Looking for some design advice.

[Tekno_Cowboy]

Hi all! I'm hoping to pick all the much more experienced people's advice on a flashlight body I'm working on. I know there are a few good threads on threading in this forum, but for some reason I just can't get my head around it. I'm sure once I get it I'm going to do this in real life.

I need suggestions for an o-ring size, and what dimensions I'll need to meet on the body and head for a good fit.

I also need some help with dimensions for the threads. I plan to use 20tpi threads, cut using the 29.5 degree method. I've also had some problems getting nice, smooth threads, so tips on how to improve the quality are very welcome.

Here's a rough drawing of what I've got so far. I'll be happy to email the CAD file to experienced members looking to help me out.

 

[Midnight Oil]

How 'bout my humble suggestions found in this thread:
http://www.candlepowerforums.com/vb/showthread.php?t=281473

 

[precisionworks]

I've also had some problems getting nice, smooth threads, so tips on how to improve the quality are very welcome.

Gotta know the material ... 6061-T6 or something else?

Are you using a threading insert, laydown style?

Is the tailstock being used to support the tube, or is the tube unsupported at some distance from the chuck or collet?

 

[Tekno_Cowboy]

I'm working with 6061-T6 on a HF 8x12. I've been threading at about 70RPM using an iscar threading bar for the inside threads, and I'm trying to use a mesa tool 4-in-1 bar for the outside threads.

The inside threads turn out okay, which I'm fairly certain has something to do with the aluminum-friendly insert. (triangular with the insert roughly horizontal)

The outside threads turned out pretty rough on a D10 conversion module I made from solid bar, about 1" out from the chuck with 1-1 1/2" in the chuck. with the stock insert that comes with the mesa tools bar. Here's a pic:

I'm turning the threads prior to boring the tube for greater rigidity, and the rod will be supported by a live center in the tailstock.

 

[olephart]

That Mesa is a dandy. I really like it. The Mesa has right hand and left hand inserts. The wrong one will make some really rough threads. Don't ask. It looks like you are off to a good start. Ya just need to vary a few things to see what works.

Ya may be taking too big of a bite for that spindle speed. Try .005 increments till ya get about 3/4 depth then use .003. You can probably do more, but it will help to determine if this is part of the problem. If that doesn't help, change the spindle speed.

I'm not sure what part of the threads are rough. If it's just the sharp part of the "V", that's fairly common. Pass a file over it or use a touch of sandpaper when you are 90% done. Makes for more accurate fitting as well.

I don't think ya need any support from the tail stock. Seems like a stout set up for threading. I've been threading away from the head stock since I got the Mesa tool. You can run the spindle at any speed without worrying about hitting something. I get better results with high spindle speeds and light feeds.

 

[Th232]

For all things O-ring (and more than you ever want to know), look up the Parker O-Ring Handbook. Section 4 in particular, although at least skimming through 1-3 would be useful. Page 4-9 in my copy has a guide for design table, starting with a desired dimension, then what to take from there.

 

[precisionworks]

I've been threading away from the head stock since I got the Mesa tool. You can run the spindle at any speed without worrying about hitting something. I get better results with high spindle speeds and light feeds.

+1

I prefer "backwards" (left-to-right) threading whenever possible. Plunge in a small relief groove just before the start of the thread, engage the feed, no need to disengage the feed & pull the tool back at the end of the thread.

6061-T6 often has a sweet spot in the speed range where the metal shears well & the finish is near mirror. You'll probably find it as you increase spindle speed.

 

[Torque1st]

Cutting the OD of the thread diameter a few thousandths undersize will provide a crest for the threads and avoid production of a sharp V. Stoning the point of the threading insert produces a flat for the root of the threads and reduces chip load on the insert. Chamfer the beginning and the end of the thread diameter at a 45° angle. Thread at a 29° angle. 29.5° will sometimes produce a poor finish on the threads. When you get down to splitting degrees sometimes inaccuracies in the compound graduations and setup lead to poor results.

Backwards threading is extremely useful for turning metric threads with a inch leadscrew or inch threads with a metric leadscrew.

Dimensions for the flat at the crest and roots of threads can be found in Machinery's Handbook. Don't turn a machine on without it. :naughty:

 

[Energie]

Some pics and drawings of making an 1" light: Link (german forum)

 

[wquiles]

Some pics and drawings of making an 1" light: Link (german forum)

VERY nice work there :thumbsup:

 

[Tekno_Cowboy]

I managed to turn some lightly better threads tonight on a practice piece. .010" compound infeed left me with some pretty lousy threads. .005" compound infeed improved things quite a bit. (I'll take a picture if I can find the time) I also think that if I had the reaction time to work at higher speeds, that would improve the quality too.

