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Kevin Brown
Sep 28th, 2010, 02:38 PM
For quite some time I have been working on a paper about the innerworkings of the random orbital. While I have not had a whole lot of extra time this year to devote to completing it, I decided to post a modified portion of it. This section pertains to MACHINE STROKE, and how it may affect overall performance. I hope it is easy to understand and will help those that are interested in maximinzing the potential of their machine. For those of you on the fence regarding which machine to purchase next, perhaps this article will help you decide.



Machine stroke is a term that identifies the diameter of the path the backing plate travels as it circumnavigates the driveshaft.
Since the machine's driveshaft is rarely level with the backing plate, machine stroke actually refers to the driveshaft axis and the backing plate or backing plate mounting spindle axis. The diagram below illustrates this point:


http://i425.photobucket.com/albums/pp334/iamwaxman/stroke-calculated-900.jpg


Machine manufacturers also refer to machine offset, backing plate offset, stroke radius, and stroke diameter. Regardless the terminology, these terms are identifiers of the machine's movement.


To clarify:

Machine offset or backing plate offset or stroke radius
The distance between the driveshaft axis and the backing plate axis

Machine stroke or stroke diameter
The distance between the driveshaft axis and the backing plate axis multiplied by two

A longer offset or stroke places the backing plate axis farther away from the driveshaft axis.


If the machine's driveshaft is rotated once, the backing plate shaft will travel around the driveshaft axis once, creating a perfect circular pattern. This motion is referred to as an orbit. Machine manufacturers often refer to operating speeds using "Orbits Per Minute" or an "OPM" rating. Some manufacturers refer to operating speeds using a "Revolutions Per Minute" or "RPM" rating. This is completely accurate, but can cause confusion because this rating is sometimes thought of as referring to the backing plate's rotational speed. Since the backing plate rotates independently of the machine's driveshaft (and does so at a random rate), an RPM rating can be confusing. The diagram below compares orbit speed to orbits per minute:


http://i425.photobucket.com/albums/pp334/iamwaxman/stroke-pencil-analogy-900.jpg


A majority of random orbital machines use a stroke diameter that measures somewhere between 1/8" to 5/16" (approximately 3.0mm-8.0mm). It is generally accepted that a smaller stroke leaves a more refined finish, but reasoning and experience tell a different story. After all, if the sanding disc being utilized features consistently sized particles that are evenly placed and proper sanding techniques are used while sanding, a satisfactory result will be realized regardless the stroke. The diagram below illustrates this point:


http://i425.photobucket.com/albums/pp334/iamwaxman/stroke-efficiency2-900.jpg


Factually, a small stroke machine does confine sanding to a smaller area. This means that for a given amount of time, sanding of any one point will be multiplied. If you plan on using the machine to sand or polish items diminutive in size, a small stroke machine is the way to go. A small stroke machine is also a great choice if you will be using the machine to work on small areas (such as touch-up jobs or similar tasks), or on confined areas that may limit machine movement. In addition, a small stroke machine is generally easier to control because the backing plate orbits around the driveshaft axis along a smaller circular path. This equates to less lateral motion of the machine as it is held in your hand (versus a similarly outfitted large stroke machine), and is therefore easier to grasp while the machine is in use.

Otherwise, a machine featuring a large stroke delivers increased speed of backing plate motion using the same RPM setting. A large stroke also increases movement of the sanding disc, so "leveling" of the area is more consistent. Increased movement may help abraded residue more readily escape from between the disc and sanded surface, so consistent sanding results and extended life of the disc may be realized. The diagram below illustrates the differences between four of the most popular stroke sizes in relation to spindle speed:


