Lathe rear parting tool

[mikey]

About how much height over the blade does one need to make this work ok?

1/4" thick top is fine.

About how wide does it need to be? More is better, but is 2.5" ok, or does the blade need more support?

2" front to back is enough.

I will need to drill my cross-slide since my lathe has no tee-slots. 2 M6 or 1/4" bolts ok, fore and aft? Or something larger? I'm thinking of machining feet (really more like flanges) so I can bolt through it into the cross-slide.

I suggest two 10-32 SHCS to hold the blade and 10-32 or 1/4" to hold the body to the cross slide.

I have some rough cut 1.625"x3.5" stock in 7075. No 6061 this big.

Go buy some 2" to 2-1/4" square 6061-T6. It isn't that expensive and I know for a fact that 6061 will not crack if you follow the general build I used. While 7075 is stronger, it is also more prone to crack with repeated stress/flexion. I would go with 6061. DO include both slots. The distance of solid stock between the slots only needs to be about 1/4" - 5/16" wide.

Be absolutely sure the bottom of the blade slot, and therefore the location of the tip of the tool, is on the exact centerline of your lathe. Find the centerline with cuts; DO NOT chuck up a pin in a collet and expect that to be accurate enough.

DO include a ledge in your design. It should butt up against the side of the cross slide. This prevents the tool post from spinning under load and allows you to lock the tool solidly to the cross slide with a single bolt.

 

[SLK001]

After a horrible parting session on my Grizzly this morning, I'm going to make one of these. I don't know what happened, this lathe used to part ok. Something changed and now it is bad. Never have seen that much movement before. Something is loose or way off. Bad enough that I gave up and used a hacksaw. I haven't had to do that in a very long time.

Before you begin on a different journey, investigate why you are having parting problems with your current setup. If you don't, you could experience the same issues with a rear mounted tool. It sounds like there will be significant mods to your cross-slide for you to mount the tool, so make sure those are mods you're willing to make.

 

[WobblyHand]

1/4" thick top is fine.


2" front to back is enough.


I suggest two 10-32 SHCS to hold the blade and 10-32 or 1/4" to hold the body to the cross slide.


Go buy some 2" to 2-1/4" square 6061-T6. It isn't that expensive and I know for a fact that 6061 will not crack if you follow the general build I used. While 7075 is stronger, it is also more prone to crack with repeated stress/flexion. I would go with 6061. DO include both slots. The distance of solid stock between the slots only needs to be about 1/4" - 5/16" wide.

Be absolutely sure the bottom of the blade slot, and therefore the location of the tip of the tool, is on the exact centerline of your lathe. Find the centerline with cuts; DO NOT chuck up a pin in a collet and expect that to be accurate enough.

DO include a ledge in your design. It should butt up against the side of the cross slide. This prevents the tool post from spinning under load and allows you to lock the tool solidly to the cross slide with a single bolt.

Thanks for the tips. I was going to make a ledge that bolted on, like @Pescadora's. When I made a plinth for your straddle knurler design, I machined the ledge in, but it wasted a bunch of material. As an aside, making that straddle knurler was such a great experience for me. I learned a whole lot from your thread. I love using it!

I do have a piece of 2" x 2.5" x 6" 6061, and a piece of 1.5 x 3 steel stock. I was thinking the aluminum wasn't wide enough once I mill away 0.75 for the screw flanges, that only makes the holder 1.75" wide, which doesn't feel wide enough to me. Centerline is approximately 79 mm above the cross-slide. This tool is for a 10" lathe. Ignore the single slot in this picture, (and the hole at the end of the slot) but look at the feet of the plinth. In this model, the width of the base is 3" and the tool holding width is just under 2-1/4". I'm playing around in CAD, and changing stuff for these simple drawings isn't difficult. Only took me 15 minutes for the whole drawing.

What do you mean by find centerline with cuts? If I made a center in the chuck, left it in the chuck, and then scribed it against the piece, would that be sufficient?

 

[WobblyHand]

Before you begin on a different journey, investigate why you are having parting problems with your current setup. If you don't, you could experience the same issues with a rear mounted tool. It sounds like there will be significant mods to your cross-slide for you to mount the tool, so make sure those are mods you're willing to make.

Will be doing that. Something obviously loosened up, or changed. I suspect a couple of things went south, but need to investigate further.

As for the cross-slide, all I expect to do is to drill and tap two holes sufficiently far away from the dovetails. There's 20 mm of space. Lots of room for an M6 screw. I've done something similar for my mini-lathe and attached a rear mount plinth for a knurler. When not in use, the cross slide holes are capped with screws.

 

[jwmay]

I did the measuring thing. I put two indicators on the carriage. One was at the top front of the parting tool block. The other was on the tailstock side of the cross slide. Both showed about .002" lift when parting. So there's that information for whomever may want to make suppositions, theories, and explanatory essays. Lol. I'll be machining.

 

[woodchucker]

Every connection between parts from the Ways on the lathe bed up to the tip of the parting tool can have some sort of flexibility or movement. On their own, they are small or possibly unmeasurable. Add them all together and all sorts of strange things can happen.

