Sanding treads with mandrel and sanding block

[FatSebastian]

David Meade's Pinewood Derby Speed Secrets advocates the sanding of wheel treads. This is done by mounting 400-grit sandpaper to a flat sanding block and then pressing the block against a spinning wheel mounted in a mandrel. Meade’s motivation is to "get rid of the defects present on the stock wheels" (p. 63), i.e., the sprue mark from the injection molding process:



With BSA wheels made after 2009, the sprue mark no longer exists and wheel roundness is on average much better than before. However, recommendations to sand the wheel tread continue to linger (for example, Session 5 of Getting Started in Pinewood Derby). But freehand sanding by a child presents an obvious risk of tread re-shaping (which is illegal under most local rules), and it is also hard to image that this technique would improve wheel runout.

However, because we never tried this technique on a new wheel, we could not say for sure whether the risks outweighed the benefits. So we decided to try tread-sanding on one pre-2009 wheel (mold #10) and one new-style BSA wheel (mold #5), both having noticeable run-out. We followed a technique described on pp. 57-59 of Getting Started in Pinewood Derby, using a variable-speed handheld drill on its lowest setting and a Pro-Wheel Mandrel.

We measured the runout before and after each sanding using a homemade concentricity gauge. Each sanding reduced the low reading on the dial indicator by ~0.002". We stopped after 3 sandings, as by then the sandpaper was pretty much used up and the tread was as close to the fluting as we dared to take it. The run-out readings were as follows:

Code: Select all

             Old-style   New-style
Unsanded      0.0090"     0.0050"
1st Sanding   0.0090"     0.0060"
2nd Sanding   0.0085"*    0.0065"
3rd Sanding   0.0075"     0.0070"

*Some sprue-mark visibility was required under many local rules to ensure that wheels were not overly lightened by tread sanding. After our second sanding, the sprue mark was still just barely detectable.

These results seemed to affirm initial suspicions that tread sanding doesn't necessarily improve the runout of new-style wheels. (What's more, it seems there's no easy way to tell what's happening short of measuring.) So we remain unconvinced that the risks of tread sanding outweigh supposed benefits with new-style wheels. I’d be curious if anyone else sands their treads in this way, and if so, if they can tell whether this technique offers any improvement with newer wheels.

[sporty]

FS,

You hit the nail on the head.

Back in 2005/2006/ we were doing this, I recall even posting pic's and videos of the kids doing this.


but that was before my skills and theres developed further. Into realizing. that its simply not worth it.

There is veral issues here, really at play.

The mandrells, with pressure can get out of round also, which can further add to run out. It's easy to do. And even easier with kids trying to do wheels this way.

The newer wheels in stock shape, are far better than the old wheels, and you risk more run out, without the proper equipment, then just leaving them alone.

Other than O.D. size and removing some material to reduce weight. There is no point to do this anymore. (unless you have good equipment, pin gages, dial indicator, ect to check run out, or lathe).


I would even go as far to say, that the shaver tool, out there, the people I know that own one, it takes a bit of practive to use it right and not have more run out then what you started off with.


I strongly feel, its better to pin gage the wheel bores, and get the good ones and a set with the same size bore. over using the old sand paper and block of wood and mandrell and dremel to true up the new wheels.


Sporty

[Noskills]

FS,
As a novice I will tell you that I read and followed the Meade book. I glued 600 sandpaper to a block and lightly saned the thread. I thought I was keeping the wheel flat on the sanding block but it turns out that I did not and I altered the trueness of the wheel. Not having much experience but some common sense I threw the wheel out and used a trued but not lightened wheel I bought from MaxV.

So Meade is not always easy for the novice to follow.

Noskills

[sporty]

The issue that comes about from this older process, is that, there is flex in the mandrell on a drill press, not allot, but a true square block with the sand paper on it, worked best.

Very hard to do right with a dremel or a hand drill.

The bigger issue, is getting 4 wheels, the same o.d and not having run out on the wheels.

Back when the old wheels were so bad, this process was a good option. but the newer wheels are good enough, that this is not needed so much.

You have to look to lathe work, to really improve the wheel or the derby worx wheel shaver might work well, once you perfect your skills in using it.

But still, a dial indicator and set up, is needed to measure the run out and know what you got.

the same goes, with gage pins. Without them, you do not know what you have for wheel bores.

So Noskills, is right along with many others, there expierence when they atempt it.

Sporty

[Stan Pope]

The run-out readings were as follows:

Code: Select all

             Old-style   New-style
Unsanded      0.0090"     0.0050"
1st Sanding   0.0090"     0.0060"
2nd Sanding   0.0085"*    0.0065"
3rd Sanding   0.0075"     0.0070"

Note that both were approaching 0.007" runout. This suggests that the chuck inherent runout was about 0.007. Your dial indicator should allow you to zero in on the culprit ... bad rotor axis, bent axis, poor chuck, bent mandrel, ...

