Portass PD5 Backgear Ratio

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

Does anyone know the raito of the backgear on a Portass PD5 lathe? I know that I can count the teeth on the gears, but the lathe is out in the garage, and it's cold out there! And counting the teeth would be a pretty tedious process… So if someone knows the answer I'd be very grateful. If they don't, then gear tooth counting is coming up in the coming days, and I will report the results here…

Thank you – Rowan

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

[Brian WoodParticipant @brianwood45127]

I don't think you have much option other than to brave the cold and count the teeth. I suspect others with the same lathe would have to do much the same!

Regards Brian

[Anonymous]

Depends how accurately the ratio needs to be known. To get an estimate just put the lathe in backgear and see how many turns of the drive pulley are needed for one turn of the chuck. Normally the ratio is on the order of 5 or 6 to 1.

Andrew

[Brian WoodParticipant @brianwood45127]

I had thought of that method Andrew but it still means Rowan getting togged up for a cold weather excursion to the fastness of his workshop, something he was hoping to avoid! By the lack of response I suspect others too are keeping warm elsewhere and he might just as well get a definitive result while he is out there.

Regards Brian

[HopperParticipant @hopper]

Do you know what RPM it does in non-backgear and in backgear? Simple calculation to arrive at the ratio.

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

All those are good suggestions thank you, but I think I will count the teeth. I am creating a wall chart showing what settings (i.e. which pulley and whether backgear or not) to use for various materials and various diameters, and I'd like the chart to be as correct as possible. I do have a tachometer, but I haven't yet made it work reliably, so measuring rpm is not as straightforward as it should be…

Thank you – Rowan

Edited By Rowan Sylvester-Bradley on 09/12/2022 13:07:55

Edited By Rowan Sylvester-Bradley on 09/12/2022 13:08:30

[BazyleParticipant @bazyle]

Don't forget to publish the result here for other to benefit in the future,

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, scrolling down close to the bottom of the page, the photo of this one PD5 (Lathes.co.uk) it looks to be about 6-1

Regards Nick.

Edited By Nicholas Farr on 09/12/2022 15:39:53

[Anonymous]

Posted by Rowan Sylvester-Bradley on 09/12/2022 13:07:35:

…like the chart to be as correct as possible.

The chart can never be correct. There is not a one to one relationship between spindle speed versus material and diameter. There are simply too many variables, not least due to variations in material. Generally cutting speed, and hence spindle speed, is a fairly broad curve and not that critical. I never use charts, just do a quick estimate in my head based on some broad rules of thumb, what sort of tooling I am using (carbon steel, HSS or carbide) and experience.

Andrew

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

I braved the cold and counted the teeth. The large gears both have 65 teeth, and the small ones 25 teeth. This means (if I've got this right) that the ratio is (13/5)^2, or about 7:1 (more precisely 6.76:1).

This means that the speeds of the main shaft (in rpm) are:

Backgear Without With
Pulley 1 526 78
Pulley 2 292 43
Pulley 3 156 28

Now I can get on and make my wall chart!

Thank you – Rowan

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

I take your point about no chart can be accurate, but if I don't have the experience (or if I'm training my grandchildren to use the lathe, who certainly don't have it), I have to start somewhere…

[Author]

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