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Colchester CHIPMASTER threading problems

Wally318

Aluminum
Joined
Oct 11, 2016
Last fall I tried to put in internal thread 2-1/4" x 8 tpi into a semi-finished chuck backplate.
It turned out horrible. Just tearing out material from not starting consistently at the same spot.
Last week I tried to turn a LH thread 7/8" x 14 tpi with the same result.
I was making a stub arbor out of a R8-JT4 drill arbor 7/8" size.
So I thought about it a quickly came up with a solution to save the project.
I turned the threaded area down to 3/4" and made a 16 tpi thread using the high range
at 400 rpm. Turned out very good.
So from that I've come to a conclusion that there is a problem in the coarse thread/low
speed train somewhere that is causing it to not start consistently at the same spot.
Even with the help of thread dial indicator.
I'm not sure where to start looking, I'm hoping some long term chipmaster users
may have some experience with this or at least be able to point me in the right direction.
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
The Chippy has the same feed box as the Bantam,and some of the gears are kinda tiny......the coarse threads being particularly feeble,and IIRC some gears having as few as five teeth,and being very liable to severe damage.....like for instance springing the shafts enough for gears to ride over each other............aaand my other advice...when engaging coarse threads,work the saddle by hand turning the chuck,and get some appreciation of the force being put thru those tiny gears.
 

Wally318

Aluminum
Joined
Oct 11, 2016
I looked in the manual and there are no gears smaller than maybe 15T except the pinion gear for the rack/saddle.
Walk over-maybe. I didn't take a close look at the gear-train output shaft, the last time I was in the headstock. And I haven't opened the front cover of the gearbox ... yet.
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
I used to buy and sell school and college lathes ...so maybe Im an extremist.......... one good feature of Colchester lathes is the gears are not very hard,and can bend teeth without breaking.I used to simply heat and bend back to shape using some formed punches................anyway,I wasnt exaggerating about trying your threading setup by hand turning the chuck......the force can be worrying.
 

Wally318

Aluminum
Joined
Oct 11, 2016
John, I'm not sure I understand the trying the set up by hand?
To what end?
How would that help me tell which part of the gear train is responsible for the inconsistent thread start?
Unless I were to do something like that with the rear cover off the headstock. and the front cover off then gearbox?
To observe any irregularities in motion.
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
Its just to get an idea of the force required to drive the saddle at the lead set,and its usually pretty substantial at a 2 1/2 tpi lead.....you can then go around the lathe and lubricate points of friction....under the saddle.,leadscrew and nut,and the thrust bearing........I suspect the inconsistent start may be wind up of the thread feed.
 

rklopp

Diamond
Joined
Feb 27, 2001
Location
Redwood City, CA USA
Sometimes it’s difficult to get the left-most threading lever to fully engage. This is the lever poking through the edge of the end cover that you shift downward to engage threading mode. I run the lathe at low speed in low gear when I do the shift to help make sure the shift goes all the way. Sometimes it takes a few tries even then. It also helps to put a little friction on the lead screw in a safe way as you shift. If the shift does not fully engage, the lead screw is just driven by friction and is not repeatable.

Your symptom could also indicate a broken shear pin or key in the end gears under the end cover. IIRC the shear pin is axial instead of the usual radial arrangement.
 

johnmontrose

Plastic
Joined
Sep 25, 2007
Location
UK
Are you using the 55/65 translation gear as an idler or is it set up for metric threading?

High range uses the toothed belt and pulleys and thus bypasses the geartrain.
 

rklopp

Diamond
Joined
Feb 27, 2001
Location
Redwood City, CA USA
Are you using the 55/65 translation gear as an idler or is it set up for metric threading?

High range uses the toothed belt and pulleys and thus bypasses the geartrain.
John's conclusion is consistent with geared threading not repeating location and belted threading repeating location properly. Check your end gearing and compare to the manual. Might as well look at the shear pin while you have the cover open.
 

Wally318

Aluminum
Joined
Oct 11, 2016
I'm using the 55/65 as an idler between the 33/66 and 66/33 combination.
No problems so far in the high range/belt combination while threading.
Both geared combination yeild similar results.
The end exposed gear train leaves little room for questioning.
The gears are meshed quite closely, with little backlash/play.
I suspect the problem is either in the headstock between the backgear and
pick off shaft/gear and past the bottom of the gear train @ the input shaft gear
and possibly inside the gearbox to the leadscrew/half nut.
 

johnmontrose

Plastic
Joined
Sep 25, 2007
Location
UK
You will have to do some systematic tests. Clamp a sharpie in the toolpost and cut a thread in each case, measuring its actual pitch.

