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Randa lathe has me much confused

How does the change gear work?

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Michael Kibby13/10/2021 19:55:28
4 forum posts

Hello everyone, not only am I new to model engineering, I also seem a bit thick in understanding how change gear works.

Let me be more specific , I have just a acquired an old Randa model A long bed, and one or two items are missing from the back gears, in trying to understand what should be there I find myself confused about the operation of the banjo lever at the back of the lathe.

When operated it engages with the small fixed gear next to the flat belt pully, at the same time it engages with the large gear fixed on the back of the main shaft. Ahead of this there is a a small 30T gear which is also fixed to the shaft.

Since the 30t gear is already fixed to the shaft, what function does the banjo serve?

It would make more sense to me if the large gear and the 30t gear shared a common rotating shaft/tube independent of the main shaft, which on my lathe doesn't seem to be the case.

Any help would be much appreciated.

Thanks, Mike.

Clive Brown 113/10/2021 20:24:15
709 forum posts
33 photos

Is this page helpful?here

Andy Carlson13/10/2021 20:39:05
397 forum posts
130 photos

Back gear and change gears are two different things.

The back gear allows the spindle to run at lower speeds and with higher torque. When engaging it you should also disengage the direct drive. This may be done using a peg on the bull gear or by loosening a grub screw in or near to the drive pulleys.

Change gears allow you to drive the feed screw at different speed ratios in sync with the spindle, either for screw cutting or power feeding.

Many (most) lathe jobs use neither so I would not worry about them until you have done some straightforward jobs.

Michael Kibby13/10/2021 20:51:19
4 forum posts

Hi Clive and Andy, thank you for your speedy responses.

I have had another look at the lathe and see the grub screw you spoke of, and sure enough on releasing it everything made clear sense to me.

I'm sure I'll not use this feature, but would like to restore the machine to how it was originally.

Again thanks for your help. Much appreciated.

Mike.

Nigel Graham 213/10/2021 22:12:47
1712 forum posts
20 photos

You probably will use the back-gear if you do any screw-cutting, as it allows the tool to amble along the work at a rate allowing you to knock the feed off at the end of the thread (unless the lathe has an automatic trip for that purpose).

I also use the back-gear when turning something awkwardly-shaped and relatively large on the faceplate.

The banjo hold the change-wheels for screw-cutting and self-acting feeds; a separate function from the back-gear. When using a change-wheel set-up it is driven by the pinion on the outside end of the spindle, and that rotates whether in direct or back-gear mode.

Andy Carlson13/10/2021 23:07:34
397 forum posts
130 photos

If your lathe uses a grub screw to disengage direct drive then it probably has a small flat on the spindle. You should turn the pulley so that the screw will line up with the flat before tightening it. The snag is of course that you can't see the flat but you can experiment to find the point where the screw will tighten down furthest. Often the flat will be in line with a more visible feature on the spindle such as a keyway but no promises.

Nicholas Farr14/10/2021 09:15:38
avatar
3001 forum posts
1371 photos

Hi, on both the RandA type A & B lathes that I have, the spindle has a shallow hole for the pulley grub screw to lock into and this is directly in line with a dot punch mark on the register flange that the chuck backplate butts up against. Neither of these have an auto knock off for for the feed for screw cutting or turning. The threaded hole at the left hand end is for a cheese head screw that acts as a key, for the large back gear wheel that fits in this position, as seen in the bottom photo below.

randa spindle 1.jpg

randa spindle 2.jpg

randa backgear.jpg

Regards Nick.

Edited By Nicholas Farr on 14/10/2021 09:23:36

Bazyle14/10/2021 17:28:53
avatar
6042 forum posts
220 photos

Use the search facility on the front page of the site to search for other threads on the Randa lathe.

Howard Lewis14/10/2021 18:33:03
5348 forum posts
13 photos

Welcome. Don't be afraid to ask questions on here.

You can find out more about the lathe by visiting the Lathes UK site.

Already, you are finding out about naming the various parts of a lathe, and their function.

Back gear is used to slow the Mandrel relative to the incoming drive. By releasing the pulley (as you already know, by slackening / removing the grubscrew ) The pulley then drives the mandrel through the two stage reduction provided by the Back Gear.

Leaving the grubscrew in place will lock everything, since the pulley id trying to drive the Mandrel at two different speeds at the same time.

Do not be tempted to use this to lock the mandrel to unscrew a stuck chuck! That is a short cut to knocking teeth off the gears.