Cutting the OD of the thread diameter a few thousandths undersize will provide a crest for the threads and avoid production of a sharp V. Stoning the point of the threading insert produces a flat for the root of the threads and reduces chip load on the insert. Chamfer the beginning and the end of the thread diameter at a 45° angle. Thread at a 29° angle. 29.5° will sometimes produce a poor finish on the threads. When you get down to splitting degrees sometimes inaccuracies in the compound graduations and setup lead to poor results.

Backwards threading is extremely useful for turning metric threads with a inch leadscrew or inch threads with a metric leadscrew.

Dimensions for the flat at the crest and roots of threads can be found in Machinery's Handbook. Don't turn a machine on without it. :naughty:

Very helpful! Thanks!
Can you recommend a particular 20TPI standard diameter thread that would work well with the drawing in Post #1?

+1

I prefer "backwards" (left-to-right) threading whenever possible. Plunge in a small relief groove just before the start of the thread, engage the feed, no need to disengage the feed & pull the tool back at the end of the thread.

6061-T6 often has a sweet spot in the speed range where the metal shears well & the finish is near mirror. You'll probably find it as you increase spindle speed.

With the backwards threading, how do I position the tool to do this? Would I just use my internal bar on the far side of the work? This is about the only way I'll be able to increase the spindle speed, as my reaction time isn't the best.

That Mesa is a dandy. I really like it. The Mesa has right hand and left hand inserts. The wrong one will make some really rough threads. Don't ask. It looks like you are off to a good start. Ya just need to vary a few things to see what works.

Ya may be taking too big of a bite for that spindle speed. Try .005 increments till ya get about 3/4 depth then use .003. You can probably do more, but it will help to determine if this is part of the problem. If that doesn't help, change the spindle speed.

I'm not sure what part of the threads are rough. If it's just the sharp part of the "V", that's fairly common. Pass a file over it or use a touch of sandpaper when you are 90% done. Makes for more accurate fitting as well.

I don't think ya need any support from the tail stock. Seems like a stout set up for threading. I've been threading away from the head stock since I got the Mesa tool. You can run the spindle at any speed without worrying about hitting something. I get better results with high spindle speeds and light feeds.

The threads are rough tip to trough. I'm still thinking I'll use the live center, as the threads will be cut about 4-6" out from the chuck jaws.

 

[olephart]

Yes, 6" overhang is different than the 1-1/2" you originally mentioned. Still shouldn't need additional support because you shouldn't be putting much pressure on the work. I get the impression you are demanding more than the equipment is able to provide. Getting better results with less feed is a clue.

Use less feed during the last 50% and see what happens. The last few passes should be .001 or less and that should clean up the threads some. Also, it is inevitable that you will have some push off. That means the threads are not cut to the depth you set.

Suppose you set .005 and had push off and only cut .003. You advance the tool another .005 for the next pass and now ya gotta cut .007 to get where ya wanna go, but you are only cutting .003 per pass. It gets progressively worse. It's a formula for rough threads. It's also why ya get to calculated depth and the nut won't go on.

All ya gotta do is make another pass without changing the feed and it should clean things up. If you can't make a 2nd or 3rd pass without cutting metal, then you haven't reached the depth you have set. A clean pass (no chips)means you are where you think you are. Try it a few times on your next thread. Start around 50% depth, then again at 75% and again at 95%. If ya have to clean up frequently, you are simply using too much feed for your machine/tooling/material.

It's one of the things we gotta plan for since we don't have the most rigid machines and fancy tooling. I usually feed too much early on, but I clean things up around the 85% point and act nice after that. If things still don't work out, we may have to look at your setup. Tool overhang, sharpness, height and square. Also the machine. Everything tight? Gibbs adjusted? Spindle bearings in spec?

Keep at it. If it was easy, a cave man could do it.

 

[wquiles]

Also, it is inevitable that you will have some push off. That means the threads are not cut to the depth you set.

+1

I was taught to call this "push off" as a situation requiring a "spring cut". Like you state, you make one (or a few) pases again but without moving the cutting bit. Assuming the cutting bit is sharp enough, each of those "spring cut passes" will take a "little" bit more to compensate for the part flexing, much as a "spring" flexes sideways.