http://i425.photobucket.com/albums/pp334/iamwaxman/stroke-speed-900.jpg


By now you may be wondering if a small stroke machine could deliver the best of both worlds by simply increasing the RPM of the machine. While this would likely increase random rotation of the backing plate, there would be several drawbacks. Most noticeable would be a decrease in user comfort, as the machine would be more difficult to control. Vibration would also increase, making the machine uncomfortable or unusable for users susceptible to fatigue. It is important to also note that since the backing plate would not have extended movement as would a long stroke machine, abraded residue and sanding debris (such as dislodged abrasive particles) might not clear away from beneath the sanding disc. The list goes on, but the most important issue that might occur due to an increase in RPM has to be a potential decrease in overall sanding performance. The following three diagrams illustrate this point:


http://i425.photobucket.com/albums/pp334/iamwaxman/stroke-pens-backing-plate-900-76q.jpg

http://i425.photobucket.com/albums/pp334/iamwaxman/stroke-patterns-900-53q.jpg

http://i425.photobucket.com/albums/pp334/iamwaxman/stroke-measurements-900-61q.jpg


Keep in mind that in terms of leveling a large surface (one that is larger than the diameter of the pad), there is no denying that a large diameter disc will level that surface more consistently than a small diameter disc. Since this article pertains to machine stroke, how best to illustrate the performance differences between small and large stroke machines? A comparison of machine stroke to a typical handheld sanding block helps to illustrate the point:

http://i425.photobucket.com/albums/pp334/iamwaxman/stroke-surface-leveling-900.jpg


What Should You Consider Before Purchasing a Machine?

There are five important factors to consider when choosing the ideal stroke size:

Control of the Sanding Area
Cushioning
Centripetal Force
Backing Plate Diameter
User Comfort

1. Control of the Sanding Area
Controlling the size of the area being sanded is accomplished primarily by the user's movement of the machine. The diameter of the sanding disc has an obvious effect, too. Finally, if the machine is held in place and throttled, a large stroke machine will sand a larger area than a small stroke machine. As an example, a machine touting a 3/32" diameter stroke outfitted with a 6" backing plate will effectively sand an area 6-3/32" in diameter, whereas an identical machine featuring a 5/16" diameter stroke will sand an area 6-5/16" in diameter. Although a 7/32" variance may seem trivial, it might not be considered as such if the 6" backing plate was swapped for a 3" backing plate.



2. Cushioning
If a majority of work is going to be done using interface pads for contouring purposes, or if a lot of paint polishing will be done using the machine, it is important to note that much of the machine's motion could be affected due to cushioning from the interface or buffing pad. This is especially true if the machine is used at a high oscillation speed, as the time frame for the foam to react is dramatically shortened. Machines featuring a small stroke are certainly more susceptible to this occurrence, so this should be considered prior to purchasing a machine.



3. Centripetal Force
Be patient reading this section- it can be hard to comprehend.
Before we discuss what centripetal force has to do with machine stroke size, it is important to understand what it is. Do not confuse centripetal force with centrifugal force- they are different! Centripetal force, loosely defined, is a "center seeking force". This means that any entity that causes another entity to form a circular pattern of motion is placing centripetal force upon that object.

An easy to understand example of centripetal force has to do with a boy holding the end of string that has a ball attached to the other end. If the boy spins in place fast enough, the ball will take flight, and string tension will direct the ball in a circular pattern of motion. The boy, through string tension, is directing the ball in a centripetal manner. Ultimately, you could say that the string is placing centripetal force on the ball.

What if the string is lengthened?
Even if the boy spins at the same rate, the ball will be moving faster because it has to travel more distance in the same amount of time.

What if the string breaks?
The kinetic energy of the ball (the energy it has while in motion) immediately forces the ball away from the boy. This reaction is known as a reactive centrifugal force, often referred to as centrifugal force. Centrifugal force is not important for this discussion- it's just good to know the difference. To increase kinetic energy, either mass or velocity must be increased. A-ha!

What does all this have to do with stroke size?
Quite a bit! If the stroke diameter is increased, all of the parts that are mounted to the backing plate spindle will travel at a faster rate of speed because they are traveling a longer distance in the same amount of time. The resultant speed increase also increases the development of kinetic energy. An increase in kinetic energy means that the backing plate (which is mounted to a free-spinning spindle and rotates at a random rate) will rotate more rapidly. Further, more resistance will be required to slow backing plate rotation.

