - I have a small benchtop lathe and my time is free so spending time stoning a lathe tool is a non-issue. If I had a production shop paying the mortgage, the bailiffs would have been round long ago. Carbide tips are about getting work through the shop fast aren't they?

Steve

Hi I I am novice to this game, and also had issue and still do getting a good finish, I spent some time with scrap metal turning down and trying to get a good finish, using this forum was inform about HSS grinding, played with grinding tools trying different things, some slight changes to tools can make a big difference. My advice would be to experiment, practice and take note on what works.

The more you do the better it becomes i do tend to keep tools that work as reference, one thing I was surprised was after grinding, use of a stone made quite a difference to finish although I thought the tool was sharp from the grinding.

That steel Murray has turned is usually supplied annealed and in black bar. Black bar is often the best thing to go for with steels because it hasn't suffered the torture of being forced through dies like bright drawn. Less stress so cuts more easily. I can also see signs of an oil film on it and would guess it was cut on a better class of machine than many have. Hard to tell without a shot that is ideally larger than life. Cutting oil will help finish even just a smear with a brush. Same with brass and aluminium even though they are usually supplied for steels.

He also makes a good point about decent sized cuts. I played with a super adept type thing some time ago, bit bigger. say +50% and best cut for finish in mild steel was around 0.050", 0.100" off the diameter. Rather excessive and needed because there was a lot of play in the head stock bearings and the saddle would rock side to side. Big machines in decent condition rather than perfect often benefit from cuts in that sort of range. HSS - Carbide I don't think it matters in that respect. I don't think carbide tips really need high speed either.

It's a very mixed area though. That one I posted is silver steel - same as the one with the shaving tool. I bought a stock of it from somewhere that had decided to get rid of it. Could be because it's a little on the hard side so tears easily. It was cheap.

Steel itself varies. They used to have a term for poor quality stuff during WWII - park railings. Steel was in short supply so they melted down any old junk.

John

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I got lucky with that stuff. I found a 15" length of it in the offcut bin and bought it for peanuts. The guy behind the counter had to check with his supervisor that it as OK to sell it at scrap value. He was obviously in a good mood that day. In fact it was a ground finish, so already cleaned up for me.

These were parted off and the groove was done with an (indexable) grooving / parting tool, so will have been oily as I part off with feed and coolant. But the final facing and counterboring will have been done fast and dry with a decent depth of cut - carbide needs the metal to get filthy hot for it to work properly.

As Jon says, every machine (and operator) is different. I experimented with feeds and speeds on my Bantam with these tools, turning the speed up and down with the VFD to see how the finish was affected. There are some combinations of feed and speed (and depth of cut) that give good finishes - and many that give a torn, matt finish or chattering.

One of the benefits of carbide is not having to keep honing the cutting edges. And when roughing out, you can shift metal in a way that would challenge HSS.

Murray

I spent a day in the shed I tried power feed carbide and HSS lots of different speeds and feeds my scrap bin is overflowing.

I think I need to find another hobby.

On a hobby lathe, I don't think you can beat a HSS tool with loads of side rake (45 degrees in this case) for actually shifting material:

Rod

All mild steel is not created equal. For instance EN1A is easy peasy, but EN3 is 'orrid stuff and will tear at the least opportunity. Have a read of this thread:

**LINK**

The thread is concerned with carbide insert tooling, but it should highlight the wide variations between even similar metals in terms of surface finish versus turning speeds.

By way of encouragement these partial (unthreaded) nuts were turned from EN1A using home ground HSS tooling and hand feeds:

They were done on a repetition lathe, which doesn't even have conventional leadscrews and dials, the slides are operated by hand using levers. For reference the AF dimension is 11mm, turning and parting off done at 1000rpm; the finish doesn't look too bad.

Andrew

turning steel to a decent finish on the ordinary old ME type lathe i think is quite an art and not easy. my workshop equipment is very basic and antiquated.

this thread reminds me of when in my teens i made a new Church key about 12" long for a Victorian lock. i was rather proud of the result and took it along to one of the ME club meetings where it was passed around. the shank was long and tapered.

'What screw thread did you turn it too?' was the response!

that incident im still reminded of 30 years later!

i think a lot depends on what type of lathe you have. i have learnt subsequently how to get a decent finish on steel on my old knackered antique equipment, with a few dodges and wheezes, and unorthodox tool angles.

has anyone else found that with HSS tools for steel the tool will cut a better finish after a bit of use after grinding up? i always stone and hone the tool after grinding. ive used the same tool for axle ends and crankpins for donkeys years in miniature loco work.

cheers,

julian

Edited By julian atkins on 01/09/2015 00:31:32

I mentioned the effect of side rake in another thread Rod. Probably a waste of time but maybe some will try playing around to see what happens. That and top rake can almost make a machine self feed - wish people luck trying to do that with carbide. It's all pretty simple really the top and side rake form a wedge. The the side rake tends to prevent the tool from just feeding itself into the work. There aren't any formulae to work out the angles and model engineering pundits as far as I am aware don't even mention what the rakes do. The sharper the wedge the less cutting power is needed which is where carbide falls down. Deep cuts with carbide tooling needs more power. The problem with hss used like this is that it will wear out more quickly because the wedge is sharper so the cutting edge is finer - that's where the various angles come from in tables that say so much of this for that material etc. They are trying to make the tool last longer.

