Threading fit issue, tapered?

[mksj]

Does the DRO show a change in position, assuming you are using the cross slide to advance the cutter and the compound is locked. Check the gibs on the cross slide/compound, also the back of the carriage. Might also recheck the head alignment, do you see the same taper when cutting a diameter and does this change under load (DOC)?

 

[WobblyHand]

A picture is worth 1000 words here .

I think a few pictures would help a lot. Real close so we can see those threads in focus and can see the crests and troughs in detail. Side A and side B, as well. As well as decent pics of your set up. Tool post and cutter, up close and personal. Might be something simple, or a habit you have acquired, that's not working out for this.

 

[benmychree]

Carbide form tools are rarely sharp, resulting in increased tool pressure, which can result in taper and size variation. I use HSS tools for threading; those made by Aloris, they are form ground and are sharpened on top only, by hand and enjoy a long working life and come in several sizes (widths) up to suitability for 4 TPI to quite fine and last for many years with regular use.

 

[GeneT45]

Carbide form tools are rarely sharp, resulting in increased tool pressure, which can result in taper and size variation. I use HSS tools for threading; those made by Aloris, they are form ground and are sharpened on top only, by hand and enjoy a long working life and come in several sizes (widths) up to suitability for 4 TPI to quite fine and last for many years with regular use.

I have, and have used, the Aloris, but have been using carbide laydown inserts from Shars lately and they are *sharp*. I suspect they're ground due to the precision form required, rather than molded like a common insert.
I have two blades for the Aloris, one of which I ground on the side so I can thread closer to a shoulder and the second blade so I don't have to grind 1/4" off the first if I need to cut larger threads...

GsT

 

[benmychree]

I might add that the Aloris tools and holder are made so as to be inverted so as to be able to cut from left to right (away from the chuck, as well as in the conventional manner (towards the chuck), and they have tools for cutting Acme threads in several pitches, for pitches not served, you cam simply surface grind the flat to the appropriate flat width. I think that I remember three available tools that fit most used pitches, I think I modified one and bought another, and that with a fixture, it is quite easy to grind accurate Acme tools from blank tool bits.

 

[mmcmdl]

I use a fixture I made that will grind regular as well as Acmes . Of course you need a surface grinder to do so . Same as the Aloris , if it gets dull , dust the top off and you're good to go . CNCs I did use inserts . I had the surface speed needed to use them and the auto threading cycles made it a breeze with no worry of crashing . If you're using low speed which I'm sure most manuals are running while threading , carbide isn't the choice .

 

[extropic]

Are you using a proper thread micrometer (or wires) to quantify the thread size/taper?

 

[jareese]

You'll get to where you can wheel the cross-slide in really fast back to zero, then add the amount you want to cut and then re-zero again, Do it that way each time and it will be muscle memory pretty quickly.

Pretty much getting there now - mostly ignore the DRO when threading except for beginning. I start fitting when I get closer.

 

[jareese]

Are you using a proper thread micrometer (or wires) to quantify the thread size/taper?

I'm fitting when i get closer - each item is a custom fit since I'm doing gunsmithing.

 

[jareese]

Does the DRO show a change in position, assuming you are using the cross slide to advance the cutter and the compound is locked. Check the gibs on the cross slide/compound, also the back of the carriage. Might also recheck the head alignment, do you see the same taper when cutting a diameter and does this change under load (DOC)?

I've done tests taking heavy (for me, .040-.050) cuts and light cuts (.002-.005) There is minimal taper over roughly six inches of a half thou or less.I haven't checked the carriage gibs, I'll check those.