The Secret Society of Lathe Trolls

Steve E.

I am trying to figure out my max possible cutting time using various lines per inch (LPI), as I figure this will help me make certain decisions about which new feed screws I want manufactured.

I came up with this data based on theory (and one research source) as opposed to experience. I am using the Presto 6N as my guide. In the case of the Presto, the absolute max inner cutting diameter seems to be 4 1/8 inches (or 10.5 cm). I am unclear as to whether the run-off groove can extend beyond this point because the lathe is not yet fully functional. (anyone know?) (EDIT: It appears that on the Presto 6N, That's IT.)

I have a xerox of a "Disc Cutting Timing Chart" from Impact Sound Recording Studio of Philadelphia. Their data looked pretty close to this chart (I've evened out the seconds so they make strictly consistent mathematical sense):

I prepared three other versions of the chart based on extreme and "safer" estimates of the potential running times at each feed screw size. Can anyone confirm which of these (if any) is most accurate? Thanks.

Doug 6N · Joined: 21 May 2007 Posts: 66 Location: Washington

Hi Steve:

I have a Presto 6N and it can only cut to around 4" on the diameter so your 4 1/8 is right. I just never precisely measured it.

Now to drive you nuts. How about a chart taking that into consideration for us 6N users, Smile

No! You really don't have to do it. But ya got to admit it would be kindof handy.

Doug

Steve E. · Posted: Tue Jul 22, 2008 5:23 pm Post subject:

After the first of the four sections, this chart _is_ for Presto users! Smile

On my unit at least, 4 1/8 is the innermost groove, not 4 inches.

I am hoping that someone who has one o these units that is fully functional can corroborate some of my timings. Thank goodness I finally got a response. Wink

more, more!

Charles Bork · Joined: 23 Sep 2006 Posts: 5

Steve wrote:

> I am unclear as to whether the run-off groove can extend beyond this point because the lathe is not yet fully functional. (anyone know?) (EDIT: It appears that on the Presto 6N, That's IT.)

There are two ways to extend the travel of the cutterhead on a 6N so that lock grooves as small as 3.5" in diameter are possible.

The problem is that most standard 6N lead screws have a thread just long enough for a 12-inch lp lead-out/lock-groove. But not long enough for a 7-inch 45 with its smaller diameter label and lock groove. However, I have two that are long enough (a 120 and a 160). I would guess these were introduced in the 50s when 45s became popular.

So, one way to solve the problem is to get or make lead screws with an extended thread range.

The other is a little clunky, but works quite well. Just attach the head to the left of the normal mounting slots. About half and inch to the left. The next slot over, as it were. This will cause the head to hang off in space to the left the standard position. But it works if you use longer mounting screws and a metal brace behind the mount--just a straight piece of metal with two holes for the head mounting screws--which functions as a clamp. Perhaps not the most elegant solution, but an easy one.

--Charles

Steve E. · Posted: Thu Jul 24, 2008 1:11 pm Post subject:

Charles, both of these sound potentially excellent. Even the extended thread screw idea is in the realm of possibility when we find a machinist and get the feed screws made.

The problem I predict is the cutting head bumping into the flange. (The flange is the 2 1/2 inch piece that fits over the label and drives the worm gear that drives the feed screw.) First, I picture the bottom of the head scraping the flat part of the flange. Does the head clear that when correctly mounted? Mine, as mounted when I received it, definitely wouldn't.

In the case of moving the head over, I also see a risk of it crashing into the sticking-up part of the flange if one is not careful. I could imagine damage to the feed screw, or to the thin blade which travels on it, if this happens. Were you simply careful or is there a safety measure to protect from this happening?

Charles Bork · Joined: 23 Sep 2006 Posts: 5

> The problem I predict is the cutting head bumping into the flange. (The flange is the 2 1/2 inch piece that fits over the label and drives the worm gear that drives the feed screw.) First, I picture the bottom of the head scraping the flat part of the flange. Does the head clear that when correctly mounted?

Yes. Presto 1-C and 1-D heads with long-shank styli and Grampian Type C and Type D heads with short-shank styli will clear the stock 6N flange. I have also conducted some experiments with Presto heads using short-shanked styli. A modified or substitute flange of with a diameter of less than two inches (or a height of .1") is required because of the low-clearance of this configuration. I have had my local 6N-friendly machinist make one of these.

> In the case of moving the head over, I also see a risk of it crashing into the sticking-up part of the flange if one is not careful. I could imagine damage to the feed screw, or to the thin blade which travels on it, if this happens. Were you simply careful or is there a safety measure to protect from this happening?

One does need to be careful. But that is true of record cutting in general. If your ruler is correctly marked you won't have a problem. I have never run the carriage into the spindle. I think I have at least 30 seconds (depending on pitch) to spare beyond the placement of the the lock groove. Also, if you make lead screws, you can have the thread end just before a collision would occur. But I probably wouldn't bother with that. As I say, I have never had a problem.

--Charles Bork

Steve E.