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View Full Version : 6 TPI leadscrew on HF 42976 mill/drill - 0.16667 inches/turn - change to 5,8,10 TPI?



PStechPaul
11-05-2014, 08:08 PM
I had not used the milling table handwheel scales much until recently, and they appeared to have graduations at 0.001" intervals to 0.165", so when I wanted to position the table, say 0.437", I would take two full turns for 0.330 and then set it at 0.107". But it just didn't seem quite right and when I looked closer I saw that the last set of graduations past 0.160" had 7 divisions with the last space about half-width. Then I realized that the leadscrews must be 6 TPI and each revolution would be 1/6 = 0.16666.. Every three revolutions would be 0.500", but with my assumption of 0.165" I would be thinking it to be 0.495". For most of the work I was doing this 0.005" error out of 0.500" was hardly noticeable, but may explain why some of my dimensions were just a bit "off". I thought it might be backlash or just error in my measurement tools (HF calipers).

What I really need is a DRO, and I was going to make one for the quill depth (Z-axis), but now it looks like I could use one on the X and Y as well. I could also use the actual 0.16667" per turn or use 0.167 for the first, plus 0.166 for the second (0.333"), and the exact 0.500" for each three turns. Then I'd always be within 0.001" and the machine is realistically only good to 0.002" to 0.005".

But I wonder how difficult it would be to replace the leadscrews and nuts with 3/8" x 8 or 10, or 1/2" x 8 or 10, 5/8" x 8, or 3/4" x 5, and then change the dials to 100, 125, or 200 divisions? My X travel is about 8" and Y travel about 4", so I might be able to use a 12" Acme rod which is about $11 to $26 depending on size for B7 Alloy steel, and half that for plain steel, from McMaster:
http://www.mcmaster.com/#general-purpose-acme-rods/=ugvf55

They are probably left-hand threads so that a CCW rotation of the wheel pulls the table toward the wheel.

I'd also need a couple of nuts, which are about $3-$6 each for plain steel
http://www.mcmaster.com/#general-purpose-acme-nuts/=ugw42w

Here is the exploded view of the mill components. Not very clear, but it's what I have:

http://enginuitysystems.com/pix/tools/Mill_42976_4.jpg

Does anyone else have a milling machine (or lathe) which such wonky non-sensible leadscrews and dials?

Thanks.

[edit] This shows how a small Taig mill table and leadscrews are configured, in this case for CNC. I might add a couple of stepper motors and do that as well.
http://www.deansphotographica.com/machining/projects/mill/mill.html

Ballscrew upgrade
http://www.projectsbyzac.com/366/rf45zay7045-cnc-milling-machine/antibacklash-ballscrew-upgrade-on-rf45-zay7045-milling-machine-part-1

Doc Nickel
11-05-2014, 08:38 PM
Closest I have is a carriage stop I bought to adapt to my big lathe. It has a 20 TPI screw, but 20 divisions on the dial- each division is therefore .0025". Nobody can tell me why they did it that way- and it's not a botch or one-off, there's two other identical stops on eBay right now, all of which have the same dial.

It would have been just as easy to mark it with 25 divisions- the knob is plenty large enough- so each mark was .002". Or even have a half-size mark in between and make each mark a nice, typical, normal .001" per.

Doc.

Doozer
11-05-2014, 08:42 PM
The wheel elevation screw on my Boyar Shultz surface grinder is 6 tpi.
There are some (miter) gears that change the ratio to make it come out
on an even (if I remember) .050 per rev. Maybe you need to add some
gears. Planetary even. Or simple, some timing belts and pulleys.

-Doozer

PS- A DRO (or adapted digital caliper scales) makes a mill 1000%
nicer to use.

mattthemuppet
11-05-2014, 09:04 PM
for the amount of time and money you could spend on this, you're a long way towards a basic iGaging DRO and you wouldn't have to take into account backlash either. I'd rather have the DRO.

