how to scribe parallel vertical lines in a cylinder

[dansawyer]

The project to to re position two mounting arms on a telescope. The arms are cast aluminum. They have been machined to have 2 new bolt holes each for attaching to a cast aluminum base. The base is a 10 inch diameter cast aluminum cylinder about 4 inches long. The walls are about 1/4 inch thick. One end is solid. The question is how to accurately mark, drill, and tap mounting holes on the sides of the base.
The base has an alignment screw on the bottom, this will be used as the starting point for the subsequent measurements. I have a marble setup flat surface plate about 12 x 18 inches, 2 v blocks, a machinist square, calipers, and a dial indicator.
I believe the first step is the identify the points on the cylinder 90 degrees on each side measured from the alignment screw. The first question is how to measure and mark points on opposite sides of a 10 diameter cylinder and 90 degrees from a base mark.
The second step is to mark lines along the cylinder axis for the subsequent measurement and marking of the actual mounting holes. This can be done by placing the end of the cylinder on the v blocks on the set up flat so the sides are vertical. The location of the lines can be located with the machinist square. The mounting hole positions can be measured with the calipers.
This is all dependent on the two initial marks on opposite sides of a 10 cylinder and aligned 90 degrees from a base mark. I am stumped on how to do this. I hope this is clear, if it is not I will answer questions. Thanks Dan.

 

[francist]

This might sound a little crude but it would be fairly quick and uses minimal setup: take a band of paper, like a length of cash register tape for example, long enough to completely encircle the cylinder. Wrap it snugly and squarely around the outside of the cylinder near one end. Carefully mark where the ends of the band overlap, remove the band from the cylinder and lay the paper down flat on the bench. You now have the circumference of the cylinder (the distance between the two marks where the paper overlaps) on a flat length of paper.

Divide that circumference into four equal parts, mark the paper accordingly, and replace the band around the cylinder taking care to align the two end marks like you had before (at the overlap). Transferring all your marks from the paper to the cylinder wall will give you four quadrants at ninety degree separation.

From there, the machinist square could carry the lines vertically up the cylinder walls as long as at least one end of the cylinder was square to the side.

Would that be accurate enough for positioning the mounting yoke, or is there further referencing downstream off of these points which would demand more precision?

-frank

 

[Tozguy]

The question is how to accurately mark, drill, and tap mounting holes on the sides of the base.

It would be very helpful to know how accurate, ex. within what range of 90 deg. from each line does it have to be?
I do not know much about telescopes but that they are precision instruments and I would expect to see several numbers after the decimal point on the drawings. One degree is huge when talking astronomy.

 

[Cadillac]

Take your tube and put on your plate horizontal and fix so it won’t spin. Take a height gauge measure the OD. Half that measurement and scribe a line on each side there’s 2 lines. Now spin you tube 90* take a square line up your first marks to the square so their vertical. Take the height gauge that’s already set on center and scribe your other 2 marks. Once you have your scribe marks you can measure off the end of tube to get your marks off the end. Make sense?

 

[dansawyer]

The accuracy question is a good one. It should be as accurate as practical. I would guess 2 to 4 thousands would be good lower objective and 8 thousands would be an upper end tolerance limit. There are 4 stages to the mount, the mounting platform, two fork arms which will have to be aligned' they create a large U cradle into which the above fits. The mounting arms above have a 1 inch diameter journal which fit into needle bearings in the top end of the U cradle. The objective of this project is to assure the journals are aligned. To do that I have connected them via holes that run through the journals and stacked them in a jig. I clamped the journals . That allowed for drilling alignment holes in the

 

[Mitch Alsup]

Can you provide a picture/drawing.

I remember my C11 got* one of the fork tines off by a couple of thou and it was painfully clear that the scope was not "on" the polar axis when viewing Polaris and rotating on the polar axis. Thus, you might be underestimating the required accuracy, especially if you want to use the setting circles to minutes of accuracy.

(*) long story.

 

[gjmontll]

Perhaps make a jig, (from a plate of aluminum?) that could clamp around the cylinder, with guide holes for the drilling and a alignment mark that could [somehow] register with the alignment screw.

 

[RJSakowski]

Another way, if you have a rotary table, would be to fasten the cylinder to the table such that it is centered on the rotational axis. Then locate the alignment screw with a fixed scribe and rotate the table 90º in either direction to locate the scribe lines. If the table is mounted on a mill, you can lock the spindle and insert a scribe to scribe your parallel lines or, if you have a vertical RT, you could actually drill the necessary mounting holes.