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View Full Version : Rear tool post for upside down cutoff tool



garyphansen
03-27-2007, 09:33 PM
I am thinking of mounting a Phase II tool post on the rear of my 9" South Bend's cross slide for mounting an upside down cuttoff tool. Has anyond done this on a 9 inch South Bend? Was it worth it? Gary P. Hansen

Mike W
03-28-2007, 01:25 AM
I made a rear mounted tool post for my 10" import. It was well worth it. Parting off is now easy.

matador
03-28-2007, 01:48 AM
who needs it?I do:D.
Homemade version that came with my lathe.works perfectly well:http://img.photobucket.com/albums/v401/bramleynz/542807ef.jpg

kap pullen
03-28-2007, 07:33 AM
Gary,
It may be better to just make a block from crs.
The qc tools are nice, but do introduce a lot of overhang.


http://img.photobucket.com/albums/v206/kappullen/Kap364.jpg

The picture is a forming operation from the rear toolpost.

This operation utalizes a Hardinge tool block
bolted on a spacer block.

kap

DR
03-28-2007, 08:13 AM
who needs it?I do:D.
Homemade version that came with my lathe.works perfectly well:http://img.photobucket.com/albums/v401/bramleynz/542807ef.jpg

Yeah, but it's an f'ing nightmare to re-grind the tool.

To me, that design is total nonsense. The bit is mounted at an extreme angle, then it's extensively ground to negate the angular mounting????

DR
03-28-2007, 08:18 AM
I made a rear mounted tool post for my 10" import. It was well worth it. Parting off is now easy.

Are you suggesting the rear parting tool is somehow better than a front mount?

The only advantage you'll see is being able to have two tools mounted at the same time, a front mounted turning tool and the rear cutoff.

garyphansen
03-28-2007, 09:11 AM
Are you suggesting the rear parting tool is somehow better than a front mount?

I believe the cutoff tool is normally mount in the rear on cnc lathes because the chips fall away from the cut and so can't get caught between the cutoff blade and the work and cause a problem. Gary P. Hansen

pcarpenter
03-28-2007, 10:10 AM
I know I am going to sound like a broken record since I bring this up every time this gets discussed, but I don't think that an Aloris style toolpost used in the rear with a reversed spindle is safe. If the wedge or piston vibrates loose (they do sometimes) then cutting forces want to lift and hurl the toolholder with a sharp cutoff blade. This puts you at risk, not to mention your lathe ways or the workpiece. In standard rotation with the toolpost at the front, cutting forces push down on the tool holders.

The type Matador pictured seems much more secure or as Kap mentioned, a block bolted down is also a simple alternative that still could use just one bolt as does the Aloris toolpost. If you set the block at the edge of the cross slide and milled a relief on the bottom creating a hook that hangs over the side of the cross slide, it could be made to bolt up squarely and kept from turning even if you only drill and tap the compound for one bolt.

Paul

BobWarfield
03-28-2007, 10:28 AM
Are you suggesting the rear parting tool is somehow better than a front mount?

The only advantage you'll see is being able to have two tools mounted at the same time, a front mounted turning tool and the rear cutoff.

Opinions vary. The rear mounting option has a large following who believe they also cut better and with less chatter.

Best,

BW

wschoenbeck
03-28-2007, 10:41 AM
I want to try this on my SB9 but first I'll have to replace the existing cross slide with one that has the T slots like this one

http://www.statecollegecentral.com/metallathe/S-4382.html

I have noticed that since I got my Aloris AXA tool post, parting off is much easier than with either the home made QC post that came with the lathe or a lantern style post. Both of those "give me the vapors" when parting. The AXA is much more rigid. I still have trouble in steel but better luck in Aluminum. It would probably be even better if I had a proper aloris holder for cutoff tools but since I have only two holders I added a section of keystock to the side of a blade type tool holder (for use with the lantern post) and now I can mount that in a standard Aloris tool holder and get to work. Sadly I have no pics of that to show.

I read about the rear mount method in one of George Thomas' books (Workshop Practise I think). He went on at length about the angle to mount the blade and different ways to sharpen the blade. Unlike Matador's version he tried horizontal or tipped down at the cutting edge. He also talks about a hook type blade as in Matador's as well as just a plain front with a relief angle (easier to sharpen). Here is a link to a kit from the UK but with the book you could build from bar stock I think.

http://www.hemingwaykits.com/acatalog/Rear_Tool_Post.html

Matador's would be much easier to build.

kap pullen
03-28-2007, 11:15 AM
I can understand Matadors toolpost design if there is a
clearance problem with the chuck when machining
with the front mounted post.

Smack that rear post with the chuck jaw and you'll know it!

Blue up the tool mounted, and put a couple of layout
lines to grind too with a surface gauge.

