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Parting off Ø3.5 8620

M

malleusmagnus

Plastic
Joined
Jan 2, 2021
I am having no luck at all parting a Ø3.5 Hot Rolled 8620 round. I have burned two HSS parting tools, one 1/16", mounted upside down, in a rear toolpost and one 1/8" in a front toolpost.

My lathe is a Myford Super7. I have no trouble at all turning or facing the stock, using carbide inserts, but I cannot get the parting tools to cut. My initial spindle speed was 505rpm. I know that's not the ideal SFM, but it's as close as I can get since I have to use the belt and pulleys.

The saddle was locked during the attempt, so it's not drifting. I tried cutting oil and dry, but both had the same effect, namely not cutting.

I'd appreciate any suggestions.

EDIT:
I originally thought there might be some work hardened "skin" on the outer surface, so I turned about 0.050" off, but that had no effect; I still burnt the tool.
 
That's 460 sfm at the OD, which is way too fast for a HSS parting tool. Switch to carbide or slow it way down.
 
Parting can be even more sensitive to tool being at centerheight than turning. Double check that, and make sure the tool has a keen edge.

Do you cut off with power cross feed? (I'm assuming a Myford can do that, but maybe not.) If so, set the feed rate for .001-.002 per revolution.
 
That's surprising; the handbook says that's the mid range SFM for 8620. What spindle speed do you suggest? I understand it'll have to change as the tool goes into the part, since the OD at the tool is diminishing.
 
malleusmagnus said:
I am having no luck at all parting a Ø3.5 8620 round. I have burned two HSS parting tools, one 1/16", mounted upside down, in a rear toolpost and one 1/8" in a front toolpost.

My lathe is a Myford Super7. I have no trouble at all turning or facing the stock, using carbide inserts, but I cannot get the parting tools to cut. My initial spindle speed was 505rpm. I know that's not the ideal SFM, but it's as close as I can get since I have to use the belt and pulleys.

The saddle was locked during the attempt, so it's not drifting. I tried cutting oil and dry, but both had the same effect, namely not cutting.

I'd appreciate any suggestions.
Click to expand...

Where to start? Get a real lathe, That one is a toy, way to much flex for parting any 3.5 steel let alone 8620. You need to cut loose with some bucks and buy a insert type parting blade and carbide inserts. Problem, you don't in any way have the HP or machine stiffness to use said carbide insert parting tool. Even if you could get your HSS blade to cut 8620 at 3.5" diameter you will need 1.75" hanging out un supported, good luck with that. If you insist with this endeavor keep the tool as short as you can, stopping the lathe to extend as required. Make sure the tool is dead parallel to the cross feed axis, use a dial indicator to make sure. Use back gear, you can cut about anything with HSS if you go slow enough. Use your power cross feed, set about as slow as it goes. When you run the spindle this slow it is hard to not feed too fast by hand. Use flood coolant. Considering the machine I'd be using a band saw.
 
The tool was as close to center as I could get (the OD is as rolled, and the ends are bandsaw cut, which makes finding the real center a trick).

Both tools were as sharp as I could want. Yes, the Myford has power cross feed; unfortunately, I couldn't get to the point where I could engage it, because the tool started burning as soon as it met the part.
 
Cole2534 said:
Maybe for a carbide tool, it's about 4x what I found for HSS.

Sent from my SM-G981V using Tapatalk
Click to expand...

you were looking in the wrong section.. He was looking in the "surface speed to instantly melt HSS" section.

You were looking in the "How to actually cut metal" section.

400+sfm with HSS in any kind of steel = Very bad time.. I don't care what you are doing.. Isn't going to work.
Period.. End of sentence... Not going to work.
 
malleusmagnus said:
The tool was as close to center as I could get (the OD is as rolled, and the ends are bandsaw cut, which makes finding the real center a trick).

Both tools were as sharp as I could want. Yes, the Myford has power cross feed; unfortunately, I couldn't get to the point where I could engage it, because the tool started burning as soon as it met the part.
Click to expand...

Finding center is dirt simple, use the little depth rod on your caliper and measure down from the top of the material to the parting blade.
 
I've got a 7.5HP 16" swing lathe, weight about 4400 pounds, and it will part 3.5" or 4", but it's working at it, and it requires noticeable cross feed thrust to keep a chip forming. moonlight machine's comments may be worth noting, because a Super 7 is not a two-ton 16x40!
 
You need to run that HSS tool about 90 SFPM- a general rule is to divide 400 by the OD of your part. This would give you a speed of about 115 RPM. This is why you are melting your tool. Also, use a good grade of cutting oil if you insist on doing this in that machine. Of course you can speed things up as your cut diameter gets smaller. Also, stick a center in the end of your part until you get near center to give the setup more rigidity. HR 8620 tends to be a little gummy, so the oil will be your friend.
 
thermite said:
3.5 mm you can do with what you have.

3.5" No Fine Way.

You need special tooling.

Saws exist. For good reason.

Simply back-mount a swing-arm 5 HP cold saw as a "live tool", drop the "LSO" into back gear as a horizontal rotab for the saw... and you are good to go.

Screw up the depth stop for it and saw the Lathe Shaped Object's puny bed in two as a "bye" product excuse for a "buy project"?

No fear. NOW you have a REAL "gap bed". Part it out.

:D

Take the first realistic step on your journey towards a BETTER solution.

EG:

3,000 Avoir, MINIMUM, of good Iron.

7 HP, MINIMUM of spindle power.

Alloy 8620, AKA "Ordnance steel" was developed to do some of the toughest jobs as can be found. It did. It does.

It doesn't give a s**t about wishful thinking, and will never.

Shorter Answer?

Yer playin' wit' yerself...
Click to expand...

8620 makes a good axle for a dump truck as well.
 
My small Logan is probably stronger than the Myford but at 3.5" it would be a chattering mess even at a low back-gear speed. I've cut off much milder 2.5" steel and it wasn't fun. Have to agree with the others, wrong tool for the job.
 
Conrad Hoffman said:
Have to agree with the others, wrong tool for the job.
Click to expand...
Even if you did D from O's suggestion and ran the tailstock center up, this is a 7" lathe. How big a partoff tool can you fit in this thing ? It will have to stick out unsupported for almost 2".

If we were on a submarine and needed this part to keep from going down forever, I'd do it. But pretty much nothing less would make me try.
 
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