The Sherline Miniature Machinist's Newsletter

Number 58, January 15, 2013

http://www.sherline.com

 


Customer Project of the Month

A fixture for checking .22LR match rounds for concentricity/Bill Young

Sherline customer Bill Young came up with a way to mount a dial indicator and run it against a .22LR cartridge to check the projectile for concentricity. When firing for accuracy, it is important to assure each bullet is as perfect as possible. Rounds that do not fall within the accuracy requirements when checked are set aside to be fired during practice, and only the best rounds are used in competition. This simple fixture makes checking each round quick and easy.

We are not presenting dimensioned plans here because Bill just used what material he had on hand, much as you probably will. It should be pretty easy to come up with dimensions from once you see how it works and keeping in mind his basic requirements stated below in the letter from Bill that follows.

1. (Above) Concentricity checker in the “closed” position.

2. Checker in the “open” position, seen from the back side.

3. Concentricity checker in the “open” position with indicator feeler in place.

4. Checker in “closed” position with indicator tip on projectile surface.

Here’s what Bill had to say:

“As you can see from the photos, this was built primarily from scraps and other stuff lying around just crying to be put to use. The design is taken from pictures on-line of a commercial product. I kept some parts close to the original and others were seriously modified to cut down on machining time. The only items purchased for this task were the bearings, the grommets, and the shrink tube for the handle. Everything else was parts I had on hand including the gauge. Speaking of the latter, this was one of the major departures from the commercial version: they used a normal dial indicator instead of the test indicator that I used. I had a dial indicator available and could have used it, but I thought that it would involve more pressure against the bullet and decided against it.

I let the handle that holds and revolves the bullet under the indicator have a long angular motion so that it could lie back like it is shown in photos #2 and #3 and thus make it easier to handle the small .22 LR bullets when putting them into the device. (Experience with the device has shown me that all the rotation I built into it is a very good thing, as most times if you leave the device “open” and put the cartridge in, it will stay in the device until you swing it to the closed position. (See picture #4.) This facility is easily obtained by correctly adjusting the height of the test indicator so that its resting position is not so low that the cartridge will fall out of the intended testing position and not so high that it doesn't properly make a measurement.)

I used the shrink tube to be the rubber on the turning rod and the grommets are carefully spaced as shown in both photos. The bearings (4mm x 8mm) are cheap ones costing $1.00 each at the local model car shop. I took a couple of gauges in with me to check them out and got four fairly good ones. When I put a length of ¼" brass rod in the device I get less than .0005" deviation on the test indicator. A good friend informed me that I could get a good bit less noise if I used class 9 bearings and he also told me where to get them. The price for them was about $5.00 per bearing, but I haven’t decided whether it’s worth it to me.

The white spacers shown are made of acetal trimmed to the correct OD and drilled to the proper ID. I also cut them off the original piece of round stock on the lathe carefully to the necessary thickness. There was an especially important OD on the one where the rim of the shell was against the brass; it didn’t want to be too large and contact the rim of the shell.

One of the lathe techniques of special interest was the fact that brass rods on which the bearings sit had to be 4mm in diameter to fit the 4mm ID of the bearings. This necessitated I go the extra mile in setting up the lathe headstock to be very accurately parallel to the bed of the lathe, a new process for me. I also jumped through a few more hoops to square up the mill when drilling the holes in the large hunk of brass to keep the rods parallel. These activities certainly cut down on the “noise” and they delivered a lot of precision. Actually, the commercial site where I found the design offered their device in two grades: one at $250 and the other at $365. Only the latter is guaranteed to deliver .0005" precision! So, if any of you builders are interested in constructing this device, you can build it cheaper and better if you take the time.”

—Bill Young

You can see more workshop projects on the Sherline web site at www.sherline.com/workshop.htm and more gunsmithing projects at www.sherline.com/guns.htm.


Shop Tips

Increase in body diameter when using knurling/Joe Martin

Knurling is often used to achieve a professional-looking finish that provides good grip on the end of a shaft. It can also be used to increase the size of a part for a press fit. Because the knurling process actually embosses the metal, upsetting the surface and raising the pointed lines, a shaft that is a sliding fit in a hole now becomes a press fit. How much of a press fit depends on the size of the part and the number of teeth on the knurl. The chart and copy below, taken from Chapter 7 on knurled finishes in Joe Martin’s book, Tabletop Machining (P/N 5301, $40.00) give some guidance on how much a diameter will increase under different circumstances.

