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Ian B
01-11-2007, 11:41 AM
I have a Harrison M400 with dual (Imperial / Metric) dials on the cross & topslides. Expensive looking things, made by GMT in the UK. They have an outer sleeve with slots to reveal either the imperial or metric calibrations; to switch you give the sleeve half a turn. That works fine.

What doesn't work is the gear drive to the metric ring. It's an aluminium ring and once had internal gear teeth. These have stripped off. What I don't understand is why the manufacturers went to all the trouble of gearing the metric ring in the first place; it has an exact 1:1 ratio with the imperial ring - ie why not just make both rings out of one bit of metal!

To describe it better, both rings have internal gear teeth. They both mesh with a wide pinion gear. The pinion is not stepped, ie the drive & driven sides have the same niumber of teeth.

I can see that this would have been a really clever setup if the metric ring needed to turn 1.06 times the speed of the imperial ring or whatever, but this problem has been solved by fixing the number of divisions on the metric ring.

So - to fix it. Right now, I can't see why I shouldn't simply pin the 2 rings together. If anyone knows where I can get spares, this may also be an option.

Thoughts?

Ian

Paul Alciatore
01-12-2007, 02:55 AM
Are you sure that the two rings turn the screw at the same rate? There would have to be some really strange number of divisons on them, like 100 and 127, for that to work out exactly correct. I have seen some wheels on cheap (HF) machines that just had two sets of divisions on the same wheel. One or the other just had a short division at the point where it went back to zero. So that set of divisions had an inaccuracy at that point and the errors would add up with every revolution of the wheel. It was just a cheap trick to make an inexperienced purchaser think he was getting both when, in truth, he wasn't.

Ian B
01-12-2007, 05:44 AM
Paul, you're right; I just counted the teeth.

The feedscrew is imperial, 5 tpi. The imperial dial is marked 0 to 0.2". The metric dial is marked 0 to 5mm. If they were fixed together, the error on the metric reading would be 1.6%

The imperial ring has 127 teeth. The metric ring has (or at least, had) 125 teeth. This *exactly* corrects the error, and the difference in number of teeth is so small that a single parallel pinion can join them both together.

Unless anyone knows of a source of spares for the dials, any suggestions on how to make an internal 125 tooth gearwheel, about 7cm diameter? It looks possible to keep the calibrated ring and machine metal out to drop a new ring gear in.

Ian

Ian B
01-12-2007, 06:12 AM
Pics of the ring:

http://smg.photobucket.com/albums/1003/IanBartlett/?action=view&current=DSCF0012.jpg

http://smg.photobucket.com/albums/1003/IanBartlett/?action=view&current=DSCF0009.jpg

John Stevenson
01-12-2007, 08:57 AM
Paul, you're right; I just counted the teeth.

The feedscrew is imperial, 5 tpi. The imperial dial is marked 0 to 0.2". The metric dial is marked 0 to 5mm. If they were fixed together, the error on the metric reading would be 1.6%

The imperial ring has 127 teeth. The metric ring has (or at least, had) 125 teeth. This *exactly* corrects the error, and the difference in number of teeth is so small that a single parallel pinion can join them both together.

Unless anyone knows of a source of spares for the dials, any suggestions on how to make an internal 125 tooth gearwheel, about 7cm diameter? It looks possible to keep the calibrated ring and machine metal out to drop a new ring gear in.

Ian

HPC at www.hpcgears.com Do a 125 internal in 48DP which is 65.09 on the ID and 66.15 on the PCD
They also do a 0.5 MOD one which is 61.5 ID and 62.5 PCD

Chances are these are decent numbers like 40, 44, or 48DP and could be of the shelf items. Last price I have is around 24 UKP less tax etc.
.

uute
01-12-2007, 04:57 PM
If you can index for the 127 teeth, should be able to cut the teeth on shaper or even shape them w/ the carrage on a lathe if you have to.

They're small teeth, even if there is a LOT of them! :D

Ian B
01-16-2007, 11:46 AM
Thanks for the replies guys. In the end, I made a new 125 tooth gearwheel on the rotary table and using a small shaping tool in the quill - see http://smg.photobucket.com/albums/1003/IanBartlett/?start=all for pics.

It's all back together, works fine. Now I need to replace noisy bearings in the main drive motor...

Ian

Spin Doctor
01-16-2007, 11:52 AM
Nice piece of work. But now I have a complaint. I clicked on the links to the original pics and got a message that the album/file/whatever no longer exists. Why is that every body has to automatically flush their 'bucket picture files all of the time. Today is the 16th, the post with the thread was put in on the 12th. That's only four days. Give us a chance to view the whole thread. Is that too much to ask.

Ian B
01-16-2007, 11:54 AM
Oops - sorry; I tried to reshuffle them into one album...

Spin Doctor
01-16-2007, 11:59 AM
Its okay. Stuff happens as they say. One trick I do on 'bucket is place pics of similiar stuff into a folder that holds just that stuff. That way if I want to share family pics with family members out of state I just email them the link to that folder