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?

Brian

Brian:

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
613 Upper Maple Street
Danielson, CT 06239

860 774 8511 x1811

http://pages.cthome.net/errol.groff/

http://newenglandmodelengineeringsociety.org/





On Thu, 12 Feb 2004 10:51:32 -0500, "Brian"
wrote:

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?

Brian

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

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.

chuck

Brian wrote:

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.

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

For setting up on a manual mill, see
http://www.metalworking.com/DropBox/...ed_reducer.pdf

Ted

Brian wrote:

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.


Whole bunch of very useful little utilities here, including
"boltcirc.zip"
http://www.geocities.com/CapeCanaveral/Hall/4425/


Tom

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.