Alternative back gears

Hi all, I have a question my portass dreadnought lathe backgear is missing lots of teeth I cannot find a suitable replacement 75 tooth gear but I have a selection of 16dp gears one of which is an 80 tooth what gear would I need to mate to this if I decided to use this as my back gear .

I'm pretty sure it is one with 5 teeth less than the existing one, to keep the sum of the two tooth-counts the same.

(You can verify that rule with a few change-wheels.)

You need to know what the gear was that it was driven by originally. then subtract 5. Or keep driver plus driven = constant.

If you don't have the original driver you can get the constant from the other two gears 'cos they too add up to the constant.

Having the exact reduction as original does not matter, getting a set of gears in the space available is your task. Having a mill would help a lot as there could be some special parts which cannot be bought. There are some companies which can make new gears, and you are able to use the old ones for patterns.

I would maintain the ratios to ensure the threading chart remains accurate

You will also find that backgear cogs tend to be of heavier construction with thicker discs and stronger more robust teeth

Edited By Ady1 on 24/04/2021 08:02:03

Backgear ratios have no effect on threading on my lathe. The threading pitch is due to a ratio between the spindle and the leadscrew that is not affected by having backgear in use or not and is independent of spindle speed in general.

Martin C

Ady -

As Martin says the back-gear does not normally affect the feeds and screw-cutting, and I doubt the Portass is any different in that regard, but the tooth-counts and DP are important to keep the centres the same.

It may be possible to buy a new stock gear of the right DP and pressure-angle, and modify its bore to suit. Either of 75T straight away, or a pair that will be of similar ratio and more importantly, the same shaft centres.

Thanks for all your input the 80 tooth gear I do have is a newish hpc steel gear so if I am correct I will need a 15 tooth to match it .

Don't forget this bit if you use different kit carl, the centre distances must match

The backgears are in no way coupled to the leadscrew of a lathe, that is entirely different, which is why I said that you can pick any set of gears that will fit in the space available with the minimum ammount of modifications.

You may have to make eccentric bushes, but at least the swing of the movable gear can be adjusted. An interesting project, which is quite rare, we look forward to hearing of your progress.

https://www.technobotsonline.com/gear-size-calculator.html

Edited By old mart on 24/04/2021 13:55:46

Edited By old mart on 24/04/2021 14:01:39

Don't forget HPC gears are 20PA

If you have now got an HPC gear it may well be 20 degree pressure angle so will need an equivalent to match it. It is likely the old gears if you have a collection are 14.5 degree. have a look at how well it meshes with your selection.

I s your lathe the one with the backgear spindle above the main spindle? Can you post some pics?

Pics of the dreadnought

I have just googled "gear shaft spacing", and the results may be of interest.

Carl,

How closely does your 80T gear match the diameter and width of the big gear in your dreadnought photos. Assuming it is close you will require to bore out and add the slot for the drive peg if you are going to use that one but it's more important to establish the centre distance between the headstock spindle and the back gear shaft because this will determine whether you can use that 80T gear. The centre distance may indicate that you need a 11 1/2 tooth gear and that's a bit impractical!!.
The sliding back gear presents a different problem. Counting the teeth I can see, I'm guessing the large gear is about 40 T, way different to the bull gear behind the chuck. It looks as though the small gear with damaged teeth on the sliding gear assembly is an integral part of the slider, it may be a press fit in the 40T? gear, but either way not an easily replaced part.
If you are looking to use standard, off the shelf gears you may consider altering the gear configuration to the simpler sliding and rotating back shaft that is used on many small lathes.This could involve pinning the 40T to its shaft and getting the mate to the bull gear similarly pinned to the back shaft.
Again, as others have said,. you must start by getting the centre distance of the gear train and then decide on your gears from there.

Regards Bob D

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Edited By Robert Dodds on 28/04/2021 20:39:57

If, the backgear shaft is above the spindle, there is a possible option of fitting a pair of eccentric bushes to allow a bit of adjustment, up to 1/16".

Hi everyone, thanks for your input I have not had much opportunity to work on the lathe but have had a friend Tig weld the backgears missing teeth , it’s not pretty but it does function the next job is to modify the spindle to carry a. Myford thread .

I've done that as well as fabricating a replacement from Aluminium

Just tack weld blobs and shape with a grinder and file

Lathe cast Iron is decent quality and seems to weld ok