Kap

DR
03-28-2007, 11:17 AM
Are you suggesting the rear parting tool is somehow better than a front mount?

I believe the cutoff tool is normally mount in the rear on cnc lathes because the chips fall away from the cut and so can't get caught between the cutoff blade and the work and cause a problem. Gary P. Hansen


The cutoff tool is normally mounted in the rear on CNC lathes because ALL tools are NORMALLY mounted in the rear. Slant bed CNC's ( the most common style) have rear mounted turrets.

But, just because the tool is mounted in the rear doesn't mean it's upside down. My slant bed machine has a rear turret with a right-side-up cutoff tool. Looking from the tailstock end the normal spindle rotation is clockwise (except for drilling, unless a left hand drill is used).

Generally regarding upside down cutoffs.....in 25 years I've had 6 automatic screw machines and two CNC lathes. All these machines used cutoff tools. They all had an easy mounting option of normal or upside down cutoff tools. I've used both ways many times. I have never seen any advantage cutting-wise of one over the other.

I much prefer the normal right-side-up mounting. It allows better visibility of the condition of the cutting edge. Being able to easily examine the cutting edge is a big deal since these tools tend to take a beating in production. They need to be changed often.

DR
03-28-2007, 11:32 AM
Opinions vary. The rear mounting option has a large following who believe they also cut better and with less chatter.

Best,

BW

Bob,

I think that's an old wives tale.

Better cuts and less chatter are a function of the rigidity of the setup. Cutting forces acting downward towards the machine bed usually result in a better situation than pulling upward on a dovetail.

The idea of the chips falling downward and out of the cut is another thing I don't buy. A good blade will curl the chip up and out.

The cutoff blades I use most are the "P" type. To resharpen like new only a grind across the front is needed.

JPR
03-28-2007, 12:08 PM
There are two separate issues here, upside down parting tool and rear mounted tool post

The reason for the upside down parting tool is if the tool grabs, it will lift and usually let go vs grab and break. Having experienced both and I would much rather go with first. The problem is the lathe needs to be run in reverse which could an issue if running a threaded chuck.

With a rear mounted tool post, the parting tool can be mounted upside and the lathe run foward.

DR
03-28-2007, 12:24 PM
The reason for the upside down parting tool is if the tool grabs, it will lift and usually let go vs grab and break.


That's an interesting theory........

I don't go along with it though. If you have a tool that'll lift during a cut you're letting yourself in for lots of problems.

Mike W
03-28-2007, 12:55 PM
Here is a picture of mine. The tool holder is a KDK with the blade mounted upside down. I made the post.

http://img.photobucket.com/albums/v110/tek798/Lathe/Reartoolholder.jpg

JPR
03-28-2007, 01:41 PM
That's an interesting theory........

I don't go along with it though. If you have a tool that'll lift during a cut you're letting yourself in for lots of problems.This is not for turning, just for parting. The tool has to over come the resistance of QC post so it is not floating or moving.

John Williams
03-28-2007, 06:58 PM
On my overly flexible Atlas/Craftsman, back side parting was/is more reliable/predictable. Parting from the front is exciting due to the age of the lathe, combined with the fexibility of the bed - would cause the tool to wedge under the work when getting close to the center - every time. Fortunately the belt drive would slip and nothing would break.

But since I bought a Newcomer I find myself parting from the front more often. Occasionally the tool will try to wedge under the work if I am not dead nuts on the height, but not nearly as bad as the HSS ones. HSS would wedge a lot.

To try backside parting, I moved my KDK/cross-slide/compound all the way to the back and cut from behind on small stock, just to try it out. See if you can do that before going to the expense of extending the cross-slide.

garyphansen
03-28-2007, 08:07 PM
Hey Evan What say you? My other reason for planning on mounting a Phase II tool post on the rear of my cross slide is to be able to mount a hunk of brass on a tool holder to use as a follower rest. Any one have any thoughts in that? Gary P. Hansen

8ntsane
03-28-2007, 09:45 PM
I had allmost went through the trouble to make up a rear mount tool post.
While I had allready drawn up the plans, and allready rough cut the stock to basic size in the band saw, I had enough time to think this over.

The more I thought about the rear mount deal, I didn,t like the idea of not seeing what was going on while cutting. The next thing I thought about was if it did grab, it would lift up on the back side. I was counting on turning the spindle forward, with the cutoff tool upside down, my lathe has the screw on chuck, so running in reverse is not an option.

More thought on this matter and it did dawn on me that it would be possible to have it dig in and try to lift the tool holder off my cheap china made QC tool holder. Wasn,t likin that thought.

The more time I spent thinking about this, I got to thinking, this must be more to do with loose lathe tollerances. So , starting with the compound, then the cross slide, then to the carriage, I snugged eveything up just a wee bit.

