Author Topic: Help please  (Read 534 times)

Offline Dell

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Help please
« on: January 14, 2023, 10:05:18 AM »
Well I have made a couple of lathe chuck keys for my Pultra four jaw chuck out of HT socket head cap bolts and happy with the finished product, my problem was or is that I started off using the crosslide but the finish was rubbish ( more like a super fine thread than a smooth finish) I tried using indexable carbide and HSS steel but no better, tried altering speed up and down, I ended up using the hand graver as I got a much better finish , I know I only have a small Pultra instrument lathe but I should be able to get a decent finish I must admit I very rarely use the crosslide because most of what I make for antique clocks is small and I much prefer using the graver, what am I doing wrong?
Dell
 

Offline John Rudd

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Re: Help please
« Reply #1 on: January 14, 2023, 11:39:05 AM »
Crap material, tool not on centre height rubbing rather than cutting.

Blunt tool, speed too fast or slow, feed rate too fast or too slow...cutting depth too great..

A case of eliminating the above....
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Offline vtsteam

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Re: Help please
« Reply #2 on: January 14, 2023, 06:24:21 PM »
Also worn bearings, worn slides in need of scraping. gibs screws not adjusted or gibs worn to curves. Too much tool overhang.

Check for play throughout.
I love it when a Plan B comes together!
Steve
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Offline Joules

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Re: Help please
« Reply #3 on: January 15, 2023, 02:18:38 AM »
Try machining a non ferrous piece of metal, do you get same or better finish using HSS.  If I remember rightly the Pultra is a small lathe, sharp HSS tooling would possible compliment it better than insert tooling.  Having noted you tried HSS, what was your tool grind like, sharp, positive or negative rake and finish on the tool ?

A picture of the setup is worth a thousand, dodgy replies 😁 

I should also have added were you machining with coolant, oil or dry ?
Hard bolts are pretty bad for surface finish cut dry with an almost scaly finish from shearing the surface.  Especially on a small not so rigid machine as already mentioned.

Stick out !!!   How far out from the chuck was the workpiece, was it unsupported.  Turning between centres is preferable, followed by fixed steady if working away from the chuck, say 4-5 times the diameter.
« Last Edit: January 15, 2023, 03:53:38 AM by Joules »
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Offline Dell

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Re: Help please
« Reply #4 on: January 15, 2023, 02:06:22 PM »
I was turning between centre and for extra support I was using a fixed steady the indexable bits are this
https://littlemachineshop.com/products/product_view.php?ProductID=3696&category=
I was using cutting oil, I think probably the bolts had something to do with it as I get a reasonable finish ( not perfect), tool on centre, I have checked the gib adjustment, donít think I had to much tool over hang but I will thank a picture of the setup tomorrow.
But thanks for suggestions so far, itís not something I do very often but it would be nice to understand why.
Dell

Offline vtsteam

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Re: Help please
« Reply #5 on: January 15, 2023, 08:31:42 PM »
I dunno, not super experienced with insert tooling, but have been using them more the last 6 months. "No chip breaker" in the one you referenced seems like it might need a lot of power and a stiff machine. All mine have chip breakers.

Lately I've been using a lot of DCMT070208 inserts for both cast iron and tough hot rolled steel. My present homemade lathe is stiffer probably than the Pultra. But I think I've also used them on the Gingery with okay results. They do have a fairly big tip radius.

However, there is no doubt in my mind that traditional lathe tool steel tools, properly sharpened, will be sharper than carbides (at least the ones I've used), and thus require less turning power and be easier on a smaller lathe. Properly sharpened is the key however The angles and the radiused tip have to be right and the honing fine. They won't stay sharp as long as carbides, especially if there is scale or casting surface skin, but they definitely will start sharper.

I also suspect the bolt steel itself as you and John mentioned will be harder to do a clean job on, but the fact that you can get a good finish on it with a graver seems to indicate it should be possible with conventional lathe tooling.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline Dell

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Re: Help please
« Reply #6 on: January 16, 2023, 07:37:19 AM »
Here is a picture of my setup but please bear in mind that I have set it up with the finished product, as I turned it down prior to milling the square, the second picture is the HSS bits I used and the carbide were the same pattern.
Dell
 

Offline Dell

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Re: Help please
« Reply #7 on: January 16, 2023, 07:39:58 AM »
 

Offline vtsteam

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Re: Help please
« Reply #8 on: January 16, 2023, 08:33:23 AM »
Tailstock:

Looks like you have a long extensions(s?) on the tailstock. That live center looks out of proportion. Try a short dead center for more stiffness. Keep the tailstock extension at a minimum and locked.

Steady rest:

That steady looks very light, and is located fairly far from the cutting location on the workpiece. That circular base makes zero sense to me. I'd make a new steady (they are not hard to make) that is solid, and keep it close to where you're cutting.

Workpiece: I assume you weren't cutting the thin end first as shown here, but did that after turning the midsection. Part stiffness and order of operations is important. But you did note that you had just set this up with a finished part.

Insert:

I can't tell anything about the insert from the pic (all pics are quite small). But again, I'd suggest using a regular hand-ground lathe tool of the largest cross section your toolholder can take. Probably 1/4" but if it can take 3/8", all the better for stiffness.

Toolholder:

That toolholder puts the location of the point of the tool far from the center of the toolpost. In other words produces a long overhang. I don't know if that is original to the Pultra. I'm guessing not.

