Thread: Harrison M250 Cross-slide Dual Scale Problems

My M250 has always exhibited slipping of the dials on the cross-slide, initially not too serious and easy too recognize and allow for.

But it got worse during a recent screw-cutting job and the decision made to dis-assemble to clean and fettle as appropriate, how hard could it be ?
Bad mistake.

These dual scaled dials are a nightmare to re-assemble onto the screw shaft.
Assembly appears to require positioning a Woodruff key at the bottom of an annular tunnel and engaging a matching piece without guides, visibility or ability too observe/check final positioning.

This is a cry for help from anyone with experience of Harrison dual scaled dials.

Thanks in advance,
John.

Are they the same as would have been fitted to a Colchester Bantam (a Gammet invention), with a ring that gets pulled out/ pushed in to change between metric and imperial?

If they are, I may well be getting to look at a malfunctioning one soon. It sounds an interesting beast, so I'll probably post about it.

Michael

I have a thread running on this subject on the Yahoo Groups site and attached is the parts break-down for the M300 that was posted to assist.
It is different to my GMT branded M250 in details, but represents the general arrangement.

It is interesting that the M300 appears to confirm my ideas that a good engineering solution is to adopt a 2-part knob with the handle part screwed on as the last step.
My M250 is also 2-part, but the two parts must be assembled before applying to the screw shaft.

My solution would put an ordinary square key at the end of the shaft, then add the knob inner part, easy adjustment of shaft end thrust and screw up tight.
Then screw the handle part of the knob on to the inner as the last action.

In my M250 I have the following backlashes to contend with when using the metric scale,
- classic cross-slide nut to imperial screw,
- Woodruff key shaft to knob is so difficult that inevitably loose to get assembly,
- very difficult assessment of thrust bearings resulting in inevitable looseness,
- knob pin to GMT assembly barrel (it should be a taper of some sort drawn up),
- epicyclic gearbox to the metric scale.

On a lathe that is sold as precision "metric", the above is unacceptable.
My long-term plan is to replace with a metric screw and single metric scale.


image1.jpg

John

I hate to get all modern on you, but a cheap dro would solve all this, no parts required. The Chinese glass scale units are surprisingly good.

Sent from my Nokia 8 Sirocco using Tapatalk

Originally Posted by caskwarrior

I hate to get all modern on you, but a cheap dro would solve all this, no parts required. The Chinese glass scale units are surprisingly good.

This.

Saw a Chipmaster locally with the dual scale dials. I was a bit intrigued by them but not enough to pay what the current owner wanted for the machine.

Must order some DRO kits...... problem then goes away as does backlash issues (more or less).

PDW

Yes, I keep considering DRO, the phamplets from H&F are always tempting.

Removed the GMT barrel from the thrust bearing housing, carefully as dis-assembly of the barrel involves springs and balls exiting to parts unknown in the shop. As long as you keep a large encircling spring clip in place, all is well.

Was then able to check all the fits and inspect/clean/fettle the thrust bearings - some wear evident that was bypassed by reversing a spacer washer.

Did some trial knob installs and determined that there is no adjustment to set the thrust bearings, it is simply the tightness of the final knob attachment capscrew.

Trialled seating the Woodruff key with 'Bluetack, with some success. Determined I could feel poor alignment and adjust as appropriate to get the keyway to slide home over the key.

So, tried a knob install with the GMT barrel in place, with success.
Cranking up the last capscrew noticeably made everything tight to move, so slacked off to get the right 'feel'.
Lathe back in service with a whole lot of knowledge gained for the next time.

John.

Hi Everybody,

The Harrison Dual Scale knobs need a bit of understanding to get them to work right. If you have an 'imperial' machine and the imperial scale showing, as the dial is locked directly to the feed screw the reading will be correct. The converse if you have a metric machine.

However, on an imperial machine, with the metric scale selected, a set of fine gears inside the handle are engaged that slows down the rotation of the metric scale in the ratio 25.4:1 so it reads correctly. Again, the converse is true for a metric machine.

The gears are better suited to a Swiss watch and easily fail. Hence, many people report one scale or the other not reading correctly.

I have yet to find a source of spare parts for the gears at a sensible cost.

Taking the assembly apart is easy. Putting it together again is a more difficult matter without a conical sleeve to deal with the balls and springs of the indent ring (easily made). Assembly then becomes relatively straight forward.

Hope this helps a little.

Len

Siromnel,
Welcome to the Forum.

Just a clarification, my M250 is a metric machine.
But, the cross-slide and top-slide screws are imperial, with the gearbox to drive the metric dial.
The imperial dial is directly coupled to the screw.

I have never seen any references to a metric screw with a gearbox to drive the imperial dial (on the M250).

Keep well,
John.

Edit: I have a parts break-down diagram for a M300 cross-slide, and it shows two assemblies.
So it looks like the metric screw with gearbox to the imperial dial does exist on the M300.
John.