What method will be more accurate?

[DennisCNC]

I'm think of doing some work for a person that makes geneva mechanisms, like the one here: http://www.flying-pig.co.uk/mechanis...es/geneva.html. I bought a BTC 2 mill and was woundering if it would be more accurate to machine the "blue" section (stop station) in the video using a rotary tabel, have it index do a cut and so on, or it will make no difference if it is bolted to the table and cut all around?

Some of the stations are up to 16 stops, some are less and about 13" diameter.

Thanks

[brent1]

forget rotary table if you want to make money u need a cnc 2 profile in 1 op.
unless u are only making 1. if you get an order 4 a 1000 u might make money
but if this job is 4 the uk you better get payed in advance

[DennisCNC]

I meant to say a cnc rotary table so it all runs by it self, I was just thinking maybe it wound make a more accurate round wise part?

[Derek.C]

Personally I think that bolting it to the table would give you less chances to accumulate error. Rotory tables are fine but remember that each time you position you have some error. It would all depend on your tolerance and your means.

[NC Cams]

Definintely would NOT use an index plate to make this. There are too many other ways that are easier and faster. Besides, the more the part moves around the more error potential you have.

If you take a square pre-form, you can bolt it down to the table and start cutting. First cut the slots - simple X and Y moves. Then, you can cut out the scallops. If you put index holes in the waste where the scallops are, you can hold down the square preform down via a bolt thru a premachined center hole. But that involves more work and multiple operations.

My neighbor has a real neat way to make a similar part. He milled the whole thing out by cutting the form/shape almost but not quite thru a piece of stock that was a bit thicker than the net finished part.

Once he cut the exact finished shape, he simply turned the part over, ran a face mill across the part and separated the finished part from the "trees" that held the thing togther. The good parts dropped away from the cutter as the "trees" got severed.

[Geof]

Originally Posted by DennisCNC I'm think of doing some work for a person that makes geneva mechanisms, like the one here: http://www.flying-pig.co.uk/mechanis...es/geneva.html. I bought a BTC 2 mill and was woundering if it would be more accurate to machine the "blue" section (stop station) in the video using a rotary tabel, have it index do a cut and so on, or it will make no difference if it is bolted to the table and cut all around? Some of the stations are up to 16 stops, some are less and about 13" diameter. Thanks

Probably either approach with a good machine will give you more than enough accuracy for a geneva wheel; these are not high accuracy devices. In either case your fixturing is the biggest challenge; at least two fixturings will be needed. Probably it would be best to do the bore and attachment holes first and then use these to locate and secure the part on a sacrificial plate which is attached to the rotary or machine table. This way you do not have to work around clamps on the perimeter or chewing grooves in your machine table. With a sacrificial plate you can do the perimeter full depth.

One advantage to using the machine table is that you can normally take heavier cuts; cutting at a large radial distance from the center of a rotary table can be less rigid because you are applying a large torque to the rotary brake. However, an advantage to the rotary is that you can machine a part with overall dimensions larger than your machine table movement because you only need move the distances associated with each geneva sector.

A disadvantage to using the machine table can be in the programming; every sector is distinct because it is at a different orientation so the program is longer and could be slower; it is almost certain you would need to use CAM for this program. With the rotary your machining is in the same orientation for each sector because each is indexed into the same place for machining; because geneva mechanisms are simple curves and straight lines this program could be hand coded without much difficulty.

If I was considering doing this job and already owned a suitable rotary I would probably use it. If I did not already own one I would probably not buy one and would just use the machine table.

[DennisCNC]

Geof,

Originally Posted by Geof these are not high accuracy devices. In either case your fixturing is the biggest challenge;

Good point! I'm stuck in a high accuracy mind set latelly

NC Cams,

That's a trick way to hold a part!!


I think these things are out of cast so machining should be quick and easy. I'll put the rotory table idea to rest not worth the trouble.

Like always THANK you all for the great advice and comments!!!

[Tru Cut Machine]

You might consider having the part water jetted leaving the bore slightly under for finish machining if it's a bearing fit. I have had very good results in many different materials and thicknesses. Shops with a water jet can work off a dxf file and are very reasonable in cost. Cheers.

Greg

[brent1]

if you have a cnc u dont need a rotary table unless u use it 4 axis

[SeaSchell]

Buy material extra thick, Cut profile complete, CNC Best mine will hold arc's within .0005 as I'm sure most newer ones will. When done invert, and fly cut to thickness.