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Cutting question

bluearc

Aluminum
Joined
Sep 21, 2012
Location
pennsylvania
I cut out a 5x3/4" round plate using oxy/acet then I ground off most the cut with a grinder.
I drilled a 3/8 hole in the center and mouted a bolt and nut to hold it in the three jaw chuck.
I am having a problem cutting the plate, the bits are getting dull, I am using high speed steel bits as well as carbide and they will not make satisfactory cuts.
Is this happening because flame cut metals are hardening to a point where they cannot be cut on the lathe.
My late is a SB9 using 1/4 in bits.
thanks,
 
would it be correct to assume that your disc was cut from a plate of structural steel? (A36??)

if so then hardening shouldn`t be a problem. I have used that stuff for backplates before and it can be hard to get a good finish -what works for me is to use backgear, and sharp HSS ground to angles like you would use for cast iron.
 
A 5" disc, and IIUC, you are trying to drive it with a 3/8" bolt?

Sounds "difficult" to me. Maybe possible with whisker thin cuts?

Sounds like you are spinning it too fast. HSS would be about (rough starting point) 80sfm or 62 rpm (back gear as suggested above) You might be able to spin it as fast as 120sfm, or ~90 - 100rpm if soft material and good HSS steel (maybe 5% cobalt)

Carbide could run quite a bit faster, maybe even 5 - 600 rpm depending. But it may chip if the cut is interupted,....or especially if the work is slipping on the arbor and stops periodically. That will immediately rip the tip off most carbide.

smt
 
I don't know what the plate alloy is, I flamed cut the disc and I am trying to cut i back gear, I also used different angles cut in the HSS.

would it be correct to assume that your disc was cut from a plate of structural steel? (A36??)

if so then hardening shouldn`t be a problem. I have used that stuff for backplates before and it can be hard to get a good finish -what works for me is to use backgear, and sharp HSS ground to angles like you would use for cast iron.
 
A 5" disc, and IIUC, you are trying to drive it with a 3/8" bolt?

Sounds "difficult" to me. Maybe possible with whisker thin cuts?

Sounds like you are spinning it too fast. HSS would be about (rough starting point) 80sfm or 62 rpm (back gear as suggested above) You might be able to spin it as fast as 120sfm, or ~90 - 100rpm if soft material and good HSS steel (maybe 5% cobalt)



Carbide could run quite a bit faster, maybe even 5 - 600 rpm depending. But it may chip if the cut is interupted,....or especially if the work is slipping on the arbor and stops periodically. That will immediately rip the tip off most carbide.

smt

There is no problem with the 3/8 bolt, the disc is not slipping I just cannot figure out why it is such a difficult cut to make.
 
I think when you get past the flame cut edge and it is running true you will get a better cut with the HSS.
 
Since you are running it your three jaw make a line aound the piece while running in the lathe. Make it with a sharpie or even a cutting tool just before finished size. Remove it from the lathe and grind to the line to stop an interrupted cut. HSS should be just fine to finish machining. I once had a piece of 5/8" steel plate from a scrap yard that I also flame cut. Yes the interrupted cut was a PITA but going slow I was able to get a good finish on the edge. The facing was another problem in that a layer of rust/scale was virtually inpenetrable with HSS. Pointy carbide was the only thing that would break that surface. Once removed the piece machined like butter witha great finish.
 
Since you are running it your three jaw make a line aound the piece while running in the lathe. Make it with a sharpie or even a cutting tool just before finished size. Remove it from the lathe and grind to the line to stop an interrupted cut. HSS should be just fine to finish machining. I once had a piece of 5/8" steel plate from a scrap yard that I also flame cut. Yes the interrupted cut was a PITA but going slow I was able to get a good finish on the edge. The facing was another problem in that a layer of rust/scale was virtually inpenetrable with HSS. Pointy carbide was the only thing that would break that surface. Once removed the piece machined like butter witha great finish.



This is exactly what I did yesterday and I am pleased to read that it was the right way to work this. I am grinding to a scribed line as close as I can then its back to the lathe, but I think I will also grind the scale to bare metal on each side because more machining will be required on it. You are right it is a PITA.
 
You may have a problem with the method you used to mount the plate. More than likely the 3/8" bolt and nut is not rigid enough to firmly hold the plate against cutting forces, especially with the large diameter you're working with. If the 3/8" bolt is flexing and causing the plate to wobble, even if you can't see it, you'll play hell trying to get a smooth cut. If you continue to have trouble, try making a thick backing plate with a large diameter axle, say 1" or more to go in the 3-jaw, then drill and thread the center and bolt the workpiece to that with a large thick washer. The backing plate wouldn't have to be perfectly round, so maybe you have a scrap that would work. Or figure out some other way to increase the rigidity of your setup - I doubt the 3/8" bolt will do it. FWIW, no offense intended. See Stephen Thomas' reply above.
 
I have done it in the past, was slow going but do-able. I welded a pipe in the center, roughed it out. tommie
 

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I guess you could silver solder a pipe or 1 inch stock to plate and have at it, like tommie did, it seems like you would get a better cut
and also use the tail stock....?
 
hmmm,
i do this pretty regularly with everything from delrin to 316 and have never had the problems you speak of.
i'm thinking the single bolt through the center is giving you problems, are you using a live cntr in the tailstock also?
my set up is usually something along the order of a couple of LARGE washers to help with deflection, with a center hole for a mandrel and another hole for a small bolt to keep disc from spinning. (assuming you can weld/grind the holes up after of course)
some kind of shop made mandrel to hold locate the piece in the 4 jaw.
a relative size disc of delrin or alum with a 60 degree hole for locating tail stock center pushing against the piece.
HSS with a left hand grind and chipbreaker.
Back gears usually on fastest rpm and power feed to face the cut 10 thou or so at a time.
as others have said, grind or band saw as close to the final size as possible.
good luck and let us know what method winds up working the best for you.
 
Along with PITA the important one to learn is SWMBO. :)
???

As to the cut... I did this as in tommied's pics. Yes, it was a pain. Had a buddy cut out a 9 inch disc of 3/4 plate...it was round within an inch:rolleyes5: (but it was free, so I couldn't complain)...anyhow, I used a carbide insert and took .075 to .100 cuts to try to "get under the scale" as quickly as possible. The scale will chew up a tool quickly... But I also understand that a .100 cut was not a option on your situation. Low rpm, and oil if possible will be your friend.
 








 
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