Bottom line:
All other parameters being equal (machine, backing plate, disc or pad, sanding surface, applied pressure, and RPM setting), a long stroke machine will spin the backing plate faster or longer than a similarly equipped short stroke machine.



4. Backing Plate Diameter
Sanding discs are available in a wide variety of diameters, ranging from 1-1/4" to 12". For the most part, 3", 5", and 6" diameter discs are the most popular sizes. Woodworkers seem to favor 3" and 5" diameter discs, while 3" and 6" diameter discs seem to get the nod for automotive sanding duties. Regardless the job, common sense should be used when pairing specific disc sizes with small or large stroke machines.

On the small side of the scale, 3" system (backing plate and sanding disc) would likely be used for spot jobs or tasks that require small diameter discs due to space limitations. Small diameter discs work well on complex or heavily contoured areas, too. While it is not unreasonable to use a large stroke machine with small diameter sanding discs (I personally use a 5/16" stroke machine quite a bit with 3" pads for the added centripetal force it creates), most times a small stroke machine is less unnerving to use because there is less side-to-side (or lateral) movement of the backing plate. Therefore, there is less likelihood that the backing plate will come into contact with surrounding parts or surfaces. If the machine is going to be used on surfaces that are not very wide (such as automotive A-pillars, stainless steel body trim, or thin strips of wood), then a small stroke machine will not be so apt to "ride off" the piece, possibly gouging an edge in the process.

On the other end of the scale, a machine outfitted with a 6" sanding system (backing plate and sanding disc) benefits from a larger stroke if the surface area to be sanded is large, or substantial amounts of material needed to be removed. As previously discussed, increased movement of the backing plate allows abraded residue to more readily detach and then evacuate from the sanding disc, keeping it and the sanded surface clean. A small stroke machine outfitted with a 6" sanding system allows the user to strategically sand and "level" larger portions of a panel compared to a similar machine equipped with a 3" sanding system. An example when this might come in handy? A desire to remove of a long but gradual sag in a paint job without removing paint outside the edges of the sag. Parts or materials that cannot withstand lots of shaking due to lateral movement of the backing plate would benefit from the movements developed by a small stroke machine.



5. User Comfort
A short stroke machine does not move the backing plate as much as a large stroke machine. For users that are susceptible to discomfort due to high frequency vibration (such as when a machine is run at a high speed setting), the small stroke machine may cause more discomfort than a similarly equipped large stroke machine. For users that are susceptible to discomfort due to lateral motions (such as encountered when a machine wiggles back and forth due to the weight of the backing plate and items attached to it), a large stroke machine may cause more discomfort than a similarly equipped small stroke machine.


In general:

A machine featuring a 3/32" diameter stroke is optimal for sanding small parts, or for sanding areas that are hard to reach due to clearance restrictions. A small stroke machine works exceptionally well on delicate parts because lateral movement of the backing plate is minimized (thus delivering a gentler movement that a large stroke machine). Random rotation of the backing plate will generally be negligible unless the machine is set to a high speed setting (which can be a benefit or drawback, depending upon the situation).


A machine featuring a 3/16" diameter stroke is ideal for most tasks because it delivers ample movement of the backing plate, but it can still be used in tight or confined areas without a problem. When used for polishing paint with buffing pads and liquids, a machine featuring a 3/16" diameter stroke will usually generate enough centripetal force to keep the backing plate and buffing pad rotating. Of course, a very tall pad will negate much of the machine's motions.


A machine featuring a 5/16" diameter stroke is the best choice if there is ample room to use the machine, and the machine is not used regularly on parts or surfaces that are not very wide.

Mark Kleis
Sep 28th, 2010, 02:52 PM
I would like to propose the adoption of a new nickname for Kevin Brown, "The Scientist."

Kevin, this is simply unbelievable. I won't even pretend to fully comprehend everything you just explained, and it is clear you know this stuff inside out and backwards.

