20mm concave radius

I have a chester vertical mill and need to

mill a 20mm concave radius x150mm long, material is mild steel, 18mm x 20mm.. radius needs to be on the 18mm.

What would be the best way to do this?

Garry..

I have made a fly cutter wich works a treat. Used in correct way it can be used to machine concave (as I have learn't ) Novice myself but I think its feasible. **LINK**

I see my error vertical machine. Feasible as I said but hey horizotal set up or lathe for the fly cutter. What abot boring a hole, cutting and final machining? I see a problem if the radius is required on a complicated dimensioned piece of work. 

ATB.

David.

Edited By woody1 on 07/02/2013 00:05:15

Edited By woody1 on 07/02/2013 00:06:22

Edited By woody1 on 07/02/2013 00:07:18

Theres a few ways to do it think i know what it is, done a fair few either way.

Bore it in lathe between centres. Just need to make up a boring bar and set the offset of cutter.

Make a cutter easier than you think or buy one. If bought chances are it will have too large a shank. Could use TCT router bits certainly on aluminium, steel be careful. Remove some material with milling cutter first.

Its a valid way David but never done it by angling the mill head to achieve a near on radius. Still in this case need a pretty large diameter cutter to begin with approaching 40mm diameter. Woodruff cutter may be worth a try.

Hi Gary,

There is a book that used to be available from Camden by Guy Lautard called Tables & Instructions for Ball and Radius Generation. It is a set of tables that enables you to machine what you are describing with a small Ball nose cutter. I have used this method myself and it does work although having a digital readout does help but I am sure a couple of well placed DTI's will do the same job. You can also machine balls in the lathe as well.

Hope this helps.

Steve....

Clamp the bar to the cross slide on the lathe with packing to bring it upto height and use a between ctrs bar to cut the concave.

If you only have a mill then a boring bar in a boring head and plunge down, come at it from both ends to avoid an overlong tool. Bit like this but with a tool in the end of the head not the side.

Edited By JasonB on 07/02/2013 08:59:23

For a 20mm radius and 150 mm long piece of work and a Chester mill Jason's method whilst good and workable won't work in this case.

How accurate does the radius have to be ?

Reason I ask is if you use a fly cutter in a boring head, with the head tilted at an angle you can get a very close approximation to an arc, in fact it will be part of an ellipse.

The more you tilt the head, the smaller the radius, same applies to the flycutter, smaller offset in the boring head, smaller radius. As an indication the rad in the picture was to fit a 1.2 metre long rack to a 3" tube.

Any specific reason why it won't work John? I've done 16mm rad 65mm long in that way here so comming from both ends is almost the same. Don't know what the mill is so can't tell if there is enough travel. And the lathe method certainly works as I have made replacements for tube benders with a 11mm rad 200mm long.

T

Edited By JasonB on 07/02/2013 13:19:11

thanks for your replys, Jasons idea is good with the boring head, but i only have 130mm depth on chester eagle mill, if i used a bullnose endmill do you think i would get a good enough finish.

garry

Jason, only commenting on the mill method in that the Chester doesn't have the required stroke and using the side of a boring head you will start off greater than 20mm, probably closer to 40 or 50mm.

Shorter length and bigger radius will work fine.

JS you must be telepathic to have known it was only a small Chester mill and I did say with the bar in the end of the head not the side as my photo showed.

Gary will you be able to hold such a big bull nose mill in you're Eagle?

If you have a full blown DRO it can be done with a small bull nose using the ARC function but it's a long tedious job so I would still suggest the lathe method if you have one. This is the method Steve suggests but the DRO works out the cutter position but for a smooth curve you will want to have maybe 0.1mm step so 180+ passes

Edited By JasonB on 07/02/2013 22:34:32

the maximum in my collets is 20mm ,any bigger i would have to get a stepped bullnose.

Garry you may hav miss my edit, yes it can be done with say a 6mm ball nose but you will need in the region of 180passes depending on the uality of the curve you want. When I'm on the PC in the morning I'll post some examples of convex curves done that way, concave are just the the same with a slight change to the calculations.

.

I think I must be !! I'm sure I read Chester Eagle but going back he did say just Chester.

Gotta run and buy a lottery ticket

Here you go garry, these convex curves were done as described above using the DRO to work out the positions but they can be worked out long hand. Your concave can be done in just the same way as the convex

Due to wanting a tight internal corner where the curve meets the flat I used the edge of a standard milling cutter so the finish was a bit rougher than you could get with a 6mm bullnose cutter.

The long edges took something like 80 passes per quater circle 160 fror the half circle and a similar method was used for the tops Bit of a clean up and paint and they look like smooth curves when complete

Hi jason, marvelous work there, this is far from my capabilities, i thought about the 20mm bullnose but concerned about the finish, looks like i will be machining it on the lathe