[4]

help with turning (or otherwise shaping) a small dome

[3]
[10] Like what you see?
Click here to donate to this forum and upgrade your account!

bbaley

Active Member
Registered
Joined
Nov 14, 2018
Messages
51
Likes
21
#1
Hi,
I am just getting started researching how one might turn or form a small "dome".
By dome I mean not a full radius half-dome (ball end, etc)

but a convex shape for which the radius would be MUCH larger than the piece being machined.
The convex dome need not be a specific dimension with an exact tolerance.
I am imagining a piece of 1" round stock on which the end is shaped a dome where the center/highest point is maybe 2.5mm / 0.1"

My first thought was to make some sort of jig to hold a cordless drill that I could use on the belt sander - where the jig bottom plate had a bolt and a slot (cut with router and large circle jig) such that I could "turn" the part on this large sweep/radius... but I am figuring there is some lathe option possibly ?

I have seen lathe ball turning attachments and the like - but those seem a) to be for small radii , and also complicated to make (for me).

what would you do ?

dome_1.png
 

RJSakowski

H-M Supporter - Gold Member
H-M Supporter - Gold Member ($25)
Joined
Feb 1, 2015
Messages
3,624
Likes
4,206
#2
If the curve doesn't have to be geometrically accurate, it can be done free hand. You can rough out the curve by calculating points along it and cuttin with a combination of the compound movement and cross slide movement. Set the compound so movement is parallel to the spindle axis and cut a series of steps. Depending on your patience and the number of steps, you can get fairly close. Once the rough profile is cut, use a file to break the steps. As you get close to your desired shape, switch to emery cloth or sandpaper. I use WD40 as a lubricant with the sandpaper. Switch to finner grades of sandpaper until you reach your desired finish. It is possible to reach a mirror finish that way.
 

Alexander McGilton

Active Member
Registered
Joined
Nov 26, 2018
Messages
49
Likes
45
#3
What kind of tolerance are we talking about and by what means are you to measure this? What is the mating part? What quantity? If moderatly low tolerance and high quantity I would say to hand grind a form tool then the profile would be cut in one operation.
 

Jimsehr

Active User
H-M Supporter - Silver Member ($10)
Joined
Apr 23, 2013
Messages
416
Likes
335
#4
Think I would grind a hand ground form tool. So I went and ground a tool. And formed this on alum on my 11 inch Logan. Might be a bit much for a smaller lathe.
But pic is of 1 in alum stock and tool I ground.
1543945580616.jpeg
 
Last edited:

bbaley

Active Member
Registered
Joined
Nov 14, 2018
Messages
51
Likes
21
#5
What kind of tolerance are we talking about and by what means are you to measure this? What is the mating part? What quantity? If moderatly low tolerance and high quantity I would say to hand grind a form tool then the profile would be cut in one operation.
sooo... I'd have to grind a form tool that is at least slightly over 0.5" wide cutting edge for a 1" dia round stock...
But I think that is pushing it for my little lathe and the little AXA tool holders you think ?

Or am I misunderstanding.
 

bbaley

Active Member
Registered
Joined
Nov 14, 2018
Messages
51
Likes
21
#6
If the curve doesn't have to be geometrically accurate, it can be done free hand. You can rough out the curve by calculating points along it and cuttin with a combination of the compound movement and cross slide movement. Set the compound so movement is parallel to the spindle axis and cut a series of steps. Depending on your patience and the number of steps, you can get fairly close. Once the rough profile is cut, use a file to break the steps. As you get close to your desired shape, switch to emery cloth or sandpaper. I use WD40 as a lubricant with the sandpaper. Switch to finner grades of sandpaper until you reach your desired finish. It is possible to reach a mirror finish that way.
What kind of tolerance are we talking about and by what means are you to measure this? What is the mating part? What quantity? If moderatly low tolerance and high quantity I would say to hand grind a form tool then the profile would be cut in one operation.
not a large quantity, but sufficient to warrant a repeatable method.
low tolerance definitely.

There is no requirement dimension - just a rough target. A half-dome has proven to be not a shallow/soft enough contour for the application, and a simple very shallow angle also does not work well, so the short dome (large radius) seems to work the best. I made a test on hardwood - by hand, which was close enough I think to prove the shape.
 