PStechPaul
11-05-2014, 10:06 PM
I found some 12" and 24" I-gaging DROs for about $36 and $45 plus about $6 shipping.

http://www.amazon.com/iGaging-Magnetic-Remote-Digital-Readout/dp/B003JULE4Y/ref=sr_1_77?s=hi&ie=UTF8&qid=1415241640&sr=1-77
http://www.amazon.com/iGaging-Magnetic-Remote-Digital-Readout/dp/B003JUII2A/ref=sr_1_65?s=hi&ie=UTF8&qid=1415242483&sr=1-65

Also a 6" unit for $28 + $10 shipping:
http://www.amazon.com/Milling-Machine-Digital-Readout-Remote/dp/B00DP495SQ/ref=sr_1_1?ie=UTF8&qid=1415242787&sr=8-1&keywords=milling+machine+digital+readout

on eBay:
http://www.ebay.com/itm/251614444105 6" unit $32.58 free shipping
http://www.ebay.com/itm/New-2014-SHAHE-0-300mm-Readout-digital-scales-External-display-/251563848009 300mm $45 free shipping
http://www.ebay.com/itm/Igaging-24-600-mm-Digital-Readout-Read-Out-DRO-w-Remote-Magnetic-LCD-display-/130985726217 600mm $47 free shipping
http://www.ebay.com/itm/Igaging-3-Piece-set-6-12-24-Digital-Readout-Read-Out-DRO-w-Remote-display-/130985862796 3 piece set 6", 12", 24" $113 free shipping

That last one seems pretty good. The 24" for my lathe, and the 6" and 12" for the mill. I can use a digital caliper for the quill Z-axis.

oldtiffie
11-06-2014, 12:08 AM
Here is the hand-book for the OP's mill:

http://images.harborfreight.com/manuals/42000-42999/42976.pdf

I think I could live with or get used to the 6tpi lead-screw - but I agree with the OP that it would be easier if the lead-screw was say 8 tpi. I could live with a right hand thread on it as well if needs be.

RichR
11-06-2014, 12:46 AM
The down feed for the quill on my milling machine is 0.080" for each turn on the crank. Not as bad as 6 TPI but still requires a little bit of thought to use.

oldtiffie
11-06-2014, 01:07 AM
Why rely on the lead-screw dial for the finishing cuts?

Use a long-ish dial indicator on the work or table wit the dial indicator mounted on a magnetic base fixed (magnetically) to the mill table or the cross slide. If the dial is accurate to 0.001" you should be very close (0.002") to the size - but I'd suggest using a good micrometer for measuring the work accurately.

David Powell
11-06-2014, 01:22 AM
Why rely on the lead-screw dial for the finishing cuts?

Use a long-ish dial indicator on the work or table wit the dial indicator mounted on a magnetic base fixed (magnetically) to the mill table or the cross slide. If the dial is accurate to 0.001" you should be very close (0.002") to the size - but I'd suggest using a good micrometer for measuring the work accurately.

My favourite lathe at home is a Standard Modern 9". I long ago equipped it with dial gauges for measuring rather than relying on reading handwheels. The one for the cross slide is adjustably mounted on an angle iron bracket. the one for saddle travel is magnetically mounted and that for the tailstock travel is also adjustably mounted on an angle iron bracket. I have a fourth permanently mounted in a tool holder to check runout when setting up, I usually have a 4 jaw on this lathe. I could have afforded digital readout arrangements but simply have not felt the need for the work I usually do on that machine. Regards David Powell.

dp
11-06-2014, 02:05 AM
I installed some of those iGaging devices on my lathe and they're kind of crappy. Not entirely useless but not something I'd use for precision work. They lack only repeatability and precision - other wise they're fine.

PStechPaul
11-06-2014, 05:07 AM
I might be able to put a 30 tooth timing pulley on the handle and an 18 tooth pulley on a dial with 100 divisions, which would be the same as using a 10 TPI screw. Or a 30 and 36 for 200 divisions like a 5 TPI screw. Then I'd need a dial with that number of divisions. I can probably rig up a digital caliper but the iGaging DROs will probably be easier to mount and good enough for my purposes. In fact, now that I know there are 0.1667" per turn, I can just keep that in mind and calculate how many full turns are needed and then what dial setting to use. Putting DROs on a little mill like that might be like putting lipstick on a toad. :rolleyes:

oldtiffie
11-06-2014, 05:56 AM
Good.

