Thread: Parting Off Blade/Tool Advice

bwal74
I have a quite a few HSS blades of different designs.The best one by a big margin is the Somma TBL532C-5/64" x 1/2" Cobalt, this is a "T" shaped blade and I use both power feed and manual with it on my AL1000 lathe, with a constant supply of oil.I purchased this blade from EE.
Bob

Originally Posted by krisfarm

The best one by a big margin is the Somma TBL532C-5/64" x 1/2" Cobalt,

Thanks for the heads-up. I just ordered one. Should make parting off SS less stressful.

HI,

Do you use the EE Soma blade with their tool holder or a generic tool holder? I'm going to order the blade today but not sure about the holder.

I parted off some 32 mm dia MS yesterday. I couldn't quiet get all the way without moving the blade out, so was happy with the hacksaw at the end.

Ben.

bwal74
Looking at your pictures, on the first one, I think your lathe speed is too low, lots of rubbing going on, did you use plenty of lubricant? That tool holder that is shown in photo 3 has way too much tool overhang, not rigid enough. Here are a couple of pictures of my tool holders.
Bob

Originally Posted by krisfarm

That tool holder that is shown in photo 3 has way too much tool overhang, not rigid enough.
Bob

Krisfarm, Can you explain that overhang statement, a bit further?

Perhaps you are noticing something that I did not see.

From what I can see, the point of view angle, in the post #33 photo, appears to elongate the amount of HSS steel stick out.


The tool holder itself, which appears to be a swan neck type has the locking nut pulled in close to the toolpost block. The position of the tool holder nut limits how much of the tool holder can be withdrawn back into the toolpost block.

By measurement if, 32mm is being cut, there should be, 16 say+ 1 mm of HSS stick out.

I am also thinking that the HSS tool being 5/8"deep would be out on its limit plus some -at 17mm. Not having used a 5/8" HSS parting tool I really can't say for sure. What do the rest of you say?

Grahame

Originally Posted by Grahame Collins

Krisfarm, Can you explain that overhang statement, a bit further?

I think Bob was referring to the amount of the toolholder that is projected beyond the tool post, rather than the blade projection. As you say Grahame, the locking nut prevents the tool holder being located any further back. My lathe came with one of those toolholders and I gave it away and bought the AXA holder that holds the blade at 7 degrees. The difference is chalk and cheese. On a lightweight lathe the induced back rake of the angled holder makes all the difference.

Hi Graham
I will try to explain my earlier observations of Ben's set up. If you look my picture showing the Cobalt HSS cut off tool and holder you can see the amount of actual blade stick out that I have, it is 15mm, I just measured it so that a comparison can be visually made. The blade is then clamped immediately behind that by the retaining wedge of the tool holder and the tool holder itself is held rigidly in place by the quick change tool post. I ran into similar parting off problems that Ben is experiencing a long time ago and after researching the issues involved made up both of my tool holders to achieve as rigid a set up that I could. The Cobalt HSS designed one I think from memory is an Aloris design.As you correctly pointed out the tool holder Ben is using has at least double that length. Parting off requires very high tool pressure and with a long lever arm as Ben set up has you are asking for trouble with light weight machines.I use the Cobalt HSS for parting off up to 30mm and the insert rear mounted tool for up to 75mm.
Bob

Thanks for the input. I bit the bullet and ordered the EE blade and tool holder (while the wife was away). Hopefully it will work better than the other set up.

The toolholder in the pictures is an orphan. I don't know where it came from or even how old it is. It's hard to keep a steady flow of coolant and part off at the same time (plus keeping my butt tight in case anything happens), but I did use cutting oil as much as I could. I think the speed was about 800 or 900 rpm's.

Anyway when the new P/O tool turns up next week (hopefully I get it before the wife see's it) I'll let you know how it goes.

Originally Posted by bwal74

...It's hard to keep a steady flow of coolant and part off at the same time...

To clarify, you don't need a steady flow of coolant, as in a continuous stream. Drops are fine, as is intermittent application, as long as the cut is being lubricated. Especially for Al, I use WD40 and give the cut a spray (with a pump bottle) when I notice that the chips are not playing nicely or the sound of the cutting changes - maybe once every 15 to 20 seconds?. Too much coolant and it just flicks everywhere.

