Internal thread cutting (the basics)

Hi,

I am making a gadget that needs to have a thread cut into the internal bore which is blind. I have the internal bore cut to the required diameter and have a recess/relief cut into the blind end of the bore. Questions are as follows;

1. Can I cut traversing away from the chuck if I reverse the spndle direction and cut on the side opposite to normal?

2. Will I need to set the top/compound slide to 55 deg in the same way as if cutting an external thread?

Thanks for any advice offered.

I do quite a lot of blind hole single point threading and generally i go forwards into the hole and use the dro to set a stop point or a tag of tape on the tool. And try and be speedy with the lead screw disengagement

You could go in reverse and upside down on the tool if your stop point is small .

What size thread are you cutting? And tpi/pitch?

I usually go straight in rather than at half the thread angle

Edited By Ian Parkin on 19/07/2018 22:52:26

Hi Ian,

Material is aluminium (not sure of grade but test cuts show good machining), ID is 0.900" and thread is 14 tpi making it a 5/8 BSP thread according to my threading table?

Thanks,

Ian

Hi Ian,

Yes you can traverse away from the chuck with the tool cutting at the back. If your chuck is screwed on beware that you risk the chuck unscrewing!

You do not need to angle your compound. If you do, set it to 1/2 thread angle not 55 degrees.

Steve.

Thank you Steve. I have a collar locking ring on threaded studs to hold the chuck so that should be ok. I will cut a few test runs first.

Thanks again chaps.

These sort of jobs are ideal for the lathe mandrel handle. And switch the power off....

Hmmmm, according to my tables the OD of a 5/8" BSP thread is 0.902". So if you want to cut a matching internal thread the ID to start with needs to be the OD of the external thread minus twice the thread depth, which I make 0.811".

I've never tried cutting a RH thread with the tool moving away from the headstock, but it is possible. Now that I've got a high speed threading unit, with auto trip, I don't bother to make an undercut. I just let the cutter make its own. For standard threads I tend to use threading inserts, so don't bother setting the topslide over by half the thread angle. Apart from anything else it messes up the trip position. This is screwcutting 3/8" BSP threads, at 280rpm and three passes plus a clean up pass for less than a minute per thread:

Andrew

I do it all the time straight in no faffing about, just hit the foot brake when near the end and rotate by hand manually.

Saying that only last week had to cut a pitch and normal proper tipped cutters were too big for the hole. Ground up for opposite hand an hss bit to power out. Heres the problem i have to leave engaged and power in close then manually in a low gear and over coming the torque get the cutter at the right start point, right pain never again abandoned the job and done it conventionally knowing exactly where the stop point is.
Come to think of it sure it was 19 tpi BSP

What was the hole size? Less than 8mm?

Have a look on joe piezynski youtube channel he did a good video explaining the merits of threading away from the chuck and why that is the only way he does it now .

Thank you gents, the depth of knowledge, experience and different ways to achieve a result expressed on this forum never ceases to amaze me.

I have bored out the internal diameter to 810" (over cut it by one thou) Thanks Andrew, I meant the OD in my previous post, and checking it shows that my mic was under reading by two thou on external diameters (now calibrated, thanks to Dave Brayford for confirming this). I am impressed that you cut those threads in just a total of four passes, very impressed with the chuck/collet as well, that looks very secure.

So all ready to cut the thread and discovered that I cannot fathom out the instructions for gear selection. I have added a thread named 'Chester DB 10 thread cutting' in the hope that someone can tell me what the hell those instructions mean.

Edited By Ian Skeldon 2 on 22/07/2018 22:30:09

1/4"BSP X 7.29mm deep - 11.5 bore blind sealing face critical.
Then found after traversing away and making a thread cutter up, fine there its using the thing right pain getting back to the start point in low gear over riding the torque by hand. Always a chance if selected mid range to do so then re selected a low range wouldnt line up exact.

Normally at 80 rpm i can stop within 0.3mm bottoming out around 19 tpi to 1.5mm pitch. Rotate chuck rest of the way to zero less than 1/2 a rev.

The only way traversing out would work is if cutting metric on a metric lathe where by can just hit the screw feed lever at any position within reason. Imperial cutting imperial have to line up exactly as well so cant just hit the traverse on or in between 1-8.

Having a VFD with Variable Speed and Jog functions really widens your options, and when combined with a DRO, exact positioning at the bottom of a bore becomes a quick, easy process, then you can thread at the tool manufacturer's cutting speed.

The ISCAR MGCH 06 boring bar is suitable for bores down to 8mm, with a huge variety of tips available including for LH and RH Metric and Imperial thread angles, it can also be used for external threading on small jobs, for internal and external grooving in the lathe and in the mill for thread milling, slotting, grooving and other operations. The MGCH 06 is expensive from dealers but they regularly come up NOS at less than £50 if you look around,

Regards,
Nick

Nick your still not with me.

Traversing out doing internal threading needs a relief where by you can use it as a start point.
If cannot have a relief (thread just ends) you have to do conventionally should i say, straight in right to left. The only piccy i have not thread mentioned above just to show internal thread ends with no undercut at the end running in to a taper. No way can traverse out cannot feed the cutter in theres metal there and will break the cutter tip.

The other point even if there is a relief at back of the internal thread you still have to manually reposition to exact start point every pass revolving the chuck by hand over coming the torque (ok in mid ranges.) Jog function absolutely useless when might be 1/8 to 7/8 of a rev to line up, guarantee breaking a tip and or scrapping the job if up against a face at start point.
 

Those Iscar bars are unreal, one of my favourite and best tools ever bought with thru coolant, totally stunning in what they can do with ease. The tips are dear over £30 each shopping around for the larger 16mm shanked one with min bore size 18mm dia. Sometimes i will whip it out the holder and mill with it and change to 2.2mm rad, 45x45 degree or flat ended parting.
MGCH 06 min bore size is 9.6mm, you have discounted the tip protruding.
Nightmare like all tooling companies deciphering the tips to go with the tooling. Some of my Iscar for that type of tool are meant for a totally different tool, key is the 3 prongue front fixing and screw work off that.

Edited By Jon on 25/07/2018 11:55:26

Edited By Jon on 25/07/2018 11:57:14

I take the easy route with such threads but take longer than Andrew.

Emgee.

Edited By Emgee on 25/07/2018 13:07:30

I extended the bore depth beyond the required depth of threaded portion so that I could cut conventionally (travelling towards the chuck) safely. However I have had to abandon the idea of cutting the thread on the lathe as I can't produce a 5/8" bsp on my metric lathe.

But I am very grateful for all the advice offered.

Thanks, Ian

Thanks XD 351 for the joe piezynski link - another youtube channel to spend hours watching. Some excellent tips and thoughts on machining there.

Chris

No it doesn't mate, I've done dead start threads with no lead groove from the bottom of a bore cutting out at full carbide cutting speeds.

What you don't understand is that if you can start at exactly the same point from the bottom you can attain a fuller thread closer to the bottom than by cutting in and stopping.

There are two ways to do this but since you know it's impossible I'll not bother you with them :D

- Nick

Ya Think?

£46 for a bar on eBay today, tips for the MGCH 06 are under £12 each retail, but why not convince yourself that you can't afford the technology as you've convinced yourself that things it could do for you are not possible either! :D

Edited By Nick Hulme on 25/07/2018 21:20:03