A follow up question from my request for trig. help. For a person with a fully manual bridgeport and a rotary table, what's the method most likely to be successful and efficient to drill hole circles, up to say 6 holes. I use the rotary table and had really not considered a coordinate approach. For 6 holes or when I know the diameter, it seems quicker to center up the table, displace say 2.5" and drill the holes by rotating the table. But for three holes when I don't know the diameter - my current dilemma - I think that the coordinate method and the jig-border table would be both quicker and easier.
What does experience and good practice have to say?
Probably six of one and half dozen of the other. For, lets say, 17 holes that would be a lot of X-Y moves. You would have to remember to take up the backlash on every move as necessary (assuming you don't have a digital readout). And you should probably be in the habit of taking up the backlash even if you do have a readout.
OTOH using a rotary table would give you an interval of 21 degrees, 10 minutes and 35.29 seconds. You would probably want to write up a chart of angular movements but would still have to be very careful with your RT movements. Very easy to mis-dial the RT.
If you use the RT you probably want to carefully calculate the center distance between holes, set up a spring caliper to that length and then after you touch off the first hole with your c'drill layout an arc that will show you the location of the next hole. That way when you index the RT you will come down on the scribe line you just made and it will show that you have correctly indexed the RT. Scribe mark the next hole location and continue on aroung the bolt circle.
This technique would also be used if you decide to use the table and saddle to move to your hole locations.
Another technique that can be used is to set up a pointer using a magnetic base. After center drilling the first hole and indexing to the next (using the RT) carefully arrange the pointer to the edge of the first hole. Thereafter each time you index the pointer will aim to the edge of the previous hole. If it doesn't you will know you have mis-indexed and can fix it before you mis-drill a hole. This assumes that you are using the quill stop to drill your center holes to a uniform depth.
Any fool can do a good job on a couple of holes. I recall jobs at P&W Aircraft years ago that had upwards of 200 holes. Every one had to be spot on, equipment was a jig borer and 48" RT. No digital readout in those days. Of course I had to walk five miles to work, up hill both ways!
Regards,
Errol Groff Instructor, Machine Tool Department H.H. Ellis Tech
I use to use a rotary table until I bought a DRO. Its much easier to use XY coordinates than to setup the rotary table. The DRO also can be used to make sure the bolt circle is related to other features on the project.
Also I read in Moores book "Precision hole location" that XY is more accurate. There are more error sources with the rotary table. However if your machine has worn leadscrews, I suspect the RT would probaby be better.
I have tried using the RT to drill the cylinder head and the DRO to drill the cylinder using XY and they always match up. Of course there is some clearance in the head bolts.
For up to 8 holes: If you have n bolts evenly spaced around a circle of radius R, the holes are R times h apart. If you know the holes are H apart, the radius of the bolt circle is H times r. n h r 2 2.0000 0.5000 3 1.7321 0.5774 4 1.4142 0.7071 5 1.1756 0.8507 6 1.0000 1.0000 7 0.8678 1.1524 8 0.7654 1.3066
Machinery's Handbook (25th) has X-Y coordinate tables for bolt circles, up to 28 holes. Their numbers are for a circle diameter of 1. Multiply by whatever size you circle is, and go.
Chris The correct way to punctuate a sentence that starts: "Of course it is none of my business but--" is to place a period after the word "but." Don't use excessive force in supplying such moron with a period. Cutting his throat is only a momentary pleasure and is bound to get you talked about.
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