Member postings for Bubble

Hi

Maybe they can't find a pen

Jim

Hi all

Joseph Whitworth's inspired invention of the method of producing a true flat surface plate using three plates scraped to all fit together. Two plates could be flat or spherical, three plates could only be flat.

He achieved accuracies of a millionth of an inch in the1840's

This invention allowed the production of precision measuring instruments and machine tools from which all else in precision engineering stemmed.

Jim

Hi Graham

I turned some valves once by taking a single deep cut at fine feed from larger diameter bar. Its wasteful in material but so is making a possibly use-once tool to do the job. You need something like free-cutting silver steel or maybe (free-cutting) EN8DM and a very sharp tool especially if using a form tool for the under-head radius. No top rake, EP lubricant such as Rocol, and make sure the tool is not above centre height, maybe slightly below. Experiment with cutting speed & feed.

It works on the basis that the uncut stock bar, being large in diameter, is stiff, and the part already turned sees no further cutting force. I finished the stems to size by diamond lapping (diamond paste, balsa wood tool) before parting from the stock material.

Worked for me!

Jim

Hi James

I hav sent you a PM

Jim

Hi Tractor Man

I found this anonymous article from Practical Machinist from 2002 which I think is very useful.

http://www.practicalmachinist.com/vb/greatest-hits-links/surface-grinding-deeper-cuts-better-finishes-110659/

regards

Jim

Hi Tractor Man

Had similar trouble when I renovated my eagle grinder (seen on www.lathes.co.uk/eagle_surface_grinder ) I fitted a powertwist belt with a belt tensioner which improved things but did not eliminate it, then fitted new spindle bearings which did likewise. Finally stripped it all down and discovered that the hardened spindle was slightly bent, about two thou in the middle. This caused shaft whirl at 2800 rpm which caused wheel wobble, even though the wheel was diamond dressed on the machine. Made a new spindle from EN8, ground all over, which provided a complete cure. Surface finish is now impeccable.

The bent shaft could have been caused by heat treatment during manufacture, but I suspect some clown lifted the machine with a sling around the shaft (not me I hasten to add).

Hope this helps

Jim

p.s. copy & paste the url, doesn't seem to work from within the post

Edited By Jim Cahill on 10/04/2012 15:16:31

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Hello Dave

I assume it is an ML7, don't know about super 7 but probably similar. Disconnect the back gear lever (lower hole). There is a little sliding key set into the bull wheel (the large gear on the spindle) on the left hand side which engages the smaller bronze back gear to lock the two together when in direct drive. You need a shortened 4mm allen key to slacken the allen cap screw, pull the sliding key outwards to disengage from the bronze gear, then tighten the allen screw again (important!!) to hold it in position. Then re-engage the back gear lever in the upper hole.

regards

Jim

Hi All

Well. I asked a question and now apparently I might not be fit to be call myself an engineer!

My bodged gear fix was (a) all I could do at the time (b) worked well enough to keep the job going (c) continues to work well many years later (d) cost next to nothing.

Sounds like engineering to me.

The whole world works on compromise, approximation and fitness for purpose. This is not to say that the best should not be strived for, but in the meantime the world needs to keep turning ( and screwcutting).

Thankyou for the answers to my question.

Jim

Chris

That's precisely why I asked the question. I need to know the PA in order to make the gears for the screwcutting clutch.

Pace the reply from Andrew Johnston, on my ML7, which I retrospectively gearbox-equipped, I had a foul-up years ago which stripped the teeth on the change gear mounted on the output from the tumbler. This was originally the input into the change-wheel train (before the gearbox was fitted). John S suggests that this stripped gear was 14.5 degrees.

I bought a cheap pattern changwheel to get me going, this was returned to the supplier as being laughably rubbish. I then made a changewheel in cast iron, using the only 20 DP gear cutter of the right tooth count that I had available. This cutter was 20 degrees pressure angle.

So apparently I was replacing a 14.5 degree gear with a 20 degree gear, and then meshing it with 14.5 degree gears. This gear has been in use when screwcutting, and quite often when surfacing (too idle to change back) for over 20 years. I have just looked at the lathe and none of the gears show any signs of distress. The meshing is not tight as it depends on the setting of the quadrant. The arrangement runs sweetly with low noise.

So I stick with my observation in the first post. Within limits of load and speed, gears of different PA but same DP will mesh without too much trouble. It may not be pretty but it works in this context.

In my days in a railway testing lab, when we often had to cut corners in order to get a testing rig built quickly, I used to say "this is fine in practice, but will it work in theory?"

Having said all this, I am still not absolutely clear what PA I have, so I will determine the PA on the gears that will be used with the screwcutting clutch modification. I will do this using the method shown in http://mikesworkshop.weebly.com/change-gear-pressure-angle.html which is accurate enough to distinguish between different tooth profiles.

Incidently, I intend to generate the gears for the clutch unit with my shaper suitably modified, using a simple rack-tooth form tool as described years ago in ME by "Base Circle", the original article can still be found on the internet.

Jim

Hi John S

So on my ML7, the spindle output gear is 14.5 degrees?

regards

Jim

Hello All

Can anyone provide a definitive answer to a long-running question? What is the correct pressure angle for Myford 20 DP change gears?

!4.1/2 degrees is often quoted, often without provenance but by reputable sources such as Hemingway, also by RDG for their gear cutters, and one would suppose that they are (now) in a position to know.

But then, equally reputable sources quote 20 degrees, including Graham Meek in his Screwcutting Simplified article in E********** in M********, who quotes the man at Myfords who used to cut the gears, and you would suppose that he also was in a position to know.

For the record, the rack teeth on my Myford are, without doubt, 20 degrees, easy to measure by digital photograph, blown up on Photoshop and laser printed.

I suppose the truth might be that, within limits, and at the speeds and loads encountered in small lathes, any old pressure angle will mesh with any other old pressure angle.

Jim