There is a lot of that "solving" of so-called "difficult" problems by bringing in bigger problems or methods when the original problem might not have really been a problem at all if the "solution" had been reduced to its simplest form - or eliminated all together.

lwalker
11-06-2014, 07:36 AM
I installed some of those iGaging devices on my lathe and they're kind of crappy. Not entirely useless but not something I'd use for precision work. They lack only repeatability and precision - other wise they're fine.

I wonder if there is a wide variation in quality. I have had them on my mill for a couple years and my only complaint is that they are slow to update. I work around that by slowing down when I get near the end of a cut, but that's not really a big deal. When I first installed them I tested repeatability/accuracy using the only thing I had on hand: a 1-2-3 block that I measured with a micrometer. I found that they were spot on.

I could repeat the test now to see if contaminants may have gotten in and reduced the accuracy, but I haven't had a problem with the parts I make fitting together.

Stepside
11-06-2014, 10:10 AM
I would wonder if the thread was actually a Metric thread with Inch dials? A Harbor Freight Lathe that a friend owns is .040 travel for each turn of the dial.With one MM = .0394 they might have decided that was "good enough for hobby work".

mattthemuppet
11-06-2014, 12:30 PM
Putting DROs on a little mill like that might be like putting lipstick on a toad. :rolleyes:

nope, that would be putting digital calipers on a HF XY vise :D

loose nut
11-06-2014, 03:46 PM
Putting DROs on a little mill like that might be like putting lipstick on a toad. :rolleyes:

Keep the lipstick handy, you never know when you might want to kiss that toad.

Paul Alciatore
11-06-2014, 07:38 PM
That was my first thought and yes, HF does that kind of thing a lot.

But 6 TPI translates to a 4.23333... mm lead. The closest round number would be 4.25mm and that is really a bit odd. Possible, but really odd.

Now, no mention was made of how the 6 TPI figure was measured. What distance was the table moved and what was used to measure that distance? Perhaps it is a 4mm lead screw and six turns will move the table 24mm or 0.945". That would be an easily noticeable difference from a full inch (almost 1/16"). So I doubt it. It is probably 6 TPI.

Why 6 TPI? Well, 6 TPI is a standard thread for 3/4" or 7/8" Acme rod. Probably just that simple. If you want a more sensible pitch, you have to use smaller or larger rod or make it special.

I have to agree that a DRO is probably the best idea here.



I would wonder if the thread was actually a Metric thread with Inch dials? A Harbor Freight Lathe that a friend owns is .040 travel for each turn of the dial.With one MM = .0394 they might have decided that was "good enough for hobby work".

oldtiffie
11-06-2014, 08:25 PM
Really, the best thing to do is to use the "depth" or "step" functions on a good digital caliper to "rough it in" and then use the jaws of the caliper to measure the job.

As you are now well within the 1/6" "to go", set the lead-screw dial to zero (which is calibrated in 0.001") and use the dial and a micrometer as normal.

Its just a matter of remembering the sequence.

PStechPaul
11-06-2014, 10:17 PM
For most purposes where actual milling is being done, I agree that it is easy enough to mill to within 0.063" and then take an exact measurement with a micrometer or caliper, and then set the dial to zero (or to the additional amount needed), and use the dials to get within 0.002" or better without crossing the transition from 0.1667 to zero. But for hole drilling operations, for instance, it is better to be able to rely on the dial readings (or the DRO of course) to position the drill to the proper coordinates, which may be several inches apart. It may be a good idea to mark the dimensions on the drawing from the datum point to each feature with the dial reading, so 0.375" would be two full turns (0.333") plus 0.042". This can be done ahead of time in the comfort of the office rather than the sometimes distracting environment of the shop.

But I think it would be a good investment to get the I-gaging DROs, and the three sizes for less than $115. :)