Start with a 4tpi thread as that should be 1:1 from spindle to leadscrew.

Pick a number letter/combination that is used on both high speed and low speed threading. Cut and measure in one mode. Change the speed lever and the gearbox input lever to the other mode and repeat.

Tabulate your expected and actual results. If you do the tests with no thought at all, there are only 54 needed. If you think a bit and first confirm the ABC lever is doing what it should, you are down to 20.

Are you aware that for any pitch that is a multiple of the leadscrew pitch, you do not need the indicator at all (i.e. 4, 8, 12, 16 etc.)? That would eliminate the possiblity of a fault in the thread dial indicator.

Count the teeth on the 33/66 to confirm they are correct.

Confirm it is a standard imperial Chipmaster, not that imposter foreign Continental thing.

I am not sure where you are in the world, but if you are in US, try the 19tpi/38tpi combination as that is unlikely ever to have been used.
 

Wally318

Aluminum
Joined
Oct 11, 2016
First of all the lathe is the standard Imperial and not continental model.
Secondly the end gears have a 66T and 33T count.
Also am aware of leadscrew multiples and the manual says:
even numbered threads-any divsion/odd number @ numbered
divisions.
So the 20 cominations you referred to would be:
16-18-19-20-22-24-28 and 30 tpi in both belt drive and geared
settings=18 combinations as well as an A & C or B & C comparison of one of the above thread pitch settings?
The clamping of the sharpie to what purpose would that serve?
Confirming that a particular combination actually cuts the approprate numerical value? If so, agreed.
But I don't think that will help confirm the inconsistent cut
starts on succesive cuts.
Before your suggestions what I had sort of mentally planned was to take off the gearbox/drain it/remove the front cover.
Then reinstall it and observe the gears/shafts in motion.
To see if I could find the culprit. And if need be also with the headstock rear cover and headstock gears.
Also I thought I might try to cut a thread with the gear train ala metric threading. w/o disengaging the half nut to see the result.
 

johnmontrose

Plastic
Joined
Sep 25, 2007
Location
UK
One of the principles of any scientific endeavor is to separate your variables. So if you do not use the thread dial indicator at all (cover it up, ignore it), that has gone away.

For threads that are a multiple of the leadscrew pitch, you can engage at any point the half nuts drop in and the thread will pick up correctly. You do not need to look at the indicator at all, you engage the nuts using mechanical rather than mathematical principles. The reason I am trying to get you to eliminate the indicator from your investigations is that I have a gear off a Chipmaster thread dial indicator in my spare parts box that has a tooth broken off it.

The twenty combinations are low range 1-9 on the sliding lever under the chuck and A on the range selector and then any single one of the 1-9 in B and C. Repeat the first nine for belt drive ( 9 + 2 + 9 = 20).

Make sure the detent from the sliding lever is properly engaged with its hole - waggle the lever up and down to confirm it cannot move. Over time the holes can fill with swarf and that can lead to issues where it is not properly in gear.

Use the sharpie as the cutting tool. It is one removed from a scratch pass and a lot easier to wipe off the stock than turning it down smooth.

If you document the actual pitches cut, you can identify if there is a consistant, reproducable error across the whole range or if it is confined to any particular setting. Your idea of trying with full reversal is good as that will eliminate the half nuts from the bucket of possibilities. Shine a light under the apron and check for no obvious problem in the halfnut or its operating mechanism.

I would be doing everything I could to narrow down the problem before dismantling anything. Look in the manual at the headstock drawing and work out the ratio between the spindle and the output gear below it (the one that's concentric with the banjo pivot). Mark the spindle and the gear, manually rotate in low gear and confirm the ratio is correct.

Mark the input gear to the gearbox and the timing pulley. Mark the leadscrew and confirm that however many rotations of the gearbox input give one rotation of the leadscrew.

The idea above about the shear pin is a possiblity - with the power turned off, drive the carriage up to a stop using the leadscrew and see if you can rotate the spindle any further. That will tell you if there is any slippage in the drive train.
 

