Exhaust thread repair on a lathe

[dobba99]

[video]http://youtu.be/qII9QMsKw68[/video]

I bought a 750 head from ebay just before christmas for the rocker gear. A bonus was the vendor delivered it personally as he only lived half a mile away and he used to race sidecars, just as I did!
It (the cylinder head that is) had the ports repaired previously by the welded insert method, one was stripped again and the other was going the same way. Looking at it I decided to repair them myself. It wouldn't fit on my cnc mill (one of the small Seig mills retro fitted with a cnc pack)
So I had to make a custom angle plate to hold it onto the faceplate on my lathe. Owning a 850 mk3 I drilled the plate to accept both 750 and 850 heads. In use it is actually quite easy to set up (I had to install jacking screws to adjust the plates position on the face plate)
In the vid you can just see the screwcutting tool used to cut the thread in the head.
This forum is a bit like show and tell!
regards
Peter

 

[john robert bould]

Nice repair, see you have plenty of counter-balance .

 

[nortonspeed]

Nice set up Peter. What is the diameter of your faceplate? IOW what minimum diameter do you need to hold an angle plate big enough to turn the Commando head :?:

 

[john robert bould]

Gap bed,,my Harrrison as a 16inch face plate ,

 

[dobba99]

Nice set up Peter. What is the diameter of your faceplate? IOW what minimum diameter do you need to hold an angle plate big enough to turn the Commando head :?:

My lathe is a Ward, Haggas & Smith flat bed with a centre height of 6.5" It has a gap bed so I can swing 23" in diameter in the gap.
the faceplate I used for this is 12" in diameter. You should be able to see from the vid that the balance weight is bigger than the plate in its 'diameter' this revolves in the 'gap'. The angle plate itself clears the bed by about 3/8"
When I was thinking about this I originally thought i could use one of the bigger angle plates I had. Not a chance, non of the holes were anywhere near and even if they were, you would not have been able to bolt it onto the faceplate as its bolt holes didn't match the angle plate!
I considered using the largest 18" faceplate i have (made out of a large brake disc from a redundant lift mechanism, elevator to our colonial cousins) but quickly ruled it out as being far too cumbersome. Hence I made the angle plate as shown. Cut from 15mm steel plate with 10mm thick gussets. After welding I stress relived it by putting it on the barbeque with a pile of sticks and set fire to it! Got it nice and hot and let it cool slowly. Set up on the mill and skim the relevant faces square to each other. What took longest is initial positioning of the head and drilling the holes to match. This involved clamping the head to the plate when in place on the lathe (ensuring that the port is running true) and then transferring the whole shebang to the mill for drilling. I had to make a long drill bit from silversteel to reach down through the head so as to spot thru to the plate. so long as you can spot the four outer bolt holes (next to the spark plug holes) by using the head gasket you can mark out the others and also mark out for the 850 (Using an 850 gasket) as well.
regards
Peter

 

[nortonspeed]

Thanks for your explanation. My lathe has only 6" centre height and no gap bed, just big enough to skim a standard Commando disc :roll: So unfortunately no tool copying for me this time :cry:

Wow, made a mistake you mentioned 12" diameter, just what I have :idea: I think I can get away without the counter weight by cutting thread at very slow speed :wink:

 

[toppy]

:idea: I think I can get away without the counter weight by cutting thread at very slow speed :wink:

I have done alot of screw cutting of out of balance jobs over the years and running at slow speed is not always a good idea. For one thing all metals and cutting tools have ideal speeds to be used and if you are screw cutting manualy your own skill and reaction time to move tool stop lathe etc. come into play. But more importantly the condition of your lathe can effect the job as the out of balance weight goes over the top if there is any play or backlash in the spindle drive the chuck will briefly speed up and you may hear a knock or see consistent fault is the pitch of the thread being cut. To avoid this and minimise unnecessary strain on your machine you should always arrange (normally by trail and error) suitable counter weight so the chuck can be left in any position an will remain stationery and not have a heavy point which will drop to the lowest point.

 

[nortonspeed]

:idea: I think I can get away without the counter weight by cutting thread at very slow speed :wink:

I have done alot of screw cutting of out of balance jobs over the years and running at slow speed is not always a good idea. For one thing all metals and cutting tools have ideal speeds to be used and if you are screw cutting manualy your own skill and reaction time to move tool stop lathe etc. come into play. But more importantly the condition of your lathe can effect the job as the out of balance weight goes over the top if there is any play or backlash in the spindle drive the chuck will briefly speed up and you may hear a knock or see consistent fault is the pitch of the thread being cut. To avoid this and minimise unnecessary strain on your machine you should always arrange (normally by trail and error) suitable counter weight so the chuck can be left in any position an will remain stationery and not have a heavy point which will drop to the lowest point.

Agreed, I will try to fix a suitable counter weight within my 12" diameter :wink:

 

[dobba99]

Thanks for your explanation. My lathe has only 6" centre height and no gap bed, just big enough to skim a standard Commando disc :roll: So unfortunately no tool copying for me this time :cry:

Wow, made a mistake you mentioned 12" diameter, just what I have :idea: I think I can get away without the counter weight by cutting thread at very slow speed :wink:

I have just been looking at these photos when i was setting this job up and I will not be able to say weather you will be able to swing the head.
When my lathe is next free (I,m modifiying harley starters at the moment) I,ll chuck the plate and head and actually measure the head to bed clearance
regards
Peter