If you have not got a coolant pump and need something while you crank handles, something like this may be the go -

https://www.machineryhouse.com.au/P220

Michael

I use WD40 delivered via the little red tube it comes with when parting aluminium. For steel I use proper cutting fluid on a small, cheap artist's paint brush (about 10mm wide) jammed right into the cut. Dip the brush frequently. I got a pack of these from Officeworks for a few bucks:

https://www.google.com.au/search?biw...vLLv1EwDw3qcM:

I put about 20mm of oil in the pot and drop a magnet into it to keep it in place. The funnel lid prevents the bristles from splaying out when I dip the brush in the pot.

Originally Posted by jack620

I think Bob was referring to the amount of the toolholder that is projected beyond the tool post, rather than the blade projection. As you say Grahame, the locking nut prevents the tool holder being located any further back. My lathe came with one of those toolholders and I gave it away and bought the AXA holder that holds the blade at 7 degrees. The difference is chalk and cheese. On a lightweight lathe the induced back rake of the angled holder makes all the difference.

Jack620 has noted here at least the 7 deg back rake which helps with chip flow. Way back some where I noted up to 25 deg back rake for mild steel.

Looking at most of the posts two words are missing: CUTTING SPEED! In another life I near had to whip some people to understand that RPM is secondary and is just a result from cutting speed. For most home machinists and even in some industry HSS tools are still great. Understand that for HSS tools the general range of cutting speed is between 25 and 35 m/min is a recommendation for M.S. For heavier broad tool operations such as parting off, around 2/3 that of turning, depending on the coolant flow. Unless you have CSS on the lathe you have to stop half way through the cut (if solid) to keep the cutting speed up which in turn gives good chip flow. Even that has limitations as many machines are sub 1500 RPM. 10mm dia M.S should start off at around 700 rpm for parting off so 5mm will be around 1500 and so on.

Now for for carbide tips. For parting tips I seem to remember about $5 at least per tip. I've witnessed some people going through 2 tips or more to part a bit of 50mm dia. M.S. Firstly by cutting speed too high........ "I use 2000 rpm they tell me"! Well for a starting cutting speed of around 150 m/min the RPM should be around 900. Increase halfway if cutting through. Not many have above that 2000 RPM on home machines unless they have a Colchester Chipmaster. These are tip manufacturers recommendations, not mine!

Two major enemies of carbide tooling: too much heat resulting from cutting speed too high and lack of cooling causing tip failure, and also from too low cutting speed causing edge buildup and once again, tip failure.

Now why did the HSS tool break. Some sort of dig in is generally the problem! There are some holder designs which hinder us all at times so we have to get smart! The ones causing major overhang (side) problems are the blocks added onto our QCTP. Too many think that a dig in is the front of the tool diving thus increasing the depth of cut and then it's the dreaded crunch. Next time anyone sets up for parting off, check firstly how the clearance is on the cross slide and adjust if necessary. Secondly, wind the top slide back so that the load from the parting operation is near over the headstock side dovetail. This stops the sideways rocking which otherwise causes the tool to dig in on the side as well as the front guaranteeing a broken tool/bent job.

Sorry to be a bit long winded! Must be some of the previous life creeping back.

Originally Posted by bwal74

Anyway when the new P/O tool turns up next week (hopefully I get it before the wife see's it) I'll let you know how it goes.

I got mine last week and had a very quick play with it. Initial impression is that it cuts about the same as my Chinese HSS blade on MS. However I haven't tried it on SS, which is where I expect it to outperform the non-cobalt blade.

The grinding is clearly to a higher standard. The blade is perfectly symmetrical, whereas the Chinese one is quite asymmetrical. What has me stumped though is the lack of hollow grind on the top surface. Somma advertise these blades as having a concave ground top. The profile diagram shows the hollow to be quite pronounced: https://www.sommatool.com/catalog/cu...off_blades.asp

So I expected it to be quite noticeable. When I couldn't see any hollow in the grind with my loupe I had a look with my UBS camera. Here's a picture of the blade sitting on a piece of square HSS tool. To me the grind appears dead flat. What do you guys think?

Originally Posted by jack620

To me the grind appears dead flat. What do you guys think?

G/day Jack, i reckon you should bang out an email informing the supplier that they have sent you the wrong parting blade,
it looks like they sent you this type.
https://www.sommatool.com/catalog/cu...id_carbide.asp

G'day Shed. My blade is labelled TBL532C which is supposed to be CO-HSS with a concave ground top. Have sent the supplier an email.

Originally Posted by jack620

G'day Shed. My blade is labelled TBL532C which is supposed to be CO-HSS with a concave ground top. Have sent the supplier an email.

Cool, I'm keen to see you spit some curly chips if it works well i might try one myself.