 Ebay is a Must... (unless you are rich!) |
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Ebay is a Must... (unless you are rich!) |
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While somewhat tongue-in-cheek, this page's title says it all. Since I decided to go with the best components, I had to face up to the projected costs. If you have $$ to spare, skip this page and go order your stuff from THK. Expect to pay $500 per axis for the ways, and $500 to $1000 per axis for custom ground screws and bearings! Oh yes, let's not forget a quality HF spindle at, oh, around $6000. Cheaper spindles are available, though, such as a Sherline ER16 spindle, or a Harbor Freight R8 mini-mill spindle.
 An assortment of ebay "kills" |
When faced with the frankly scary costs of precision
components, the only real alternative is to lurk ebay,
and get really good at setting up searches for the
desired items. I also recommend a good sniping
program like Auction
Sentry. While sniping feels dishonorable, it is a
fact of life in the online auction world, and you may as
well be armed with the best. For those wondering, to snipe an auction is to either manually (or automatically, with Auction Sentry) insert a kiler bid with seconds remaining. If you guess right, your bid will win the auction, and leave no reaction time for bid responses from your competition. This hopefully avoids the back and forth bid wars which can really jack up the price. Ebay is excellent for obtaining several critical components: THK recirculating ways (NSK and Thomson also make them), ground ballscrews, expensive bearings, and other sundries. Spindles on Ebay are not so common, but I did score one which we shall see momentarily. |
The blocks themselves come in different flavors, short/long, tapped holes, through holes, etc. Look for either the HSR prefix, or the SHS prefix. Both of these are 4-way rails and blocks, with the SHS having polymer spacers between balls, creating a better lube environment and quieter running than the HSR. Thus, the desirable rails/blocks are named HSR20, SHS15, etc. I would suggest a requirement for SHS or HSR blocks for the Z axis, and preferably all around. Prefixes SR and SSR are designed to take loads perpendicular to the rail, and would be OK for horizontal use, but not vertical. THK Prefix summary of the common
block types |
Unfortunately, the SR and SSR blocks are
far more common on ebay than HSR and SHS. Rails - The length of the rails vary; getting the correct length is desirable, but they can be cut with an abrasive cutoff saw. Blocks within a series are interchangeable. In other words, any SHS15 block will ride any other SHS15 rail. All blocks above the 12 series (except RSR) in size are removeable; the balls are contained within the block and will not fall out. Below 15mm, the balls will fall out of the blocks if they are removed from the rails. This frankly sucks. Don't use blocks smaller than the 15 series unless you must, as this makes assembly and disassembly very difficult. I have had balls fall out, and while they can be reloaded, it is tedious and unpleasant. Of course, several of the balls will never be found as they scatter across a shop floor like frightened mice. Grrrrr. Avoid the RSR series of block and rail; these have only a single set of balls per side rather than two, and cannot handle the loads that the other series can. The balls also are not retained if the blocks are removed from the rails. |
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Don't hesitate to make purchases of these rails and
blocks. If they are priced right, but they don't fit your
application, you can turn right around and auction them
back on ebay, often at a profit. 4 ea. SHS15
blocks and 2 ea 18" rails: 8 ea. HSR20 Blocks, NIB |
 One of three identical ground 12mm screws from THK, C3 accuracy (0.0003" variance / 300mm), zero-backlash preloaded nut. 3
ea. THK ground ballscrews, identical, 15" long: |
Ballscrews: Not as
common as THK rails on ebay. Beware of rolled ballscrews,
which can be purchased new through MSC and similar for
decent prices, but simply aren't that much better than
ACME leadscrews. It's tough to find ground screws in the correct length and diameter. Most are too big, being designed for large machines. Look for 12mm to 20mm diameter, pitch 4mm or 5mm, length as required. 16mm diameter is ideal. Almost all of these ballscrews are metric in pitch, diameter, and input shaft size, being from Japan, so be sure your CNC control can be set up with metric leadscrews. This shouldn't affect the output. A good controller can digest metric pitches and output inch product all day long. Likewise, the direction of travel is irrelevant. |
 Be sure the ID of the bearings is correct for your ballscrew journal. |
To go with the ballscrews, you will need support
bearings and blocks. Often, the ballscrews come to you
with the correct block. If the ballscrew you are bidding
upon has a fixed bearing block pre-installed, this is a
choice item, and don't hesitate to value the block at
least as much as the ballscrew. If not, you can make your
own, or snap up a tidbit like this, advertised as an "NSK
bearing thing, NIB". In actuality,
this is a costly fixed support block for ballscrews. Inside
are two angular contact bearings of great precision. When
I saw this particular auction, I basically foamed at the
mouth, and crossed my fingers. Luck was with me. 1 ea. NSK fixed support, c/w lock nut
and hardware, NIB: |
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While the Flashcut servo system I ordered came with three nice servo motors, they don't hold a candle to these Parker NEMA23 motors, either in quality or power. The guy I bought from had 75 of these as part of a severance package from his motion-control company. Sorry for this gloat, but I snapped up 5 new in box, Parker SE-232-A-E-N-FL-N deluxe rare earth brushless servos for $60 each. These retail at $600+ each. |
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Obviously, be aware of what types of
servos (voltage, brush/brushless) your controller can
use, and bid accordingly. 5
ea. Parker brushless servos, NIB: Muhahahaha! |
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Finally, the spindle. I will go into details later about spindle woes, but I did buy, after much though, the KaVo 4041 HF spindle shown in the middle, along with a KaVo HF controller, from a fellow in Colorodo. He had 3 others. The spindle has new bearings. 5,000 to 50,000 RPM. You can hold this spindle in your hand at 50,000 RPM and feel essentially zero vibration; a real top-shelf unit. |
 KaVo 4442 HF spindle controller |
1 ea. KaVo
spindle + controller, mint: I meant no gloating (well a little perhaps), but simply wanted to show what patience and persistence can do. There's some bargains out there, guys!
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Blocks - You will
want blocks known as "4-way", which support
loads evenly in all directions. Since I know THK rails
and blocks fairly well, and I think they are the best,
may I make some observations... First, the rails and
blocks are named by the width of their rails. The 25
series of THK rails have a 25mm wide base, the 15 series
15mm, etc. 20mm rails are a good all-around size for a
medium benchtop mill. I am using 25's for the Y-axis,
15's for the X (due to tighter space requirements) , and
25's for the Z. Both the X and Y axis use 4 blocks, and
the Z will use a pair of long blocks.
