How to create a circular scribe

[WolfieParticipant @wolfie]

OK not sure if that title is making sense but basically I have a disc of metal into which I need to drill a circle of 6 holes at a certain distance from the centre.

 

Now obviously this requires 3 lines drawn across the disc at the correct angles and also a line scribed around the disc at the required distance from the centre.

 

How do I mark this out? The disc has a hole in the centre so I can’t use a compass and my protractor is way too big to scribe the correct angles. I’m probably missing summat obvious but help please!

 

Oh yeah and the plans don’t give the correct distance for the ring of holes, but they do say 7/8″PCD whatever that means

[WolfieParticipant @wolfie]

[JasonBModerator @jasonb]

7/8″ PCD means that the holes are placed around a 7/8 diameter diameter so your scribed circular line will be 7/8″ diameter.

 

There are several ways of doing this.

 

It can be done using co ordinate drilling using the tables in a Zeuz book. basically this gives a a constant to multiply the PCD by and how far to move from the centre point, best done on a mill but possible on a vertical slide on the lateh. Or easier still with a DRO on a mill as it works it out for you.

 

Another way is to draw your 7/8 scribed circle and set your dividers to just over half that distance and then step out 6 divisions, you will need to adjust the setting a few times until you get six equal parts.

 

If the part can still be held in the 3 jaw then by using a square off the cross slide you can set each jaw to the square which gives you 3 lines right accross the work so six divisions.

 

Have a look about half way down this page for a way to scribe the lines in the lathe.

 

J

Edited By JasonB on 13/10/2011 19:27:23

[Terry LaneParticipant @terrylane]

Turn a well fitting plug for the hole in the centre, with an accurate centre dot in that, and use your dividers in the usual way. Other than that – Wot Jason said!

[WolfieParticipant @wolfie]

Thanks all, that link was very useful

[TerrydParticipant @terryd72465]

Hi Wolfie,

 

PCD is shorthand for ‘Pitch Circle Diameter, i.e. the diameter of the circle on which the holes are located. Jason and Tel both offer good advice, but I’m not sure why Jason said to increase the dividers a little as the radius of a circle will go round the circumference of that circle exactly 6 times. I can still remember that as a way of constructing regular hexagons in tech drawing at school and that was over 50 years ago. I also remember using a similar technique to draw 6 petalled flower shape by drawing a series of radii through the circle from the circumference

 

To fill the hole temporarily I would use a tapered wooden plug, even a square section that is forced into place would suffice.

 

Regards

 

Terry

[JasonBModerator @jasonb]

Yes Terry you are right, moment of brain fade got chord and arc length muddled.

 

J

[colin hawesParticipant @colinhawes85982]

I very rarely mark anything out but use the lathe or mill to locate holes.This eliminates human error due to eyesight but needs care with sums.I have made 24 equally spaced vee slots on the lathechuck backplate and can index from this,using a small milling spindle to centre drill the holes.

[TerrydParticipant @terryd72465]

Posted by JasonB on 14/10/2011 07:39:20:

Yes Terry you are right, moment of brain fade got chord and arc length muddled.

 

J

Hi Jason,

 

Don’t worry, you’re not alone, I suffer quite a lot of ‘Alzheimer Moments ‘ these days .

 

V Best regards,

 

Terry

[TerrydParticipant @terryd72465]

Hi Wolfie,

 

Another and perhaps simpler route to solving the problem, depending on the accuracy required, is to create a paper template by constructing the pcd with a pencil compass (sharp lead!) or cad program to size, including your centre hole then stick it to the component with Pritt Stik or similar, carefully centre punch hole positions through paper, remove paper.

 

Sounds crude but can be surprisingly accurate. A useful tip to keep in mind.

 

Regards

 

Terry

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