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Blog entry by tyskkvinna posted 06-25-2012 01:47 AM 1701 reads 0 times favorited 10 comments Add to Favorites Watch

I was asked about my turning process. Specifically, about the limitations that I have when turning on a CNC (metal) lathe.

This is a diagram of my setup, it will look pretty familiar if you’ve ever used a metal lathe.

this is a picture of the exact same setup in real use so perhaps it will be a little easier to understand.

The toolholder runs on a track that moves on two different axises. “Z” moves left and right, “X” moves forward and back. Between these two, you have the coordinates for every move you can possibly make on a lathe.

You cannot use a tool outside of the toolholder – for example, a hand-held chisel. It would be extremely dangerous. There’s no rest for your hands and the toolholder won’t work as a rest because it is at a dramatically different height and you can’t change it (or the angle). It’s very rigid.

You take the tool and mount it in the holder, with grub screws. Your tool has a cutting end that resembles a scraper, and the rest of it is a rectangular block.

Now, I hope you can see where some of the limitations come in.

Something I want to make clear: I’m not complaining about how this works, but each method has its advantages and disadvantages, and this particular instance is a distinct disadvantage.

The absolute maximum diametre you can have of something in the chuck (however you mount it) is a tiny bit over 7 inches. Let’s just call it seven. (I think it is 7.12”. Might be 6.88”—I remember it is about .12” off but not which way…) At seven inches, your cylinder (let’s assume you threw a perfectly round cylinder in the lathe) is touching the toolholder when it turns and you may have a tool length of zero.

So now let’s put a cylinder in the lathe that is six inches in diametre.

You may now have a tool length of one inch poking out.

This is an example of what you can do with that much tool. On the back, you are fairly limited to depth (one inch) because the toolholder will hit the wood. If you have nothing that will collide, you can go in deeper. As long as at no point the geometry of the tool collides on either side (or into the live centre) you can get into some pretty big detail.

I’m sure at this point you are wondering why I don’t just cut the back and flip it like most of you do.

Because I cannot cut a stub into it and stick that into the chuck—chuck is too big, too forceful. It just throws them out. You either turn on centres, or on a flange/faceplate.

Trying to keep everything on centre this way is a monumental pain. I have done it before, but it’s rarely worth the effort.

And now, how you turn the interior of the bowl.

The same deal applies. As long as your wood won’t wobble (or won’t wobble enough to matter) you can dig in as deep as you can clamp a tool. In this piece the tool is comically short, usually I have 3-4 inches. I just ordered some tools that are six inches long.. look forward to doing some stuff with them.

It does go without saying that you can’t do things like hollow forming on this lathe. It’s fully enclosed.

Any questions? :)

-- Lis - Michigan - http://www.missmooseart.com - https://www.etsy.com/people/lisbokt



10 comments so far

View CharlieM1958's profile

CharlieM1958

15706 posts in 2908 days


#1 posted 06-25-2012 01:58 AM

Now I understand, Lis.

Thanks for the very clear and well-illustrated response.

-- Charlie M. "Woodworking - patience = firewood"

View BertFlores58's profile

BertFlores58

1646 posts in 1612 days


#2 posted 06-25-2012 02:21 AM

Hi Lis,
Now I understand too…. using a faceplate (as you mentioned above) will add more difficulty as you have bigger lathe chuck. Again adding to an endless idea…. Is there any chance of having a reducer chuck. It is just using a lathe dog… Clamp a dead center on the chuck then use the dog to lock the workpiece. Just a suggestion.

-- Bert

View tyskkvinna's profile

tyskkvinna

1308 posts in 1676 days


#3 posted 06-25-2012 02:27 AM

I have been messing around with the idea of various reducers. It can be done, but every addition you put on it becomes part of the overall length, and you have to try and keep that length-diametre ratio as tight as possible if you’re going to turn without a live centre. Jim is actually coming over to my shop tomorrow to show me an idea of how he does it. We’ll see if I can adapt it!

