Reading deg min and seconds

Discussion on all milling machines vertical & horizontal, including but not limited to Bridgeports, Hardinge, South Bend, Clausing, Van Norman, including imports.

Moderators: GlennW, Harold_V

1949DC
Posts: 39
Joined: Thu Apr 02, 2020 6:31 pm

Reading deg min and seconds

Post by 1949DC »

Can someone help with a video or a link that best instructs how to dial in seconds on a rotary table? I understand finding degree and minutes but making that final adjustment on dial to line up marks for seconds is tricky even with good eyesight. Oddly enough not very many YouTube videos on this subject or at least my search comes up empty.
User avatar
mklotz
Posts: 428
Joined: Thu Jan 09, 2003 11:35 am
Location: Los Angeles, CA, USA
Contact:

Re: Reading deg min and seconds

Post by mklotz »

I find it hard to believe that you need to be concerned with that sort of accuracy. An arcsecond is 4.85 microradians. At a distance of 100 yards that's an offset of only 0.017 inches. Round off the arcseconds to the nearest arcminute and go with that.
Regards, Marv

Home Shop Freeware
http://www.myvirtualnetwork.com/mklotz
1949DC
Posts: 39
Joined: Thu Apr 02, 2020 6:31 pm

Re: Reading deg min and seconds

Post by 1949DC »

Your probably right about that. I don’t mind trying to hit my mark. Been around lathes all my life and .0005” is the difference between a light press fit vs a sliding fit. I just got a 8” rotary table for my Millrite and learning.
Patio
Posts: 1369
Joined: Wed Oct 06, 2010 3:14 pm
Location: Centralia Wa

Re: Reading deg min and seconds

Post by Patio »

Thanks Marv, for putting that into perspective, for us limited math guys! It helps a lot!
Live for the moment!
Prepare for tomorrow!
Forgive the past!
armscor 1
Posts: 311
Joined: Sun Jun 12, 2016 3:12 am
Location: Philippines

Re: Reading deg min and seconds

Post by armscor 1 »

Watch Keith Ruckers latest video, calculating fine degree movement to make a bevel gear.
1949DC
Posts: 39
Joined: Thu Apr 02, 2020 6:31 pm

Re: Reading deg min and seconds

Post by 1949DC »

Keith is a favorite of mine. Thanks. My signal at the moment is weak for video viewing to watch entire clip but I did heard him say fine degree cuts are not needed for low rpm. Pinion gear on a NASCAR car now that’s a different story. Good info. I’ll relax on my hobby work.
pete
Posts: 2518
Joined: Tue Feb 10, 2009 6:04 am

Re: Reading deg min and seconds

Post by pete »

When dialing in fine divisions on a R/T I've found it helpful to add a little drag to the table using the table locks partially applied. That way it's a bit less likely to go past your marks if your careful. And the worm and wheel then always has a bit of load so the small amount of backlash is fully taken up in the direction your going.

Fwiw and it's a bit outside your question. But to be honest, and it would depend on the quality of the rotary table your using. I don't trust any of the off shore R/T's and D/H's to be all that accurate including the Vertex that seem to get good reviews on the forums. Years ago there was a post by John Stevenson on the HSM forum that his order of twenty brand new 6" Vertex R/T's all had the exact same numbers on there "certificate of accuracy". Some of the numbers I have been able to check on my 6" Vertex such as the MT bore run out, table flatness and it's vertical squareness to the world didn't seem to quite agree with the factory "numbers" either. Plus just like feed screws, no matter who made it there's always going to be some degree (pun intended) of measureable lead/lag errors in the worm and worm wheel. And I know of no easy, fast and cheap method of verifying the accuracy of the worm and worm wheel down to the minutes and seconds level to be sure of what you bought or already have. Yes the degrees, minutes and second markings are there and can be used as intended. Much quicker and with possibly less mistakes would be adding the dividing plates, index pin and sector fingers to a R/T if you've got one that either takes the dividing accessory like those Vertex R/T's, or adapting one to another manufacturer's R/T if you end up needing to do a fair amount of high number dividing in your shop. I'd fully agree with Marv's point that for what most of us are doing seconds level dividing is splitting some really fine hairs. But depending on what you are doing, it's not impossible that for some that seconds level dividing may be required with higher count divisions as the rounding off errors might still have enough of a cumulative effect with something like gear making for telescope tracking. But I don't know that for sure since I know almost nothing about telescope making or just how accurate there tracking might need to be. Because of the little bit I think I might know, dividing to that seconds level might still require a lot better than the common rotary tables. A good condition old school optical R/T that were mostly used on jig borers and grinders could be a lot better choice if you really did need that level of accuracy. Then of course there's the accuracy of cut verses hardened and ground gear teeth to even get dependable seconds level accuracy onto the part your making. :-)
armscor 1
Posts: 311
Joined: Sun Jun 12, 2016 3:12 am
Location: Philippines