Very small lathes are not improved in stiffness by miniaturized typical quick change tool posts. These tend to reduce lathe stiffness and increase twisting stress on the carriage and slides compared to simpler traditional toolholders. They are really suited to larger lathes.

I think an ideal toolholder system for a small lathe is the "Rose" type, which appeared quite awhile back in Model Engineer. Invented by a Dr. Rose who actually lived in the New Orleans in the US. These are very simple to make, and have the greatest stiffness possible on a small lathe.

Many of the modern "must haves" for lathes: insert tooling, quick change tool posts, live centers, and three jaw chucks are counterproductive for small lathes where stiffness is the greatest concern. Faceplates and collets, HSS tools, dead centers, and a simple short traditional tool holding system work better

The name of the game here is solidity, stiffness, short overhangs everywhere, sharp tools, and proper feeds and speeds.
« Last Edit: January 16, 2023, 09:01:12 AM by vtsteam »
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline Muzzerboy

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Re: Help please
« Reply #9 on: January 16, 2023, 08:57:34 AM »
As noted above, these inserts have zero top rake angle, which is the last thing you need on a small machine. If anything, you can afford to run more top rake than on a "normal" machine, perhaps as much as 30 degrees or more. This results in lower forces and cleaner cutting. Even a large machine would struggle to get a decent finish in high tensile steel with a zero angle.

Personally I don't understand the logic of using HSS on indexable inserts when there are far better carbide inserts available. I'm also at a loss to understand how LMS can charge $11 per insert - but I guess if you don't ask you don't get.

For non-interrupted machining like this, you should try a CCGT060202 insert or similar (that's the metric nomenclature but the first 4 letters should be the same in the imperial system). The "G" indicates a higher tolerance which in practice means a polished (sharp!!) edge and the last digit indicates the tip radius - a smaller radius will result in lower axial and radial forces which can be helpful on a small machine. These are often described as "aluminum cutting" but they are also sold for "finishing" or "semi-finishing" in steel.

The more common CCMT inserts have a noticeably duller edge, which is a tradeoff between robustness and sharpness but makes life more tricky on a smaller machine.

If this doesn't work for you, grind some HSS tools with plenty of top rake and make sure they are razor sharp.

Offline Joules

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Re: Help please
« Reply #10 on: January 16, 2023, 09:56:44 AM »
Well there you go I learned something, didn't know you could get HSS inserts, dreamed of having or making them myself.   Saying that I haven't looked at tooling inserts in ages as I built a stash over the years.   The nearest size lathe I use to yours would be a Taig.  I use their tool posts as quick change and it keeps the tool close to the carriage centre.    Not much more I could add that others haven't said except.   Would you be prepared to set up a grinding spindle to get a quality finish.   My Taig gets used for that a lot as I can set it up outside or under the carport to keep grinding dust away from other equipment.  Wrap the lathe in polythene or newspaper to catch the dust.  I use a Proxxon spindle with little grinding stones, very light loading so even a not so rigid setup can produce good results.   Easy to then polish if that's a finish you are looking for.

LOL sorry if that's telling you how to suck eggs, by the sound of it you have good graver skills and a very clean looking lathe.

I just saw your tiny top slide, that also won't add to rigidity, could your lathe be fitted with a miniature Gibraltar style tool post directly on the cross slide, looking at the casting in your picture ?  In fact a rear toolpost might be a better option as you already have the mountings for one and can keep the tool post you have, as is.
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Offline Joules

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Re: Help please
« Reply #11 on: January 16, 2023, 11:37:17 AM »
Just watched Stefan Gotteswinter's latest video, BNC style connector.    Quite applicable to what you are trying to achieve or anyone else that's looking for a good finish on small parts, and his method keeps grinding dust away from your machines.
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Offline Dell

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Re: Help please
« Reply #12 on: January 16, 2023, 12:02:39 PM »
Thanks for all the suggestions and advice, as I said earlier itís not something I need to do very often as most of what I do is with a graver, IE make and blue screws out of blue steel, arbors with silver steel, other parts with brass, next time I need to do something similar I will try the advice above .
Many thanks Dell

Offline Joules

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Re: Help please
« Reply #13 on: January 16, 2023, 02:45:23 PM »
You're welcome Dell, the questions you asked are applicable to lots who start out or come on the site looking for help and advice, so we tend to answer in a more general fashion.

Be good to see some of the work you do, and details on your graving activities, not something we see as much these days.

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Offline Dell

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Re: Help please
« Reply #14 on: January 17, 2023, 04:36:53 AM »
Some work I have done using a graver
Making a screw

Making a replacement pendulum adjuster 

Making a new hand washer
 

Dell

Offline Joules

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Re: Help please
« Reply #15 on: January 17, 2023, 07:08:16 AM »
You're a Ringer Dell  :lol:

Had me convinced you were new at this, got some skill with a graver...   For anyone else's interest, you should take a look at Dell's YT channel.  I subscribed so I can take a better look over time as he has some very interesting looking videos.

Have you sorted a bigger rest yet ?

Really nice work Dell, thanks for posting and glad I asked.
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Offline Dell

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Re: Help please
« Reply #16 on: January 17, 2023, 08:18:04 AM »
Yes Joules
Someone on the model engineering site kindly offered to make me a 20mm. Extension so I could use my original T rest designed for a 50mm centre lathe.
Glad you enjoyed the videoís and thanks for subscribing.
Dell