Make And Octagon Without A Rotary Table?

[RVJimD]

Guys,

I am working on a new set of drive tumblers for my model crane. I made the first one using the RT and clocked around one the 45 to cut the 8 flat sides from round stock. Is it possible to make an octagon without using the RT? I only used it for the flats and then I need to go back to the vise for the rest of the part. I am looking for an alternate method so I don't have to lift the RT up and down again.

Here is the first prototype part.



I crushed in the vice as I was trying to thin down the spokes and side walls. For me (low time hobby guy) this is one of those parts where I wish I had an old timer I could walk up to with the part and ask, "what order of operations would you use to make this?"

It needs a center bore, eight flat sides, every other side is slotted as I hope you will see in the photo. Good head scratcher for me. I have to try and think three steps ahead to figure out how I might need to hold it for the next operation.

Here is one from a full scale crawler crane. I would guess the full scale part is cast, but even that make me scratch my head.



Thanks for any advice.

Jim

 

[RJSakowski]

You could make an octagon using a Vee block of suitable size. On a recent post, I describe making a Vee block for the drill press by welding two pieces of angle iron to a base plate. It will not be as accurate as using the RT but with care you should be able to get the angle close to 45º. Once you cut the first 45º side, you can cut the rest of them in the milling vise.

 

[RVJimD]

RJ,

Thanks! I did manage to also finally find a YouTube video that showed your suggestion as well. Worked great and saves a BUNCH of messing around, switching from the RT to the vise.



Thanks for the tip!

 

[TOOLMASTER]

easy in a vise

 

[karim]

Maybe I'm wrong, but it looks like you're boring out semi-circular pockets at 45 angles alternating faces... have you considered milling the octagonal flats last?

It seems you could do virtually all of your operations on the RT:

1. turn center bore as needed on lathe
2. mount the blank, center on the RT on mill
3. offset on one axis by distance: RT center to pocket center.
4. bore pockets at 90 deg
5. flip part, recenter on RT, rotate RT 45 deg, repeat step 4
6. offset further by end mill radius
7. mill off each octagonal flat, rotate by 45 deg, repeat.

 

[RVJimD]

I will study your list of steps, I'm always trying to learn and understand how to machine something.

Here is a photo of a mostly finished tumbler. It will have a delrin center in the bore and a set screw thru one "flat" thru to the center of the bore to lock onto a brass axel shaft.



Jim

 

[karim]

Here's a video I found which kinda-sorta illustrates the process I described above:

This isn't exactly the same part you're after (to be honest, it's not even remotely the same part... ) but if you use your imagination and squint a little, you might be able to find the part you're building in this process somewhere

An advantage of an approach like this (if it does indeed work for you) is that it maintains most of the structural strength of the original blank until the very last operation.

 

[RVJimD]

I think I have seen that video before. I actually want to try that very procedure, just haven't gotten there yet.

Thanks!

Jim