Kudos sir, you are a walking detailing Encyclopedia.

Kevin Brown
Sep 28th, 2010, 02:56 PM
I would like to propose the adoption of a new nickname for Kevin Brown, "The Scientist."

Kevin, this is simply unbelievable. I won't even pretend to fully comprehend everything you just explained, and it is clear you know this stuff inside out and backwards.

Kudos sir, you are a walking detailing Encyclopedia.

Thank you!!! :dance1

BTW- I would hope you get a little more time to browse through it all. I just posted it and you are already DONE a' reading!!! :nervous1

Michael Stoops
Sep 28th, 2010, 03:32 PM
This is some pretty heady stuff Kevin! Not a light read, but great stuff for anyone interested in the dynamics of what's really going on when we put pad to paint.

It's going to take some time to digest and get through fully - probably multiple readings as well - but we did want to comment on all your hard work. Great, great information here and it is very much appreciated!

Tim Lingor
Sep 28th, 2010, 03:38 PM
Hey Kevin,

Simply outstanding!!! I am extremely impressed!!!! :worship

Tim

Shawn T.
Sep 28th, 2010, 04:06 PM
Fantastic work! I will have to read this a few times over the week.

Thank you for posting such great work!

Peter_Klim
Sep 28th, 2010, 04:17 PM
WOA! Great technical post! I love reading them! Somewhere in there I think is the cure for cancer!

Pkloz2010DC
Sep 28th, 2010, 04:50 PM
Wow, this could take awhile to get through. Kevin, is there any way you could post this as a PDF so we could print it ouy and do a better read of it?

Mark Kleis
Sep 28th, 2010, 09:27 PM
Thank you!!! :dance1

BTW- I would hope you get a little more time to browse through it all. I just posted it and you are already DONE a' reading!!! :nervous1

Hah, I am "working" so I only had time to skip through the main stuff. This one will need a free hour or two :D

dvtldav
Sep 29th, 2010, 03:32 AM
Good info Kevin. As a woodworker one thing I have learned using a random orbital sander is that the smaller the stroke, the better the finish or less sanding marks. Maybe the sanding marks are not less they are just closer together so less noticeable. I learned in a wood finishing seminar that all finishes have scratches it is just that the smaller the scratches the shinier the finish. Again, great article.

Dave

RDVT4ME
Sep 29th, 2010, 06:29 AM
Wow Kevin, great stuff!

I think there are a number of papers you could write and we would love to see them.

Kevin Brown
Sep 29th, 2010, 09:03 PM
This is some pretty heady stuff Kevin! Not a light read, but great stuff for anyone interested in the dynamics of what's really going on when we put pad to paint.

It's going to take some time to digest and get through fully - probably multiple readings as well - but we did want to comment on all your hard work. Great, great information here and it is very much appreciated!

Yes, it's not the type of info designed to skim over... THANKS!!


Hey Kevin,

Simply outstanding!!! I am extremely impressed!!!!

Tim

I hold your opinion in high regard, and appreciate the kind words, Tim. :woot2


Fantastic work! I will have to read this a few times over the week. Thank you for posting such great work!

I hope you get a lot out of it- Thanks!


WOA! Great technical post! I love reading them! Somewhere in there I think is the cure for cancer!

Thanks for your enthusiasm! :dp:


Wow, this could take awhile to get through. Kevin, is there any way you could post this as a PDF so we could print it ouy and do a better read of it?

I have it in PDF now, but as one PDF file it's about 3.6 megabytes. I broke it into individual pages, but still, some of them are 500k.
Don't think I can upload... but can e-mail. Thanks!


Good info Kevin. As a woodworker one thing I have learned using a random orbital sander is that the smaller the stroke, the better the finish or less sanding marks. Maybe the sanding marks are not less they are just closer together so less noticeable. I learned in a wood finishing seminar that all finishes have scratches it is just that the smaller the scratches the shinier the finish. Again, great article.

