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 Post subject: Re: More progress
PostPosted: Sun Apr 29, 2018 4:42 pm 
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Joined: Thu Nov 19, 2009 2:51 am
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Location: Colorado USA
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No problem there guys,

The casting was set up using a 1-2-3 block in one corner then leveled and indicated with the casting supported by two additional machine jacks on corners opposite, the remaining corner's jack was "kissed off" under the diagonal corner for additional support & clamping force.

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Ken


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 Post subject: Re: More progress
PostPosted: Mon Apr 30, 2018 12:52 am 
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Joined: Fri May 13, 2016 2:42 am
Posts: 142
Location: Da Nang City Vietnam
Boat Name: Alphington
Quote:
Castings are naturally annealed when cooled slowly and typically have very little internal stress, unlike a weldment.


While it is true that a casting would be highly unlikely to have the levels of stress that are seen in a weldment, most castings will have some amount of stress, and some will have significant stress. There is an entire art written around the ways to reduce and later remove the stresses in castings. Simple green sand iron casting with varying thicknesses often have significant stress. Green sand castings (the worst case) do not cool at even rates though out the casting as the evaporating water has an uneven cooling effect on sections of differing thickness. If the manufacturing process of an important iron casting is unknown the best way is to have the casting stress relieved, the next best thing is to use 3 point clamping and remove most of the metal from every machined surface on the casting and then go back and finish cut everything. All good machine and engine building companies have their castings stress relieved prior to machining. Many years ago I machined the first cylinder of a new racing IC engine block only to see it move 0.003" out of round after I machined the second and third cylinders. It can be very disappointing. Never assume that a job shop foundry took particular care with the filling or cooling rate of the casting unless you organized the casting with them yourself.

Lionel


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 Post subject: Re: More progress
PostPosted: Mon Apr 30, 2018 1:58 pm 
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Joined: Fri Nov 27, 2009 5:56 pm
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Location: Northwest Detroit
Boat Name: Iron Chief
Quote: "the best way is to have the casting stress relieved, the next best thing is to use 3 point clamping"

In my experience, a large flat part held only on three points invites chatter and excessive deflection from cutting force resulting in inaccurate/unattractive surfaces. It's really case by case basis, if the casting has enough mass for vibration dampening weight and strength, then three points is probably sufficient, but in my experience, it typically isn't for other than establishing an initial plane of reference. We all work differently, but we all get the job done.

Machining some of these weldments that these "Cad operators" (inexperienced kids) draw up, Ive had to use stabilizers like those used on compound bows, clamp bars on with with weights (1-2-3 blocks) out on the end to dampen vibration. Well, "a calm sea never made a good sailor", so I guess they were helping me get better at what I do :lol:

-Ron


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 Post subject: Re: More progress
PostPosted: Mon Apr 30, 2018 2:31 pm 
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Joined: Wed Dec 07, 2011 6:41 am
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Location: Lopez Island, Washington State, USA
Boat Name: S.L. Folly
I had a flat thin casting that was so flexible that I had to bed it in plaster before I could even think of starting a cut or two. It worked so well that after I spot machined a few reference places I turned it over and, after supporting it accurately on those points I bedded it in again. Just a plastic bag on the mill table and then playing mud pies.

It was a dumb design but the customer went away happy.

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 Post subject: Re: More progress
PostPosted: Mon Apr 30, 2018 3:17 pm 
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Joined: Thu Nov 07, 2013 9:16 pm
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Location: Northeast Ohio, USA
Boat Name: SL Nyitra
Lopez Mike wrote:
I had a flat thin casting that was so flexible that I had to bed it in plaster before I could even think of starting a cut or two. It worked so well that after I spot machined a few reference places I turned it over and, after supporting it accurately on those points I bedded it in again. Just a plastic bag on the mill table and then playing mud pies.

Thanks Mike, I never heard that trick before. Makes perfect sense. I'll keep that in the back of my mind! :idea:

-Andy


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 Post subject: Re: More progress
PostPosted: Wed May 02, 2018 6:29 am 
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Joined: Fri May 13, 2016 2:42 am
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Location: Da Nang City Vietnam
Boat Name: Alphington
Our new Machining center has some spooky vibration control software. The machine detects and measures harmonics in the cutting cycle and automatically varies the RPMs and feeds in order to avoid harmonics. The system is self teaching. If you find that a part starts to chatters you simply turn on the software and the machine will determine the optimal speeds and feeds, usually still leaving some evidence of chatter marks. On the second cut the machine will predict the harmonics in advance and set the speeds and feeds to avoid harmonics all together in most cases. RPMs are continually changing through the cutting path and in a pattern that causes the harmonics to be avoided and even self cancel. Like everything it has it's limitations but still quite amazing. We have yet to run castings on this machine but we have a product in the pipeline with a few small iron castings that will put it to the test.


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 Post subject: Re: More progress
PostPosted: Wed May 02, 2018 1:32 pm 
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Joined: Fri Nov 20, 2009 5:34 am
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Location: Phila PA USA
Boat Name: Margaret S.
Wow, almost like magic! The old way, changing feeds and speeds, clamping a heavy piece of metal on the work piece, or on the tool holder, fooling around with the positioning, to improve vibrations while machining has been used for well over 100 years. The old method has highly variable success, and generally depends on the experience of the machinist to get good results.


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 Post subject: Re: More progress
PostPosted: Wed May 02, 2018 2:18 pm 
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Joined: Wed Dec 07, 2011 6:41 am
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Location: Lopez Island, Washington State, USA
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Cave man machining here. I have a hunk of heavy wall lead pipe I can hang on either the exposed end of a boring bar or on the tool post to damp vibrations. A heavy rubber strap I can wrap around larger cylindrical things.

Of course my new lathe was bought by my dad in 1946.

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 Post subject: Re: More progress
PostPosted: Wed May 02, 2018 4:19 pm 
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Yeah that is impressive, unfortunately oftentimes it doesn't matter what the feedrate, RPM or depth of cut, it's just going to chatter and if it can chatter, it's not secure and still deflecting from cutter force, chattering or not. It goes back to the design of the part and the processes necessary to complete it. It's poor design or not secured properly.

Who makes that control?

We're discussing milling here, but for boring bars, the typical cause is too much radius on the tool. Near sharp pointed ground bits or inserts (if they can be found) will produce the least amount of vibration in the boring bar. For boring inserts, I always get the smallest tip radius possible.

-Ron


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 Post subject: Re: More progress
PostPosted: Wed May 02, 2018 5:57 pm 
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Location: Lopez Island, Washington State, USA
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Oh, yes. A dull or too wide a cutting edge is a sure route to chatter. That said, there are often situations where I have to use a boring bar that is small in diameter and long for that diameter. And there are limits to how much time I'm going to spend on light cuts. Dampening has its place.

On a rather different subject, I'm not in sympathy with CAD, CAM, and CNC. I want to make one part and get the hell out of the shop. The setup time and learning curve for all of those things is just not worth it.

To the intense irritation of my spouse, I feel the same way about food and cooking. If it takes longer for me to cook something than to eat it, things are out of whack. Now if you like to cook or draw or program machines, go for it. But to argue that it will get my steam boat or motorcycle out in the world operating sooner just doesn't fly. I, of course, keep my thoughts to myself when she is doing the cooking!

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