Wally318

Aluminum
Joined
Oct 11, 2016
John K. The force you speak of when rotating the chuck has little to do with the carriage or even the quick change gear box.
If you rotate the chuck with and w/o the half nut engaged, you
will find the force has to do with engagement of the back gear
in the headstock. On the Chipmaster it has a 10-1 ratio
compared to other lathes of 6-1 or so. Our Lance/Zubal lathe
that we had at work was a 7.14-1 and has less resistance in
low range than the chipmaster even though it was a 14 x 40"
lathe and heavier.
There is no wind up to speak of in threading except maybe
taking a deep cut on a 2 or 4 thread pitch.
And in turning even taking a 1/4 inch deep cut in low range
look at how much smaller the feedscrew is compared to the
leadscrew.
The feedscrew uses a wormgear to deak with the loads
and leadscrew size is to deal with wear because of metal on metal contact and greater contact area with the half nut,
 

Wally318

Aluminum
Joined
Oct 11, 2016
rklopp: I find that my Hi-Low range lever only occasionaly is
difficult to engage. When I went through the spindle timing belt
change. I decided to go through the outboard gear train completely and found there was no drive pin so installed
one, and i is still intact. The 8 tpi on the backplate was done
w/o the pin. And the 7/8 x 14 tpi done with pin in.
I'm having a hard time understanding what you were refferring
to/implying re: belt drive threading is consistently repeatable
compared to gear train threading.
Most lathes whether belt drive or gear head have an outboard
gear on the spindle that drives the gear train to the quick change gearbox. And so by definition are fully gear train
threading whether in hi range or backgear. No having any real issues. And as such gear train threading is beyond contestation.
 

Wally318

Aluminum
Joined
Oct 11, 2016
Now to impeach myself.
Yesterday I turned some alum rods 5/8" 7/8" and 1"
for 1" long threaded section of 8/9/11/12 and 14 tpi.
and tried each dia. with appropriate thread and did the threading with the half nut constantly engaged and retracting the tool at end of cut and using the reverse tumbler to get back to the start. The thread pitches being cut were not measurable
with my thread pitch gages. One even being something like 9-1/3 tpi. nor were they starting the cut at the same point.
Each one the same-all 5. So I thought about it for a while
and realized that there was incorrect gear ratio happening. At first imagining that maybe the gearbox had been overhauled at some point and gears were out of order and then realized it couldn't be in the lower shaft because all the holes lined up with the plunger. Then all of a sudden I realized that it was probably the 65/55 gear so I went to the shop to check and sure enough
it was. This aft. I re-turned the same blanks and started threading, this time with all the gears in the outboard train
in-line. I turned 2 threads using the 33T gear(top) and the other 3 using the 66T gear. First with the halfnut engaged but got inconsistent start points. And that was on the 8 tpi so shouldn't matter where I engaged, but switched to using the threading dial and from there turned 5 good threads in the above thread pitches.
I guess somehow I got into the erroneous thinking its an
idler gear not thinking that by engaging on both the 55 and 65T
gears I was changing the ratios. Lesson learned.
Thank you all for your input.
 
Last edited:

johnmontrose

Plastic
Joined
Sep 25, 2007
Location
UK
I am glad you have sorted it out. I am thankful you did not question me too closely concerning how I knew that was the problem...

Please could I pick up on one thing in your post, the part "using the reverse tumbler to get back to the start"?

I am concerned that by 'revese tumbler' you mean the lever that reverses direction of the feed and lead screws. If so, that lever should never be touched when screwcutting as there is nothing in the way it functions that assures the relationship between leadscrew and spindle is preserved during its operation.

The only way to reverse when threading is to switch off the motor and then start it up again in reverse (using the chromed arm at the back with the light in it).
 

Wally318

Aluminum
Joined
Oct 11, 2016
I totally agree on that one.
I came to the conlusion after re-reading the Chipmaster manual
section on threading dial.
So at the start I was essentially threading in metric mode, but with incorrect ratios.
Then, for my first tests I was threading in metric mode as well
as disengaging the leadscrew with the tumbler and not knowing
where I was re-engaging. Making it even more difficult to see the problem, or adding to it. Then coming to realize that at the
begining of the last set of tests-success.
 








 
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