[Project 1] M300 screwcutting gearbox repair

After ~2 years of searching, I finally managed to source a suitable lathe.

She's a 1995 Harrison M300 - Long Bed [40"]

Pretty, no?

I actually took a bit of a punt on this one, as Covid-19 prevented a viewing.

Most things turned out well. It's in pretty good condition overall.

However, when I tested the screwcutting gearbox, the feed selector was jammed and the leadscrew skipped.

I feared the worst; chewed-up gearing, missing teeth etc. My heart sank.

After some guidance from the wonderful folks over on the Harrison Lathe Group (https://groups.io/g/HarrisonLathe), it transpires that there are a couple of known issues with the M300 SC gearboxes. Firstly, the oil can gum up the workings. Secondly, the delrin cams can expand over time, ceasing the feed selector. Hopefully it's one of these issues.

Well, there's only one way to find out.

The gears look like they're in good shape - no missing teeth. Phew!

Gave it a good clean and fished what turned out to be one of the external plugs out from under the cams. No idea how it would have made it into the gearbox. I had hoped that this was the cause of the jamming. No such luck I'm afraid.

Anyway, gave her a good clean.

I'll save you from all the gory details (for those wishing to delve into those you can see the link just below). The issue was expanded delrin cams. There are a couple of options available to fix this. Firstly, one can mount them on an arbor and turn them down. However, since I have never used a lathe before, let alone turned delrin (requiring a honed HSS tool turned not too fast so as to melt it, etc) and the fact that they are an expensive part to replace if I mess the up (£160 - thanks Harrison!), I decided on the second option.

https://groups.io/g/HarrisonLathe/topic/m300_newer_type_screw/73407135?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,73407135

Option 2 was to capitalise on the fact that any movement (i.e. any direction) of the cam follower shaft seemed to free up the binding. So first I made up a new cam follower shaft (so as not to chance scrapping the original part), turn down one end and place it into a brass cam, which can be tuned.

This is my first ever turning operation.

Faced:

Turned down to the same diameter as the original part (bang on ).

Centre then twist drilled:

Reamed:

Hex milled on the end for fine tuning (this is to be removed later):

Tuned:

Hole for grub screw drilled:

Tapped:

Fit-up:

And ... it verked! \o/

Phew!

Edited By Lee Jones 6 on 17/05/2020 09:48:21

Edited By Lee Jones 6 on 17/05/2020 09:48:37

Edited By Lee Jones 6 on 17/05/2020 09:49:33

Well done Lee,

An ingenious solution I must say but isn't the same problem going to arise later on at the remote end to your fix where the shaft comes out through the gearbox? The follower shaft is now no longer parallel with the cam shaft or the gears it shifts

I can see you may be forced to be more radical next time and make a new eccentric support inboard of the point of exit for the follower shaft so that it can be made to move truly parallel with the cam shaft below.

Enough for now of course but maybe a point to ponder on. Enjoy using the lathe, it looks like a very capable machine.

Of course, by then, your confidence will be more than sufficient to tackle the skimming of the delrin cams instead

Kind regards Brian

Edited By Brian Wood on 17/05/2020 10:30:57

Lee,

I agree entirely with Brian. I also own a M300 which is VFD controlled, a mod I have found useful on all my machines. The M300 is a very capable machine, treat it with respect as they can bite!

What on earth was it "lubricated" with? Does not look just like any old oil. With the SC box in that condition, it will be worthwhile taking the top plate off and looking at the main head gears and their oil, clean it all out and replace with clean ISO 30 hydraulic oil (the main bearings cost considerable more than £160, probably nearer £1000)

I like the covers below the front shear, will keep all that swarf off!!

K

Brian, he has not modified a layshaft it is only the shaft on which the cam followers slide axially. A clever mod.

Interesting that you say you have no lathe experience - bit like a person without a driving licence getting a Ferrari to learn on. yes I am jealous

It would surprise me if the delrin swelled further. It took 25 years to get it to the point of binding.

However, if this does happen again, I can either add the same mod to the other end or turn down the cams.

The odd thing is, the cams on the right of the picture (3 and 4) were the worst. 3 was causing the bind.

I will be careful, promise.

That's a good question. This was the muck that came out of the SC gearbox.

Head gearbox looked okay, but I dropped and changed the oil anyway.

What is in the last photo on the yellow paint, right hand side 2nd shaft up?

In the headstock gearbox? The little shiny spot on the dirty section of paint? Or something else?