Alexander McGilton

Active Member
Registered
Joined
Nov 26, 2018
Messages
49
Likes
45
#7
Forgot to ask, what is the material? A four way tool post is more ridged then the QCTP. There is no better way than to try.
 

Aaron_W

H-M Supporter - Gold Member
H-M Supporter - Gold Member ($25)
Joined
Nov 14, 2016
Messages
327
Likes
360
#8
Is the dome mostly cosmetic you do you need it to be repeatable with some precision? If it is just for appearance I've had good lock using a file to shape.

I have a radius cutter, and I don't see why it couldn't be set to cut a much larger radius. I don't think it is limited to cutting 1/2 ball ends.
 

petertha

H-M Supporter - Gold Member
H-M Supporter - Gold Member ($25)
Joined
Jan 15, 2016
Messages
615
Likes
406
#9
Here is how I did the nose case on my model radial engine. The section shape had to be pretty accurate. If there is a way for you to generate a table of convenient X-Y step-over coordinates corresponding to your curve shape (ideally a CAD program which makes quick work of it). Then its basically a series of cuts, blue the surface & careful profiling with file, then abrasive paper until the blue is gone. Depending on your desired resolution & how the work is mounted, I would make one dimension nominal (like 0.020", 0.040", 0.060...) and then let the corresponding intercepts be whatever odd dimensions they come out as.
 

Attachments

mikey

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 20, 2012
Messages
4,477
Likes
4,842
#10
A graver would be an excellent tool for this purpose. I could knock that out in a few minutes.
 

RJSakowski

H-M Supporter - Gold Member
H-M Supporter - Gold Member ($25)
Joined
Feb 1, 2015
Messages
3,624
Likes
4,206
#11
A spherical surface meeting your specification would have a radius of 1.3". This is not out of the question for a ball turner. If you elect to make the domes by hand, you can make a template out of sheet metal to check your curves. This should ensure reasonable repeatability.
 

C-Bag

Ned Ludd's bro
H-M Supporter - Gold Member ($25)
Joined
Feb 9, 2017
Messages
505
Likes
343
#12
I went through a couple iterations to try to make concave and convex, and finally domes. I used 80/20 because it was reusable and pretty much infinitely scalable. And now totally repeatable.

The first one was based off my 10" rotary table with a stationary arm opposite. It worked ok but was hard to get accurately setup and that RT is just too heavy. It was also cumbersome to switch the cutter and work to make convex and concave. So the present one is two independent arms on one rail central rail with little 3" cheap rotary ables as the pivots. When the cutter is swinging it cuts concave, when the cutter is locked and the work is swinging it cuts convex. I put another 3" on the work head and it cuts domes. Sorry for the small pics, I didn't know what I was doing then. image.jpg image.jpg image.jpg image.jpg image.jpg image.jpg
 

bbaley

Active Member
Registered
Joined
Nov 14, 2018
Messages
51
Likes
21
#13
I went through a couple iterations to try to make concave and convex, and finally domes. I used 80/20 because it was reusable and pretty much infinitely scalable. And now totally repeatable.

The first one was based off my 10" rotary table with a stationary arm opposite. It worked ok but was hard to get accurately setup and that RT is just too heavy. It was also cumbersome to switch the cutter and work to make convex and concave. So the present one is two independent arms on one rail central rail with little 3" cheap rotary ables as the pivots. When the cutter is swinging it cuts concave, when the cutter is locked and the work is swinging it cuts convex. I put another 3" on the work head and it cuts domes. Sorry for the small pics, I didn't know what I was doing then. View attachment 281494 View attachment 281495 View attachment 281496 View attachment 281497 View attachment 281498 View attachment 281499

This is similar to what I was envisioning if I used the belt sander with a fixture and a curved slot (albeit less complex, not adjustable and not as cool as yours)
 

C-Bag

Ned Ludd's bro
H-M Supporter - Gold Member ($25)
Joined
Feb 9, 2017
Messages
505
Likes
343
#14
Thanks. Your description sounded somewhat like mine, that's why I posted. The coolest parts is the 80/20 which I was lucky to bumble onto on the local CL. There is a long way between an idea and something that works. It makes it possible to have an idea evolve. It's why they call it big boy erector set :)
 

bbaley

Active Member
Registered
Joined
Nov 14, 2018
Messages
51
Likes
21
#16
Thanks. Your description sounded somewhat like mine, that's why I posted. The coolest parts is the 80/20 which I was lucky to bumble onto on the local CL. There is a long way between an idea and something that works. It makes it possible to have an idea evolve. It's why they call it big boy erector set :)
And I have had an 80/20 "kit" in my Amazon cart for ....ohhh, a long time now !
 

