A grinder for HSS toolbits

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Harold_V
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A grinder for HSS toolbits

Post by Harold_V » Thu Jan 25, 2007 3:55 am

I've commented often on grinding toolbits without the use of a rest. The process requires a little skill, but that comes quickly with practice. The benefit is full access to the wheel, which, if you're grinding chip breakers, can often be a necessity. The work rest often limits your ability to address the wheel with the tool such that you can view the grind as it progresses, and limits the degree of mobility available. In both cases the end result is often a poorly configured tool or chip breaker, with less than good machining characteristics, and lots of frustration. Should you choose to explore this method of grinding tools, you'll come to realize that tools are easier to grind, with or without a chip breaker.

Being able to see clearly what you're grinding is key to success. Leaning over a low mounted grinder, trying to view the grind when it's wrapped towards the back of the wheel to avoid the rest, tends to be troublesome, but there's a simple solution. A small grinder, with no rest, can be mounted easily to a wall or on a rigid post, with the center of the wheel at approximately chest height. The height of the grinder is particularly enticing when grinding a larger tool (½", for example), which takes considerable time. The height allows for the operator to stand erect, eliminating fatigue from leaning over.

Considering you have no point aside from the wheel to steady the grind, the wheel should run dead true, with no bounce. Once it starts breaking down, it's desirable to dress it with a dressing stick to keep the surface smooth and free cutting. It's been my experience that once a wheel starts shedding media, it tends to be somewhat selective, shedding more in one area than from the balance of the wheel, quickly forming a wheel that is not round. Grinding with such a wheel becomes almost impossible by the no-rest method. Remember----the wheel becomes your rest. It must run true.

The typical bench or pedestal grinder rarely has a wheel that is acceptable for successful grinding of HSS, and one that is acceptable can be difficult, if not impossible, to obtain. In order to get around that problem, wheels that are intended for cutter grinder and/or surface grinder use are an excellent choice, although they won't mount directly to the typical bench grinder. These wheels are sized 7"x ½" x 1¼", are readily available in a broad selection, and are stocked by most machine shop supply houses. They're also commonly found on ebay. A little work in building a dedicated grinder that provides for mounting them can be accomplished at reasonable cost.

To build such a grinder, a ½ horse, 3,600 RPM motor with ½" shafts protruding from each end is more than adequate. Larger is better, but not necessary. By choosing a motor with double shafts, adapters with an integral fixed flange and locating boss can be made that accept the 1¼" bore wheels, providing a grinder that can be equipped with a roughing and finishing wheel, or even one side fitted with a diamond wheel and table for grinding brazed carbide tools. If the grinder is used in that fashion, it's desirable to be able to reverse the motor, and to have a double pole double throw switch for turning the motor on, with or without the coolant pump. Diamond wheels for such use should not be operated dry.

Wheel guards can be fabricated from large pipe, and mounted with appropriate brackets. If there is no place to attach the brackets to the motor, the motor can be mounted to a base plate, which in turn would be mounted to the wall or pole. Some welding would be required to enclose the inside of the guards. ¼"' plate can be used for the purpose. Note: The typical 7" grinding wheel has a maximum recommended speed in keeping with a 3,600 RPM motor. To assure your safety, limit the guard size such that a larger wheel can not be mounted.

Considering you grind lower on the wheel, the bottom edge of the guard is best held back such that it does not interfere with your grinding. The top end can closely approximate the typical grinder, considering you now use the centerline of the wheel instead of the upper portion, so visibility is not an issue.

The importance of running vibration free can't be emphasized too much. Thus, adapters that are adjustable to a small degree are advised. Boring the hole for the shaft should be accomplished concentric with the balance of the adapter, with a slight amount of oversize provided. The adapter then has a series of two sets of three holes spaced @ 120°, drilled and tapped either 10-32 or ¼"-28 where they will contact the shaft of the motor near the end of the motor shaft, and again near the bearing of the motor. Wall thickness of the adapter where it mounts to the shaft should be no less than ¼", and set screws should finish below the surface to avoid harming one's hand should they use the adapter to stop the grinder. The adapter can then be dialed in perfectly by adjusting the six socket head set screws while monitoring the adapter with a DTI. Excellent shop practice should be exercised when making the adapters. They should be dead concentric and perpendicular in order to minimize vibration and runout. The opposite end of the adapter should have a flange diameter of 2¾", no less than 3/16" thick. The flanges should be relieved so only the outer ½" bears on the blotter. The movable flange should also be no less than 3/16" thick, and have a small shoulder where the nut bears. It should be bored with minimal clearance on the threaded shaft so it is self aligning, minimizing vibrations.

The locating boss portion of the adapter (the part that accepts the wheel) should be held to 1.250" +.000/-.002" in diameter. That allows for precise centering of the wheel without risk of cracking should the bore not be perpendicular to the wheel. Length of this diameter should be held short of the widest wheel you intend to run. Considering these wheels are available as narrow as ¼", you may wish to keep the length as short as 7/32". Likewise, you should allow for enough thread so that a wheel that is ¾" or 1" wide can be mounted. That, along with the thickness of the nut of choice, will dictate the length of the thread you'll generate. I've found that a ¾"-16 thread serves very well, but any thread larger than ½" in diameter would be adequate.

