How to mark out PCD

[Allthegearnoidea]

I bought a dividing head the other week and have just ordered a small 80mm chuck off amazon. I need to make a back plate. This needs three holes for the chuck and four holes to suit the T nut slots in the table.

What’s the easiest way to mark out a PCD. I have a surface plate and v blocks and various height gauges etc and I could definitely figure it out, but I want to know how a machinist would do it. I even have a DRO on the mill with a PCD function but kind of want to do it long hand.

Thanks

Regards Chris

 

[eLuSiVeMiTe]

Fit the backplate on the dividing head and divide it into 3?

I'd use my recent aquisition.

Divides into 24 divisions.
Can move up to 6 divisions of 15 degrees with one lever pull.

 

[ronan]

I'd use the DRO, its an excellent feature. The figures to do pcd's are on the back of the zeus machinists pocket book.

 

[Allthegearnoidea]

LOL I didn’t think of using the dividing head. Well maybe I should try all your suggestions it’s just a learning exercise. I do have a Zeus so will look at that also.

Thanks Regards chris

 

[ronan]

http://www.ptsintl.com/pdfs/Multihead Centinel/PitchCircle.pdf

 

[eLuSiVeMiTe]

I intend on putting 24 indexing holes on my lathe chuck backplates for ease when turning in the future.

 

[MattH]

Scribe a circle with a pair of deviders, set the deviders to a 3rd of the circumference and place one point on the circle. Use the second point to scribe the first bolt hole. Centre punch this mark, put the point in the punch mark and scribe the second bolt hole. Rinse and repeat.

 

[ronan]

Scribe a circle with a pair of deviders, set the deviders to a 3rd of the circumference and place one point on the circle. Use the second point to scribe the first bolt hole. Centre punch this mark, put the point in the punch mark and scribe the second bolt hole. Rinse and repeat.

Problem with that is distances between the holes will be too long. Dividers measure in a straight line, not along the curve of the pcd circle you have scribed. Might not matter, but he has the simple and accurate way of doing this job with the dro.

 

[MattH]

Problem with that is distances between the holes will be too long. Dividers measure in a straight line, not along the curve of the pcd circle you have scribed. Might not matter, but he has the simple and accurate way of doing this job with the dro.

Yes, sorry that was a mistake. You need to use this formula: (cut and pasted);

B = D x sin( 360 / (2 x N))

where

B = distance between bolt holes

D = diameter of bolt circle

N = number of holes in circle

 

[optima21]

I used this, it does the calculations for you

https://littlemachineshop.com/mobile/bolt_circle.php

 

[Allthegearnoidea]

Thanks for all the great replies. As I said the learning experience is just as important to me as the finishing product. I spent most of today converting a slab of aluminium into tinsel but I learned how not to do it which is a very important lesson.

Thanks

 

[eLuSiVeMiTe]

Thanks for all the great replies. As I said the learning experience is just as important to me as the finishing product. I spent most of today converting a slab of aluminium into tinsel but I learned how not to do it which is a very important lesson.

Thanks

I'm in the same boat mate.
Quite often do things twice.
Carpenter by day.

 

[graffian]

Three holes should be fairly obvious, when someone points it out.
A hexagon is made up of a load[6] of triangles, a circle fitting to the
points of a hexagon has the same radius as the sides of the little triangles in the hexagon.
to mark out a 6 point.
Scribe your circle
Then scribe around the circle from point where the lines intersect, the compasses still
open to the radius of the original circle,
You have 6 points. 3 points is clearly every other point.

4 point, I don't know how your meant to do it. But.
Scribe a circle.
Scribe a diametrical line. Two points are where the circle and diametrical line intersect.
Open your compasses and scribe some marks from the points, first two marks on the circle,
open the compases large enough that the scribes lines from the two points intersect. Scribe
a diameter though the centre and where the two scribe lines you just put down intersect.

Much easier to do it than describe it.

 

[MattF]

Much easier to do it than describe it.

No kidding. I was going to try describing the compass trick, but then uncertainty set in and I couldn't remember for the life of me whether that gave 5 or 6 points around the circle, and I wasn't digging a compass out at this time of night, to check.

 

[ronan]

I don't think in many years i have seen anyone manually mark out a pcd. Its so much easier to clamp the part to the mill table, find centre, pop the details into the dro, and let it do the work for you.

 

[MattF]

I don't think in many years i have seen anyone manually mark out a pcd. Its so much easier to clamp the part to the mill table, find centre, pop the details into the dro, and let it do the work for you.

It's a two minute job to do it manually, and how do you use your method without a mill?

 

[ronan]

It's a two minute job to do it manually, and how do you use your method without a mill?

The op has a mill, and a dro. Seems daft not to use it.

 

[addjunkie]

Most DROs on mills have an option to use the coordinates system, allowing you to mark, or drill etc a pcd.

These are also in a zeus book.

Things like marking out , grinding tools, are all explained in any old apprenticeship type engineering books. Handy to keep for reference if you can find them at a car boot etc

 

[Allthegearnoidea]

Looks like it’s going to be the DRO. The DRO was the main reason I bought the new milling machine and got sick of constantly swapping drive belts for the speed
http://www.warco.co.uk/milling-machines/303310-super-major-vario-milling-machine.html

 

[addjunkie]

Looks like it’s going to be the DRO. The DRO was the main reason I bought the new milling machine and got sick of constantly swapping drive belts for the speed
http://www.warco.co.uk/milling-machines/303310-super-major-vario-milling-machine.html

I have its older sibling!