Tuesday, January 3, 2012

PD400 - Tailstock Locking Lever

I'm on leave today to bring my little girl to and back from school on public transport. She is In secondary one this year and has never been out on her own. Tomorrow on, she'll be on her own. I believe I'll get used to her growing up... though she is still my baby.

While she was in school, I had a few hours to myself after running some errands, so it was shop time! The objective this session is to turn down an M8 bolt with a lever to lock the tailstock. The only piece of unknown steel I have in my stock pile was about 25mm in diameter 50mm in length and it was covered with rust. The rest are too small. Didn't make it to completion as I got to pick up my girl to take public transport back. Will do a part 2 if I can finish the job in the next session, or even 3 if I can't.

Here goes:

Please ignore the diagram on the top. I took the measurements from the existing locking bolt. The cap will be taller to allow a tommy bar of sort to be inserted.

I set the pulley to the 2nd position for 330 rpm (stage 1) and 660 rpm (stage 2). I'll begin with the slower speed of 330 and increase as the diameter is reduced.
Cleaning up the rust. The entire length of the stock was 65mm and about 50mm was sticking out of the chuck (2 times it's diameter).
I started to feed the tool by hand. The surface finish was very bad. Power feed was used at 0.07mm/rev for the next few passes. The surface finish was so much better. I tried, at one point in time, set a depth of 0.2mm. The cut didn't sound too good to me. At 330 rpm, 0.15mm depth seems right, but i thought this lathe can do more?
I did a facing cut to end the session. In my next, hopefully this Friday night, I'll flip the stock around to clean up the other end.

One thought came to my mind on the way to pick up my daughter - if I'm to cut M8x1.25 threads for a length of 30mm, how would I solve the problem of the stock flexing during cut?


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Anonymous said...

Wong, about the 8mm flexing, I would center drill the end of the stock and use the tail stock center to support the right end.
8 mm diameter and 30 mm length is only about a 4:1 ratio. Most books I remember recommend no more than 6:1. But it is always nice to be safe.
I know modern day home machinist like you and I use the 3 jaw chuck for centering. However when I need concentric turnings I tend to turn between centers with 60 degree centers. I got used to this in watchmaking where things have to be centered.
Turning between centers will let you take the work out of the lathe for testing and return it to finish, just don't take the driver dog off, and keep track of the slot in the face plate.


Wongster said...

GeneK, will the flex happens in the middle when the stock is held between centers?

I just finished a session turning down a 22mm piece of steel rod to 13mm. This will be made into an M8 screw with a taller cap to insert a tommy bar of sort - my first project making a tailstock locking lever. During the turning operation, I tested with carbide insert tool and HSS. Speed was fixed at 660 rpm with power feed at 0.07mm/rev. The carbide tool gave a nicer finish while the HSS tool thread like finish. But as the stock got under 18mm, I started to see better finishes using the HSS.

One observation; I've the usual 0.02mm error (stock at headstock bigger) when using HSS and larger error using carbide insert. I was puzzled as that shouldn't be the case.

Anyway, I finished up this part of the job with HSS getting 13.00mm at the tailstock end and 13.02mm at near to the headstock.

The stock was held in the 3-jaw with no support at the other end.

Will be ordering the face plate soon.


GeneK said...

>GeneK, will the flex happens in >the middle when the stock is held >between centers?

If I remember correctly, flexing is related to the length by a square function, if you double the length of a rod, same diameter, it will flex four times as much. since supporting at both ends is cutting the distance from support in half I would expect one fourth the deflection at the center.

The taper you are seeing does seem to be backward. If the part is flexing away from the tool at the unsupported end, that is where it would be larger. Carbide is not as sharp as HSS, lathe inserts are not pure carbide. They are made of sintered materiel finely ground carbide mixed with a binding metal then melted together. That means the carbide will not have as fine an edge as HSS, but it is tough and lasts longer.

Keep your notes, the next time you may find the taper going the other way. If over time there is a consistent error look for a solution to improve your work.


Wongster said...

Come to think about it, you're right, I should get the larger end at the tailstock end.

The flexing in the middle was on the Sherline when I was turning to 4mm in diameter on stainless steel. about 50mm+ was sticking out of the 3 jaw. It may well be the carbide tool pushing the stock towards the middle.

I just finish turning 30mm length of the stock to slightly less than 8mm to take the M8 threads. Roughing cut was done with the roughing tool from Proxxon (didn't know there is such till I saw from the catelogue. I can reach 0.5mm per pass with ease. Removing 1 mm off the diameter each pass really speed things up quite a bit!

Will be posting the pics after church.