Suggested Material For Qctp

[smarrocco]

I'm planning a round column-based QCTP similiar to this

http://www.gadgetbuilder.com/ToolHolders.html#Round_QCTP
....for my Sherline CNC lathe. My main reasons for the round column
format (vs. square dovetails-based versions) is the ease of tool holder
creation.In designing it to keep the cutting tools within the same space
the Sherline standard tools inhabit, while require the column to be
approximately 3/4" diameter, with a flat 1/8" square base for mounting
to the T-slots. As this is a bit narrower than the standard 1.25"
diameter square column Sherline tool holder, I am a bit concerned about
rigidity. My original plan was to go with aluminum, but knowing the
column diameter will be thin in encouraging me to go with steel. I use
this lathe for steel and stainless, so I think the aluminum would be
tool weak.

I'm looking for suggestions as to what steel I should select for the
tool post (and tool holders). I've considered stainless (merely because
of the corrosion resistance) but I'd rather lean on the side of
strength. I have no surface grinding capabilities but was also
considering steel that can be hardened (via propane/acetylene). I also
haven't ruled out that I'm over-engineering this and that simple
unhardened steel would be enough.

Any thoughts on the choice of metal?

 

[mikey]

As this is a bit narrower than the standard 1.25"
diameter square column Sherline tool holder, I am a bit concerned about
rigidity.

I'm looking for suggestions as to what steel I should select for the
tool post (and tool holders). I've considered stainless (merely because
of the corrosion resistance) but I'd rather lean on the side of
strength. I have no surface grinding capabilities but was also
considering steel that can be hardened (via propane/acetylene). I also
haven't ruled out that I'm over-engineering this and that simple
unhardened steel would be enough.

Any thoughts on the choice of metal?

I would share your rigidity concerns, too. However, if I understood you correctly and the central post will have a projection into the T-slot then it shouldn't turn. Then the only question is if the tool holders will turn on the post.

If this was my project, I would use 1144 semi-hardened steel. It turns nicely, has a decent finish and can be hardened if you choose.

 

[smarrocco]

Yes, Mike, there will be a projection into the T-Slot to prevent the toolpost from turning.

From what I've read about this type of design, a good fit can prevent the toolholders from turning, but there are alternates that involve a flat/key on the post to prevent it.

I'm also thinking about thickening the post a bit (3/4" seems pretty thin). That will involve making the tool hang a bit further over the carriage, which might also involve making a longer tailstock for clearance when cutting. The tailstock on the Sherline always seems at least a 1/4" short anyway.

Thanks for the feedback. I'm going to check my local steel place for some 1144 drops.

 

[mikey]

Hope this turns out well, but I'm curious to know why you aren't just using a standard QCTP.

As to the design of the tailstock, yeah, it gets in the way so the extension to the ram becomes almost mandatory sometimes. The standard live center gets in the way when you turn small work, too. I love everything about the Sherline lathe except for the tailstock.

 

[mikey]

My main reasons for the round column
format (vs. square dovetails-based versions) is the ease of tool holder
creation.

Sorry, you already answered my question as to why you chose this over a standard QCTP design. I hope you don't mind if I share my concerns in a bit more detail.

This design relies on a relatively small base diameter to handle the vertical and horizontal forces the post will experience, and it all anchors on a very small area of the T-slot of an aluminum table. All the cutting forces of a cantilevered tool are borne in one small area - that makes me nervous. If this were a manual lathe you might feel the load but with CNC, the cut will go regardless so if the slot is overloaded you won't know it until it breaks.

In contrast, a standard square QCTP, the cutting loads are spread over the relatively huge area of the base. The T-slot only has to hold the post down solidly, which it can do without any fuss. This would seem to be a wiser choice, especially since tool holders are readily available.

I'm not trying to dissuade you from your project. I just wanted to express my concerns.

 

[smarrocco]

No apology necessary....Other than the possible rotation, it does seem as though a round post will give more area of potential grip surface than even a wedge-type design, which has less surface area. A key in the post would eliminate the slip possibility, complicating the toolholders a bit, but I haven't given up on that possibility yet.

I share your concerns about loads and stress, hence my original post before completing the design phase.

I'm modifying my design a bit since getting more feedback. I'm going to increase the diameter of the post to between 1" and 1.25", pretty close to the diameter of the existing Sherline toolposts. That allows me to increase the base (which is one piece with the post, but now can be at the edge of the carriage, giving it a base width of 1.25". I'm going to make it a bit longer, to 1.5" along the carriage. This will give me a stronger base, now 3/16" thick as well. The base will have a t-slot male groove in it as well to prevent rotation.

I ordered some 1144 2" diameter stock for the toolpost and 1018 bar for the toolholders today. I plan to turn it down to toolpost diameter, them mill the base square, all as one solid piece.

I also ordered some 1144 stock for making a replacement tailstock ram, since the toolholder will overhang the carriage a bit more now. I've been wanting to replace that thing with another inch for a long time anyway. This will be good justification for it.

 

[mikey]

Okay, if you make the base that large then this would distribute the load over a much wider area and that should reduce the risk. The Sherline is capable of taking relatively huge cuts and the resultant load on the post is significant. Hope this turns out well for you. It sounds like you've put a lot of thought into this and I'll be watching for a followup when you're done!

When you have some time, make a replaceable tip live center and make an el0ngated small diameter tip for it. You will be very happy you did this.



This one is also 1144 steel with hardened O-1 steel tips and extended arbor to reach over the carriage. It is vastly more accurate than the Sherline live center.

 

[smarrocco]

Nice design! Is that one you've come up with, or one that can be purchased?

 

[mikey]

Nice design! Is that one you've come up with, or one that can be purchased?

Thanks. No, you cannot buy one in a size that fits the Sherline lathe. I looked at the live centers from several well known makers and figured out how to make one. If you're interested in seeing a write up, PM me.

 

[smarrocco]

In an effort to take this thread to its conclusion I present the results of this project. My thanks to mikey for his contributions that led me towards completing the QCTP & toolholders, all of which are seeing daily use.

As a side effect of the QCTP project I strayed into a second involving a close duplicate of his improved live center for the Sherline lathe. It was a serious adventure into boring from which I have learned a lot (yet still very little I'm certain) about boring and precision measurement. My finishing techniques were improved along the way as well, and I can't thank him enough for the help and suggestions. Here are a couple of pics of the finished tools.