How to center a piece of work on a rotary table?

[ScrapMetal]

It's been bit slow in here lately and as soon as the raw materials show up I'm going to need this information.  

Centering the rotary table on my mill is simple using my DRO and Haimer.  Getting the work piece in the proper position on the rotary table becomes a bit of a problem though. 

I will need to "center" a portion of an oddly shaped piece of work on the rotary table in order to round off an edge that is an arc and in another operation cut a section out of it in a circular arc.

Anyone have effective methods for centering/positioning work on rotary tables?

Thanks,

-Ron

[Dr Stan]

Without seeing the part or a drawing of the same my best guess is to use a tooling plate on top of the RT.  By a tooling plate I mean a piece of material (usually 1" thick aluminum) with equally spaced drilled and tapped and drilled and reamed holes.

[Hawkeye]

Once the tooling plate is set up, mark the centre of the arc with a prick punch. I made a couple of centres out of broken endmills with 3/8" shanks, ground to a 60 degree point. With one of these mounted in the mill's collet chuck, you can move the work, loosely clamped to the rotary table and tooling plate, until the point of the centre fits exactly in the punch mark. Then tighten the clamps.

Another advantage of the centre point is that you can press it down into the punch mark and rotate the work piece to get a critical line of the piece aligned with the X or Y axis, as required, then clamp it.

[ScrapMetal]

Sorry, I've been away from the 'puter for a couple of days. I'll post a pic of what I'm working with when I'm at the office tomorrow.

I'm not quite following you guys on using the tooling plate and how that would accurately position the work in the situations I'm working with.

Got the raw materials delivered today so, if I'm lucky and get in to the shop for a decent amount of time, I can start to get the pieces roughed out.

I'll provide more info tomorrow.

Thanks guys,

-Ron (feels like the village idiot ;) )

[ScrapMetal]

Don't look at my drawing too critically.  It's not complete and even if it was I only put in enough detail to get me through making it.

I'll be making a matching set of these for mounting a newer Harley fender to a set of '82 front sliders.  The part will be somewhat visible.  I was planning to round off certain edges and I figured the easiest way to do that would be a concave end mill with a 1/8" radius.  CNC - no problem.  Manual mill with rotary table, a bit more.



Comments?

Thanks,

-Ron

[Hawkeye]

Ok, Ron. Looking at the drawing, Stan's tooling plate and my centering point will do the job. Mark out and punch the centre points for the 4 through holes and the centre of the concave curve. With the rotary table accurately centred under the spindle and the DRO zeroed, clamp the tooling plate to the RT. The plate should have enough threaded holes to clamp the work piece in a variety of positions.

With the centering point in the spindle, place the work on the plate and bring the point down into the centre for the concave curve. (At least that's where I would start.) Clamp the spindle in that position to keep the point in the punch hole and rotate the work to line up one of the straight lines for the concave section with the X or Y axis. Clamp the work firmly to the plate, leaving the first working section clear. You can then cut the concave section.

Pick an outside curve and move the work to that centre, again using the point. That's where the zeroing of the RT comes in - lets you go back to 0,0. Again rotate the piece to align a flat side with X or Y. Cut, move, repeat.

After the top side is complete, the centering point will help you line up the piece to counterbore the back side.

At least that's how I'd do it if I couldn't use my CNC.

[ScrapMetal]

That makes sense. I will give it a try as soon as I get/make a tooling plate large enough.

I should have a plate ready to go in a week or so and then I'll post my progress.

Thanks very much guys,

-Ron

[ScrapMetal]

Just an update to show how I'm doing on these.  There is still a lot more work to be done, a few edges to round off, and a lot of grinding/file work to make up for some sloppy cuts.



The "lump" on the end of the large radius is due to me not calibrating my rotary table to the mill before making the cut.   I little grinding here...



The pair of them - left and right front fender mounts for my shovelhead motorcycle build.  Here you can see one of the edges (the outer ones that end in a radius) that still need to be radiused.



This side just needs a little polishing.



Same here, a little grinding and polishing to get rid of the machining marks and it'll be good to go.

I'll take some more pics when I have them finished and installed.

Thanks,

-Ron

[Dr Stan]

If you have access to a sandblaster use it as the first step.  It will help blend the surfaces before final polishing.

[ScrapMetal]

(07-23-2018, 07:53 AM)Dr Stan Wrote: If you have access to a sandblaster use it as the first step.  It will help blend the surfaces before final polishing.

I was considering that but was undecided.  You may have just tipped the scale toward using the blaster.

Thanks for that,

-Ron