120 degree indexing

[roofman]

I am having an old age moment here...I have a 75mm diameter steel slug 25mm thick and i need 3 holes drilling at 120 degree increments, don't have a rotary chuck so what is the best and easier way to mark and dot the holes prior to drilling?

 

[addjunkie]

Dividers at the radius for the pcd. one step-is 60 degrees, two is 120

 

[Brad93]

Chord length can be used to check your marking

 

[Redsailor]

Wrap a piece of paper or masking tape around the outside and mark the overlap. Remove and measure out and mark points then re attach and centre pop the hole locations.

 

[Agroshield]

...I have a 75mm diameter steel slug 25mm thick and i need 3 holes drilling at 120 degree increment

Where are the holes? On a pitch circle on the 75mm dia. part or equally spaced around the perimeter (i.e. on the 25mm edge)?

Dividers at the radius for the pcd. one step-is 60 degrees, two is 120

Pick any random piece of 75mm dia. steel. Is its centre marked out? You need the centre in order to mark the pitch circle, so you need to tell us how to find the centre first.

Wrap a piece of paper or masking tape around the outside and mark the overlap. Remove and measure out and mark points then re attach and centre pop the hole locations.

That is very good. It works directly if the holes are on the perimeter. If the holes are on the face, all you would have to do is mark _six_ points and join opposite ones. The intersection of the three lines will automatically define the centre of the piece, from wich you can draw the pitch circle.

 

[roofman]

Holes to be drilled are on the flat face, i can get an accurate center dot by putting it in my lathe

 

[daleyd]

If you want a rotary table I have one here if you want to borrow it , but you should just be able to scribe something - I think Zeus tables have pcd calculations in them??

 

[roofman]

If you want a rotary table I have one here if you want to borrow it , but you should just be able to scribe something - I think Zeus tables have pcd calculations in them??

Cheers Dale for the offer but i have found the need for one a few times recently so i think its my next purchase,nothing to expensive though.

 

[JMS1972]

If it's in a 3 jaw chuck you can index using the chuck jaws.

Put a screw jack or suitably sized bit of bar between one of the (almost horizontal) jaws and something horizontal on the lathe rotate the chuck until it hits this stop.

Use a sharp tool in the tool post to scribe a horizontal line.

Then rotate the chuck until the next jaw hits your screw jack. Scribe again. Repeat for the last jaw

Obviously this needs you to move the screw jack out of the way whilst rotating so some errors can appear. But it's quick and dirty

 

[addjunkie]

Pick any random piece of 75mm dia. steel. Is its centre marked out? You need the centre in order to mark the pitch circle, so you need to tell us how to find the centre first.

lathe, or a centre square. Or if really pushed there are a few ways using geometry.

 

[Agroshield]

lathe, or a centre square. Or if really pushed there are a few ways using geometry.

I'm not sure about the sentiment behind the 'really pushed' part. Compass (dividers) and straight edge, known to the Greeks 2000+ years ago shirly is on an equal footing to an expensive, non-portable machine (lathe) or one-trick pony tool (centre square). Simplerer is betterer.

Note also that the band wrapped around the part and divided using the same method carpenters have used for years can as easily do 7, 11 or 13 holes, which is quite difficult when you are stepping off chords using the dividers (first mark the centre using a band divided into four by folding). Also note that the method of dividing does not need any kind of measurement device - the only place you need a dimension is the radius of the pitch circle.

 

[addjunkie]

Id only use arcs and a straight edge, if the centre square is too small.

 

[Morrisman]

Dot the centre, mark the pcd radius with your dividers, then swing arcs around that circle with the unaltered dividers, which gives you six equal spacings, drill 3 of them. Schoolboy geometry.
Depends on the tolerance you require as well I guess.