C-Bag

Ned Ludd's bro
H-M Supporter - Gold Member ($25)
Joined
Feb 9, 2017
Messages
505
Likes
343
#17
while 80/20 is amazing stuff, it's also a very "deep" system. There is a lot to it like "series" besides dimensions. 10 series is in inches, 15 series is metric. I was lucky mine was an old CNC router 4x8 table that the electronic's had been salvaged and originally they were just going to scrap the frame and all it's hardware. So I got a total of 74' of 2x2 10 series extrusions and a bunch of angles, braces and a ton of hardware. All this stuff is proprietary but there are some outfits on eBay that sell "equivalent" and that helps with the cost. If I would have bought it new it would have been 4 or 5x's as much. I've since tried to buy some local and it's basically impossible so all the other stuff like linear bearings, the gear racks and pinions and more hardware has all been off eBay. All of it new.
 

mikey

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 20, 2012
Messages
4,477
Likes
4,842
#18

bbaley

Active Member
Registered
Joined
Nov 14, 2018
Messages
51
Likes
21
#19
Ever see or do any wood turning? Same thing, except in metal. It is all done free hand and can create almost any shape you need. Look on Youtube. We also discussed them here: https://www.hobby-machinist.com/threads/gravers-for-turning.56828/
Yes I do turn on a wood lathe.
How would you compare the safety / level of control and dexterity needed to use safely ?
Wood is fairly forgiving considering the big difference between it and the metal tool being used, and also the turning tools tend to be very large levers/handles in relation to the cuts/depth being taken - I guess what I am saying is that this seems a bit scary of a proposition at first blush to me.

It is a very interesting idea I would like to learn more about though...
 

Cadillac

Brass
Registered
Joined
Mar 12, 2018
Messages
513
Likes
543
#20
Put in the lathe and use a file. short work if one offs.
 

mikey

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 20, 2012
Messages
4,477
Likes
4,842
#21
Yes I do turn on a wood lathe.
How would you compare the safety / level of control and dexterity needed to use safely ?
Wood is fairly forgiving considering the big difference between it and the metal tool being used, and also the turning tools tend to be very large levers/handles in relation to the cuts/depth being taken - I guess what I am saying is that this seems a bit scary of a proposition at first blush to me.

It is a very interesting idea I would like to learn more about though...
Actually, it is very safe except that you are often working near the chuck. I try to use an ER chuck so there are no spinning jaws to whack my knuckles with. The tool rest is only 1/8" away from the work so although gravers are somewhat short, their length is huge compared to the gap that exists between the work and tool. Graver rarely dig in if used properly. Brass can grab if you don't keep your approach angle flat but that isn't hard to do. I haven't had a dig in for many years.

If you can turn wood, you can turn metal. The process is identical to what you already know how to do. I have turned all kinds of metals and plastics, including hardened steel and stainless - no problem for a graver.

I think gravers are the most unappreciated tools out there. Most don't know what they are and very few have even used them. We need to change that.
 

bbaley

Active Member
Registered
Joined
Nov 14, 2018
Messages
51
Likes
21
#22
Actually, it is very safe except that you are often working near the chuck. I try to use an ER chuck so there are no spinning jaws to whack my knuckles with. The tool rest is only 1/8" away from the work so although gravers are somewhat short, their length is huge compared to the gap that exists between the work and tool. Graver rarely dig in if used properly. Brass can grab if you don't keep your approach angle flat but that isn't hard to do. I haven't had a dig in for many years.

If you can turn wood, you can turn metal. The process is identical to what you already know how to do. I have turned all kinds of metals and plastics, including hardened steel and stainless - no problem for a graver.

