Unimat Problem

[Melkal]

Hello, I have been learning how to use a metal lathe with an inherited Unimat.
I have stumbled trying to shape a 2” steel round into the flywheel. Making the face cuts are producing a lot of chatter.
Side cuts are ok. I can face aluminum, brass etc.and get a nice finish. Steel seems to be the problem.
I’ve ground new bits, tried brand new carbide tools. I’ve tightened up everything I can, the cross slide, spindle, endplay, etc.
Am I missing something, have I exceeded the capacity of the machine? The steel came as part of a kit from Little Machine
shop for a mini steam engine, so I don’t know what it is, but some hardware store steel works
The same. Any thoughts would be appreciated. I should point out that I can cut the steel but I can’t stop the chattering leaving a lousy finish.
Thanks in advance.

[Paul Jones]

Dear Melkal,

I have a Unimat that I purchased new in 1970 and used it until 2014 when I started to make some major modifications from its original design (see http://www.homemadetools.net/forum/m...00-lathe-10111 ).

I too have had chatter problems when machining a 2" diameter part with the 3" swing Unimat. I solved the chatter problem by slowing down the lathe spindle RPM using another Unimat accessory designed to add a third intermediate pulley that further steps the speed down. You can also make your own slow speed attachment like I did for the vertical milling head (see the link). I also recommend only using HSS turning tools with a positive rake. I do use carbide tools but for any larger machined part the Unimat motor does not have enough motor power to machine with carbide. Also, the standard formulas and charts for determining turning speeds do not work very well with the Unimat when machining large parts because the lathe is too under-powered.

I reviewed some of my previous posting at HMT to find an example of machining a steel part larger than 2" on my Unimat 3" swing lathe. Here is a good example of machining a 2.5" diameter faceplate ( http://www.homemadetools.net/forum/u...3953#post17439 ) made from 12L14 steel. I used a HSS indexable tool holder and HSS T15 insert from A. R. Warner (see http://www.arwarnerco.com/ ) to do the machining and did not encounter any chatter using the slower speeds. I like the A. R. Warner HSS T15 inserts because the insert tops can be quickly honed on a fine grain stone and kept ultra-sharp. Also the High Speed Steel AISI T-15 is a very tough steel that can take interrupted cuts and used for some of the hardest to machine materials like titanium alloys and Inconel.

I use the 12L14 steel for projects machined on the Unimat because this type of steel has a machinability index of 170% which is excellent. See this chart to compare the machinability indices of other steels http://www.carbidedepot.com/formulas-machinability.htm .

I hope this helps,

Paul

[mklotz]

I must agree with Paul. Reduce the speed and your depth-of-cut; use only HSS - the Unimat is not made for carbide.

[thehomeengineer]

Hi
I am not sure what the slowest speed on the Unimat lathe is?
Generally, speeds on smaller lathes tend to be on the higher side even in the lower range, as the diameter work is normally of a smaller diameter so higher speeds are needed.
In a perfect world to find the cutting speed the formula is:
RPM = cutting speed of material x 1000/π x Diameter of material/tool (mm).
RPM = cutting speed of material x 12/π x Diameter of material/tool (inch).
Cutting speed of a few materials in m/min and feet/min when using High Speed Steel (HSS):
Mild steel: 20 -30 m/min 60’-90’/min
Aluminium: 70-90 m/min 210’–270’/min
Stainless steel: 10-15 m/min 30’-45’/min
Therefore, a 50mm diameter piece of steel (2”) would be:
Metric 20 x 1000/3x50 = RPM - 20000/150 = 133 RPM
Imperial 60 x 12/3x2 = rpm – 720/6 = 120 RPM
I also agree with the other comments made by Paul and Marv, a nice sharp HSS tool and a suitable feed rate
0,05mm/Rev or 0.001”/Rev
Also remember as the diameter is reduced the speed can be increased
Hope this helps
The Home Engineer

[Melkal]

Thanks to all. Very good suggestions. I am not wanting to invest more money into this machine, had to buy a bunch of stuff to make it usable.
I may skin the cat another way, by starting with a chunk of alum or brass, and insert some steel rods around the circumference or sandwich a piece of steel that does not need to be worked, between them. Or maybe just a 2” round brass blank, which even sounds better and easier.
I appreciate all the prompt replies from this group.

[Paul Jones]

Melkal,

If you can find the book "The Shop Wisdom" by Rudy Kouhoupt for Volume I (edited by Joe D. Rice), you will find many tips and inspirations to using the small 3" swing Unimat and all sorts of unique homemade tool adaptations for this lathe. The book also proves the capability of the small Unimat. In this book Volume I, Rudy also had a Perris lathe (Page Title) and with both lathes he created some very nice small steam engines.

