What's the Differance? Tool Post Styles, Piston or Wedge

T Bredehoft said:
I've used both, have no preference from experience. Having said that, I note from the descriptions above, that while the piston type pushes the tool away from the post, putting pressure on the dovetails, the wedge type pulls the tool toward the tool post putting pressure on one of the dovetails (the wedge is not rigid, usually formed sheet metal) and the flat surfaces of the tool post, being somewhat more secure. This is from logic, I'd want to study both with indicators before making a positive judgement. In other words, which cutting tool would wobble more when clamped.
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Neither should wobble at all when clamped... It is about repeatability. The piston pushes the tool holder, and the two dovetails hold the toolholder in place. If you do it twice, the tool holder might be twisted very slightly up on one dovetail and down on the other dovetail, compared to the previous mounting. The wedge style tries to minimize that by only moving on one side, while the other side settles better into the opposite dovetail. We are talking about a tiny amount of possible difference in repeatability. Once they are clamped down, both styles are solidly rigid to all but major crashes.
 
Wedge, pressure on 2 Ddovetails, better than one
 
Without question I would spend the extra for a wedge, I have a Dorian AXA on my 13X40 and rate it a 10 for size and performance.
 
The prevailing wisdom is that wedge is better, and when there was a substantial price difference, many of us went for the piston. However, that difference seems to be eroding, so, now, I would go for the wedge.
 
Contact surface when the tool holder is locked in place on a wedge style is greater than a piston style so theoretically it'd hold better. Piston style makes contact on the dovetails and the piston itself. The wedge style makes contact on the dovetails and the surface of the tool holder/post.

That being said, as Bob mentioned above, you'd be fine with a piston style if it's that much cheaper. I have a Grizzly G0709 lathe which came with a piston style holder. Have a wedge style on my Clausing 12" and my Rockwell 10". I've added a taper attachment, 5-C lever style collet closer and DRO to the Grizzly (plus around 35 tool holders), never felt the need to replace the stock piston-style tool holder with a wedge style on that lathe.

Bruce
 
I have done testing on both types (BXA size), using various brands of holders, using a TDI and actual cutting.

The piston type is more prone to repeatability errors depending on the fit of the holder to the tool post. A loose fitting holder can be locked up slightly tilted fore or aft several thousands of an inch where a tighter fitting holder is more likely to true up when tightened. This can be dealt with by applying pressure to the front or back of the tool holder when locking to fully seat the dovetail.

Some holders from other manufactures may not fit the piston type holders due to that large variation in dovetail dimensions between manufacturers. I have not run across this problem with the wedge type.

As stated above the contact area between the holder and tool post is greater with the wedge type but in the testing I have done, taking maximum turning and parting cuts, the real world performance was negligible between the two.

That being said my personal preference is the wedge type because any brand of holder will fit and there is less chance for repeatability errors.
 
Rd2nowr said:
Ok, so what's the difference between a piston QCTP and the wedge type? Pro's and cons of both? Shar's has the AXA piston type in a set for $114. and the wedge type for $183.
Thanks, Tim
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Tim, first thing, my vote goes to the wedge type. But my reason for replying is that I think you should shop around a little more. The price difference you quoted seems way too wide. When I bought my Phase II wedge toolpost set I'm pretty sure the difference was only about 25 bucks, off coarse that was from the now (sadly) gone Enco. Hope this helps, JR49

EDIT:

https://www.amazon.com/dp/B01GVWBBA8?m=A3H367CBPE368U&ref_=v_sp_detail_page


https://www.amazon.com/dp/B002YPDQ4G?m=A3H367CBPE368U&ref_=v_sp_detail_page
 
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I bought the Shars AXA wedge type kit and and happy with it. Solid and well made in my opinion especially for the hobbyist.
You can save a bit on the Shars AXA kit on Amazon or even more on Ebay. I just looked and it's $120 plus shipping, or you can get the toolpost only for about $67 and add a just one or two holders, but the kit is a pretty good deal and the mix of holders and the knurling tool (works fine for aluminum and mild steel) is nice to have. There are other import brands but I haven't personally seen or tried other than the Shars so like anything I'm sure quality varies.
 
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