rotary table

[cabbage]

Er strange one this! I have a 40 turn rotary table for my mill and lathe. For some reason the central brass gear has come free of the shaft and now simply goes around and around on the side screw....

I have examined the base and the one allen screw just holds the centring plate in place.

Anyone any ideas of how I can take it apart to fix it???

regards

ralph

[IanT]

I have a 6" and a 3" table Ralph - and on both of them you can adjust the worm for backlash - or take the worm right out of engagement to enable the table to "free-wheel". Are you sure you haven't missed some form of 'adjusting' screw on your table? It might look like a 'lock-screw' for instance.

Otherwise if you can unscrew the table from its base, then perhaps you will be able to examine the worm mechanism and see what the problem is.  Do you have a photo (or two) of your table we can peek at?

IanT

[cabbage]

Ian,
I can remove the screw and push the brass spur gear around with my finger while the top plate is still -and visa versa!

I will take a few shots in the morning -I can barely focus at this time of day!!!

regards

ralph

[cabbage]

The FTP server has decided to go down on me! IanT could you send me an e-mail and I will attach the JPGs to it. Nothing very interesting to look at I am afraid as it is a std 4inch table. It seems devoid of any ways into it. Removing the allen screw at the base simply allows you to remove the centre plate. The only other things on it are the brass screw to lock rotation and the thumb lever to lock inclination.

A simple twist removes the screw part of the table. The threads look OK but grimy!

The last shot shows the brass gear in the centre of the table looking down the screw hole. Finger tip inspection of the teeth on the wheel show there is some wear (it was 2nd hand!) but nothing obvious that I can feel?

regards

ralph

[cabbage]

RDG have got the rotary table apart and shipped the pieces back to me. It seems that the brass central gear is knackered! I also need a new ball race bearing for the axis. RDG do not supply the required "spares" that I need. Does anyone know anyone who does?

regards

ralph

[IanT]

Not off-hand Ralph - but the bearing should be a 'standard' size and I'd measure it and look at Euro Arc Trade for a replacement. Bearings used to be their main business, and their prices are reasonable and they will sell you one-off.

The gear is a bit harder - so you really need to measure it and see if you can figure out what size it is. Anyway - if it's a 'regular' DP or Mod gear - you may be able to find a replacement at HPC or somewhere similar. If you look at the HPC website, they have some very useful data sheets you can download in PDF format that you might find useful.

I'm assuming here that it's the 'worm' that's probably gone (it is a worm and wheel set-up is it?) - and it will probably be a 'single-start' type but not necessarily so. So you really need to figure out the MOD size (I doubt it will be DP these days). Perhaps a photo would help us give you some more informed advice too.   

Regards,

IanT

[cabbage]

The steel worm is fine! I could not tell you what DP or MOD the brass wheel should be -as it was junked at RDG... I agree that getting a new bearing for it is no real problem. But HOW do I deduce the MOD or DP of the new gear required?     

The steel screw has a diameter of 0.75 inches and 6 turns in 0.75 inches -this look suspiciously Imperial!!!

I will continue digging....

regards

ralph

[IanT]

They didn't return the wheel? Well, I'm surprised (and I'd want it back) but Ok.

Let's just assume for now that the worm is like a 'rack' (or screw) if you measure between the outside "teeth" (but on the same side of them) and divide this distance by the number of teeth - this is the (simple) tooth pitch. I think from what you've said - that this is 0.75"/6 = 0.125 (3.175mm)? But (for instance) if this measurement was actually 0.7422" (less than 8 thou difference) - that works out at 3.142mm pitch and most likely it's MOD 1.0

If so - you can then probably estimate the size of the wheel (number of teeth) to match by the amount the rotary table (e.g. worm wheel) should turn per rotation of the worm...?  (the number of worm 'starts' also effects the gear ratio btw..)

Check your measurement very carefully and have a look at HPC's 1.0 MOD worm and gears - and you might get lucky Ralph.

Regards,

IanT

[cabbage]

I have tried one of my plastic MOD1 gears against the screw. It is not what I would call a "good" fit -but it seems as if it "wants" to fit... I am going to order a MOD1 gear based on that assumption.

Thank you for your help.

regards

ralph

[IanT]

A spur gear will not have a lead angle Ralph - whereas a worm wheel will have, so you should notice a difference. Try tilting the spur gear against the worm - should fit better (but still not perfectly).

As another thought - an 'ungeared' table is still quite useful when machining things - sometimes more so, as it can be swung easier. I don't know what you intend to use it for.

You might also want to fit the bearing first and see if everything's OK before lashing out on the replacement wheel.

IanT