Changeears allow the Leadscrew to be driven at different speed from that of the mandrel, so that threads of different pitches can be cut on the material in the chuck.

The gear train required for a given thread will depend up the pitch of the Leadscrew on a particular machine.

Many (But not all! ) lathes produced to Imperial standards used a Leadscrew with 8 Threads Per Inch (tpi )

So, purely as an example, if a 12 tpi thread needed to be cut, the changewheels would be arranged to give a ratio of 1.5:1 between mandrel and Leadscrew. So the gear vtrain might be set up Mandrel 30T, to drive a 45T gear on the Leadscrew with an Idler in between. The number of teeth on the idler will not matter since it does not affect the overall ratio. The essential thing is that the mandrel revolves 1.5 turns while the leadscrew rotates for each turn of the Leadscrew.

In some cases the Idler may have to be a compound gear (Two, of different tooth counts, joined together )

But those are lessons to learn further along.

by arranging a suitably large ratio, between Mandrel and leadscrew, a power feed can be arranged to provide a fine finish.

But in early days, you need to become familiar with the controls, and how to set tools at centre height (To ensure that they cut properly ) and to operated the handles / handwheels to provide a steady feed.

Reading L H Sparey's Book "The Amateur's Lathe", Ian Bradley's "The Amateurs Workshop", Harold Hall's "Lathework", or Neil Wyatt's "Lathework" be explain a lot of things.

Being an older design of lathe, you would probably be better using High Speed Tools, usually referred to as HSS. This will then let you in for buying a bench grinder, so that you learn how to grind tools.

A set of Zeus charts will be useful. You will refer to them often (I still use mine, bough as an Apprentice in 1958 )

Another book that you may well come to find a useful reference is Tubal Cain's "Model Engineers Handbook"

When you eventually come to cutting threads with Taps and Dies, yo will find No 12 in the Workshop Practice Series, "drills, Taps and Dies" helpful. For this work always use a cutting lubricant such a Trefolex or Rocol RTD, and back the Tap or Die every half turn, to break the swarf; rather than the Tap! taps break through bending rather that excess torque (But if gets tight, back off anyway )

Find a Model Engineering Club near you, if possible, and join. In this way, you can get face to face advice, and possibly demonstrations.

HTH

Howard

Michael Kibby14/10/2021 19:32:59
4 forum posts

Thank you all for your thoughts and advice, it's both enlightening and appreciated.

I guess I should add a few details about myself and where I'm at so to speak. I'm a full time self employed watchmaker, and as such use a Boley lathe for staff turning, drilling etc. I have cut a few wheels from time to time using a vertical slide and milling attachment with an appropriate cutter. Needless to say a Boley watch lathe doesn't have back gears or screw feed etc, hence my lack of knowledge in this area. When I left school I did a seven year apprenticeship as a coppersmith...(not much call for that now!) During Covid I got hold of a few stationary engines which then led to mowers and now an RandA lathe. It would be very handy to get command, even in a small way, of using it in the restoration of the engines etc.

So the plan is to clean and restore the RandA and make it function as it was intended...and learn the name and function of its parts along the way Lol.

I'm seventy three now and hoping that that particular old adage is wrong...you can indeed teach an old dog new tricks!

Thanks again to all you guys for taking the time to explain the various elements of what is for me a huge lathe.laugh

Michael Kibby14/10/2021 19:33:00
4 forum posts

Thank you all for your thoughts and advice, it's both enlightening and appreciated.

I guess I should add a few details about myself and where I'm at so to speak. I'm a full time self employed watchmaker, and as such use a Boley lathe for staff turning, drilling etc. I have cut a few wheels from time to time using a vertical slide and milling attachment with an appropriate cutter. Needless to say a Boley watch lathe doesn't have back gears or screw feed etc, hence my lack of knowledge in this area. When I left school I did a seven year apprenticeship as a coppersmith...(not much call for that now!) During Covid I got hold of a few stationary engines which then led to mowers and now an RandA lathe. It would be very handy to get command, even in a small way, of using it in the restoration of the engines etc.

So the plan is to clean and restore the RandA and make it function as it was intended...and learn the name and function of its parts along the way Lol.

I'm seventy three now and hoping that that particular old adage is wrong...you can indeed teach an old dog new tricks!

Thanks again to all you guys for taking the time to explain the various elements of what is for me a huge lathe.laugh

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