I think gravers are the most unappreciated tools out there. Most don't know what they are and very few have even used them. We need to change that.
ok - so now I have to try it...
one of the things we always try to keep very close tabs on on the lathe is cutting height/tool edge on center of part (usually)
is that just a matter of "feel"? meaning, as in wood I am guessing you can tell when the work likes your angle of attack with the cutter (whereas in the tool holder you pretty much get your feedback by chatter, noise, heat, chip shape, etc)
 

ttabbal

H-M Supporter - Silver Member
H-M Supporter - Silver Member ($10)
Joined
Jun 12, 2017
Messages
726
Likes
722
#23
I keep meaning to try gravers. One thing that's held me up is lack of a tool rest. Would hanging a HSS bit out to act as a rest work? I could probably mill up something as well...
 

mikey

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 20, 2012
Messages
4,477
Likes
4,842
#24
ok - so now I have to try it...
one of the things we always try to keep very close tabs on on the lathe is cutting height/tool edge on center of part (usually)
is that just a matter of "feel"? meaning, as in wood I am guessing you can tell when the work likes your angle of attack with the cutter (whereas in the tool holder you pretty much get your feedback by chatter, noise, heat, chip shape, etc)
When turning brass, the top of the tool is always horizontal and roughly on center height. You adjust the tool rest height for that. When turning all other materials, the tool rest is lowered so that the tool is at about a 10-15 degree angle. As you say, its a feel thing but when its right, the tool cuts. I mean it really cuts, not scrapes. You will be amazed at how fast you can remove material with a graver.

In that thread I linked to there is some info on making gravers by WR Smith. Have a look.
 

mikey

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 20, 2012
Messages
4,477
Likes
4,842
#25
I keep meaning to try gravers. One thing that's held me up is lack of a tool rest. Would hanging a HSS bit out to act as a rest work? I could probably mill up something as well...
Now that you have a plinth, why not make something up? I should talk; I need to make one for my larger lathe, too. Basically, you need a smooth, hardened surface upon which the graver can slide. It should be very solid so there is no give; this helps avoid chatter. It has to be able to turn to parallel the work and it has to be height adjustable. I need to work on a final design and I'll share it. Maybe you can do the same, Trav.
 

ttabbal

H-M Supporter - Silver Member
H-M Supporter - Silver Member ($10)
Joined
Jun 12, 2017
Messages
726
Likes
722
#26
I'll have a think on it. I'm sure I can figure something out. The hardened bit throws me off as I don't have a forge or oven I can heat treat things with. Suppose I could torch it and go by color. :)
 

mikey

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 20, 2012
Messages
4,477
Likes
4,842
#27
Got a Mapp gas torch? Even a butane torch might do it. If you use O-1 and the part is small, a butane torch will do it but Mapp gas is better. You can also use two butane torches and it will work. Heat the part until it is non-magnetic and then swirl it around in a can of ATF (trans fluid) or used motor oil or peanut oil. Then temper it in your toaster oven until amber. Not hard to do in the home shop.
 

bbaley

Active Member
Registered
Joined
Nov 14, 2018
Messages
51
Likes
21
#28
Put in the lathe and use a file. short work if one offs.
Which brings up another thing I need, to buy some new, decent files (or even some very good ones).
Someday, I'll have somewhere near the same quantity and quality of metalworking tools as I have for woodworking, but I'll need a good lottery plan or a new way to keep my wife from seeing where the other $200k went...
 

bbaley

Active Member
Registered
Joined
Nov 14, 2018
Messages
51
Likes
21
#29
I'll have a think on it. I'm sure I can figure something out. The hardened bit throws me off as I don't have a forge or oven I can heat treat things with. Suppose I could torch it and go by color. :)
Off topic but....

No man should ever go without a forge! even a tiny one.
here you go;

 

mikey

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 20, 2012
Messages
4,477
Likes
4,842
#30
Which brings up another thing I need, to buy some new, decent files (or even some very good ones).
Brands to consider: Pferd, Glardon Vallorbe (used to be Grobet Vallorbe; Glardon V is Swiss-made, Grobet is no longer Swiss-made), US-made Nicholson (excellent long pattern lathe files). Buy good ones. Junk files are ... junk.
 
[6]
[5] [7]
Top