Re: Reading deg min and seconds

Post by armscor 1 »

My 72:1 Rotary table has dial plate with 150 graduations which gets me to only 2'.
RSG
Posts: 1541
Joined: Fri Feb 04, 2011 9:59 am
Location: Ontario, Canada

Re: Reading deg min and seconds

Post by RSG »

I agree with Pete, I'd question the ability of most rotabs to be that accurate. I use a rotab all the time and never bother getting in the seconds, As Marv mentioned is just not necessary even for the stuff I do were radial balance is important.
Vision is not seeing things as they are, but as they will be.
User avatar
GlennW
Posts: 7284
Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: Reading deg min and seconds

Post by GlennW »

An Arcsecond would be about 19 millionths at the perimeter of an 8" rotary table. (If my math is correct...)
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
User avatar
mklotz
Posts: 428
Joined: Thu Jan 09, 2003 11:35 am
Location: Los Angeles, CA, USA
Contact:

Re: Reading deg min and seconds

Post by mklotz »

Here's another way to get a feel for such a small number...

Let's say you have an RT with a 90:1 ratio and a crank on the input shaft that has a 1.5" radius. How far must the end of the crank move to rotate the table by one arcsecond ?

4.85E-6 * 90 * 1.5" = 0.00065"

or less than a thousandth of an inch.

Here's another consideration...

The standard NATO and US armed forces angular unit is the mil*. You might think that is an abbreviation of milliradian but military types aren't mathematicians so they "rounded" things off to 6400 mils in a circle instead of the mathematically correct 2*pi*1000 = 6283. Put another way, one NATO mil is 360*(3600/6400) = 202.5 arcseconds.

Despite using a unit that corresponds to a bit more than 200 arcseconds, they seem able to put their artillery rounds where they need to be. Maybe that's because a one mil offset at 1000 yards corresponds to

(2*pi/6400)*36000 inches = 35... inches

which is a bullseye if you're using a howitzer.

--
*This is also why the practice of referring to a thousandth of an inch as a 'mil' should be avoided. The inferial system has enough confusion of units without adding to the mess.
Regards, Marv

Home Shop Freeware
http://www.myvirtualnetwork.com/mklotz
Rwilliams
Posts: 1048
Joined: Thu Dec 22, 2005 2:45 pm
Location: Central California

Re: Reading deg min and seconds

Post by Rwilliams »

Most smaller home shop rotary tables are lucky to divide arcs into minutes let alone seconds. Such accuracy is going to require some serious dollars and even size of table including weight.

Recently we had to start planning for a big project that called for some large parts to be drilled accurately. We dug out a 15.75 inch diameter tilting table Pratt & Whitney rotary table that must weigh at least 600 pounds. With some cleaning of dust and rust, it was determined that the seldom used rotary table was accurate to within 2 seconds of arc. Far more than what we need.

Could not find any instruction manuals in the files for the rotary table and will have to use trial and error to sort out its operation. Is equipped with two different crank handles, one for the tilt and one for the rotation of the table. several different holes for the location of the rotation of the table crank location which we have yet to sort out.

Anyone out there have any experience with one of these monsters?
Attachments
Pratt Whitney 15.750 inch tilting rotary table.jpg
Pratt Whitney identification plate.jpg
Post Reply