Big threads on small lathe???

I have an Axminster/ SEIG SC4 lathe.

I would like to be able to cut a M 33X3.5 mm thread in aluminium. (I would like to make a vacuum chuck for my wood turning lathe, so effectively Im making a ‘cup’ with thick base which has a female through thread to allow it to screw onto the spindle). It will be made from a piece of aluminium approximately 100 mm diameter.

The maximum pitch thread the lathe can screw-cut with change wheels / lead screw i believe is 3 mm so I don’t think through cutting it direct is an option on this lathe. I do have a tap that is the correct size but given the significant thread depth, I fear probably too ambitious for the lathe? Is there any way I can complete this project on this lathe?

thanks

simon.

You could probably work out a gear train that would cut it, may need to buy an extra gear. Pitches like this also put a lot of load onto the lathe so probably best to turn the leadscrew by hand and let that drive the spindle not the motor

3.0/3.5 is 6/7 so take any driver gear that has teeth which divide by 6 and swap it for one with teeth of that result x 7.

So if you have a 30 driver in your train swap it for a 35. If you have a 36 driver swap it for a 42 etc.

Edited By Pete Rimmer on 23/11/2021 17:21:21

I would also pay heed to Jason's warning about driving the lathe with a high step up ratio.

Nice easy calculation Pete, one to remember.

A train of leadscrew 30T driving the spindle at 70T will give a pitch of 3.5 mm and set up that way it will be much kinder to the machine when cutting that thread.

Brian

May not be very helpful to you, but I would mill it, would be quite benign on a CNC mill. Start with the aluminium blank, pocket out the hole, then mill the thread.  How long is the threaded portion you want?

Edited By John Haine on 23/11/2021 18:37:23

Looking at the image of the SC4's threading chart in Neil's book I can see that to get a pitch of 1.75 the train is 49T driving 120T and then the 120T driving a 56T which given the 2mm pitch screw works out right.

So if we do away with the 120T and have the first pair as a 49T (gearA) driving a 56T (gearB) and then gears C & D can be whatever fits to give an answer of two so that could be 100T driving 50T, 80T driving 40T or 60T driving 30T. Which look to all be supplied gears

Maths is

49/56 = 0.875

80/40 = 2

So 0.875 x 2 = 1.75 turns of the leadscrew per turn of the chuck so 1.75 x 2mm leadscrew pitch = 3.5mm

Edited By JasonB on 23/11/2021 18:46:05

Thanks John - so only a £10k part then !

70-ANY-40 would also do it

Edited By JasonB on 23/11/2021 19:10:39

I expect the limitation is the 150rpm minimum speed - and the power available at that speed. It may start at 1100W at 2000rpm, but at minimum speed there won’t be much available, hence only 3.5mm pitch catered for.

I expect belt drive machines, with back gear, of half that specified power would manage the job with power to spare.

I suggest you ask someone at your local modei engineering society with a myford, or similar, to cut it for you.

Better than risking your lathe electronics.

It's not going to be a long thread is it?

Use the mandrel handle that is commonly recommended here, or a spanner on one of the chuck jaws like I use. It will be a bit tedious, but much quicker than scrounging the use of another lathe. And if you're going to do that, surely it would make more sense to use a machine that could handle the job easily?

Heres an idea for others to blast away at:-

Set the top slide to half the included thread angle. Relieve the cutting face and the back faces of the tool such that they are only 1mm wide. proceed as normal. Once the thread reaches final depth, replace the tool with a full faced tool and as others say, wind it along by hand. Yes, you will have to carefully align the tool with the cut thread, but this tool would take out any tiny imperfections of the cut faces of the thread.

Bob

The SC4 has a 1 Kw brushless DC motor, which I would not risk by cutting a 33 x 3.5 mm thread, since the motor speed will be low, giving minimal cooling.

My lathe has a 1.5 hp three phase motor and a back gear, which was used to cit a 28+mm (1.125" actually ) x 4 mm pitch half round thread, for a 225 mm length, on mild steel . So the motor speed was fairly high for cooling air flow.

With a less powerful motor, I would start by making a Mandrel Handle, and then using that to provide the power.