Dave

Thanks, Dave. The general consensus is as you described.
But as discussed in the paper, the graphs and the reasoning point us towards a different conclusion.


Wow Kevin, great stuff!

I think there are a number of papers you could write and we would love to see them.

I can always count on you for a nice compliment, Bob. Appreciate it! :wavey

the other pc
Sep 29th, 2010, 09:51 PM
:xyxthumbs

rapport25
Sep 30th, 2010, 06:56 AM
Time to covert to PDF and some light bed time reading. Great write up :D.

Kevin Brown
Sep 30th, 2010, 07:00 AM
I have it in PDF now, but as one PDF file it's about 3.6 megabytes. I broke it into individual pages, but still, some of them are 500k.
Don't think I can upload... but can e-mail. Thanks!



Time to covert to PDF and some light bed time reading. Great write up :D.

Happy to e-mail it if yo PM me.

Kevin Brown
Sep 30th, 2010, 01:12 PM
Also have in a PDF.

Easier to read but it's bout 3.6 megabytes, so e-mail me and I'll fire off a copy to you! :xyxthumbs iamwaxman@aol.com

rapport25
Sep 30th, 2010, 02:27 PM
Also have in a PDF.

Easier to read but it's bout 3.6 megabytes, so e-mail me and I'll fire off a copy to you! :xyxthumbs iamwaxman@aol.com

Kevin you have mail.

Many Thanks Simon :xyxthumbs

Kevin Brown
Sep 30th, 2010, 02:39 PM
Kevin you have mail.

Many Thanks Simon :xyxthumbs

On the way... happy reading.

rapport25
Sep 30th, 2010, 02:42 PM
:thankyou1
On the way... happy reading.

sontakke
Oct 1st, 2010, 07:35 AM
I guess the hell has finally frozen over!! :-)

Kevin Brown
Oct 1st, 2010, 09:59 AM
I guess the hell has finally frozen over!! :-)

:huh1

ShinyChevy
Oct 7th, 2010, 01:22 PM
Thanks Kevin, great article as usual! :goodjob2

Could you post, or tell us where to find, the stroke lengths of various DA sander/polishers such as as the PC, G110, UDM, large and small air DAs and the Flex DA?

Thanks again! :patriot

Kevin Brown
Oct 7th, 2010, 10:14 PM
...Could you post, or tell us where to find, the stroke lengths of various DA sander/polishers such as as the PC, G110, UDM, large and small air DAs and the Flex DA?...

Well, most of the machines list their stroke, orbit, or offset measurements.
The major players in the air-tool business generally offer their machines in 3/32", 3/16", and 5/16" diameter orbit.

Here's a list of some of the most popular 5" & 6" electric-powered machines.
"RO"=random orbital, "FRO"=forced rotation orbital, and a machine listed as both is switchable between the two.

1/8" stroke diameter
(RO) Griot's 10925 (original machine) - 1/8" stroke diameter (not verified and not likely that small)


7/32" stroke diameter
(RO/FRO) Makita BO6040


9/64" stroke diameter
(RO/FRO) Festool Rotex RO125 FEQ


3/16" stroke diameter
(RO) DeWalt DW443

(RO/FRO) Bosch 1250 DEVS (specs list 3/32" offset)
(RO/FRO) Festool Rotex RO150 FEQ


5/16" stroke diameter
(RO) Meguiar's G100
(RO) Meguiar's G110
(RO) Meguiar's G110v2
(RO) Porter Cable 7335
(RO) Porter Cable 7336
(RO) Porter Cable 7424
(RO) Porter Cable 7424XP
(RO) Griot's 10765 (new machine)

(FRO) Flex XC 3401 VRG

(RO/FRO) Festool Rotex RO125 FEQ
(RO/FRO) Festool Rotex RO150 FEQ


3/4" stroke diameter
(RO) Dynabrade 61374 (attachment for rotary buffer)
(RO/FRO) Dynabrade 61379/61384 (attachment for rotary buffer)