Provide a set of shallow flats on the body diameter of the adapter to allow for a wrench to remove stubborn wheels. Hand holding the wheel while tightening the nut with a wrench will provide more than adequate holding power, even when reversing the motor. The motor should not be capable of instant reverse, however.

Commercial nuts may or may not have threads that are perpendicular to the faces of the nuts, so it's a good idea to single point a thread that is short of the length of the nut you choose, then mount the nut on the thread and face both sides so it will run without attempting to tip the wheel. Break the corners of the nut, or if you desire it to mount the nut in only one direction, face the corners until you have a shoulder that's about 1/64" long, then chamfer the corners of the opposite side. Deburr well.

Always run proper blotters on your wheels, and never mount a wheel that doesn't ring when tapped. Any wheel found to yield a dull thump instead of a clear ring should be destroyed to avoid an unsuspecting person from mounting a wheel that could very well kill them.

Always stand clear of grinding wheels when spooling them up. ALWAYS! Even when you know that the wheel was good when mounted. You never know when a wheel might be bumped and cracked by the activity of others. Allow the wheel to run a half minute or so before standing in front of it. Wear eye protection.

If you've been struggling with tool grinding, give this process a try. You may be pleasantly surprised. I know I was when I was introduced to it by my peers.

Harold

Edit: corrected spelling
Last edited by Harold_V on Wed Apr 15, 2009 11:47 pm, edited 2 times in total.

RET
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Post by RET » Thu Jan 25, 2007 10:31 am

Harold,
As always, very good. I'm always impressed by your choice of words and the clarity of your language. Clear, simple, to the point and always dead on topic.

One thing, and I'm sure you know this, but others might not. There are two types of bond (the matrix that holds the grinding particles) used in grinding wheels. The wheels you are recommending are all likely to be vitreous bond and any wheels of that type MUST ring to be safe.

The other type of bond used in wheels won't ring even when the wheels are new because it is a resin or other slightly flexible material that dampens any vibration. Vitreous means glassy and is hard & brittle whereas the resinoid bond while rigid, has a little "give" to it.

Richard Trounce.

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Victor_R
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Post by Victor_R » Thu Jan 25, 2007 12:54 pm

Excellent write up as usual Harold! I can only think of one thing to add which is that the left side of a dual shaft grinder should have a left handed arbor thread with matching nut, lest it work itself loose during use.

Victor
The machines are gaining ground upon us; day by day we are becoming more subservient to them ~ Samuel Butler (1863)

Harold_V
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Post by Harold_V » Thu Jan 25, 2007 3:06 pm

RET wrote:There are two types of bond (the matrix that holds the grinding particles) used in grinding wheels. The wheels you are recommending are all likely to be vitreous bond and any wheels of that type MUST ring to be safe.
Truth is, there's five kinds of bond, which I touched on briefly in my post on wheel selection. The five varieties are vitrified, the wheel that is of most interest to the home shop type, and used for grinding of HSS, shellac, rubber, synthetic resin (resinoid) and silicate (of soda). Each have particular properties, and lend themselves to specific operations. Vitrified wheels are most common, and will serve the needs of the home shop type with rare exception.

Resinoid bonded wheels have incredible strength, and are designed to operate in a range that exceeds 9,000 SFPM, yielding exceptionally fast metal removal rates. Parting wheels are generally this type of construction, and are designed at operating speeds as high as 16,000 SFPM. As you allude, a ring test has no value for this type of wheel.

Harold

Harold_V
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Post by Harold_V » Thu Jan 25, 2007 3:31 pm

Victor_R wrote:Excellent write up as usual Harold! I can only think of one thing to add which is that the left side of a dual shaft grinder should have a left handed arbor thread with matching nut, lest it work itself loose during use.

Victor
Thanks, Victor.

The nut issue.

Because I use my grinder in both directions, I didn't worry about a left hand thread, although it would have been better because the wheel in question, when in use, runs the wrong direction for the thread. Had I had my wits about me when I built the grinder, I'd have reversed sides, with the diamond wheel on the left side of the grinder. It mounts by a different system and poses no risk of loosening.

I have always tightened the nut with a wrench, holding the wheel by hand. That has served with no problems in all the years I've used my grinder (over 39 years). I don't use it for heavy grinding, however. It's a dedicated tool grinder, and uses a wheel that is properly suited for the purpose, so grinding pressure is very light.

I would highly recommend a left hand thread for the left side of the grinder. The force of the grind could loosen a wheel, particularly if there's considerable power available, and a rest, on which pressure can be applied.

Harold

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Greg_Lewis
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Post by Greg_Lewis » Thu Jan 25, 2007 10:59 pm

Great info, appreciate the time you put into it. Could you post a pic of the adaptor? THANKS!
Greg Lewis, Prop.
Eyeball Engineering — Home of non-interchangeable parts.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

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Marty_Escarcega
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Post by Marty_Escarcega » Thu Jan 25, 2007 11:37 pm

How about a 2900 RPM motor :?:
Dual Shaft motors at 3450rpm are a little tough to come by....

