Harrison Horizontal/Vertical Mill Restoration

Fastdruid · Nov 30, 2022

Martin-Sy said:
I'm a little perplexed by the use of 63T. This must be an approximation to the imperial pitch.
Presumably it will be out by (2 * 63)/127. Over the length (2.375" ish) of the nut that amounts to a 20 thou error.
That sounds too much to work, but perhaps I'm making some wrong assumptions.
Is there no 127T gear available for the Harrison?

The older Harrisons used a 127T gear, I think the newer ones don't have the room in the changewheel train and hence use a 63T.

Martin-Sy · Nov 30, 2022

Fastdruid said:
The older Harrisons used a 127T gear, I think the newer ones don't have the room in the changewheel train and hence use a 63T.

Ah, OK, thanks. Same as with Monarch 10EE and CVA then. Their solution was to use a smaller DP just for the metric gear set. 32DP in place of 16DP.

Agroshield · Nov 30, 2022

Martin-Sy said:
Presumably it will be out by (2 * 63)/127.

It is a highly unfortunate coincidence that 63 is nearly half of 127. It causes people to be misled in their minds about what is going on.

When you do an (exact) imperial to metric conversion with a 127t gear, it is always, always the driven gear.

When you do an (approximate) imperial to metric conversion with a 63t gear, it is always, always the driver gear.

Martin-Sy · Dec 1, 2022

Yes, apologies, a wrong assumption on my part.
@Carl Wilson does say in one of the videos that the pitch error is very small.

Carl Wilson · Dec 2, 2022

It can be the driven gear. If it is driving the leadscrew directly, then it has to be the driver. In the case of the Harrison M250 it is then driving through the feed gearbox to the leadscrew.

Here is the handy calculator I use from Dr Al's Miscellany to see what gears that I have could be used to get 5tpi on my metric M250.

CGTK - Harrison M250 Change Gear Calculator

www.cgtk.co.uk

The error for 5tpi is very small, the combination of gears I'm using along with the feed gearbox settings gives 5.00063tpi.

Thanks for watching my films guys, I hope you enjoy them. Please like them if you do and consider subscribing. Also, I'd love to see these types of comments on my channel. The channel is all about learning and sharing technical information and knowledge.

Agroshield · Dec 2, 2022

Carl Wilson said:
It can be the driven gear.

...to get 5tpi on my metric M250.

I was very specific above in saying that the 63t is a driver for IMPERIAL to METRIC conversions.

Hence, if you are doing a metric to imperial conversion, of course it will be the driven gear.