One word of advice with the original Unimat SL 1000 or DB200 are the motors. The original motors are are NOT continuous duty and require approximately eight minutes running on and then about two minutes off cooling period or risk destroying the expense motors. When the Unimat motor temperature exceeds 130 F or 140 F (60 C) shut it down. You can just feel the temperature by hand and know it is way too hot. Years ago, I converted my Unimat lathe motors to the continuous duty Unimat U-100 motors.

Good luck with your Unimat and please check the books by Rudy Kouhoupt,

Regards,

Paul Jones

[pfredX1]

How do I break this to you gently? Steel be hard. Brass, aluminum and plastic are nothing like steel is. When you step up to steel machining gets real, real fast. Then as you've found out your machine's inherent limitations become clear quite quickly too. There's a reason large machine tools weigh as much as they do. It's not because people are just trying to get rid of iron either. You need that mass to absorb the vibrations cutting harder materials generates. Your Unimat doesn't weigh what a motor in a large machine does. So of course it is going to scream when you try to cut steel on it. People selling you carbide tools for your little hobbyist machine are just ripping you off and taking your money too. Which I'm sure they're completely aware of. The only chance you stand of working steel is with well sharpened HSS tools. if the edge is perfect, the angles are perfect you might be able to take a cut. A little nibble at any rate. You're punching so far out of your weight class now though it isn't even funny. The bottom line is for the kind of work you want to do you need a much bigger lathe. Heck to do a peripheral cut on a 2" round you should be turning 220 RPM. Which is a tad slower than Unimat's can go.

[Aussie48]

All very good suggestions chatter on small lathes especially is usually caused by a sloppy saddle, check to make sure it is adjusted up tight and have another go bearing in mind the suggestions of speed, tool material choice etc.

Dick

[Paul Alciatore]

I have had and used my Unimat DB-200 for over 50 years. It is the original model made with cast iron, not the aluminum of the later models. So that may help with difficult cuts like you are talking about. Facing a 2" diameter is probably at or beyond the reasonable work envelope for this machine. The carriage and cross slide are not really precision fits on the round ways. And the ways themselves can flex in a hard cut. And, of course, the linear cutting speed changes on a facing cut as you progress from the OD to the center, where it will be zero.

What can you do?

1. Tighten down the carriage lock while facing. You are not moving the carriage while facing and you don't want it to move in any direction on the ways. This will only make it tight on one of the two ways, but every bit helps.

2. Forget carbide. Get a HSS tool and use a fine stone with some oil to hone it DEAD SHARP. You want a very small radius on the tip, no more than 0.004" and preferably around 0.002". This can be easily accomplished on that fine stone. Now, I am talking about the radius at the tip, as viewed from above, not the radius of the cutting edge, which should be ZERO, including where it wraps around that tip radius.

3. Use a generous amount of a good cutting OIL. Keep the cut wet. Dripping wet. I use an acid brush (small cheap brush) or a squirt bottle while cutting.

4. A tip I found while using my 9" SB lathe is that the standard handwheels on lathes are too small. The feed can be somewhat jerky and that can cause the tool to dig in. I found that adding an extension handle can often make a manually feed cut a lot more constant. This can work on the Unimat also. An extension to the original hanwheel or a new handwheel that gives a two or three inch cranking distance can work wonders.

5. The slow speed accessory, which adds a second intermediate pulley, is a very good idea. Get one if you can. I have one and it is great. The pulley plate is just a flat plate with a bunch of holes in it so it could be easily made in the home shop. I would be happy to provide dimensions if you want.


Note: The problem with carbide tools on a small machine like this is they often have a slight radius on their cutting edges. On a multiple horsepower machine, this rounded edge just blunders through the cut. But on a small, low powered machine there is not enough power to do that so it will just skim over the surface if a shallow cut is attempted or it will dig in too much if you go deep enough for it to actually cut. Then it springs back and you see a line in the surface finish. You can hone HSS to a dead sharp edge and it can peel off whiskers that are less than 0.001" thick. So you can make the kind of cuts that the Unimat's power level is good with. If you must use carbide, get one of those diamond stones with several different grits and use ALL of them, ending with the finest. And use some water on it to promote a better finish. But HSS or carbide; you want a mirror smooth finish on your tool's front surface and top rake surface, the two surfaces that form the cutting edge. MIRROR FINISH.

[Melkal]

Dear Paul A,
Thank you for your detailed reply, I appreciate the suggestions. I think I am going to have to scale back my expectations and build in aluminum and brass. At 74, I am not going to outfit a metal shop. I have nothing I need to build, it s strictly hobby and enjoyment. Having online groups like this are tremendous sources of real knowledge. Thanks again to all.