Large Radius on Mill

[Stan]

Am I missing something obvious on how to do a set up? I have a Benchmaster Vertical Mill with a 6" x 36" table. I want to cut an arc 6" long on an 18" radius. How do I do it?

 

[Jasonb]

Assuming you don't have a massive rotary table the method used for expansion links is to mount the work on a long piece of flat bar. Make a pivot at one end and fix to the mill table, the other end of the bar has a pin that can slide in the tee slot of a rotary table. By turning the rotary table the end of the bar will move in an arc, set your work 18" from the pivot plus 1/2 x tool dia and it will but your radius.

If that does not make sense then pm me your e-mail, I think I can find a sketch or photo.

Jason

 

[shred]

I think sorta like how these guys do it, but in a slightly smaller scale. There's a couple threads on profiling rod-ends around a pivot that might be useful. You'll have to have the pivot point further away than most though.

 

[Stan]

Thanks for the replies. I thought about borrowing a 24" rotary table but I don't have a crane to lift it onto my mill table. I also though about the flat bar method and considered mounting a lead screw to move it back and forth. I have a tool for rounding over by hand that I use on small aluminum pieces but this is mild steel.

My concern with the flat bar method was how to keep it rigid enough to mill without chatter. I will be milling a 3/4" slot.Does the use of a rotary table with a pin in a T slot solve that problem?

 

[Cedge]

Jason has the right idea for getting the radius, so I'll leave that alone. I'd drill the ends of the arc to proper diameter and then mill the arc in two cuts, using a small diameter end mill. The holes will give you room for escape without running away as you cut the inner and outer diameters using shallow cuts. I'd leave just enough material to make finish cuts after the heavy work is done. A 36 inch diameter piece of steel should allow you to control things by hand while using the small end mill.

Steve

 

[bob ward]

Another thought is to dispense with the rotab and do the job on the mill table, with your part mounted on a piece of pivoting flat of course. If you are already thinking of making a lead screw to provide rotary motion, all (all?) you need to come up with after that is a hold down out near the cut that will provide vertical restraint but allow horizontal movement.

EDIT After properly reading and thinking about Jasonb's suggestion, that seems to be a very clever solution to the problem.

 

[hammers-n-nails]

is stepping it out leaving many flat spots acceptible for this part? if so thats an option, it wont be real quick working it out on paper but it is possible, i do it all the time but the dro makes it so easy its like cheating. something to think about.

 

[Jasonb]

The rotary table on the moving end will act as a lead screw, you need just enough clearance between the bar & table to allow them to slide but not jump about. You can get a very good finish, the expansion links I mentioned have to have an accurate slot for a die block to slide in the curved slot.

Stan I have found the pictures, let me know your e-mail and I'll send the scans.

Jason

 

[Stan]

Thanks Jason. That sounds like it would do what I need. Please send the pictures to sdmcw at shaw.ca

 

[Stan]

Thanks Jason. The pictures arrived and answered any questions I still had. Now I have to survey the metal pile and see what is available is build a fixture.