Marking out a Pitch Circle Diameter (PCD)
Apr 27, 2015 8:00:58 GMT
jem, Midland, and 4 more like this
Post by springcrocus on Apr 27, 2015 8:00:58 GMT
Marking out a pitch circle diameter (PCD) can be a bit awkward at times and it is often easier to either use a template or use co-ordinate drilling on a DRO-equiped machine. However, some of us can't afford a DRO....
One of the essentials for marking out a PCD is having two edges at 90 degrees to each other as these are the two reference points needed to acurately find the centre of the circle. After this it is just a matter of following the formula and scribing the work accordingly.
This is how I marked out my Britannia pump body which uses six of an eight-hole PCD. As can be seen, it's not the easiest of shapes to work around but there is always a way. Following the formula in the Zeus book I have a PCD of 1.250" so the heights of the holes above and below the centre line are 0 (zero), 0.183" (1.250 x 0.14615), 0.442" (1.250 x 0.35355) and 0.625 (half of the PCD). These are the only four dimensions I need although I only actually used the 0.442".
I held the work in my milling vice as shown and, using my vernier height gauge, I first found the height of the top of the spigot. Since the O/D of this is 1.000" I lowered the height gauge by half of this to give me the centre of the PCD and then reset the scale to the nearest whole number.
Now it was just a case of working plus or minus the worked out co-ordinates and using the height gauges facility for scribing.
Next I turned the vice on end and did the procedure all over again, resetting the centre and marking out as before.
Now that I had marked out all my points, it was a simple matter to just centre-pop each position and then to spot-drill, drill and tap them. Because of the distance I used a long spotting drill, and the 2.3mm drill I held in a pin chuck. Tapping was done freeehand from the other side.
Similar to above, I could have "blued" up the face of the work and set a scribing block with the various co-ordinates and marked them that way. I won't bother describing that in detail, though, as I will probably get told off for using the wrong type of "blue".
I hope this little demo is helpful to someone.
Steve
One of the essentials for marking out a PCD is having two edges at 90 degrees to each other as these are the two reference points needed to acurately find the centre of the circle. After this it is just a matter of following the formula and scribing the work accordingly.
This is how I marked out my Britannia pump body which uses six of an eight-hole PCD. As can be seen, it's not the easiest of shapes to work around but there is always a way. Following the formula in the Zeus book I have a PCD of 1.250" so the heights of the holes above and below the centre line are 0 (zero), 0.183" (1.250 x 0.14615), 0.442" (1.250 x 0.35355) and 0.625 (half of the PCD). These are the only four dimensions I need although I only actually used the 0.442".
I held the work in my milling vice as shown and, using my vernier height gauge, I first found the height of the top of the spigot. Since the O/D of this is 1.000" I lowered the height gauge by half of this to give me the centre of the PCD and then reset the scale to the nearest whole number.
Now it was just a case of working plus or minus the worked out co-ordinates and using the height gauges facility for scribing.
Next I turned the vice on end and did the procedure all over again, resetting the centre and marking out as before.
Now that I had marked out all my points, it was a simple matter to just centre-pop each position and then to spot-drill, drill and tap them. Because of the distance I used a long spotting drill, and the 2.3mm drill I held in a pin chuck. Tapping was done freeehand from the other side.
Similar to above, I could have "blued" up the face of the work and set a scribing block with the various co-ordinates and marked them that way. I won't bother describing that in detail, though, as I will probably get told off for using the wrong type of "blue".
I hope this little demo is helpful to someone.
Steve