Marty
"Jack of all Trades, Master of None"

Harold_V
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Post by Harold_V » Fri Jan 26, 2007 12:24 am

Marty_Escarcega wrote:How about a 2900 RPM motor :?:
Dual Shaft motors at 3450rpm are a little tough to come by....

Marty
That's starting out with a serious reduction in surface speed (roughly 20%), so wheels wouldn't perform as intended. That's not to say that they won't work, they'll just behave softer---and break down prematurely. You could get around that issue by buying harder wheels (M, N), or even going to slightly larger wheels. The same wheel bore is also available in 8" diameter wheels, which would be a generous increase in surface speed (slightly over 6,000 sfpm, in fact). That might be the best solution, assuming you can find the proper range of wheels. They aren't nearly as popular as 7" wheels. I've managed to score a few wheels that size on ebay for a song. I'll use them on my surface grinder (some day, anyway! 8)).

While I can't speak for all of them, the 8" ones I bought are rated @ 3,600 RPM.

Harold
Last edited by Harold_V on Fri Jan 26, 2007 1:10 am, edited 1 time in total.

Harold_V
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Post by Harold_V » Fri Jan 26, 2007 1:09 am

Greg_Lewis wrote:Great info, appreciate the time you put into it. Could you post a pic of the adaptor? THANKS!
Greg,

I don't have a pic available, but I pencil sketched cross sections of the pieces a couple weeks ago for our very own SteveM, so I've copied the sketches, below. They're poorly done, chopped out in haste, buy convey the message. Please note that the thread, in this case, is ¾"-16. Thread size is, of course, optional.

The dimensions presented are adequate for mounting a wheel no thinner than ½" thick, but allow for a wider wheel, up to 1". You would narrow the 7/16" dimension if you wanted to mount wheels of less than ½" thickness. Should you make any alterations, make sure that the nut can tighten the flange against the narrowest wheel you choose to run. The grip length in any case should be slightly shorter than the thickness of the moveable flange.

You may have to do a compete redesign for your needs, but adhere to the flange diameters, relief and locating boss diameter, to keep you out of trouble. The balance of the features aren't critical. Concentricity and perpendicularity is always important.

There are almost no dimensions on the side that mounts to the motor, although I did dimension it for a ½" shaft diameter. The OD of the adapter should be adjusted so regardless of motor shaft diameter, you can install set screws that are at least ¼" long. Regardless of diameter, insure that the set screws are flush, or below the surface, when tightened. Lengths should be determined by the length of the shafts on your motor, thus there are none included.

Hope this helps. :oops:

Harold
Attachments
Adapter.jpg
Flange.jpg

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MikeC
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Post by MikeC » Fri Jan 26, 2007 2:19 am

Kind of an aside, my big 10" grinder runs at 1750 rpms. This only works out to about 4800ft/min. I have found two big benefits to such a low speed grinder.

First, as Harold says about the harder wheels, they work much better at lower speeds. I have a typical gray Bay State 24grit pedestal grinder rock on the roughing side. It'll remove HSS at an amazing rate with very little pressure, but doesn't tend to load or burn at all due to the low speed. The fine side is a tan Norton 80. It doesn't move material quite as quick, but can provide a razor sharp edge in seconds. It also has no tendency to load or burn.

Second thing is that 1hp 1750rpm motor. Electric motor hp is basically a measure of torque. I have never even slowed this grinder, much less stalled it, even when sharpening bushog and mower blades. I was fearing it would be somewhat underpowered, considering I was used to the older Craftsman 3/4hp 3450rpm 8"er at work. Point is, a slow turning grinder is not a bad thing if you do as suggested and go to harder grades of wheels to make up for the reduced speed.

On the use of left hand threads, my T&C grinder has right hand threads on both ends of the spindle. The flange washer in Harold's drawing has a tooth and the threaded arbor has a keyway, like the front wheel bearing on a car. That way, the nut is just holding the wheel in place, while he key takes any rotational loads. You can plug reverse to stop the wheel without fear of it spitting a wheel off either side.

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Greg_Lewis
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Post by Greg_Lewis » Sat Jan 27, 2007 10:10 pm

THANKS for the sketches, Harold. They really help.
Greg Lewis, Prop.
Eyeball Engineering — Home of non-interchangeable parts.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

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JimGlass
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Post by JimGlass » Sun Jan 28, 2007 8:12 am

Great information here. We are fortunate to have people like Harold willing to share his knowledge with us.

"Standing back when wheel starts up"
Best safety advise anyone can give. I have always done that. It took 30 years of starting up grinders for it to payoff. Finally, a wheel came apart unexpectedly at start up. I was at a safe distance away.

Jim
Tool & Die Maker/Electrician, Retired 2007

So much to learn and so little time.

www.outbackmachineshop.com

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