The torque required is going to increase quite markedly as the thread depth increases.

better to risk overheating you than the motor!

Howard

It might be worth thinking about an alternative material - such as Delrin. This is a stiff plastic which is a good engineering material but a lot easier to turn and to cut threads.

Delrin is a trade name - so if it means nothing try Acetal, polyoxymethylene, Hostaform C, Ultraform.

Cheers, Tim

It is not so much the power of the motor that is the issue it is the forces going through the drive train and leadscrew if you power the spindle either by hand or with a mandrel handle which increase as the gearing up increases.

If you further want to reduce the cutting load then the topslide can be set parallel to the lathe axis and a quick bit of cad or back of the envelope maths will have it done with no more load than if cutting a 1.75mm pitch thread (14tpi) Cut the yellow bit first, advance topslide 1.75mm and cut the green then retract topslide 0.875 and remove the orange. As OP has a tap that can be used as a final chaser.

Drive the leadscrew and you can get very large pitches which would be so scary even on the slowest backgear of a Myford as the carriage will be moving at one hell of a rate. A better handle would help as the allen key shown is not ideal

Edited By JasonB on 24/11/2021 18:31:29

I wouldn't be too worried about overheating, the thread is fairly short and the cutting period won't be long. 3.5mm is a coarse thread, but if you work things right the cutting edge will only be about 2mm, well within the lathe's capabilities, even in steel. In aluminium, it should be a doddle.

Use the in two units/across one unit method of infeed and you will only be making a significant cut on half the thread width. No need to angle the topslide.

The lathe is still in storage, so I don't have the change gears to check, but I would be surprised if 3.5 can't be achieved.

The biggest issue is that the carriage will move pretty fast, but with a through thread you have plenty of runout.

1kw is 1.34hp, so not significantly different power from your 1.5hp lathe Howard, I'm sure they will both handle the job similarly.

Neil.

Edited By Neil Wyatt on 24/11/2021 18:54:37

Torque increases as speed decreases for a brushless motor, there shouldn't ban any issues other than relatively lower cooling air flow, which is why the minimum speed is limited.

But each cut will be relatively short so as long as common sense is used overheating ought to be easy to avoid. I'm conifident the SC4 can handle bigger cuts than a Myford.

Neil

Even the brushless do lack grunt at low speed particularly as the diameter of work (or tool on a mill) goes up. Simple example here where running at book surface speed stalls the spindle but faster will allow the saw to cut.

The old brushed X3 would cope with the slower 100rpm despite a less torquey motor but that is because it has a low range in the gearbox. I susppect Howard has a range of gears or belts too.

.

I am genuinely surprised, Neil

… Could you please explain ?

Thanks

MichaelG.

Hi Michael,

I will try to explain in simple terms as I have understood it from SIEG, and I hope it is understandable.

For the brushless range, the torque and speed are programmed onto a chip for the SIEG machines.

In the programming, improved torque is available at low speed, with a specific arrangement of belt drive, or a combination of belt and gear drive, providing a maximum speed of around 2000 rpm.

However, if the maximum speed is programmed on the chip at around 6000 rpm, the torque available at the lower speed is low.

So, for example: the same 1000w output brushless motor is used in the SC4 lathe, and the KX3 CNC Mill.

The SC4 lathe delivers higher torque starting at the lower speed, which is carried through to its max speed of 2000rpm.

The KX3 CNC mill chip on control board programing delivers high max speed, but the torque is low at low speed, gradually increasing as the speed increases.

So two different torques curves. On certain machines such as the SX2.7 mill which uses a 750w output brushless motor, in addition to the programing of the chip, a hi/low belt drive is also used to control the speed range and in turn deliver better torque on the lower speed belt arrangement.

The correct balance in programming/set up of belt/gear is difficult to achieve, and it would be wrong to presume that all brushless motor based machines from different manufacturers will perform in the same way, even though the principal concept is broadly similar for most brushless motor arrangements.

Hope above explanation is understandable.

Ketan at ARC

.

Many thanks, Ketan ... I think I understand [as much as I need to] now

The torque at low speed is a feature of the system, rather than being inherent to the nature of a brushless motor.

Much appreciated.

MichaelG.