Threading fit issue, tapered?

[jareese]

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 .

I thread away from the headstock anywhere from 300-500 rpm on the readout on most threads.

 

[mmcmdl]

'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.

Are you threading in the chuck without the tailstock ? If you are checking taper with the tailstock over a long distance , it's different from turning a part in your chuck alone . That's why I was asking about the set-up . Even if your chuck is out to your ways , you can pull the taper out with the tailstock . Test the headstock with a light cut over 4" or so and verify it's aligned correctly .

 

[mmcmdl]

I thread away from the headstock anywhere from 300-500 rpm on the readout on most threads.

Thats low surface speed for carbide on such a small thread . Also , If you're starting out in an undercut your leadscrew needs time to catch up . This may be the problem . Even on CNC machines , it's common practice to start the thread 3Xs the thread pitch away from the part . You'll get it !

 

[jareese]

Are you threading in the chuck without the tailstock ? If you are checking taper with the tailstock over a long distance , it's different from turning a part in your chuck alone . That's why I was asking about the set-up . Even if your chuck is out to your ways , you can pull the taper out with the tailstock . Test the headstock with a light cut over 4" or so and verify it's aligned correctly .

No, I am not using the tailstock when checking taper.

As for threading, I've tried both with and without the tailstock - still getting a slight taper of several thou. Action threads are 1.0625x16 - 1.0625x20 and muzzle threads are 1/2x28, 9/16x24 and 5/8x24.

Never thought anything about the under cut. Good point.

 

[mksj]

If you are not seeing a taper on normal turning toward the headstock and only seeing it when threading away from the headstock, two things may be happening depending on how you engage the thread. If you are threading w/o releasing the half nut then there is an issue of the slow acceleration and not cutting full depth when accelerating. If using the threading dial and the headstock is at speed, unless you have a relief at the engagement point the cutter may take a few threads before it is taking a full thickness cut as others have mentioned. You also may be getting some initial deflection in particular if your cross slide gibs and carriage gibs are not properly adjusted. You threading speed with carbide should be fine, I been using Carmex carbide laydown inserts for quite a few year, they are quite sharp and the SFM rating is less than typical carbide turning inserts. I use them over a wide range of SFM and never had a cutting depth or finish issues. I thread at between 250-600 RPM depending on the material and diameter, as I use an electronic stop I thread toward the headstock. Typically my relief grove is the thickness of the thread and +0.001" over planned thread depth.

 

[Harry Knutz]

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

These inserts make beautiful tenon threads even at slow speeds.

Carmex 16 ER A60 BXC Carbide Laydown Threading Insert (16ERA60BXC), 1 Piece | eBay

TiN Finish, RH External Partial Profile 60 degree Thread, 48-16 TPI, 3/8" Inscribed Circle, 0.146" Thick. Insert is priced and sold as each (1).

www.ebay.com

 

[extropic]

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

My point was to quantify the magnitude of the taper with a micrometer rather than the "fit" vs no-fit method.

 

[GeneT45]

Yes, the inserts from Shars seem to work well even at very slow speeds too.

GsT

 

[mmcmdl]

If you are not seeing a taper on normal turning toward the headstock and only seeing it when threading away from the headstock, two things may be happening depending on how you engage the thread. If you are threading w/o releasing the half nut then there is an issue of the slow acceleration and not cutting full depth when accelerating.

Thanks for putting this into better words . Most don't realize the play/force on the leadscrew will F with your pitch .

 

[jareese]

If you are not seeing a taper on normal turning toward the headstock and only seeing it when threading away from the headstock, two things may be happening depending on how you engage the thread. If you are threading w/o releasing the half nut then there is an issue of the slow acceleration and not cutting full depth when accelerating. If using the threading dial and the headstock is at speed, unless you have a relief at the engagement point the cutter may take a few threads before it is taking a full thickness cut as others have mentioned. You also may be getting some initial deflection in particular if your cross slide gibs and carriage gibs are not properly adjusted. You threading speed with carbide should be fine, I been using Carmex carbide laydown inserts for quite a few year, they are quite sharp and the SFM rating is less than typical carbide turning inserts. I use them over a wide range of SFM and never had a cutting depth or finish issues. I thread at between 250-600 RPM depending on the material and diameter, as I use an electronic stop I thread toward the headstock. Typically my relief grove is the thickness of the thread and +0.001" over planned thread depth.

I'm seeing it threading both ways, towards and away. Both are on the headstock side, or the end of the thread. I also release the half nut each time, and start back up on the same number every time. When threading away from the headstock, I do have a relief cut.
I much prefer to thread away from the headstock as I have a hard time discerning the correct stopping point except when threading very slowly. (with or without my glasses)