If you turn a wheel on the mandrel and get 0.000" runout while still mounted on the mandrel, you can then rotate the wheel 180 degrees on the mandrel and recheck the runout. The true runout will be 1/2 the 2nd measurement! That measure shows both the inherent runout at the mandrel and of the finished wheel.

0.005" runout is certainly inferior to your goal, so improving is worthwhile. Probably have to use a different machine though.

[FatSebastian]

If you turn a wheel on the mandrel and get 0.000" runout...

When we say "turn a wheel on the mandrel", I suppose that we mean turn in the machine-shop sense of cutting on a lathe.

The issues of the inherent runout of the turning device and mandrel are all valid; these caveats generally went unmentioned when recommendations were published regarding the sanding of treads with a mandrel and sanding block. Most of those who would resort to sanding treads with mandrel and sanding block probably do not have access to machine-shop tools and methods. Also, I am not confident that a spinning hand-held drill, to which pressure is also being applied transversely from the sanding block, will always be held steadily enough at speed (to within a few thousands of an inch), even if its inherent runout was superior.

0.005" runout is certainly inferior to your goal, so improving is worthwhile. Probably have to use a different machine though.

Or different wheels. Because tread modifications are illegal under our local rules, our practice has been to simply purchase extra wheels, avoiding the ones with the worst run-out (per the concentricity gauge).

[Stan Pope]

If you turn a wheel on the mandrel and get 0.000" runout...

When we say "turn a wheel on the mandrel", I suppose that we mean turn in the machine-shop sense of cutting on a lathe.

I'm actually referring to the concept of turning ... whether it is a commercial lathe or a hand drill + mandrel + sandpaper block the concept is the same! And the test I described is equally effective.

[FatSebastian]

I suppose that we mean turn in the machine-shop sense...

I'm actually referring to the concept of turning ... whether it is a commercial lathe or a hand drill + mandrel + sandpaper block the concept is the same!

Let me rephrase: I suppose that "the concept of turning" implies the removal of material from a rotating object. (For those unfamiliar with machining, "turning a wheel" might be mistaken as "rotation" -- without removing material.)

[*5 J's*]

what if you chuck the wheel directly into the drill press and bypass the mandrel? You will still have the runout of the chuck though but that should be less then the combined mandrel/chuck runout.

[FatSebastian]

Good to hear from you again *5 J's*.

what if you chuck the wheel directly into the drill press and bypass the mandrel?

We've never done this. If this involves grasping the wheel by the hub, then wouldn't the concentricity of the hub relative to the bore become an added factor?

[*5 J's*]

Good to hear from you again *5 J's*.

what if you chuck the wheel directly into the drill press and bypass the mandrel?

We've never done this. If this involves grasping the wheel by the hub, then wouldn't the concentricity of the hub relative to the bore become an added factor?

yes - but I prefer this method over the commercially available mandrels.

[macd]

I found some Big Rig kits on clearance at the scout store this year, so my son and I have a 36 wheels to sort through.
Is picking out matching mold numbers (looking for the documented higher quality numbers) a close enough proxy to taking the measurements mentioned above?

Does anyone have additional comments to add to the thread about the pros/cons of the sanding the treads?
I've become convinced that I'm more likely to do harm than good sanding the wheel.
My rules prohibit 3rd party wheels.

[5kidsracing]

With the new wheels I would either shave them with a DerbyWorx shaver or lathe them, but not sand them. You will most probably make them worse by sanding, I know I would.

[FatSebastian]

Is picking out matching mold numbers (looking for the documented higher quality numbers) a close enough proxy to taking the measurements mentioned above?

After measuring hundreds of wheels, and after being significantly educated by others about the injection molding process, we no longer trust mold numbers and just prefer to measure the runout with a dial indicator.

FYI, Troy Thorne's latest book includes some nicely illustrated instructions on assembling a gauge. Weren't you going to go shopping for a dial indicator?

Does anyone have additional comments to add to the thread about the pros/cons of the sanding the treads?

I liked 5kidsracing answer. Of course another option besides lathing or sanding is to just leave the treads alone.

[SirStorm]

I have stopped "turning" my wheels for the run out issue. I do not have a metal lathe. I bought a dial indicator and measured my new wheels. Since I cant my axels I measured the inner rim edge (not inside!). All of my tools had more run out in the chucks than the amount of run out in the unmodified wheel (MVwheels). I own several drills, a dremel, a wood lathe and a wheel shaver. In no case was I able to improve the run out. The best I achieved was to move the highspot to a different location. My conclusion was, buy lots of wheels. Pick the best set using the dial indicator. Give your rejects to your competitors.

The only true way to true the wheel is to take a lathe, cut a custom mandrel to mount the wheel. Cut the wheel. If the mandrel is removed, you must cut a new one next time.

Also, if you use a commercial mandrel, you are guaranteed to add more error to your chuck run out.