I have to admit that the rake tipped carbide inserts do improve carbides performance in this area because the allow a more acute wedge but the degree of side rake that can be added is limited. If some one want to use indexed tips though they are ideal for small lathes and allow big sods to work better as well. They also wear out. The only time I have had chipping problems is on a lathe with rather loose head stock bearings. I have had the coating wear off, the finish deteriorates. In fact the drop off is noticeable even before that can really be seen. All HSS needs is a quick polish with stone which can be done in the lathe taking care not to round the cutting edge off. That's best done before there are real any signs of wear. Say just before every finishing cut.

What I have done in the past when I need to remove a lot of metal is grind some top rake into the cut. Say 20 degrees. I then grind the side rake on it and hand feed. If it seems to take a lot of effort I add more side rake and so on. I don't go too far because the tool will tend to grab and self feed at some point especially on softer materials. Fortunately I don't have to do it very often as I hate all of that swarf - it's wasteful. The last time I did it was on a Peatol which really did need help removing a lot of aluminium from a 6in dia start. I should have made the part from 2 pieces really.

John

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Edited By John W1 on 01/09/2015 00:35:00

Just twigged what this is - the 'traditional' design is a ground bit ground half way with a bit of top rake and rotated by about 80-degrees. Unlike a square tool you can adjust it to get the best setting.

Neil

The best thing to ask for if finish is really important on mild steel is leaded free cutting. Ideally black bar as bright drawn will have a skin on it that wont machine as well. The only catch with it is that it can't be case hardened. I assume it's still available. It will vary according to where it comes from and the state of the moon etc. All steels do so both good and bad can turn up. I think standardisation hasn't helped that aspect. I worked for a large company and they produced a booklet on metals listed for specific purposes and brand named rather than ISO numbers etc. It worked well because quality of what came in was more controlled.

I suspect the speed aspect often mentioned in relationship to carbide tips really relates to feeds and speeds and cutting pressure removing play and sources of vibration.

One thing that is bugging me at the moment is why my grinder in the garage produces better tools than a cheapo one I have bought for use in the house. The grade of wheel in the garage is obviously not ideal for HSS. The one in the house appears to be better. It cuts a lot cooler so more can be done in one go. More slowly as well. I reckon it has something to do with wheel wobble. Brief periods when the wheel cuts more and also generates more heat that I don't notice. Great fun because not much can be dressed of a wheel at a time otherwise the diamond is likely to fly out. The wobble also makes it more difficult to feel the angle being ground accurately back onto the wheel.

John

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More likely to do with increased production rates? Referring to the link I gave in an earlier post with some metals carbide gave as good, or better, finish at HSS speeds. In others, EN8 in particular, it needed to be run at high speeds to get anything like a good finish. In theory the cutting power needed per unit of metal/minute removed goes down as the speeds increase. The idea being that the metal in the shear zone is hot (red hot ideally) and so is softer and less force is needed to shear it.

Andrew

When I first started with my new lathe (twenty odd years ago), an engineer friend (for the Kiwis, he did his apprenticeship with Andersons, engineers in Christchurch, they built stationary engines), told me that if I wanted to get a really good finish, a carbon tool steel treated carefully would out do HSS any day, just halve the speed, and keep the feed down, and it will keep it's edge sharper than HSS.

Ian S C

Production rates do rule but having heard huge machining centres being set up the job sometimes limits that and speeds a feed rule again. They can only go so far and need balancing according to the work and the machine.

The red hot aspect is interesting. Hadn't thought of that but would wonder about some alloy steels. I once asked some one at work about what steel supplier they used, just to try another. He phoned up and asked them to supply a certain alloy steel as a joke. I'd guess it was some sort of relatively low temperature low distortion hardening material so you can guess what happened when I treated it like mild steel. It's one of those materials where HP really matters and I hadn't got enough. It also made short work of HSS due to that and brazed tip carbide wasn't keen on it either after it had hardened. I'd wonder if that sort of stuff could be got red hot quickly enough. I have machined nitriding steels at work with HSS without any problem at all. Even moderately hardened plug gauges.

When he asked me how I got on just said fine and he looked surprised and walked away. I think the piece is still in the garage. At times it might be useful to know what it was.

Another link that shows what Pb free cutting and others do machining wise.

**LINK**

John

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