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Making a tap guide...with problems.

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Russel:
Hello everyone,

A few days ago I posted how I made a T-nut to use with a small camera mount for my little bitty Sherline Lathe and Mill. At the end of that thread I decided that some kind of Tap guide for the mill would be very usefull. The spindle on the Sherline lathe and mill is hollow and will allow a 3/8” diameter rod to pass through it, but not much bigger. So, what I was thinking was to make a guide that has 2” of 3/8” rod at one end, a half inch of 1/2” diameter rod in the middle and 3/8 – 24 threaded on the other end for a drill chuck. With a hole drilled through the middle half inch section, I could use a tommy bar or small diameter rod to turn the device for tapping like a common tap T- handle has. The best material that I have for this is some half inch stainless steel rod. So, the first thing that I do is hacksaw off a three inch length of the stainless rod.



Now, I am by no means very experienced with machining. I've spent decades working in shops as a mechanic so I have experience drilling and such. I've done a lot of reading and have used my little Sherline lathe and mill off and on over the last few years. But I haven't tackled any major projects. Madmodders has inspired me to really start putting these little machine tools (and the one between my ears) to good use. So, my purpose here is to learn from the advice of others as well as maybe provide a tidbit or two that might help someone else. Keep that in mind as you read, and feel free to comment, good or bad.

Anyway, the first operation is to face and center drill the end of the part. First I center it in my 4 Jaw chuck. Something that I gleaned from one of the posts here is to use two wrenches on the 4 jaw chuck and center it by eye.



Then I use a DTI to finish centering the part till the needle barely moves. Once I got the hang of it, I began to use the 4 independent jaw chuck about 99% of the time. I love the repeatable precision you get with it.



Then, I faced off the end of the part:



Then center drilled it:



I'm thinking that a bushing mounted in the 3 Jaw self center chuck on the mill spindle with act as a good guide for this part. So, I figure that I can drill and ream the bushing for 3/8” or .375”  That being the case, I figure that the 2” of guide shaft that I'm going to turn down should be about .370”  I zero the Z axis on the lathe to the end of the rod, then move the saddle 2” closer to the headstock and mark the spot with a groove.  Because of cutting bit limitations, I either have to turn this part of the shaft unsupported, or with a bit that I can't face  the headstock end of the cut with. So, I decide to do this in two stages. Turn the shaft down to .370”, then change cutting bits and face the end near the headstock.

Marking the 2” spot.


Start turning down the shaft:


This is what I end up with at one end of the cut when I'm done because I can't face with the turning bit that I used:


So, at this point I switch bits. I zero the z axis of the DRO to the end of the shaft again, as well as the x-axis and set up to remove the “steps.”


About halfway done removing the steps:


And done removing the steps:


The next thing that I want to do is put a little radius in the end. This will be the end that slips into a chuck mounted bushing in the mill, so I wanted to make it as easy as possible to get it started in the bushing. So, I set up the radius cutter.





and when I'm all done, here is the end:


Next I flip the part over to machine the other end. I face the end and turn 1/2” inch of it down to .375” to be threaded to 3/8” - 24.


Basically, I am trying to copy the threaded end of this MT1 drill chuck adapter.


So I shoot for .050” undercut for .075” of the shaft at the base of where the threads go.


I then bevel the end so that the 3/8” 24 die can start threading.


The best method that I can think of to start the die straight is to use a tool post as a guide, while I turn the chuck and hold the die with a wrench. First, I tighten the tool post square.


Then, I place the die between the part and the tool post and crank the saddle until the tool post is gently holding the die against the part.


Add a 1” wrench to hold the die.


So, in order to thread the part I turn the chuck with my left hand while turning the lead screw with my right hand to keep the tool post up against the back of the die, keeping is straight. Once the part is about 3 /4 of the way through the die I back the tool post away and just use the 1” wrench to advance the die. Unfortunately, it appears that a small chunk of one of the teeth on the die broke off midway and I was backing the die back off of the threads.


I'm really not happy with these threads, but it is stainless steel after all. The stuff can be a little tough at times. So, I try treading on a drill chuck:


No problem at all. The threads may look ugly, but they work. The chuck seated nice and square, threading on smoothly. So, I chuck the part back in the lathe (wondering why I took it out without checking the run out) and carefully center it. Then check the run out at the far end of the chuck and got .0045”


I'm thinking that for what I am going to use this part for that should be fine. What do you folks think? Also, the die that I used was one that I bought from Matco tools. They are of good quality. I'm wondering what I could have done differently to prevent the problem with the threads. Should I have threaded a shorter section of the part, say 1/4” or 3/8” of the length instead of 1/2”?  What is the best cutting fluid for working with stainless steel? Why did the die lose a tooth? (or a small part of one anyway) I have a thread cutting attachment, should I have cut the threads with a single point tool?

At this point I figure it is time to clean up and put everything away. The next operation is to drill a hole perpendicular to the length of the part in the middle thick section of the part. That means using the mill. So, after I work on that and maybe give the tapping guide a test to see how it works, I'll post the results.

Russ

bogstandard:
Russ,

You asked for comments, and I hope the ones I am going to pass on will help you understand and progress a little further.

With regards to your threading die.

Dienuts, as they are called, are not really for cutting new threads, but for cleaning up old threads that have been damaged in some way, and are usually used in auto shops.
What you really require is a split die and holder. With those you start with the die sprung fully open, then gradually come down to perfect size.
Also it seems that your dienut was made of carbon steel, which really is too brittle to be used for cutting threads on harder materials. For stainless you really need HSS dies to do a good job, and also a good high pressure threading lubricant.

Next is the sequence of cutting for the arbor you have made.

When cutting a precision item such as you have made, where you have die'd the thread on, rather than single point cut, normally you would get the basic holding spindle made, but slightly oversized. Then when the two parts are put together, you use the assembly to fine cut to perfect runout.
If you go to this article, and go down until you find the two consecutive blue tins in the pics, the two bits following those explains how to true everything up at the very end. I didn't point you directly at the said parts, as really, reading the whole article will maybe help in the future.

http://madmodder.net/index.php?topic=721.0

If your holding spindle is not too critical on size, you are not too late to use this method to really true up what you have already done.



I hope this has helped.


Bogs

Gerhard Olivier:
Russel that camera stand of yours is getting you very good pics and a nice write-up as well.
Sorry about the thread it is a pain when all your steps has gone so well and then the last one gives trouble - one more advantage of what Bogs suggested is that the difficult part gets done first and all the easier steps follow after. ( somehow if the first step goes wrong it is not so bad as the last one.)

Gerhard

raynerd:
Russel - thanks for the info regarding the tap guide and look forward to the progression in this build.

Totally  :offtopic: if you ever find chance to give us a little review of the Sherline in the Machine Shop forum I would be really greatful. I`ve been looking at the Sherline 4000 (4100), the Unimat 3 and the Taig as potential mini lathes for clockmaking in the future and getting rid of the Clarke cl300m. Sorry to be  :offtopic:

Bernd:
Russel,

I went back and looked at that die you used. From what I can see it looks similar to a Vermont American Tool Co. die. I had bought some taps and dies at the local hardware store made by the company. No matter what I did they just won't cut a decent thread.

For a machine as small as the Sherline I would get some better dies from MSC, Enco or McMaster Carr. Less frustration and they turn out better work.

 :offtopic: Chris I own a Sherline lathe with a milling column option. I like it very much. It will do presion work.  :offtopic:

Bernd

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