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Sieg C1 lathe autofeed and screw cutting

Sieg C1 lathe autofeed and screw cutting

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Rob Athome02/11/2020 17:16:15
3 forum posts

Hello,

I have been asked if I can help a friend who has a Sieg C1 lathe. He wants to start using it after it has sat in the box ( crate? ) for a good number of years.

Two things he asked about were single point threading and using the auto feed to produce a good surface finish on the material which will be aluminium or brass.

AFAIK the C1 lathe has gearing from the main spindle to the leadscrew via a on/off clutch switch so for creating a good finish it is simply a question of engaging the function and letting the machine get on with it. How do you use this function when machining upto a shoulder ? Same goes for threading - if you want to put an M6 thread on the end of a shaft down to a shoulder.

As there doesn't appear to be a threading dial I assume it is a case of using the reversing switch to get back to the start for subsequent passes.

Thanks for any info.

Rob

Howard Lewis02/11/2020 22:47:07
4102 forum posts
3 photos

It is unlikely that there will be a clutch, as such, between mandrel and Leadscrew. More likely a handle that operates the tumbler reverse to allow the feed to be engaged, for Forward, Neutral, or Reverse.

The technique for machining up to shoulder is to have fantastically quick reactions to open the half nuts, just before the tool hits the shoulder!.

The practical and sensible alternative is to stop the machine just before the shoulder and then rotate the Mandrel by hand, either pulling the chuck round by hand, or using a mandrel handle.to bring the tool upto the shoulder, stopping JUST short of it.

Your friend could spend his time well, initially, by making a Mandrel Handle.

It could save him a lot of broken tools and scrapped workpieces.

For a fine feed, with the aim of getting a good surface finish, the Leadscrew needs to be geared down so that it rotates as slowly as possible compared to the chuck. If it needs to be said, a small radius on the nose of the tool will improve surface finish.

The C2 and C3 mini lathes include two 20T gears and 2 x 80T gear, among their changewheld, so a 16:1 reduction is available Probably the C1 does as well.

With a 1.5 mm pitch leadscrew, this will give a feed rate of 0.09375 mm per rev ( 0.00369" per rev in old money)

If the lathe is Imperial, the Leadscrew is likely to be either 8 tpi or 16 tpi.

With a 16 tpi leadscrew this will give a feed rate of 0.00391" per rev.

For screw cutting, the changewheels are selected to give the required ratio between Spindle and Leadscrew

So for a 0.5 mm pitch thread, with a 1.5 mm pitch Leadscrew tjhte ratio would need to be 3:1. Probably using a 20T on the Mandrel, and driving via an Idler, to a 60T on the Leadscrew.

To cut a 1 mm pitch thread the ratio would need to be 1.5:1 so probably a 40T driver to a 60T on the Leadscrew , with an Idler to fill the gap between the gears.

If your friend is unsure, he needs to buy a couple of books, for a deeper explanation and examples of set ups.

Martin Cleeve's "Screwcutting in the Lathe" which is No. 3 in the Workshop Practice Series, and Brian Wood's "Gearing of Lathes for Screwcutting" The latter gives table of changewheel set up for a number of lathe, so that probably no calculations would be needed, just set up according to the tables for your particular lathe, or one with a Leadscrew of the same pitch.

Obviously if cutting an Imperial pitch thread on a Metric lathe, or vice versa, ideally a 127T gear would be needed, but a 63T gear will produce threads with acceptable pitch errors,being a it is low powered, so shallow depths of cut will be the order of the day.

The manual for the C2 or C3 regards 0.010" (0.254 mm ) as a roughing cut, although many exceed that figure without problems.

HTH

Howard

Rob Athome03/11/2020 00:44:38
3 forum posts

Hello Howard,

Many thanks for the detailed reply. The Sieg C1 has a mechanical lever to engage the autofeed and a separate switch to set the motor rotation to 'forward' or 'reverse'. There aren't any half nuts as in most mini lathes to dis-engage the saddle from the leadscrew. I checked out a parts diagram/listing for the C1 and what they refer to as a clutch is more of a forked coupling.

You are quite correct about a radius on the tool being sensible - I use this method myself for finishing cuts - it works well on brass parts but no quite so well on aluminium etc in my experience.

The manual for the C1 that I tracked down indicates a feed rate of 0.025 or 0.05mm per chuck revolution depending on which of the standard gears you have bolted on. There is also an extra set of gears available that whilst mainly intended to give metric thread pitches of 0.5 to 1.5mm can also function to make a coarser auto feed rate.

I suspect I should find out how much threading he wants to do and consider if just a tailstock die holder may be better suited.

Thanks again.

Rob

Ronald Morrison03/11/2020 11:00:21
56 forum posts
4 photos

The fact that the lathe motor can be run in reverse makes threading to a shoulder for imperial threads much easier. With the motor running in reverse use a boring bar with a thread cutting end on the backside of the stock , bring it to the shoulder, then engage the "half nut'. Now you cut threads away from the shoulder so you don't have to have lightning reflexes. Making a stop for the carriage lets you set the starting point so the tool never hits the shoulder.

woody111/11/2020 21:38:24
avatar
91 forum posts
21 photos

Threading can sometimes turn into a "bain of your life thing", if I can get away with a die on smaller diameters happy days. On larger diameters I have found roughing with a single point then finish with the die, a far better operation all round, in both speed and finish. My machine is a cl300m I don't know how that compares to a C1?

Woody.

Rob Athome11/11/2020 21:41:52
3 forum posts

Thanks Woody and Ronald for replying.

Rob

not done it yet12/11/2020 07:00:37
5354 forum posts
20 photos
Posted by woody1 on 11/11/2020 21:38:24:

Threading can sometimes turn into a "bain of your life thing", if I can get away with a die on smaller diameters happy days. On larger diameters I have found roughing with a single point then finish with the die, a far better operation all round, in both speed and finish. My machine is a cl300m I don't know how that compares to a C1?

Woody.

In addition to the above points, threading larger diameters (particularly of coarse threads) can be very hard work if using a die. Also single pointing assures the thread is properly aligned with the material being cut, before finishing with a die.

HOWARDT12/11/2020 08:32:05
652 forum posts
15 photos

You could leave the lead screw and nuts engaged and rotate the spindle by hand. Whilst time consuming with small depth of cuts it is one way to cut to a consistent point. Like most I thread with split dies, most of mine are small, M2 etc but I do go up to M16. Some odd pitches will require some additional gears which can be easily obtained.

woody112/11/2020 19:26:10
avatar
91 forum posts
21 photos
Posted by not done it yet on 12/11/2020 07:00:37:
Posted by woody1 on 11/11/2020 21:38:24:

Threading can sometimes turn into a "bain of your life thing", if I can get away with a die on smaller diameters happy days. On larger diameters I have found roughing with a single point then finish with the die, a far better operation all round, in both speed and finish. My machine is a cl300m I don't know how that compares to a C1?

Woody.

In addition to the above points, threading larger diameters (particularly of coarse threads) can be very hard work if using a die. Also single pointing assures the thread is properly aligned with the material being cut, before finishing with a die.

Tell me about it, I machined an entire air rifle block with a thread of 1/2" X 14nps so quite course. Left this until last got lazy didn't rough enough and the tap went in not even remotely perpendicular. 2 days of work lost. It happens but learn from your mistakes and all thatcrying

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