PM1340 Solid Tool Post for Dorian BXA QC

[bretthl]

What I'm trying to understand is what portion of the compound contributes the most to cutting tool instability. Is it the base (where it attaches to the cross slide, ie the cirular T-slot arrangement) or is it the compound dovetail ways? Since I have been reading about these conversions I have paid more attention to the compound on my 1340GT and have seen (and measured) displacement across the compound ways. Snugging up the lock eliminates some of it, but not all.

I built a scissor knurling tool about a month back and have had the Dorian tool post move a couple times while working 4130. When I made the T-nut for this post I did not incorporate the registration pins.

If you bolt the tool post riser block to the cross slide at four points, why do you need a registration bar? And if you use the registration holes in the Dorian post then why do you need an external registration for that post?

I'm not trying to beat a dead horse, it's just my insanity for optimization.

 

[ddickey]

For the post you would not. Any slight adjustment would be made from the block as the clearance holes will allow some movement.
I think the reason for the register on the bottom of the block is to ensure it squares up every time you put it back on. I didn't do this on my second one I made. I also relieved the bottom of my block to it sits on four pads. I spot faced my tapped holes on the cross slide as well. None of that is probably necessary but I don't think it hurts.

 

[Nogoingback]

What I'm trying to understand is what portion of the compound contributes the most to cutting tool instability. Is it the base (where it attaches to the cross slide, ie the cirular T-slot arrangement) or is it the compound dovetail ways? Since I have been reading about these conversions I have paid more attention to the compound on my 1340GT and have seen (and measured) displacement across the compound ways. Snugging up the lock eliminates some of it, but not all.

I think it's movement in the dovetails. I turned a solid tool post for my Logan some time ago and bolted it down exactly like the compound with the t- bolts. The improvement in rigidity was still substantial. Nice to work around as well, and keeps me from bumping into the handle of the compound just before I make my last pass.

 

[bretthl]

I think it's movement in the dovetails. I turned a solid tool post for my Logan some time ago and bolted it down exactly like the compound with the t- bolts. The improvement in rigidity was still substantial. Nice to work around as well, and keeps me from bumping into the handle of the compound just before I make my last pass.

And I have done that (the middle age belly?). How many T-bolts did you use? DDickey pointed out that the register could be used to true up the block to the cross slide but with T-bolts along with the center registration boss mentioned by Jbolt then the block could be rotated true before tightening down. You got to be dead square with a knurling tool that David shows in his video. Many different ways I suppose.

 

[Nogoingback]

I just used two T-bolts, because that's what the compound uses though I could have used three if I'd thought about it
at the time. The tool post is just a
cylinder turned from 1144. The top of the Logan cross slide isn't flat, so I had to match the compound mounting surface.

 

[jbolt]

At least on the PM GT series the weakest link is the compound t-nuts

What I'm trying to understand is what portion of the compound contributes the most to cutting tool instability. Is it the base (where it attaches to the cross slide, ie the cirular T-slot arrangement) or is it the compound dovetail ways? Since I have been reading about these conversions I have paid more attention to the compound on my 1340GT and have seen (and measured) displacement across the compound ways. Snugging up the lock eliminates some of it, but not all.

I built a scissor knurling tool about a month back and have had the Dorian tool post move a couple times while working 4130. When I made the T-nut for this post I did not incorporate the registration pins.

If you bolt the tool post riser block to the cross slide at four points, why do you need a registration bar? And if you use the registration holes in the Dorian post then why do you need an external registration for that post?

I'm not trying to beat a dead horse, it's just my insanity for optimization.

The weakest link in the GT compounds is the t-nuts. Additionally the access hole for the t-nut interrupts the circular t-slot which is a terrible design. Access should be through the bottom. In the testing I did, flex was measured in the compound bolts, dove tail, tool post bolt/t-nut and tool. While these are relatively small movements, some are greater than others and have an accumulative affect which varies with compound angle and extension/retraction. When we add a QCTP we create a cantilever of the tool vs the factory tool post which also contributes.

Locking the gibbs for any axis not moving during an operation lends itself to greater rigidity.

As to registering the riser and tool post. For the riser, having some form a registration that is repeatable saves time when removing and reinstalling. It also aides in preventing rotational slip which could occur in some circumstances with bolts alone due to the nature of thread fit. If one were to use the existing circular t-nuts only then it becomes even more important.

For the tool post, most QCTP's have a hole for a pin which can be used to prevent rotation. The additional clamps such as the Mitee-Bite clamps I use are for fine tuning to get the tool post perfectly perpendicular to the spindle axis and take up any fit tolerance of the anti-rotation pin.

All of this is really unnecessary for 90+% of what most people do but the increased rigidity can have a profound affect on depth and speed of cuts, surface finish, parting capacity, knurling, boring bar size & extension and power drilling with the carriage.

Some people see this a major modification when it reality it is just another accessory to be used as needed. I rarely put the compound on.

A perfect setup for me would be a wider/thicker cross slide with straight t-slots the length of the slide to allow fore and aft positioning of the riser and the use of other accessories.

 

[davidpbest]

Jay has done an excellent job describing all the aspects. I can confirm that I treat my solid tool post like any other lathe accessory. And when I do take it off and put it back on again, I'm very glad to have the registration stops to ensure it goes back on square to the CL of the spindle - saves a lot of time by eliminating the need to tram in the tool post. Jay used Mitee-Bites to capture the alignment of the tool post on the solid mount, whereas I used a permanently mounted corner L-bracket that was pre-loaded in both directions and pinned (and screwed) to the solid mount (it's all detailed on the drawing I posted:

https://flic.kr/p/UstqZd

). When the Dorian BX QC tool post is sucked down onto the solid mount with the top nut, that L-bracket forces the tool holder rigidly against the two alignment dowel pins from underneath which registers the holder square and unable to rotate under load.

I routinely use an Eagle Rock scissors-type knurling tool, and it hangs out to the spindle side of the tool post a good 3 inches, and thus puts a tremendous amount of twisting force on the tool post, so having the solid mount secured at the corners in conjunction with the index pins into the bottom of the QC holder is really necessary to keep the tool post from rotating when knurling into 4130, 316 stainless, etc. And I haven't broken a single parting insert since I installed the solid tool post. Surface finish is night and day different too, and depth of cut limits have about doubled.