Those desktop mills -- 16 TPI or metric?

I'm thinking about getting one of those small mini-mills at HF, Grizzly, Homier, etc. and converting it to CNC for machining PC boards. The ones
I've seen at HF have X-axis handwheels with 62.5 graduations. One revolution moves the table 0.0625". 16 revolutions moves the table 0.0625*16=1". So it seems the threads on the X-axis leadscrew are 16 TPI. But I thought the strange 62.5 graduations was because the leadscrew was actually metric and retrofitted with a strange handwheel for the US market. 0.0625" converted to metric is 0.0625*25.4=1.5875mm. If the leadscrew was metric, shouldn't one revolution be some nice round mm number? Am I misunderstanding something? I plan on adding one of those cheap digital linear scales so it doesn't really matter, but I just want to make sure.
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AL sez: "I'm thinking about getting one of those small mini-mills at HF, Grizzly, Homier, etc. and converting it to CNC for machining PC boards... and, Am I misunderstanding something?"
Yeah, AL, likely you are misunderstanding quite a bit. IMO, you are letting yourself in for a lot of grief in attempting to convert such a machine to CNC.
Bob Swinney
The ones

market.
adding one of those cheap digital

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Right on there, Bob.
I see in HSM magazine, Sherline is advertizing a CNC ready mini mill - Just add the PC and stepper drivers. I've not used Sherline but I know its an order of magnitude higher in quality.
Karl
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    Not to say that it can't be done.
    Steve Stallings -- the operator of the dropbox, has converted two of them now (with the assistance of another member of CAMS, who owns the larger of the two.)
    The conversion, in part, was done as a testbed for some rather nice stepper drivers which Steve is building and selling. One of the machines was demonstrated at Iron Fever last weekend.
    I've seen both machines (the smaller one pre-conversion) at a recent CAMS meeting.
    Yes -- it required some machine tools to make some of the conversion parts. However, I think that they were within the capability of the un-converted mill, and a lathe of similar size range.
    Enjoy,         DoN.
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You may be disappointed in the lack of precision of these mini-mills. Not that you can't convert them to CNC (several people have), but you will have to make quite a few modifications to the machine *PRIOR* to the CNC conversion: supporting the z-axis for greater rigidity, replacing the screw-adjusted gibs with tapered gibs, possibly replacing the z-axis rack-and-pinion feed with a screw feed, etc.
But to answer your question...

That is correct: the X & Y axis leadscrews are 16 TPI (except for the MicroMark version which has 20 TPI leadscrews).

market.
Nope. With leadscrews of 16 TPI, one revolution = 0.0625" of feed. So in order to graduate the handwheel into 0.001" increments, you end up with 62.5 divisions. Very goofy indeed!
MicroMark advertises their version of the Mini-Mill as "True Inch" which is a bit of a misnomer (since the 16 TPI lead screws on other mini-mills are also also in true inches). But the 20 TPI lead screws on the MicroMark allows them to graduate their handwheels with 50 even divisions (50 div * 0.001"/div = 0.05" per rev = 20 TPI).
The good news is that there is a company called www.LittleMachineShop.com. They have a complete stock of all replacement parts for these mini-mills, including true 1.5mm pitch metric leadscrews and 20 TPI leadscrews (and all the related parts, including the handwheel dials).

I'd say that adding some form of DRO is a requirement for the mini-mill, even if you are not doing a CNC conversion. These mills are not very repeatable just using the dials.
Regards, Michael
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DeepDiver wrote:

I don't know about the others but the Sherline machines have tapered gibs and screw feed. http://sherline.com/MillExPN.pdf
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market.
They will work nice for this, but be aware that converting a machine is a considerable Job that will require machine tools to make the parts. The cost of a conversion is also much higher then what it appears, a lot of smaller items are often overlooked when estimating the cost. The 16 Tpi of the original Leadscrews are not a problem the software can be calibrated to match, any pitch inch or metric dose not matter here. However using acme Leadscrews may impose some limitations on the performance of the cnc you are building it is difficult to control backlash and many people convert to Ballscrew for better performance. Most inexpensive drives for cnc will not be able to make use of the digital readout, they are not needed anyway the cnc software will display positions. Good Luck
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