When I am threading "D" bodies on my 12x36, even with the "D" body really deep inside my 6-jaw chuck with only about 1/2" or so sticking out, I can "still" do a very light spring cut. So if the body here is "several" inches away from the chuck's face, I would certainly expect having to do various spring cuts to reach the proper/target depth.

 

[Torque1st]

Very helpful! Thanks!
Can you recommend a particular 20TPI standard diameter thread that would work well with the drawing in Post #1?

ANY diameter can be used for 20TPI threads (as long as it is large enough). You need to use a matching diameter for the mating threads. Check out the machinery handbook for threading dimensions, classes, clearances, tolerances, etc. There is no need for a "standard" pitch diameter for a custom product unless you must mate up with other parts.

It would be advantageous to cut your threads before the part was extended.

 

[precisionworks]

With the backwards threading, how do I position the tool to do this? Would I just use my internal bar on the far side of the work?

The only difference is that the carriage feeds left-to-right, instead of the normal right-to-left.

Reverse the carriage feed, bring the threading tool forward inside the tube, move it to the starting position in the groove you've made for that purpose, start the spindle & engage the half nuts. This allows threading at any speed your lathe has available.

I'm still thinking I'll use the live center, as the threads will be cut about 4-6" out from the chuck jaws.

No way to use the live center for internal threads, but you should be using the steady rest.

 

[olephart]

This should help.

www.kennametal.com/images/pdf/techRef/turning/kpLTThreading.pdf

Set the compound so the tool feeds into the direction of travel.

 

[Tekno_Cowboy]

ANY diameter can be used for 20TPI threads (as long as it is large enough). You need to use a matching diameter for the mating threads. Check out the machinery handbook for threading dimensions, classes, clearances, tolerances, etc. There is no need for a "standard" pitch diameter for a custom product unless you must mate up with other parts.

It would be advantageous to cut your threads before the part was extended.

Before the part was extended?

The only difference is that the carriage feeds left-to-right, instead of the normal right-to-left.

Reverse the carriage feed, bring the threading tool forward inside the tube, move it to the starting position in the groove you've made for that purpose, start the spindle & engage the half nuts. This allows threading at any speed your lathe has available.

No way to use the live center for internal threads, but you should be using the steady rest.

I'm not 100% sure, but I'm pretty sure that I can't reverse the leadscrew on my HF 8x12, so I guess this option is out until I upgrade my lathe. I'm thinking a Grizzly lathe might hold me over until I can afford the PM1236 I'm hoping to get someday.

I would love to use a steady rest, but I don't have one.

This should help.

www.kennametal.com/images/pdf/techRef/turning/kpLTThreading.pdf

Set the compound so the tool feeds into the direction of travel.

That helps, but it also shows me that I don't have the tooling or the machine capability to turn a set of threads backwards right now.

I'm more than a little overloaded by the sheer volume of information available in Machinery's Handbook. Can someone point out a specific part of the threading section I should look for?

 

[precisionworks]

I would love to use a steady rest, but I don't have one.

EBay usually has a number of steadies of unknown origin, and many are not hard to modify to fit your machine.

There is a steady kit from MLA fits a 9" machine, meaning you'd need to take only 1/2" off the bottom to make it fit your 8" lathe: http://www.statecollegecentral.com/metallathe/MLA-9.html

 

[Torque1st]

Before the part was extended?

Cut your threads close to the chuck before you position the part way out for finish operations.

I'm more than a little overloaded by the sheer volume of information available in Machinery's Handbook. Can someone point out a specific part of the threading section I should look for?

Start with the first page in the "Threading" section and read about thread systems. In my book the first table gives you the dimensions you need. You MUST read and understand the information on threading systems and nomenclature.

 

[olephart]

No problem. You don't need fancy stuff to cut threads. Take a deep breath and forget about flashlight parts and non-standard threads and learn how to do it. Learning to run a lathe well is not on the list of instant gratification items. Start at the beginning.

Get a standard nut and thread a rod to fit it. Do it till ya get clean, nice fitting threads. Then get a bolt and make a nut for it. Leave yourself room, so you are not worried about crashing into something.

All ya need is a sharp tool on center and square to the work with the lathe gears set correctly for the pitch. Set the spindle speed at whatever you are comfortable with and feed in small increments.

Make some clean-up passes along the way. Calculate the depth of feed and when ya get within .005, clean up again and try the nut. Proceed in .001 increments till you like the fit. I guarantee it will work. The only "art" in the process is learning how to sneak up on a nice, snug fit.