Advice on best approach to milling recess in end bar

I have to machine a Delrin bar end to shape three recessed areas that will couple with a socket. The diameter of the bar is 80 mm and the recessed areas are 8 mm deep and spreading through sectors of 60* and 17 mm width.

I am mounting the bar on a rotary table and I was thinking about machining it in a mini milling machine.

I don't have much experience with milling and when I was thinking about how to set the work, I was wondering what would be the best approach:

1. Setting the RT horizontally, part vertically, and then mill with an end-mill from the top of the part while rotating the table.
2. Setting the RT vertically, part horizontally, and then mill with an end-mill from the side of the part while rotating the RT.

Please, could someone share wisdom with me and advise regarding the best approach and the reasons?

I am think about firstly using a 6 or 8 mm cutter and then later use a 2 mm mini-cutter to work on the corners that are left rounded (either depending on how I approach the part).

Thank you very much on advance.

Just a personal opinion but I think it would be better as per your suggestion No 1. Your idea of firstly using a 6 or 8 mm cutter and then later use a 2 mm mini-cutter to work on the corners that are left rounded.

Please let us know how you get on.

Brian

Agree with Brian - though either would be a suitable approach the vertical way will enable you to see the radial positioning easier. Given the size of the pockets though personally I would use a smaller cutter for the initial cuts 5mm diameter at most but preferably 4mm

Don't forget to off set the mill table by the cutter radius once you have moved the RT radially.

Tug

You need to decide what is acceptable in regards to fillet size and where they will be. The part can be approached axially or radially or both.

If you do it axially you will need align the RT to the spindle then offset either the x axis or the y axis half the milling cutter diameter to do one side of a cut out, do all three edges first. Offset the RT the opposite way to do the other edge and with that offset rotate the table round to meet the first cut. Then machine away any remaining material towards the edge.

I would make a paper or card cut out of the required part and stick it to the top as a guide to make sure you were going in the correct directions as you go.

I'll knock out a diagram.

Martin

Brian, thank you very much for your advice. With learning in mind, please, could you advise the reasons (control, finish, stress, ...)?

For the sake of clarity, here are two pics depicting 1 and 2. Please, do not pay attention to the slot cutter,. it is the only one long enough I had for the photo.

1

2

Martin, thank you very much for your description with steps. It makes a lot of sense!

I was actually thinking on doing one slot at a time, but the way you describe requires far less movements of the spindle in the X or Y axis. Bravo!

It is a difficult shape to produce with only a mill and RT but I would proceed with your first option as Brian and Ramon have also said.

How important is the radius that is left in the corner when you finish with a 2mm cutter ? Can you make a small undercut as shown below ?

If this can't be done the radius could be cut out by hand with a sharp chisel. Easy as the material is delrin.

Paul

Thank you so much to all. SO much help. This is a brilliant community!

Paul, the radius is not critical as far as it is small. The part is bolted down and the fit is on the walls of the slots. The counter part has does not have sharp corners, all rounded.

As per the undercut. It would not be a problem, actually it will take away stress from the engagement, if the part wouldn't have to be bored hollow. Sorry, I did not disclose that part in my original post as I thought it would distract from the question but I can see now that it is relevant.

Paul, you mention that " It is a difficult shape to produce with only a mill and RT".

What tool or machine (manual) would you use for this job?

The difficulty with the part is the solid centre. If it were possible to remove it, machining becomes easy. If it is crucial for the part's function, consider removing it, milling the three teeth and then gluing in a disc to replace it. With a hollow centre, the part is just like a dog clutch, see:

https://www.youtube.com/watch?v=0bwRUf2wCyE

Setup as shown with cutter in position A. Move table to give position B then back to A. Repeat for the other two cut outs. Change offset to the second diagram, move to position B then rotate the table to meet first cut. Remove the remaining material towards the edge. Repeat for the other two cut outs. If you want to remove the fillets completely you could run a small cutter down them to give an undercut.

Martin C

The biggest problem I see if using a 2mm cutter for the internal corners will be length, 8mm is  more than the flute length on most 2mm cutters so you may need to shop around for a long series one. may be worth drilling out the corners first before milling then you won't get excess cutter engagement in the corners though with plastic it should not be too much of an issue

Edited By JasonB on 18/01/2021 10:09:25

DC31k, indeed. I have updated the sketch to represent the part better. The centre is bored but there is a 2 mm wall at this end and, to make it more complicated, the other end bore is tapered. I guess I could glue the 2 mm straight wall section. I think I will leave that option in case something goes wrong.

It made sense to me to machine the slots first before boring the centre in the lathe. Also, a lot easier to align the centres in the mill as it is using a centre finder.

Martin, that's brilliant! Thank you very much for the sketch, very illustrative. I am ordering a 4 mm extra long (75mm) end mill and will hopefully machine the part tomorrow or the day after.

Jason, thank you! Another brilliant suggestion. I will indeed drill the corners first with a 2 mm drill bit.

That will also present a very good guide when cutting.

^^^^^ Yes. If I were doing this is how I would proceed. Bore the centre out for the full length of the piece, machine the three slots with nice square corners using a good sized end mill cutter, then install a piece of round in the bored hole up the centre. Retain it by the method of your choice, Loctite, pinned, threaded, silver soldered, soft soldered etc.

Much easier, to my way of thinking anyway. But there is always more than one way to skin a machinist's cat.

Edit: Although, just saw Jason's drilling suggestion. Good idea if rounded corners suits your application. And probably the easiest way of all.

Edited By Hopper on 18/01/2021 10:12:58

Thank you all for your advice. I would say a success.

Jason, I could not do the drilling. Run out of vertical space for the chuck and drill. It would have helped no doubt.

Have to dull the corners... like a knife.

Edited By Adan Santander on 18/01/2021 19:44:03

What if you start with he job vertical and finish up as per the last post then position job horizontal you would only be left with two tiny radiussed corners.

I thought I would post the final part ready to install.

Again, thank you very much all for your sound advice.

Is that an impellor with the vanes removed; or a flinger disc for a road salter spinner?

Just curious..

George.

It is the top jaw of a self-tailing sailing winch.

The lower jaw is preloaded with a spring and the helix in the top jaw bites in the rope.