OK, that fixed about 90 percent of the chatter that this lathe was producing when parting off. Thinking allright, now I can part off without having to cringe every time I do it. I normally would use cutting oil on a paint brush, and it worked OK, But decided to turn on the flood coolant to keep the chips washed out the best I could. Well that made up that last bit of differance.

For years I had been avoiding parting off in the lathe, and would remove the part and bandsaw, then face off to size. A major pain in the butt to say the least. Ive snapped off parting blades so many times trying, that I was starting to build the rear tool post.

Now, parting off steel and aluminum is no problem. I resolved all the problems on my import 12x37 lathe without biulding a rear tool post, too boot, the parting blade was the same one the wouldn,t do anything but chatter on previous attemps.

On a last note, the surface finishes turning and facing did improve some, nothing drastic, but improvement none the less. I guess one day I,ll find somthing to make out of those blocks I had sawed up. So far I only feed the tool in by hand, but I might get brave enough to feed it in under power in the near future.

Cheers Paul

BobWarfield
03-28-2007, 11:46 PM
Bob,

I think that's an old wives tale.

Better cuts and less chatter are a function of the rigidity of the setup. Cutting forces acting downward towards the machine bed usually result in a better situation than pulling upward on a dovetail.

The idea of the chips falling downward and out of the cut is another thing I don't buy. A good blade will curl the chip up and out.

The cutoff blades I use most are the "P" type. To resharpen like new only a grind across the front is needed.

You may be right. OTOH, you are the first guy I've read who actually claimed to have tried it and didn't see any difference. Every other post I've read the fellow swore that running upside down from the backside made the parting work a lot better. There's no end of these posts if you search these boards, PM, and whatever other literature you care to research.

The argument that the tool doesn't dig in when run upside down certainly resonates with my experience parting off. When I've had a problem with it, it has been due to digging in more than anything else. In the end, a lot of lathes do have problems with rigidity for many reasons and if running upside down cures them, why not? I have read one individual who felt that if running upside down made the difference you ought to check whether your bearings had worn out.

Best,

BW

JCHannum
03-29-2007, 10:44 AM
The rigidity of the lathe and the tightness of it's components will have a definite effect on the cutting off operation.

The smaller, lighter lathes usually encountered in the home shop, often coupled with inexperience and lack of confidence of the operator, all contribute to problems with cutting off.

The rear mounted toolpost helps eliminate some of these problems. Since it is usually purpose made, it adds rigidity, and the direction of forces is more favorable. Armed with the "solution", operator confidence increases.

The rear mounted cutoff will perform better on the lighter, more flexible machines. On a heavy, commercial grade machine, the difference will be negligible.

BobWarfield
03-29-2007, 02:36 PM
The rigidity of the lathe and the tightness of it's components will have a definite effect on the cutting off operation.

The smaller, lighter lathes usually encountered in the home shop, often coupled with inexperience and lack of confidence of the operator, all contribute to problems with cutting off.

The rear mounted toolpost helps eliminate some of these problems. Since it is usually purpose made, it adds rigidity, and the direction of forces is more favorable. Armed with the "solution", operator confidence increases.

The rear mounted cutoff will perform better on the lighter, more flexible machines. On a heavy, commercial grade machine, the difference will be negligible.

Confidence is key too. I find increasing the feed rate is the first thing to try when parting off is misbehaving. Fixes my problem 9 out of 10 times, but seemed scary at first.

Best,

BW

LarryinLV
03-29-2007, 04:28 PM
Gary,
You don't say if you're having trouble parting in the normally accepted front tool post mounted fashion.

If you're having problems parting and it sends chills up your spine when you think about it, then I would say that you are looking for a cure-all and it will not make you a better machinist having a rear mounted parting tool.

If you just want to do it because it is one of the little things on your to-do list; go for it.

Remember, on anything less than a very rigid set-up, there is a tremendous downward force on the tool that will push it down off center causing chatter, or the tool will be pushed under the work (the work will ride over the tool). So set the tool a little above center. Too light a feed will cause chatter, too little lube will cause chatter. HSS tools need a little lighter touch than carbide cutters, but watch the color of the chip; tan colors are nice; you can run a carbide 'til the chip is blue and smoken'

Paul Alciatore
03-30-2007, 10:31 AM
I have a SB-9, not the biggest or solidest lathe in the world and it does have some wear. My original attempts at cutting off with a lantern style post were a disaster. Way too much flexture in the setup. And it would run under the work easily.

I made a QC holder of my own design that I believe is more solid than the commercial ones and made a couple of cutoff holders for it. I also read about using a fairly agressive feed instead of trying to go in slowly. Now, I have no trouble with the cutoff operation on the front side.