Approximate Increase in Knurl Diameter

TPI

Tooth Angle

Straight

Diagonal

Diamond Male

Diamond Female

12

90°

.034"

.034"

.038"

16

90°

.025"

.025"

.029"

20

90°

.020"

.020"

.023"

.014"

25

90°

.016"

.016"

.018"

.011"

30

90°

.013"

.013"

.015"

.009"

35

90°

.011"

.011"

.013"

40

90°

.009"

.009"

.010"

.006"

35

70°

.014"

40

70°

.012"

.010"

50

70°

.009"

.009"

.010"

.006"

60

70°

.007"

.007"

70

70°

.006"

.006"

80

70°

.005"

.005"

 

Two types of knurls

There are two basic types of knurls available. One is based on TPI (threads per inch) and the other is based on diametral pitch, which is how gears are defined. Machinery’s Handbook recommends using the DP, but if you try to purchase DP knurls, you will find that what is really available are TPI knurls. The end result is most people use the TPI type knurls because they are readily available and less expensive. The Sherline knurling tool uses knurls that are ½" in diameter with a 3/16" center hole. These knurls are based on the TPI system.

Knurls for press fits

When the specifications of a knurl are placed on a drawing in detail, there is always a chance the desired outside diameter cannot be met and produce a good knurl at the same time. The process has too many variables to be that exact. You can’t get in too much trouble if the knurl is put on for appearance or grip, but if the purpose of a knurl is to control a press fit, it may take some experimentation. This can be accomplished with simple test parts that can be pressed together and taken apart. A good setup would involve tooling that aligns the parts involved as they are pressed together. If these parts are for a customer, the customer should determine what is acceptable before the parts are run. The OD of the knurl may have less impact on the amount of “press” required when joining two parts than one would believe. The shape of the knurl at the OD determines the press as much as the outside diameter because a greater amount of material may be displaced during the press. Straight knurls have to be more carefully selected for the job if they are to be used for enlarging a shaft diameter for a press fit. In closer tolerance production work, special knurls have to be made to accomplish this, so the finer the knurl, the better your chances of success. When you are dealing with “one-off” parts, it may be easier to machine the part in one piece rather than pressing the two parts together.


Product Spotlight

Knurling Tool, P/N 3004 ($75.00)

Knurling tool and T-nuts. The standard included set of knurls produces a diamond pattern but straight knurls are also available.

Using the Sherline Knurling Tool

Sherline’s knurling tool can add that “professional” look to your parts. Knurls are pressed or embossed into the surface of a part rather than cut. Material is moved from one position to another under pressure. Doing so can involve quite a bit of force, which is hard to achieve from a single direction (known as “bump knurling”) on a small machine. For this reason, Sherline developed a tool holder that squeezes two knurling wheels against the part from opposite sides, relieving stress on the machine itself.

The holder consists of a right and left side that evenly tighten down on your part, creating a knurled pattern. The holder is supplied with one set of spiral knurls that creates a diamond pattern. Other size knurls for diamond or straight patterns are also available as options. The tool will handle diameters up to 1" (25mm).

The knurling holder is designed to mount directly to the crosslide’s tee slot groove. The tee nuts that run in the groove should only be tightened enough to eliminate “play”, but not so tight as to keep the holder from moving freely in the groove. This allows the holder to self center on the part to be knurled. (We recommend using aluminum for your first practice knurl, approximately 1/2").

The part to be knurled or an experimental part should be running true with a chamfered corner at the end of the knurled section. Adjust the top and bottom clamping bolts so the knurls are lightly touching the part without the spindle turning. The knurls should be located at the beginning of the section to be knurled.  Apply a liberal amount of cutting oil to the knurls and have the spindle run at a slow speed (approximately 500 RPM for 1/2" diameter of soft material).  Start tightening the top and bottom clamping bolts evenly, one at a time until the knurls are engaged with the work in a positive manner. Back the knurls off the part with the feed handwheel. Stop the spindle and carefully examine the quality of the knurl. It should be full depth, clean, and sharp. The finished diameter should be larger than the starting diameter by approximately the amount shown in the chart on the next page. If not make the necessary adjustments.

 Complete instructions for use of the Sherline knurling tool can be found at www.sherline.com/3004inst.pdf.

The above photo from Tabletop Machining shows a part after knurling. The tool is fed onto the rotating part using the leadscrew handwheel.


Did you know?

