Thread Dial indicator

[jaybie]

Hi all
I know threading is discussed a lot and I should trail through posts to help with my question, which I have and found no answer to this puzzling conundrum so here I go, I'm afraid. I have restored a Harrison M300 imperial lathe recently. Levelled it to a reasonable cutting standard, minimal taper over 100mm. Happy.
Now I am trying to thread. I must tell you, my other lathe I used only had the keep the half nut engaged,rewind and cut again method. The only option for my small hobby lathe. Competent and confident doing this. Cut many threads up to 3mm.
This fine cutting machine I have now has my hair falling out. So as not to confuse I will explain what's going wrong. The Harrison has a 4tpi lead screw. It has a 16 tooth imperial thread counter,all good. To cut a 14 tpi I engage the half nut at position 1 or 8 cut the thread disengage the half nut pull out the cross slide wind apron back to start wind cross slide back into zero, wind in cut on compound, engage half nut on number 1 or 8 as first done and repeat until thread fits my thread gauge. HAPPY. Also using the 30 degree angle method while cutting threads. But No. Every time I re-engage the half nut at number 1 for example it does not go into the thread precisely, it will cut another thread just in front of the previous one. I have tried many different thread sizes and all seem to do the same. I think someone has had the dial off the counter to paint it and may not have put it on correctly but I can not see any datum marks to do this either. So number one may be number 7 for all I know. That would put things out of sync. I also cut a 3mm metric thread using the engaged half nut and reverse method as manual states and this never fit my 3mm thread gauge very well. Not like my other lathe has done. Gearing is also correct as displayed on front screw cutting chart I don't know if this is due to wear of cogs but I saw very little wear while dismantled for a 35 year old machine. Any help will be greatly appreciated.

Regards
John

 

[nnam]

I suppose the scratch cut shows correct thread pitch?

Have you try rolling the head by hand with some marker of its angle position, and count how many turns the thread dial goes back to a same number? Did the head goes back to a same place you mark?

Try that with different thread pitches and see if any pattern standout.

Maybe you can put a round label on the dial with your own positions.

Count how many times the head rotates to get one thread dial to have one turn.

Maybe prevous owner switched gears between metric and standard, that is diff than your lathe default setup?

 

[benmychree]

If your lead screw is Imperial, you should be able to close the half nuts on any line on the dial for an even numbered thread, and any number for odd numbered pitches, if it is the normal four numbers and four intermediate lines; half threads can be cut on opposite numbers, quarter threads on only one number; all the thread dial does is indicate one inch distances on the lead screw. If your tool is not tracking consistently, I'd look at the end train of gearing on the end of the lathe, for some inappropriate gear being substituted for the regular gearing as posted on the charts; many special pitches may be cut by extra gearing. Wear in the feed/threading gear train does not effect tracking once all the backlash is taken up, you may also check for slippage caused by a sheared key or pin.
Special pitches may be cut by substituting a gear of the number of teeth the same as the desired pitch (on the feed box driving gear, with the quick change box set for the number of teeth on the regular driving gear. Check to see if the end train may be set up for metric pitches when you are cutting imperial; threads.

 

[MontanaLon]

Does the lead screw have a clutch and if so is it slipping? I have also seen them with a shear pin which sheared but had enough friction against the shaft to make it turn but only after it slipped 1/2 or full turn and got around to the grippy spot again and engaged. Your thread dial will read correctly but the lead screw is out of time with the spindle at that point and it won't work.

 

[Tozguy]

it does not go into the thread precisely, it will cut another thread just in front of the previous one.

Could we see a picture of the doubled threads please?

 

[Mitch Alsup]

The Harrison site says that both 4TPI and 6TPmm were available.
These will measure very similarly.

 

[jaybie]

Hi guys
Thanks for all your replies. It has a clutch and I will check it's function later. Gears on head of lathe are 40,88,44 and correct for the threads attempted. I will get pictures of the double,triple threading and do tests suggested later today after work. Thanks again. I'll report back results.
John

 

[jaybie]

Hi all
Sorry for the delayed update. I have discovered the problem with the threading. It was working on only 3 sprockets in the thread cutting gearing setup. There was a spacer which had one cog number 88 not in the mesh at all. See picture, bottom cog was set back and orange spacer in the front So it was going 40/95/44. No wonder the pitch's and sync was all over the place. Corrected late last night and is now cutting threads correctly from both the thread dial and just the engaged half nut method for metric threading. PHEW! Thought I'd bought some frankenstein model made up of different model parts. Thanks for all the suggestions and advice. Still learning and making silly mistakes. Worst thing is I checked and re-checked them sprockets.

John