-- Lis - Michigan - http://www.missmooseart.com - https://www.etsy.com/people/lisbokt

View BertFlores58's profile

BertFlores58

1646 posts in 1612 days


#4 posted 06-25-2012 02:50 AM

Good Luck Lis,
The most important is you are enjoying your work.

-- Bert

View whitewulf's profile

whitewulf

447 posts in 1627 days


#5 posted 06-25-2012 05:38 AM

Wood & CNC 6/24/12

Lis, As I understand, you are trying to do a bowl shape?

I have turned Large Hemi’s on cnc machines. I/we used vaccum chucks exclusively.

1> using flat faceplate(vaccum) we rough out the inside hemi with short parallel waist.
2> adding plug to faceplate that macthes waist diameter.
3> rough & finish outside.
4> replace faceplate with hollow form, countour id to fit outside countour.
5> fit hemi into hollow form, align part true, then finish inside, and face off waist.

If wall is to be thin use lower vaccumm pressure as not to distort shape. I have done wall thinner than 1/8” this way.

You can do many hollowforms this way think “amphora” with this system.

Before I retired , I was involved in Turning a large Hemi from Plastic Explosive, over 40 “ in diameter! I /we used A vertical CNC milling machine converted for turning.

Paul

-- "ButI'mMuchBetterNow"

View whitewulf's profile

whitewulf

447 posts in 1627 days


#6 posted 06-25-2012 05:54 AM

A few more details,

Use a set spindle speed, less than 900 rpm

A slim tool with less surface area at the cutting edge, 1/16”/to 3,32” or so & light cuts

Paul

-- "ButI'mMuchBetterNow"

View HalDougherty's profile

HalDougherty

1820 posts in 1927 days


#7 posted 06-25-2012 11:36 AM

You’ve made some great items with your metal working setup so far. Check Craig’s List in your area for a cheep woodworking lathe. I’ve bought 4 lathes I found on the list in the last couple of years. I got a Delta Midi lathe for $125 and a Grizzly (G1495) and the duplicator attachment (G2891) for $200! At full retail, the two lathes would have cost over $2000 with tax and shipping. Find a cheep woodworking lathe then you can choose which type of cutting you need for your project.

I just pulled up the Chicago Craig’s List and searched the entire list using “Lathe” as the search key… I’m so glad I don’t live near Chicago. It would cost me a fortune in used tools… There are some great tools at a price that would tempt me.

-- Hal, Tennessee http://www.first285.com

View tyskkvinna's profile

tyskkvinna

1308 posts in 1676 days


#8 posted 06-25-2012 01:18 PM

Well if I had a vacuum chuck that’d be easy! ;) I don’t have one, and it’s not worth putting one on my lathe because we use it for metal and wood combined, every day, and the chuck on there has through-holes. I can’t take the chuck off, everybody else in the shop would yell at me. :)

I can’t imagine why I’d use a slow, set spindle speed. What are your thoughts behind that?

We’re working on getting a woodworking lathe for the shop, because a lot of people want to use it. I don’t imagine I will use it much though, I just don’t have the time.

-- Lis - Michigan - http://www.missmooseart.com - https://www.etsy.com/people/lisbokt

View whitewulf's profile

whitewulf

447 posts in 1627 days


#9 posted 06-26-2012 05:10 AM

Even tho I am comfortable with CNC, I am much happier with Hand turning, just go with the flow.
You always know beforehand what it is going to be with CNC. Once you have written the program, it’s over, no fun for me!

-- "ButI'mMuchBetterNow"

View cosmicturner's profile

cosmicturner

403 posts in 2085 days


#10 posted 12-26-2012 01:48 AM

Vacuum chucks are more for RE-chucking and finshing a bottom or sanding, not really for ruffing out a bowl they slip out of round and power outage is real big fun with a vac chuck…flying bowls…vac chucks are nice but not the be all to end all…in my opinion

-- Cosmicturner

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