• The current month’s “Internet Special” can be found at www.sherline.com/special.htm. Each month a selected product is offered at a savings of 20%. This month the featured products are inserted tip carbide tool holders.

Continuity, loyalty and commitment are rare at many companies today. Not so at Sherline. Average length of employment at Sherline is about 12 years, and this is despite a long-time employee retiring recently and the addition of several new hourly employees in assembly in the past few years. On the salaried side, the average is over 28 years, with Karl and Charla both coming up on 35 years service. Craig is the “new guy” dragging down the average with only 17 years next month, although before coming to work here full time in 1996 he worked as an outside contractor doing all of Sherline’s ads and literature over the years starting with the line art for the Sears instruction manual about 38 years ago.

Sherline history—Where does the “Sher” name come from?

 Ron Sher

Sherline is named after Ron Sher, the owner of Sherline Pty, Ltd, the original company to produce the lathe in Australia before Joe bought it and moved production to the USA in 1974. The lathe was originally designed by Australian Harold Clisby using stiff extrusions to overcome the inherent design weakness of the then-popular Unimat lathe. Joe Martin introduced a longer lathe, three mill models, more powerful motors, digital readouts, cnc control and a huge line of accessories.


Upcoming Model Engineering Shows

2013…

Cabin Fever Expo, York, PA, April 12-14, 2013. The east coast’s largest annual model engineering show has moved from its former January date. See http://www.cabinfeverexpo.com for more information.

North American Model Engineering Society Expo (NAMES), Wyandotte, MI, April 20-21, 2013. This is the oldest Model Engineering show in the USA and where the Joe Martin Foundation presents its annual award for excellence in metalworking craftsmanship. Sherline will also have a booth featuring our tools at the show. See http://www.namesexposition.com. This show retains its traditional late April date.

Send us your show details and we will post them here…


Joe Martin Craftsmanship Foundation News

• New Craftsmen added to the museum

The foundation’s Craftsman of the Year award winner for 2013 was just announced. Aircraft model builder Guillermo Rojas-Bazan of Argentina and now a resident of Royal Oak, MI has been selected as this year’s winner. A new page on him was just added to the Joe Martin Foundation’s on-line Craftsmanship Museum web site at www.CraftsmanshipMuseum.com. CLICK HERE or on his name above to view his page and learn about our 17th winner of the foundation’s top award.

—Also added to the on-line museum this past month is watch maker George Thomas. He is most noted for his work on historic watches owned by Abraham Lincoln and George Washington, making custom watches of his own from scratch and most recently founding the Towson Watch Company in Towson, MD with his partner Hartwig Balke.

• New Museum Displays—A temporary display of computer memory and some vintage mechanical calculating devises has been added to the museum display in Carlsbad, CA. It starts with slide rules and early electronic calculators and goes through a 4KB core memory board with thousands of wires coming out of it up through a Processor Technology computer that was Sherline’s first computer. Other examples of storage from various floppy and zip disks through CD, DVD and a thumb drive are included. We are still seeking the donation of a nice abacus to start the display timeline.

• Group Visits—A group of founders of the sport of R/C flying and old friends of Joe Martin gathered at the museum on January 9th. Present were Chuck Hayes, Joe Bridi, Jerry Bruce, Benny Taguchi, Jerry Nelson, Larry and Peggy Leonard. A tour of the Sherline factory was followed by a catered lunch and tour of the museum hosted by Joe Martin.

• Docent ProfileJim White

  

Jim at work at the museum front desk. (His Mustang can be seen outside the window…)

For the past several months, Jim White has been volunteering his time each Tuesday at the museum. He was born in Los Angeles and was interested in architecture in school. He served in the US Air Force during the Korean War in the engineering department. After leaving the service he worked as a mechanical engineer for an aircraft parts manufacturer and then as a sales rep. for another aircraft parts company in Southern California before moving to Seattle, WA to work at the Boeing Aircraft Company during development of the 747. Six years later, he moved into the industrial market as Regional Sales Manager for a company that serviced a wide market including four northwestern states. Twenty years after that he retired and continued working part time as a small business consultant.

His special interests include woodworking, the stock market and model cars. He considers himself a “true car nut” having owned more that 25 automobiles. His current ride to and from the museum is a rather wicked looking black-on-black Saleen Mustang Convertible.

Anyone interested in volunteering your services at the museum, please contact craig@craftsmanshipmuseum.com or call 760-727-5857 and ask for Craig.