Making a solid connection with no adjustment, (center height is guaranteed) removes all the other parts and their connections, lubrication and things from the equation. many problems go away and you likely will have a larger operating envelope in some other areas for parting.. more materials, bigger/smaller diameters..

on a machine tool, stiffness and rigidity is everything.

Here's a different take on why it works, which I agree with.
I don't agree with that sentiment that the cross slide is tighter in the rear, since it's more tight at the front of the cross slide (our side) vs the back..

 

[mikey]

Thanks for the tips. I was going to make a ledge that bolted on, like @Pescadora's. When I made a plinth for your straddle knurler design, I machined the ledge in, but it wasted a bunch of material. As an aside, making that straddle knurler was such a great experience for me. I learned a whole lot from your thread. I love using it!

Glad that tool worked out for you. It is a learning experience, that's for sure.

I do have a piece of 2" x 2.5" x 6" 6061, and a piece of 1.5 x 3 steel stock. I was thinking the aluminum wasn't wide enough once I mill away 0.75 for the screw flanges, that only makes the holder 1.75" wide, which doesn't feel wide enough to me.

Why bother with flanges? You only need the ledge plus a single bolt in the center of the tool body to hold it solidly in place.

What do you mean by find centerline with cuts? If I made a center in the chuck, left it in the chuck, and then scribed it against the piece, would that be sufficient?

The centerline I'm referring to is the exact centerline of your specific lathe's spindle. The tip of the parting tool has to be on that centerline or very, very close to it. You can maybe be half a thou or less below it but you should not be above it. Here is a simple way to determine this critical dimension:

• Chuck up a piece of stock, maybe 3/4-1" OD. Aluminum or mild steel is fine. Use a sharp HSS tool and take a skim cut of 0.010" deep to take off the skin, then another cut of 0.003" to get a clean surface, then a final cut of 0.001-0.002" deep with a fine finish. You can use a simple 3 jaw to hold the work.
• Use a height gauge if you have one and measure from the surface of the cross slide to the top of that turned work piece.
• Measure the OD of the work piece with a micrometer and divide that dimension in half.
• Subtract 1/2 the OD from the height to the top of the work piece and that gives you your centerline or spindle height above the cross slide. Write this down someplace accessible.

You can approximate this dimension with a precision pin and collet but you will be off; cutting it is more precise. When you make your tool post, the bottom of the blade slot must be on this precise dimension or as close as you can get it.

An add-on ledge is fine. You do not need to cut one.

Let me know if any of this is unclear.

 

[jwmay]

it. Here is a simple way to determine this critical dimension:

All I did was set my parting tool holder on the carriage and slid it across a tailstock center. That left a layout line. Then I measured it. I don't know if it's a fool proof method, but it proved fine for this fool.

 

[mikey]

Here's a different take on why it works, which I agree with.
I don't agree with that sentiment that the cross slide is tighter in the rear, since it's more tight at the front of the cross slide (our side) vs the back..

The debate on how a rear mounted tool works has been going on for decades ... literally. Almost everyone involved has their guess, including Myford, and nobody has proven the mechanism behind it to universal satisfaction. So many guys have argued vigorously over this and the most vehement of them have been those who have never used a rear mounted tool post before. This is why I don't like to get involved in these discussions. There is no end to it.

I have taken the stance that it just works. Why it works is a matter of personal belief and I have mine but that is of no consequence. What matters is that it works and it works better than parting from the front. My Emco Super 11 parts just fine from the front and I have no issues doing so but I cannot part at the speeds that I can from the rear and cutting loads are higher when parting from the front.

I can tell you that I can part from the rear in almost any machineable material on a little Sherline lathe at very high speeds with zero chatter. I've parted 1.25" OD 304SS with almost no heat in the part and no work hardening at all. I've done 2" OD Delrin, mild steel, brass, 1144, 4140, wood and I forget what else with zero issues. After thousands of problem-free cuts I can say that it works ... it just does.

 

[WobblyHand]

Why bother with flanges? You only need the ledge plus a single bolt in the center of the tool body to hold it solidly in place.

I'd rather not do flanges, but the plinth height is 4". That's a long hole. Was thinking of using a long M8 SHCS. An extended length P drill (which I don't have) will only drill to 3.64", a jobber length P drill drills to 2.82". Guess, I could flip the piece to try to make the hole longer. Haven't tried that before, but why not. If I flip the piece for drilling, probably need to up the through hole size a smidge. An M8 is as large as I dare drill into my cross-slide. Want to have sufficient spacing from the edge of the dovetail, since there is a groove cut in that corner with a slitting saw.

Use a height gauge if you have one and measure from the surface of the cross slide to the top of that turned work piece.

No height gauge. Did buy one, but returned it, since it did not match the photos. Pics showed a carbide scriber, I got a steel one. Sort of a bait and switch. Got a refund, but haven't found a replacement. Some day...

Could measure the height from the bottom up, with some 123 blocks, ground stock and shims on the cross-slide. Not sure how accurate that would be with tolerance stack up, but it should allow a measurement.

I'll redraw the tool holder, assuming a 2.5" x 2" x 4" piece of 6061. (Have a chunk on my desk.) My drawing is parameterized so if I need to change a dimension, I just type it into the spreadsheet and whamo, the drawing is updated.